Sel SEL-787 - Basic Protection

4.5
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
When you set SINGLEV equal to Y, the relay performance changes in the
following ways:
➤ Power and Voltage Elements. When you use one voltage, the
relay assumes that the system voltages are balanced in both
magnitude and phase angle. Power, power factor, and positive-
sequence impedance are calculated assuming balanced
voltages.
➤ Metering. When you use one voltage, the relay displays
magnitude and phase angle for the measured PT. The relay
displays zero for the magnitudes of the unmeasured voltages.
Balanced voltages are assumed for power, power factor, VG,
and 3V2 metering.
Relays that are not equipped with phase voltage inputs hide these settings and
disable voltage-based protection and metering functions.
VNOM Range Check
The relay performs a range check for the VNOM setting that depends upon the
voltage-input delta or wye configuration. Valid nominal voltage is 100–250 V
secondary (l-l) when DELTA_Y is DELTA; it is 100–440 V when DELTA_Y
is WYE.
Note that the VNOM setting is always in line-to-line primary kV, even when
set for a wye configuration. You should be careful to use a solidly-grounded
wye system for VNOM inputs greater than 250 V (l-l, secondary) to avoid a
1.73 increase in terminal voltages from a line-to-ground fault.
Basic Protection
Differential Element
Protect your apparatus with dual-slope percentage differential protection.
Percentage differential protection provides more sensitive and secure
protection than traditional differential protection; the dual-slope characteristic
compensates for steady-state, proportional, and transient differential errors
within the zone of protection. Steady-state errors are those that do not vary
with loading through the differential zone. These include transformer
magnetizing current and unmonitored loads. Proportional errors are those that
vary with loading. These include relay measuring error, CT ratio errors, and
errors because of tap changing. Transient errors are those that occur
temporarily due to transients such as CT saturation.
The relay allows you to choose harmonic blocking, harmonic restraint, or
both, providing stability during transformer inrush conditions. Even-
numbered harmonics (second and fourth) provide security during
energization, while fifth-harmonic blocking provides security for
overexcitation conditions.
Operating Characteristic
The SEL-787 has three differential elements (87R-1, 87R-2, and 87R-3).
These elements employ Operate (IOP) and Restraint (IRT) quantities that the
relay calculates from the winding input current. Figure 4.1 shows the relay
characteristic. You can set the characteristic as either a single-slope,
percentage differential characteristic or as a dual-slope, variable-percentage
differential characteristic. Tripping occurs if the Operate quantity is greater
than the curve value for the particular restraint quantity. A minimum pickup
level for the Operate quantity must also be satisfied.