SEL-351A Relay Phase Overcurrent Trip - CacheFly

Introduction
Event Analysis Introduction Session
Wind Farm Circuit: SEL-351A Relay
Phase Overcurrent Trip
Basic collector feeder protection includes an inverse-time overcurrent element (51). This element protects
the feeder from phase faults that occur on the collector feeder and are fed from the utility source.
Date and Time of Trip Event
Date: February 27, 2012 (Monday)
Time: 05:55:36
Targets: 51 (phase time overcurrent)
Event: three-phase fault trip
Currents and Voltages at Time of Trip
IA = 369 A at 194 degrees (34.5 kV, A-phase current)
IB = 372 A at 73 degrees (34.5 kV, B-phase current)
IC = 367 A at 313 degrees (34.5 kV, C-phase current)
I1 = 369 A at 193 degrees (34.5 kV, positive-sequence current)
VA = 19.7 kV at 0 degrees (A-phase voltage)
VB = 19.6 kV at 240 degrees (B-phase voltage)
VC = 19.5 kV at 120 degrees (C-phase voltage)
V1 = 19.6 kV at 0 degrees (positive-sequence voltage)
Settings That Caused Trip
CTR = 160 (current transformer ratio is 800/5 or 160/1)
PTR = 300.00 (potential transformer ratio is 300/1)
Z1ANG = 78.07 (positive-sequence impedance angle set to 78 degrees)
E51P = 2 (two sets of inverse-time overcurrent elements are being used)
E32 = AUTO (directional element set to automatically calculate)
51BP = 1.50 (B-phase overcurrent set to pick up at 1.5 • 160 = 240 A)
51BC = U2 (inverse overcurrent curve)
51BTD = 1.25 (overcurrent time dial)
DIR1 = N (overcurrent made directional forward with 51BTC torque control)
51BTC = F32P (B-phase inverse-time overcurrent set for forward direction)
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50P32P = 2.25 (directional phase-to-phase supervision = 2.25 • 160 = 360 A)
TR = … + 51BT + … (B-phase inverse-time overcurrent set to trip)
OUT101 = TRIP (output 101 used to trip the breaker)
Summary of Trip
There was an increase in feeder current after the wind farm went into noncurtailment. The current was
above the inverse-time overcurrent trip point and was seen to be in the forward-looking direction from the
perspective of the SEL-351A. After about 5 seconds of increased current, the relay tripped the feeder
breaker on the inverse-time overcurrent element set in the SEL-351A.
Cause of Trip
According to the Sequential Events Recorder (SER) report, the directional element F32P picked up in the
forward direction at 5:55:31. The inverse-time overcurrent element picked up and then timed out at
5:55:36, about 5 seconds after the directional element picked up. Once the inverse-time overcurrent
element timed out, the trip output closed to trip the breaker. This event information is shown below.
05:55:31.340 F32P Asserted
05:55:36.342 51BT Asserted
05:55:36.342 OUT101 Asserted
The directional element phase-to-phase current magnitude was at about 369 • 1.73 = 638 A, which is
above the 360 A threshold of this element. The positive-sequence impedance (Z1) measured by the relay
was as follows:
Z1 = V1/I1 = (19600 at 0/369 at 193) = 53 at –193 ohms primary or 28 at 167 ohms secondary
The relay considers anything within ±90 degrees of Z1ANG to be in the forward direction. The relay
considers Z1ANG + 90 = 78.07 + 90 = 168 degrees to Z1ANG – 90 = 78.07 – 90 = –11.93 degrees to be
in the forward direction. The Z1 impedance for this trip was just within this range at 167 degrees, so the
relay declared this event to be in the forward direction. According to a wind farm drawing, the relay is
connected so that the forward direction points into the wind farm and away from the utility.
The B-phase current was 372/240 or 1.55 times the pickup of the B-phase overcurrent element. With this
amount of overcurrent, the relay trips in a little over 5 seconds with the set curve and time dial.
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Graphs and Pictures
The graph below shows the currents and voltages recorded by the SEL-351A.
The plot below shows the magnitudes and angles of the currents and voltages at the time of the trip.
Conclusions
The SEL-351A tripped the breaker from an overcurrent condition caused by the SEL-351A seeing the
increased current from the noncurtailment condition to be in the forward direction and above the
overcurrent set point. Other feeder operations show that this same type of relay trip is occurring quite
frequently at this wind farm. The relay seems to be operating correctly per the relay settings; however,
this type of tripping operation should be evaluated according to the following recommendations.
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Recommendations
The SEL-351A settings should be reevaluated with a full fault, load flow, and relay coordination study on
the collector circuit feeders.
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