Transformer Differential Protection Scheme

Introduction to Differential Protection

Differential protection is a unit-type protection for a specified zone or piece of equipment. It is based on the fact that it is only in the case of faults internal to the zone that the differential current (difference between input and output currents) will be high. However, the differential current can sometimes be substantial even without an internal fault.

This is due to certain characteristics of current transformers (different saturation levels, nonlinearities) measuring the input and output currents, and of the power transformer being protected.

With the exception of the inrush and overexcitation currents, most of the other problems, can be solved by means of the percent differential relay, which adds to the normal differential relay two restraining coils fed by the zone-through current, by proper choice of the resulting percent differential characteristic, and by proper connection of the current transformers on each side of the power transformer.

Percentage restraint differential protective relays have been in service for many years.

Figure 1 shows a typical differential relay connection diagram. Differential elements compare an operating current with a restraining current. The operating current (also called differential current), I_d, can be obtained from the phasor sum of the currents entering the protected element:

Figure 1 – Simple diagram connection for differential power transformer protection

I_d is proportional to the fault current for internal faults and approaches zero for any other operating (ideal) conditions.

There are different alternatives for obtaining the restraining current, I_RT. The most common ones include the following:

Where k is a compensation factor, usually taken as 1 or 0,5.

The differential relay generates a tripping signal if the differential current, I_d, is greater than a percentage of the restraining current, I_RT :

Differential relays perform well for external faults, as long as the CTs reproduce the primary currents correctly. When one of the CTs saturates, or if both CTs saturate at different levels, false operating current appears in the differential relay and could cause relay male-operation.

Some differential relays use the harmonics caused by CT saturation for added restraint and to avoid male-operations. In addition, the slope characteristic of the percentage differential relay provides further security for
external faults with CT saturation. A variable-percentage or dual-slope characteristic, further increases relay security for heavy CT saturation.

Cause Of False Differential Current

Specific events can induce a significant differential current to flow in the absence of a fault, and these differential currents are typically sufficient to trigger a percentage differential relay to trip. In such circumstances, the differential protection should refrain from disconnecting the system, as it does not constitute an intrinsic malfunction of the transformer.

Such events may result from the non-linearities within the transformer core. The following situations are examined below.

Inrush currents

The magnetizing inrush current in transformers is caused by any sudden alteration in the magnetizing voltage. While typically seen as a consequence of powering up a transformer, magnetizing inrush may also be induced by:

1. Occurrence of an external fault,
2. Voltage recovery after clearing an external fault,
3. Change of the character of a fault (for example when a phase-to-ground fault evolves into a phase-to-phase-to-ground fault).
4. Out-of-phase synchronizing of a connected generator.

Moreover, from the perspective of transformer longevity, tripping during inrush conditions is highly undesirable, as interrupting a purely inductive current produces significant overvoltage that can compromise transformer insulation and potentially lead to internal faults.