Differential Protection for Transformers

Differential Protection for Transformers

Differential protection is a vital element in the reliable operation of electrical power systems, particularly when it comes to protecting transformers. Transformers are essential components of electricity transmission and distribution networks, serving the purpose of efficiently transferring electrical energy between different voltage levels.

The primary role of differential protection for transformers is to quickly and accurately detect internal faults and initiate a trip command to isolate the faulty transformer from the system. Such faults can occur due to insulation failures, winding-to-winding short circuits, or transformer core faults. By detecting and isolating faults promptly, differential protection plays a crucial role in preventing further damage to the transformer and the power system.

To understand the principles of differential protection, one must first consider the operating principles of transformers. A transformer comprises two or more windings, each wound around a common magnetic core. When properly functioning, the current entering the primary winding creates a magnetic field that induces a voltage in the secondary winding. The voltage ratio between the primary and secondary windings determines the transformation ratio of the transformer.

Differential protection for transformers relies on the comparison of the currents entering and leaving the transformer windings. Ideally, the sum of currents entering the transformer windings should be equal to the sum of currents leaving the windings in an ideal system. If there is a fault within the transformer, such as a short circuit or an insulation failure, the current entering the winding will differ from the current leaving the winding. This discrepancy is the basis for differential protection.

In practical terms, differential protection for transformers involves the use of current transformers (CTs) to measure the currents entering and leaving the transformer windings. These CTs convert the high currents flowing through the windings into manageable magnitudes suitable for relay protection devices. Differential relays compare the currents using the following formula:

$$I_d = I_{\text{in}} - I_{\text{out}}$$

where (I_d) is the differential current, (I_{\text{in}}) is the current entering the transformer winding, and (I_{\text{out}}) is the current leaving the winding.

For a normally operating transformer, the differential current (I_d) will be close to zero or within a specified tolerance range. If a fault occurs within the transformer, the differential current (I_d) will deviate from this expected range, indicating the presence of an internal fault.

The setting of differential protection for transformers involves choosing a suitable current magnitude that corresponds to a fault condition, while also accounting for magnetizing inrush currents during transformer energization. Additionally, restraining functions are implemented to avoid undesired tripping due to external faults or transformer on-load tap changer operations.

To illustrate the application of differential protection for transformers, consider the following numerical example. Suppose a 50 MVA, 132/33 kV transformer is protected using a differential relay. The CT ratio on the high-voltage (HV) side is 400/1 A, and on the low-voltage (LV) side is 1000/1 A. The operating current on the HV side is 400 A, and on the LV side is 1000 A. The relay setting is chosen as 10% of the transformer rated current.

To determine the differential current setting for the relay:

1. Calculate the rated current for the HV and LV windings:

HV rated current:

$$I_{\text{HV,rated}} = \frac{50 \, \text{MVA}}{\sqrt{3} \times 132 \, \text{kV}} \approx 201.58 \, \text{A}$$

LV rated current:

$$I_{\text{LV,rated}} = \frac{50 \, \text{MVA}}{\sqrt{3} \times 33 \, \text{kV}} \approx 805.91 \, \text{A}$$

2. Determine the differential protection setting:

HV side:

$$I_{d,\text{HV,setting}} = 10\% \times I_{\text{HV,rated}} = 0.1 \times 201.58 \, \text{A} = 20.16 \, \text{A}$$

LV side:

$$I_{d,\text{LV,setting}} = 10\% \times I_{\text{LV,rated}} = 0.1 \times 805.91 \, \text{A} = 80.59 \, \text{A}$$

These calculated values determine the differential current settings for the relay. Any increased differential current beyond the set threshold will indicate a fault within the transformer, prompting the relay to initiate a trip command, isolating the faulty transformer.

It is worth mentioning that differential protection for transformers is standardized by several organizations, including the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE). The IEC standard IEC 60076-7 and the IEEE standard C37.91 provide