# Switching Overvoltage Analysis of Parallel Shunt Reactor Sets

##### Kantanen, Aleksi (2017)

Kantanen, Aleksi

2017

Sähkötekniikka

Tieto- ja sähkötekniikan tiedekunta - Faculty of Computing and Electrical Engineering

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##### Hyväksymispäivämäärä

2017-06-07**Julkaisun pysyvä osoite on**

https://urn.fi/URN:NBN:fi:tty-201705161402

##### Tiivistelmä

Shunt reactors are an essential part of the modern power grid, as they are used to balance off the capacitive reactive power produced for example by the increasing amount of transmission cables. However, the switching of shunt reactors is challenging for the insulation of the shunt reactors, as the sudden changes in current trough the reactor may result in very high transient voltage peaks. In addition, if an extinguished arc in the circuit breaker re-ignites, the resulting overvoltages can be even more severe. The aim of this thesis is to develop a reactor model, by which the total capacitance of a reactor and its effect on both the reactor nominal parameters and switching overvoltages could be studied. In addition, the switching of two parallel-connected three-phase shunt reactors are simulated to find estimates for the overvoltage peak values.

In the first chapters, the equations and the underlying theory for calculating the internal resistances, inductances and capacitances are presented. Next, a model for the connecting substation including the circuit breaker re-ignitions is discussed. In addition, equations for estimating the overvoltage peak values and the resonance frequencies of the voltage oscillations are provided.

The equations were coded with Matlab into a script, by which the total values for an equivalent circuit of the reactor were calculated. Dimensions of a sample reactor manufactured by Grid Solutions Ltd. is used as an input for the model and the results are compared to routine test reports of similar reactors. Furthermore, the substation model is built and several switching operations are simulated in Simulink.

According to the results, the presented equations can be used to calculate the reactor parameters with good accuracy. It was found, that reactor capacitance does not have a significant effect on the reactor parameters with supply voltage frequencies below 1 kHz, nor on the switching overvoltages. The switching voltage peaks did not exceed the basic lightning impulse rating of the reactors if the interruption occurs during a relatively low current, but the circuit breaker performance seems to be in a major role as a disconnection at high load current resulted in peaks exceeding this rating.

Furthermore, the most important oscillation frequencies resulted from oscillations involving the whole switching circuit. Oscillations of such a high frequency were observed, that the switching voltages had rise times shorter than the standard lightning impulse and of those stated in the transient recovery voltage testing requirements of circuit breakers.

In the first chapters, the equations and the underlying theory for calculating the internal resistances, inductances and capacitances are presented. Next, a model for the connecting substation including the circuit breaker re-ignitions is discussed. In addition, equations for estimating the overvoltage peak values and the resonance frequencies of the voltage oscillations are provided.

The equations were coded with Matlab into a script, by which the total values for an equivalent circuit of the reactor were calculated. Dimensions of a sample reactor manufactured by Grid Solutions Ltd. is used as an input for the model and the results are compared to routine test reports of similar reactors. Furthermore, the substation model is built and several switching operations are simulated in Simulink.

According to the results, the presented equations can be used to calculate the reactor parameters with good accuracy. It was found, that reactor capacitance does not have a significant effect on the reactor parameters with supply voltage frequencies below 1 kHz, nor on the switching overvoltages. The switching voltage peaks did not exceed the basic lightning impulse rating of the reactors if the interruption occurs during a relatively low current, but the circuit breaker performance seems to be in a major role as a disconnection at high load current resulted in peaks exceeding this rating.

Furthermore, the most important oscillation frequencies resulted from oscillations involving the whole switching circuit. Oscillations of such a high frequency were observed, that the switching voltages had rise times shorter than the standard lightning impulse and of those stated in the transient recovery voltage testing requirements of circuit breakers.