STANDARDS/MANUALS/ GUIDELINES FOR SMALL HYDRO DEVELOPMENT

Transcription

1 STANDARDS/MANUALS/ GUIDELINES FOR SMALL HYDRO DEVELOPMENT Electro-Mechanical Works Guidelines for Power Evacuation and Interconnection with Grid Sponsor: Ministry of New and Renewable Energy Govt. of India Lead Organization: Alternate Hydro Energy Center Indian Institute of Technology Roorkee Sep 19, 2008

2 CONTENTS Sl. No. Items Page No. 1 OVERVIEW Objective 1 2 REFERENCES & CODES 1 3 VOLTAGE LEVELS Generation Voltages Transmission Voltage 2 4 GENERAL CONSIDERATION FOR SWITCHING ARRANGEMENTS IN 2 SWITCHYARDS 4.1 Inter Connected Transmission System Voltage Level Site Considerations General Miscellaneous Considerations 3 5 SWITCHING SCHEMES FOR SWITCHYARD Double Bus and Single Breaker Scheme Single Bus Schemes Double Bus Schemes Ring Bus Scheme 7 6 POWER EVACUATION AND INTERCONNECTION WITH GRID Power Evacuation Modes Requirements Power Evacuation Provision in Generating Equipment Isolated Operation Mode Minigrid Operation Grid Interconnections 11 7 ISLAND OPERATION General Hazards of Islanding Common Cause Tripping Sources of Common Cause Tripping of Generators 16 8 ISLANDING DETECTION AND PROTECTION General Islanding Detection Systems and Settings Islanding Network Protection 20 9 IMPLEMENTION OF ISLANDING PROTECTION General 22

3 9.2 Interconnection Protection Requirements SURGE PROTECTION General Location of the Arrester Earthing Selection of the Lightening Arrester Generators Directly Connected to LV Overhead Lines HARMONICS POWER LINES SPECIFICATION (EXAMPLE 33 K POWER LINE) Design Consideration Installation Guidelines 39 Annexure I Explanatory Notes on Earthing 44 Annexure II Explanatory Notes on Islanding Detection & Protection 58 Annexure III Explanatory Notes on Neutral Voltage Displacement (NVD) Protection 63 Annexure IV Central Electricity Authority (Grid Standards) Regulations Schedule Grid Standards for Operation and Maintenance of Transmission Lines 74

4 GUIDELINES FOR POWER EVACUATION AND INTERCONNECTIONS WITH GRID 1.0 OVERVIEW Small hydroelectric power plants are mostly located far away from load centres. It becomes, therefore, necessary to step up generation voltage through step up transformers in switchyard located near power plant and connect the same to grid substation through transmission line at a suitable point. 1.1 Objective The intent of this guideline is to provide guidance for selecting voltage level for power evacuation, bus bar arrangement for connected switchyard, interconnection with isolated load or grid, selection of necessary protection scheme for the selected grid interconnection. The guidelines also explain importance of various earthing systems of generators as well as distribution/transmission lines emanating from the switchyard. Central Electricity Authority (Grid Standards) Regulation-2006 has also been annexed for guidance of generation as well as transmission companies. 2.0 REFERENCES & CODES I. CEB guide for grid connection of embedded generators II. Central Board of Irrigation and Power (India) Manuals. III. IEEE Standard C IEEE Guide for generator grounding. IV. IEEE IEEE guide for protective reclosing. V. IEEE Standard 242, 1996 IEEE recommended practice for protection. VI. REC Specification 30/1984. VII. Central Electricity Authority (Grid Standard) Regulation VIII. For Power Line reference of relevant ISS are given in section VOLTAGE LEVELS 3.1 Generation Voltages Generation voltages are generally limited to following levels (CBI&P Manual): Generation Voltage Level Upto 750 kva 5.15 Volt 751 to 2500 kva 3.3 kv 2501 to 5000 kva 6.6 kv Above 5000 kva 11.0 kv Generally terminal voltage for large generators is 11 kv in India. Generator with high terminal voltages upto 20 kv is being made. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 1

5 3.2 Transmission Voltages Step up voltage for transmission system depends on the following factors Length of transmission line for interconnection with power system Power to be transmitted High voltage increases cost of insulation and support structures for increased clearance for air insulation but decreases size and hence cost of conductors and line losses. There are some empirical formulae to calculate economic voltage for power evacuation few of these are as follows: Voltage in kv (line to line) = Where: * L is length of transmission line * kva is power to be transmitted For the purpose of standardization in India transmission lines are classified as under: Transmission Sub-transmission Distribution 66, 132, 220, 400 kva above 33 kva 11 kv & below Higher voltage system is used for transmitting higher amounts of power and longer lengths and its protection becomes important for power system security and requires complex relay system. 4.0 GENERAL CONSIDERATION FOR SWITCHING ARRANGEMENTS IN SWITCHYARDS Main considerations for selecting a suitable and economical switching arrangement are as follows: Inter connected transmission system Voltage level Site limitations General & special considerations 4.1 Inter Connected Transmission System The switching should fit in the planning criteria used to design transmission system. System should remain stable if a fault occurs on line. It is therefore, important to avoid system un-stability caused by outage of line, transformer or generator due to fault in switchyard substation. Sustained generation outage by such faults should not exceed available spinning reserve. This could exceed the spinning reserve only to the extent by which important loading be connected, to be dropped automatically by under frequency actuated relays. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 2

6 4.2 Voltage Level up. Following points must be considered at the time of selection of voltage level for step Power carrying capability of transmission line increases roughly as the square of the voltage. Accordingly disconnection of higher voltage class equipment from bus bars get increasingly less desirable with increase in voltage level. High structure are not desirable in earth quake prone areas. Therefore in order to obtain lower structure and facilitate maintenance it is important to design such switchyards preferably with not more than two levels of bus bars. 4.3 Site Considerations Practical site consideration at a particular location e.g. lack of adequate flat area of layout of equipment in the switchyard may also influence the choice in such locations. Pollution caused by location near to sea or some other contaminated atmosphere may also effect layouts. At some location completely indoor, switchyards even at 400 kv level have been made while at some other location high type (both buses one above other) layout have been adopted in view of lack of space available at site. 4.4 General Miscellaneous Considerations Other consideration in the selection of suitable arrangement and layout are given below: Repair and maintenance of equipment should be possible without interruption of power supply Future expansion of switchyard should be easily possible In seismic prone zones height of structures should be as low as possible. The outgoing transmission line should not cross each other. 5.0 SWITCHING SCHEMES FOR SWITHYARD Following schemes are considered for planning and design of different types of switchyards A double bus single breaker scheme A single bus single breaker scheme - Single & transfer bus - Sectionalized single bus A double bus one and half breaker Double bus double breaker scheme Ring bus 5.1 Double Bus and Single Breaker Scheme This scheme is quite common on large and medium station upto 220 kv in India being economical and maintenance of breaker is possible by utilizing bus coupler. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 3

7 This scheme has following disadvantages Selection of bus is by isolators which is a weak link. Inadvertent operation on load inspite of interlock, may damage switch. Utilising bus coupler during maintenance will necessitate transfer of tripping circuits through auxiliary contacts Discretion of operators to select bus is not desirable. If machines are on one bus then entire power generation will be lost in case of bus fault which is not desirable for large generating station. 5.2 Single Bus Schemes Single Bus Single Breaker All units and outgoing lines are connected on single bus. In case of bus fault entire generation is lost. This is generally provided on small generating station. FIG. 2 SINGLE BUS SINGLE BREAKER SCHEME AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 4

8 5.2.2 Single Bus with a Transfer Bus This scheme is useful for feeder breaker maintenance, but involves transfer of tripping circuits through auxiliary switches. Generator breakers are maintained along with unit maintenance outages Single Sectionalized Bus This scheme is very commonly used being economical; generation outages can be controlled by sectionalizing. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 5

9 Simple arrangements, does not require isolating switches to select bus. This system is adopted even on large hydropower stations and most medium power stations where parallel outgoing feeders are provided. 5.3 Double Bus Schemes Double Bus One and Half Breaker This scheme is very reliable and used on EHV system. Cost is high as one and half breaker is used for each element. In case of fault not more than one element is lost on any outage. Maintenance of any breaker is possible without outage Double Bus Double Breaker Scheme This scheme is very reliable but very costly as it requires two breakers for each element. Generally this is recommended for medium and large power stations where reliability is of highest priority. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 6

10 5.4 Ring Bus Scheme This scheme is reliable and economical as only one element is lost in case of any fault, but protection is complicated and hence not used in India. It is used in British grid system for sub-stations and not in switchyards of generating stations. 6.0 POWER EVACUATION AND INTERCONNECTION WITH GRID 6.1 Power Evacuation Modes (a) Micro hydropower stations upto 200 kw unit size with electronic load controller Isolated operation Grid / mini grid connected operation (b) Mini and small hydropower plant (unit size 0.2 to 5 MW) with integrated governor and plant control Isolated operation Grid / mini grid connection (c) Small hydropower plant (SHP) unit size 5 MW to 25 MW size with integrated governor and plant control with conventional manual facility Grid connected operation only 6.2 Requirements Power Evacuation Step up voltage at generating station to be fixed in accordance with para 3 and detailed economic studies. Interconnected transmission and switching scheme to be designed in accordance with para 5 above. Transmission line protection to be provided in accordance with prevalent schemes for different voltage levels. The high voltage transmission lines must be disconnected both at receiving end as well as sending end by carrier or other communication signals. Provide for no voltage closing for receiving end breakers and synchronizing check relay closing at sending end breaker. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 7

11 It is normal practice to provide synchronizing facilities at sending end breaker of transmission line. 6.3 Provision in Generating Equipment Isolated and islanding operation will require adequate flywheel for stable operation for commercial load changes. This may be checked by full load rejection. The speed rise should not be more than 35% or even lower in case of special (large motor) load characteristics. In case isolated operation is not required flywheel effect could be reduced and the criteria of speed rise on full load rejection can be increased upto 55% - 60%. Excitation system for generator should have a provision for power factor control in grid connected mode. Voltage control is required for isolated mode operation as well grid connected mode operation. Before synchronizing machine with the grid voltage control mode is required and after synchronizing change over from voltage control mode to power factor control mode is required. In case of micro-hydels manual excitation control with excitation limit could be provided. The transformers for micro hydels for interconnection with the grid should be cast dry outdoor type as per REC specifications 30 / 1984 which corresponds to IEC 726. The transformers are suitable for harsh conditions and require less rigid maintenance schedule. The transformers should be connected with grounded star on low voltage side and delta on high voltage side, so that grounded neutral is provided for local load in case the SHP is shut down. The generator breaker and bus bar at generator voltage be provided for islanding operation. For 33 kv and above line side breakers should be electrically operated circuit breakers. For islanding operations synchronizing arrangement should be made from generator breakers as well from interconnecting LV/MV grid breaker. Reclosing on receiving end grid breaker should be prevented/ blocked. 6.4 Isolated Operation Mode Isolated power supply systems using renewable technologies are emerging as technically reliable and economical option for power supply to remote and secluded places. For electrification of remote and secluded rural areas there are two general methods; grid extension and diesel generators. In remote areas both options are extremely costly as such renewable resource such as river based or stream based micro mini or small hydropower plant with isolated operation mode provide low cost alternative. For such systems following provisions in generating equipment are essential: AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 8

12 Adequate fly wheel affect for stable operation for commercial load changes on full load rejection speed rise should be less than 35% Excitation system for generator should have provision of voltage control. In case of micro hydels manual excitation control with excitation limit can be considered. Electronic load controller or Induction Generator controller can be used for load variations. Fig 8 & 9 shows standard connection for isolated operation. 6.5 Minigrid Operation Provision for Mini Grid Operation A mini grid is the connection of two or more generating system without main grid. So no reference of voltage & frequency for synchronizing. Stabilizing main grid is also not required. Load balance between generating facilities and load required in mini grid. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 9

13 Synchronization and load balance can be achieved by using ELC system on each generator. Need to provide Black Start where mini grid is dead. Fig 10 shows connection of mini grid system Synchronization with ELC Procedure Run up turbine ELC regulates speed/ frequency Mains and hydro side protection sets, Auto synchronizer than controls ELC to bring generator and put mini grid in synchronism. When synchronization occurs main contactor/ breaker is closed and system is connected to grid Advantages of synchronization with ELC Provides smooth synchronization No need of fine control of water flow to turbine Can synchronise at low power to use small ELC e.g. 1 MW system would synchronise at 50 to 100 kw. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 10

14 6.6 Grid Interconnections Types of Grid inter Connections There are mainly following type of grid connections of SHP Power plants with substantial local load These power plants will regularly operate in parallel with connected grid, but supply the requirements of localized industrial or other facility. The power plant and load when considered together may either be a net importer or net exporter of electrical energy from grid Power plants with minimal local loads These power plants are built to harness a source of energy. These power plants are typically connected to grid through a dedicated line. Typically there will be no other customer load between the grid substation and power plant. The generators of these power plants will always operate in parallel with the grid. There are several plant presently in operation with minimal local load which supplies the requirement of generating pant and associated residential colonies and drinking water system System Arrangement Typical arrangement of grid connection and local feeders is shown in fig.-11. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 11

15 6.6.3 Voltage Levels for Interconnection The voltage at the point of supply from the small hydropower plant shall be determined as follows: Micro HPs with installed generation capacity upto and including 250 kva shall be inter connected at LV distribution voltage level, if the nominal voltage of generator is 415 V line to line, three phase or 240 V line to neutral, three phase SHPs with an unstalled generating capacity exceeding 1000 kva shall be interconnected at 11 kv or 33 kva. For installed capacities between 250 kva to 1000 kva, it can be decided on case by case basis, depending on the capacity of transformer sub-station SHPs with an installed capacity exceeding 5000 kva shall be interconnected at 66 kva or 132 kv depending on voltage level of nearest grid sub-station Other Miscellaneous Considerations Voltage rise The voltage rise due to generation at grid sub-station bus bar must be within operational limits. The target bandwidth for voltage for 33 kv bus bar of grid sub-station is 3%. The target voltage on 33 kv bus bar of the grid sub-station is 33 kv Earthing Guidelines for earthing are enclosed as Annexure-I Protections Protections and islanding based on IEEE C are recommended for these guidelines. A typical simple scheme of interconnection fo an SHP with 33 kv grid with local 415 volts feeder is shown in Fig. 11. Minimum protection relays to ensure adequate protection of both generator as well as interconnection are shown in the figure. The transformer is connected grounded star on low voltage side so that grounded neutral system is provided for local load in case SHP is shut down. HV (33 kv) side is grounded from sending end side. HV side is protected by HV breaker installed at receiving end transformer grid sub-station for ground fault. 7.0 ISLAND OPERATION AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 12

16 7.1 General Islanding is the process whereby a power system is separated into two or more parts with generators supplying loads to some of the separated systems. In such system part of the electrical system is disconnected from main grid and is energized by one or more generators connected to it. Islanded operation improves the availability of electricity to the consumers connected to the distribution system. However prolonged islanded operation is unsafe and should be prevented by suitable means of protection. In an islanded group, there will be stable balance between load and generation resource. However, in few situations generator and prime mover control can establish a new equilibrium between generators and load of islanded group. The sequence of conditions prior to and during islanding are: i. Pre-islanded Condition The active and reactive power outputs of the generator are exported to the grid. The grid voltage and frequency are controlled by the grid and therefore stable conditions of voltage and frequency are achieved. ii. Initiating cause of islanding There are many possible causes of islanding but they fall in two categories fault or operational. In the case of an operational cause such as under frequency tripping, the circuit does not see significant initiating event. iii. Instant of Islanding At the instant islanding occurs, there is an instantaneous change in the power flow of the islanding circuits and outputs of generators connected. iv. Islanded Operation For steady state conditions to continue, the islanded electrical generation must equal the electrical load (neglecting electrical storage which is minimal). Since the probability of pre-islanded generation and load being equal is extremely unlikely, then power imbalance must cause an acceleration or deceleration of the rotating plant in the islanded network and result in drift of frequency. 7.2 Hazards of Islanding The potential hazards presented by operating a generator in an islanded situation are: Unearthed operation of the distribution system Lower fault levels Out of synchronization reclosure AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 13

17 Voltage levels Quality of supply Risk to maintenance personnel Unearthed Operation of the Distribution System It is usual for distribution line systems to be earthed at the grid sub-station only, with an HV delta unearthed arrangement at the generator connection. This will always be the case where multiple earthing is not allowed. When part of a distribution network is disconnected from the grid sub-station earth point, the unearthed system will be energized by SHP generators unless the generators are disconnected by islanding protection. The line to earth voltages of the three phases may then drift, or in the case of a line to earth fault, will be forced to shift. One phase will be referenced to earth causing the other phases to have a phase to earth voltage equal to the nominal phase to phase voltage. In this case, the normal design voltage limits for insulation and other equipment may be exceeded. This may cause damage or cause a hazard. This situation can be detected using a Neutral Voltage Displacement relay, (NVD), Annex II Lower Fault Levels The fault level of an islanded system is likely to be lower than the minimum fault level of the grid sub-station during normal operation. This means that the settings of protection relays may be inadequate to protect the islanded network Out of Synchronization Reclosure In the period following the initiation of islanding, a phase difference arises between the grid and islanded side of the isolating circuit breaker or recloser. The grid and islanded network are said to be out of synchronism. If the circuit breaker or recloser recloses before the generator is disconnected, then a severe surge of current may flow between the grid and the generator, causing severe voltage disturbances. The event can cause severe mechanical shock to the generator and prime mover, and there is risk of damage to the generator and equipment on the electrical system. Out of synchronization connection may be avoided by the use of dead line check or synch check facilities of the grid sub-station breakers Voltage Levels The operation of a generator supplying a distribution feeder in an islanded situation may cause the system voltage levels to be outside normal operating limits. The voltage will be determined by the output of the generators which are primarily designed for connection to the grid and may be operating in power factor control mode with little control of output voltage. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 14

18 The generator voltage protection limits of nominal ± 10% may not protect consumers from voltage excursions substantially outside operating limits but will limit the duration of such occurrence Quality of Supply The islanded network will have a significantly lower fault level. This is likely to increase the voltage fluctuations during the period of islanding, caused by consumer electrical equipment Risk to Maintenance Personnel The connection of a generator to a distribution line that has been disconnected from the grid for the purposes of maintenance or repair may cause harm to personnel through the energisation of a line that is expected to be dead. 7.3 Common Cause Tripping General Common cause (or common mode) tripping is where an event which causes a grid disturbance leads to the tripping of several generators, some of which may be on separate distribution line from the one where the fault occurred Risks to the Network Risks to the grid network of the loss of generation can be classified as: Loss of a single generator or site with a maximum generation capacity of less than 10 MW should have no significant effect on grid network if the connection meets voltage step requirements. Loss of multiple generators or sites with a maximum generation capacity totaling between 10 MW and 50 MW will cause local voltage disturbances and possible instability of other generator plants. Loss of multiple generators with a maximum generation capacity totaling between 50 MW and 100 MW will cause significant grid frequency dip and possible under-frequency tripping of feeders. Under low demand conditions, it may also trigger a total failure of the grid. Loss of multiple generators with a maximum generation capacity totaling greater than 100 MW is likely to cause grid instability resulting in possible total failure of grid. Note that the levels of generation defined here are subject to change as the grid generation and loads change in the future. The loss of a single generator is not significant for the operation, stability or security of the grid. However, common cause tripping of many generators may have serious consequences for the grid. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 15

19 7.3.3 Risks to the Generator The consequence of excessive and unwanted tripping to the generator is mostly financial due to the unnecessary loss of generation. There may also be an affect on mechanical and electrical equipment life in the case that trips are occurring frequently. 7.4 Sources of Common Cause Tripping of Generators The following devices and conditions are identified and assessed as possible causes of common cause tripping: Loss of mains relays are the main possible cause of common cause tripping of generation. The avoidance of common mode tripping is therefore a major consideration for loss of mains detection methods Loss of Mains Relays It is difficult for protection relays to distinguish between loss of mains events and general grid disturbances which may cause nuisance or unnecessary tripping Voltage Relays It is improbable that non-fault voltage excursions will exceed ± 10% for more than 0.5 second. Such an event would also result in substantial loss of motor load, offsetting some loss of generation Frequency Relays It is unlikely that general non-islanded frequency excursions will exceed 5% unless there is a condition under which tripping of generating plant would have no further impact Local Generator Instability A fault at or close to the primary busbars or on the local transmission system, could result in pole slipping of one or more generators on the distribution network. Pole slipping can cause damage to the generator, and with larger machines, will cause extensive current surges or swings and severe customer voltage fluctuations. This in turn could cause tripping of other generators. The potential for pole slipping of large generators should therefore be assessed and protection provided if the risk is confirmed. It is recommended that generators above 5 MW should be provided with pole slip protection or assessed to confirm that this is not required. 8.0 ISLANDING DETECTION AND PROTECTION 8.1 General Protection relays should be used to detect abnormal electrical conditions on the grid which may indicate that an islanded condition has occurred. They may also indicate that the AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 16

20 grid system is, for some other reason, outside operating conditions, and generators should be disconnected. After a distribution line, to which a Generator is attached, is disconnected from a grid substation, there will be some disturbance in the electrical condition on the distribution line. This disturbance may take the form of, a change in voltage or frequency or a single shift in the voltage vector or a change in reactive power flow The stand-alone detection of an islanded situation is based upon the detection of this electrical disturbance. The magnitude and type of disturbance will depend upon the type and capacity of the generating plant, the distribution line and transformer characteristics, and the nature of the connected loads. If at the time of disconnection, the remaining connected load and generated power are in balance and the feeder reactive power demand and generated reactive power are in balance and the feeder line to earth capacitance are equal for all phases, Then there will be no disturbance on the line and there will be no immediate way, apart from detection of the change of fault level, to detect an islanded situation. This balanced Generator and load situation is extremely unlikely to occur. 8.2 Islanding Detection Systems and Settings General The types of protection relays and recommended settings that are to be used to detect an islanded situation are as follows: i. With the exception of the voltage and frequency types the protection is known as loss of mains protection. ii. The settings of relays should be agreed with Grid control and the settings shall not be changed without their consent. iii. It must be noted that the total tripping time given under each type of protection includes timing period of the protection relay as well as auxiliary relay and circuit breaker operating times Over and Under Voltage The voltage of each phase is monitored and any excursions outside preset limits on any one phase should cause the relay to operate. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 17

21 The relay should have time delay of operation based on it being long enough to avoid spurious trips due to remote faults, but short enough to ensure disconnection before out of synch reclosure. Settings Level for HV point of connection: ± 10% Level for LV point of connection: +10%, - 14% of nominal 230 V Note: limits should not exceed or be less than the maximum and minimum statutory voltage by more than a few percent and should be based on declared nominal voltage) Total tripping time shall be less than 0.5 second Over and Under Frequency The frequency on a single phase is monitored and any excursions outside preset limits will cause the relay to operate. Settings Level +5%, -6% (i.e Hz to 47.5 Hz) This range has been selected on the basis that they are outside the normal range of frequency variation of the grid but close enough to nominal frequency to allow tripping of generator in islanded mode. It may be possible for the low frequency to be reduced to 46 Hz given the possibility of grid recovery from 46 Hz. Additional generator low frequency protection may be required to ensure that there is no damage to the generator at low frequency. Time delay There is no requirement for a time delay. Total tripping time shall be less than 0.5 second Rate of Change of Frequency RoCoF relays operate by measuring the zero crossings of successive sliding cycles of the measured voltage, establishing the apparent rate of change of frequency (RoCoF) and detecting when the applied setting is exceeded. Some RoCoF devices detect a cumulative angular shift and are also sensitive to initial change or vector shift of the measured voltage. True RoCoF detects the islanded condition rather than its onset or pre-condition. Settings True RoCoF operation is specified for some situations in Section 8. Limits: 2.5 Hz/second This has been specified to ensure there is minimum spurious tripping. Time delay There is no requirement for a time delay. Total tripping time shall be less than 0.5 second AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 18

22 8.2.5 Voltage Vector Shift Vector shift relays operate by measuring half cycle voltage and detecting a step change exceeding an equivalent vector shift setting. It detects a voltage vector shift arising when there is a step change in the current through the generator internal impedance. Such a condition will be caused by a fault or the onset of islanding. Voltage vector shift is inherently a protection to disconnect a generator from disturbances and is susceptible to spurious tripping during faults because it detects voltage angle disturbance rather than the characteristic of the islanded condition. Setting Level 6 0 in a half cycle. This can be de-sensitized upto 12 0 where spurious tripping is experienced. Time delay There is no requirement for a time delay. The maximum total tripping time shall be less than 0.5 second Reverse VARs Reverse VAR relays operate by detecting a flow of reactive power from the generator into the grid. The technique depends on the islanded network having the typical net VAR demand and the generator having a normal operating reactive power demand (i.e. the power factor controller is set leading). Reverse VAR protection can provide a simple and reliable method of islanding detection when there is only a single generator connected to a distribution line and where other conditions are suitable. The Captive Line Load must have a Var demand which cannot be met by other system components such as cables and capacitors. Generators must have stable power factor control to enable reverse VAR protection to be used. The generator shall be importing reactive power under normal operation. Tripping must be delayed to avoid loss of generation during short voltage dips (due to faults) or transient excursions of the power factor controller (especially following synchronization or voltage disturbances). Setting Level This is to be agreed between the Generating Company and Grid Control Typically levels of 1-5% of the magnitude of the maximum export kw may be used. The setting will be dependent upon the generator and local load conditions. Time delay The lesser of upto 5 seconds or 50% of the time delay of auto reclose devices. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 19

23 8.2.7 Intertripping Intertripping is a direct means of islanding protection, which provides a reliable method of tripping islanded generators without any unwanted common cause problems. It is therefore recommended where practical for larger generators (see Section 9 for protection requirements). It operates by sending a trip signal from the circuit breaker or recloser responsible for the islanding, to all the generators which use this method as the loss of mains protection. The reliability of intertripping is dependent on the security of the transmission method. Pilot wires, public telephone, radio or satellite communications are possible but the reliability should be assessed on an individual basis. Voltage barriers may be required on communication paths. Consideration must be given to the routing of intertrip signaling, to minimize the risk of interference, particularly during line fault conditions. The communication link should be reliable and fail-safe. If the communication link fails, the generator should automatically trip Fault Thrower This is a special application of a fault thrower. It is a form of intertripping using the phase conductors as the medium for communication. The fault thrower would be installed at the source substation and would be operated following opening of an appropriate source circuit breaker. The device would either put a short circuit on the islanded feeder, creating detectable over current or undervoltage and causing operation of generator protection, or it could put an earth fault on one phase, causing the operation of generator NVD (Neutral Voltage Displacement). Operation would be delayed to allow generator relays to operate if sufficient load imbalance exists. It would only be effective for generators connected between the source breaker and the first auto recloser Restoring All relays should have a minimum restoration time of one minute after the grid supply is restored should be within specified limits Future Developments The Guide does not preclude the use of novel methods of achieving a dependable and reliable loss of mains function. An example of a possible option is the use of sensitive true RoCoF blocked by vector shift to prevent operation during general grid instability. 8.3 Islanding Network Protection AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 20

24 8.3.1 General The primary protection specified in the Guide is designed to detect the islanding condition and trip the generators within a safe period. Detection of the islanded condition is not possible in all cases, especially when the load may closely match the generation on the islanded network. Secondary protection such as dead line and synch checks on auto reclosing devices and neutral voltage displacement (NVD) relays is recommended as back-up in cases where loss of mains protection is not adequately dependable to protect against the more severe and likely consequences of islanding. In addition, automatic low frequency disconnection relays on feeders with and average net export capability shall be rendered inoperative. The cumulative generation capability of a feeder shall be updated and compared to metered loads. This information shall be used to ensure that the current flow when restoring a tripped feeder is within the feeder and protection capability Inhibiting of Reclose Devices Two standard methods are available to prevent closure of a circuit breaker or recloser into a live or out of synchronized circuit voltage restraint (dead line check), and / or synch check relay In most applications, the dead line check will be adequate for radial lines, since the probability of prolonged islanded operation is extremely small. Dead line check relays inhibit automatic or manual circuit breaker closure onto a live circuit. This is not islanding protection and therefore does not replace loss of mains or ensure a safe islanded condition. However, it does mitigate the risk of islanding if loss of mains does not operate. This is a simple and dependable method of preventing out of synch closure where loss of mains protection is not secure Neutral Voltage Displacement (NVD) Protection An NVD scheme measures the displacement of the neutral on the HV side of the generator transformer and operates with delay when the displacement exceeds the trip setting. This device does not detect islanding, its purpose is to detect an earth fault and an unsafe islanding condition. NVD is a dependable means of satisfying safety requirements and mitigating the risk of islanding when generators operate without parallel earthing. NVD is implemented by having three voltage transformers (VTs) on the primary, HV, side of the connection transformer which are connected phase to ground. If HV metering voltage transformers are available, these may be of an appropriate type and may be utilized to reduce the incremental cost. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 21

25 The detection relay will be used to trip either an HV or LV breaker to disconnect the generator from the line. Voltage transformers providing NVD protection will require a voltage factor of 1.9. The time rating under voltage factor will depend on the type of system earthing at the Grid substation and the fault clearance time of the NVD protection system. The NVD protection should grade with the earth fault protection at the grid substation, so that feeder earth faults are cleared at the grid substation before NVD protection in other feeders operate. In case of Arc Suppression coil (ASC) earthing, the ASC should not be allowed to maintain an earth fault for a long time. It should be shorted-out in a few seconds and the earth fault protection in the faulty feeder allowed to clear the fault. The NVD should grade with the operation of this earth fault relay. The NVD relay should be capable of withstanding three times the secondary phase voltage of the voltage transformer. Further details of NVD protection are given in Annex-II. Setting Level Typically 25% of phase voltage on effectively grounded systems This setting may need to be increased for impedance earthed systems to avoid tripping on distant earth faults. Time delay 1 to 3 seconds depending on speed of line earth fault protection on other feeders. 9.0 IMPLMENTATION OF ISLANDING PROTECTION 9.1 General Protection relays shall be of suitable quality to provide reliable and consistent operation. The performance levels of the relays shall be declared by the manufacturer. It is preferable to use proven protection equipment supplied by a reputable manufacturer with a track record in this type of application. The performance of all protection relays shall be within the scope of IEC protection product family Standard IEC (formerly IEC 255). It is recommended that control and protection panels are soak tested (i.e. the protection relay panels are energized for several hours or days) prior to being put into operation. During the soak test the operation of the protection relays should be checked periodically. Protection relays and the associated sensing circuits must be designed to maintain accuracy and operation in fault conditions. Particular consideration should be made of the requirements for current transformers to sustain operation when fault currents occurs. Current and voltage transformers should be appropriately selected and comply with product standards IEC (formerly IEC 185) and IEC (formerly IEC 186) respectively. All protection relays to have indication of operation. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 22

26 It is useful if the relay system can indicate which relay or function operated first to disconnect a generator during a fault condition. The indication may be reset at next breaker or contactor closure. 9.2 Interconnection Protection Requirements In order to specify the type of interconnection protection required, Embedded Generator interconnections may be classified into one of five cases. The protection requirements for these five cases are illustrated in Figures 12 to 15. A summary of the Case definitions and protection requirements is shown in the following table. Summary of Minimum Protection Requirements for SHP Generator Interconnection Case 1 Case 2 Case 3 Case 4 Case 5 Generator Type All All See Case 3 description All See Case 5 description Self commutated static inverters Minimum captive L L L L load Maximum <0.5 x L <0.8 x L > 0.8 x L > 0.8 x L cumulative installed capacity Maximum site < 5 MW < 5 MW < 5 MW < 5 MW installed capacity Under and over voltage protection Under and over frequency protection Vector shift * protection ROCOF protection * True ROCOF * protection NVD protection *(1) Intertripping * Loss of Phase Other * * * * * Mandatory minimum requirement * For other requirements and alternatives see the descriptions under the respective case descriptions and requirements (1) NVD or parallel earthing The above table summarizes only the mandatory requirements of protection. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 23

27 9.2.1 Case 1 Generator Type All types Conditions The installed generating capacity is less than half the minimum Load (see note below), and The installed generating capacity is less than 5 MW Note: Minimum Load is the sum of Minimum Line Load and the Minimum Local Load. The minimum Line Load may be difficult to establish, in which case it may be assumed to be 50% of average line load. In this case, following distribution line disconnection, the Generator speed and voltage will fall rapidly as the generator(s) will be unable to supply the load. Protection Required Under and over voltage Under and over frequency Optional, at the Discretion of the Generating Company Three phase vector shift Design Criteria The minimum Load is subject to change due to insertion of sectionalisers, reclosers, reconfiguration or reduction in customer load. ALDC (Area Load Despatch Centre) shall be informed any modification made to the Generator for review, and to assess the need for any retrospective enhancement of protection. Similarly, whenever any changes to the LCC system affects the Line Load, ALDC shall review the interconnection protection and advice the Generating Company accordingly. Auto reclosers must have a minimum reclose time of 1 second to minimize the possibility of out of synch reclosure. ALDC shall verify that this reclosing time is adequate for interconnection protection to operate prior to reclosing. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 24

28 Figure-12 Interconnection Protection Arrangement for Case Case 2 Generator Type All types Conditions The installed generating capacity is less than 80% of the minimum Load (see note below), and The installed generating capacity is less than 5 MW Note: Minimum Load is the sum of Minimum Line Load and the Minimum Local Load. The minimum Line Load may be difficult to establish, in which case it may be assumed to be 50% of average line load. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 25

29 Protection Required Under and over voltage Under and over frequency 3-phase vector shift Optional, at the Discretion of Grid Control, when a balance between Load and the installed generating capacity is very likely to occur True RoCoF may be used as well as vector shift. Design Criteria As per Case 1 Figure-13 Interconnection Protection Arrangement for Case 2 AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 26

30 9.2.3 Case 3 Generator Type Conditions: All types except mains excited generators defined in Case 5. The installed generating capacity is more than 80% of the minimum Load (see note below), such that load/ generator balance is possible, and The installed generating capacity is less than 5 MW. Note: Minimum Load is the sum of Minimum Line Load and the Minimum Local Load. The minimum Line Load may be difficult to establish, in which case it may be assumed to be 50% of average line load. Protection Required NVD Under and over voltage Under and over frequency 3-phase vector shift Dead-line check ( true RoCoF may be used in place of vector shift) or as a replacement for the combination of Vector shift and NVD, any one of the following may be used: Intertripping Fault throwing Reverse VAR protection with synchronous generators, where only one generator is connected to the circuit. NVD protection is not required where the maximum site installed capacity is less than 1 MW, if the cumulative generating capacity on a distribution line that does not have NVD protection is less than 0.8 times the minimum captive load. The fitting of deadline check relays on upstream breakers or sectionalisers, or disabling of all upstream automatic reclosing devices should be considered. Design Criteria The total generating capacity connected to a single grid substation using the vector shift method for loss of mains protection shall not exceed 20 MW. This is to prevent possible common cause tripping of local generation exceeding 3% of grid minimum load. This limit will be increased with the increase in grid minimum load. Whenever any changes to the system affects the Line Load, Grid Control shall review the interconnection protection and advice the Generating Company accordingly. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 27

31 Figure-14 Interconnection Protection Arrangement for Case 3 AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 28

32 9.2.4 Case 4 Generator Type All types Conditions The installed generating capacity of an Generating Station is greater than 5 MW. Configuration If is preferred that Generator is connected directly to the primary bus rather than direct connection to HV distribution feeder. Protection Required Under and over voltage Under and over frequency Intertripping from grid substation bus intake Parallel earthing or NVD protection If the Generator is directly connected to a distribution feeder, the following is also required: Intertripping from feeder breaker or Fault throwing or Reverse VAR protection, if applicable. Design Criteria Generators larger than 5 MW will be encouraged to obtain more secure connections. Insecurity is mainly a factor of the length and exposure of overhead lines to lightning and vegetation. For large generators remote from the primary bus, adequate security may only be achieved by double circuit connection to the primary bus. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 29

33 Figure-15 Interconnection Protection Arrangement for Case 4 AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 30

34 9.2.5 Case 5 Generator Type Mains excited asynchronous generator with local power factor correction less than the reactive power demand, or a line commutated inverter. The network/circuit capacitance is not sufficient to self excite the generator. Conditions The installed generating capacity is more than 80% of the minimum load (see note below), such that load/generator balance is possible, and No synchronous generation or self-excited generation are connected. Note: Minimum Load is the sum of Minimum Line Load and the Local Load. The minimum Line Load may be difficult to establish, in which case it may be assumed to be 50% of average line load. Protection Required Under and over voltage Under and over frequency 3-phase vector shift Design Criteria The total generation connected to a primary substation using the vector shift method for loss of mains protection shall not exceed 20 MW. This is to prevent possible common mode tripping of local generation exceeding 3% of minimum grid load. This limit should be increased with the increase in grid minimum load Self Commutated Static Inverters The general requirements for protection of this type of generation are covered with synchronous machines in cases 1 to 5 above. However, inverters commonly include proprietary protection methods including RoCoF. The Generating Company should submit to utility details of protection to ascertain that the generation is not susceptible to tripping for RoCoF less than 2.5 Hz/s. If it is proposed that the protection replaces any protection specified in the cases above, it is the responsibility of the Generating Company to demonstrate that the protection meets the acceptable levels of dependability and reliability. 10. SURGE PROTECTION 10.1 General Equipment associated with a generator requires to be protected from hazards effects of transient over-voltages. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 31

35 Occurrence of transient over voltages can be due to external as well as internal causes. Lightning is the most common source for transient over-voltages. However, damaging transients could originate from within the grid system itself, due to switching operations, ferro resonance, etc. Adequate measures should be taken to protect the insulation and equipment from being damaged due to the above conditions. An LV/HV transformer usually connects the generator to the grid system. It is essential that the HV side of the transformer be protected from the transient over voltages by installing gapless metal oxide surge arresters with polymer housings. The connections to earth electrodes and the design of the earth system should be appropriate for the surge protection function Location of the Arrester Location of the arrestor is a critical factor. In order to maximize the arrester effectiveness, all lead lengths should be made as short as possible. Arresters require connection to the system phase conductor and earth. Inherent inductance in the leads produces a voltage build up during impulse discharges. It is usual to assume a value of kv/m for the lead voltage build up. During surge conditions, the net residual voltage that will appear across the transformer will be equal to the sum of the arrester residual voltage and the voltage drops across the leads. The ability of the surge arrester to protect the transformer can be assessed by comparing the net residual voltage with the withstand voltage of the equipment Earthing General practice is to provide a separate earth for the transformer LV neutral and to link the lightning arrester earth with the transformer tank earth. When this method is adopted, under surge conditions, a potential stress could develop across the two windings. Also due to capacitive effects, transient voltages can be transferred to the LV side. Hence providing arresters on the LV side will ensure better protection for the equipment. However, in multiple earth systems, it is advantageous to interconnect the arrester ground terminal and transformer tank earth with the secondary neutral earth. With this connection, possibility of a voltage stress developing between the two winding is minimized. It is recommended standard practice as per relevant ISS must be followed Selection of the Lightning Arrestor Higher rated voltages or higher maximum continuous operating voltages and the best surge protection of the equipment are contradictory requirements. Many other system and equipment parameters have to be considered and an optimization process has to be followed to select the basic characteristics of the arrester which will provide the best protection, to the AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 32

36 equipment to be protected. It is recommended that the procedure given in IEC 99-5 should be followed for the selection of the arresters. Due consideration should also be given to the following aspects when selecting the arrester. Service life Environmental aspects Polymer characteristics Energy handling capacity. Arresters chosen shall comply relevant ISS Generators Directly Connected to LV Overhead Lines Small generators directly connected to the distribution LV overhead lines could also be subjected to transient overvoltages. It is recommended that surge arresters chosen according to the guidelines laid down in IEC 99-5 and capacitors should be installed between phase and earth, as close as possible to the machine terminals. Protection performance could be improved by installing a second set of arresters on the overhead line. 11. HARMONICS Harmonic voltages and currents produced within the system may cause excessive harmonic distortion on the grid system when the systems are connected. To avoid excessive harmonic distortion on the grid system, the generator installation should be designed and operated in such a way that if testing for harmonics is carried out with the generator connected to the grid, the results obtained should be comparable to those measured at the same point of grid when generator is not connected. 12. POWER LINES SPECIFICATION (EXAMPLE 33 K POWER LINE) 12.1 Design Consideration General i. All electrical installation shall confirm to the Indian Electricity Act, IE RuleS and Regulation in force, in the state, by electrical inspectorate. ii. Before charging the line/equipment, contractor shall submit the completion report for each part/equipment indicating rectifications/modifications carried out during erection, site test certificates with observations, rectifications carried out. Contractor shall also indicate the correctness of operational and safety interlocks. Site test certificates shall also indicate the corresponding values obtained in the factory test. iii. The conductor/jumpers shall be correctly and effectively connected to the terminals of equipment. The connection shall be flexible to withstand stresses during switching operation. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 33

37 Relevant IS/IEC: 1. Aluminium Conductor for overhead Transmission Purposes IS 398 (ACSR/AAAC) 2. Conductor and earthwire accessories for overhead power line IS Design and construction of foundation for transmission line IS 4091 poles 4. Hot-dip galvanizing coatings on round steel wires IS Hot-dip galvanizing coatings on structural steel & allied IS 4759 products 6. Porcelain insulators for overhead power lines with a nominal IS 731 voltage greater than 1000 V. 7. Solid core insulators IS 2544 IS 5350 IS Electric power connectors IS Method of testing weights, thickness & uniformity on H.D.G. articles IS Recommended practices for hot dip galvanizing of iron & steel IS Insulator fitting for overhead power lines with a normal voltage IS 2486 greater than 1000 V 12. Use of structural steel in overhead transmission lines IS Rolled steel beams, channels and Angle sections IS Nuts & threaded fasteners IS High tension structural steel IS Hexagonal bolts & steel structure IS Washers Spring- IS 3063 Plain-IS 2016 Heavy-IS Terminal connectors IS ACSR Conductor Construction Conforming to IS 398 (Part-II), i. Aluminium wire made from at least 99.5% pure electrolytic aluminium rods of EC grade with copper content less than 0.04%. ii. Steel wires uniformly coated with electrolytic high grade, 99.95% pure zinc. iii. Steel strand hot dip galvanized with minimum coating of 250 gm/sq.m. after standing. iv. No joints permitted in the individual aluminium wires and steel core of the conductor. v. Standard length of conductor shall be 2500 mtr. with a tolerance of + 5% Connectors Bi-metallic connectors shall be used for connecting equipment terminals made of copper or brass, bolts, nuts and washers for connector shall be made of mild steel and shall be electro-galvanized and passivated to make them corrosion resistant conforming to requirements of BS AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 34

38 H.T. Insulators String insulators (Constructional Features) Suspension and tension insulators shall be wet process porcelain with ball and socket connections. Insulators shall be interchangeable and shall be suitable for forming either suspension or strain strings. The insulator shall be such that stresses due to expansion and contraction in any part of the insulator shall not lead to deterioration. All ferrous parts shall be hot dip galvanized, the zinc used for galvanizing shall be grade Zinc 99.5% a per IS: 209. The zinc coating shall be uniform, adherent, smooth, reasonably bright, continuous and free from imperfections such as rust stains, bulky white deposits and blisters String Insulator Hardware (Constructional Features) Insulator hardware shall be of forged steel. The surface of hardware must be clean, smooth, without cuts, abrasion or projections. No part shall be subjected to excessive localized pressure. The metal parts shall not produce any noise generating corona under operating condition. Insulator tension string hardware assembly shall be designed with Electromechanical strength of kg. Tension string assembly shall be supplied along with suitable turn buckle (one turn buckle per string). alloy. All hardware shall be bolted type. The tension/suspension clamp shall be Aluminium 12.l.7 Steel Tubular Poles The Swaged Type Steel Tubular Poles Shall conform to IS: 2713 Part-I to Part-III (1980) including subsequent amendments thereof in every respect. Poles shall be made of steel tubes having minimum tensile strength as 42 kg/mm 2 and minimum percentage elongation as specified in IS: 1161 (1979). Technical Parameters Sl. Description Type of Pole No. SP-21 SP-23 SP-33 SP-45 SP Total length in Met Outside Diameter & Thickness of section in mm Bottom x x x x x 4.85 Middle x x x x x 4.5 Top 88.9 x x x x x Min. weight of Pole in kg Breaking load in kgf >=612.0 >= Crippling load kgf >=435.0 >= Max. permissible working load F.O.S. of 1.5 m crippling load with point of application of load at 0.3 M from top of the Poles in kgf >=290.0 > AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 35

39 Hot Dip Galvanized MS Stranded Wire The hot dip galvanized MS stranded wire of sizes 7/8, 7/10 and 7/16 mm etc. SWG shall conform to the following specifications: 1. Material a) MS Wire: Used for each strands shall have the chemical composition maximum sulphur & phosphor 0.055%, Carbon-0.25%. b) Zinc shall conform to grade Zn 98 specified as per IS: and IS: with up-to-date amendments. 2. Zinc Coating: Shall be in accordance with IS: (heavily coated hard quality grade 4 as per table-1). 3. Galvanizing: Shall be as per IS: , IS: with up-to-date amendments. 4. Uniformity of Zinc Coating: Shall be as per IS: (Col to 4.2.3) with up-to-date amendments. 5. Tensile Properties: Of each strand ensuring MS wire, mechanical properties as per IS: Cl. 8.1 to 8.3 and after galvanizing each wire shall be of tensile strength minimum 700 N/mm 2 (71 kg/mm 2 ). Tensile strength breaking load and elongation of each wire and full stand shall conform to IS: , IS: in the tensile grade given above. 6. Construction: Shall be as per IS: Test on Wire before manufacture: As per IS: (Cl. 7.1 to 7.2.2). 8. Test on Complete Strand: Test shall be conducted in accordance to IS: Packing: Each coil shall be between kgs packed as per IS: (Cl ) and (Cl. 11). 10. Marking: As per IS: (Col. 8.1 and 8.1.1), IS: Cl. 10 and 10.1). 11. Danger board for 33 kv voltage and danger mark conforming to IS: shall be fixed on each location MS Stay Sets of 20MM DIA GS Stay Sets For Ht Lines (Galvanized Non-Painted) (20x 1800 MM) a) Anchor Rod with one washer and nuts :- Overall length of rod should be 1800 mm to be made out of 20mm dia MS Rod end threaded upon 40 mm length with a pitch of 5 threads per cm and provided with one square MS washer of size 40 x 40 x 1.6mm and one MS hexagonal nut conforming to IS : and IS : 1363 :1967, and as per latest version. Both washer and nut to suit threaded rod of 20mm dia. The other end of the rod shall be made into a round eye having an inner dia of 40mm with best quality welding. b) Anchor Plate size 200 x 200 x 6mm : To be made out of MS plate of 6mm thickness. The anchor plate shall have at its centre 18mm dia hole. c) Turn Buckle: i) Eye bolt with 2 nuts : To be made of 20mm dia MS rod having overall length of 450mm, one end of the rod to the threaded upto 300mm length with AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 36

40 a pitch of 5 threads per cm and provided with two MS Hexagonal nuts of suitable size conforming to IS : and IS : The other end of rod shall be rounded into a circular eye of 40mm inner dia with proper and good quality welding. ii) Bow with welded angle: To be made out of 20mm dia rod. The finished bow shall have an overall length of 995mm and height of 450mm. The apex or top of the bow shall be bent at an angle of 10R. The other end shall be welded with proper and good quality welding to a MS angle 180mm long having a dimension for 50 x 50 x 6mm. The angle shall have 3 holes of 22mm dia each. d) Thimble: To be made of 1.5mm thick MS sheet into a size of 75 x 22 x 40mm shape. e) Entire stay set shall be hot dip galvanized as per relevant IS. f) Using stay wire of 7/10 SWG GS grade Structural Work Design and fabrication of structural parts shall conform to the applicable provisions of the DIN standards, including DIN 19704, Hydraulic steel structures : criteria for design and calculations and DIN 4114, Stability of steel structures, unless otherwise prescribed elsewhere in these Specifications. All embedded metal shall be at least 12 mm thick and all other metal shall be at least 10 mm thick. Dimensions without tolerances shall be according to DIN 7168, Deviations for dimensions without tolerances, class "mittel", unless otherwise specified Welding The minimum strength of welding provided on various components of 16mm dia stay sets shall be 3100 kg. Minimum 6mm filter weld or its equivalent weld area shall be deposited in all positions of the job i.e. at any point of the weld length. The welding shall be conforming to relevant IS : 823/1964 or its latest amendment Excavating pits for erection of 410 HT poles i. After the pit locations are finalized and peg marked on the ground, the pole pit of size 750 x 750 x 1500/1850mm be dug (9/11 Mtr. pole). The base padding of 200mm thick with 1:4:8 cement concrete shall be done before erection of pole. The earthing coil shall also be grounded 800mm below ground level by digging a separate pit minimum 4 mtr away from pole and filling the pit with soil. The pole in the pole pit shall be erected in truly vertical position and the pit is filled with 1:3:6 cement concrete mixture for size 600 x 600 x 1500/1850 mm and muffing be provided on pole upto 400 x 400 x 400mm above ground level of 1:2:4 RCC. ii. Painting of pole with on one coat of red oxide and two coats of approved aluminium paints on portion above ground level shall be applied. iii. For the portion buried under ground, additional two coats of Bitumen paints shall be applied. iv. Each pole shall be earthed with MS earth rod 2500x20mm below the 800mm below ground level including fixing of 6 SWG Gl wire between rod and pole. At 60cm height above ground level by putting hole in pole and bolting with 16mm size nuts and bolts. The earth coil shall be grounded 800mm below ground level by digging AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 37

41 separate pit and filling the pit with soil. All materials are included in tenderer's scope, as part of erection work Fixing cross arm, top clamps, channels etc. on the poles The fitting such as V cross arm, top clamps, channel etc. shall be fixed on poles as per standard practice. The fabrication of above fittings shall also be done as per approved standard drawing submitted by tenderer. The general specifications of steel sections are given below: i) V cross arm shall be made of MS angle of size 125 x 65 mm. ii) Top clamp shall be made of MS flat of size 50 x 8, mm. iii) iv) Double Cross arm shall be made out of the MS channel of size 100 x 50 x 6mm. Other special fittings if required may be got fabricated as per the standard drawings of utility. The clamps for holdings the fittings shall be fabricated out of MS flat 65 x 8mm size. All nuts and bolts used shall be of MS with combination of plain and spring washer and machine made Clamps and Connectors (Constructional Features) The clamps and connectors shall be made of materials listed below: For connecting ACSR Conductors destination For connecting equipment terminals made of copper with ACSR conductors For connecting G.I. Shield wire Bolts, nuts and plain washers Spring washers for items a to c Aluminium alloy casting conforming to destination. A6 of IS: 617 and shall be tested for all test as per IS: 617. Bimetallic connectors made from aluminium conforming to destination A6 of IS: 617. Galvanized mild steel. Hot dipped galvanized mild steel except for sizes below M12 for which electrogalvanized ones shall be used. Electro galvanized mild steel as per service conditions at least 3 of IS: All casting shall be free from blow holes, surface blisters, cracks and cavities. All sharp edges and corners shall be blurred and rounded off..no current carrying part of a clamp or connector shall be less than 12 mm thick. All ferrous parts shall be hot dip galvanized conforming to IS: For bimetallic clamps and connectors, bimetallic strip shall be used. Flexible connectors, braids or laminated straps shall be made from tinned copper sheets or aluminium laminates depending on the clamp. The terminal clamps for bus posts shall be suitable for both expansion as well as fixed / sliding connection as required. Fixed / sliding feature shall be possible just by reversing the top gripper without the necessity of any additional components. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 38

42 Code number for the clamp / connector shall be indelibly marked on each component of the clamp / connector, except on the hardware. Clamp shall be designed to carry the same current as the conductor and the temperature rise shall be equal or less than that of the conductor at the specified ambient temperature. The rated current for which the clamp / connector is designed with respect to the specified reference ambient temperature, shall also be indelibly marked on each component of the clamp / connector, except on the hardware. All current carrying parts shall be designed and manufactured to have minimum contact resistance. Clamps and connectors shall be designed corona controlled. The welding sleeve for the aluminium tube shall match with that of the aluminium tube to avoid unnecessary work at site after the despatch. The length of the sleeve shall be minimum seven times the OD of the main aluminium tube. Sleeves along with distancing pins (4 nos. per sleeve) shall be supplied Fixing of insulators and Connected Hardware i. Insulator shall be handled carefully in all stages of loading and individually checked for cracks, damages, loss of glaze etc. before assembling and erection at site. ii. The 33kV galvanized steel pins made by process of forging suitable for 33kV pin insulators having maximum failing load of 10KN with small steel head as per fig. IB of IS: shall be used. The pin shall be provided with nut (hot-dip galvanized) one plain washer and one spring washer (electro galvanized). iii. The disc insulators shall be fitted with 33kV Hardware for tensioning the conductor 33kV hardware should be fixed in the disc insulators as per the standard practice and in the correct position to bear the tension of conductor. The 33kV strain hardware fitted of aluminium alloy suitable for required conductor (ACSR) shall be used conforming to IS: 2486 (Part-II) Installation Guidelines General The Power Line work includes Survey, Profiling, Alignment each including preparation of schedule of material, check survey profiling etc., manufacture/ procurement of electrical equipment, shop testing, packing, transportation, loading & unloading, delivery, storage at site, handling, erection, Laying Stringing & Sagging of 3 phase ACSR (Required) conductor with 7/16 SWG Gl earth wire including the hosting of Disc insulator, Disc Fitting & jumpering of line by fixing of PG clamp and erection of poles pre-commissioning test and commissioning of all equipment/ system including preliminary acceptance test, performance guarantee and post commissioning services. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 39

43 Relevant Standard I.S CODE NO. TITLE IS: Installation of Grouting/Earthing of Power Line. IS: Installation of Danger Board IS: 398 (Part II) 1996 Stringing of Conductor IS: 2486 (Part II) 1989 Stringing of Conductor IS: 209 Installation of Insulators IS: 2544 Installation of Insulators IS: 731 Installation of Insulators IS: 1248 Installation of Insulators IS: 2713 (Part I to III (1980) Installation of Steel Tubular Pole IS: Structural Steel (fusion welding quality) Structural steel (fusion welding quality) Installation Guidelines: Excavating pits for erection of poles After the pit locations are finalized and peg marked on the ground, the pole pit of size 600 x 900 x 2250mm be dug. The base padding of 200mm thick with 1:3:6/1:2:4/1:1 1 /4:2 as per design cement concrete shall be done before erection of pole. The earthing coil shall also be grounded 800mm below ground level by digging a separate pit and filling the pit with soil. The pole in the pole pit shall be erected in truly vertical position and the pit is filled with 1:3:6/1:2:4/1:1 1 / :2 cement concrete mixture for size 450 x 600 x 2050mm and muffing be provided on pole upto 400 x 400 x 400mm above ground level. The poles shall be erected normally with a span of 80 to 90 meters or as per standard design. Painting of pole with two coats of red oxide and two coats of aluminium paints on portion above ground level shall be applied. For the portion buried under ground, additional two coats of Bitumen paints shall be applied. Each pole shall be earthed with Gl pipe electrode of 50mm dia / 115 turns of 4mm dia Gl wire at 60cm height above ground level by putting 18mm hole in rail/pole and bolting with 16mm size nuts and bolts. The earth coil shall be grounded 800mm below ground level by digging separate pit and filling the pit with soil Fixing cross arm, top clamps, channels etc. on the poles The fitting such as V cross arm, top clamps, channel etc. shall be fixed on poles as per standard practice. The fabrication of above fittings shall also be done as per standard drawing. The general specification of steel sections are given below: i) V cross arm shall be made of MS angle of size 75 x 75 x 6mm. ii) Top clamp shall be made of MS angle of size 75 x 75 x 6mm. iii) Double Cross arm shall be made out of the MS channel of size 100x50x6mm. All nuts and bolts used shall be of MS with combination of plain and spring washer and machine made. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 40

44 Fixing of insulators and Connected Hardware Insulator shall be handled carefully in all stages of loading and individually checked for cracks, damages, loss of glaze etc. before assembling and erection at site. The 33kV galvanized steel pins made by process of forging suitable for 33kV pin insulators having maximum failing load of 10KN with small steel head as per fig. IB of IS: shall be used. The pin shall be provided with nut (hot dip galvanized) one plain washer and one spring washer (electro galvanized). The disc insulators shall be fitted with 33kV Hardware for tensioning the conductor 33kV hardware should be fixed in the disc insulators as per the standard practice and in the correct position to bear the tension o conductor. The 33kV strain hardware fitted of aluminium alloy suitable for Dog conductor (ACSR) shall be used conforming to IS : 2486 (Part -11)1989. The pit 0.4 x 0.6 x 1.6 meter shall be excavated and anchor plate with stay rod shall be suitably aligned in such a manner that the stay wire when bonded with - anchor rod and stay clamp at pole, the same shall make on angle of 30o to 45o from the pole. Cement concrete mix of 1:3:6/1:2:4/1:1½:2 shall be poured in the pit, rammed adequately and cured properly. The conductor shall be laid out in such a way that there is no damage to conductor/ Reels of conductor shall be handled carefully so that no damage to conductor strands occur Stringing of Conductor Conductor shall be laid out from rotating wheel supported on jacks for easy unwinding of the conductor. Snatch blocks shall be used for stringing the conductor and shall have grooves of a shape and size to allow early flow of conductor and ensure damage free operation. Clamps shall be used to grip the conductor at the time of stringing Sagging of conductor Guarding All conductors sagging shall be in accordance with the sag and tension tables as per relevant Indian Standards. After the conductors have been pulled to the required sag, inter mediate spans shah be checked to determine the correct sag. The conductor shall be installed on insulators secured to it by means of 6 SWG aluminium binding wire. The jumpers at the tension locations shall also be bound by 6 SWG aluminium binding wire. Before fixing the conductor on insulator and strain hardware, aluminium tape shall be wrapped on the conductor. The 33kV cross arm fitted on the pole guarding line with 6SWG Gl wire guard wire and 8 SWG Gl wire for lacing. Guarding cross made of 75 x 75 x 6mm angle, 8 feet long shall be claimed at 300mm below the bottom arm of V cross arm. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 41

45 Anti Climbing Devices. Barbed wire weighing 35kg per pole shall be wrapped at a height of 3000mm above ground level stretching in 900mm length. Both ends of barbed wire shall be clamped suitably to avoid coming down from its location Danger Board Danger board for 33kV voltage and danger mark conforming to IS: shall be fixed on each location Survey and Markings for Construction of Overhead lines i. The preliminary survey of the line shall be done and plotted on the map. ii. During preliminary survey, crossings / proximity to buildings and to all categories of power lines as well as telecom lines under P&T Department shall be clearly indicated in the route map. iii. The detailed survey shall be undertaken only after finalizing the route alignment iv. The pit marking shall then be done at the locations. Any likely discrepancy in respect of ground / building clearance shall be sorted out first, and then the work shall be started. v. Some sites are under forest department, safety of forest property, for which forest clearance from Government, shall be got approved ACSR Conductor Construction Conforming to IS 398 (Part-ll), i. No joints permitted in the individual aluminium wires and steel core of the conductor. ii. Standard length of conductor shall be 2500 mtr. with a tolerance of ± 5% shall be used. Clearances The net clearance in air for conductor, bus bars, Jumpers etc. shall not be less than as per Indian Electricity Act H.T. Insulators String insulators (Constructional Features ). Suspension and tension insulators shall be wet process porcelain with ball and socket connections. Insulators shall be interchangeable and shall be suitable for forming either suspension or strain strings. Rated strength of each insulator shall be printed on the porcelain before firing. Polymeric insulators may be used as per the guidelines of the Engineer in charge String insulator hardware (Constructional Features) Insulator hardware shall be of forged steel. The surface of hardware must be clean, smooth, without cuts, abrasion or projections. No part shall be subjected AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 42

46 to excessive localized pressure. The metal parts shall not produce any noise generating corona under operating condition. Insulator tension string hardware assembly shall be designed with Electromechanical strength of kg. Tension string assembly shall be supplied along with suitable turn buckle (one turn buckle per string). All hardware shall be bolted type. The tension / suspension clamp shall be Aluminium alloy. Each Insulator string shall comprise of 4 nos. disc insulator for 33kV to meet the required creepage / dry arc distance requirements. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 43

47 ANNEXURE-I EXPLANATORY NOTES ON EARTHING 1 DISTRIBUTION NETWORK EARTHING 1.1 General This informative annex is designed to provide background information on earthing in general. The information is provided for reference and possible future use. Information provided in this annex shall not necessarily be taken as a requirement for a particular installation. Utility networks will be earthed at the source transformers that connect the network to a higher voltage network or to the main generators. There will normally be at least two points of earthing, because transformers are usually duplicated for security. Other earthing points may also be paralleled from time to time as a result of operational parallel switching. Design of earth systems will take account of the most severe earth fault current. Also any significant increase of earth fault current attributed to generator will be considered in respect of the design of existing earthing systems. Typical arrangements used to earth utility distribution networks with a range of voltages are shown in Figure 16. The diagram also shows typical connection points where generators may be "earthed or not-earthed". Earthing is effected by connecting either the neutral point of a star winding or a derived neutral point to an earth electrode system. The connection will be either solid or through an impedance. Medium voltage distribution networks, such as 66 kv, 33kV and 11kV, may be impedance earthed to control earth fault current, minimize cable ratings and achieve economy in the design of earth systems. Utility 33kV networks are derived from transformers with delta windings to phase correct and limit circulating currents at mains and third harmonic frequencies. An additional 2 or 3 winding earthing transformer is then required to produce the neutral point. This may be directly earthed, earthed through a resistance or through an arc suppression coil. Normal utility practice is to connect the earthing transformer neutral directly to ground or through a resistance. The total impedance is designed to reduce earth fault current to 1 pu. and the dimensioning of earth systems. The star point of a power transformer star winding is the normal method to obtain a neutral connection for networks operating with a voltage less than 33 kv. This applies to networks providing 11kV and LV supplies and allows solid, resistance or reactance earthing. Networks with an operating voltage greater than 33kV have solid earthing at all supplying and distributing transformer windings. The larger number of network earthing points usually lead to earth fault current at utility/generator connection points greater than phase fault currents. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 44

48 Figure-16 Typical Earthing Options for Networks with Generators Multiple earthing reduces the zero sequence impedance possibly below the positive sequence impedance. Earth fault current may then exceed the 3-phase fault current. Calculation of currents must consider the impedance of the network source and circuits to zero sequence currents, any impedance in the neutral to earth connection, and the grounding resistance at the point the system is earthed and at the point of fault. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 45

49 The impedance of return paths and hence the magnitude of earth fault current is heavily dependent on presence of metallic earth wires or sheaths of cables and/or resistance of earth electrodes on route. This enhancement is unlikely at lower voltages where only the source transformers are earthed, even with solid and multiple system earthing. Low voltage networks normally have metallic earth return paths to ensure operation of overcurrent protection devices such as fuses, where these are the sole means of protection from electric shock. This metallic path must be continuous to the point the system neutral is earthed to ensure that sufficient current is available to isolate the faulty equipment within the required time for safety. 1.2 Earth Leakage Currents in Distribution Networks The magnitude of current resulting from an earth fault in distribution networks is primarily determined by the network voltage, the method used to earth the neutral point of the network and the construction of circuits feeding the substation or connection point (unearthed/earthed lines or cables). Where it is found necessary to restrict the prospective earth fault current from the network or a generator by inserting an impedance between a star point and the earthing system, the value appears in the earth fault current calculation formula as 3 times its actual ohmic value. This also applies to the effects of the substation earth system resistance. Contributions and an increase of earth fault current attributed to generator must be considered in respect of the design and capacity of existing earthing systems. In applications where an increase of earth fault current is tolerable, solid connection of the generator neutral to an earth system is the simplest and most reliable solution. However, this will require the relevant technical factors to be considered. Generators typically have a low zero sequence impedance compared to the positive sequence impedance. Should a generator neutral be directly connected to earth, then the single-phase prospective earth fault level at the generator terminals, may exceed its 3-phase prospective symmetrical fault level by typically 20%. This assumes typical parameters, and within the utility network, the generator earth fault contribution will be attenuated by the relatively high zero sequence impedance presented by the power lines. Maximum enhancement of prospective earth fault levels will normally apply in the utility network close to the generator connection point. Where a utility earth system and a generator's earth system are interconnected, generator earth fault current will flow through the interconnecting connections and will not contribute to voltage rise of the utility substation earth system. Typically an impedance of 0.85 pu inserted in the neutral of a generator restricts the earth fault level to the full load current of the generator. The impedance can take the form of either a resistor or a reactor. The legal requirements for safety are satisfied by compliance with relevant Standards on earthing plant and equipment in electricity networks. It is important to note that existing AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 46

50 earthing systems, possibly designed to outdated standards, are usually compliant provided there is no record or history of danger and no significant changes have taken place. The effects of significant change to earth fault current attributed to generators must be considered as part of the connection negotiations. 2 DESIGN OF EARTH SYSTEMS Earth systems must be designed to handle safely the maximum earth fault current. They must also restrict site ground potential rise and ensure that touch and step voltages within and around earth systems do not exceed values permitted in Standards. If this is not practical, precautions must be taken to remove danger. The general principle in Standards is the shorter the duration, the higher the body current which may be tolerated without risk of ventricular fibrillation. General practice is to base the design of an earthing system upon the fault clearing time of the primary or main protection, as the risk of a relay failure occurring at the same time as other adverse factors necessary for electric shock to occur are a very low order. Standards generally require improved designs to restrict the severity of rise of voltage on earth systems and also the resulting touch voltages in terms of magnitude and duration. This is particularly important where operators may have wet footwear or make low resistance foot contact with ground. To comply with Standards, it is firstly required that all practical steps must be taken to keep the potential rise of the earth grid below the level which otherwise would require special precautions within a site. 2.1 Transfer Potential and Limits Voltage rise on a utility or generator 33 or 11 kv earth system will be transferred to LV neutral conductors and LV earth terminals where these are connected to the earth system. Limits for permitted voltage rise on shared or interconnected 33 or 11 kv earth systems and LV neutral earth systems now appears in Standards. Figure 17 shows the earth systems that may be present for a LV generator connection to a 33 kv overhead line. AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 47

51 Figure-17 Connection of Earthing Systems Associated with LV/33 kv Generators AHEC/MNRE/SHP Standards/E&M Works Guidelines for Power Evacuation & Interconnections with Grid 48