Delayed Current Zero Crossing Phenomena During Switching of Shunt-Compensated Lines

Transcription

1 Delayed Current Zero Crossing Phenomena During Switching of Shunt-Compensated Lines David K Olson Paul Nyombi Xcel Energy Pratap G Mysore Pratap Consulting Services Minnesota Power Systems Conference St. Paul, MN November 7-9, 2017

2 Acknowledgement American Transmission Company (ATC) study group for bringing up DCZ issues during the line design studies on one of the CAPX lines. This prompted three utilities associated with CAPX to determine the impact of shunt reactors on their lines and to determine mitigation methods. 2

3 Transmission Line Representation Long Transmission lines can be represented as several series connected modules made up of series resistance, Series Inductance and Shunt capacitance. Lines are also represented with Lumped parameters where the line capacitances are split between two ends. 3

4 Line Parameters Typical 345 KV System Line Resistance: Ω/Mile Line Reactance: Ω/Mile Line Inductance: mh/mile Line Charging MVAR: MVAR/Mile Line Capacitance: µf/mile Resistance can be ignored for our discussions. 4

5 Transmission Line Behavior During Light load or no load conditions, Transmission line has the same effect as capacitance connected to the system. System voltage increases with the connection of open ended transmission Lines. Capacitance of the line is distributed over the entire length. Remote end voltage increases with increase in length Also Known as Ferranti Effect 5

6 Shunt Reactor Application To Keep System Voltage below allowable maximum value System voltage needs to be within the allowable range to prevent connected Equipment failures. - IEEE (R2004) IEEE Standard Preferred Voltage Ratings for Alternating-Current Electrical Systems and Equipment Operating at Voltages Above 230 kv Nominal Installed on Tertiaries of transmission Transformers On Transmission Lines Either at both Ends or at only one end. Middle of the line 6

7 Shunt Compensated Lines Transmission Lines with shunt reactors connected. Total shunt reactor MVAR_R = Sum of all shunt reactors connected on the line. Degree of Compensation, M = Where, MVAR_C is the Line Charging MVAR Degree of Compensation is determined by planning studies- Can exceed 100% 7

8 Effect of Shunt Reactors Decreases the voltage by compensating for the capacitive charging currents. Reactor current, I L lags voltage by 90 0 Capacitor (line charging) current, I C leads voltage by 90 0 Decreases the current through the breaker current ( I C - I L ) * (f ile Paper1.pl4; x-v ar t) v :I_L1 c:i_c2 - c:i_l1 -I_L2 c:xx0002-i_l1 factors: E offsets: Steady State current and Voltage phase relationship

9 Line Currents During Energization Voltage waveform: V Sin (ωt +φ) where φ is the delay angle of switching on the voltage waveform from voltage zero. 300 * (f ile Paper1.pl4; x-v ar t) v :XX0006 Charging (Capacitive) Current: I Sin(ωt+φ+90) Reactor Current: MI [Sin (ωt+φ-90)+ Cosφ e -t/τ ] 9

10 Line Breaker Current I C = I Cos(ωt+φ) I L = MI[-Cos(ωt+φ)+ Cosφ e -t/τ ] Breaker Current, I BRφ = I L +I C I BRφ = I L +I C =(1-M)I Cos(ωt+φ) + MI Cosφ e -t/τ Maximum DC offset is seen when the line with reactor is switched at Voltage zero Crossing. I BR0 = (1-M)I Cos(ωt) + MI e -t/τ 10

11 Line Breaker Current Components AC sinusoidal wave has a peak value of (1-M)I DC component is exponentially decaying with time constant of τ sec. X/R of oil filled reactors are in the range of ; τ can be as high as 2 seconds (~=750/377). DC component will decay to less than 2% of the initial value after 4τ time. 11

12 Line Breaker Delayed Current Zero If AC component Peak (1-M)I is greater than DC component MI, AC waveform will always cross current zero axis or else, Current zero occurs after a delay Delayed Current Zero (DCZ) DC Component 2 Breaker Current Delayed Current Zero

13 Criterion to Prevent DCZ during Normal Switching Boundary Condition: (1-M)I = MI; M = 0.5; Under normal switching, the degree of compensation, M cannot exceed 50% to prevent Delayed Current Zero on Breaker Current. 13

14 Time to first Current Zero If the Degree of compensation is greater than 50%, the AC waveform will cross current zero line when the peak equals or exceeds the decayed DC component (1-M) = M * e -t/τ Solving for t, t Seconds τln Degree of Compensation VS Time to first Current Zero (τ =2 sec) 1.2 ; Degree of Compensation Time for first current zero

15 Effect of Source Impedance The time constant of DC Component is the overall time constant of the system: (X total /R Total) ) If the source is another transmission line behind the line, X/R is typically around 17. Overall X/R reduces. DC component decays faster. Z Source = j47.5 Ohms for 2500 MVA, 345 kv source (X/R=17) Z Reactor = j (X/R =750) Z total = j2428 (X/R=428.6) 15

16 Energizing Faulted Line Fault Location: at the shunt Reactor Voltage Shift on un-faulted phase is the maximum at the fault location and is maximum for double line to Ground Fault Voltage of B-Phase (PU) for A-G fault V B = e -j240 - ; K= 16 B-C-G fault, V A (PU) = For K=2.8, A-Phase voltage increases to PU for B-C-G fault B-phase voltage increases to 1.23

17 M Limit - Effect on DCZ due to Faults Voltage rise on un-faulted phase of the shunt reactor results in higher DC component due to Voltage zero switching and also due to voltage rise. Capacitive current doesn t increase in the same proportion due to distributed Capacitance of the line. Is affected by inter-phase capacitance Transient studies are needed to determine the reduction in M from 50% to prevent DCZ 17

18 18 System Simulation

19 Energization of Shunt Compensated Line Simulated case 1a: Radial line energization from South with A- and B- phases poles closing at voltage zero a. With both shunt reactors connected (approx. 86% compensation) b. With both shunt reactors connected and utilization of 425ohm pre-insertion resistors inserted for 13ms (f ile North_South_345kV.pl4; x-v ar t) c:sth_ba-sth_la c:sth_bb-sth_lb c:sth_bc-sth_lc (f ile North_South_345kV.pl4; x-v ar t) c:sth_ba-sth_la c:sth_bb-sth_lb c:sth_bc-sth_lc

20 Energization of Shunt Compensated Line Simulated case 1b: Radial line energization from South with A- and B- poles closing at voltage zero a. With only North line-end shunt reactor connected (approx. 43% compensation) b. Energizing the transmission line into a close-in BC-Ground fault with only one reactor connected. A-Phase current is shown below (f ile North_South_345kV.pl4; x-v ar t) c:sth_ba-sth_la c:sth_bb-sth_lb c:sth_bc-sth_lc (f ile North_South_345kV.pl4; x-v ar t) c:sth_ba-sth_la

21 Energization of Shunt Compensated Line Observations: Energization of the line with only one reactor connected eliminates DCZ 43% shunt compensation DCZ may also be minimized by using pre-insertion resistors and/or increasing pre-insertion time Several faults may need to be studied as their impact on DCZ depends on other factors like system parameters and line configuration. 43% compensation was found to be adequate in this case for minimizing DCZ for all switching scenarios. 21

22 Switching of Shunt Reactor on an energized line a. Switching South line-end shunt reactor during limited/no loading on the line. South line end breaker currents are shown below: b. Switching of the shunt reactor with at least 46MW flowing from South to North substation. DCZ on South line-end currents is eliminated (f ile North_South_345kV.pl4; x-v ar t) c:sth_ba-sth_la c:sth_bb-sth_lb c:sth_bc-sth_lc (f ile North_South_345kV.pl4; x-v ar t) c:sth_ba-sth_la c:sth_bb-sth_lb c:sth_bc-sth_lc 22

23 Switching of Shunt Reactor on an energized line Observations: Switching of shunt reactor at voltage zero crossings creates DC offsets at line-end breakers, with the strongest source experiencing the greatest offset. Line breakers experience DCZ if the offset is greater than the capacitance from the reactor up to the breaker location. A minimum of 46MW, power flow from South to North, is found to be adequate in mitigating the delayed current zeros when energizing the second shunt reactor 23

24 Switching of line/shunt reactor for transmission line fault events Simulated case 3a: South line-end A-Phase reactor current following a close-in A-G fault on transmission line Under this study: A-Phase to ground fault is considered to occur at A-Phase voltage zero crossing Both reactors are connected with the line closed through Shunt reactor current takes several cycles to decay to zero; X/R ~ 1.68s (f ile North_South_345kV.pl4; x-v ar t) c:sth_la-sth_ra 24

25 Transmission line de-energization under no fault conditions Simulated case 3b: South line-end B-Phase line voltage following line de-energization with both reactors connected Trapped energy in the shunt capacitance and shunt reactors creates high frequency transients that may take several seconds to decay A- and C- Phases also do experience voltage transients. Only B-Phase is shown for clarity. 400 * (f ile North_South_345kV.pl4; x-v ar t) v :STH_LB 25

26 Switching of line/shunt reactor for transmission line fault events Observations: Shunt reactors should not be tripped for faults on transmission line. Disabling high-speed reclosing is recommended especially for lines that have more than 50% compensation. Adequate time may be required before safely reducing the shunt compensation for re-energization. 26

27 Delayed Current Zero Mitigation Methods 1. Use of pre-insertion resistors on line breakers Limitations: Breaker manufacturers do typically guarantee only 8-12ms of pre-insertion time. Pre-insertion time of at least 13ms was required in our case study during normal energization. The time duration was not enough to eliminate DCZ during faulted line energization. Depending on the breaker closing mechanism, increasing pre-insertion resistor size created new transients when the main contact by-passes the pre-insertion resistor 27

28 Delayed Current Zero Mitigation Methods 2. Limit degree of line compensation during line energization Limitations: Reduced line compensation increases remote end line voltages. Remote end line connected equipment must be rated adequately Surge Arrestors, CCVTs, etc 28

29 Delayed Current Zero Mitigation Methods 3. Utilization of controlled closing on shunt reactor breakers Limitations: Implementation of this may be challenging on existing shunt reactor breakers as this may require breaker upgrade 29

30 Delayed Current Zero Mitigation Methods 4. Consider moving reactors (or some of them) from transmission line to substation buses Limitations: Implementation of this may be challenging for existing substations 30

31 Tripping and reclosing considerations for shunt compensated lines Radial energization of a transmission line with less 50% shunt compensation is recommended. Disabling high-speed reclosing is recommended on lines with more than 50% shunt compensation. 31

32 Tripping and reclosing considerations for shunt compensated lines Delaying line breaker tripping for transmission line faults is suggested allows for appearance of current zero crossing for faults (during light load periods) on lines with more than 50% compensation. South line-end currents are shown below for close-in A-G fault (f ile North_South_345kV.pl4; x-v ar t) c:sth_bb-sth_lb c:sth_bc-sth_lc 32

33 Conclusions a. On line energization to prevent DCZ: If shunt reactor(s) on the line is not required to limit open end voltage to acceptable level, energize the line without shunt Reactors. Keep Shunt Compensation below 50% during line energization. Energize the line through breakers equipped with preinsertion resistors that provide enough damping to produce current zeros within breaker interrupting time. This is dependent on the resistor value and duration of insertion. It may not work under all the cases if the degree of compensation is close to 100%. 33

34 Conclusions b. On Shunt reactor energization onto energized lines, to prevent DCZ: Switch shunt reactors less than 50% of the total charging current. Switch shunt reactor at voltage maximum point on the wave. Switch shunt reactors if their inductive MVAR is not greater than the minimum load on the line, in MW. 34

35 Conclusions c. Tripping for line faults: Trip only the line breakers and not shunt reactors. Shunt reactors need to be tripped only after several seconds delay to allow full decay of reactor current on the faulted phase. Total interrupting time of faults on lines with shunt compensation greater than 50% may need to be at least few cycles to allow presence of current zeros on healthy phase(s) during faults. This is dependent on the zero sequence currents flowing on the healthy phases or the minimum load on the healthy phases. 35

36 Conclusions d. Tripping for shunt reactor faults: Trip the shunt reactors only if they are equipped with breakers. 36

37 Conclusions e. Reclosing on Lines: Instantaneous reclose is disabled on lines where shunt reactor switching is required to reduce the degree of compensation. Time delay reclose is enabled after the reactor is switched out. Synch-check reclosing at the other end after energizing the line may generate offsets on the currents. DC offset is dependent on the load picked up after restoration. 37

38 38 Questions

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