4.4
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Group Settings (SET Command)
Neutral CT ratio setting is used to convert the current input to primary, similar
to the phase CT ratio settings.
The remaining settings shown in Table 4.3 configure the optional relay voltage
inputs to correctly measure and scale the voltage signals. Set the phase PT
ratio (PTR) setting equal to the VT ratio (see Example 4.2 for sample
calculations).
EXAMPLE 4.2 Phase VT Ratio Setting Calculations
Consider a 13.8 kV Transformer application where 14400:120 V rated
voltage transformers (connected in open delta) are used.
Set PTR := 14400/120 := 120, VNOM := 13.8, and DELTA_Y := DELTA.
VNOM is the rated transformer line-to-line voltage in kV at the protected
winding determined by setting VIWDG. Consider the transformer tap when
calculating this setting. For example, a 345 kV transformer set on the 327.75
kV TAP would have VNOM := 327.75. VNOM is used by the Volts Per Hertz
Elements and the Loss-of-Potential (LOP) Protection.
When phase-to-phase potentials are connected to the relay, set DELTA_Y to
DELTA. When phase-to-neutral potentials are connected to the relay, set
DELTA_Y to WYE.
Use the VIWDG setting to tell the relay which winding current to use when
calculating power for the meter report. For example, set VIWDG := 2 when
the voltage inputs to the relay are derived from Winding 2 side.
COMPANG setting adjusts the power metering to account for angular
differences that may be present resulting from delta connected CTs. Set the
COMPANG equal to the angle by which the current input lags the
corresponding phase voltage input (e.g. IA and VA inputs) at unity power
factor.
In applications where only a single voltage is available, set SINGLEV equal
to Y. As shown in Figure 2.16, the single voltage must be connected to the
A-phase input, but it may be an A-N or an A-B voltage. Be sure to set
DELTA_Y equal to WYE for an A-N input or DELTA_Y equal to DELTA for
an A-B input voltage.
When you set SINGLEV equal to Y, the relay performance changes in the
following ways:
➤ Voltage Elements. When you use one phase-to-phase voltage,
the relay overvoltage and undervoltage elements use the
applied phase-to-phase voltage only. When you use one phase-
to-neutral voltage, the relay voltage elements use the applied
phase-to-neutral voltage only.
Table 4.3 Configurations and Ratings (Optional Neutral CT, Phase PT)
Setting Prompt Setting Range
Setting Name :=
Facto ry Default
NEUT 1 CT RATIO 1-5000 CTRN1 := 120
PHASE PT RATIO 1.00–10000.00 PTR := 120.00
NOMINAL VOLTAGE 0.20–10000.00 kV VNOM:= 13.80
PT CONNECTION DELTA, WYE DELTA_Y := DELTA
VOLT-CURR WDG 1, 2 VIWDG := 2
COMP ANGLE 0–360 Deg COMPANG := 0
SINGLE V INPUT Y, N SINGLEV := N
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