DC Override Tripping in Substations? Portable Ampere-Second Tester for Reliable Verification

In the secondary electrical systems of modern substations and power plants, the DC power supply system is known as the "heart" of the whole station’s relay protection and automatic devices. The circuit breaker closing and tripping circuits, microcomputer protection and measurement & control devices, emergency lighting and communication power supplies within the station all rely on safe and stable power supply from the DC system.

To guarantee independent power supply for each load circuit, the DC feeder network of substations generally adopts a tree-shaped three-level power distribution structure. A large number of DC circuit breakers or DC fuses are used in each feeder circuit as protection components against overload and short-circuit faults. Nevertheless, during long-term grid operation and maintenance, "DC override tripping" has always been one of the most feared serious faults for frontline relay protection engineers. When occasional short-circuit or ground faults occur in protection & measurement devices or control circuits at the end of a lower-level distribution panel, the fault should be cut off instantly by the lower-level circuit breaker of this circuit. However, due to mismatched operating characteristics between upper and lower protective devices or tripping setting drift, the upper-level main incoming circuit breaker of the bus trips first before the lower-level breaker clears the fault! This trip directly causes voltage loss of the entire section of DC bus, which in turn deprives the main transformers and high-voltage lines of the substation of power supply for protection tripping, and may easily lead to large-scale power outages and equipment damage.

To address the technical difficulty that the selectivity coordination of DC circuits in substations is hard to verify on site, Baoding Huazheng Electric Manufacturing Co., Ltd. has launched the HZDK-1000 DC Switch Ampere-Second Characteristic Tester, providing a lightweight and practical test tool for on-site inspection of substations.

No more reliance on national-level testing institutes — bring high-current tests to the substation site Previously, the ampere-second characteristics (current-operating time characteristics) of DC protective devices could only be tested with large high-current test benches in national laboratories or provincial metrology centers. These fixed devices occupy a large area and require bulky dedicated charging transformers and high-current dummy load boxes, making them impossible to transport to substation sites. Some operation and maintenance teams tried series discharge tests using on-site storage batteries, yet the discharge current fluctuated greatly and the millisecond-level current duration could not be precisely controlled, so it was difficult to obtain accurate time-current coordination curves.

The launch of HZDK-1000 fills the gap of on-site ampere-second testing tools for substations: Portable integrated structure: Power electronic conversion control, high-precision sampling sensors and intelligent control core are integrated into a portable desktop case, so testers can carry the equipment directly into the substation power distribution room; Wide current coverage: The test current stably ranges from 0 to 1000A, fully applicable to ampere-second characteristic verification of DC circuit breakers with rated current from 1A to 1000A, and also supports fusing characteristic tests of DC fuses rated 6A to 150A; Fully compliant with State Grid specifications: It satisfies the mandatory requirements specified in State Grid’s Operation Code for DC Power Supply Systems, Technical Supervision Regulations for DC Power Supply Systems and Measures for Preventing Accidents of DC Power Supply Systems, which require sampling inspection of ampere-second characteristics and operating current for DC circuit breakers before commissioning and during periodic maintenance.

Convenient sampling inspection and accurate verification. The HZDK-1000 enables on-site routine maintenance for selectivity coordination verification of substation DC circuit breakers, building a solid safety line of defense for the secondary control hub of the power grid.