ELEC5204 Power System Analysis Experiment 3 – Distance, Bus and Transformer Protection 3.1 Objectives The objectives of this laboratory exercise are: Study distance protection for transmission line. Summarize common bus faults, Identify methods for bus protection, Summarize common transformer faults, Identify methods for transformer protection, Discuss issues with the protection of transformers in power systems, Present combined bus and transformer protection methods. 3.2 Distance Protection Transmission protection using over-current relays is difficult and may not be possible, even if the over-current relays are directional. In such cases a more selective relay is required. The distance relay is often the preferred solution to this problem. It meets the requirements of reliability and speed needed to protect these circuits, and for these reasons is extensively used on power system networks. Distance protection is a non-unit type of protection and has the ability to discriminate between faults occurring in different parts of the system, depending on the impedance measured. Essentially, this involves comparing the fault current, as seen by the relay, against the voltage at the relay location to determine the impedance down the line to the fault. The main advantage of using a distance relay is that its zone of protection depends on the impedance of the protected line that is a constant virtually independent of the magnitudes of the voltage and current. Thus, the distance relay has a fixed reach, in contrast to overcurrent units where the reach varies depending on system conditions. 3.2.1 Distance relays Distance relays can have characteristics as shown in Fig. 1. Fig. 1a shows a Zone-2 impedance characteristic with a balance point, that is, a circle, centered at the origin of an R-X diagram. If the circuit impedance falls within the circle that describes the balance point for the relay, the relay actuates and closes its output contact. If the circuit impedance is outside the circle, the relay does not operate. Such impedance relays are not directional. Directional supervision should be added to instantaneous impedance relays. Fig. 1b shows a Zone-2 mho (offset impedance) characteristic with a balance point, that is, a circle, centered on the “line angle”. The operation is similar to the impedance relay. Mho relays are inherently directional. When applying electromechanical distance relays, the diameter of the relay balance point is located at 60 or 75 , as the angle of the diameter is not adjustable. Fig. 1c shows a Zone-2 reactance relay characteristic with a balance point that is a straight line above, and parallel to, the R-axis. If the circuit impedance falls below the line that describes the balance point for the relay, the relay operates The University of Sydney Page 1
ELEC5204 Power System Analysis otherwise if the circuit impedance is above the line, the relay does not operate. Reactance relays are usually used in combination with impedance relays. Different relay characteristics can be configured based on the protection
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