Zero Sequence Current Compensation

In power distribution, perfectly balanced systems are generally considered only in ideal conditions. This is because connected loads in the system are mostly unbalanced, if not all. There are more single phase loads than three phase loads and power systems are very dynamic. Although planning engineers see to it to keep the degree of unbalance minimal, uncontrollable conditions such as faults can cause a significant unbalance to the system.

A Review on Symmetrical Components

Unlike balanced systems, unbalanced systems are analyzed using symmetrical components. Symmetrical components are n sets of balanced phasors for any n phase system. The phase displacement between the phasors in each set is determined by the following equation,

where,

Ɵ_k is the phase displacement between the phasors in each set

k is an integer from 1 to n (1, 2, …, n)

n is the number of phases in a system.

In a three phase system, there are three balanced phasors, n = 3. The three sets of phasors are the positive, negative, and zero sequence components. This is shown in figure 2.

Figure 2. Symmetrical Components

Among these set of phasors, the zero sequence is of particular interest. This is because they are in phase, which means that they can behave differently in wye and delta systems. In wye systems, zero sequence currents add up to 3I₀ to flow in the neutral line while in delta systems, zero sequence currents only appear on the phase windings and not on the line.

Figure 3. Zero Sequence Behavior in Delta or Wye

Zero Sequence in Differential Protection

In transformer differential protection, not accounting the zero sequence currents into consideration may result to misoperation. This is especially true for wye-delta, and delta-wye connected transformers on ground through-faults where zero sequence currents are present.

“Imagine if zero sequence currents are present at one side of the transformer winding while missing in the other. How will this affect the security of the protection?”

The recommended practice for zero sequence compensation is to compensate at the wye side through

physical DAB or DAC CT connection or
through numerical compensation in modern relays (wye connected CTs).

Consider the winding configuration in figure 4 (see previous article), by applying DAB compensation, we can effectively remove the zero sequence currents present in the wye-side winding of the transformer.

Figure 4. Zero Compensation in a DYn1 Transformer Configuration

This is zero sequence compensation.

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 https://selinc.com

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