Compensating CT Ratio Mismatch | Differential Protection

Compensating CT ratio mismatch is a very important step in configuring a differential protection. This may be achieved by specifically selecting the CT ratios or in modern relaying, configured through the relay software.

In the previous article, the concept of zero sequence currents was discussed including how they affect the performance of transformer differential protection and how to compensate for them. Zero sequence compensation is a form of phase compensation. While zero sequence compensation is very important in differential protection, compensating CT ratio mismatch through magnitude compensation or commonly known as tap compensation is equally important.

When applying any kind of protection scheme, power system parameters are measured using instrument transformers. Current transformers (CTs) are used in transformer differential protection. These CTs are selected based on the amount of current they are expected to measure up to certain value of current, during fault conditions, in which they can measure without loss of accuracy. In their application to transformer differential protection, CTs are usually selected based on the full load rating of circuit. Therefore, it is normal that CTs on both sides of the power transformer have different CT ratio. By having different CT ratio, measured values in the relay will not be equal and will yield an I_OP not equal to zero.

Learn about CT and Relay Connection here!

Compensating CT ratio mismatch using TAP compensation is based on the concept of selecting CT ratio based on the power transformer turns ratio. Let I_P and I_S be the transformer primary current values at the transformer high and low side, respectively, I₁ and I₂ be the secondary currents measured by the relay, CTR₁ and CTR₂ the current transformer ratio, and N₁ and N₂, the transformer winding turns. To make I₁ equal to I₂ in all normal conditions, the ratio of CTR₁ to CTR₂ should be equal to the transformer turns ratio N₂/N₁. For delta connected CTs at the secondary side, CT₂, since

In most cases, selecting standard CTRs based on the required conditions is very difficult if not impossible without making any compromise.

Compensating CT ratio mismatch for transformer differential protection using numerical relays is quite straightforward. Relay measured currents are expressed to their primary values by multiplying CTR₁ and CTR₂ to I₁ and I₂, respectively. The primary values are then expressed to their per unit values based on the power transformer MVA and kV ratings. This process allows the current values used in the calculation of I_OP to be equal in magnitude during normal conditions.

To consider the CT connection in tap compensation, a constant is included in the tap equation as shown below,

Compensating CT ratio mismatch works with phase compensation to ensure the reliability of transformer differential protection.

While setting the relay is a straightforward process, it should be kept in mind that understanding the basic concepts are very vital in the practice of power system protection.

References:

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.

SEL-387A Instruction Manual. Available at SEL website.