Translated Abstract
Voltage source converter based high voltage direct current (VSC-HVDC), using full-controlled switching devices and high frequency pulse width modulation (PWM) technology, is a new DC transmission and distribution technology. The VSC-HVDC system offers many advantages, such as power feeding into passive networks, control active and reactive powers independently, immunity to commutation, and power inversion without changing voltage polarity, and it is very promising in the fields of the grid connection of renewable energy, the island power supply, the urban power supply, the interconnection between synchronous grids, multi-terminal DC transmission and constituting DC grid.
Generally, the DC transmission lines, operating under harsh weather conditions, have a high fault rate. Therefore, to ensure the safe operation of the whole system, a set of perfect and reliable relay protection devices are necessary and crucial. In the current actual project, the VSC-HVDC transmission lines still adopt protection principle on CSC-HVDC transmission lines, without taking into account the characteristics of VSC-HVDC system. So, we should study novel protection schemes according to the characteristics of VSC-HVDC system. The main research works are described as follows:
1) Study on pilot protection for VSC-HVDC transmission lines based on parameter identification
The shunt large capacitance, on both sides of voltage source converter HVDC (VSC-HVDC) transmission lines, will rapidly discharge to the fault point when a fault occurs. According to this characteristic of the VSC-HVDC system, a novel pilot protection for VSC-HVDC transmission lines based on parameter identification method is proposed in this paper. The principle, using time-domain algorithm, can distinguish internal faults and external faults by identifying the capacitance parameter at both ends. The capacitance at both ends can be accurately identified simultaneously when an internal single pole ground fault occurs, while the capacitance at both ends cannot be accurately identified simultaneously when an external fault occurs. According to this characteristic, a pilot protection criterion is constructed. The simulation results show that the method is not affected by transition resistance and line distributed capacitance, and it can identify internal faults and external faults reliably and rapidly.
2) Study on novel pilot protection for VSC-HVDC transmission lines using model identification
In order to solve the problems in existing protection for VSC-HVDC transmission lines, such as insufficient sensitivity for main protection and slow operation for backup protection, a novel pilot protection for VSC-HVDC transmission lines based on model parameters is proposed in this paper. In the principle, external fault is equivalent to a positive capacitance model, which is the positive line-to-ground capacitance, while the internal fault is equivalent to a negative capacitance model, which is the negative shunt capacitance. Internal faults and external faults can be distinguished by judging whether the identified capacitor is positive or negative. Simulation experiments show that this principle, not affected by transition resistance and line distributed capacitance, having sufficient sensitivity, can identify internal faults and external faults within 10ms reliably in various operating conditions.
Protection schemes proposed in this thesis, not affected by transition resistance and line distributed capacitance, take action quickly, and it can not only be used as a supplement to the dc transmission line main protection, but also to accelerate backup protection.
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