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TIPS ENGINEER ZONE� POWER SYSTEM PROTECTION�TYPES OF FAULTS

TIPS ENGINEER ZONE

www.tipsengineerzone.in

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EARTH FAULT/GROUND FAULT

  • Earth fault protection also requires current transformers and senses an imbalance in a three-phase circuit. Normally the three phase currents are in balance, i.e. roughly equal in magnitude. If one or two phases become connected to earth via a low impedance path, their magnitudes will increase dramatically, as will current imbalance. If this imbalance exceeds a pre-determined value, a circuit breaker should operate. Restricted earth fault protection is a type of earth fault protection which looks for earth fault between two sets of current transformers .(hence restricted to that zone).

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DISTANCE (IMPEDANCE RELAY)�

  • Distance protection detects both voltage and current. A fault on a circuit will generally create a sag in the voltage level. If the ratio of voltage to current measured at the relay terminals, which equates to an impedance, lands within a predetermined level the circuit breaker will operate. This is useful for reasonably long lines, lines longer than 10 miles, because their operating characteristics are based on the line characteristics.

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DISTANCE (IMPEDANCE RELAY)

  • This means that when a fault appears on the line the impedance setting in the relay is compared to the apparent impedance of the line from the relay terminals to the fault. If the relay setting is determined to be below the apparent impedance it is determined that the fault is within the zone of protection. When the transmission line length is too short, less than 10 miles, distance protection becomes more difficult to coordinate. In these instances the best choice of protection is current differential protection

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OVERLOAD AND BACK-UP FOR DISTANCE �(OVER CURRENT)�

  • Overload protection requires a current transformer which simply measures the current in a circuit and compares it to the predetermined value. There are two types of overload protection: instantaneous over current (IOC) and time over current (TOC). Instantaneous over current requires that the current exceeds a predetermined level for the circuit breaker to operate. Time over current protection operates based on a current vs time curve. Based on this curve, if the measured current exceeds a given level for the preset amount of time, the circuit breaker or fuse will operate

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TIPS ENGINEER ZONE� POWER SYSTEM PROTECTION�TYPES OF FAULTS

TIPS ENGINEER ZONE

www.tipsengineerzone.in