Modeling Metal Oxide Varistors (MOV) in Short Circuit Calculations

Introduction

A typical MOV(Metal Oxide Varistor)-protected series capacitor arrangement is shown in Figure 1. The MOV protection is connected directly in parallel with the series capacitor and performs its function by holding the maximum capacitor voltage within the designed protective level. MOV conduction normally occurs only during faults because the capacitor protective level is usually specified above the maximum voltages expected during overload or swing conditions. In normal operation, all current flows through the series capacitor and none through the MOV protection.

An MOV is a nonlinear variable resistance that has very high resistance for low voltages across the varistor, and then above the threshold voltage its resistance decreases rapidly. Figure 2 illustrates a typical VI characteristic of a 120 kV rms rated MOV. Notice that the unit conducts little current up to a voltage of about v3 x 120 = 208 kV and then, as the voltage across it increases further, the current increases very rapidly.

Solving a network containing a nonlinear impedance Zeq requires an iterative scheme. An initial value of current entering this equivalent impedance is estimated, based on the system condition, and then the network with the nonlinear impedance is solved using the values corresponding to this estimated current. This procedure is repeated until the estimated line current change between two consecutive iterations is within a desired tolerance. This procedure is described by the equation below where I is the MOV-protected series capacitor branch current, a is an accelerating factor, and k is the iteration number.

The ability to model Metal Oxide Varistor (MOV) protection for series compensated lines in short circuit calculations is now available in PSS®E Version 33.1. This MOV modeling will allow PSS®E users to evaluate the short circuit capability of breakers used in the switching of series compensated lines and series capacitor banks for symmetrical and asymmetrical fault currents. PSS®E fault current calculation activities ASCC, IECS and SCMU model MOV protection.

The use of series capacitors as a solution to increase the power transfer capability of existing extra high voltage (EHV) transmission lines has been tried extensively in recent years. The general idea is to reduce the inductive reactance of the line, thereby increasing the electrical length and surge impedance loading of a series compensated transmission line.

An important design issue of series capacitors in transmission lines is their overvoltage protection. A widely used protection scheme based on metal (zinc) oxide varistors (MOV) has proven to be very effective and reliable. MOVs present a nonlinear resistance characteristic that is approximated by fundamental frequency nonlinear impedance during short circuit calculations. The series capacitors and MOV protection are modeled together as equivalent branch impedance, Zeq= RCeq –j XCeq, controlled by the branch current I flowing into the MOV-protected(Metal Oxide Varistor) series capacitor branch.