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Non-standard Effects in Networks and their Impact on VT

Non-standard Effects in Networks and their Impact on Voltage Instrument Transformers

General Principles of Prevention and Protection


This effect has already been described in many publications. In principle it refers to an exchange of electrical energy between the capacitance and inductance of a transformer. In the course of the ferroresonance effect, the magnetic core of the transformer becomes oversaturated, followed with overcurrents causing destruction.

The prevention consists in:

Loading the voltage transformer always with a burden, only a small one, consisting from an electronic W-meter or protection relay. In this a way, the startup of resonance effects will be curtailed.

Damping of the already arisen ferroresonance phenomenon can be done using e.g. a special resistance (KPB Intra sells a resistance of 68 Ohm/2000 W for this purpose), or the AFR 31 device, which is to be embedded in the auxiliary windings connected in opened delta.

In special cases, in particular when this phenomenon has arisen repeatedly, it is necessary to notify the manufacturer of the transformers, i.e. the company KPB Intra. Based on experiences gathered with the so called undersaturation of the magnetic core the principle mentioned above can be augmented and the occurrence of ferroresonance be prevented or its effects limited.

Atmospheric overvoltages

External protection consists in the use of surge arresters

Switching overvoltages

Structural protection consists in the reinforcement of insulation of the input turns of the MV transformer winding.

External protection consists in the usage of surge arresters.

Short-circuits in the secondary winding

When connecting the windings it is necessary to avoid a double earthing of the same, e.g. by connecting one terminal of the secondary winding to the earth in the secondary terminal block of the transformer, and the other terminal to e.g. an earthed energy meter.

Auxiliary windings connected in delta may be earthed at one point, only.

Short-circuit can be prevented also by using adequate circuit breakers or fuses, connected to the secondary terminal block (the VPT 25 does include fuses incorporated in the secondary terminal block).

General protection

Generally, voltage transformers are protected from explosion by fuses mounted on the MV side. The manufacturer recommends their usage and the fuses are delivered by him with appropriate parameters. It is important to realize that the fuse above is not capable of saving the transformer, but it may reduce the effects of the transformer explosion on the environment.

The above recommendations refer in particular to projects installed at exposed points of the network, such as mines, railways, transition points from HV onto MV, and recently also in the photovoltaics and biogas stations.