Transformer X/R ratio - Effect on Instantaneous Overcurrent Setting for Transformer HV side Relay

This post describes the guidelines for choosing an instantaneous setting with respect to the X/R ratio for Transformer protection.

TRANSFORMER X/R RATIO – EFFECT ON INSTANTANEOUS OVER CURRENT SETTING FOR TRANSFORMER HIGH VOLTAGE SIDE RELAY

Transformer X/R ratio affects the Instantaneous over current (IOC) (ANSI code 50) setting of transformer high voltage side relay. This happens due to the asymmetrical nature of the fault current at the instant when the relay interrupts the fault current. Thus understanding the nature of asymmetrical fault current is essential.

ASYMMETRICAL FAULT CURRENT:

Refer to Transformer X/R ratio Video

Figure 1 shows a typical fault in an AC system. From the illustration, it can be seen that there is an initial DC current component (IDC). This initial DC component decays over time, eventually reaching zero. The addition of the DC component to the symmetrical short circuit current gives the asymmetrical fault current.

From the above equations, it can be noted that the DC current (IDC) component decays exponentially and its decay depends on the X/R ratio (Time constant) of the system. Hence the magnitude of the DC current component is based on two scenarios:

1. At what voltage angle, the fault is happening

With respect to voltage and current waveforms, since the power system is an RL circuit, it consists of both inductance and resistance. However, the power system is generally considered to be inductive in nature. Current in inductance cannot change instantaneously. When a fault occurs at a voltage angle of zero degrees, then the current is Maximum and the current can’t change instantaneously, so the DC component comes into existence. So the waveform will be offset for the first few cycles. If the fault occurs at a voltage angle of ninety degrees, then there won’t be a presence of a DC current component.

2. What is the X/R Ratio at the time of fault?

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