The latest Engineering Recommendation (EREC) P28 issue 2 makes specific reference to the switching of capacitor banks, and the subsequent voltage disturbances this may cause. Furthermore, there are certain Grid Code connection conditions that call for reactive compensation measures to have the ability to be switched in and out at higher frequency than would normally be possible. Due to this, the application of Point-on-Wave switching with capacitor banks is particularly relevant.
When a capacitor bank is energised there is commonly a large and high frequency inrush current spike. This inrush current can lead to a voltage increase at the PCC. The magnitude and frequency of the voltage rise depends on the inrush current, network fault level and X/R ratio. Furthermore, when a capacitor bank is de-energised a residual DC voltage will be left on the capacitors. This commonly means there must be a 6-10 minute delay period while the voltage decays before the bank can be re-energised.
When switching capacitors, inrush current occurs when there is a rapid change of voltage across the capacitors. The theory of Point-on-Wave capacitor switching is to ensure that this voltage change is avoided, or at least kept to an absolute minimum. When a capacitor bank is de-energised and completely discharged, there is 0V across the capacitors, so for this state POW aims to switch the phases at 0V. Conversely, when the capacitor bank is de-energised and fully charged, there is maximum voltage across the capacitors, for this state POW aims to switch the phases at peak voltage. Finally, for a discharging capacitor bank, POW aims to target the voltage that remains on the capacitors, until it reaches a fully discharged state and once again targets 0V.
The full paper presents waveforms of this action and then displays some on-site comparisons between capacitors switched with and without PoW. The full paper is available to view here:
Find out more about Enspec’s Point-on-Wave switching here.