4.7.3.3 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
(DT).
This function contains a blocking functionality. It is possible to block the function outputs,
reset timer or the function itself, if desired.
4.7.3.4 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.
I_A
I_C
I_B
Blocking
logic
BLOCK
TRIP
PICKUP
t
Timer
Under
impedance
detection
Impedance
calculation
V_A_AB
V_C_CA
V_B_BC
GUID-572750B7-5217-44AE-BEA1-380D25531343 V1 EN
Figure 351: 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.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 707
Technical Manual
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