The current transformer (CT) of the 220kV II-IV bus section switch (No.224) in a substation was replaced. An incorrect wiring connection of the CT to the bus differential protection, such as reversed polarity, may cause maloperation of the bus differential protection. To reduce the operational risks of the power grid and switching operation risks, the primary current injection method was adopted to complete the testing and verification of the secondary current circuit of the CT for the 220kV II-IV bus section switch (No.224). The load simulation method was used to separately check the current circuits of the No.224 switch bay for the No.1 and No.2 bus protection devices of the 220kV I-II and III-IV buses. The check verified the correctness of the phase, amplitude of the current and voltage input into the protection device, as well as the relative polarity relationship of the primary wiring of the current transformer, ensuring the secondary current circuit is correct and complete.

Test Equipment: HTA-3300 Three-phase Digital Intelligent High-current Source, YD-300E Intelligent Current and Voltage Source Wireless Remote Control Host

1. Pre-test Preparation

No personnel shall conduct line testing work at the test site.

The secondary side of the voltage transformer (PT) shall not be short-circuited, and the secondary side of the current transformer (CT) shall not be open-circuited.

The access point of the test equipment shall be supplied with power.

During the test, the No.1 and No.2 bus protection devices of the 220kV I-II and III-IV buses shall be shut down in turns. Inadequate safety measures on the secondary side of non-current-injection CT windings may trigger the operation of the in-service bus protection devices.

Confirm that all trip output and failure start link plates in the tested bus differential protection panel have been removed.

2. Test Wiring

The primary test wires of the HTA-3300 Three-phase Digital Intelligent High-current Source shall be reliably connected to the polarity terminal P1 on the primary side of the CT for the section switch (No.224). The primary injection current flows through the CT and exits from P2, forming a reliable circuit of "P1—CT—P2" (see the figure below).

△ HTA-3300 Three-phase Digital Intelligent High-current Source Wiring Diagram

△ Three-phase Current Source Application Position for No.224 Section Switch Bay

Voltage Reference WiringTake the AN-phase voltage of the tested bus differential protection device as the reference, and connect it to the voltage terminal of the YD-300E Intelligent Current and Voltage Source Wireless Remote Control Host (see the figure below).

3. Parameter Setting

Taking the No.2 panel of the 220kV I-II bus protection as an example:

Select a branch with relatively small load current in the No.2 panel as the reference. Through on-site observation, the No.262 bay on the II bus was deemed suitable. Short-circuit the current of this branch at the terminal block inside the protection panel (at this time, the No.2 bus protection device of the 220kV I-II bus will display differential current, and the value of this secondary differential current is the primary load current of the short-circuited branch). The differential current values and branch current values on the protection device before and after short-circuiting are shown in the figures below.

△ Display Values of No.2 Panel of 220kV I-II Bus Protection Before Short-circuiting

△ Display Values of No.2 Panel of 220kV I-II Bus Protection After Short-circuiting

 

The transformation ratio of the No.262 bay is 2500:1, and that of the No.224 section bay is 4000:1. The differential value display is based on 4000:1. The data analysis is as follows:

 

Before short-circuiting: The secondary current value of the No.262 bay is about 0.026A, which is converted to a primary value of approximately 65A. The total differential current is 0.01A, the differential current of the II bus is about 0.003A, and the differential current of the I bus is 0.013A.

After short-circuiting: The secondary current value of the No.262 bay is 0A. The total differential current is 0.028A, the differential current of the II bus is about 0.021A, and the differential current of the I bus is 0.013A.

 

Use the HTA-3300 Three-phase Digital Intelligent High-current Source to inject current to simulate the current of this bay:

 

Set the HTA-3300 to the remote control mode. Search for the corresponding device with the YD-300E.

After the connection is completed, set the output amplitude and phase angle, then return to the remote control main interface and click Start to control the connected device for output.

The injected current values and the display results of the protection device are shown in the figures below:

 

△ Secondary Current Value of No.224 Bay After Current Injection

△ Bus Differential Protection Differential Current Display Value After Accessing No.224 Bay Current

Operate the No.224 section bay to be connected to the II bus for operation, and start the test equipment. Since the secondary current wiring of the No.224 bay connected to the bus protection is reversely connected, the primary injected current flows from P1 to P2. As can be seen from the above figure, the current phase angle of the No.262 bay is in phase with the voltage. Therefore, to simulate the injection of primary current with the same amplitude and phase as the short-circuited branch into the CT of the section switch (No.224), the output phase angle of the current source device shall be increased by 180° based on the voltage reference phase angle. After current injection, check whether the differential current of the bus protection device is zero. Stop the test after confirming no errors.

4. Test Result Analysis

The test data of the No.2 panel of the 220kV I-II bus protection is summarized in the table below:

Condition	No.224 Bay Current	No.262 Bay Current	Total Differential Current	II Bus Differential Current	I Bus Differential Current
Before short-circuiting No.262 bay	0A	0.026A	0.01A	0.003A	0.013A
After short-circuiting No.262 bay	0A	0A	0.028A	0.021A	0.013A
After accessing No.224 bay current	0.016A	0A	0.013A	0.003A	0.013A

If the CT installation and wiring of the No.224 section switch are correct, the differential current generated by short-circuiting the No.262 bay should be offset by the injected current in this test method. It can be concluded from the table above that when a 65A current with a phase angle 180° different from the voltage is applied on the primary side, the differential current caused by short-circuiting the No.262 bay is restored to 0A (the total differential current is not restored to 0 due to a small differential current on the I bus). The final displayed total differential current and II bus differential current are both close to 0. Thus, the test result is correct, and the vector of the No.224 section bay connected to the No.2 panel of the 220kV I-II bus protection is verified to be correct.