In power systems, the DC power supply system is the heart of the substation secondary system, providing the fundamental guarantee for the correct operation of measurement, control, protection and other systems. Currently, DC feeder networks in substations mostly adopt a tree-like structure. Power is distributed from the battery bank to on-site electrical equipment generally through three-level power distribution, with DC circuit breakers used as protective devices at each level. During the operation of the DC system, if the protective action characteristics of upper and lower level DC circuit breakers are mismatched, a short-circuit fault in lower-level equipment can cause an overstep trip of the upper-level DC circuit breaker, leading to power outages in other feeder circuits.

To verify the correct probability of differential coordination of protective actions for DC circuit breakers at all levels in the substation DC loop and calibrate the differential coordination selection of DC circuit breakers at all levels based on test result reports, Wuhan Haomai Electric Power has developed the MDJ-800 DC System Differential Coordination Characteristic Tester. It meets the performance requirements of DL/T 5044-2014 Technical Specification for Design of DC Power Supply Systems in Electric Power Engineering, featuring high precision, high performance, portability, and suitability for on-site use in substations. Taking the differential test of a newly built 220kV substation by XX Power Transmission & Transformation Company using the MDJ-800 as an example, this article introduces the short-circuit test and differential test methods for switches.

Pre-Test Preparation

1. The charging device is out of service or temporarily replaced, on-site safety measures are complete, and a battery bank matching the DC power supply system is available.
2. A fixed or mobile distribution panel is provided, equipped with a single-phase 220V AC output with a power of not less than 1kW, and the input is protected by an automatic air switch.
3. Review the DC system wiring diagram and differential coordination configuration table of the substation.
4. Before wiring, disconnect the DC switch of the circuit under test. Connect the input terminals and voltage sampling terminals of the control device to the lower port of the DC circuit breaker under test using positive/negative connection cables and voltage sampling lines, ensuring the polarity of the connection cables and voltage sampling lines is correct. Then connect the control device to the computer using a network cable.

△ Test Schematic Diagram

Test Procedure

1. Parameter Setting: After connection, connect the working power supply of the control device, close the circuit breaker, run the host computer program, enter the initial interface, and set parameters according to the operation instructions.
2. Estimation Test: Click "Confirm" to enter the estimation test interface. Fill in "System Information" and "Switch Information" in the information setting bar; the system automatically sets the estimation test current reference points based on the filled switch information.

Set the system measurement and control protection time, click the "Start" button to automatically perform the estimation test. After completion, the system estimates the line-to-line internal resistance, estimated short-circuit current, and differential coordination probability.

3. Short-Circuit Verification Test: Enter the short-circuit verification test interface, fill in the short-circuit test current value based on the estimated short-circuit current from the estimation test, and set the measurement and control protection time. Click "Start" to perform the test.

Example: According to the setting sheet of a 110kV battery bank, the rated current of the upper-level switch is 100A, the rated current of the local-level switch is 32A, the measurement and control protection time is 5.00s, and the short-circuit current is set to direct short-circuit.

4. Waveform Recording and Report Generation: After the test, the system automatically records the current breaking characteristic curve, calculates the circuit breaker action current, pre-arcing time, and arc extinction time. After manually confirming whether an overstep trip has occurred, the system automatically generates a test report.

Test Result Analysis

Analyze the results such as action current, pre-arcing time, arc extinction time, and current breaking action curve based on the test data report.

Estimation Results:

Estimated Short-Circuit Current: 827.019A

Estimated Differential Coordination Probability: 93.260%

During this on-site test, the MDJ-800 DC System Differential Coordination Characteristic Tester verified the selectivity of differential coordination through direct short-circuit tests with preset short-circuit currents. It estimated the short-circuit current of the branch using small loaded currents, calculated the estimated differential coordination probability via look-up tables, and displayed parameters such as short-circuit action current, pre-arcing time, arc extinction time, and switch breaking characteristic curves. This solves the problems of cumbersome procedures and heavy workload in traditional differential tests, greatly shortens the operation time, improves the operational reliability of the DC system, and ensures the safe and reliable operation of the power grid.