Schweitzer Engineering Laboratories SEL-351A, Manual

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8.20
SEL-351A Relay Instruction Manual
Date Code 20080213
Breaker Monitor and Metering
Demand Metering
Demand Metering
The SEL-351A offers the choice between two types of demand metering,
settable with the enable setting:
➤
EDEM = THM (Thermal Demand Meter), or
➤
EDEM = ROL (Rolling Demand Meter)
The demand metering settings (in Table 8.3 ) are available via the SET
command (see Table 9.1 ). Also refer to MET Command (Metering Data) on
page 10.18 ).
The SEL-351A provides demand and peak demand metering for the following
values:
Depending on enable setting EDEM, these demand and peak demand values
are thermal demand or rolling demand values. The differences between
thermal and rolling demand metering are explained in the following
discussion.
Comparison of
Thermal and Rolling
Demand Meters
The example in Figure 8.11 shows the response of thermal and rolling demand
meters to a step current input. The current input is at a magnitude of zero and
then suddenly goes to an instantaneous level of 1.0 per unit (a “step”).
Currents I
A,B,C,N
Input currents (A primary
I
G
Residual-ground current (A primary;
IG = 3I0 = IA + IB + IC)
3I
2
Negative-sequence current (A primary)
Power MW
A,B,C
Single-phase megawatts
(wye-connected voltages only)
MVAR
A,B,C
Single-phase megavars
(wye-connected voltages only)
MW
3P
Three-phase megawatts
MVAR
3P
Three-phase megavars
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