Investigation of ungrounded 72,5 kV shunt reactor switching transients

Power systems with predominantly cable-based transmission and distribution may experience an excess
of reactive power. To remedy this, shunt reactors are implemented in order to absorb some of the excess
reactive power. When de-energizing the shunt reactors, transient voltages occur due to the line loading
being close to purely inductive and the small current chop by the circuit breaker when opening a phase.
This leads to a transient recovery voltage (TRV) across the circuit breaker terminals. An industryoriginated
proposed solution to reduce the TRV is to connect a short cable between the shunt reactor
and the busbar. Therefore, this project investigates the influence of different cable lengths on the TRV
of an ungrounded shunt reactor. Additionally, a sensitivity analysis on the influence of three different
parameters on TRV is conducted, these are the chopping current level, and the stray capacitance at the
busbar and shunt reactor. A model suitable for transient simulations is developed and implemented in
PSCAD.
The simulations show that inserting a short cable between busbar and shunt reactor greatly decreased
the rate of rise of recovery voltage (RRRV), while having less influence on the peak TRV and that the
standard practice cable length of around 200m in the substation is sufficient to reduce the RRRV.
Furthermore, it was observed that due to the shunt reactor being ungrounded, two current phases may
align after all breakers open, resulting in a substantial current increase in the last phase. This caused a
higher voltage at the shunt reactor terminals, leading to an increased TRV. Therefore, phasor alignment
must be considered when evaluating the worst-case TRV scenario.
The case study is based on a real installation of a 72,5 kV shunt reactor in 150/60 kV substation Bredkær
BDK owned and operated by Danish utility company Nordenergi Net. Single line diagram and overview
of modeling depth for EMT simulations are shown in figure 1. The results presented are obtained in a
MSc students project by authors Simon, Jakob and Kaare in cooperation with Nordenergi Net.