Function and regulation of controllable shunt reactor

With the rapid development of EHV systems and construction of UHV lines in China, controllable shunt reactors are expected to be put into operation in the near future as one of the important electro-apparatus. They have the ability to compensate for the dynamic capacitive power of transmission lines smoothly, suppress the power frequency overvoltage effectively, and avoid the overvoltage in the case of empty line interruption. They are also be able to prevent the power frequency resonance and impel the secondary arc to suppression when a small power inductance is connected to the neutral of reactors. These functions have been partly presented in the open literature. This paper will develop some further insights into the functions of the controllable reactors from different viewpoints of the mentioned problems. From the conditions for reactive power balance in long EHV and UHV lines, the regulation mode of shunt reactors was presented in this paper for optimal operation of the lines. It was realized that the suppression of the power-frequency overvoltage through using a shunt reactor was due to the weakened capacity effect resulted from the reduction of the equivalent length of the empty line. This requires a quick response from the controllable reactor in order to recover its full capacity immediately in the presence of empty line. When the empty line is interrupted, residual charges on the isolated lines will discharge with low frequency oscillation, this delays the rise of the recovery voltage between the contacts of the breaker and thus avoids restrike. While the neutral inductance of a shunt reactor plays an important role in restraining resonance phenomenon and secondary current, our research in this paper reveals that the value of neutral inductance should be determined according to the maximum degree of the compensation of the shunt reactor, implying that this value should not be set in controllable form, which narrows the range of the power regulation of the shunt reactor and requires high level of insulation of the neutral itself.