Percent Differential (87) Element Testing
STEADY-STATE PICKUP TEST
A) Understanding the Test-Set Connections
Wye-Wye and Delta-Delta connections are not a problem as there is no phase shift and the current in any phase-winding is reflected in the same phases on the other windings.
However in a Delta-Wye or Wye-Delta transformer, the current in one Wye phase CT is reflected into two phases of the Delta CT as shown in Figure 95.
PHA H1
H2 H3 PHB PHC
X1 X2 X3
H0
400:5CT1 CT2
1200:5
IA@0ºIB@-120ºIC@120º Ic@120ºIb@-120ºIa@0º IH1=0º
IH2@-120º
IH3@120º
IX1=Ia-Ic@-30º
IX3@90º IX2@-150º
W1 normally measures. For the example in Figure 95, connect the W1 test current to IAW1 and ICW1 in series and the W2 test current is applied to IAW2 for an A-Phase test.
Connect the B-Phase W1 test current to IBW1 and IAW1 and W2 test current to IBW2.
Connect the C-Phase W1 test current to ICW1 and ICW1 and W2 test current to ICW2.
Another problem with 1-Phase testing is the unbalance or zero sequence current that is applied to the relay. Zero-sequence is a problem in transformer differential protection because a phase-to-ground fault outside the transformer on the power system can cause zero-sequence current to flow through one winding of the transformer and not appear in the other winding due to a delta or tertiary winding. Most digital relays apply zero-sequence filtering to prevent a transformer differential trip for ground faults on the power system. Zero-sequence filtering is applied on the SEL-387 relay when the W_CTC setting equals 12. A phase-to-phase connection is required to compensate for the zero-sequence compensation. When both W_CTC settings are equal, both windings will use a Phase-Phase connection.
Use the table in Figure 96 to determine the correct 1-Phase test connections.
GE T-60, T-30 SR-745 Beckwith SEL-587
W1 W2 W1 W2 W1 W2 W1 W2
W1=0, W2=0 Y/y0° Yyyy YY 12 12 A-B A-B B-C B-C C-A C-A
W1=0, W2=30° Y/d30° YDACyy YDAC 12 1 A-C A-N B-A B-N C-B C-N
W1=0, W2=330° Y/d330° YDAByy DACY 12 11 A-B A-N B-C B-N C-A C-N
12 0 A-B A-B B-C B-C C-A C-A
D/d0° D/d0° DACDACyy DACDAC 0 0 A-N A-N B-N B-N C-N C-N
0 12 A-B A-B B-C B-C C-A C-A
W1=0, W2=30° D/y30° DABYyy DABY 0 1 A-C A-N B-A B-N C-B C-N
W1=0, W2=330° D/y330° DACyy DACY 0 11 A-B A-N B-C B-N C-A C-N
1 0 A-N A-C B-N B-A C-N C-B
1 12 A-N A-C B-N B-A C-N C-B
1 1 A-N A-N B-N B-N C-N C-N
11 0 A-N A-B B-N B-C C-N C-A
11 12 A-N A-B B-N B-C C-N C-A
11 11 A-N A-N B-N B-N C-N C-N
B Phase C-Phase
SEL-387 A Phase
Figure 96: Transformer Relay Connections for Single-Phase Differential Testing
B) Test-Set Connections
The connection diagram for the SEL-387 is as follows:
Figure 97: Schweitzer Electric SEL-387 Transformer Protective Relay Connections
The winding settings for this transformer are W1CTC=12 and W2CTC=1. Using the table in Figure 96, the following test-set connections will be required for each phase:
SEL-387 RELAY RELAY TEST SET
Magnitude Phase Angle W1AØ Test Amps 0°
Frequency Test Hz
W2AØ Test Amps 180° Test Hz
0.00A 0° Test Hz
C1 Amps
C2 Amps
C3 Amps WINDING 1
WINDING 2
+ + +
+ + +
+ +
+
Alternate Timer Connection IAW1 Z01
Z02 IBW1 Z03 Z04 ICW1 Z05 Z06
IAW2 Z07 Z08 IBW2 Z09 Z10 ICW2 Z11 Z12
SEL-387 RELAY RELAY TEST SET
Magnitude Phase Angle W1BØ Test Amps 0°
Element
Output Timer
Input
Frequency Test Hz
W2BØ Test Amps 180° Test Hz
0.00A 0° Test Hz Alternate Timer Connection IAW1 Z01
Figure 99: 1-Phase Restrained-Differential Slope Test-Set BØ Yd1 Connections
SEL-387 RELAY RELAY TEST SET
Magnitude Phase Angle W1CØ Test Amps 0°
Element
Output Timer
Input
Frequency Test Hz
W2CØ Test Amps 180° Test Hz
0.00A 0° Test Hz Alternate Timer Connection IAW1 Z01
Figure 100: 1-Phase Restrained-Differential Slope Test-Set CØ Yd1 Connections
C) 1-Phase Differential Slope Test Procedure
Follow these steps to test the Differential Slope settings using the Post-Test Calculation method from the previous “3-Phase Differential Slope Testing” Section:
1. Determine how you will monitor pickup and set the relay accordingly, if required.
(Pickup indication by LED, output contact, front panel display, etc…See previous packages of The Relay Testing Handbook for details.)
2. Apply a restraint current into the first test phase of Winding-2. The pickup indication should be on.
3. Apply an equal, but opposite current in W1 using the Tap ratios to determine what the current magnitude should be. For example:
TAP1 A1 W1 W2
TAP2 A2 2.41 1 W1 5 4.61 1.732
W1 5 0.302 W1 1.510
= × ×
×
= × ×
×
= ×
=
4. The pickup indication should now be off. Raise the Winding-1 current until the relay operates. Record the value on your test sheet. (Remember that you can also use the pulse or jog method to minimize the amount of current applied to the relay. Review the Instantaneous Overcurrent (50-Element) pickup procedure in previous packages of The Relay Testing Handbook for details.)
5. Calculate the IRT, IOP, and Slope using the formulas in the Post-Test Calculation section of the “3-Phase Differential Slope Testing” in this document. Use a 1.732 correction factor for windings with odd W_CTC settings and use the IRT calculation to determine if the test was O87P, Slope-1, or Slope-2. Raise the W2 current and repeat 2-5 until all elements of the differential protection have been tested.
¾ IRT= IAW2 / TAP2 A2
( (
×) )
+(
IAW1 / TAP1 A1(
×) )
/ 2¾ IOP= IAW1/ TAP1 A1 - IAW2/ TAP2 A2
(
×)
(
×)
6. Compare the pickup test result to the manufacturer’s specifications and calculate the percent error as shown in the following example.
1 1.732
1.38
15 15.0 A 5.535 2.088 0.418 20.00 20.02 0.12
0.904 0.304 0.30 0.30
14 6.0 A 2.544
1.08
9 15.0 A 5.544 2.090 0.422 20.00 20.19 0.93
0.778 0.303 0.30 0.30
8 5.0 A 2.24
0.94
3 15.0 A 5.54 2.089 0.420 20.00 20.11 0.57
0.778 0.303 0.30 0.30
2 5.0 A 2.239
0.28
18 27.0 A 11.035 3.980 1.197 60.00 60.23 0.38
3.441 0.871 60.00 60.17
17 24.0 A 9.342
1.24
16 21.0 A 7.753 2.924 0.587 20.00 20.08 0.38
0.152 0.304 0.30 0.30
13 0.0 A 0.732
IOP EXPECTED ACTUAL ERROR (%)
TEST IAW2 (A) IBW2 (A) IRT
0.26
12 30.0 A 12.711 4.516 1.517 60.00 60.17 0.28
3.620 0.978 60.00 60.16
11 25.0 A 9.902
1.24
10 20.0 A 7.382 2.784 0.558 20.00 20.05 0.26
0.152 0.304 0.30 0.30
7 0.0 A 0.732
IOP EXPECTED ACTUAL ERROR (%)
TEST IAW2 (A) IBW2 (A) IRT
0.54
6 30.0 A 12.726 4.519 1.523 60.00 60.26 0.44
3.624 0.986 60.00 60.33
5 25.0 A 9.923
1.80
4 20.0 A 7.394 2.786 0.563 20.00 20.21 1.06
0.153 0.305 0.30 0.31
1 0.0 A 0.736
PICKUP TESTS
TEST IAW2 (A) IAW1 (A) IRT IOP EXPECTED ACTUAL ERROR (%)
W1CTC
SLOPE 2 60% TAP2 4.61 W2CTC
12 1
SLOPE 1 20% TAP1 2.41
DIFFERENTIAL TEST RESULTS
PICK UP 0.3 TIME DELAY 0 CORR
7. Repeat steps 2-6 for all three phases.
8. Repeat the pickup test for all windings that are part of the differential scheme. If more than two windings are used, change all connections and references to W2 to the next winding under test. (W2 becomes W3 for W1 to W3 tests)
8. Harmonic Restraint Testing
Transformers are inductive machines that require a magnetic field to operate. Under normal operating conditions, there are very small losses inside the transformer that are typically less than 3%. A large inrush of current is required in the first few cycles after energization to create the magnetic field necessary for transformer operation. This current only occurs in the first-energized winding and can be up to 10x the transformer’s nominal current that, in any other circumstance, would be the definition of transformer differential and the differential relay should operate.
We do not want the transformer differential to operate every time a transformer is energized, and there are several options available to prevent transformer inrush from tripping the differential relay. It is possible to block the 87-Element for a preset time after a circuit breaker is closed, but an internal transformer fault would not be isolated until the time delay had passed that will cause additional damage for the extended time caused by this workaround.
You could increase the pickup setting, but this would prevent the relay from operating during high-impedance faults.
The most common technique used to prevent differential operation during inrush conditions is called Harmonic Restraint or Harmonic Blocking. Figure 101 displays a typical transformer inrush waveform. As you can see, this is not your typical sine wave. There is a significant DC offset in the first few cycles where the center point between the positive and negative peaks is not the x-axis. Also the waveform is extremely distorted and doesn’t display the nice round peaks we normally see.
Protective relay designers analyzed these waveforms and realized that the distortion was caused by even harmonic content that typically only occurred during transformer energization and not during transformer faults. Harmonic detectors were built into transformer differential relays which measure the percent of 2nd,4th and/or 5th harmonics in a waveform. These harmonic detectors will prevent differential operation if the harmonics exceed a pre-defined setpoint.