IEEE TRANSACTIONS ON POWER DELIVERY, VOL. 24, NO. 3, JULY 2009 1037 Fast and Reliable CT Saturation Detection Using a Combined Method Hamed Dashti, Majid Sanaye Pasand, Senior Member, IEEE, and Mahdi Davarpanah Abstract—Busbar differential relays may operate incorrectly for external faults due to current-transformer (CT) saturation. In this paper, two different techniques are suggested to detect CT satu- ration. The first proposed technique is based on the fact that the waveshape of the CT secondary current changes significantly at the instant of the saturation. Based on this feature, an algorithm which uses the second derivative of CT output current is devel- oped. It uses an adaptive threshold to detect CT saturation fast. The second proposed technique utilizes two criteria based on the zero crossing principle. Utilizing the combination of the second derivative and zero crossing techniques results in a powerful and reliable scheme which is able to detect various CT saturation cases correctly and quite fast. The proposed method is able to detect even small CT saturation events. A real 400-kV busbar is simu- lated using PSCAD/EMTDC for evaluating the performance of the proposed algorithm. The obtained results demonstrate precise op- eration of the proposed algorithm in different conditions. Index Terms—Busbar differential protection, current-trans- former (CT) saturation detection, second derivative, zero crossing. I. INTRODUCTION W HEN a short-circuit fault occurs on a transmission line close to a busbar with high short-circuit capacity, all of the currents provided to the fault are passed through the cur- rent transformer (CT) connected to this line. If this CT satu- rates, fictitious differential current appears in the busbar differ- ential relay. The differential relay can declare an internal fault condition and misoperate incorrectly [1]. To avoid this mis- take, low-impedance busbar differential relays usually utilize a CT saturation detection unit to avoid false tripping for external faults [2]. A method for detecting the CT saturation onset is discussed in [3]. It relies on the abrupt change in the current when the CT saturates. However, this method may operate incorrectly when the current does not change instantly after inception of satura- tion. Moreover, the antialiasing filter will cushion the collapse of the secondary current. Another approach proposed in [4] requires a function for cal- culating the core flux from the secondary current and compen- sates it. This method is developed based on the given CT param- Manuscript received February 14, 2008; revised December 01, 2008. Current version published June 24, 2009. Paper no. TPWRD-00103-2008. M. Sanaye-Pasand is with the Control and Intelligent Processing Center of Excellence, School of Electrical and Computer Engineering, University of Tehran, Tehran 14395/515, Iran (e-mail: msanaye@ut.ac.ir). H. Dashti and M. Davarpanah are with the School of Electrical and Com- puter Engineering, University of Tehran, Tehran 14395/515, Iran (e-mail: h.dashti@ece.ut.ac.ir; davarpanah_ma@yahoo.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.2009.2022666 eters to approximately represent the nonlinear core characteris- tics of a specific model of the CT. Furthermore, this method is based on the assumption that the remanent flux at the beginning of the calculation is zero. Another CT saturation detection method was proposed based on evaluating the mean of error and the mean and variance of current amplitude [5]. The error is derived on the assumption that if a current is a perfect sinusoid, the summation of the cur- rent and its second-order derivative should be zero. Reference [6] proposed an impedance-based CT satura- tion detection algorithm for busbar differential protection. Impedance calculation is based on a first-order differential equation, or RL model of a transmission line, with three con- secutive signals of voltage and current which are measured at the relay location. A symmetrical component-based method for CT saturation detection is suggested in [7]. This method utilizes the zero-se- quence differential current gradient with respect to the bias cur- rent to detect saturation in a numerical current differential feeder protection relay. Each of the aforementioned proposed algorithms includes some shortcomings in detecting CT saturation. Some of these algorithms might maloperate for short-circuit currents which include a high decay dc component. Some others should be blocked for a fraction of a cycle after fault inception. The performance of some of the algorithms is not evaluated for the remanence flux of CT core and some others require a voltage signal in addition to the current signal to detect CT saturation. Furthermore, a CT compensation algorithm is disclosed in [8], which is capable of converting from a current waveform distorted by CT saturation to a compensated current waveform. This method provides accurate results, independent of CT pa- rameters/characteristics and secondary burden. However, it re- quires about one-and-a-half cycles after fault initiation to calcu- late compensated current. This means accurate current measure- ments can be expected about 25 ms after fault initiation. There- fore, this method cannot fulfill requirements of busbar protec- tion relays. One other suggested algorithm is the CT saturation detection based on the second or third derivative of the CT output current. This method is implemented by using a preset threshold [9]. In this paper, the second derivative technique is improved by using an adaptive threshold to detect CT saturation fast in different conditions. Moreover, another new algorithm based on the zero crossing technique is proposed. Using tow criteria, the proposed technique is able to detect CT saturation in various cases. Utilizing the combination of the second derivative and zero crossing techniques results in a powerful and reliable scheme which is able to detect various CT saturation cases correctly and 0885-8977/$25.00 © 2009 IEEE