PPA45xx Quick User Guide
Page 54
When using synthesised neutral, the elementary measurement of the voltage
rms will usually be higher than that measured with a true neutral. For this case,
there is a conversion option “star to delta” which accurately computes the phase
to phase voltages, then derives a phase to neutral value from the computed
phase to phase values. This gives a phase to neutral value which is much closer
to that measured with a real neutral. The phase to phase (or “delta”) value,
Vd(t) is derived from the instantaneous “star” voltage values, Vs(t) (eg. Vd1(t)
= Vs1(t) – Vs3(t)) so the voltage present on the synthesised neutral has no
effect.
Consider the delta wired configuration:
In this case the phase to phase voltages are measured directly and the phase to
neutral values are derived (set conversion to “delta to star”). The power
measurements are also adjusted to show the individual “star” phase powers. The
delta/star conversion assumes that the impedance of the load is equal on all
three phases.
Wiring
conversion
Phase rms voltage Phase power
Star with neutral
Phase to phase
rms
Measured
Measured
Star without
neutral
Phase to phase
rms
Slightly high
Measured
Star to delta
Derived
Measured
Delta
Delta to star
Derived
Derived
or
or
A
CH1
A
CH2
A
CH3
or
or
V
CH1
V
CH2
V
CH3
HI
HI
HI
HI
HI
HI
LO
LO
LO
Ph1
Ph1
Ph2
Ph3
Ph3
Ph2
LO
LO
LO
Three
Phase Source
Three
Phase Load
