Analysis and Improvement Measures of DC System Grounding Detection Device

   1　Hazards of DC System Grounding

　　In operation practice, it is found that DC grounding not only can cause relay protection misoperation or refusal to operate, but also can cause equipment controlled by DC to misoperate or refuse to operate, even damage equipment, resulting in large-scale power outages and system collapse.

   2　Current Status of DC Insulation Detection and Monitoring System

　　At present, the technical principle adopted by the insulation detection and monitoring device of Huaibei Guo'an Power Co., Ltd. has a certain gap with the actual situation on site, resulting in the device's function being difficult to fully meet the actual needs of the site.

   2.1 Problems in the Technical Principle of Insulation Monitoring Device

　　For the production site, after years of operation of the power plant, the cable insulation generally declines, various terminal boxes, mechanism boxes, and auxiliary contact boxes of disconnectors rust and damage, sealing performance decreases, and grounding is prone to occur in rainy, snowy, and humid foggy weather; moreover, it is often non-metallic grounding (high resistance to ground), multi-point grounding, grounding on both positive and negative poles, and the difference in insulation resistance between positive and negative poles is small, forming symmetrical grounding faults. However, the current DC insulation monitoring device uses the bridge balance principle for persistent monitoring and alarm of DC systems, which cannot effectively detect the above-mentioned high-resistance symmetrical grounding. Due to the limitation of the bridge balance principle, the device can only monitor asymmetric DC grounding faults, and cannot respond when the insulation resistance of both positive and negative poles decreases equally or when their values are close. Moreover, if the insulation resistance difference between the two poles is large, but the insulation level of either pole is not actually below the allowable value, it may also alarm, causing detection personnel to mistakenly believe that the insulation level has declined.

   2.2 Problems in Branch Detection Principle

　　With the widespread installation and use of anti-interference capacitors in microcomputer protection and the adoption of open-loop radial power supply operation mode for DC systems, the capacitance current to ground of DC systems increases. The current microcomputer-based insulation branch line selection device that injects signals into the system can no longer effectively find the grounding branch. When the capacitance current is greater than the setting value of the detection device for insulation resistance leakage current, it will cause false signals, affecting the correct judgment of the device. Operation practice also proves: Huaibei Guo'an Power Co., Ltd. has installed a grounding line selection instrument from a domestic manufacturer. When the control power DC grounding occurs in the 380V working IA section, it reports DC grounding in the central control room circuit, causing great interference to operation and maintenance personnel in finding the grounding, endangering the safe operation of the power grid.

   2.3 Existing Insulation Monitoring Devices Cannot Automatically Meet the Requirements of Changes in DC System Operation Mode

　　According to the newly issued anti-accident technical measures of the State Power Corporation, "25 Key Requirements for Preventing Major Accidents in Power Production", the DC systems of hub substations widely adopt the wiring mode of dual battery banks and single busbar segmented connection. When the two DC busbars are operated in parallel (such as during capacity test of a single battery bank), it is required to promptly stop the DC insulation monitoring device of one busbar section to ensure that the DC system's insulation resistance to ground is not reduced, otherwise it may cause relay misoperation during a single-point grounding; when the two DC busbars are operated in split mode, it is required to promptly put into operation the insulation monitoring devices of both busbar sections, otherwise one DC busbar section will lose insulation monitoring. Taking the conventional DC insulation monitoring device as an example: when the two DC busbars are operated in split mode, they are two independent DC systems, and each busbar section is equipped with a monitoring device. To measure the potential to ground, each insulation monitoring device has an artificial grounding point. To prevent relay misoperation caused by another single-point grounding anywhere in the DC network, the coil of the insulation monitoring relay is required to have a sufficiently large resistance value. (For a 220V DC system, the coil has a sufficiently large resistance value of 30KΩ, and its starting current is 1.4mA. The starting currents of other relays in the system should be selected to be greater than 1.4mA). When operated in parallel, it is equivalent to one DC busbar and one DC system. Before paralleling, the insulation monitoring device of one busbar section must be stopped, otherwise two 30KΩ resistors will be connected in parallel, and the insulation resistance to ground becomes 15KΩ, causing a single-point grounding (for 220kV DC system, the insulation alarm value for grounding is 20KΩ). At this time, if another point is grounded, the grounding current is sufficient to cause some relays to misoperate. At the same time, after the two busbars are switched from parallel operation to split operation, the insulation monitoring device of the stopped busbar section should be promptly put into operation, otherwise that busbar section and its system will lose insulation monitoring to ground. Existing DC insulation devices cannot automatically adapt to the split and parallel operation modes of the two DC busbars. Generally, manual switches or auxiliary contacts of bus tie switches are used in the secondary wiring to switch and stop the grounding mode of one device, or the device is stopped by disconnecting the fuse connected to the DC busbar.

   3　Countermeasures and Effects

　　To solve the above problems, our plant has selected a new type of microcomputer-based DC grounding line selection monitoring device - GYM DC grounding line selection monitor.

   3.1 Working Principle of GYM

　　By combining balanced bridge and unbalanced bridge, it can effectively detect simultaneous grounding of positive and negative poles, and the insulation resistance to ground is not affected by whether the grounding resistances of positive and negative poles are the same or close.

　　Its working principle: When the equipment operates in a balanced state, K1 and K2 are closed, providing a grounding point for section I busbar, and recording the positive busbar voltage to ground, negative busbar voltage to ground, and the leakage current values of each branch in section I to ground at this time. If a single-point grounding occurs at this time, then IV positive 1 ≠ IV negative 1, and the grounding resistance value can be obtained based on the voltage deviation.

　　When simultaneous grounding of positive and negative poles occurs, this method cannot accurately measure the grounding resistance, and it is necessary to use the unbalanced method to detect the busbar insulation to ground. When the equipment is in automatic detection mode, it first uses the balanced bridge with K1 and K2 closed. When the grounding resistances of the positive and negative busbars are not equal, or not simultaneously equal, it will cause voltage deviation of the positive busbar to ground and negative busbar to ground. Once this deviation exceeds the set value (10V), the equipment will initiate an unbalanced detection, that is, close K1 and K2 separately once, record the positive and negative busbar voltages to ground and branch leakage currents when K1 is closed; and the positive and negative busbar voltages to ground and branch leakage currents when K2 is closed; based on the four voltage values of the busbar to ground, the grounding resistances of the positive and negative busbars can be calculated.

　　R+=（V3-V1）RO/V1 R-=（V2-V4）RO/V4

　　Combined with the two leakage current values of the branch, the branch grounding resistances Rn+ and Rn- can be calculated. The detection method for section II busbar is the same as that for section I busbar. To overcome the influence of system capacitance, we adopt delayed sampling after switching to avoid the influence of the transient process of capacitor charging and discharging.

3.2 Impact on DC System Operation Mode

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