Influence of Power Quality on Value of Switching Overvoltages in Networks 6-10kV

The article considers influence of power quality on multiplicity of switching overvoltages. Currently, electrical equipment that distorts waveform of current curve and highspeed circuit breakers is widely used. In this regard the value of switching overvoltages increases when turning on and off electric motor and power transformers from network. Dependencies of switching overvoltages multiplicity on the type and power of electric motor, are given. These dependencies were obtained when the electric motor were disconnected from the network in idle mode. Influence of higher harmonics of current and voltage on degree of switching overvoltages is considered. It was established that 5th and 7th current harmonics have the greatest influence on value of switching overvoltages. This effect is demonstrated by computer simulation of switching overvoltages at terminals of synchronous motor when it is disconnected from 6 kV network by a vacuum circuit breaker. It was also confirmed by real measurements of switching overvoltages. Dependencies of correction factor of switching overvoltages multiplicity on total harmonic distortion factor (THD) of switching current are given. Comparative oscillograms of process of electric motor disconnection from network without higher current harmonics and network with presence of higher harmonics are presented. It is shown that when the current waveform distortion factor changes from 0.2 to 0.6, the overvoltage multiplicity can increase from 1.25 to 2 times depending on power of electric motor. It has been proved that higher current harmonics contribute to increase in the multiplicity of switching overvoltages, which can cause breakdown of insulation of electric motor and power transformers. It is recommended in networks with the presence of higher harmonics of current and voltage during the operation of electric motor and power transformers to use protection against switching overvoltages in a mandatory manner.