Valhalla 2300, Operation Manual

Page: 90 / 115

102-3. Potential Transformers
Potential transformers (PT's) are used to
extend the voltage measurement capabilities
of Valhalla digital power analyzers. PT's are
available in many ratios, maximum voltage
ratings and isolation voltages.
PT's can be used to isolate the digital power
analyzer from high voltage systems. Some
high voltage systems operate at voltage levels
directly measurable by the power analyzer but
at common mode voltages beyond the power
analyzer's limits. The isolation voltage rating
should be at least 1½ times the common mode
voltage or peak input voltage.
When selecting a PT, try to use decade ratios,
e.g. 10:1, 100:1, etc. Decade ratios make for
easier power analyzer measurements. The
user just mentally moves the voltage and
power display decimal points to the right the
appropriate number of places. Of course,
non-decade ratios may also be used. If the
power analyzer is remotely controlled via the
IEEE interface, the controller can apply the
PT ratio to the measurements automatically.
The maximum voltage rating of the PT should
be selected 1½ times the expected voltage
level to allow for high crest factor waveforms.
The ratio accuracy of PT's is usually about
2%. This is quite a bit wider than the
accuracy of a digital power analyzer. The
easiest way to improve the accuracy is to
measure the actual ratio and use this value
when correcting measurements. PT's are low
frequency devices, typically 50-400Hz. At
high frequencies, their phase shift will cause
the power measurement to be in error. Figure
2 illustrates the method of connecting a PT to
a digital power analyzer. Other methods are
shown in Section 6.
102-4. Using Both CT's and PT's
Most applications using PT's will also use
CT's as well. In this case, the power
measurement must be multiplied by both the
CT and PT ratios. Figure 3 illustrates a
method of connecting a CT and PT to a digital
power analyzer. Section 6 contains other
variations on this method.
102-5. Poly-Phase Systems
When measuring poly-phase systems,
multiple CT's and/or PT's are required.
When calculating total power, the individual
phase powers should be scaled by the CT/PT
ratios and then summed as shown below:
Total Power =
(W
a
x PT
a
x CT
a
) + (W
b
x PT
b
x CT
b
) +
(W
c
x PT
c
x CT
c
) +...+ (W
n
x PT
n
x CT
n
)
Where:
W
n
= Phase n watts
PT
n
= Phase n PT ratio
CT
n
= Phase n CT ratio