- 1-DIGITAL DIFFERENTIAL RELAYS FOR TRANSFORMER PROTECTION USING WALSH SERIES AND LEAST SQUARES ESTIMATORS

Two estimators have been developed, using Walsh series and Least Squares algorithms. The design philosophy and procedure have been presented. Current restraint concept has been used for inrush current recognition. Both methods have been tested using real data. The results are presented including illustrations, graphs and discussions. Index-terms: Differential Relay, Transformer Protection, Digital Signal Processing, Walsh Series, Least Squares, Pattern Recognition, Inrush Current. INTRODUCTION Power transformers are exposed to a wide variety of abnormal conditions and faults that may cause damages. They are usually protected by percentage current differential relays that use power frequency signal measurements [1-3]. There is a general consensus that digital relays form an increasing segment of electric power system protection base. A signal estimation algorithm is necessary for determining input waveform characteristics. Digital Signal Processing (DSP) algorithms are utilized for this purpose [4-9]. Many researchers have focused their studies on power transformer protection utilizing new theories and technologies. Kalman filter [10-11], block pulse functions [12] and RLS algorithm [13–16] have been applied to protection function in literature. This paper describes the design, computational aspects and experimental results of a microprocessor based differential relay for transformer protection. Two estimator algorithms: Walsh series and Least Squares have been implemented and compared. The application of sine and cosine data windows involves time consuming computations due to many multiplication operations. Walsh function data windows can only assume the values of +1 and/or –1. It significantly reduces the computational requirements. A combination of Walsh and Fourier series with some simplification is applied for phasor estimation. Also, Least Squares algorithm have been developed and applied to this estimation problem. Using real data input, it has been shown that both methods do their job very well. A comparison of the two algorithms has been presented in this work, too. A differential relay has been developed then. Transformer inrush current may cause some confusion for differential current recognition procedures. So, current restraint concept has been utilized using second harmonics for recognizing transformer inrush current. The designed relay can distinct inrush from internal short circuit currents perfectly. The performance of the proposed relay is tested for internal and external faults and inrush currents of transformer in playback mode, using actual recorded data. Effectiveness of the techniques is verified for three mentioned situations. Test and comparison of two algorithms shows that both operate for internal transformer faults. Besides, they do not operate for any other non-fault condition. i.e., the presented differential relay does its duty in a secure and dependable manner. The paper includes test-input waveforms and experimental results for each stage of relay, using illustrations, figures and discussions. TRANSFORMER PROTECTION It has long been recognized that differential protection can provide unit protection for two ended and multiple ended feeders and equipment without the problems associated with other forms of protection [17-18]. The fundamental principle of differential protection is that the sum of the currents entering a device through normal paths should be zero: Kirchhoff's Current Law (KCL). If the currents enter (or leave) through abnormal paths, namely fault paths, then the sum of the currents through normal paths will not be zero. This is the method of differential fault detection. This method is applied to various parts of power system, for example bus-bars and transformers. There are problems in transformer differential protection, caused by natural phenomena like as: inrush current, CT inaccuracy, CT saturation and over-excitation. These problems produce fault trips in transformer protection function [19-20]. Studies have shown that inrush currents contain harmonic components of the fundamental waveform. Conventional philosophy has been to utilize the second harmonic current to provide inhibited operation of differential relays because of its dominance. Hence, harmonic analysis is considered immensely in transformer protection area. [4, 20-23] DIFFERENTIAL RELAY IMPLEMENTATION Presented digital differential relaying scheme comprises of C I R E D 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003 UTI_Fereidunian_A1 Session 3 Paper No 72 2 these parts: current sensor (CT), filter, pre-processor, Data Acquisition System (DAS), estimator, and a decision maker. The latter one is the equivalent for popular word "protective relay", which performs the duty of making decision about the faulted or non-faulted situations. Its job is pattern recognition, or in other words, classification. FIGURE 1. Digital differential relay scheme. Current Sensor (CT) The saturation nature of magnetic cores of current transformers (CTs) causes distortion in the waveforms of power system current. Figure 2 shows such an effect. It may cause some false trips in relaying function. Efforts have been reported in literature on correction of CT errors [24-25]. In the presented differential relay, the essential part of the signal is derived by the estimator sub-system, and it is used for judgment on fault occurrence. So, the saturation phenomenon has less effect on relaying function. -3 -2 -1 0 1 2 3 -5 0 5 Fl ux [ V . S ] Magnetizing Curve 0 1 2 3 4 5 6 7 -1 0 1