734 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 25, NO. 2, APRIL 2010 Power-Frequency Voltage Withstand Characteristics of Insulations of Substation Secondary Systems Jinliang He, Fellow, IEEE, Zhanqing Yu, Rong Zeng, Senior Member, IEEE, Bo Zhang, Shuiming Chen, Senior Member, IEEE, and Jun Hu Abstract—The insulation of secondary system in substation, including secondary cables and devices, have to endure the high ac power-frequency voltage during the short-circuit fault. The ac withstand characteristics of the secondary cables and devices are experimentally investigated and discussed in this paper. The insulation breakdown mechanism of relays can be classified into three modes: air gap breakdown, creeping discharge, and solid insulation breakdown, that of the microcomputer protection device is solid state component damage. The ac power-frequency breakdown voltages of the microcomputer protection device, the small-size electromagnetic and solid-state relays all are as low as about 2 kV. The statistic method to obtain the withstand voltage of long communication cable from experimental data of short cables is proposed. The minimum 3-s withstand voltages of secondary system are 6.5 kV and 2 kV, respectively, this should be considered in substation design. What’s more, the volt-second characteristics of the power-frequency withstand voltage of the secondary cables and devices are analyzed and compared in detail, which would be important in the selection of secondary cables and devices. At last, the method how to guarantee high ac breakdown voltage of electromagnetic and solid-state relays and microcomputer protection device is suggested. Index Terms—Creeping discharge, gap breakdown, ground potential rise, microcomputer protection device, relay, secondary cable, secondary device, statistics, withstand characteristics. I. INTRODUCTION S UBSTATION control cables are used to transmit electrical signals and power with low voltage levels and low cur- rent levels, between apparatus and secondary devices, including protection, control and monitoring devices. Instrumentation ca- bles are used to transmit low-energy electrical signals with low voltage levels and relatively low current levels between elec- tronic equipment, such as monitors and analyzers, and control equipment for apparatus [1]. In the U.S., secondary cables are usually designed and constructed in accordance with NEMA and ICEA standards [2]–[4]. Low voltage cables and devices serving substations are often required to withstand the effects of fault-produced ground po- tential rise or induced voltages, or both. Some of these cables Manuscript received October 20, 2006; revised June 26, 2009. First pub- lished February 02, 2010; current version published March 24, 2010. Paper no. TPWRD-00673-2006. The authors are with the State Key Lab of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China (e-mail: hejl@tsinghua.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRD.2009.2037231 and devices are used for control and protective relaying pur- poses and may be called upon to perform critical operations at times of power system faults. This presents a major chal- lenge in the design and protection of the secondary system be- cause power system short-circuit faults, lightning and switching surges can result in the introduction of interfering voltages and currents into the secondary circuit at the very time when the cir- cuit is most urgently required to perform its function [5]. Any variation of the electromagnetic field induces voltages in nearby conductors [6]. Induced voltages in control cables can lead to false operations of the relay protection or even damage solid state components. Many papers had discussed the elec- tromagnetic interferences of the secondary circuits in substa- tion [1], [7]–[9]. Thorough measurements and computer simula- tions were conducted in order to determine magnitudes and fre- quencies of the transient voltages and currents in the secondary circuits [10]–[19]. C. M. Wiggins et al. [16] and EPRI report [36] quantified experimentally and theoretically the electromag- netic interference (EMI) levels on sensitive electronic equip- ment in air and gas insulated substations of different voltages generated due to lightning, faults, or switching operations, such as bus is energized or de-energized by an air-break switch or a circuit switcher. On the other hand, different measures were proposed to minimize the EMI in substation secondary system [1], [7]–[9], [20]–[24]. CIGRE Working Group 36.04 had com- prehensively summarized the EMI in substation and protection measures in the Guide on EMC in power plants and substa- tions in 1997 [25]. Ordinarily, all grounded structures within the substation, including the shielding layers of secondary ca- bles and the shells of secondary devices, can be connected to the common grounding grid of substation according to IEEE Std 80 [26], thereby the voltage applied on the insulation of secondary system would be minimized, in the meantime, the electromag- netic interference entering into the secondary system would be small. Several field and laboratory tests show that grounding the shield at both ends reduce the common mode voltage between 50 and 200 times [21], [22]. On the other hand, the secondary electronic devices are vul- nerable to damages due to a phase-to-ground power-frequency short-circuit fault with a large magnitude of power-frequency fault current. Incidences involving erroneous operation of relay circuits are known to occur under these conditions [1], or di- rectly breakdown the insulation of the secondary cables and de- vices. When a short-circuit fault takes place in the substation, a high power-frequency voltage is applied between the signal line or power line of secondary device and ground, as well as the voltage applied on the insulation layer between the core wire and the metal sheath of secondary cable. 0885-8977/$26.00 © 2010 IEEE Authorized licensed use limited to: Tsinghua University Library. Downloaded on March 28,2010 at 09:55:28 EDT from IEEE Xplore. Restrictions apply.