Simulation of Numerical Distance Relays

Utility engineers and consultants use relay models to select the relay types suited for a particular application, and to analyze the performance of relays that appear to either operate incorrectly or fail to operate on the occurrence of a fault. Instead of using actual prototypes, manufacturers use relay model designing to expedite and economize the process of developing new relays. Electric power utilities use computer-based relay models to confirm how the relay would perform during systems disturbances and normal operating conditions and to make the necessary corrective adjustment on the relay settings. The software models could be used for training young and inexperienced engineers and technicians. Researchers use relay model to investigate and improve protection design and algorithms. However, simulating numerical relays to choose appropriate settings for the steady state operation of over current relays and distance relays is presently the most familiar use of relay models (McLaren et al., 2001) Numerical relay models can be divided into two categories. The models of the first category consider only the fundamental frequency components of voltages and currents. Phasorbased models were the first to be widely used to design and apply relays. The models of the second category take into consideration the high frequency and decaying DC components of voltages and currents in addition to the fundamental frequency components. These models are called transient models of relays. (McLaren et al., 2001) The goal of this chapter is to explain the building process of MATLAB model of a distance relay and validating the relay behavior when the input data that describes the voltage and current signals at the relay location is generated by simulation of the power network using EMTP–ATP. Voltage and current signals during faults are severely distorted; this is why EMTP is used as a power simulator during faults. EMTP would present voltage and current signals during fault with their dc decaying components and high frequency oscillations. However the model was validated by a similar input data generated by the simulation of the power network using MATLAB. The validation process extended to include the cases where the measured impedance is changed due to a change of fault location, due to an existence of resistive faults or due to an existence of more than one in-feed. The chapter began by introducing the principle of operation of distance relays and reviews the functionality of each of the internal modules of numerical relay such as, analog antialiasing filtering module, analog-to-digital conversion module, and phasor estimation algorithm. 9