The main part of this web-page is a reprint, with minor stylistic alterations to suit the internet, of a paper presented to the MATES 99 (Metering and Tariffs for Energy Supply) Conference held at Birmingham (England), 25-28 May 1999. IEE Conference Publication No CP462.
Copyright (c) Institution of Electrical Engineers, United Kingdom, 1998 and 1999.
Early diagnosis of tariff metering faults by a systematic analysis of Main/Check metering discrepancies.
R G Chambers.
Chambers Metercare (Consultant) Chambers Metercare Home Page
SUMMARY
A method is described by which the integrity of MWh tariff metering systems in a Power Station can be monitored remotely by a systematic analysis of the discrepancies between the Main and Check meter readings. The method is cheap to implement, using only metering data that is already in existence; it is also highly effective in detecting tariff metering faults at an early stage of development. The technique is recommended for metering systems on large generators, where the financial losses resulting from inaccurate metering can be substantial.
INTRODUCTION
The electrical energy registered by the tariff metering on a base-load 500MW generator normally yields an annual income of around US $170M. A metering error of -0.1% might therefore entail a loss of US $170,000 per year to the generating company. For this reason, there is a strong financial incentive to maintain the accuracy of generator metering to a high standard at all times. The method described here is an analysis tool that will help the power station to identify cost-effectively any metering system whose accuracy is drifting, so that repairs can be effected at an earlier stage.
The Station (or the Generating Company) keeps a database of the Main and Check MWh meter readings for each Generator, for every half-hour metering period; this is the duplicate of the metering database that is accessed daily by Settlements to arrange for tariff payment. In the analysis method to be described here, the discrepancies between the Main and Check MWh data are analysed by a database macro, linked manually to a separate spreadsheet macro. The resulting graphs and analysed summaries give a clear indication when a metering system has started to drift out of calibration. The method is both cheap and effective in detecting the onset of calibration drift, for the following reasons:
DEFINITION OF MAIN/CHECK METERING DISCREPANCY.
The Main/Check discrepancy of a sister pair of meters, in any half-hour metering period, is defined by:-
discrepancy = 100 ( M - C )/M percent
where M and C are the values of MWh registered by the Main and Check meters respectively during the metering period under study.
PRINCIPLE OF THE DISCREPANCY ANALYSIS TECHNIQUE
A metering system basically consists of the CTs, the VTs, wiring (including fuses, switches and electrical joints) to connect these to the meter, and the meter itself. If the instrument transformers remain stable, if the wiring remains in good condition, and if the calibration characteristic of the meter itself remains constant, then there should never be any drift in the overall calibration of the metering system over all time. If both the Main and the Check MWh metering systems remain in this perfect condition, the plot of the Main/Check metering discrepancy against time, measured in years, should be a horizontal straight line running parallel to the time axis of the graph. Any drift in the overall calibration of any component of either the Main or the Check metering system will cause the graph to become a sloping line. The degree of slope indicates the rapidity with which calibration accuracy of one of these metering systems is drifting.
The method is at its most useful where both the Main meter and the Check meter is fed by its own individual set of VTs and CTs. In an inferior metering system where the Main and Check MWh meters share a common VT (or CT), a drift in the calibration accuracy of the VT would affect both metering systems equally, not producing any change in the Main/Check discrepancy.
LONG-TERM DRIFT OF THE MAIN/CHECK METERING DISCREPANCY.
The errors of a MWh metering system vary
naturally, sometimes by as much as 0.2%, according to the value
of MW and MVAr load being measured. If these natural variations
were ![[Selection criterion for data validity]](../images/mate99_1.gif)
allowed to exist in the data, they would confuse any
attempt to produce a meaningful long-term graph of the variation
of Main/Check meter deviation with time. For this reason, each
metering data point is accepted as valid for analysis only if it
falls within the box ABCD of the machine capability diagram,
Figure 1. Any metering data that falls outside this box is not
used in the analysis. This policy ensures consistency of the data
in graphs that may cover periods of several years.
Figure 2 shows the plot of MWh Main/Check
metering discrepancy against time, over a four-year period, for
Generator A and its associated Unit Transformer. Each plotted
point on the graph is the average of all the metering data
collected over a one-week interval that satisfies the validity
criterion described above. Each ![[Graph: Long-term]](../images/mate99_2.gif)
plotted point in
Figure 2 is therefore the average of between 10 and 200 readings,
the actual number depending on the load regime of the generator
for that week. Over a one-year period between April 1994 and
April 1995, the Main/Check discrepancy of the Generator MWh
meters gradually changed from -0.1% to +0.3%. At this time, it
was not normal practice to save the metering data of the Unit
Transformer for long. To the extent that data is available in
Figure 2, the Main/Check discrepancy of the Unit Transformer was
changing in a manner that mimicked the behaviour of the Generator
metering. In August 1995, the Check MWh metering of both the
Generator and the Unit Transformer failed altogether.
Investigation by the Station revealed that the HV fuse protecting
the 22kV side of one phase of the Check VT had become
open-circuit. They replaced the fuse (see Arrow A of
Figure 2); the Main/Check discrepancy of both items of plant
returned to approximately the same value as had existed in April
1994, before the fuse-failure had started to develop.
At the time of this fault, the Station had the routine of looking at their Main/Check discrepancies once a week or once a fortnight. Nobody ever drew a graph showing the long-term variation of the Main/Check discrepancy. The fault grew slowly, over a fifteen-month period from May 1994 to August 1995 (Figure 2). The changes developed so slowly that nobody noticed them. The final complete failure of the Check metering was a surprise to the Station staff. If they had had access to graphs shown in Figure 2, they would have been aware of the developing fault as early as December 1994, and the failure would have been rectified before it caused any appreciable trouble.
The failure of an HV fuse protecting a VT is often a slow process taking months or years. Wright and Newbery (1) describe the corona-discharge erosion process which attacks the fuse element and is responsible for the failure. This type of failure is possible at VT voltages of 12kV (line) or above, but becomes an increasingly likely mode of failure for higher voltages. Because failures can take place so slowly, a week-by-week visual inspection of the Main/Check discrepancy figures is never likely to detect the growing fault. A graphical technique is the only method by which a slowly-developing fault can be reliably detected at an early stage of development.
MEDIUM-TERM DRIFT OF THE MAIN/CHECK METERING DISCREPANCY.
In some investigations of metering accuracy, a
medium-term or short-term plot of Main/Check ![[Graph: Medium-term, with mimicking]](../images/mate99_3.gif)
discrepancy may be required. The spreadsheet macros we
have developed are able to do this. A medium-term plot for
Generator B and its associated Unit Transformer is shown in
Figure 3. The plot covers a period of six months; data for
the graph was selected by the criteria of Figure 1. Each plotted
point is the nine-point moving average of the measured point
itself, the four preceding, and the four succeeding points. The
graph of Figure 3 was drawn because the routinely-produced
long-term graph showed the start of an excursion, and the Station
wished to discover more details about what might be the cause.
The excursion of Main/Check discrepancy of the Generator metering
in late February was -0.25%, and was mimicked by the behaviour of
the Unit Transformer metering. Once again, a fault in the VT
signal is indicated, since this is the only input signal shared
by both sets of metering. The cause of the problem was quickly
diagnosed: a knife-switch on the VT secondary signal had a
high-resistance electrical contact.
NON-MIMICKED CHANGES OF THE MAIN/CHECK DISCREPANCY.
This Section assumes that the metering system under study includes separate Unit Transformer and Generator metering. The Unit Transformer Main MWh meter shares a VT signal with the Generator Main MWh meter; and similarly for the Check metering.
When a fault develops in an individual
Generator meter, the changes of Main/Check discrepancy would not
normally be mimicked by the Unit Transformer metering. The same
is ![[Graph: Non-mimicked discrepancy]](../images/mate99_4.gif)
(theoretically)
true for any fault that might develop in the CT signal, although
the present author has not yet observed a CT fault using the
techniques described here. The presence of mimicking is therefore
diagnostic of a fault in the VT signal, while the absence of
mimicking indicates either a meter fault or a fault in the CT
signal.
Figure 4 shows the abrupt change of Main/Check discrepancy that occurred when an out-of-calibration Generator MWh meter was replaced by a newly calibrated one. As described in the previous paragraph, the change of meter characteristic affected only the Generator metering, while the Main/Check discrepancy of the Unit Transformer metering continued at its previous value.
USE OF THE TECHNIQUE FOR ASSESSING MAINTENANCE WORK ON THE METERING.
Figure 4 illustrates another valuable feature of the spreadsheet macros. The ability to easily plot out Main/Check metering discrepancies can be a useful tool for assessing the calibration of a metering system after maintenance work has been done on any of its components. The metering engineer responsible for the maintenance can easily assess whether the jump in the graph in Figure 4 corresponds with the expected outcome of the work.
DETECTION OF A PHASE-ANGLE ERROR IN THE METER OR IN A METERING SIGNAL.
If the phase-angle compensation of a Generator
MWh meter is perfectly set up to cancel the (actually existing)
values of phase-angle errors of both the CT and the VT, the meter
should ![[Graph vs tan(phi); no fault.]](../images/mate99_5.gif)
theoretically
give accurate measurements at all values of power factor angle
phi. Furthermore, if both the Main and the Check meter are
perfectly set up in this way, the plot of Main/Check discrepancy
against tan (phi)
should yield a line of best fit which runs parallel to the tan (phi) axis, as in the
observed results of Figure 5. The graphical data in Figure 5 is
from Generator B for the interval from January to early February
1998. This was the period (see Figure 3) when the Main/Check
discrepancies were stable, and when the metering was fault-free.
The validity criterion for data in the graph of
Figure 5 is different from what has been used up to now. To be
valid for the present analysis, the load must fall in the box
WXYZ of Figure 1. This extension beyond the original
MW/MVAr validity box ABCD is necessary if we are to obtain a wide
range of power factor angle phi for the graph.
![[Graph vs tan(phi); with fault.]](../images/mate99_6.gif)
Now consider the situation when the phase-angle compensation of the MWh meter becomes mismatched with the actually existing CT or VT phase errors. The resulting meter error is theoretically proportional to tan (phi). Figure 6 shows the result of a linear regression analysis for Generator B in the interval from late February to early March 1998; this was a period when the metering was known, from Figure 3, to have problems. At the 95% statistical confidence level, the slope of the line of best fit in Figure 6 is 0.16 ± 0.06 percent/unit of tan (phi). A theoretical analysis has shown that a slope of this magnitude would be obtained if one (electrical) phase of the VT signal had a phase (angle) error of 0.49 centiradians. This amount of phase-angle error is unacceptable for the class of metering used on Generator B.
We have therefore seen that the method of analysis can detect metering faults by a graphical technique of plotting Main/Check discrepancy against tan (phi). Graphs of this type can easily be obtained from the metering database by use of the spreadsheet macro.
ANALYSIS OF THE STATISTICAL SPREAD (STANDARD DEVIATION) OF THE METERING DISCREPANCIES.
The vertical scatter of the data points in Figure 6 has a full range (maximum - minimum) of 0.42%. This compares with a full-range scatter in Figure 5 of 0.09%.
The value of 0.09% occurred when the metering was in a healthy state, while the value 0.42% occurred when there were problems caused by a high-resistance contact in the knife-switch. Any loose or dirty electrical contact, whether in an electrical connection or (hypothetically) in a dry solder joint on a meter circuit board, has a contact resistance which varies unpredictably. At any given time, the resistance might be zero, or it might be sufficiently high to affect the signal at or in the meter. This explains why some types of fault may be accompanied by (and diagnosed by) an increased statistical variation of the metering discrepancy values. Our software automatically calculates the standard deviation of the discrepancy values for each meter, and reports any meter which has an unusually high spread.
COMPANY-WIDE /HISTOGRAM OF METERING DISCREPANCY VALUES.
Figure 7 is a histogram of the MWh discrepancy
values of the generators of two companies combined, as found in
April 1998. The histogram is produced annually, and is a useful
tool for ![[Main/Check histogram]](../images/mate99_7.gif)
planning the meter maintenance programme for the year ahead.
Special maintenance attention is normally planned for those
metering systems that lie at the extreme left or the extreme
right of the histogram.
World-wide extension of the histogram.
Other companies are invited to contribute data,
confidentially and free of charge, to build a combined world-wide
histogram covering a greater number of generator metering
systems. The resulting histogram will give a better idea to each
participating company of what is achievable and where their own
problems might lie. For further details on the benefits of
membership of this scheme, and on how you can participate in the
world-wide histogram, click the hyperlink below:-
Hyperlink: Details of membership of the scheme.
CONCLUSIONS
The database and spreadsheet macros are effective in identifying metering faults at an early stage of development. The technique is cheap to implement and simple to understand.
REFERENCE
Wright, A. and
Newbery, P.G., 1982.
"Electric Fuses", First
Edition, Peter Peregrinus Ltd, London.
Contact the author.
Join the World-Wide Histogram Club
Hyperlink:- What is this histogram all about?
Advantages of membership.
By contributing data, your company will be entitled to receive a
copy of the resulting world-wide histogram. This will give you a
bench-mark comparison by which you can judge the accuracy of your
own metering against the performance of comparable metering
systems in other companies around the world. The service of
preparing the data and issuing the resulting histogram to
participating companies will be free of charge. All data
submitted will be confidential. The published histogram will be
anonymous, in that it will not be possible to identify which data
originated from which company.
The Histogram.
Two histograms will be prepared:-
To Join.
Apply to dick.chambers@metercare.co.uk ,
stating which class of histogram is applicable, and how many
metering systems are involved. Include your full postal address,
telephone and fax numbers, and e-mail address. I will send you a
simple questionnaire for all the relevant data.
Contact Information
Dr Richard Chambers
Chambers Metercare
58 Primley Park Avenue
Leeds
LS17 7HU
England.
e-mail
dick.chambers@metercare.co.uk
Telephone
+44
(0)113-268-4406
Fax/Voice Mail +44 (0)113-295-9116
Chambers Metercare Home Page http://www.metercare.co.uk/index.htm