What is a meter compensation?
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(c) Copyright: R G Chambers, 1999
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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.
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
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. In some installations, the metering of a generator is performed at the generator terminals. 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 (see Figure 1). 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 mistakes being made by the calibration laboratory when setting up the meter.
2. The Official Technical Auditor has the right (and occasionally the duty) 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 the following information that the Auditor might need for a full verification:-
(i) Details of the serial numbers (as assumed by the calculation) of the CTs and VTs whose known errors are to be compensated. [The Auditor might wish to check that the CTs and VTs actually installed have the same serial numbers as has been assumed by the calculation.]
(ii) Identification of the CT and VT calibration certificates that are being used as data in the calculation.
(iii) Similar identifications to fully justify the compensations for power losses in relevant power transformers.
(iv) The calculations are presented in a clear way, so that the Auditor can independently check them against his/her own calculations, if required.
(v) The combined effect of all the applied compensations are calculated for a range of standard load conditions. This enables the Auditor, if he/she wishes, to use his/her own precision calibrator to verify on site that the actual performance of the meter corresponds (within the allowed tolerance) with the theoretical optimum.
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 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 firstname.lastname@example.org .
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, the compensation is 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 is to calculate the combined magnitudes of all the relevant compensations for all the load conditions of interest. The calibration laboratory then adjusts 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.
Some manufacturers 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.
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Modification dated 1 November 2001
[Addition of a Section on why compensation calculations are valuable and important.]
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