The corresponding standards are very similar with a few exceptions. The
allowable instrumentation errors for IEEE 112 2004 and NVLAP 150 standards are
identical. However, CSA C390 2006 has some differences in terms of temperatures
and readings.
For example, the input power requirement for CSA C390 2006 is ±0.5% of the
reading and must include the CT and PT errors, whereas those for IEEE 112 2004
and NVLAP 150 both require only ±0.5% of full scale (FS).
Current sensors are usually required for testing because high current cannot
be brought directly into the measuring equipment. There is a variety of sensors
available to match specific applications. Clamp-on sensors can be used with
power analyzers. Scope probes can also be used, but they must be used with
caution to ensure the instrument is not exposed to high currents.
For CTs, the feed wire can be connected through the window (CTs are
typically donut shaped or oblong, with the hole or inner portion referred to as
the window), or low current connections can be made to the terminals on the top
of the device. Shunts are typically used for dc applications but not ac or
distorted frequencies, although they can be used for synchronous motors up to a
few hundred Hz. Specialized CTs are available that work well for high
frequencies, more commonly found in lighting applications than in motors and
drives.
A new type of CT system that delivers high accuracy from dc to the kHz range
is now available. It features an active-type transformer that uses a power
supply conditioning unit. This type of CT system provides highly accurate
measurements-approximately 0.02% to 0.05% of the reading-especially for VFDs,
which can have frequencies that range from 0 Hz to the operating speed of the
connected motor. Voltage transformers simply transform a voltage from one level
to another. In measurement applications, step-down transformers are sometimes
required to reduce the voltage delivered to the measuring instrument. However,
many instruments can accommodate relatively high voltages and don't require a
step-down transformer.
An instrument transformer is usually combination of a CT and a voltage
transformer. Instrument transformers can reduce the number of required
transducers in certain measurement applications.
Selection considerations, cautions
When deciding which device to use, the first question is the frequency range
of the parameters to be measured. Typically, dc-to-line frequency sine waves
can use dc shunts, which offer high accuracy and simple installation. For ac
and dc applications, the Hall effect or active type instrument transformer can
be used. Hall effect technology has a lower accuracy rate, while the active
type provides more precision. Various instrument transformers can operate at
high frequencies of 30 Hz or more, but they can't be used for dc.
The next consideration is the level of accuracy required. For an instrument
transformer, it's typically specified as the "turns ratio" accuracy.
Phase shift is another important factor because many transformers are designed
for current measurement only and aren't compensated for phase shift.
Phase shift is basically an effect of power factor for power measurement,
and it will thus have an influence on the power calculation. For example, a CT
with a 2-deg maximum phase shift as part of its specification would introduce
an error of Cos 2 deg, or 0.06%. The user must decide if that percentage of
error is acceptable for the application.
A CT is a current source. According to Ohm's Law, voltage (E) equals the
current (I) through the conductor multiplied by the resistance (R) of the
conductor in units of ohms. Opening the secondary of a CT effectively drives
the resistance to infinity. This means the internal current will saturate the
coil, the voltage will approach infinity as well, and the unit will be damaged
or will destroy itself. Even worse, a CT with an accidentally opened secondary
could seriously injure workers. For more information visit our website allindiayellowpage.com.