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PQSensor™
(Texto en ingles)
The PQSensor™ offers a unique, convenient and cost
effective method of faithfully reproducing harmonic
content using Capacitive Voltage Transformers (CVT)
without the need for Resistive-Capacitor Dividers (RCD)
− The PQSensor™ offers a practical and economical solution
for wide bandwidth measurements using CVTs.
− Eliminates the need for special high voltage instrument
transformers or wide bandwidth voltage dividers.
− Can be retrofitted to in-service CVTs or installed in new
units.
− CVTs can continue to be used in normal ways to feed re-
lays and other standard devices and at the same time to be
used for power quality monitoring.
− It can be used to detect CVT internal ferroresonance.
General description
A power quality sensor is available to very accurately measure
power quality parameters such as harmonics and flicker over
a wide bandwidth from sub synchronous to high frequencies.
The PQSensor™ current probes are installed in capacitor
voltage transformers (CVTs) at the ground connection points
in the secondary terminal box. The PQSensor™ signal-con-
ditioning unit can be mounted on the CVT support structure
making the retrofit installation simple.
Background
Power quality assessment has become an increasingly impor-
tant requirement in the management of electric supply sys-
tems. This recognition has led to the introduction of several
standards for power quality measurement and monitoring. All
standards such as IEEE 519, IEC 61000-4-30 and 61000-4-7
and UK Engineering Recommendation G5/4 require measure-
ments up to the 50th harmonic. Flicker standard IEC 61000-
4-15 require measurement of modulating frequency between
0.5 Hz to 33 Hz.
Options for measuring harmonics
If utilities and users are to monitor ferroresonance oscillations,
power quality and other wideband transients in high volt-
age systems, there is a need for cost effective and accurate
means to do so. Sophisticated power quality monitors are
now available from various manufacturers. The challenge,
however, is to provide inputs to these monitors which ac-
curately reflect the phenomena occurring on the monitored
system in a cost effective and safe way.
The best performance from a conventional device in terms
of a wide bandwidth frequency response is offered by a
resistive-capacitor divider (RCD), which is very expensive, has
a very limited output and would not normally be present in
60 Transformadores de medida exteriores | Guía para el comprador
a substation environment and does not meet safety require-
ments for isolation between high voltage and low voltage
parts.
Most power quality monitors are currently receiving their
inputs from wound or inductive type voltage transformers
(VTs). The advantage with VTs is that they can also provide
inputs into conventional revenue meters and relays and
therefore may already be present if harmonic measurement is
being considered as an add-on. What is not well understood
is that wound VT's have a limited frequency range. Graph
1 shows the performance of typical wound VT (Reference:
CIGRE Working Group 36). It can be seen that the frequency
response becomes unacceptable around 500 Hz, well below
the frequency limit established in the major standards. The
upper limit increases for lower voltage class units but worsens
for higher voltage class units.
Capacitor voltage transformers have become the dominant
technology for voltage measurement at transmission voltage
levels because they provide reliable and accurate perfor-
mance at reasonable cost. CVTs, because they are essentially
tuned to the system frequency, are not in themselves capable
of harmonic measurement. Because of the prevalence and
reasonable cost of this technology, much effort has been ex-
pended to add the functionality of harmonic measurement.
There is a solution for employing CVTs for harmonic measure-
ment. A power quality sensor, to be used in conjunction with
CVTs, has been developed and patented which overcomes
the aforementioned objections. One of the important advan-
tages of the technology, in addition to its cost effectiveness, is
the speed of installation in in-service CVTs.
Relative transformation ratio
3.0
U = 400 kV
N
U = 220 kV
N
U = 20 kV
N
2.0
1.0
0.0
50 100 200 500 1000 2000
Frequency
5000