CHAPTER 3 OVER CURRENT AND EARTH FAULT RELAY COORDINATION

Transcription

1 18 CHAPTER 3 OVER CURRENT AND EARTH FAULT RELAY COORDINATION 3.1. OVERVIEW OF RELAY COORDINATION Relay coordination has to be carried out based on many other power system studies and Equipment Sizing calculation, are listed below a. Load Flow Studies b. Short Circuit Studies c. Motor Starting Studies d. Transient Stability Studies e. Harmonic Analysis f. CT Sizing Hence it is recommended to give a brief introduction of all the above studies before going for the relay coordination. Types of Faults There are two types of faults. 1. Symmetrical faults. 2. Unsymmetrical Faults.

2 19 1. Symmetrical faults. The symmetrical fault is also called as balanced fault. The Faults occurs only when all the three phase are short critical simultaneously. There are two types of symmetrical faults. 1. (L.L.L.G) all the three phases are short circuited to ground. 2. All the three phases are short-circuited. The symmetrical faults will occur only 2 to 5% of the total system faults. The analysis of symmetrical faults is mandatory for the resulting the capacity of the circuit breakers, for choosing relays of corresponding type and other protective switchgear. Overview of Protective Relays There are different types of relays which are used to isolate the devices under critical conditions 1. Magnitude Relays. 2. Impedance Relays. 3. Directional Relays. 4. Pilot Relays 5. Differential Relays 6. Latching Relays. 7. Reed Relays. 8. Bucholz Relays. 9. Over Load Protection Relays. 10. Solid State Relays. 11. Inverse definition Minimum Time Relays (IDMT Relays). A few relays related to the thesis are explained as below.

3 20 Differential Relays Differential relay get in operation when two (or) more equipment electrical exceed certain predetermined value. Whenever there is a difference between the magnitude and phase difference of the currents. Figure 3.1 Schematic Diagram and Images of Differential Relay

4 21 The above figures shows the two CT Transformer connected on both sides of the transformer the one CT will be linked on the primary side and another CT will be connected to the secondary side of the transformer. The relay present in the system will compare the currents on both sides of the transformer, whenever there is a difference in current occurs then the relays get in operation. The difference relays can be either of voltage balance differential relays, (or) bi-used differential relays. Buchholz Relays Their relays are gas operated. These relays will come to into operation whenever there is imminent fault occurs. The early failure is nothing but small mistake in which due course will develop into significant faults. It will be accommodated in between the transformer tank and conservator. These are used only for oil immerged relays. Figure 3.2 Bucholz Relay Schematic Diagram and Images The above figure shows the complete picture of bucholz relay. Whenever the fault occurs there change in the top at the relay gas is occupied at the top. The float will be fitted as soon as the fault has occurred. The closing of the mercury contacts indicates that the fault has occurred and in which form the alarm will get into operation.

5 22 Solid State Relay Solid state relays are mainly composed of BTT, SCR, IGBTC, MOSFETS s, AND TRIAC for the switching operation. The power required to operate the relay is lesser when compared to other relays. The control energy needed for the functioning of a relay is deficient. There is no mechanical contact, Hence the relay is operated with high speeds. Figure 3.3 Schematic Diagram and Images of Solid State Relays

6 23 In these above picture photos, a sensitive semiconductor device is used for the switching operation. The control signal will be given to the LED, which is turn gives supplies to the photo sensitive device get into conduction, using LED. Due to this installation of SSR, the relays operate in a faster mode and its life span is higher when composed to the other relays. The advantages of this relays are make very less noise. Latching Relays Latching Relays is a relay which maintains its state each after the actuation of the relay. The name of the relay is impulse relay. The latching relay is mainly chosen for the low power consumption. It consists of internal magnets in which the current supplied to the coil, maintain the contact position and there is no separate power to support the holding. a). Current Flows (No position). b). Current stop and relay stays in position c). Current flows in opposite Direction. Latching relays are consists of this coils, and the latching relays does not have any default position since these coils are responsible for the position of the armature. In the loop type relay, the armature position is usually determined using direction of current flow in the coil. But in the case of two coil type, depending on the current flow, the armature position is determined.

7 24 Reed relay The reed relay produces of mechanical actuation of physical contacts to open or close a circuit. These relays have much smaller, and the mass of the relay is lower when compassed to they relays. The reed relay ends consist of contacts through which input and output terminals are connected to them. Whenever the power is supplied to the coils it act as an magnet and the connections make a closed path the contacts again separated using spring which it is attached to them. The switching speed of the reed relay is very much faster that is 10 to 20 times quicker than the other determining hierarchical relay.

8 25 Figure 3.4 Schematic and images of Reed Relays The main disadvantages of reed relay are electrical arcing since it has smaller contacts. The arcing process makes the contacts to melt and makes it in welded position, since the contacts are melted when it is in closed position. The demagnetizing not operates in a proper manner hence the contacts coil spring not in a position to separate them. The problem that is existing to over come by means of placing the resistor (or) capacitor so that the input currents which leads to arc will be reduced and the main advantages of these relays due to small size and high speed operation. Polarized Relay These relays are very sensitive to the direction of the current flow, since the names itself identify the process. If it is a type of electromagnetic relay then it uses magnetic forces to attract the armature and the same for the other side. The armature used in this type of relays uses magnetic force which attracts (or) repeats the armature whenever the current flows through the electromagnet, it produces a magnetic flux. The magnetic force extended by the magnet exceeds certain limit, then the armature change is occurred, the force is reduced than permanent magnet force and due to this effect, the armature return to its original position.

9 26 Figure 3.5 Schematic and Images of Polarized Relays The flux ᶲ will be produced by the magnet when it passes between the two parts namely ᶲ1 and ᶲ2. The flux which is produced ᶲ1, will passes through the left gap of the magnet. The flux which is produced as ᶲ2 will passes through the right gap of the magnet. Due to this a current will be produced in the coil, and then the armature will

10 27 stay in their own position. Depends on the parameters combination and the interactions, a force will effect on the magnitude of the current power and the value of the working gap. The two famous types of relays of this type including differential and bridge type relays. In medium and normal size of the relay model, only this type of relay is widely used. Overview of Earth Fault Relay These are different types of earth fault relays. This occurred during star point treatments of generators. a. Isolated star point. b. Compensated star point. c. Earthed via Reactor (Or) transformer. d. Earthed through Registrar RE. e. Directly Earthed star point. f. Star point treatment through neutral earthing Registrar RE. g. Protection is needed in case of earth fault external. h. Protection is required in case of internal faults. a. Isolated Star Period Depending upon the cable capacity of the earth fault currents is isolated at star points. This is the only method which is possible when the cable sizes are within the network system and do not vary. In most of the power stations, however these types of network are not available on most of the cable network system. If the fault occurs in one conductor then the voltage of system will increase by factor. 3 times of the good conductor. IE = 3.I0 3. Ui. ῶ. CE. with IE earth fault current at earth fault location. CE earthing capacitance = zero capacitance of the network. ῶ. 2πf = angular frequency.

11 28 b. Compensated star point This method is possible only when the cable capacities are within the network systems. In major power stations, the condition that is the cable dimensions is not within the network. In the case of earth fault, a fault occurs at any one conductor, then the voltage will increase by 3 factor of the remaining two good conductors and thereby the line to line voltage is achieved. IE = 3.I0 3. Ui. ῶ. CE. With IE - Earth fault current at earth fault location CE Earthing capacitance = zero capacitance. ῶ = 2π f (angular frequency). c. Earthed via Reactor In the case of earth fault, the residual current which present are very low and the cable capacities of the entire network system are mainly paid using independence. d. Indirectly Earthed Star Point The reactor is used almost of the generator to determine the earth fault current value. Due to this method the earth fault will depend on network system. IE 3.Ui ῶ CE ῶ LD IE 3.Ui ῶ CE - 1 With 3 ῶ LD

12 29 IE earth fault current at earth fault location. LD Inductance of reactor (or) transformer. CE earthing capacitance of the network. ῶ = 2π f = angular frequency. The system voltage will increase to 1.4 factors due to earth factor FE when compared to the other two good conditional conductors. e. Earthed Through Resistor The protection is carried out by means of earthing at one point instead of isolating the whole system method. The fault current depends on the cable capacities of the system. IE 3.Ui 1 + I ῶ. CE 3RE IE 3.Ui 1/ 3RE 2 + ῶ. CE 2 Ui good conductor 1.4. Ui. With IE RE CE - earth fault current at earth fault location. - earthing resistance - earthing capacitance of the network. ῶ = 2π f = angular frequency.

13 30 In the case of earth fault at the conductor, the voltage will increase approximately of a max. Factor 1.4 due to earth fault factor F E against the two good conditional conductors. f. Directs earthed star point In the case of directly earthed star point the high one phase short circuit currents, which is the multiple of the nominal generator current. T2 T2 G1 G2 T1 T1 QE RE QE TE Figure 3.6 Start Point Treatments through Neutral Earthling Resistor RE With RE - ear thing resistance with temperature suspension

14 31 RE 318 Ω IRE = 20 A rated for a period of 10 s and IRE (count) = 15 A; URE = 11KV / 3. QE - single phase vacuum contactor of the series 12,400A, with short switching lines. IE - cable type current transformer for backup earth fault detection at neutral earthing resister RE. 50/1A with 2.5 VA for max 120 min at 10% IN. T1, T2 - cable type current transformers for earth fault detection within the generators. In the above case the generator is connected to the bus bar through the earthing resistor. The star point of the generator which is connected to MV switch gear is switched to neutral earthing. If the circuit breaker of the generator, whose start point is neutral resister RE, then the corresponding circuit breaker will be in closed position. The start point of generator only one should be earthed through neutral resistor RE. Otherwise, third harmonic will flow across the star points. To protect the system against fault, the inductance and capacitance will be introduced in the system. The introduction of inductance and capacitance will make the system in stable condition. g. Protection is needed in case of external earth faults If the fault occurs beyond the generator range, then the earth current IE will be determined by the neutral earthing resistance and the source will be secondary. If the fault is of selection theory, then the relays IDMT are used for solving the earth faults problem. The IDMT relays are used on transmission lines to ensure that the line current will not exceed the safe value. During fault if it exceeds then IDMT takes minimum time to trip the circuit breaker. h. Protection in case of internal earth faults Figure 3.7 Wiring of Stator Earth Fault Protective Devices

15 32 In the internal earth fault, the stator earth relay will has to disconnect the generator which is not in running condition. The cable type transformer T1 and T2 are installed to find out the earth faults within the generator protection range. The current will be same for the fault that is occurring outside of the protection field of the transformer, and the rating will be same. If the earth fault occurs within the protection range, then the current rating will be added. If the earth fault current exceeds more that 5A, them the generator, circuit breaker will be automatically disconnected from the network and the generator circuit breaker tripping will be done not more than 10 sec maximum. If the generator not grounded then the stator earth fault will to the grounded generator which is usually in operating condition. The earth fault current of the generator will be identified when, the primary connection (L1, L2, L3) are solved by means of earth fault relay. This earth fault relay will detect. Protection of the Neutral Earthing Resistor RE TE is used to find out the residual currents that are available in the path of resistor. The neutral earthing resistor overload condition will be prevented by means earth fault relay, and the relays are root picked in correct faulted state. The pickup rate of the earth fault relay is determined using adjusting the pick up time10 s after the latest. The over temperature detecting facility is provided so that the overheating of the resistor will be reduced, this over temperature detecting equipment is another source for isolating if the earth fault do not pick up at the right period. The pre request will be given for both overload protection and over temperature relays, to isolate the generator from location of the faults. Following the above case the earth fault relay is used to operate for a specified time.

16 33 After the expiry of the time, the earth fault relay will get in operation and the fault current will be grounded properly through the neutral grounding resistor. Figure 3.8 Images of Solid State Relay 3.2. Load Flow Analysis The planning, design, and operation of power systems require load flow calculations to analyze the steady-state (quiescent) performance of the power system under various operating conditions and to study the effects of changes in equipment configuration. The basic load flow question gives the load power consumption at all buses of a known electric power system configuration and the power production at each generator to find the power flow in each line and transformer of the interconnecting network and the voltage magnitude and phase angle at each bus are founded. Based on the Analyzing the solution of this problem for various conditions helps ensure that the power system is designed to satisfy its performance criteria while incurring the most favourable investment and operation costs. The load flow studies that were carried out determines the following a. Component or circuit loadings b. Steady-state bus voltages

17 34 c. Reactive power flows d. Transformer tap settings e. System losses f. Generator Exciter/regulator voltage set points g. Performance under emergency conditions Modern systems are complex and have many paths or branches in which power can flow over the systems. Such systems form networks of series and parallel paths. Load flow Analysis provides the maximum load current and minimum load current in both the directions it constitutes the basis for selection of CT Ratios and Plug settings of Relays. Plug setting multiplier of relay is the ratio of fault current to its pick up current. The selection of required current setting will be given by plug setting multiplier plug and the plug is withdrawn inorder to adjust the current setting at the time of on load condition, due to this the maximum current taping is connected default and the opening of the secondary side of the CT is avoided. Computer programs are used to solve load flows are divided into two types static (offline) and dynamic (real time). Most of the load flow studies for system analysis are based on static network models. Real-time load flows (online) that incorporate data input from the real networks are typically used by utilities in automatic Supervisory Control and Data Acquisition (SCADA) systems. Such systems are used primarily as operating tools for optimization of generation, var control, dispatch, losses, and tie line control. This discussion is concerned only with only static network models and their analysis. Tie line control in power system which will regulate the active and reactive power flow between the micro grid and the bulk grid at the point of interconnection. By means of this tie line control the micro grid is allowed to behave as a aggregated power entity that can be made dispatch able by the utility. This tie line method is used

18 35 to associate the renewable energy sources, wind energy which push the management burden inside the micro grid To balance the load flow problem pertaining that are to be balanced, steadystate operation of power systems, a single-phase, positive sequence model of the power system are used. Three-phase load flow analysis software is available, but they are not needed for routine industrial power system studies. A load flow calculation determines the state of the power system for a given load and generation distribution. It represents a steady-state condition as if that state has been held fixed for some time. In actuality, line flows and bus voltages fluctuate constantly by any small variation in loads status lights, motors, and other loads whenever turned on and off. However, these small fluctuations can be ignored in calculating the steady-state effects on system equipment. As the load distribution, and possibly the network, will vary considerably during different time periods, it may be necessary to obtain load flow solutions representing different system conditions such as peak load, average load, or light load. These solutions will be used to determine either optimum operating modes for standard conditions, such as the proper setting of voltage control devices, or how the system will respond to abnormal conditions, such as outages of lines or transformers. Load flows form the basis for determining both when new equipment additions are needed and the effectiveness of new alternatives to solve present deficiencies and meet future system requirements. The load flow model is also the basis for several other types of studies such as short-circuit, stability, motor starting, and harmonic studies. The load flow model supplies the network data and an initial steady-state condition for these studies.

19 Short Circuit Studies Electrical power systems are, in general, fairly complex systems composed of a wide range of equipment devoted to generating, transmitting, and distributing electrical power to various consumption centres. The very complexity of these systems suggests that failures are unavoidable, no matter how carefully these systems have been designed. The feasibility of designing and operating a system with zero failure rates is, if not unrealistic, economically unjustifiable. Within the context of short-circuit analysis, system failures manifest themselves as insulation breakdowns that may lead to one of the following phenomena: i. Undesirable current flow patterns ii. Appearance of currents in excessive magnitudes that could lead to equipment damage and downtime iii. Extreme over voltages, of the transient and/or sustained nature, that compromise the integrity and reliability of various insulated parts iv. Voltage depressions in the vicinity of the fault that could adversely affect the operation of rotating equipment v. Creation of system conditions that could prove hazardous to personnel The short circuits cannot always be prevented, but we can only attempt to mitigate some degree contain their potentially damaging effects. One should, at first, aim to design the system so that the likelihood of the occurrence of the short circuit fault becomes very small. If a short circuit occurs, however, mitigating its effects consists of a. Managing the magnitude of the undesirable fault currents b. Isolating the smallest possible portion of the system around the area of the mishap in order to retain service to the rest of the system. A significant part of system protection is devoted to detecting short-circuit conditions in a reliable fashion. Considerable capital investment is required in

20 37 interrupting equipment at all voltage levels that is capable of withstanding the fault currents and isolating the faulted area. It follows, therefore, that the primary reasons for performing short-circuit studies are the following i. The adequacy verification will be preferred for the existing interrupting equipment. The same type of studies will form the basis for the selection of the interrupting equipment for system planning purposes. ii. Determination of the system protective device settings, which is done primarily by quantities characterizing the system under fault conditions. These amount also referred to as protection handles, typically include phase and sequence currents or voltages and rates of changes of system currents or voltages. iii. Determination the effects of fault currents on various system components such as cables, lines, bus ways, transformers, and reactors during the time of fault persists. iv. Thermal and mechanical stresses from the resulting fault currents should always be compared with the corresponding short-term, first-cycle, operation with stand capabilities of the system equipment. v. The assessment of different kinds of short circuits in varying severity may have effect on the overall system voltage profile. These studies will identify areas in the system for which faults can result in unacceptably widespread voltage depressions. vi. Conceptualization, design and refinement of system layout, neutral grounding, and substation grounding will lead to short circuits Short Circuit study provides information related to maximum fault current, minimum fault current, Maximum through fault current etc., for various kinds of faults which are all input for setting the relays.

21 38 Short circuit studies are as necessary for any power system, like other fundamental system studies such as power flow studies, transient stability studies, harmonic analysis studies, etc. Short-circuit studies can be performed at the planning stage to help finalize the system layout, determine voltage levels, and size cables, transformers, and conductors. For existing systems, fault studies are necessary in the cases of added generation, installation of extra rotating loads, system layout modifications, rearrangement of protection equipment, verification of the adequacy of existing breakers, relocation of already acquired switchgear to avoid unnecessary capital expenditures, etc. Postmortem analysis may also involve short-circuit studies in order to duplicate the reasons and system conditions that led to the system s failure. The requirements and extent of a short-circuit study will depend on the engineering objectives sought. In fact, these objectives will dictate what type of shortcircuit analysis is required. The amount of data required will also depend on the extent and the nature of the study. The great majority of short-circuit studies in industrial and commercial power systems addresses one or more following short circuits i. In three-phase fault conditions, it may or may not involve ground, but all three phases shorted together. ii. In single line-to-ground fault, only one phase is shorted to ground. iii. In line-to-line fault. Any two phases are shorted together. iv. In Double line-to-ground fault. Any two phases are connected and then one line to ground These types of short circuits are also referred to as shunt faults, since all four exhibit the common attribute of being associated with fault currents and MVA flows diverted to different paths from the prefault series ones. In conventional Three-phase short circuits often turn out to be the most severe of all. Hence It is thus customary to perform only three phase-fault simulations when

22 39 seeking maximum possible magnitudes of fault currents. However, importance exceptions do exist. For instance, single line-to-ground short-circuit currents can exceed three-phase short-circuit current levels whenever they occur in the vicinity of the following i. A solidly grounded synchronous machine. ii. The solidly grounded wye side of a delta-wye transformer of the three-phase core. iii. (three-leg) design. iv. The grounded wye side of a delta-wye autotransformer. v. The grounded wye, grounded wye, delta-tertiary, three-winding transformer. For systems, where any one or more of the above conditions exist, it is advisable to perform a single line-to-ground fault simulation. The fact that mediumand high-voltage circuit breakers have 15% higher interrupting capabilities for single line-to-ground faults should be taken into account if elevated single line-to-ground fault currents are found. Line-to-line or double line-to-ground fault studies may also be required for protective device coordination requirements. It should be noted that, since only one phase of the line-to-ground fault can experience higher interrupting requirements, the three-phase fault will still contain more energy because all three phases will experience the same interrupting requirements. Other types of fault conditions that may be of interest which included are so-called series faults (Anderson [B1]) and pertain to one of the following types of system unbalances: i. One line open. Any one of the three phases may be open. ii. Two lines open. Any two of the three phases may be open. iii. Unequal impedances. Unbalanced line impedance discontinuity. The term series faults is used because the above unbalances are associated with a redistribution of the pre-fault load current. Series faults occur when assessing

23 40 the effects of snapped overhead phase wires, failures of cable joints, blown fuses, failure of breakers to open all poles, inadvertent breaker energization across one or two poles and other situations that result in the flow of unbalanced currents. Industrial and commercial power systems are usually multimachine systems with many motors and possibly more than one generator, all are interconnected through transformers, lines, and cables. There should also be one or more locations at which the local power system is interconnected to a larger grid. These areas are commonly known as utility-interface points. The objective of the short-circuit study is to correctly determine the short-circuit currents and voltages at various system locations. Given the dynamic nature of the short-circuit current, it is essential to relate any calculated fault currents to a particular instant in time from the onset of the short circuit. AC decrement analysis serves the purpose of correctly determining the symmetrical RMS values of the fault currents, while dc decrement analysis will provide the necessary dc component of the fault current, thus yielding a correct estimate of the total fault current. It is the total fault current which, in general, must be used for breaker and switchgear rating and in some cases for protective device coordination. System topology considerations are equally important because the system layout and electrical proximity of the rotating machinery to the fault location will determine the actual magnitude of the short-circuit current. It therefore becomes necessary to devise a model for the system, as a whole and analyze it as such in a flexible, accurate, and computationally convenient manner. The AC decrement analysis will able to determine the system subtransient, transient and steady state of a generator during the fault condition. Whenever a fault is occurred in the generator the current in the all three phases rises rapidly about 10 to 18 times of full load current during the first cycle of operation and starts decreases during the second cycle of operation. The change in RMS value of the current and reactance are determined The symmetrical components theory dictates that for a three-phase system, three sequence systems need, in general, to be set up for the analysis of an unbalanced fault condition. The first is the positive sequence system, which is defined by a balanced set

24 41 of voltages and currents, equal in magnitude, following the standard phase sequence of a, b, and c. The second is the negative sequence system, which is similar to the positive sequence system but it is defined by a balanced set of voltages and currents with a reverse phase sequence of a, c, and b. Finally, the zero sequence system is a system defined by a set of voltages and currents that are in phase with each other and not displaced by 120 degrees, as is the case with the other two systems. The topology of the zero sequence system can be quite different from that of the Positive and negative sequence systems because it depends heavily on the power transformer connections and system neutral grounding, factors which are not of importance when determining the topology of the other two sequence networks. Static system equipment like transformers, lines, cables, bus ways, and static loads present, under balanced conditions, the same impedances will flow to the positive and negative sequence currents. In case of the same components present, then different impedances will flow through the zero sequence currents. Rotating equipment like synchronous generators, motors, the positive sequence impedances are the ones typically used for balanced power flow studies. All sequence impedances must be calculated, measured, provided by the equipment manufacturers, or estimated. The zero sequence impedance may not exist for some rotating equipment, depending on the machine grounding Motor Starting Studies Motors on modern industrial systems are becoming increasingly larger; some are considered large even in comparison to the total capacity of large industrial power systems. In starting large motors, especially across-the-line, can cause severe disturbances to the motor due to locally connected load, and also to buses electrically remote from the point of motor starting. Ideally, a motor-starting study should be made before a large motor is purchased. A starting voltage requirement and preferred locked-rotor current should be stated as part of the motor specification. A motorstarting study should be made if the motor horsepower exceeds approximately 30% of the supply transformer(s) base kva rating, if no generators are present. If generation

25 42 is present, and no other sources are involved, a study should be considered whenever the motor horsepower exceeds 10 15% of the generator kva rating, depending on actual generator characteristics. The study should also recognize contingent condition(s), i.e., the loss of a source (if applicable). It may be necessary to make a study for smaller horsepower sizes depending on the daily fluctuation of nominal voltage, voltage level, size and length of the motor feeder cable, amount of load, regulation of the supply voltage, the impedance and tap ratio of the supply transformer(s), load torque versus motor torque, and the allowable starting time. Finally, some applications may involve for starting large groups of smaller motors of sufficient collective size to impact system voltage regulation, during the interval of starting. Motor starting study provides the essential information of time vs current during motor starting. This is very necessary for the setting of the relay curves should be above these motor starting curves to prevent the maloperation of relays during motor starting Transient Stability Studies For years, system stability was a problem almost exclusively to electric utility engineers. Small independent power producers (IPPs) and co-generation (co-gen) companies were treated as part of the load and modelled casually. Today, the structure of the utility industry is going through a revolutionary change under the process of deregulation. A full-scale competition in the generation market is on the horizon. Increasing numbers of industrial and commercial facilities have installed local generation, large synchronous motors, or both. The role of IPP/co-gen companies and other plants with on-site generation in maintaining system stability is a new area of interest in power system studies When a co-generation plant is connected to the transmission grid, it changes the system configuration as well as the power flow pattern. This may result in stability problems both in the plant and the supplying utility.

26 43 Transient stability studies provide the information regarding the critical clearing time. Relay setting should be done in such a way that relay will isolate the fault well before this critical clearing time. 3.6 Harmonic Analysis Traditionally, the primary source of harmonics in power systems has been the static power converter used as rectifiers for various industrial processes; however, the static power converter is now used in a variety of additional applications such as adjustable speed drives, switched-mode supplies, frequency changers for induction heating, etc. Semiconductor devices are being increasingly used as static switches that modulate the voltage applied to loads. Examples of these are soft starters for motors, static var compensators, light dimmers, electronic ballasts for arc-discharge lamps, etc. Other examples are devices with nonlinear voltage-current characteristics such as arc furnaces or saturable electromagnetic devices. Since nonlinear loads represent an ever-increasing percentage of the total load of an industrial or commercial power system, harmonic studies have become an important part of overall system design and operation. Fortunately, the available software for harmonic analysis has also grown. Guidelines for the acceptance of harmonic distortion are well-defined in IEEE Std By modelling power system impedances as a function of frequency, a study can be made to determine the effect of the harmonic contributions from nonlinear loads on the voltages and currents in the power system. The harmonic study provides the information on the maximum distortion for which relay should not maloperate. 3.7 CT Sizing Proper selection and sizing of Current Transformers are first important step in over current relay coordination. CT ratio should be selected based on the Full load current of component with the overload margin. Protection CT shall be sized in such a way that it is not saturated for maximum fault current. However new numerical relays allows CT saturation without any maloperation.

27 Setting Time for Over Current and Earth Fault Introduction Conventionally overcurrent (OC) relay settings are provided based on full load current of power system components. Time Dial Setting (TDS) and Type of curve are chosen to ensure that the coordination with the downstream relays. This conventional procedure for setting the relays went well for a long time. However introduction of Embedded Renewable Generation, Cogeneration in plants in process industries and islanding from the grids, Change in the Grid Topology etc. results in the drastic change in the fault current. This leads to problems like nuisance tripping of relays, improper coordination or longer time taken to operate the relays for a fault. The Situation got worse in the continuous addition of renewable generation, most sophisticated grid islanding schemes, Energy efficient motors which draw high starting current, technological advancements in controlled switching for transformer and reactor to reduce the inrush current etc. made the conventional method of relay setting obsolete. These technical improvements along with new feature in the new numerical relays provide a better platform to coordinate the relays to reduce the operating time of relay, prevent the nuisance tripping and ensure the coordination between the relays in all the grid topologies. Overcurrent Relays utilized in power systems protection as economical protective devices. Overcurrent relays are used as primary protection devices in Low Voltage Radial systems and as backup relays to distance and differential in the High voltage interconnected transmission and sub-transmission system. Over current Relays are categorized as Instantaneous, definite time and inverse time relays. Modern numerical relays from famous manufacturers like ABB, Areva, GE, Siemens etc. has three stages of Protection. Stage 1 & 2 shall be either Inverse curve or definite time whereas Stage 3 is Instantaneous without any time delay. Also these relays have the additional feature of multiple group setting depending on the digital input to the relay which may corresponds to particular topology of grid

28 45 conditions. In addition to these relays have inrush or starting multipliers which may be effectively used to prevent the mal-operation of relays during motor starting or transformer charging. In addition to standard curves such as Normal Inverse (NI), Very Inverse (VI), Extremely Inverse (EI), Long Time Inverse (LTI), Standard Inverse (SI) etc. relays also have the feature of developing user defined curves based on the user requirements. All these features available in the new relay are utilized with sophisticated software program to reduce the fault clearing time and prevent the nuisance tripping of the relays Over Current Relay Coordination Stage: 1&2 (Inverse Definite Minimum Time Delay IDMT Relay 51) a. Pickup up Setting or Plug Setting Plug setting for inverse relays shall be selected based on the maximum possible load current and over load margin. In HV substation this depends on the worst case power-flow current with some future margin. b. Time Dial Setting (TDS) or Time Multiplier Setting (TMS) Choosing TDS is more involved task which provides the necessary coordination with downstream relays. This depends on the how many factors including maximum fault current, minimum fault current, starting Current, Inrush Current, through fault current, Type of curve selected etc., c. Curve Selection Selection of curve for the relay is also involved task. Conventionally normal Inverse or Stand Inverse is selected for plain feeders and Extremely Inverse is used for Transformer Feeders and Motor Feeders. Stage: 2 (Definite Time Delay DT Relay 50)

29 46 a. Pickup up Setting or Plug Setting Plug setting for Definite Time relays depends upon whether time discrimination is adopted or not. If time bias is not used i.e. Definite Time is set as minimum time then the pickup setting shall be higher than the starting or inrush or through fault current. However present modern of numerical relays provides many options related to starting and inrush, hence pickup setting can be lowered in normal operating condition which reduces the damage. i.e. Even when there is fault with minimum short circuit current, this options make to fall in the Definite Time region of the relay characteristics, otherwise which falls in Inverse characteristics of the relay in conventional setting. This result in significant reduction of fault clearing time and leads to damage. b. Time Setting (TS) Proper Time discrimination is proposed if the relay is used to coordinate with the downstream relays. Otherwise the minimum available time delay the relay will be used. Stage: 3 (Instantaneous stage 50) a. Pickup up setting Pickup setting shall be higher than the starting or inrush or through fault current. However present modern numerical relays provides many options related to starting and inrush and hence pickup setting which can be lowered in normal operating condition that reduces the damage. I.e. whenever fault occur with minimum short circuit current this options enable to fall in the instantaneous region of the relay characteristics which falls in Inverse characteristics of the relay in conventional setting. This result in significant reduction of fault clearing time will leads to damage. Earth fault settings also Similar to the over current relay setting except the below facts

30 47 a. Plug settings are based on the unbalanced current in residual connected type current transformers and minimum possible setting shall be adopted in Core Balancing Current Transformers b. Earth fault current depends on the type of earthing (Solid, Resistance Earthed) and hence care shall be taken to ensure the fault current available. c. Earth fault current also depends on the winding configuration. For an example of fault on the star side of the Delta Star transformer, the same will not be reflected in the Delta side and hence the same need not be coordinated Table 3.1 Relay Characteristic Grading Margin The current and time settings selection shall start at the load end and worked back towards the power source. Grading margins between protection relays shall be typically as follows: Grading Margin : (Er1 + Er2 + Ect) x T/100 + Tcb + T0 +Ts =( ) x 0.25/ = s Where,

31 48 Er1, Er2 = Relay Timing Errors Ect = CT Error T = Nominal Operating time of relay nearest to the fault Tcb = Circuit Breaker Opening time T0 = Relay Overshoot time Ts = Safety Margin 3.9. Summary In this chapter the basics of Relay coordination and steps involved in the relay coordination were discussed.