7SR18 Applications Guide
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9.3.2 Close Circuit Supervision Connections
Figure 9.3.2-1 Close Circuit Supervision Scheme
Close circuit supervision with the circuit breaker Open or Closed.
9.4 Inrush Detector (81HBL2)
This element detects the presence of high levels of 2nd Harmonic current which is indicative of transformer Inrush
current at switch-on. These currents may be above the operate level of the over-current elements for a short
duration and it is important that the relay does not issue an incorrect trip command for this transient network
condition.
If a magnetic inrush condition is detected operation of the over-current elements can be blocked.
Calculation of the magnetising inrush current level is complex. However a ratio of 20% 2
nd
Harmonic to
Fundamental current will meet most applications without compromising the integrity of the Over-current
protection.
There are 3 methods of detection and blocking during the passage of magnetising inrush current.
Blocking only occurs in those phases where Inrush is detected.
Large, Single Phase Transformers – Auto-transformers.
All 3-phases are blocked if Inrush is detected in any phase.
Traditional application for most Transformers but can give delayed operation for Switch-
on to Earth-fault conditions.
Composite 2nd Harmonic content derived for all 3-phases and then compared to
Fundamental current for each individual phase.
Provides good compromise between Inrush stability and fast fault detection.
Table 9.4-1 Magnetic Inrush Bias
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