28
Sub Meters
GE Multilin
3. AC POWER MEASUREMENT
This Chapter describes the following:
1. Fundamentals of Three-Phase Measurements
3.1. Three-Phase Systems
A three-phase system delivers higher levels of power for industrial and commercial
applications; the three phases correspond to three potential lines. A 120° phase shift exists
between the three potential lines.
A typical configuration has either a Delta connection or a Wye (Star) connection (see
Figure
3-1
, below).
In a three-phase system, the voltage levels between the phases and the neutral are ideally
defined by V1 = V2 = V3 = V12 / 3 = V23 / 3 = V31 / 3. In practice, there will be some
unbalance (difference).
Figure 3-1: Three-Phase System: (1) Delta, (2) Star (Wye)
Voltages between the phases vary depending on loading factors and the quality of distribution
transformers.
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 Star (Wye) connection (see
Figure 3-2
,
below).
