Simple Percent Differential (87) Element Testing
A) Test-Set Connections
The test-set connections for a simple slope test are as follows. Some tests require large amounts of current which will require the connection in Figure 12-28.
A Phase Amps
W2 Test Amps 180° Test Hz
W1 RELAY INPUT
Figure 12-27: Simple 87-Element Slope Test-Set Connections
A Phase Amps
W2 Test Amps 180° Test Hz
Figure 12-28: Simple 87-Element High Current Slope Test-Set Connections
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www.valenceonline.com B) Slope Test Procedure
The test procedure seems straightforward. Apply equal current into Winding-1 and Winding-2 with a 180º phase shift between windings, and raise one current until the relay operates. The difficult part of this procedure is determining what starting current to apply and what the expected pickup should be. We can start by re-plotting the characteristic curve shown in Figure 12-29 using the following relay settings.
• Phase Differential Trip = Unlatched
• Assign Trip Relays = 1
• Differential Trip Minimum Pickup = 0.1 x CT
• Differential Trip Slope-1 = 20%
• Differential Trip Slope-2 = 80%
• Differential Trip Delay = 0 cycles
Differential Protection 20/80% Characteristic Curve
0.00 0.50 1.00 1.50 2.00 2.50 3.00
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00
Restrain Current (Ir) x TAP Operate Current (Iop)xTAP
Trip Area
Restraint Area Slope 1
Slope 2
Figure 12-29: Percentage Differential Protection Dual Slope Characteristic Curve
We first need to change the restraint and operate current values to amps instead of multiples of Tap. Then we can calculate the transition point between the Minimum Pickup and Slope-1 operation. The Minimum Pickup is 0.50A (0.1 CT× ) as we calculated earlier in this chapter. Slope-1 will be enabled when the operate current is greater than the minimum pickup. Different relays have different slope calculations and we can refer to the GE 489 relay instruction manual to determine the relay’s differential calculation as shown in Figure 12-30.
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FUNCTION:
The 489 percentage differential element has dual slope characteristic. This allows for very sensitive settings when fault current is low and less sensitive settings when fault current is high, more than 2 x CT, and CT performance may produce erroneous operate signals.
The minimum pickup value sets an absolute minimum pickup in terms of operate current. The delay can be fine tuned to an application such that it still responds very fast, but rides through normal operational disturbances.
The Differential element for phase A will operate when:
Operate Restraint
Differential elements for phase B and phase C operate in the same manner.
Figure 12-30: GE 489 Differential Formulas Notice that the Ir current (IRestraint) is defined as
2
I + I for our terminology. The bars on either side of each current indicate that we use
absolute values when calculating the restraint current. We can drop the bars and re-define Ir as 1 2
W 2 W
I +I . The Iop current (IOperate) is defined as IA −Ia which can be translated into
1 2
W W
I −I for our calculations. The bars above each current indicate a vector sum calculation and we can redefine the Iop calculation as IW1−IW2 because our vectors will always be 180º apart.
Use the first IOperate formula to calculate the transition between Minimum Pickup and Slope-1 where Slope-1 begins when Iop exceeds the Minimum Pickup setting (0.5A) defined previously. Notice that our generic formula on the right also works for this relay. Based on these calculations, our restraint current must be greater than 2.505A to test Slope-1.
0.501 20%
100 0.501 50.1 2.505
20% 20
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Figure 12-31 displays the revised characteristic curve so far.
Differential Protection 20% Characteristic Curve
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Restraint Current (Ir in Amps) Operate Current (Iop in Amps)
Trip Area
Restraint Area Slope
1
Min
Transition 1
Figure 12-31: Example Characteristic Curve in Amps with 1st Transition Defined
We also need to determine the maximum restraint current before we accidentally start testing Slope-2 when testing Slope-1. We can use the slope formula (Slope(%) 100 Iop
= × Ir ) to calculate the restraint current (Ir) at the transition point between Slope-1 and Slope-2 but we first need to define the transition point. This is usually the Breakpoint setting, but this setting is not available in the GE 489 relay. A quick review of the manufacturer’s operate description (Figure 12-32) of the manual informs us that the breakpoint is pre-defined as 2 times CT.
where:
IOperate
IRestraint
k IA
Ia
= operate current = restraint current
= characteristic slope of differential element in percent (Slope 1 if IR < 2 X CT, Slope 2 if IR > = 2 X CT) = phase current measured at the output CT = phase current measured at the neutral end CT Differential elements for phase B and phase C operate in the same manner.
Figure 12-32: Percentage Differential Protection Dual Slope Characteristic Curve
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You can use the same Minimum Pickup formulas to calculate the transition from Slope-1 to Slope-2. This time we’re going to expand the formula to determine the actual W1 and W2 currents instead of operate and restraint currents.
1 2
The maximum amount of Ir
W W
is defined as Ir<10A
2 1.222
9.99 2
19.98 2.222 8.99
Slope 1 expected test currents can be defined by:
1.222
The maximum amount of Ir is defined as Ir<10A
W W W
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The new characteristic curve will look like Figure 12-33.
Differential Protection 20/80% Characteristic Curve
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Restraint Current (Ir in Amps)
Operate Current (Iop in Amps)
Trip Area
Restraint Area Slope 1
Min Transition 1
Slope 2 Transition 2
Figure 12-33: Percentage Differential Protection Dual Slope Characteristic Curve
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