Ferroresonance in Voltage Transformer (VT) Circuits

October 1, 1997 (corrected)

In the last couple of months we have received several different questions as to what is ferroresonance in a VT circuit, when does it occur and how do we protect against it.

Ferroresonance can occur when the primary of a voltage transformer is connected line to ground in an ungrounded circuit. This configuration results in the magnetizing reactance of the VT being in a parallel loop with the coupling capacitance to ground of the system (see Figure 1).

Figure 1

The coupling capacitance is primarily made up of the capacitance of the system dielectric between the phase conductor and ground. The value of the voltage transformers magnetizing reactance varies as a function of the amount of flux going through the iron. This results in an LC circuit and requires only a simple voltage transient to excite the resonant frequency. Once the ringing begins the voltage across the individual components of magnetizing reactance and coupling capacitance can reach high levels and the ringing can go undamped if the voltage transformer is lightly loaded. The loading of the VT has a very important part to play in limiting the magnitude of current in the oscillation circuit since the resistance of the load will act as a current divider and send a portion of the current to ground. This graph from the IEEE Red Book shows the impact of load on the magnitude of the current in the ringing circuit (see Figure 2).

Ferroresonance in Voltage Transformer (VT) Circuits page 2

Figure 2

During the oscillation, the current can drive the magnetizing force to saturate the VT. When the VT is saturated, the reactance to ground will diminish and the current to ground through the primary of the VT will go high. At the end of the sinusoid the VT will drop out of saturation, but with a low loss system the stored charge remains relatively high across the system coupling capacitance. As the polarity of the sinusoid changes the process repeats itself. The current surges, through the VT primary during the periods of saturation, can be much greater than full load rating but not approaching fault current levels, making it very difficult for the fuses on the primary of the VT to interrupt. Thus current surging may result in a blown VT fuse but often results in a shorted VT.

To keep the resonance magnitude down, the secondary side of the VT circuit can be artificially loaded. There are two common methods of loading used to minimize the effects of ferroresonance. One is to install the VT’s with their secondary windings connected in a broken delta and with a resistor completing the broken delta circuit. The watts of the resistor should equal 50% of the VA of a single VT.

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The second and most popular method is to put a resistor across the secondary of each VT. The rule of thumb from several old references is that the resistive loading should range between the VA required to excite the core at no load and 50% of the thermal rating of the VT. For specific VT’s, the manufacturer can recommend a precise value of resistance.

Due to the varying frequency of the transient and the magnetizing reactance this is not a problem that occurs in every system or even every time a voltage transformer is connected to ground on an ungrounded system. If the resonant frequency of the LC circuit is excited the swamping resistor will dampen the ringing to prevent long term effects.

If we can be of help on this or any other topic please don't hesitate to call.

Jim Bowen Technical Director