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1: Three-Phase Power Measurement
This introduction to three-phase power and power measurement is intended to
provide only a brief overview of the subject. The professional meter engineer or meter
technician should refer to more advanced documents such as the EEI Handbook for
Electricity Metering and the application standards for more in-depth and technical
coverage of the subject.
1.1: Three-Phase System Configurations
Three-phase power is most commonly used in situations where large amounts of
power will be used because it is a more effective way to transmit the power and
because it provides a smoother delivery of power to the end load. There are two
commonly used connections for three-phase power, a wye connection or a delta
connection. Each connection has several different manifestations in actual use.
When attempting to determine the type of connection in use, it is a good practice to
follow the circuit back to the transformer that is serving the circuit. It is often not
possible to conclusively determine the correct circuit connection simply by counting
the wires in the service or checking voltages. Checking the transformer connection
will provide conclusive evidence of the circuit connection and the relationships
between the phase voltages and ground.
1.1.1: Wye Connection
The wye connection is so called because when you look at the phase relationships and
the winding relationships between the phases it looks like a Y. Figure 1.1 depicts the
winding relationships for a wye-connected service. In a wye service the neutral (or
center point of the wye) is typically grounded. This leads to common voltages of 208/
120 and 480/277 (where the first number represents the phase-to-phase voltage and
the second number represents the phase-to-ground voltage).
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1: Three-Phase Power Measurement
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