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

Test Instrument: Relaystar-7000 Digital-analog Integrated Relay Protection Tester

The diagram below shows 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 has a 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, and the reference direction of each side current is defined as positive when flowing into the generator-transformer unit.

△ Generator-transformer Unit System Diagram

△ Differential Operating Characteristic Diagram

Percentage Differential Protection Operating Equation:

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

 

01 Protection-related Settings

1. Protection Setting Value and Control Word Configuration

Serial Number	Setting Name	Value	Serial Number	Setting Name	Value
01	Rated Capacity of Main Transformer	360MVA	09	Secondary Side of Generator TA	5A
02	Primary Rated Voltage of High-voltage Side	242kV	10	-	-
03	Primary Side of High-voltage Side TA	1250A	11	Generator-transformer Unit Differential Pickup Value	0.4Ie​
04	Secondary Side of High-voltage Side TA	1A	12	Generator-transformer Unit Differential Instantaneous Value	5Ie​
05	Rated Capacity of Generator	330MVA	13	Initial Slope of Percentage Restraint	0.1
06	Power Factor of Generator	0.94	14	Maximum Slope of Percentage Restraint	0.7
07	Primary Rated Voltage	20kV	15	Differential Instantaneous Protection Enable	1
08	Primary Side of Generator TA	15000A	16	Percentage Differential Protection Enable	1

2. Protection Press Plate Configuration

In the "Press Plate Settings", set the "Generator-transformer Unit Differential Protection" soft press plate to "1", set the "Maintenance" hard press plate to "1", and disconnect the "Trip Output" hard press plate.

 

02 Test Wiring

The YD11 current phase compensation of the generator-transformer unit differential protection in the PCS-985 device adopts the method of compensation from the Y side to the Δ side:

△ PCS-985 Phase Conversion Vector Diagram and Formula

For the YD-11 main transformer connection mode, the PCS-985 device adopts the method of phase correction from the main transformer high-voltage side currents A-B, B-C, C-A to the generator neutral point side. During the differential protection test, currents are injected from the high-voltage side and the generator neutral point side respectively.

The corresponding relationship of the injected currents between the high-voltage side and the neutral point side is: A - ac, B - ba, C - cb.

When a current IA​=∠0∘, IB​=0, IC​=0 is injected into the high-voltage side, the converted currents are IA′​=1∠0∘, IB′​=0, IC′​=1∠180∘.The corresponding currents injected into the low-voltage side are Ia​=1∠180∘, Ib​=0, Ic​=1∠0∘.

∘.

△ PCS-985 Differential Wiring Diagram

03 Test Method

 

1. Calculate the Secondary Rated Current of Each Side

2. Inject Balancing Current
   According to the above test wiring and analysis, the two-phase current output of the Relaystar-7000 can be used to inject the generator-transformer unit differential balancing current. Apply a current IA​=1.732×0.687=1.19A∠0∘ to phase A on the high-voltage side, inject a current IB​=3.378A∠180∘ into phase A on the neutral point side, and output a current of 3.378A∠180∘ from phase C on the neutral point side. After applying the currents in accordance with the above settings, check the protection sampling data: the generator-transformer unit differential current is 0Ie​, and the restraint current is 1Ie​.

△ Generator-transformer Unit Balancing Current Parameter Settings

Percentage Differential Boundary TestConduct the test according to the table below. Note that the high-voltage side current needs to be divided by 1.732 when converted to the rated current during the test. The test data are shown in the table below.

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	1.19	1	1.689	0.5
2	1.87	1.57	3.378	1
3	3.4	2.85	6.756	2
4	5.13	4.31	10.134	3
5	7.12	5.98	13.512	4
6	9.41	7.98	16.89	5

04 Test Conclusion

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