Analysis of grounding detection method for DC system and improvement of detection device

DC system of power plant and substation is an independent power from generators, power plant, station transformer and change the operating mode of the system, provide a reliable and stable uninterruptible power supply for control circuit, signal circuit, power system relay protection, automatic devices and emergency lighting, it is switching circuit breaker provides operating power. Therefore, the DC power system plays a vital role in the safe operation of the substation, and is the prerequisite for the safe operation of the substation. DC power supply system in power system with the insulation operating mode on the ground, when the occurrence of the DC system grounding point, does not cause direct harm, can continue to run, but potentially dangerous, must immediately give the alarm, and carry out search; otherwise, when another grounding point, there may be a serious consequences.

1. on the grounding and harm of DC system

1.1 what is called DC system grounding

The DC power supply is a power supply with polarity, that is, the power cathode and the power supply negative pole. The "ground" of the DC power supply is only a neutral point to the DC circuit. If the insulation resistance of the DC power system positive pole or negative pole decreases to a certain setting value or is lower than a specified value, we call the DC system a positive earth fault or a negative grounding fault.

Why 1.2 DC system will be grounded

The DC system of power plants and substations is equipped with many devices and complex circuits. In the long run, there will inevitably be DC system grounding due to environmental changes, climate changes, cable and joint aging, equipment problems and so on. Especially in the process of construction or expansion of power plants and substations, because of various problems of construction and installation, it is difficult to avoid hidden troubles of DC system grounding.

Classification and harm of 1.3 DC system grounding

The DC feeder network system connected by more complex grounding polarity can be divided into positive and negative grounding grounding; according to ground types can be divided into direct grounding (or metal grounding or earthing) and indirect grounding (also known as non metallic grounding or semi ground); according to the ground can be divided into single point grounding, multi point grounding, grounding loop and insulation reduction. According to the research, it is indicated that the earthing fault may cause the circuit breaker to trip wrongly. Because the circuit breaker's tripping coil is all connected to the negative pole power, it may cause a tripping trip when the positive grounding occurs. Negative grounding may lead to the rejection of the circuit breaker.

Operation practice also found that DC grounding not only causes relay protection misoperation and refusal to move, but also causes the malfunction of DC controlled equipment, malfunction and even damage to equipment, resulting in serious consequences of large power outage and system collapse. In the case of heavy rain in July 6, 2000, a 220kV hub substation in Hebei took place in the station. 273-1 electric knife sluice is automatically cut off in operation without any signal. After the analysis, the 273-1 knife gate control box seepage and dampness, confirmed that the DC two points grounding, resulting in the mistake of the knife sluice. A 500kV junction substation, because of heavy snow weather, DC two points to ground, causing the station to use 380V AC power control switch trip. In a 220kV junction substation in Guangxi, the circuit breaker is misoperated because of the DC two point grounding. These are the faults of the power grid caused by the insulation of the DC system.

2. search, the usual method of removing the DC system

There are many detection techniques for grounding fault in DC system, and the principle of the realization is not the same. It can be summed up in the following ways:

2.1 DC bus bridge method

Detection device developed by the bridge method is relatively simple, the equivalent of two balance resistance in positive and negative DC bus, forming the balance bridge; only the DC system grounding fault alarm, fault location and straight no indication of grounding resistance function is too monotonous. Manual road pull method is usually used when the maintenance personnel are excluded from the field. In turn, the DC load circuit for DC load is opened in short time. When a circuit is removed, the fault disappears, indicating that the fault is in the loop. The maneuverability is poor, especially for the important load, short time sluice is not allowed. Therefore, the detection device used in this method is only applicable to the DC cabinet system of a low end distribution house.

2.2 low frequency signal injection method

After the grounding fault occurs in the DC system, the low frequency signal is injected between the fault bus and the ground, and the low frequency current flows out of the signal generator and flows through the ground fault feeder and returns from the grounding point. The clamp type current detector is used to detect the feed line by bar. Find the grounding feeder and then find the place.

The method has successfully realized the direct current connection location without power stop, but the accuracy and sensitivity of the detection are greatly influenced by the distributed capacitance of the DC system. The feeder tributary capacitance up to a few method. When the probe at a point measurement, because the capacitor current exists, will make the operator is difficult to determine the capacitance current or grounding current. The detection of grounding resistance by this method is often misjudged or measured, and the grounding resistance error is very large.

2.3 frequency conversion signal injection method

The frequency conversion signal injection is actually a low frequency signal input, but the injected signal is a low frequency signal with alternating frequency. Then through the clamp current probe, the change of the amplitude of the resistive current of the branch is detected to determine the grounding branch and the fault point. The resistive current in the feeder branch is indirectly calculated by injecting the variable amplitude and frequency conversion signals. However, through the field test, the effect is still not ideal, the reason is the distribution of capacitance. Other low-frequency signals