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To further improve the performance of distance protection in renewable energy systems, a distance protection method is proposed, which considers the effects of control system and the distributed capacitance effect of the line. When an asymmetric fault occurs in the system, combined with the control characteristics of the converter, the sequence impedance characteristics of the grid and the pure resistive feature of the fault resistance, three unknowns in the fault loop equation can be eliminated, and the solution of the fault distance can be solved in the R-L model. Then, the setting point is set based on the calculation results in the R-L model, and the real electrical quantity at the setting point can be deduced by the calculation formula in Bergeron model. Finally, the equation from the setting point to the fault point is simultaneously solved to reduce the distributed capacitance effect on the distance protection. The simulation results testify that the method is less influenced by the fault features of the converter and fault resistance. It still has good performance in long line.

The coal mine power supply and distribution system usually adopts the neutral point non-effectively grounded method. Most of the power supply lines in this method use cable lines. The underground power supply system is complex and uses many voltage levels. When a single-phase to ground fault occurs, it is easy to form intermittent arcs at the grounding point. Moreover, the existing single-phase to ground fault arc suppression technology of the neutral point non-effectively grounded distribution network has the problem of poor arc suppression effect and difficulty in accurately measuring the parameters of the distribution network to the ground. The most common connection method of transformer winding in 10 kV substation of coal mine in China is delta connection. This mode needs to lead out neutral point and connect arc suppression coil artificially through special grounding transformer (mostly Z-type grounding transformer). In order to solve the above problems, a flexible arc suppression system for single-phase to ground fault of distribution network based on Z-type grounding transformer is designed. When the initial time of grounding fault occurs in the distribution network, the three-phase power supply electromotive force and the neutral point voltage of the power distribution network are firstly measured in real time. When the amplitude of the neutral point voltage is greater than 15% of the three-phase power supply electromotive force, it can be judged as a grounding fault. Compared with the three-phase power supply electromotive force, the phase with the smallest power supply electromotive force is the fault phase. After a single-phase to ground fault occurs, the fast switching switch corresponding to the fault phase is quickly closed. And the active inverter and the Z-type grounding transformer power conversion module are simultaneously put into operation. The Z-type grounding transformer power conversion module clamps the voltage of a neutral point in an opposite phase to be close to the electromotive force of the fault phase power supply. And the amplitude and the phase of the difference between the neutral voltage and the fault phase supply voltage are precisely compensated by the active inverter. Therefore, the control target of suppressing the voltage and the current at the fault point to zero is achieved. After a certain time delay, the fast switching switch is cut off. If the voltage of the neutral point is effectively reduced, it can be judged that the fault is a transient ground fault. And the normal operation of the power distribution network system can be restored. Otherwise, it is judged as a permanent fault. The measures to isolate the fault feeder are taken to restore the normal operation of the distribution network. The simulation results show that when the single-phase to ground fault transition resistance is 500 Ω and 3 000 Ω, the power conversion is only through the Z-type grounding transformer. The voltage and current suppression rate at the fault point reaches 79%-83%, which cannot fully achieve the effect of arc suppression. The flexible arc suppression system can effectively suppress the voltage and current at the fault point, with a suppression rate of more than 98%, realizing reliable arc suppression of single-phase to ground faults in the distribution network.

Due to the change of the ground parameters of the distribution network after the single-phase grounding fault, the traditional active arc suppression method based on the ground parameters in the normal operation state has a deviation between the compensation current reference value and the actual value. In consideration of the problem that the fault residual current cannot be fully compensated, which leads to difficulty in arc extinguishing, the characteristic frequency current signal was injected into the distribution network through the active arc suppression device connected to the neutral point, to eliminate the influence of the system frequency on the measurement accuracy of the equivalent parameters to the ground of the distribution network. By constructing a characteristic signal circulation loop, the measurement of the equivalent parameters to the ground of the distribution network after ground fault was realized. Then, according to the real-time measured ground parameters and zerosequence voltage, the compensation current was dynamically calculated, and the active arc suppression device was controlled to inject current into the neutral point to realize the fault arc suppression. And the method of injecting zero-sequence current was proposed to obtain the amplitude and phase of the faulted phase voltage which can be used to detect the faulted phase. Finally, it is verified in PSCAD/EMTDC that the proposed method can reflect the changes of ground parameters of the distribution network in real time, realize full compensation of fault branch current, and achieve the purpose of rapid arc extinguishing of ground faults in distribution network. [ABSTRACT FROM AUTHOR]