Q: We are seeing voltage swells caused by switching operations and load disconnections. Phenomena is unpredictable and largely random. The neutral of a medium-voltage distribution network is grounded through a resistor and the neutral of the low voltage network is directly connected to the ground (TN-C scheme). We know there may be an increase in the voltage in the low voltage network when there is a significant unloading of the transformer. But perhaps there is an increased voltage in low voltage network when the transformer is reconnected after a disconnection (intentional or by protection), that causes a dangerous overvoltage to the customer? What would be the physical cause? EN 50160 standard does not exclude the possibility of voltage swells due to switching operation in network: 126.96.36.199.
-Question from Romania
A: Determining the root cause of intermittent phenomena can be especially challenging. That is particularly true in an electric distribution system where important parameters such as load levels are subject to significant changes over time.
It is presumed that the transformers used on a resistance- grounded medium voltage distribution system have either delta or ungrounded-wye primary windings. It is further presumed that these MV-LV transformers are three-phase with TN-C low voltage connection. One cause of voltage swells could be ferroresonance when transformers are connected to or disconnected from the MV system – particularly if the switching is performed with single-phase devices. Ferroresonance can result in significant voltage swells on both the MV and LV side of the transformer. This condition is made more likely by a variety of conditions. These include:
- Delta or ungrounded-wye primary transformer windings.
- Relatively high distribution system voltage
- Significant phase-to-ground capacitance on source due to shielded cables
- Light load on transformer being switched
On U.S. multigrounded-wye distribution systems ferroresonance can be a problem if shielded cables are present in the circuit supplying a delta-connected transformer. This is why U.S. practice has migrated to grounded-wye connections on the primary winding of three-phase transformers for underground distribution systems. Long overhead circuits in rural areas, particularly on 25 kV-class systems, can also experience ferroresonance even during switching of ungrounded-wye overhead transformers. Concentrated MV capacitor installations can also contribute to ferroresonance on overhead systems. Small (<150 kVA) transformers are more susceptible, especially when there is no load on the secondary winding.
Another possible, but completely different, cause could be intermittent turn-to-turn shorting of high voltage winding(s) in an oil-filled transformer. This could be due to previous breakdown of the winding insulation under lightning surges. Of course, this could be intermittent but may not be strongly correlated with switching operations. This condition usually evolves into a sustained overvoltage condition on the affected winding. -
Edward S. Thomas, PE
Utility Electrical Consultants, PC