CHAPTER 1: AC POWER MEASUREMENT
GE Multilin EPM 5000 series Advanced Power Meters 3
Voltages between the phases vary depending on loading factors and the quality of distribution
transformers. The three-phase system is distributed in different voltage levels: 208V AC, 480V AC, 2400V
AC, 4160V AC, 6900V AC, 13800V AC, and so on.
Power measurement in a poly phase system is governed by Blondel's Theorem. Blondel’s Theorem
states that in a power distribution network which has N conductors, the number of measurement elements
required to determine power is N-1. A typical configuration of poly phase system has either a Delta
connection or a Wye connection (see Figure 1.4, below).
N
A
B
C
2) Wye
X
E I
CN C
X
X
P = E I + E I + E I
AN
BN CN
E I
BN B
E I
AN A
B
A
C
1) Delta
X
X
E I
CB C
E I
AB A
P = E I
AB A
+ E I
CB C
ABC
Figure 1.4: Poly Phase System: (1) Delta, (2) Wye
1.3: Consumption, Demand and Poor Power Factor
CONSUMPTION:
T W=WH × W = instantaneous power T = time in hours
The total electric energy usage over a time period is the consumption of WH.
Typically, the unit in which consumption is specified is the kilowatt-hour (KWH): one thousand watts
consumed over one hour. Utilities use the WH equation to determine the overall consumption in a billing
period.
DEMAND: Average energy consumed over a specified time interval. The utility determines the interval,
typically 15 or 30 minutes. The utility measures the maximum demand over a billing period. This
measurement exhibits a deviation from average consumption, causing the utility to provide generating
capacity to satisfy a high maximum consumption demand. The highest average demand is retained in the
metering system until the demand level is reset.
POOR POWER FACTOR: Results in reactive power consumption. Transferring reactive power over a
distribution network causes energy loss. To force consumers to correct their Power Factor, utilities
monitor reactive power consumption and penalize the user for Poor Power Factor.
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