Ieee std 1159-1995, ieee recommended Practice for Monitoring Electric Power Quality

6.6 Instrument power
6.6.1 Power supply and monitoring compatibility
As mentioned earlier in this clause, whatever one measures will be affected by what devices are used to
make the measurement. The goal when using power monitoring instruments should be to minimize the
instrumentÕs impact on the measurement of the power system. By employing voltage connections and CTs,
this is relatively simple to accomplish. However, problems may enter into the measurement process not
through the connections, but through the power supply of the instrument. Even though most power monitor-
ing instruments require little power, their power supplies may signiÞcantly distort the measurements. The
following issues should be considered before choosing to power the instrument at the monitoring location.
Refer to the instrumentsÕ application manual to resolve these issues.
a)
What is the level of isolation between the instrument power supply and the instrument measurement
circuits?
b)
Does the instrument power supply generate noise or cause additional power system variations?
c)
Will the power consumption of the instrument inßuence the measurements?
d)
Does the instrument power supply contain transient protective devices that may affect the measure-
ments made with the instrument?
e)
Will the instrument operate correctly during power system variations that may induce power quality
problems?
f)
Are parasitic power cables being used that may introduce false readings in the instrument?
Many of the issues discussed may become irrelevant when powering the instrument and measuring from dif-
ferent locations. However, other concerns, such as the following, come to the forefront with this approach:
a)
What is the quality of the power supplied to the instrument and will it affect the measured data?
b)
Have Òground loopsÓ been introduced into the measurement setup?
c)
Does the placement of the instrument require excessively long power cords, voltage, and/or current
leads which might degrade the accuracy of the measurements?

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IEEE
MONITORING ELECTRIC POWER QUALITY
Std 1159-1995
35
6.6.2 DC power
Depending upon the location of the power system to be measured, a power monitoring instrument may use
dc power for instrument operation. Further, dc power may be employed internally to the instrument to
provide emergency backup power. DC power may be used internally or externally to the instrument. When
using dc power, several points should be considered. Once again, refer to the device application manual for
guidance.
a)
If using external power, are the power cables properly sized?
b)
Has the instrument been properly grounded?
c)
Does the dc power come from a battery pack that can build up a Òmemory,Ó which will shorten the
backup time available from the batteries?
d)
If an external charger is used and connected to an ac outlet, what is the chargerÕs isolation capability
and what effect will the charger have on the power system?