GE Multilin EPM 2200 - Power, Energy and Demand

1–6 EPM 2200 POWER METER – INSTRUCTION MANUAL
POWER, ENERGY AND DEMAND CHAPTER 1: THREE-PHASE POWER MEASUREMENT
Figure 1-6: Three-Phase Wye Load Illustrating Kirchoff’s Law and Blondel’s Theorem
Blondel's Theorem is a derivation that results from Kirchoff's Law. Kirchoff's Law states
that the sum of the currents into a node is zero. Another way of stating the same thing
is that the current into a node (connection point) must equal the current out of the
node. The law can be applied to measuring three-phase loads. Figure 1.6 shows a
typical connection of a three-phase load applied to a three-phase, four-wire service.
Kirchoff's Law holds that the sum of currents A, B, C and N must equal zero or that the
sum of currents into Node "n" must equal zero.
If we measure the currents in wires A, B and C, we then know the current in wire N by
Kirchoff's Law and it is not necessary to measure it. This fact leads us to the
conclusion of Blondel's Theorem- that we only need to measure the power in three of
the four wires if they are connected by a common node. In the circuit of Figure 1.6 we
must measure the power flow in three wires. This will require three voltage coils and
three current coils (a three-element meter). Similar figures and conclusions could be
reached for other circuit configurations involving Delta-connected loads.
1.5 Power, Energy and Demand
It is quite common to exchange power, energy and demand without differentiating
between the three. Because this practice can lead to confusion, the differences
between these three measurements will be discussed.
Power is an instantaneous reading. The power reading provided by a meter is the
present flow of watts. Power is measured immediately just like current. In many digital
meters, the power value is actually measured and calculated over a one second
interval because it takes some amount of time to calculate the RMS values of voltage
and current. But this time interval is kept small to preserve the instantaneous nature
of power.
Energy is always based on some time increment; it is the integration of power over a
defined time increment. Energy is an important value because almost all electric bills
are based, in part, on the amount of energy used.
Phase B
Phase C
Phase A
A
B
C
N
Node "n"