In response to the operation and maintenance requirements of a 220kV substation in a power plant, Wuhan Haomai successfully assisted the user in completing the mutual inductor testing and DC system testing projects, winning unanimous praise from on-site technical personnel. The following briefly introduces the application of the CTP Mutual Inductor Tester, FDT Series DC System Comprehensive Tester and other devices in the operation and maintenance of this substation.

Mutual Inductor Testing

△ On-site Application of CTP Mutual Inductor Tester

The IEC6044-6 standard specifies that CT testing can be carried out at a frequency lower than the rated frequency to prevent windings and secondary terminals from withstanding unacceptable voltages. The CTP Mutual Inductor Tester, a newly developed product of Wuhan Haomai, adopts square wave method technology. It can complete CT testing with an inflection point voltage of 50,000V and an inflection point current of 4A, and is competent for comprehensive testing of PTs of various types (e.g., protection-type, metering-type TPY, TPX bushing CTs, etc.) and all voltage classes. It can accurately measure test items including transformation ratio, polarity, volt-ampere characteristics, with an accuracy of 0.05%.

The tester has a built-in printing function. After the test, it automatically prints data such as transformation ratio, polarity, inflection point voltage, phase, ratio error and angle error. The saved test data including 5% and 10% error curves can be exported for easy viewing and analysis by technical personnel. In this test, the latest CTP Mutual Inductor Tester was used to test 5P30 and TPY class CTs with a transformation ratio of 4000:1. The excitation saturation test time was shortened from the previous 107 seconds to 65 seconds, which greatly improved work efficiency.

Battery Charging and Discharging Test

△ On-site Application of FDT Series DC System Comprehensive Tester

At present, most DC system testers are designed according to the maximum battery capacity. When conducting a discharge test for a battery pack with a total capacity of only 500Ah, it may be necessary to carry equipment weighing up to 35kg to the site. The FDT Series DC System Comprehensive Tester integrates battery pack discharging, charger ripple coefficient, voltage and current stabilization accuracy characteristic testing. It is compatible with discharge testing of 220V/110V/48V battery packs, and can be freely configured according to the on-site battery capacity.

It can independently meet the discharge test requirements of battery packs with a capacity of 500Ah, and can satisfy discharge tests of up to 1800Ah when matched with a load expansion box. When paired with a wireless single-cell battery monitoring system, it can collect data of 2V/6V/12V battery cells, visually display the voltage and current stabilization adjustment process and voltage-current change curves, display ripple waveforms in oscilloscope mode, and automatically generate test reports.

Differential Coordination Verification

△ On-site Application of MDJ-800 DC System Differential Coordination Characteristic Tester

At present, the DC power supply network of substations mostly adopts a tree structure. From the battery pack to the electrical equipment in the station, it generally goes through three-level power distribution, and each level of power distribution mostly uses DC circuit breakers as protective electrical appliances. During the operation of the DC system, if the protective action characteristics of the upper and lower level DC circuit breakers do not match, a short-circuit fault in the lower-level electrical equipment will cause the upper-level DC circuit breaker to trip across levels, which will then lead to power outages in other feeder lines.

The MDJ-800 DC System Differential Coordination Characteristic Tester can test the correct probability of the protective action differential coordination of DC circuit breakers at all levels of the substation DC circuit, and verify the differential coordination type selection of DC circuit breakers at all levels according to the test result report. In this test, the tester was used to test the final-stage DC air switches of three specifications (3A, 4A, 6A) in the substation. The short-circuit current was preset, and the short-circuit test was directly carried out to verify the selectivity of differential coordination. The short-circuit current of the branch was estimated according to the on-load small current, the estimated value of the differential coordination probability was calculated by looking up the table, and the results such as operating current, pre-arcing time, arc-extinguishing time and current breaking action curve were analyzed in combination with the test data report. This greatly shortened the operation time and ensured the operational reliability of the DC system.