Experimental testing of stand-alone digital relay for power transformers

Abstract

The differential protection scheme is one of the most widely used techniques in power system protection, especially for power transformers. This method extracts harmonic components from current signals via current transformers using various digital filtering algorithms to make protection decisions. Different researchers reported that they have found the best algorithms for their transformer differential protection schemes. However, since their test results are usually based on different equipment, it is difficult to compare the relative performance of the test results. For the protection of various types of transformers, both the hardware and software of the earlier protection units may have to be changed. -- Presented in this thesis is a stand-alone microprocessor based digital relay that has been designed and implemented. Only the software modification is required to accommodate the protection of different types of transformers. The hardware portion of the design consists of seven identical scaling circuits, a sixth order Chebyshev anti-aliasing filter, a sample-and-hold circuit, and a multiplexer which is connected to an analog-to-digital converter on the TMS32010 boards. The TMS32010 signal processor is used to compute the power transformer data. The digital relay is designed in a modular form such that the hardware part is a "black box" to the users. -- The software has been written in TMS assembler language which includes second and fifth harmonic restraints with ground fault plus internal and external protection. Five digital filtering algorithms are used to extract the harmonic components. These algorithms are: Discrete Fourier Transform, Walsh Functions Algorithm, Rectangular Transform Algorithm, Finite Impulse Response filtering, and Least Squares Algorithm. The digital relay was tested extensively in the laboratory using the five algorithms. By comparing the real time performance of these algorithms, Discrete Fourier Transform (DFT) was found to be the best among the algorithms presented. As a result, a transformer protection test set-up has been developed using DFT. This test set-up can be used for the protection of any kind of three phase transformer with minor changes on the relay software.