Protection Device: PCS-985 Generator-transformer Unit Protection Device

Test Equipment: Relaystar-7000S Digital-analog Integrated Relay Protection Tester

For the generator-transformer unit system of a power plant, the main transformer is a three-winding transformer, and the low-voltage side of the station transformer is of double-branch structure. The generator-transformer unit differential protection includes four branch currents, namely the 220kV side current I1​, 110kV side current I2​, high-voltage station transformer high-voltage side current I3​ and generator neutral point side current I4​ of the main transformer. The polarity terminals of TA on each side are shown in the figure below. The reference direction of each side current is defined as positive when flowing into the generator-transformer unit.

△ Generator-transformer Unit System Diagram

△ Generator Partial Simplified Diagram

△ Percentage Differential Operating Characteristic

 

Percentage Differential Protection Operating Equation:

This paper briefly introduces how to use the Relaystar-7000S Digital-analog Integrated Relay Protection Tester to commission the PCS-985 generator longitudinal differential protection.

△ Relaystar-7000S Digital-analog Integrated Relay Protection Tester

1 Protection-related Settings

1. Generator Protection Setting Value and Control Word Configuration
2. Protection Press Plate Configuration
   In the "Press Plate Settings", set the "Generator Differential Protection" soft press plate to "1", set the "Maintenance" hard press plate to "1", and disconnect the "Trip Output" hard press plate.

Serial Number	Setting Name	Value	Serial Number	Setting Name	Value
01	Generator Rated Capacity	330MVA	11	Differential Instantaneous Protection Enable	1
02	Generator Power Factor	0.94	14	Maximum Slope of Percentage Restraint	0.5
03	Primary Rated Voltage	20kV	16	Percentage Differential Protection Enable	1
04	Primary Side of Generator TA	15000A	-	-	-
05	Secondary Side of Generator TA	5A	-	-	-
07	Percentage Differential Pickup Value	0.2Ie​	-	-	-
08	Differential Instantaneous Pickup Value	2Ie​	-	-	-
09	Initial Slope of Percentage Restraint	0.1	-	-	-

2 Test Wiring

For the generator protection wiring mode, currents are injected into the generator terminal and the generator neutral point respectively during the test. The currents applied to the generator terminal and the neutral point are equal in magnitude and identical in direction.

△ PCS-985 Generator Differential Wiring Diagram

3 Test Method

1. Calculate Generator Rated Current

2. Inject Balancing Current
   According to the above test wiring and analysis, the two-phase current output of the Relaystar-7000S can be used to inject the generator-transformer unit differential balancing current. Apply a current IA​=3.378A∠0∘ to phase A at the generator terminal, and inject a current IB​=3.378A∠0∘ into phase A at the neutral point side. After applying the currents, check the protection sampling data: the generator-transformer unit differential current is 0Ie​, and the restraint current is 1Ie​.

△ Generator Balancing Current Parameter Settings

3. Percentage Differential Boundary Test
   Conduct the test according to the table below. By fixing the current on one side and changing the current on the other side, starting from the balanced state until the protection operates, record each operating point.

Serial Number	Current on Side 1 (A)	Current on Side 2 (A)	Restraint Current Ir​ (A)	Differential Current Id​ (A)
A	Ie​	Ie​	Ie​	Ie​
1	0.337	0.1	1.007	0.32
2	0.674	0.2	1.464	0.44
3	1.685	0.5	2.635	0.78
4	3.337	1	4.647	1.38
5	6.74	2	9.15	2.71
6	10.11	3	14.19	4.21
7	13.48	4	19.78	5.87

4 Test Conclusion

Based on the restraint current Ir​ and differential current Id​ from the above test data, the generator longitudinal differential percentage boundary curve can be plotted.

△ Generator Longitudinal Differential Percentage Boundary Curve