Incident Energy On The Low Voltage Main Breaker
The electromechanical relays usually had only one time-overcurrent and one overcurrent element to protect the transformer and coordinate with the inrush current. The overcurrent element is set to higher than the through-fault current so that it clears the transformer fault instantaneously and does not trip on the load side faults. This results in higher incident energy on the low-voltage main breaker due to higher fault clearing time. Figure 1 below shows the traditional way to set up line side protection with one time-overcurrent and one overcurrent element.
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Based on the fault current on the load side of the transformer, the time-overcurrent will take approximately 656 msec to clear the fault.
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The new digital relays have multiple overcurrent elements, which helps to improve coordination. As shown in Figure 2 below, two overcurrent elements are being used to reduce the fault-clearing time on the low-voltage side of the transformer. The first overcurrent element is set lower than the load side fault current with a 200 msec delay to coordinate with the low voltage breaker and transformer inrush current. The fault clearing time during a load side fault has been reduced to 200 msec from 656 msec. The second overcurrent is set higher than the through-fault current and lower than the fault current on the line side of the transformer, so it trips instantaneously on the transformer line side fault and does not trip on the load side of the fault.
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The incident energy has been reduced from 71 cal/cm2 to 31 cal/cm2 by adjusting the settings.
