Evaluating Polarizing Quantities For Quadrilateral Ground Distance Protection: Field Insights and Practical Guidelines

The reactance element characteristics of the quadrilateral distance function are defined using both the operating and polarizing quantities. While the operating quantity is fixed for a given reach of the reactance element, there is flexibility in choosing the polarizing quantity. The most commonly used polarizing quantities in the industry are negative-sequence, zero-sequence, and loop currents.

Traditionally, negative-sequence current has been the preferred choice for polarizing the reactance element because the negative-sequence network is typically more homogenous, and the negative-sequence current is less susceptible to mutual coupling in parallel line applications. With the introduction of inverter-based resources characterized by unreliable negative-sequence source impedance, the negative-sequence current polarization is no longer a preferred choice. As a result, attention is now shifting towards the use of zero-sequence or loop current for polarization.

In choosing zero-sequence current for polarization, utilities often have concerns about using this polarization for parallel line applications with heavy zero-sequence mutual coupling. This paper shows that zero-sequence current polarization is reliable in these applications. It is the zero-sequence voltage drop caused by mutual coupling, rather than the choice of polarizing current, that leads to under-reach or over-reach of the reactance element. The paper also demonstrates, through field events, instances where zero-sequence current is not an appropriate choice for polarization. For example, depending on the grounding method used for underground cables, zero-sequence current may not be a viable option for polarization. Through field events, the paper also illustrates scenarios where loop current polarization is beneficial, while emphasizing the dependability impact and considerations in applying it.

In summary, this paper explains why zero-sequence current is a viable choice as a polarizing quantity for the ground reactance element, even in parallel line applications. It offers clear criteria for selecting the appropriate polarizing quantity for quadrilateral distance elements. The conclusions are supported by field events that illustrate and validate the findings.