5
3.3 NGR Monitoring
The Sigma 3 monitor relay also monitors the health of the NGR to ensure that it does not become open or short
circuit. If the Sigma 3 determines that the NGR is not healthy the NGR FLT LED illuminates, the NGR FLT form-C
contact energizes, and the trip relays energize.
The Sigma 3 monitor relay monitors the NGR using one of two methods: measurement mode and injection mode.
If the leakage current through the NGR is over 5% of the rated let-through current, the Sigma 3 operates in
measurement mode. In measurement mode, the leakage current through the NGR is measured by the ZSCS and
the voltage across the NGR is measured by the NGRS. The Sigma 3 uses these values to calculate the apparent
resistance of the NGR. If this calculated value is determined to be over 150% of the nominal value or less than 66%
of the nominal value, the Sigma 3 annunciates an NGR fault.
If the leakage current through the NGR is below 5% of the rated let-through current, the Sigma 3 operates in
injection mode. In injection mode, a high frequency signal is injected into the system through a band-pass
filter (NGRX). The Sigma 3 uses the current and voltage of the high frequency signal to calculate the apparent
resistance of the NGR. If this calculated value is determined to be over 150% of the nominal value or less than 66%
of the nominal value, the Sigma 3 annunciates an NGR fault.
3.4 Pulsing
Pulsing is a technique that is employed in high resistance grounded systems to locate a ground fault. The NGR
is divided into a non-pulsing section and a pulsing section that includes a contactor. The pulsing section can be
inserted into the system by the operation the contactor. This cycles the let-through current of the NGR between
different values (usually from 50% to 100% of the rated let-through current) and this signature can be used to trace
the location of the ground fault. Various pulse arrangements are shown in Figure 3.3 below.
Pulse signals are available from the Sigma 3 that can be used to create the signature by driving an NO solid state
relay that is used as the pulsing contactor. These are available at terminals 19, 20, and 21. If using pulse down
signals use terminals 19 and 21. If using pulse up signals use terminals 20 and 21. Terminals 19-21 are normally
energized with +12VDC and terminals 20-21 are normally de-energized. When pulsing is enabled by closing
terminals 25-26 or by enabling at the Sigma3-TDM touchscreen HMI, the pulse signals cycle at a frequency of 1Hz.
The cycle frequency can be changed using the Sigma3-TDM touchscreen HMI.
Figure 3.3: The creation of a pulse signature by cycling the let-through current under various arrangements
