engleski

PROTECTION RELAY FOR SHAFT CURRENT AND VOLTAGE

Electric power system is a backbone of contemporary technical civilization and today’s economy. Functioning is not possible without stable, uninterruptable supply of electric energy that is needed both in industry and in most areas of our private lives. According to its importance, electric power system has to be maintained, controlled and protected against possible faults and other abnormal phenomena. The most important parts of every electric power system are electrical rotating machines, especially those involved in the production of electrical energy. Constant increase of the reliability and availability of such machines is of utmost importance. Also, the current practice is to extend periods between planned outages of the machines. Any malfunctions and unnecessary and unplanned outages can cause extensive material damage. The way to alleviate or even prevent such events is implementation of various modes of condition monitoring. One of the problems that can occur in electrical machines are shaft currents and voltages. Due to the asymmetry of the magnetic field in the machine, a voltage in the shaft can be induced, which, depending on the type of machine, its size and load, varies in amplitude and frequency composition. Low impedance of the circuit consisting of shaft, bearing, oil film and other structural components, can cause shaft current flow which leads to bearing destruction. Continuous measurement and analysis of shaft currents and voltages can prevent major faults. Through detailed processing of the measured data the causes of occurrence of shaft currents and voltages can be identified. We have conducted a detailed research on the subject of shaft currents and voltages. Complex FEM (Finite Element Method) models have been developed and used to analyse a number of factors that can affect the occurrence of the shaft currents and voltages. Also, the effect of the harmonic content and signal form of the shaft currents to the occurrence of faults and defects has been researched. Through cooperation with the generator factory, laboratory models of the rotating electrical machines have been used to test the results obtained by FEM calculations and analyses. Based on these researches a system called "Shaft Current and Voltage Protection Relay" (SCVP) was developed, with all the acquired knowledge implemented in it. The main purpose of SCVP is to detect shaft currents and voltages that may damage the generator bearing, and thus prevent greater economic damages. It can be concluded that SCVP system allows improved asset management, by providing insight into machine fault conditions, which can cause major damage and thus major repair costs.

Hydro generator; shaft current; shaft voltage; relay

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