Analyzing high impedance differential protection scheme

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Prikler László
Department of Electric Power Engineering

Differential protections are world – widely used tools to protect electrical equipments (eg. bus station, transformer) on distributed networks. These protections use the Nodal Law to operate during faults inside and don’t operate faults outside the protected objects.

The problem with this protection happens when the core of current transformer saturates during faults outside the protected object. This means the differential protection will operate needlessly and it causes a wrong detection.

In order to avoid transformer saturation, a new solution started to spread: if the relay resistance is increased, the effect of transformer saturation can be eliminated. That’s why it is called ‘high – impedance differential protection’.

We need to consider that transient fault current occurs. These transient behaviors have an effect on the level of saturation and this has a significant influence on setting the relay resistance. The problem is that we don’t understand the transient behavior of this protection; furthermore, we cannot give dimensioning viewpoints on how to set the current transformer knee point, the relay resistance and the voltage – dependent resistor correctly. So in this thesis I make an attempt to model the high – impedance differential protection scheme. I try to understand the transient behavior and how it affects the protection considerations.

I use a transient simulation program (EMPT ATPDraw) to model the high – impedance differential protection and I observe the protection behavior from these simulation results. During the simulations I study the common influences of setting the current transformer oversize factor, the relay impedance and voltage – dependent resistor parameters knowing the parameters of fault current. I recommend some dimensioning viewpoints from these tests that will hopefully help users to set the elements of high – impedance differential protection. I also investigate the setting levels of using current relay or voltage relay in the protection. The protection algorithm (in this paper it is RMS measure) can also work on different saturation levels, so I will analyze the transient behavior with different transformer knee points.


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