Differential Protection for Feeders

Differential Protection for Feeders in Electrical Power Networks

Differential protection is an essential component in the design and operation of electrical power systems, particularly in the protection of feeders. Feeders are transmission and distribution lines that deliver electrical power from a substation to an end-user or to a localized area. The primary role of differential protection for feeders is to detect and isolate faults that occur within the protected zone, ensuring the safe and reliable operation of the electrical network.

Differential protection operates on the principle of comparing the currents entering and leaving the protected zone. Under normal operating conditions, the sum of the currents entering the zone should be equal to the sum of the currents leaving the zone. However, in the presence of a fault, there is an imbalance in these currents due to the fault current flowing into the protected zone. This imbalance is detected by the differential relay, which initiates a trip signal to isolate the faulted section from the rest of the network.

To implement differential protection for feeders, several factors need to be considered. One key factor is the selection of appropriate current transformers (CTs). CTs are responsible for converting high currents flowing through the feeders into proportional low currents that can be safely measured by the differential relay. It is essential to ensure that the CTs are properly sized and combined with suitable burden resistors to achieve accurate and reliable current measurements.

Another critical aspect of differential protection for feeders is the setting calculation of the differential relay. The relay must be set to discriminate between faults inside the protected zone and faults outside the zone. Discrimination is vital to prevent unnecessary tripping of the feeder during faults occurring outside the protected zone. The setting calculation involves determining the operating current and the restraining current thresholds, which define the sensitivity and selectivity of the differential protection scheme.

To illustrate the concept of differential protection for feeders, let’s consider an example. Suppose we have a feeder transmitting power at a line voltage of 220 kV and a line current of 300 A. The differential relay is designed to operate with a percentage differential characteristic and has a nominal operating current of 5 A.

To calculate the operating current threshold, we can use the formula:

$$I_{\text{op}} = I_{\text{line}} \times K_{\text{op}}$$

where:

$I_{\text{op}}$ is the operating current threshold, $I_{\text{line}}$ is the line current, $K_{\text{op}}$ is the operating current multiplier.

Assuming $K_{\text{op}}$ is set to 20% (0.20), the operating current threshold would be:

$$I_{\text{op}} = 300 \, \text{A} \times 0.20 = 60 \, \text{A}$$

Next, we need to calculate the restraining current threshold, which is typically set higher than the operating current to provide selective coordination within the power network. Assuming a restraining current multiplier of 140% (1.40), the restraining current threshold would be:

$$I_{\text{restr}} = I_{\text{op}} \times K_{\text{restr}}$$ $$I_{\text{restr}} = 60 \, \text{A} \times 1.40 = 84 \, \text{A}$$

In this example, we have determined the operating and restraining current thresholds for the differential relay. During normal operation, the sum of the currents entering the protected zone should be equal to the sum of the currents leaving the zone, with the differential relay in a balanced condition. Any fault occurring within the protected zone that causes an imbalance in these currents beyond the set thresholds will initiate a trip signal and isolate the faulted section from the rest of the network.

Differential protection schemes for feeders are essential to ensure the safe, reliable, and selective operation of electrical power systems. They play a vital role in the overall protection strategy, working in conjunction with other protection elements to detect and isolate faults, thereby minimizing damage to equipment and reducing downtime. International standards such as IEEE C37.2 and IEC 60255 provide guidance on the design and implementation of differential protection schemes, helping ensure consistent and reliable protection in power network transmission and distribution.