Schweitzer Engineering Laboratories SEL-700G Series, Instruction Manual

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4.6
SEL-700G Relay Instruction Manual Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
EXAMPLE 4.2 Phase VT Ratio Setting Calculations
Consider a 13.8 kV Generator application where 14400:120 V rated
voltage transformers (connected in open delta) are used.
Set PTRX := 14400/120 := 120, VNOM_X := 13.8, and
DELTAY_X := DELTA.
The relay performs a range check for the VNOM_X and VNOM_Y settings
that depends upon the respective PT connection setting (DELTAY). When
setting DELTAY is DELTA, then the allowed range of the VNOM is 20–250 V
(L-L secondary). When setting DELTAY is WYE, then the allowed range of
VNOM is 20–440 V (L-L secondary).
Note that the VNOM_X and VNOM_Y settings are 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.
When DELTAY_X : = DELTA, the relay allows you to connect external zero-
sequence voltage (3V0) by setting EXT3V0_X. Set EXT3V0_X to VS or VN
to connect external 3V0 to
VS-NS
or
VN-NN
voltage inputs, respectively, for use
with different elements. Refer to Figure 2.16 for the voltage connection and
Figure 2.25 for an application example.
Generator Protection
Phase 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 transformer
energization, while fifth-harmonic blocking provides security for
overexcitation conditions.
Operating Characteristic
The SEL-700G1 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 4.2 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.