Wind farms usually have no load or minimal load before commissioning, making it challenging to conduct vector and polarity checks on main transformer protection. The HTA-3300 Three-phase Digital Intelligent High-current Source can simulate primary-side load operation before power transmission, achieve synchronous current injection on both high and low voltage sides. The correctness of the vector and polarity of the main transformer protection can be directly judged by the differential current value, which greatly improves the efficiency of power transmission.

1. Test Mode

The transformer of a wind farm adopts the Y/Δ-11 connection mode. The high-voltage side has a single busbar with one 220kV outgoing line, one high-voltage side bay and one busbar PT bay; the low-voltage side has a single busbar with an independent grounding transformer.

For the high-voltage side: One HTA-3300 Three-phase Digital Intelligent High-current Source injects current from the earthing switch near the busbar side, which flows through the CT and is grounded at the line side.

For the low-voltage side: According to the on-site conditions, current is injected from an outgoing line bay, flows through the low-voltage side bay, and is short-circuited at the grounding transformer.

The primary current on both sides flows into the main transformer from the busbar. The YD-300E Intelligent Current and Voltage Source Wireless Remote Control Host adopts the internal synchronization mode to control the synchronous output of the two current sources on the high and low voltage sides, completing the vector verification project of differential protection. Sampling data is checked on the protection device to verify the vector correctness of the main transformer protection.

2. Test Wiring

Since the high-voltage side uses GIS fully enclosed equipment, it is necessary to remove the short-circuit connection at the side of the main transformer close to the busbar before test wiring, so as to apply three-phase current. The current flows through the high-voltage side switch and the CT to the ground, forming a current loop.

△ Current Injection Point Wiring Diagram

The low-voltage side adopts a switchgear structure. After unlocking the cabinet door, current is injected from the CT P2 side of an outgoing line bay, flows through the busbar, the low-voltage side switch and the CT, and is grounded via the earthing switch of the grounding transformer to form a current loop.

3. Test Process

After completing the primary circuit wiring, the on-site personnel first performed single-machine current output tests at 10A, 20A, and 30A respectively to check whether the primary wiring and phase sequence are correct. After confirming no errors, synchronous output was enabled, and the two current sources were controlled for synchronous output through YD-300E wireless connection.

According to the on-site conditions, the primary current applied on the high-voltage side was:

Phase A: 60A∠0°

Phase B: 60A∠240°

Phase C: 60A∠120°

The primary current applied on the low-voltage side was:

Phase A: 60A∠210°

Phase B: 60A∠90°

Phase C: 60A∠330°

This indicates that the high-voltage side current flows into the main transformer, while the low-voltage side current flows out of the main transformer.

4. Test Results

After the current was boosted, the sampling current and differential current of each side could be checked on the main transformer protection device.

△ High-voltage Side Primary Current               △ Low-voltage Side Primary Current

△ Differential Current Value

The on-site test data is sorted out as follows:

Protection Type	Transformation Ratio	Phase A (A/V)	Phase B (A/V)	Phase C (A/V)
Main Transformer Protection Set A (CSC-326) - High-voltage Side	1200:1	0.05∠0°	0.05∠-119.413°	0.049∠120.722°
Main Transformer Protection Set A (CSC-326) - Low-voltage Side	3000:1	0.02∠-145.555°	0.02∠93.913°	0.02∠-25.555°
Main Transformer Protection Set A (CSC-326) - Differential Current	/	0.42	0.42	0.42
Main Transformer Protection Set B (PCS-978) - High-voltage Side	1200:1	0.118Ie	0.119Ie	0.119Ie
Main Transformer Protection Set B (PCS-978) - Low-voltage Side	3000:1	0.019Ie	0.019Ie	0.019Ie
Main Transformer Protection Set B (PCS-978) - Differential Current	/	0.1Ie	0.1Ie	0.1Ie

It is known that the rated secondary current of the high-voltage side on site is 0.417A, and that of the low-voltage side is 1.039A. The balance coefficient of the high-voltage side is 1, and the balance coefficient of the low-voltage side is 0.417/1.039 = 0.4. The calculated differential current is 0.05 - 0.02×0.4 = 0.42, which is consistent with the differential current displayed by the protection device.

△ Differential Current Value

If the primary current applied on the low-voltage side is changed to:

Phase A: 60A∠30°

Phase B: 60A∠270°

Phase C: 60A∠150°

This indicates that both the high-voltage side and low-voltage side currents flow into the main transformer. The calculated differential current is 0.05 + 0.02×0.4 = 0.58, which is consistent with the differential current displayed by the protection device. Therefore, the polarity and vector of the differential protection are correct.