The coil of the grounding relay is connected to a zero-sequence current transformer (cable type, busbar type, or zero-sequence current filter composed of three-phase current transformers). When the zero point of the protected motor is grounded through impedance, the relay is connected to the differential circuit of the converter. When the relay is connected to a zero-sequence current filter composed of three current transformers, it should also be connected to a blocking relay to prevent malfunctions that may be caused by unstable currents caused by external penetrating short circuits.
1. Research status of grounding relays
Improving the ability to withstand transition resistance of grounding distance protection was once a research hotspot in relay protection. The traditional directional impedance relay, when the forward two-phase and single-phase are short-circuited by the external grounding of the transition resistance, may have an over-range action phenomenon, and the allowable transition resistance is also small, so the directional impedance relay is used as a measurement element for ground faults. Not ideal. For this reason, it is necessary to research and develop a grounding distance relay with a new principle.
Someone proposed a grounding reactance relay scheme based on the differential equation algorithm. In this scheme, it is suggested to use the sum of the fault components of the positive sequence and negative sequence currents to approximately replace the fault current flowing through the fault point, and to determine the fault distance by solving the differential equation. Whether the fault is in the protection zone, the experimental results show that the ability of this grounding reactance relay to respond to high-resistance ground faults is significantly improved, but the ability of the relay to withstand transition resistance is greatly affected by the change of positive sequence and negative sequence operating parameters. In the actual power system, the zero-sequence network of the system is relatively stable, so it is recommended to replace the fault current flowing through the fault point with the zero-sequence current flowing through the relay.
Because the algorithm for solving differential equations is based on the calculation of instantaneous values, the mathematical model is simple, the calculation speed is fast, and the protection has a strong ability to withstand transition resistance, so it becomes the preferred algorithm for computer grounding distance protection. However, the grounding reactance relay based on the differential equation algorithm has the problem of out-of-area steady-state exceeding in some operating modes of the power system. Aiming at this problem, a new algorithm of apparent distance based on the principle of phase compensation is proposed. The grounding distance relay realized by this algorithm can select different phase compensation values of zero-sequence current according to the change of system operating conditions, so as to ensure the protection when the fault occurs outside the zone. No misoperation beyond the range. However, the performance of the phase compensation principle, the apparent distance, the new algorithm, and the grounding distance relay are greatly affected by the angle setting value. Polarized ground distance relays show excellent performance in reflecting high-impedance ground faults, and are widely used in line protection devices as measurement components for ground distance protection.
Through the analysis and research on the mechanism of steady-state overriding of polarized grounding distance relays, a new scheme for the problem of steady-state overriding of polarized distance relays outside the area is proposed, that is, the protection scheme for double-polarized grounding distances. When a single-phase grounding fault occurs, this scheme can Under the premise of ensuring that the fault protection beyond the set point does not malfunction, it has the highest possible ability to withstand transition resistance for faults in the zone. However, it is not difficult to see that in the development process of ground distance relays, all kinds of relays are based on simple double-sided power supply single-circuit line system as the research model, and have developed from traditional directional impedance relays to double down-polarization ground distance protection schemes. The advantage of the double lower polarization ground distance protection scheme is that it has the highest transition resistance resistance capability among the ground distance protection schemes for the model studied.
However, the model it researches also determines its inherent deficiencies. For more complex system models, especially when there are ring networks, parallel double-circuit lines, or the next line is a branch line, the phase of the fault current flowing through the fault point and the compensation voltage The relationship can no longer reflect the faults in the area and the faults outside the area as in the single circuit line. Therefore, it is very necessary for the action characteristics of the relay to coordinate the allowable transition resistance capability and the overshoot capability.
2. Adaptive grounding relay
Positive-sequence voltage is used as the polarization quantity, and the reason why the distance relay has poor tolerance to transition resistance is that the action boundary of the action equation is fixed. If the grounding distance relay's ability to respond to grounding resistance is to be enhanced, the action boundary of the grounding distance relay must be adaptive so that it can follow the grounding resistance.
This change can be attributed to the counterclockwise rotation of the protective action boundary with the O point as the center. The ideal rotation angle is to make the rotated action boundary coincide with the voltage vector of the ground fault point, so that it can be completely It is not affected by the transition resistance, but in fact, because the grounding distance relay uses the voltage and current on the side of the line for protection calculation, this ideal rotation angle cannot be measured, so only some approximate means can be used get.