User Manual Chapter 1
GFK-2749A Jan 2020
PSM Module Description and Specifications 9
Frequency Measurement Notes
PSM preferentially chooses Grid 1 VA zero crossing as the reference for signal sampling.
In the absence of Grid 1 VA, PSM uses Grid 2 VA zero crossing.
In the absence of either VA signal, grid frequencies will be reported as 0Hz, and the
remaining inputs will be sampled at a rate of 128 samples per 25ms. New data will be
updated at 25ms intervals. This may create sporadic looking individual readings, but a 10-
cycle average will be fairly steady and accurate.
When the difference in frequency between Grid 1 and Grid 2 is large, the Grid 2 RMS
readings will vary from cycle to cycle, however a 10-cycle average will be fairly steady and
accurate.
1.9 ANSI Functions Supported
The Grid 1 and Grid 2 functions are similar, but with differences in implementation. Grid 1 is
assumed to be the most stable power source, such a utility. Grid 2 is assumed to be a less
stable power source, such as a back-up generator. Either or both grids can be connected for
load monitoring, such as motors, lighting, HVAC or server room power.
1.9.1 ANSI Functions for GRID 1 (Public Grid/Load) Protection
ANSI 27 – Under-voltage Protection
Under-voltage Protection monitors voltages to protect motors against voltage sags. The
PSM monitors the phase-to-phase voltage in a delta power system, or phase-to-neutral
voltage in a wye power system, as a percentage of the Nominal Voltage parameter. An alarm
will be triggered if any voltage falls below the specified UNDER_VOLT_THR value for a time
period longer than UNDER_VOLT_DELAY. The alarm is cleared when the grid voltage rises
above the UNDER_VOLT_THR value for a time period longer than UNDER_VOLT_DELAY.
The UNDER_VOLT_THR is a percentage of the nominal voltage.
ANSI 32 – Reverse Power Protection
Reverse Power Protection provides protection from excessive reverse active power
produced by motors running as generators (supplying power). An alarm will be triggered if
a circuit’s active power exceeds the REV_PWR_THR for a time period longer than
REV_PWR_DELAY. The Alarm will be cleared when the active power exceeds the
REV_PWR_THR for a time period longer than REV_PWR_DELAY. The REV_PWR_THR is a
percentage of the nominal power, which in a 3-phase system, is three times the nominal
voltage multiplied by the nominal current.
ANSI 47 – Voltage Phase Sequence Protection
This function monitors the sequence of the phase A, B and C voltages. Detection of a wrong
sequence (A, C, B for example) triggers an alarm.
An entire cycle of waveforms is required to make this Phase Sequence determination. There
is no user settable delay parameter, so the moment a Phase Sequence error is detected, the
alarm is set. In both systems (Delta and WYE), the signal at the VA terminal is expected to
lead the signal at the VB terminal by 120 degrees. The signal at the VB terminal is expected
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