Advertisement
1.1.3
Blondell's Theorem and Three Phase Measurement
EPM 6010 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE
120 V
Vbc
120 V
FIGURE 1–5: Three-Phase, Four-Wire Delta Phasors
In 1893 an engineer and mathematician named Andre E. Blondell set forth the first
scientific basis for poly phase metering. His theorem states:
•
If energy is supplied to any system of conductors through N wires, the total power in the
system is given by the algebraic sum of the readings of N wattmeters so arranged that
each of the N wires contains one current coil, the corresponding potential coil being
connected between that wire and some common point. If this common point is on one
of the N wires, the measurement may be made by the use of N-1 wattmeters.
The theorem may be stated more simply, in modern language:
• In a system of N conductors, N-1 meter elements will measure the power or energy
taken provided that all the potential coils have a common tie to the conductor in
which there is no current coil.
• Three-phase power measurement is accomplished by measuring the three
individual phases and adding them together to obtain the total three phase value. In
older analog meters, this measurement was accomplished using up to three
separate elements. Each element combined the single-phase voltage and current to
produce a torque on the meter disk. All three elements were arranged around the
disk so that the disk was subjected to the combined torque of the three elements. As
a result the disk would turn at a higher speed and register power supplied by each
of the three wires.
According to Blondell's Theorem, it was possible to reduce the number of elements under
certain conditions. For example, a three-phase, three-wire delta system could be correctly
measured with two elements (two potential coils and two current coils) if the potential coils
were connected between the three phases with one phase in common.
In a three-phase, four-wire wye system it is necessary to use three elements. Three voltage
coils are connected between the three phases and the common neutral conductor. A
current coil is required in each of the three phases.
CHAPTER 1: THREE-PHASE POWER MEASUREMENT
Vca
Vnc
Vbn
Vab
1–5
Advertisement
