Biased Differential Protection for Transformers

The purpose of biased differential protection is to detect faults with high selectivity and sensitivity. This means that it can trip with no intentional time delay for faults within its zone of protection, the reason why differential protection is often applied on high value equipment such as transformers. Generally, differential protection is applied to transformers with capacity greater than or equal to 10MVA. However, in some applications, differential protection is applied on 5MVA transformers.

The operation of a typical differential protection is illustrated in figure 1. Differential protection is based on the Kirchhoff’s current law which states that the currents flowing into a node is equal to sum of the currents flowing out of that node. For normal operation and external faults, the difference between currents entering and leaving the zone of protection is zero. If the difference is not zero, then there could be a fault within the zone of protection. Here, the zone of protection is defined by the location of the current transformers (CTs).

Click here to read about how to connect CTs to relays.

Sources of Differential Currents

Differential protection application in transformers however allows a differential current during its normal operation. This is practically because of the magnetizing current present in the transformer core. Other sources of differential current which impacts the application of the differential protection include tap compensation mismatch (prevalent on electromechanical relays), transformer tap changer, relay measurement error, and CT error. Figure 2 shows the differential currents for normal operation and external faults with

1. CT error at nominal tap, and
2. CT error at transformer maximum tap.

As shown in the tables accompanying figure 2, differential current increases proportionally with through current. This presents a difficulty in setting the differential protection. A fixed pick-up would have to be set higher to avoid relay operation on excessive through-current but doing this would significantly decrease the sensitivity, rendering the protection inadequate. The concept of percentage biased differential comes into play wherein relay operation is based on differential current as a function of through current. In this way, pick-up setting is raised as through current increases.

Percentage Biased Differential Protection

The percentage biased characteristic operates on the ratio of operate-to-restraint current. The operate current is defined as the magnitude of the differential current in the zone of protection.

For the restraint current, several methods are defined which varies among relays. In most cases, the restraint current is

There are two (2) basic settings required to set-up the percentage biased differential protection. These are the minimum pick-up and the slope. The minimum pick-up covers the transformer magnetizing current, typically 1-4% of the transformer rating. A pick-up of 0.2 to 0.3 times the TAP is generally recommended. The slope setting is defined by determining the potential sources of differential current. This could come from the tap variation (10%), CT error (3% at nominal to 10% at 20 times nominal), and relay measurement error (5%). Figure 3 shows these errors stacked to together to form a single slope characteristic.

To provide security for higher levels of through-current where CT errors reach up to 10%, a second slope is introduced forming a dual slope characteristic. This is shown in figure 4.

A slope safety margin is set by applying a negative aggregate error (including excitation) on either the current entering or leaving the zone of protection. Thus, for an aggregate error of 22% applied on the current entering into the zone, the slope is computed as follows,

From the assumption of subtractive error, the general formula to determine minimum slope setting is,

The safety margin offers the security needed on excessive through-currents where CT saturation is mostly likely.

References:

M. Thompson, “Percentage Restrained Differential, Percentage of What?”, proceedings of the 64th Annual Conference for Protective Relay Engineers, College Station, TX, April 2011.

G. Pradeep Kumar, “Principles of Transformer Protection”, proceedings of Power System Protection Training, Visayan Electric Company, Cebu City, Philippines, December 2016.

J. Blackburn, T. Domin, “Protective Relaying Principles and Application, 4th ed.”, CRC Press, Boca Raton, FL, 2014.