Download Maintaining Voltage-Current Phase Relationships in Power Quality

Maintaining Voltage-Current Phase Relationships in Power Quality Monitoring Systems
Brian Kingham, Utility Market Manager, Schneider Electric, PMC Division
Abstract:
Historical power quality measurement devices focused on
voltage-only measurements for analysis, survey and
troubleshooting purposes. In order to increase their
return on capital investment, utilities are looking for
measurement devices capable of meeting more than one
business need:
for example, a device capable of
simultaneously monitoring power quality and providing
revenue-accurate data.
This desire drives a new requirement to maintain phaseangle relationships between previously independent
voltage and current circuits so that energy accuracy is
preserved. This paper describes the system components
(such as transformers and PQ monitor components) that
can affect V-I phase relationships and methods to
eliminate or compensate for these errors. In conclusion,
it is shown how requirements for leading revenue
accuracy standards and IEC 61000-4-30 Class A Power
Quality compliance can not only be met but exceeded.
I. Introduction
increasing cost of energy has intensified
THE
the need for more accurate end-to-end metering
systems. At the same time, utilities are looking to
leverage new technologies to reduce capital
expenditure costs, leading them to install multifunction devices within their measurement and
monitoring systems.
This paper describes how total system
measurement accuracy can be affected by
components both internal and external to metering
devices. The paper then introduces available
technologies that, with proper application, are
capable of mitigating the error introduced to
provide system measurement accuracy that is
orders of magnitude greater than was historically
possible.
II. Sources of Phase Error
There are many potential sources of phase and
magnitude errors within an electrical network. For
the purposes of this paper, we are focused on those
components near the metering point that can affect
the accuracy of the metered parameters. As shown
in Figure 2, these sources are identified as the
power
transformer
(A),
the
instrument
transformers (B), the current transformers inside
the meter (C), and the analog front-end of the
meter (D).
B
A
C
Front-End
D
CPU
Figure 2. Sources of phase and magnitude error in metering
systems
Figure 1. Rise in US energy price, 1993-2006
Brian Kingham is with Schneider Electric in Victoria, BC
(email: [email protected])
A. Power Transformer Error
Substation power transformers provide highly
efficient voltage and current magnitude
transformation but still introduce both ratio error
(the difference, expressed in RMS volts or amps,
between the primary and secondary magnitudes
after the turns ratio has been accounted for) and
phase error (the difference in phase, expressed in
degrees, between the primary and secondary
sinusoidal waveforms) .
Both these sources of errors are due to the
magnetizing inductance required by the core and
the impedance of the windings (see Figure 3). The
resistive losses will cause ratio error and the
inductance of the windings and core will cause
both ratio and phase error from the primary to the
secondary side of the transformer.
of the core, phase shift through a CT will be nonlinear.
C. Metrology CT Error
An often overlooked component of the metering
system is the current transformer used inside the
meter itself. Like any other transformer, this
internal CT introduces another source of ratio and
phase error.
Passive
CTs
are
inexpensive
magnetic
transformers which produce output current
proportional to input current but which incorporate
both a magnitude loss and phase error.
Figure 4. Equivalent circuit of a passive CT
Figure 3. Equivalent circuit of a power transformer
B. Instrument Transformer Error
Instrument transformers, including current
transformers (CTs) and potential transformers
(PTs), are used to provide low-voltage and lowcurrent inputs to meters. Like power transformers,
instrument transformers are a source of both ratio
and phase error.
In order to maximize the accuracy of voltage
transformation, PTs are designed to minimize
voltage drop in the transformer windings and
magnetization current. The equivalent circuit for a
PT is essentially the same as shown in Figure 3 for
a power transformer.
CTs, however, are more sensitive to the
magnetization current flowing through the
transformer core. Due to the complex impedance
Phase error introduced by passive CTs varies with
the impedance of the core, and is therefore
susceptible to changes in load or temperature.
Because of this multi-variable dependency, it is
difficult to fully compensate for passive CT phase
error through calibration.
Active CTs are also a source of magnitude error,
but use a feedback loop to replace lost current and
maintain a zero-flux core.
Because of this
feedback loop, there is no phase error though an
active CT.
Figure 5. Equivalent circuit of an active CT
D. Analog Front-End Error
Finally, a source of error particular to
multifunction power quality and energy meters is
the introduction of low-pass RLC filters in the
analog front-end. These filters are required by
international power quality standards such as IEC
61000-4-30 and IEC 61000-4-7 to ensure that high
frequency signals are not aliased into the reported
values for individual harmonic magnitudes or total
harmonic distortion.
The insertion of any RLC filter will produce ratio
and phase shift error between measurement
channels which must also be corrected if power
and energy accuracy is to be maintained.
III.
Phase Error Correction Methods
A. Power Transformer Correction
Rather than attempting to compensate for power
transformer ratio and phase errors measured at a
meter’s three-phase voltage and current input
channels, the industry-standard approach is to
correct for errors in the resulting Watt and VAR
totalized values. One of two sets of calculations is
used to achieve this, dependent on the input
coefficients available.
Method 1 uses manufacturer-supplied test sheet
data to calculate the Watt and VAR losses caused
by the transformer. The test sheet data (VATXtest,
LWFeTXtest and LWCuTXtest) are entered into the
loss compensation calculations along with actual
voltage and current levels, %Excitation and
%Impedance. The results of these calculations can
then be either programmed into a meter for realtime compensation or used for post-processing of
energy data.
Method 2 uses empirical data from the in-situ
transformer to determine the amount of
compensation necessary. The measured Watt and
VAR losses are entered into the loss compensation
calculations along with system resistance and
reactance information. Like Method 1, these
coefficients can either be programmed into the
meter for real-time correction or used in postprocessing applications.
B. Instrument Transformer Correction
Unlike power transformer correction, instrument
transformer correction (ITC) corrects for error in
the individual voltage and current circuits in real
time. This produces the same benefit to power and
energy calculations as the power transformer
correction method but with the advantage of also
correcting the individual voltage and current
readings. Additionally, since each phase of a
three-phase PT or CT can have a unique error
correction curve, per-phase rather than total error
correction is preferred as it will be more accurate
in unbalanced systems than total error correction.
To implement ITC, a series of ratio correction and
phase angle correction data points are provided
from transformer manufacturers’ specifications or
from test results (see Figure 6). These coefficients
are programmed into an algorithm that interpolates
between data points to best fit the error curve of
the transformer.
curves can be calculated for the entire current
range and for multiple power factors. Because the
error curves vary by power factor, an
approximation is always employed to compensate
for points not covered by the calibration process.
Because of the active compensation circuit, active
CTs have no phase error and are thereby immune
to changes in power factor.
This allows
manufacturers to use standard calibration
techniques to compensate for the magnitude error
of the CT in real time.
An additional benefit of an active CT’s dynamic
compensation is correction for core magnetization.
Half-wave signals (such as transients) can shift the
magnetization of a CT core away from its ideal
state. For passive core CTs, this will result in
additional phase error. The feedback circuit in an
active CT will compensate for the magnetization
effect by injecting the necessary current to
maintain zero magnetic flux.
D. Analog Front-End Correction
Figure 6. Phase angle error curve
C. Metrology CT Correction
To correct for errors introduced by the meter’s
metrology CT, passive correction is dependent on
the ability to correctly model the phase and
magnitude error. Given a highly accurate
reference, sufficient memory and processing power
to handle high-order polynomials, and enough time
during the calibration process, error correction
The amount of correction needed to compensate
for ratio and phase errors introduced in the analog
front-end depends greatly on the architecture being
used. At a minimum, to comply with international
harmonics measurement standards such as IEC
61000-4-7, sufficient filtering must exist in the
signal chain to prevent aliasing of higher
frequency components into lower harmonic
measurements. Low-pass filters, introduced to
prevent aliasing, will cause undesirable phase shift
errors between voltage and current channels which
then affect power and energy calculations.
These phase shift errors can be corrected through
calibration in a similar manner to instrument
transformer correction. By modeling the error
curves using laboratory-precision devices,
compensation curves can be programmed into the
meter to correct for errors in real-time. If matched
filter packs are used to keep ratio and phase error
consistent between voltage and current channels,
the correction can completely mitigate the error.
IV.
Results of Phase and Magnitude Error
Correction
A. Power Transformer Correction Results
By its nature and the formulas used, power
transformer correction applies correction factors
directly to the Watt and VAR values of interest.
This means that the effect of correction is
dependent on the accuracy of the input variables
used rather than an empirical measurement as in
other correction methods.
The ability to use high-voltage, highly-accurate
current sensors has allowed empirical observations
of ITC results. Table 1 shows the results with and
without ITC, where it can be seen that an
improvement of up to 1.5% has been realized
through error compensation of an instrument CT.
Voltage
V RMS
120
120
120
120
120
Current
A RMS
20
20
50
100
50
Power
Factor
Unity
0.6 lag
0.6 lag
0.6 lag
0.6 lag
Error (%)
Without ITC
Error (% )
With ITC
-0.92
1.63
0.89
0.65
0.55
-0.017
-0.10
-0.003
0.056
0.067
Table 1 Effect of instrument transformer correction
Using data supplied by the transformer
manufacturer, a typical 7500 kVA transformer
operating at 3780 kVA and unity power factor will
introduce an error of 37.2 kW or 0.98%. This
includes a phase error of 2.9 degrees. Power
transformer correction will completely eliminate
this error.
B. Instrument Transformer Correction Results
Because the voltage input to a PT tends to be fairly
stable, PT accuracy limits are typically defined
between 80 and 120% of nominal voltage. Within
this range, PT phase error for a Class 0.5 PT is
limited to about 0.35 degrees. The allowable ratio
error is included in the accuracy class number
(0.5% at nominal voltage for a Class 0.5 PT).
These errors are fairly small, and unless in-situ
data is provided the effect of instrument
transformer correction on a PT will be limited.
CT error, particularly phase error, can be much
higher. CTs are expected to be subjected to most
of their dynamic range as loading levels change.
Because of the relationship between current,
magnetic flux and inductance within a CT, phase
error is non-linear and larger than within a PT.
The same accuracy Class 0.5 CT can introduce
phase errors between 0.5 degrees and 1.5 degrees
and a Class 1 CT can be up to 3 degrees.
C. Metrology CT Correction Results
With passive CT error approximation and
correction, power and energy accuracy can be
brought to within industry standard requirements
of 0.2%. Due to the non-linear phase error, it is
difficult to improve error correction beyond this
level except at discrete calibration points (e.g.
unity power factor and full load).
Active CT correction results are much better and
more consistent over the entire operating range of
the meter due to the zero phase error. Using active
CT correction, the error of the meter can be
brought to within the bounds of calibration
reference standards; typically four to ten times
more accurate than the most stringent 0.2% energy
error tolerance. Empirical results of active CT
correction show an improvement in metering error
(and thereby system error) of 0.15%.
D. Analog Front-End Correction Results
The total benefit of analog front-end correction
depends greatly on the architecture chosen for the
meter. If all filter components are perfectly
matched on all input channels, phase error is
negated.
Using a combination of matched
component
filter
packs
and
calibration
compensation, improvements in power and energy
accuracy of up to 0.2% have been achieved.
V. Conclusion
With system measurement error becoming
increasingly important as energy prices rise, power
transformer correction, instrument transformer
correction, active CTs and analog front-end
calibration can all contribute to increased accuracy
in power and energy measurements.
Proper selection and application of these methods
has shown total system error improvements of over
3%, and should be considered integral to the
implementation of a Class 0.2 metering system.
VI.
Author Biography
Brian Kingham received his Bachelor of Electrical
Engineering from the University of Victoria in
1995. In 1995 he joined Power Measurement
(now Schneider Electric) and is currently the
Utility Market Manager for the company’s Power
Monitoring and Control (PMC) division.
VII.
References
[1] P. Doig, C. Gunn, L. Durante, C. Burns & M.
Cochrane, "Reclassification of Relay-Class
Current Transformers for Revenue Metering
Applications", IEEE 2005