| Chinese energy and power load distribution is unbalance. For overcoming this problem, extra high voltage (EHV) and ultra high voltage (UHV) is used to transport power. Improving transmission efficiency is an important issue to realize EHV/UHV large-scale long-distance transmission project. Series compensation devices can effectively compensate the line reactance and short electrical distance. At the same time, using series compensation can reduce transmission line’s loss and improve the stability of power system and transmission power. It is widely used in EHV/UHV transmission lines. However, using series compensation is equivalent to adding a negative impedance in transmission lines. It will destroy the transmission lines’ uniform. And when the transmission line causes fault, it will occur current reverse, voltage reverse, and generate low-frequency and high-frequency component which can affect the correct operation of protection.This paper is based on the UHV project. Firstly, this paper analyzes the reason of series compensation transmission line causes current reverse, voltage reverse and generates low-frequency and high-frequency components when the line occur different type of faults in the different positions. And it points out that when the line occur three-phase fault or adjacent line occur ground fault through resistance, the protection devices of transmission line are unable to measure current reversal phenomenon. If the line occur asymmetry fault or three-phase fault through resistance or adjacent line occur metal fault, the protection devices may appear current reversal phenomenon when the power system impedance is small and series compensation degree are large. Voltage reversal associated with fault point location and series compensation degree. When different position occur fault, the line and adjacent line can detect voltage reversal phenomenon one by one. When UHV transmission line installs series compensation, it will appear low-frequency and high-frequency components. And this low frequency and high frequency components are more close to the power frequency. It will make this components are difficult to remove.Then, considering of the transmission lines electrical characteristics changes after installing the series compensation. This paper analyses the influence of UHV transmission lines’relay protection after installing series compensation. Because UHV transmission line has larger distributed capacitance, it may affect the sensitivity of phase current differential protection. If the line cause current reversed, it will affect the reliability of phase current differential protection. When the line access series compensation, it is equivalent to add a negative impedance, and it may cause distance protection occur transient overreach phenomenon and protection malfunction. When the transmission line have increasing current, this effect will be more obvious.According to the impact of relay protection after installing series compensated, this paper proposes an adaptive protection scheme for series compensated line. Firstly, the program will first do UHV circuit partitioning, using the multi-fault characteristic difference between different regions of protection. For the bus side protection device, using the different fault impedance angle range of area1and area2, the area2’ protection malfunction problem will solved. Using the different inductance variation rate of area3and area4, the area4’protection malfunction problem will solved. For the line side protection device, using the different inductance variation rate of area2, area3and area4, the area2and area4’protection malfunction problem will solved. The protection scheme has good adaptability to the transition resistance. Through the PSCAD simulation, the reliability of this protection algorithm is verified. |
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