8-20 Breaker Monitor, Metering, and Load Profile Functions Date Code 20001006
SEL-351 Instruction Manual
DEMAND METERING
The SEL-351 Relay 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 in
Section 9: Setting the Relay and also Settings Sheet 10 of 22 at the end of Section 9). Also refer
to MET Command (Metering Data), MET D – Demand Metering in Section 10: Serial Port
Communications and Commands).
The SEL-351 Relay provides demand and peak demand metering for the following values:
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
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”).
Courtesy of NationalSwitchgear.com
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