Kappenman pointed out in 1990 that GIC will distort the magnetizing current waveform of power transformer, thus destroying the normal operation of power grid. Therefore, it is very important to study the detection, compensation and elimination methods of GIC. In 199 1, Kappenman first proposed to connect a capacitor in series between the neutral point and the grounding point of the power transformer. The method of isolating GIC flowing into transformer [8], and the effectiveness of this method is proved by simulation experiments. Kappenman finally pointed out that if this capacitive isolation method is improved, a bypass device needs to be installed. In 1992, literature [9] takes Radisson/LG2 complex of Quebec Hydropower Station as an object, and puts forward several schemes to suppress DC current: connecting capacitors with small power frequency impedance in series with neutral line to realize quick overvoltage protection of neutral point gap capacitors, trigger gap capacitors and MOV, and this scheme is also equipped with mechanical bypass switches. In 1994, the document [10] puts forward a method of connecting a DC power supply in the opposite direction of GIC in series between the neutral line and the grounding point of power transformer, and it is proved by experiments that the DC power supply can effectively start and inject DC current in the opposite direction when GIC exists, thus achieving the effect of restraining DC bias. At the beginning of 1996, American DEI company developed four sets of very complicated transformer DC isolation devices. In 2003, according to the parameters provided by Siemens, 10 transformer DC isolation device was developed and used in a transformer of a HVDC transmission terminal near India. The device consists of 1 4000μ f DC/ AC blocking capacitors (Xc = 0.8Ω at 50hz), two groups of anti-parallel high current bypass channels and mechanical switch bypass [1 1]. In 2005, the document [12] developed a DC isolation device with transformer neutral point capacitance and thyristor bypass by using power electronics technology. John Ka Man Pen simulated the common series capacitors of transmission lines in American power grid. The results show that GIC can be effectively suppressed, but it will produce some reactive power. John Ka Man Pen also carried out the experiment of installing neutral point resistance on some high voltage transformers. The results show that installing neutral point resistors with different resistance values on some high-voltage transformers can suppress GIC as a whole, but it can't completely isolate GIC channels. It may not be possible to achieve the goal of global governance only by dividing whether transformers are equipped with suppression resistors according to voltage grades, and certain distribution strategy demonstration is needed [13].
The research on DC deviation control in China started late. In 2000, when considering the DC bias caused by geomagnetic induced current, Xue Dang and others analyzed the mechanism of compensation and weakening GIC for the first time, and theoretically put forward methods to eliminate or suppress geomagnetic induced current, including external DC power supply method, self-excited compensation GIC method, series capacitor or resistance method and so on. Xue et al. also pointed out in the document [14] that the common problem of compensation and elimination is that the neutral point of transformer cannot be effectively grounded.
In 2005, some scholars put forward the method of injecting reverse DC current into transformer neutral point [15]. The main principle is that the power supply is stabilized by voltage regulator, rectified by silicon, and injected with reverse DC current through auxiliary grounding electrode and transformer neutral point loop. In 2006, based on the principle of potential compensation, Du Zhongdong and others proposed a new method [16] to reduce the DC current of the neutral point of transformer: inject a certain reverse DC current into the grounding grid of substation to reduce (or increase) its potential, which will reduce the DC current flowing through the neutral point grounding transformer through the grounding grid and restrain the DC bias of transformer to some extent. Du Zhongdong and others also carried out the experimental simulation of positive and negative potential compensation, and the results showed this suppression method. Qin et al. [17] pointed out that the method of injecting reverse current into neutral point does not change system parameters, and has no influence on relay protection, automatic device and insulation coordination. Negative potential compensation can play a cathodic protection role in grounding grid, and its shortcomings can also be properly solved by taking the following measures: 1) shunt of overhead lightning conductor will affect compensation efficiency, which can be solved by increasing the capacity of compensation equipment; 2) The problem that positive potential compensation will cause slight corrosion to grounding grid can be ignored; 3) Improper position of auxiliary grounding electrode will aggravate the DC bias of other substations, which can be solved by setting the auxiliary grounding electrode far away from other substations considering the network layout of the system. The field application of reverse injection in Wuhan shows that the change of substation operation mode [18] will lead to deviation of current direction, and the injected reverse current cannot be adjusted at any time.
The research and development of capacitor DC isolation device in China began in 2005. Zhu Yiying and others first studied three suppression measures: neutral point series resistance grounding, AC transmission line series capacitance grounding and neutral point series capacitance grounding, and then applied three suppression measures according to the actual distribution of neutral point current. Finally, the research results show that the optimal method of neutral DC current is transformer neutral point series capacitance [19].
However, although the capacitor DC isolation device has a good governance effect, it is necessary to install a bypass device because of overvoltage. The device developed by Liu Zhao adopts DC blocking capacitor+silicon rectifier bridge thyristor bypass+mechanical switch bypass, and it was put into field operation in June 2007. In recent years, with the development of power electronics technology, the structural design of capacitor DC isolation device is also developing constantly, mostly focusing on the improvement of capacitor bypass device, such as installing bypass protection system. The main reason is that the zero-sequence current flowing through the capacitor caused by a large AC system grounding fault will produce transient overvoltage, which may damage the insulation of the capacitor and transformer. For another example, the mechanical switch is not fast enough and needs thyristor bypass or better performance materials.
Neutral series resistance is also one of the effective methods to suppress DC bias. In 2006, the DC bias suppression device with small resistance in series with neutral point of transformer in China was officially put into use. Zhao Jie et al. [2 1] first demonstrated the feasibility of limiting underground DC inflow by series resistance at the neutral point of transformer, and analyzed and checked the influence of neutral point electric queue on the system from the perspective of restraining neutral point DC and overvoltage and relay protection. The field installation test shows that this is a simple and effective measure to suppress the neutral DC current of transformer, and it has obvious inhibitory effect on the neutral DC current. However, in order to ensure the safe operation of the system, first of all, there are high requirements for the voltage resistance and heat capacity of the neutral point series resistor; Secondly, the series resistance of neutral point changes the system structure, the relay protection and automation devices need to be reset, and the insulation coordination should be checked accordingly; From the point of view of current limiting, the resistance of the installed current limiting resistor should be large enough, but from the point of view of system operation, the series large resistor cannot guarantee the reliable grounding of the system. If the resistor is bypassed by the discharge gap under fault condition, the grounding impedance of the system will be discontinuous and the relay protection configuration will be complicated. Finally, when the system fails, it will also lead to a series of problems such as overvoltage of transformer neutral point, so the setting of resistance parameters of neutral point series resistance needs further study.
There are also some theoretically proven methods to suppress DC deviation. For example, based on the principle of potential compensation, Ma Zhiqiang proposed a new method to weaken the DC of the transformer neutral point [22], that is, a small resistor (0.5 ~ 2.0 Ω) is connected in series with the transformer neutral point, and then an external DC power supply is connected in parallel to form a DC potential, so that the DC current flowing into the transformer winding can be reduced by adjusting the DC potential of the neutral point. The resistor needs to protect the bypass, and its effectiveness is verified by an example.