What is a
meter compensation?
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Copyright: R G Chambers, 1999
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http://www.metercare.co.uk/compens/comp01.htm
Chambers Metercare, 58 Primley Park
Avenue, Leeds LS17 7HU. Tel +44(0)113-268-4406. Fax
+44(0)113-295-9116
Aim and Readership
This
webpage provides a brief introduction to the compensation of MWh tariff meters
used at bulk electricity supply points, such as power stations or sub-stations.
The page is designed to meet the needs of professional electrical engineers who
have no previous experience of working on meter compensation.
Definition
A compensation is a precise
adjustment to the calibration characteristics of a meter, to cancel the
quantifiable effects of errors which provably exist elsewhere within the
metering system. Compensations are applied to meters in strict accordance with
the guidelines issued by the Authority responsible for the metering
regulations.
Examples of meter
compensations
EXAMPLE 1. Suppose
that we have a three-phase, three-element meter whose voltage signals are
provided by a three-phase Voltage Transformer (VT) of nominal ratio
22kV : 110 V (line). Suppose that the VTs are class 0.2, but
that their actual ratio error (as reported by the VT calibration laboratory) is
+0.1% for all three phases. For a primary voltage of 22kV, the VT secondary
signals of all three phases would therefore be 110.11V instead of 110.00V. This
would cause the meter to over-register. In the absence of any compensation,
there would be an overall metering error of +0.1% from this cause. The
compensation that must be applied to the meter to correct this quantifiable
source of error is
-0.1%.
EXAMPLE 2. There is a
similar compensation that must be applied to the meter to correct for the
calibrated ratio errors of the Current Transformers (CTs).
EXAMPLE 3.
Phase-angle errors of the CTs and VTs must also be allowed for in the meter
compensation. The effect of a phase-angle error (on the meter compensation) is
zero when the metered power supply operates at unity power factor (ie zero power-factor angle); the effect becomes
considerable (even dominant) when the metered power factor angle becomes
strongly leading or strongly lagging.
EXAMPLE 4. Refer
to Figure 1. In some installations, the
authorities may issue a dispensation to allow the metering of a generator to be
performed at the generator terminals, instead of at the commercial boundary.
However, the power losses of the generator transformer are counted as a loss to
the owner of the transformer. The quantity of electricity exported and sold is
the quantity flowing across the Commercial Boundary, which is less than the
quantity generated. Hence, compensations must be applied to the meter to
correct for the Ohmic I-squared-R loss in the transformer windings, and the
iron-hysteretic loss in the transformer core. These compensations can be
calculated from the load-loss and the no-load loss reported by the transformer
Test Report.
There are also various other
types of compensation which sometimes need to be applied, in addition to the
four examples described above. Different formulae must be applied for
calculating VT and CT compensations for two-element and three-element metering
systems.
The magnitude of meter
compensation varies with MW and MVAr load
In Example 3, we saw that the magnitude of the meter compensation for CT and VT
phase errors depends on the power factor angle of the supply being metered. In
other words, at a given value of MW, this type of compensation takes different
values for different values of MVAr. Similarly, the I-squared-R loss described
in Example 4 is small when the metered MW and MVAr are low, but becomes
appreciable at high values of MW and MVAr (because of the higher value of
current I). As a result of these effects, the magnitude of the overall meter
compensation often varies considerably across the range of MW and MVAr values
that the meter might be expected to operate in. The person calculating the
compensation is responsible for specifying the requirements of these
compensations at standard MW/MVAr calibration points; the calibration
laboratory is responsible for adjusting the meter compensations to these
specifications.
The need for meter compensation
In a privatised or de-regulated
electricity market, it is important to ensure fair competition between
producers. For example, it would be unfair to allow a power station an
advantage of perhaps +0.3% over its competitors simply because the generator
metering at that station happens to be installed at the generator terminals
instead of being correctly positioned on the commercial boundary.
Meter compensation therefore
ensures that all producers of electricity are subject to identical criteria of
competitiveness. It gives us "a level playing field" with "goal
posts of equal width at each end".
The need for compensation
calculations
There are three reasons why you
ought to have auditable meter compensation calculations for your tariff
meters:-
1. The main output page of the
calculation displays, on one sheet of paper, all the settings the calibration
laboratory needs to apply to the meter in order to achieve the correct
compensation values. A compensation calculation, with a results page that has
been well set out, therefore increases the efficiency with which the
calibration laboratory can adjust the meter to the required values. The clearly
specified compensation values displayed by a professional calculation also
reduce the risk of human error while the calibration laboratory is setting up
the meter.
2. The Official Technical
Auditor may occasionally exercise his/her right to verify that the compensation
values applied to the meter are fully justified and correctly set. An auditable
compensation calculation assists the Auditor in this task. The compensation
calculation sheet is not just a calculation, but it also provides all the
information and primary data that the Auditor needs for a full verification of
the meter settings.
3. To protect the income of his
Power Station or Sub-station, the on-site Metering Engineer needs similar
information to that described above for the Auditor. Clearly presented
compensation data help the Engineer to assess the metering, and to correct any
defects that might develop in service.
If you would like to mail you
some samples of compensation calculations performed by Chambers Metercare, please contact me at the e-mail address [email protected] .
Compensation calculation for
smart meters
Nowadays, many types of high quality meter have their
compensations programmed into the software of the meter. A frequently asked
question is:- "Why is it necessary to perform a compensation calculation,
when the compensation values are automatically calculated by the meter itself?".
Chambers Metercare recommends compensation
calculations, even for a smart meter, because the input data displayed within
the calculation document provides the justification for your meter
compensation, making your meter fully auditable. Furthermore, the compensation
calculation document is a considerable aid when you need to check the accuracy
of your meter by an on-site calibration measurement.
Meter compensation in
countries other than Britain
In the United Kingdom, the
compensation of tariff meters is (usually) required by the regulations for
large-scale electricity metering, such as at power stations or sub-stations.
Many other countries have similar rules, but in some countries the regulations
make no requirement for compensation. If your metering system is to be
installed in a country other than Britain, you should check with the relevant
authorities before you decide whether or not to compensate the meters. In most
countries, it is either the National Grid or the Electricity Regulating
Authority (or perhaps the Pool, if they have one) which issues these
regulations.
The financial implications
of meter compensation
A base-load 500MW generator would
typically produce electricity valued at around US $170M per year, with
fuel costs of around US $100M per year. In cases where the metering is
installed on the low voltage (LV) side of the generator transformer, the
compensation for power losses in the transformer typically amounts to around
-0.3%. The equivalent economic value of the compensation would therefore be
around US $0.5M per year if judged against total production value, or
US $0.3M if judged against fuel costs. A 2000MW station with four
base-load 500MW generators would have compensations valued at a total of
between US $1.2M and US $2M per year.
It would be incorrect and illogical to describe the compensation as a financial
loss caused by the metering. It is merely an adjustment made necessary by the
fact that the metering is achieved at a point other than the commercial
boundary.
The importance of the
mathematical sign
The fundamental equation of meter
compensation is:-
C
= -E
.... (1)
where C is the compensation
that must be applied to the meter, and E is the quantifiable error that
provably exists elsewhere within the metering system. The minus sign in
equation (1) is vitally important, since it produces the
error-cancellation we are seeking. For this reason, it is important to ensure
that there is a clear understanding of the difference between an error
and a compensation. All documentation should be punctilious in clearly
stating this distinction. Above all, it is important to audit the procedures of
your meter calibration laboratory, to ensure that the personnel understand the
distinction and are applying compensations with the correct intended
mathematical sign. Because of the very large amount of money registered by
generator meters, these devices should be calibrated only by an experienced
senior calibration engineer.
Applying the meter
compensation
In Britain in the 1990s, the
compensation was normally applied directly to the meter, usually as a
hard-wired or lockable potentiometric adjustment to the circuit board which
controls the calibration characteristics. The standard procedure was to calculate
the combined magnitudes of all the relevant compensations for all the load
conditions of interest. The calibration laboratory then adjusted the meter
characteristics to comply as closely as possible with the results of the calculation,
for all of the MW and MVAr load conditions listed on the meter calibration test
sheet. The procedure used
for making these adjustments depends on the make of the meter being calibrated.
As already mentioned in the
paragraph on smart meters, many manufacturers nowadays apply the compensation
as a calculated value by the software of the meter. Where this is done, the
software must be made rigorously tamper-proof. There must also be adequate
facilities to enable the generating company, or the Official Auditor, to check
the compensation parameters programmed into the meter. They should also be able
to spot-check the performance of the meter against the theoretical values of
compensation, under the range of actual load conditions at which the meter
operates. The most convenient method for a general spot-check
would be by a tamper-proof read-only modem link to the meter. The Station
Metering Engineer (as well as the Official Auditor) would certainly also
require facilities to directly check on site both the calibration and the
compensation of the meter against a portable calibration standard.
In principle, an alternative
method of compensating a metered value would be to calibrate the meter with no
compensation at all, and then to apply a calculated adjustment by the central
computer at the Financial Settlements Authority. The central computer would
repeat the compensation adjustment calculation for each item of metered plant,
for each half-hour metering period. This scheme was considered for a while in Britain
during the privatisation/deregulation process, but the idea was rejected. The
main reason for rejection was that the scheme was insufficiently tamper-proof.
Chambers Metercare
Home Page English Français Español
R G Chambers
[email protected]
58 Primley
Park Avenue
Leeds LS17 7HU
United Kingdom
Tel +44 (0)113-268-4406
Fax +44 (0)113-295-9116
Modification dated 27 January
2003.
[Addition of a Section on why
compensation calculations are valuable and important.]
Further modification dated
22/12/2023, to make the page compatible with my retirement from the industry.