Schweitzer Engineering Laboratories SEL-734 - Page 84

4.38
SEL-734 Meter Instruction Manual Date Code 20090730
Metering
Metering Calculations
Form 5 (2-element meter)
This calculation provides vector apparent power.
Per-Phase Reactive
Power (VAR)
This calculation is the equivalent of IEEE 1459-2000, 3.1.2.15. Under
sinusoidal conditions, this is equivalent to reactive power, VIsinθ. Under
nonsinusoidal conditions, the value for Q will be larger due to the distortion
power of the harmonics.
Form 9 (3-element meter)
x = a, b, or c
Form 5 (2-element meter)
Per-phase values not calculated for delta
Three-Phase Reactive Power (VAR)
Form 9 (3-element meter)
Q = Q
a
+ Q
b
+ Q
c
Form 5 (2-element meter)
Energy Calculations The SEL-734 calculates energy by integrating power over time. This is
accomplished using a numerical approximation of the integral. As an example,
using active power, P, as described above, the SEL-734 calculates watthours
by adding watthours for the most recent interval to the previously accumulated
watthour value.
In the example above P can be a per-phase or polyphase quantity. Separate
registers are used for bidirectional and 4-quadrant energy metering. These
calculations are performed in a similar fashion for reactive and apparent
power.
SV
ab
I
a
• V
ab
I
a
–[]V+
cb
I
c
• V
bc
I
a
–[]∠∠=
Q
x
S
x
2
P
x
2
–=
QS
2
P
2
–=
where:
P = the active power measured over the time interval Δt
n = the index for the most recent, non-overlapping interval Δt
Wh(n-1) = the accumulated energy from all prior intervals
Δt = the time interval in seconds, normally 1-second
Wh n()
x
PtΔ•
3600
--------------- Wh n 1–()watthours()+=
Courtesy of NationalSwitchgear.com