Extensive LV cable network. Figure 1: Simplified SLD of the transformer and associated LV network

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1 Copyright 2017 ABB. All rights reserved. 1. Introduction Many distribution networks around the world have limited earth-fault current by a resistor located in the LV winding neutral point of for example an 132/X kv infeed transformer. LV voltage level (i.e. X kv) depends on a particular utility and country and can have different value. Some typical values are 6.3kV, 13.8kV, 20kV and 35kV. Simplified single-line diagram of such distribution network and the relevant fault points for the LV side low-impedance restricted earth-fault protection function (i.e. REF) are shown in Figure 1. Note that for such installations special attention regarding LV side low impedance REF protection setting and ACT configurations shall be taken. This is even more important if only one single grounding resistor is used for the entire LV network and at the same time the LV network consist mainly of cable feeders. Namely the capacitance to ground of such cable network is quite large and this can cause difficult operating condition for LV REF function in case of internal earth-fault as described in this document. 132kV BUS Transformer 132/X kv YNynd11 P2 F1 P1 F2 LV BUS (X kv) Extensive LV cable network Figure 1: Simplified SLD of the transformer and associated LV network Note that internal earth-fault location (i.e. F1) and external earth-fault location (i.e. F2) are also marked in Figure 1. Based on ; Prepared Zoran Gajic Approved Cost Patrick ABB Power Grids, GA Products 1MRG A en 1/8 File: 1MRG029011_REF function in distribution network.docx; 1.0 Saved Date: :55; Template: TECHNICAL DOCUMENT STANDARD, PORTRAIT.DOTX

2 The following shall be noted about such networks: 1. Resistor R 1 limits the earth-fault current to a value typically in-between 300A to 2000A on the LV side depending on the practice of a particular utility 2. The entire LV network has only one grounding point via the R 1 resistor located in this transformer secondary winding neutral point 3. The LV network has relatively large capacitance to ground due to extensive use of cable feeders (common practice for distribution networks within densely populated urban areas or in industrial installations). As well in many countries cablification of distribution network in general is becoming quite popular. 2. Theoretical explanation of earth-fault in such network The REF function is measuring zero sequence currents. Namely IN in the star point and 3Io at the LV winding terminal side. Zero-sequence currents during an earth fault can be calculated by using equivalent zero-sequence circuit of the distribution network. The simplified, zero-sequence equivalent circuit during the internal fault is shown in Figure 2. Internal Fault Point F1 LV Busbar IN_W2/3 Io_Net L T R E L E 3R 1 Uo 3Io C E IN Ground Figure 2: Simplified equivalent circuit for the zero-sequence system during internal single phase-to-ground fault The simplified, zero-sequence equivalent circuit during the external fault is shown in Figure 3. LV Busbar External Fault Point F2 IN_W2/3 Io_Net L T R E L E 3R 1 3Io Uo C E IN Ground Figure 3: Simplified equivalent circuit for the zero-sequence system during external single phase-to-ground fault ABB Power Grids, GA Products 1MRG A en 2/8

3 The following symbols are used in above two figures: 1. R 1 is the neutral point resistor in the LV winding star point. Note that its value must be multiplied by three because Io currents and Uo voltages are used in this equivalent circuit. 2. L T is the phase leakage inductance of the 33kV transformer winding. 3. R E is the equivalent resistance in one phase of the LV cable network. Note that it has very low value. 4. L E is the equivalent reactance in one phase of the LV cable network. Note that it has very low value. 5. C E is the equivalent capacitance to ground of one phase in the LV cable network. Note that it has relatively large value. 6. Uo is zero-sequence voltage component at the fault point. Practically its magnitude can be taken as rated phase-to-ground voltage on the LV side. Note that this voltage will drive the two zero-sequence current components through their respective branches. 7. _ 2/3 is zero-sequence current component which flows through the transformer LV winding neutral point and associated resistor during a single-phase-to-ground fault. Note that this is only one third of total IN current which will be measured through the resistor. 8. _ is zero-sequence current component which flows through the LV cable network during a single-phase-to-ground fault. By looking into the equivalent circuits (i.e. above two figures) the following can be concluded: 1) IN_W2 current component will be slightly inductive due to dominant resistance (i.e. R 1 ) in that circuit branch (i.e. it will lag behind the Uo voltage for several degrees) 2) Io_Net current component will be dominantly capacitive due to dominant capacitance (i.e. C E ) in that circuit branch (i.e. it will lead the Uo voltage for almost 90 ) Conclusion: During an internal earth-fault, the angle between these two zero-sequence current components will be around 90 or even somewhat bigger (see Figure 2). This can cause problems if REF function utilizes directional measurement for its operation. Note that for external faults the REF function will see 180 between two measured zero-sequence current components. The reason is that IN_W2 component will be measured in both measurement points by the function (see Figure 3) due to a fact that the fault location (i.e. F2 in Figure 1) will be outside of the protected zone. 3. Evaluation of an actual internal earth-fault in such network Internal earth-fault in an actual installation was captured by the disturbance recorder in RET650. Figure 4 shows the entire fault duration. Obviously the fault started as L1-Gnd fault on the LV side and then after approximately 275ms it has involved into a three-phase fault which was quickly tripped by 87T function. Main question is why the LV winding REF protection did not operate during the single- phase-to-ground fault stage? Figure 5 and Figure 6 zoom-in into the recording part when only the single-phase-to-ground fault has existed. Figure 7 shows the phasor diagram for LV winding currents during the earth-fault. Note that current magnitudes in all figures are given in primary amperes. ABB Power Grids, GA Products 1MRG A en 3/8

4 Figure 4: Overview of the entire fault duration Figure 5: Zoom into the instant when single phase-to-ground fault has happened ABB Power Grids, GA Products 1MRG A en 4/8

5 Figure 6: RMS value of the currents during single phase-to-ground fault Figure 7: Phasor diagram for LV currents at t=+0,04s (see Figure 3 or 4 for time scale) ABB Power Grids, GA Products 1MRG A en 5/8

6 Figure 8:W2 REF function Diff and Bias current as recorded by RET650 Figure 9: Used settings for W2 REF function (Base current was set to 2187A primary) From the previous six figures the following can be concluded: 1. The fault started as L1-Gnd fault on the LV side and then after approximately 275ms it involved into a three-phase fault. After that the fault was tripped quickly by 87T function. 2. IN_W2 magnitude was 858A primary during the entire single-phase-to-ground fault. 3. 3Io_W2 magnitude was 233A during the steady-state part of the single-phase-to-ground fault. Transiently 3Io_W2 magnitude went up to 384A primary. 4. From captured DR it is obvious that a lot of transients were present in recorded 3Io_W2 current waveform. 5. Recorded Differential and Bias current for W2 REF functions are shown in Figure 8. Diff current had magnitude of 866A, while the magnitude of Bias current was 933A primary. The differential current was almost equal to the bias current. Thus the REF function should operate based on the set operating characteristic. 6. Used settings for W2 REF functions operation are given in Figure The phase angle between IN_W2 and 3Io_W2 phasors was slightly bigger than 90 (see Figure 7). By using more precise calculation it was concluded that the value of this angle was 93, as seen by the REF function in RET650, during this internal earth-fault. This angle was outside of the set ROA value of 60 (see Figure 9). This was actually the main reason for missing operation of the W2 REF function. Conclusion: The LV REF function did not operate because the measured angle between IN_W2 and 3Io_W2 phasors was bigger than the set value of 60 which is the default value for the REF function. The reasons why this angle had value of 93 is already explained in the Section 2 of this document. Thus, this captured recording just confirms the previously given theory in this document. ABB Power Grids, GA Products 1MRG A en 6/8

7 4. Proposed Solution for REF function in RET670/RET650 The directional criterion in REF function is used in order to prevent unwanted operation of the function during CT saturation when installed in a directly grounded network. Because in this type of installation the earth fault current is limited by the resistor, the CT saturation is not really a problem. Thus for all installations where only one grounding resistor is used as only grounding point for the entire network and the LV network consist mainly of cables the following two actions shall be done in order to have secure operation of the low-impedance REF function: 1. Set parameter ROA to 90. However please note that this will still not guaranty the secure operation of the W2 REF function. Therefore the second point given below is as well strongly recommended to be implemented in such installations. 2. Trip tripping logic for such REF function shall be arranged in ACT as shown in Figure 10. Figure 10: Proposed trip logic in ACT for W2 REF function Practically the START signal from the W2 REF function will be allowed to trip if there is no start of the overcurrent protection on the LV side and no other protection function has already given a trip signal. The trip will be also blocked by presence of the second harmonic in the neutral point current. As additional security during switching operations in the LV network this trip shall be also time delayed for two cycles. Note that above configuration is given for RET650, but similar configuration shall be arranged even when RET670 is used. The above two corrective measures shall ensure proper operation of the low-impedance REF function for an installation in such type of distribution network. ABB Power Grids, GA Products 1MRG A en 7/8

8 Rev.ind. Page (P) Chapt. (C) Description Date Dept./Name A Document created ZG ABB Power Grids, GA Products 1MRG A en 8/8