UProtection Requirements. Ufor a Large scale Wind Park. Shyam Musunuri Siemens Energy

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1 UProtection Requirements Ufor a Large scale Wind Park Shyam Musunuri Siemens Energy Abstract: In the past wind power plants typically had a small power rating when compared to the strength of the connected electrical network and the behavior of the wind mills during faults in the network was considered non critical and wind power plants were simply pulled out of the system. Hence protection requirement of a wind mill was just restricted to simple current and voltage based measurement. This paper identifies the areas of concern where proper protection has to be introduced apart from the basic wind park requirements. Owing to the increase in demand for renewable energy large wind parks are constructed in deep seas. Such wind parks are connected to the electrical grid on the land by comprehensive underground cables. The underground cables are a potential for a fault occurrence. The next area of concern would be in the multiple mechanically switched capacitor banks or FACTS used for dynamic reactive compensation. The voltage flicker produced by large wind parks owing to the varying speed of the wind mills on the interconnected grid has a deteriorating effect on the other connected equipment and also on the grid as well. To maintain voltage stability as per NERC guidelines and to mitigate voltage flickers, dynamic reactive compensation can be provided with multiple mechanically switched capacitor banks or FACTS. Therefore the paper will discuss in detail the protection possibilities for the underground cables and the reactors for large wind parks located offshore. INTRODUCTION The exorbitant and phenomenal rise in oil prices in early 2008 and the drastic need to protect the environment from the climatic changes due to burning of fossil resources for power generation led to the increase in wind generation. Although wind generation is explored extensively on the shore, however the visually abhorrent pollution created by large wind mills on the scenic beauty of the land, led to the fast emerging alternative - the location of wind parks offshore in deep seas. The penetration of the wind mills in generating clean energy is on the increase every day and these wind mills when integrated into a large existing grid may require a redesign of the existing power system and the operation approaches. The challenge of integrating the windmills into the grid has to address the following pertinent questions. a) How to maintain acceptable voltage level at all times to the consumers? b) How can windmills co-generate with the other power plants according to the customer energy needs? Therefore this paper will focus on the following 1. Introduction and analysis of the various electrical fault possibilities in an offshore wind park. 2. Detection and prevention against Ferro resonance due to inductive reactance introduced by wind generators, reactors and capacitive reactance introduced by the long running cables and capacitor banks. 3. Automatic fault collection and analysis, and its benefits /09/$ IEEE 478

2 Figure 1 indicates the typical single line diagram with the conventional protection elements in the wind park. I) ANALYSIS OF THE VARIOUS ELECTRICAL FAULT POSSIBILITIES IN A WIND PARK The wind park faults shall be split broadly into (a) Network faults occurring in 1.1 Sea Submarine cable connection between the Onshore and the Offshore systems. 1.2 HV Transformer and cable connection to the collector bus 1.3 Bus bar faults of 34.5kV system (b) Wind park faults occurring in 1.4 Cable connection between the collector bus and the Wind turbines 1.5 Cable connection between Wind turbines 479

3 (a) Network faults 1.1- Sea Submarine cable connection between the Onshore and the offshore systems On-shore Off-shore COMMUNICATION CHANNEL(Dual) Figure 2 The conventional AC transmission via sea cables, turns out to be technically and economically attractive, however the limitation being the distances over which the power can be transmitted. This limitation of the ac power transmission capacity can be suitably compensated by the FACTS and HVDC system. However in the HVDC system it will be difficult to determine the fault current fed from the network into the wind park during the design stage. The fault typically varies from 0.0 to1.0 pu and the system behavior without the actual fault current will not be accurate enough to draw any conclusions. Alternate to the conventional current, voltage and frequency protection, the technically superior and cost effective solution will be 2 in 1 protection, where there exists two main protections like Line Differential and Distance protection. The sea submarine cable is protected by 3 terminal differential protection connected in a ring, with one relay connected to the feeder connecting the submarine cable at the on-shore and the other two relays connected to the secondary side of the step-up transformer located on the off-shore. For any faults in the submarine cable, the relays consider the fault as an in-zone fault, all the three 2 in 1 relays will trip on differential protection and isolate all the fault feeding possibilities. 480

4 1.2- HV Transformer and cable connection to the collector bus 1 A-G 2 Figure-3 The step up grid transformer typically used for the windmill is a 3 winding transformer which is star connected on the primary and connected on both the secondary as shown in figure-3 The dashed line indicates the fault trajectory and it can be seen that only when the single line to ground faults occur between the H102-T1 CTs and the transformer-t101, the fault current passes through the CT and if it occurs other than the above mentioned zone then the fault current will not pass through the CTs. Conventionally a transformer differential protection is provided for the transformer; however there will be a serious limitation with respect to the sensitivity of the settings on the differential relays during an SLG fault. Let us start analyzing the same. 481

5 In our case study, the earth fault current is limited by the grounding transformer to 400A. Primary MVA of the Transformer = 220MVA Secondary MVA of the transformer = 110MVA CT ratio of the Transformer Secondary = 2500/1A Earth fault current = 400A Secondary voltage = 34.5kV I BSec B= U220,000 U= 3682A 3*34.5 I BDiff B= U400 U= IB0 B= U400 U= The minimum setting for the differential relay is set at 0.2, to take care of the CT errors, errors due to the variation in the transformer taps etc. When the relay is set at 0.2 and if the actual differential current works out to be or 0.16, the relay will get desensitized and will fail to trip resulting in huge over voltage in the transformer, which can ultimately lead to a catastrophe. The solution for such an application will be to use the 3IB0 Bsetting of the 51N function and set according to the utility standards. TRANSFORMER FAULTS Faults in the Step up Transformer can also be identified, utilizing the distance protection function available in all the 2 in 1 relays. On the primary side, the 2 in 1 relay located in the on shore could be set to cover 80% of the transformer primary side windings with suitable Zone-1 / Zone-2 settings and the 2 in 1 relays connected in the secondary sides can be set to have an offset reverse Zone-3 protection to cover 80% of the transformer secondary s. A REF protection can be provided for the transformer primary to locate sensitive earth faults and any transformer winding faults 482

6 Figure-4 Busbar protection of 34.5kV system The bus bar fault is the most severe fault in any power system and bus bar protection could be realized by the reverse interlocking function functionally available in the over current relays.iec61850 GOOSE helps in the intra communication between relays exchanging information.. The busbar faults can be realized by the following logic and condition Condition-1 : Relay in Circuit No2 should see the faults in reverse direction Condition-2 : Relay in Circuit No4 up to Circuit-8 should see the faults in forward direction 483

7 67(P) Reverse pickup 67(N) Reverse pickup 2 67(P) Forward pickup 67(N) Forward pickup 4 Trip all fault feeding Circuit Breakers 67(P) Forward pickup 67(N) Forward pickup 8 Reaction of a Wind park for network faults For network faults, immediate disconnection of the large wind parks is not advisable as the disconnection would put additional stress on the already troubled system. As a rule the wind parks are not disconnected as long as certain voltage and frequency limits are not exceeded as defined by the utility. Each utility has its own definition for the LVRT( Low Voltage Ride Through )and the commonly adopted characteristics is as shown below 484

8 (b) Wind Park Faults 1.4 -Cable connection between the collector bus and the Wind turbines Figure-5 The wind mills connected radially ends up in the collector bus. Therefore the collector bus carries a huge power and removal of this feeder on fault is equivalent to the aggregate sum of the wind generation connected to this feeder getting lost. The collector bus is protected by main and backup protections. The main protection could be supplied by the distance element of the 2 in 1 protection and the backup provided by the Directional / nondirectional relays. The distance element can be suitably graded to take care of all the faults and the backup protection as well. The collector bus faults can be realized by the following logic and condition Condition-1: Relay in circuit no-2 should see the faults in reverse direction Condition-2: Relay in circuit no-4 should see the faults in reverse direction Condition-3: Relay Nos 5-8 should see the faults in forward zone 485

9 When the above conditions are met then the relay identifies the fault as a fault on the collector bus feeder. 67(P) Reverse pickup 67(N) Reverse pickup 2 67(P) Reverse pickup 67(N) Reverse pickup 4 Trip the collector bus CB 67(P) Forard pickup 67(N) Forward pickup Wind mills and their interconnections Let us understand what happens when a single phase to ground fault occurs in an isolated system. A healthy isolated system is as shown in fig-(a). When there is a solid A-phase to ground fault, the voltage at phase-a equals the neutral voltage. Because of this shift in the neutral we observe that the phase to neutral voltages of the other two healthy phases equals the phase to phase voltage. Hence, during ground faults the phase voltage equals the line voltage. If the system continues operation and when the system gets stressed due to over voltage, it can lead to a catastrophe. 486

10 NETWORK 150/34.5kV 0.69/34.5kV 0.69/34.5kV 34.5kV Collector bus The individual wind mill after the step up transformer is connected radially to the other wind mills and the system is isolated without the grounding transformer. There are only CTs in the windmill switchgear and no VTs. When there is an earth fault in the radial feeder, then the collector bus feeder detects an earth fault from the broken delta VT. However this does not exactly identify the position of the earth fault but just an indication that there exists an earth fault somewhere in this feeder. Each wind mill is taken out of the collector bus circuit and when there is an indication in the broken delta, and then it is identified as a feeder with an earth fault. The minimum the time spent on the offshore windmill, the better it is for the maintenance engineer, because the offshore plants are wet and are even dangerous. Let us assume a radial feeder with an earth fault connected to a collector bus and to a bus bar as shown below. These CTs will not see this fault as the zero sequence currents will not flow through the step up transformer as the earth fault current will circulate between the circuit where the fault lies and the grounding transformer. So, if we place a core balance CT in the radial circuit which senses the earth fault and if the CT outputs can be given as the fourth input of the relay in the switchgear then the relay senses the earth fault. 487

11 G G G G Collector Bus Grounding Transformer Busbar Red dotted line shows the fault trajectory in case of an earth fault. II) Ferro-resonance in Windmills- Is there a possibility? When the system capacitance is in parallel to the inductance of the voltage transformer and if there is an initiating event such as a transient overvoltage due to switching or a phase to ground fault on the ungrounded system, this can drive the voltage transformer into saturation. The ferro magnetic circuit during saturation can lead to a condition when the inductive reactance is exactly equal to the capacitive reactance and ferro resonance results. The saturation of the core is maintained by the continuous over voltage and the ferro resonant condition is stabilized. Induction generators, reactors in a wind park are a source of inductive reactance and cables, capacitor banks contribute capacitive reactance. This combination of inductive reactance and capacitive reactance can lead to a complex electrical phenomenon called Ferro resonance, characterized by the sudden onset of a very high sustained over voltage concurrent with high levels of harmonic distortion. The following are the conditions under which Ferro-resonance is likely to occur. 1. A sinusoidal voltage source A power system generator 2. Saturable Ferromagnetic inductances- Can be power transformers or instrument transformers 3. Capacitance- The large capacitance from the cables, or the capacitance to ground of an ungrounded system 4. Low Resistance- Unloaded Transformer, low short circuit power source 5. Existence of at least one point in the system whose potential is not fixed. 488

12 Refer to figure-1 and assume that there is no grounding transformer. Let us start analyzing bay no-h103 the collector feeder circuit if Ferro-resonance can be a possibility and see how the above conditions are met. 1. A sinusoidal voltage source- The wind generators feeding thro Kabel-C Saturable Ferromagnetic inductances- The voltage transformer- ( T5 ) in the collector circuit 3. Capacitance- The capacitance to ground of the 34.5kV cables 4. Low Resistance- The voltage transformer is probably very lightly loaded. 5. The existence of at least one point in the system whose potential is not fixed.-the inadequately grounded section of the 34.5kV system. The effect of Ferro resonance on a power system is excess overvoltage and harmonic distortion. The over voltages can exceed the normal phase to phase voltage and damage the insulation of the connected equipment and the harmonics confuse the protection systems from taking the right decision Prevention, Detection and Mitigation. There are relays in the market which detect the ferro resonance and alarms the condition. The following conditions can be practiced for elimination of the ferro resonance effect. 1. To prevent the system from becoming ungrounded at any point of time with suitable grounding systems 2. Introduce Losses by means of load Resistances. III) Automatic Fault Analysis As the off-shore wind parks are wet, dangerous and many other inconveniences, minimum effort should be exercised at the site for rectification of faults. The control engineer or the utility engineer would be highly benefitted with the following 1. Automatic retrieval of fault records from all devices installed. 2. Centralized Data archiving 3. Automated data analysis required for Fault analysis including distance to fault location, monitoring facilities like device monitoring and also system monitoring like communication monitoring. 4. One analysis tool instead of multitude of software packages-this reduces significantly the analysis time and the crew training times. The automatic fault analysis systems must be able to process all kinds of data recorded by digital devices like numerical protection relays, digital fault recorders and power quality recorders installed. Diagnostic results and PQ reports. Now let us see how a best result is obtained from an automatic event analysis 489

13 The following tasks are being carried out for the automatic fault analysis. Grouping of fault records Transient records are collected from different equipment capable of producing fault records. The transient records are grouped based on the trigger time information. The goal of this action is to group all records related to the same network event in one folder and to facilitate by this the searching of records. Automatic diagnosis of faults in the network The automatic diagnosis is started immediately after receipt of a new record file. The diagnosis considers all records pertaining to the same power system event. Principles of Fault Location: Basically a single ended measurement of the reactance is used to determine the fault location. This principle requires measured data from the three phase to ground voltages and the three line currents during the fault in one single record. In the next step frequency and the phasor of all analogue signals are determined. Finally depending upon the fault type the distance to the fault and the fault resistance are computed. 490

14 The advantages of an automatic fault analysis system is as enumerated Feature Explanation Benefit Fast and reliable fault location after fault clearance Fast and reliable fault location after successful auto reclosing Automatic data grouping and storage Automatic analysis of fault recorder and numerical protection device records and messages Identification of weak points in the network like Identification of fault cause Minimizing outage times (down times) Identification of weak points in the network like defective Insulator Grouping of all recorded data involved in network event. This allows Availability of all concerned data sets without searching in data bases Corrective action may be started immediately after data analysis Identification of weak points in the electrical system like Ferro resonance Breakers (switching time monitoring) Frequent transient faults caused by trees Saves Money Saves Time Saves Money Saves Time Saves Money Saves Time Gives peace of mind Saves Money Conclusion With today s Multifunction relays and standard open communications protocol like IEC61850, the investment is secured. Due to higher capability with modern algorithms in the numerical relays, a reduced number of relays are required for the same protection requirement which otherwise will require many conventional relays. The 2 in 1 relay is one of such a type which can lead to a better asset management because of usage of less number of relays. Apart from the above mentioned advantages, the fault recording capabilities of the numerical relays can be used as inputs to an automated fault analysis system. The fault recordings from different relays for a same fault can be grouped and analyzed to find the exact fault location in a wind park. The automated fault analysis system also helps in maintaining records and archiving it as per NERC or other regulators storage guideline. The paper also points out to a must do in a wind park that the system cannot be left ungrounded at any point even if it is to forego some benefits as this could lead to a complex phenomenon called Ferro resonance. References 1. Siemens Power Engineering Guide Siemens Line Differential protection with Distance protection-7sd52/3 3. Book: Wind Power in Power Systems by Ackermann.T 4. Paper: Offshore Wind farm electrical connection options by W.Grainger,N.Jenkins 491