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Why do phasor diagrams for a 3-Phase High-Leg Delta systems look incorrect in SMS?

Issue
The Fundamental Phasor Readings Table in SMS displays incorrect values for a 3-Phase High-Leg Delta system, also referred to as: Red-leg, Stinger-leg, Center-Tap grounded, or Wild Leg. For a normal (3-wire) delta only L-L readings are available, however, a center tapped grounded connection delta system is considered to be a 4-wire system type (40 or 41 depending upon installation) and is treated as such by the meter. As such it is expected that the L-L voltage phases will have a 120 degree separation.


Resolution

For a 3-phase 4-wire system (monitored by a PM800, CM3000, or CM4000), the fundamental phasor readings table will display L-N Voltages and not the L-L Voltages (Note, the neutral as shown is actually the ground). The A-N Voltage will have 180 degrees of separation from the C-N Voltage (assuming that B is the High Leg). Additionally, the B-N voltage (the high-leg) will have 90 degrees of separation for the other two L-N readings. See Figure 1 below for a typical example.

Figure 1: Phasor Readings for a 4-wire Delta

Figure 2 depicts the L-N/L-G waveforms for a Center-Tap Grounded Delta system. The as seen from ground waveforms are the only ones that we are interested in. Notice how the two 120V sine waves are 180 degrees out of phase from each other and the 208V sine wave is 90 degrees out of phase from the 120V sine waves:

Figure 2: Phasor waveform of a 4-wire Delta

Note: When monitoring a High-Leg Delta system, the single phase power factor will be incorrect.

High-Leg Delta Phasor Diagram.doc

Legacy KB System (APS) Data: RESL208714 V1.0, Originally authored by MaTh on 07/25/2012, Last Edited by MaTh on 07/25/2012
Related ranges: System Manager 4.3, System Manager 4.2, System Manager 4.0, PM800 series, CM4000 series, CM3000 series
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