Operation Principle - ABB 615 series Technical Manual

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Protection functions

By default, 51NHA is used as a standalone mode. Substation-wide application can be

achieved using horizontal communication where the detection of a faulty feeder is done

by comparing the harmonics ground-fault current measurements.

The function picks up when the harmonics content of the ground-fault current exceeds the

set limit. The operation time characteristic is either definite time (DT) or inverse definite

minimum time (IDMT). If the horizontal communication is used for the exchange of

current values between the protection relays, the function operates according to the DT

characteristic.

The function contains a blocking functionality. It is possible to block function outputs,

timer or the function itself, if desired.

Operation principle

The function can be enabled and disabled with the Operation setting. The corresponding

parameter values are "Enable" and "Disable".

The operation of 51NHA can be described using a module diagram. All the modules in the

diagram are explained in the next sections.

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Functional module diagram

Harmonics calculation

This module feeds the measured residual current to the high-pass filter, where the

frequency range is limited to start from two times the fundamental frequency of the

network (for example, in a 50 Hz network the cutoff frequency is 100 Hz), that is,

summing the harmonic components of the network from the second harmonic. The output

of the filter, later referred to as the harmonics current, is fed to the Level detector and

Current comparison modules.

1MAC059074-MB A

615 series ANSI

Technical Manual

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