Functionality - ABB 615 series Technical Manual

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1MAC059074-MB A

615 series ANSI

Technical Manual

Functionality

The three-phase underimpedance protection 21G is generally applied as a backup

protection for generators and transformers against short circuit faults.

The protection is based on the origin-centric circular characteristics on the impedance

plane. The function calculates the impedance value from the voltage and current phasors.

If the impedance vector enters the origin-centric circle, the function produces the tripping

signal after a set delay. The operating time characteristics are according to definite time

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

reset 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 21G can be described with a module diagram. All the modules in the

diagram are explained in the next sections.

GUID-572750B7-5217-44AE-BEA1-380D25531343 V1 EN

Functional module diagram

Impedance calculation

This module calculates the impedance based on the selected voltages and currents. The

Impedance Meas mode and Phase Sel for Z Clc settings determine which voltages and

currents are to be used. If Impedance Meas mode is set to "1Phase-phase", the Phase Sel

for Z Clc setting is needed for determining which phase-phase voltages ("AB", "BC" and

"CA") and currents should be used for calculating the impedance.

Protection functions

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