1670 IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 27, NO. 3, JULY 2012 Compensation of CVT Increased Error and Its Impacts on Distance Relays Mahdi Davarpanah, Majid Sanaye-Pasand, Senior Member, IEEE, and Firouz Badrkhani Ajaei Abstract—Increased ratio error of capacitor voltage trans- formers (CVTs) has been recently encountered in several sub- stations in Iran. The results of the performed diagnostic tests indicate that short-circuited capacitor elements, especially in the low-voltage capacitor, have increased the CVT error. In this paper, some simple and effective techniques are suggested to solve this problem. One of the proposed methods to compensate the increased error is utilization of an auxiliary voltage transformer (AVT), which does not require disassembling the CVT. The practical aspects of this method, its effects on the CVT transient response, and the influence of the AVT improper design on the performance of distance relay are scrutinized in this paper. Fur- thermore, the design requirements of the AVT to avoid distance relay malfunctions due to the AVT ferroresonance are discussed. Index Terms—Auxiliary voltage transformer (AVT), capacitor voltage transformer (CVT), distance relay, ferroresonance, tran- sient response. I. INTRODUCTION V ARIOUS problems have recently arisen in some of the high-voltage (HV) substations in Iran. These problems in- clude distance relay blocking due to the fuse failure unit oper- ation [1], failure to synchronize the transmission line with the grid, and unequal measured voltage magnitudes of the feeders connected to the same bus. Initial investigations indicated that the increased error of the capacitive voltage transformer (CVT) is the main reason for these problems. A CVT is comprised of a capacitive voltage divider (CVD), a compensating reactor (CR), an intermediate voltage transformer (IVT), and a ferroresonance suppression circuit (FSC) (Fig. 1). The CVD consists of an HV capacitor and a low-voltage (LV) capacitor , both consisting of some capacitor elements connected in series [2]. In recent years, some investigations have been reported on the CVT temporary error due to the transient voltage appearing at the CVT output terminal as a result of a nearby fault [3], [4]. However, few reports regarding the increased CVT steady- state error or the causes of CVT failure have been published. An increase in CVT error due to a change of the CR reactance and more important, change of the CVD capacitance is reported Manuscript received January 11, 2012; revised March 19, 2012; accepted April 22, 2012. Date of publication June 11, 2012; date of current version June 20, 2012. This work was supported by the University of Tehran under Grant 8101064-1-02. Paper no. TPWRD-00042-2012. The authors are with the Electrical and Computer Engineering School, College of Engineering, University of Tehran, Tehran 14395-515, Iran (e-mail: m.davarpanah@ut.ac.ir; msanaye@ut.ac.ir; f.badrkhani@gmail.com). 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.2012.2197422 Fig. 1. Circuit diagram of a typical CVT. by a CVT manufacturer [5]. Old capacitors were insulated by mineral oil-impregnated kraft paper. It is concluded in [5] that the increased ratio error was caused by the insulation material, which is prevented in modern CVTs by using polypropylene film. Although this insulation material is a proper substitute for kraft paper, investigations on ten impaired CVTs demonstrated that the polypropylene film was the major insulation for these CVT capacitor elements [6]. Consequently, the CVT increased error is not completely prevented even by using better insulation materials. A statistical assessment of CVT failures is accomplished for the Brazil grid and the steady-state increase of CVT ratio error is introduced as a serious problem of operating CVTs [7]. Fur- thermore, comprehensive in-situ diagnostic tests are presented in [6]. In this study, ten CVTs with the rated primary voltages of 230 or 400 kV, which have voltage ratio errors higher than 5%, are chosen for diagnostic studies of the increased ratio er- rors. Experimental analyses reveal that the excessive ratio error is mainly caused by the short-circuited capacitor elements of CVTs. In other words, the measured capacitances of , or both, for nine of the investigated CVTs are considerably dif- ferent from their rated values, taking into account their per- missible tolerances. Moreover, an auxiliary voltage transformer (AVT) is widely utilized to adjust the input voltage of synchro- nizing and directional earth fault (DEF) relays. The occurrence of ferroresonance due to saturation of the AVT connected to the CVT secondary side is introduced as a main reason for short circuits of capacitor elements. This phenomenon may only take place if an improper AVT is employed [6]. In this paper, several practical methods for compensating the CVT error are discussed. Besides replacing the entire CVD or just the defected capacitor elements, which require a consider- able amount of time and expense, two other methods are sug- gested. The first method is the utilization of a series capacitor at the end of the terminal. As the second method, an AVT is recommended to be connected to the CVT secondary side. To investigate this method, the transient response of a CVT, which 0885-8977/$31.00 © 2012 IEEE