CT Connection for Transformer Differential Protection

The previous discussion on the transformer vector group showed how transformer connection can cause phase displacement between the HV and LV winding. This phase displacement will result in the misoperation of differential protection if left uncompensated. In this article, CT connection for transformer differential protection will be discussed. The analysis of different CT connections will be presented in order to lay down the basis for the matrix equations used in modern numerical relays.

Phase Angle Compensation by CT Connection

Figure 1 shows a wye-delta transformer connection of vector group YNd11. 

Based on the previous discussion, the connection shown in figure 1 indicates that the LV winding leads the HV winding by 30°. Figure 2 shows a delta-wye transformer connection of vector group Dyn1. This connection indicates that the LV winding lags the HV winding by 30°.

 

This phase shift needs to be compensated to avoid the misoperation of the differential protection. Phase compensation is done by means of wiring current transformers either delta or wye, or through internal relay compensation in numerical relays. To do just that, CTs are wired such that the 30° phase shift is reversed. This is shown in figure 3.

It can be seen from the CT connection for transformer differential protection that wye-connected CTs are used in the delta side of the transformer while CTs on the wye side of the transformer is DAB-connected. Using DAB-connected CTs, secondary currents seen from the relay will lead the actual currents by 30°, thus offsetting the 30° phase displacement (LV lagging HV) introduced by the transformer vector group Dyn1.

To illustrate further, if we let the transformer wye-side currents be

we can calculate the delta side current I_A as shown below,

We can then calculate the secondary currents seen from the relay,

These values are adjusted using I_AW1 as a reference.

With the measured secondary currents displaced at 180°, the differential (operate) current, IOP, will yield a value equal to zero. See discussion on The Operate Quantity.

Derivation of Matrix Equation

If we investigate how we derived I_AW2, we can create an equation that relates the actual currents to the secondary currents seen from the relay.

Doing the same for I_BW2 and I_CW2, we can obtain a matrix that relates the actual currents to the secondary currents seen from the relay. The derived matrix represents how DAB-connected CTs compensate for a 30° phase angle displacement in a Dyn1 transformer.

Phase Angle Compensation in Numerical Relays

The process that has been discussed so far involves modifying physical CT connections by wiring current transformers in such a way as to compensate for the phase displacement. In modern, microprocessor-based relays, phase compensation is done numerically. Figure 5 shows the same transformer connection with wye CT connection for transformer differential protection on both sides.

To derive the secondary currents seen from the relay, we let

and solve for I_AW1 and I_AW2,

These values are adjusted using I_AW1 as a reference.

Without compensation, secondary currents seen from the relay are displaced by 150°. This would yield an I_OP ≠ 0 during normal operation. To do phase compensation numerically, we use the previously derived matrix to solve for I_AW2C, I_BW2C, and I_CW2C. Since our CTs are wye connected, we can see that I_AW2, I_BW2, and I_CW2 are equal to I_a, I_b, and I_c, respectively.

I_AWC2­ is computed as follows,

These values are adjusted using I_AW1 as a reference.

After the phase angle compensation using the derived matrix equation, the secondary currents seen from the relay are now displaced by an angle of 180° and will result in a differential (operate) current, I_OP, equal to zero.

Key Point Summary

• Different transformer connections (vector groups) result to phase angle displacement between HV and LV windings
• This phase angle displacement may result to a differential (operate) current if left uncompensated
• Phase angle compensation is conventionally done by connecting CTs either in wye or delta so as to compensate for the phase angle displacement
• Numerical relays provides flexibility by using matrix equations to compensate for angle displacement instead

Phase angle compensation is very important in implementing transformer differential protection. However, this is just one part of securing your protection so stay connected for the next topic.

Reference

SEL-387A Instruction Manual. Available in SEL, Inc. website.