Occasionally, the transient overvoltage resulting from circuit breaker (CB) switching can be hazardous to the electrical insulation in power systems. The magnitude of such overvoltages depends, to a great extent, on the phase angle at which the switching takes place. The phase angle of conventional switching is random for each operation, which means the resulting overvoltage may be quite high. This is especially true when the CB is closing into a large capacitance (such as a shunt capacitor bank or a no-load long transmission line) or when breaking a large reactance (such as a shunt reactor or a no-load transformer).

In such cases, CB operations most likely will result in high amplitude overvoltages dangerous to the insulation of power apparatus. As the switching overvoltage level is essential to insulation coordination, especially for the extra-high-voltage (EHV) power systems, Chinese utility companies pay great attention to the techniques for suppressing the switching overvoltage in power systems.

Conventional Measures

In most Chinese power systems rated 220 kV and below, no special measure is adopted to suppress switching overvoltages; therefore, the switching overvoltage level cannot be reduced to below 2.5 per unit. China now has two voltage ratings for EHV power systems, 330 kV and 500 kV (330 kV is used only in Northwest China, where the 750-kV system soon will be constructed). The circuit breaker used in EHV systems is generally equipped with a closing resistor to reduce the switching overvoltage to 2.3 per unit for 330-kV systems and 2 per unit for 500-kV systems. Because CBs equipped with closing resistors are bigger, more expensive and less reliable, many Chinese utility companies are reluctant to use them.

Phase-Controlled Switching

The technique of phase-controlled CB switching was first developed in the mid-1970s. Theoretically, it is ideal because it enables the CB to switch at a selected phase angle to eliminate the switching transient or to limit its magnitude to a low value. Phase-controlled switching has been studied in China for more than a decade, but there was no application in China until 1998. In 1998, the first unit of a phase-controlled switching device was installed in a 500-kV system for switching a 350 km (217 miles) no-load transmission line instead of using the CB closing resistor. That phase-controlled unit was incorporated with the operating system of the 500-kV gas-insulated switchgear (GIS).

Site Testing Result

Site testing of phase-controlled CB switching was carried out in a 500/220/66-kV substation in the city of Changchun in November 2000. The shunt reactor to be switched off was installed in the 66-kV bay of the substation. Each CB bay was field tested at least 10 times, and the same tests were performed on the capacitor bank bays. Test results showed that the switching overvoltage could be dramatically suppressed compared to using the conventional CB. When switching by the conventional CB (without the phase-controlled unit), a maximum of 2.5 per unit overvoltage was recorded. By using the phase-controlled unit, the site test showed that the overvoltage can be reduced to lower than 1.3 per unit.

Operation Performance

Each of the eight phase-controlled switching units used in China has shown satisfactory performance. One unit is incorporated with GIS for switching a 500-kV long transmission line (350 km). Five units are used with SF₆ CBs for switching 66-kV shunt reactors (breaking capacity of 60 MVA each). Two units are used with the SF₆ CBs for switching 66-kV shunt capacitor banks (breaking capacity of 60 MVA each).

After several hundred operations with no reported failures, the use of phase-controlled switching CBs has proven to effectively suppress switching overvoltages.

Liuling Li received his BS and MS degrees from Xi'an Jiaotong University in 1985 and 1988, respectively. He has been working on design and testing of SF₆ circuit breakers and GIS for 15 years, and is currently also pursuing his Ph.D. at Xi'an Jiaotong University.
liuling.li@cn.abb.com

Yuchang Qiu graduated from Jiaotong University in 1955. He is professor of electrical engineering at Xi'an Jiaotong University and senior consultant at the State Key Laboratory of Electrical Insulation for Power Equipment. His main research interests include high-voltage insulation, overvoltage protection, gas discharges and their applications. He is an IEEE Fellow.
ycqiu@xjtu.edu.cn

Anjia Hou received his BS degree from Wuhan Hydro-Electrical Institute in 1991. He is a senior engineer for the Extra-High-Voltage Administration Bureau.
jlsi026@163.com