ABB 615 Series ANSI, Technical Manual

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Section 4
1MAC050144-MB C
Protection functions
344 615 series ANSI
Technical Manual
are not present in the differential current, it can be suspected that there is a fault in the
transformer. This second harmonic deblocking method is used, for example, in the case of
switch on to a fault. This feature can also be enabled and disabled using the Harmonic
deblock 2.H parameter.
Fifth harmonic blocking
The inhibition of 87T operation in the situations of overexcitation is based on the ratio of
the fifth harmonic and the fundamental component of the differential current (Id5f/Id1f).
The ratio is calculated separately for each phase without weighting. If the ratio exceeds the
setting value of Pickup value 5.H and if blocking is enabled through the Restraint mode
parameter, the operation of the restrained differential in the concerned phase is blocked.
The fifth harmonic ratios I_5H_RAT_x are given in monitored data.
At dangerous levels of overvoltage, which can cause damage to the transformer, the
blocking can be automatically eliminated. If the ratio of the fifth harmonic and the
fundamental component of the differential current exceeds the Stop value 5.H parameter,
the blocking removal is enabled. The enabling and disabling of deblocking feature is also
done through the Harmonic deblock 5.H parameter.
Figure 164: The limits and operation of the fifth harmonic blocking when both
blocking and deblocking features are enabled using the Harmonic
deblock 5.H control parameter.
The fifth harmonic blocking has a hysteresis to avoid rapid fluctuation between "TRUE"
and "FALSE". The blocking also has a counter, which counts the required consecutive
fulfillments of the condition. When the condition is not fulfilled, the counter is decreased
(if >0).
Also the fifth harmonic deblocking has a hysteresis and a counter which counts the
required consecutive fulfillments of the condition. When the condition is not fulfilled, the
counter is decreased (if >0).