Fast auto-reclose function for 765 kV lines in the proximity of resonant line voltages

摘要

Shunt reactors are installed on long lines (in this case a 435km 765kV line) to reduce over voltages experienced when the load is suddenly lost and to control voltages under lightly loaded conditions. When the compensated line is opened to clear a fault, it is found that the open-phase voltage does not disappear. A transient voltage waveform with a resonant frequency between 35Hz and 45Hz can be seen which gradually reduces in magnitude. This transient phenomenon depends on the line parameters and the level of compensation and without any intervention can last for greater than 15seconds. During the auto-reclosing stage, the transient voltage will be superimposed on the source voltage creating dangerous over voltages along the line and on the shunt reactors. The over voltages are very high and as such represent a danger for the line and reactor insulation. In most cases, surge arrestors together with closing resistors are installed to limit the switching over voltages. This is expensive solution and prone to failure which is very expensive to repair and maintain. A cost effective solution not based on expensive pre-insertion resistors is therefore the ideal solution. An improved auto reclose function is proposed which controls the switching of the EHV breaker to limit over voltages and to achieve faster reclosing. The algorithm described in this paper detects the resonant frequency very fast and accurately depending on the compensation on the lines and predicts the correct closing instance of the breaker. The main characteristics and performances of this technique under real dynamic conditions are presented. The technique is shown to improve the resultant over voltages on the power system and allows for faster reclosing thus aiding stability. Also described is the model created in Simulink (Matlab) [1] for developing this technique.