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Gross Volume Correction (GVC)

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Gross Volume Correction (GVC)

Guidance Note

What is Gross Volume Correction?

Once a Settlement Day has been subject to the Final Reconciliation Volume Allocation Run (RF Run), you shouldn’t change any data for that day unless the Metering System in question is subject to an upheld Trading Dispute.

Gross Volume Correction (GVC) is a technique used to correct errors relating to Meter Advance Periods during which some Settlement Dates have already been subject to the RF Run. The effect of using GVC is to reallocate the lost or gained energy volume to a range of Settlement Dates for which RF Runs have not yet taken place. This process ensures that the total gross volume of energy is correct, although it will be allocated to the wrong Settlement Days/Settlement Periods. The process also ensures that consumption data for Settlement Days for which RF Runs have taken place isn’t changed.

When do we use GVC?

Where an erroneous Meter Advance Period spans the date of the latest RF Run, you cannot withdraw the AA, EAC and reading(s) for that period. We usually refer to this as a ‘part crystallised’ error. You can, however, use GVC to correct the total volume of energy during the Meter Advance Period, without changing the volume of energy that has already been subject to RF Runs.

NHHDCs can also use GVC where an energy error for a given Metering System is affecting the NHHDC‟s ability to process subsequent Meter readings. For example, you can use GVC where the latest EAC is out of line with the expected consumption for the Metering System to the extent that later valid readings for the Metering System are failing validation, or should be likely to fail validation.

You should not use GVC to compensate for errors that are having no ongoing impact on the NHHDC‟s ability to validate readings for the Metering System. You should not use GVC solely as a mechanism for correcting historic over- (or under-) payments.

GVC requirements

Where can we find the GVC requirements?

You can find the requirements for applying GVC in BSCP504 sections 3.4.3 and 4.14.

Who applies GVC?

Non Half Hourly Data Collectors (NHHDC) carry out the GVC process.

Who initiates GVC and how?

Suppliers request GVC. This is a manual process, by phone or email. NHHDCs can also initiate GVC, but only with the approval of the relevant Supplier. Suppliers can approve NHHDC-initiated GVC for each instance or on a more general basis through some form of delegated authority. Data Collectors who want to initiate GVC should agree the preferred approval method with Suppliers.

Is GVC mandatory?

GVC is an optional requirement for Suppliers. NHHDCs must be able to carry out GVC if requested by the Supplier. NHHDCs will carry out GVC when this has been requested by the Supplier or agreed with the Supplier. NHHDCs should only apply GVC under the circumstances described above (in ‘when do we use GVC?’).

If, as a NHHDC, you receive a request from a Supplier to apply GVC, this should be to resolve a partially crystallised error or to resolve validation issues. If the requested use of GVC does not meet either of these conditions, you can refer the request back to the Supplier. You should provide the Supplier supporting rationale for why you believe GVC is not appropriate.

GVC terms

Before looking at how GVC is applied, there are a number of terms that we use in the GVC process that you will need to be familiar with. These are listed below. Please note that we use a number of additional terms in relation to the audit requirements for GVC and Dummy Meter Exchanges. You can find these definitions in BSCP504 section 4.14.2.



Crystallised Period

A range of Settlement Dates for which RF Runs have already taken place and for which data cannot be amended unless subject to an upheld Trading Dispute.

Error Correcting Reading

Where you have a valid actual reading in the ‘Fluid Period’ (see below), you can deem an Error Correcting Reading at an earlier date within the ‘Fluid Period’. You can then use this as the end reading of the compensatory AA. This has the effect of compensating for the error over a shorter period. To avoid excessive negative values in the compensatory period, BSCP504 recommends that, where possible, the Error Correcting Reading should be at least 60 working days after the Error Freezing Reading.

Error Freezing Reading

A reading deemed in the current RF Window to prevent energy volumes for a crystallised period being amended. You calculate an Error Freezing Reading using a Deemed Meter Advance for the period between the last valid reading and a date within the RF Window. You should calculate the Deemed Meter Advance using the AA or EAC that was included in the RF Runs for the period of the advance.

Fluid Period

A range of Settlement Dates for which RF Runs (or Post-Final Settlement Run (PFSR)) have not yet taken place and for which data can still be amended.

RF Window

This is the window of time between 5 and 20 working days before the RF Run being carried out for a particular Settlement Day. The window allows time for the Error Freezing Reading and its associated AA to be processed by the NHHDC, Non Half Hourly Data Aggregator (NHHDA) and the Supplier Volume Allocation Agent (SVAA) without changing data for Settlement Dates in the Crystallised Period.

Realistic Reading

Where a Meter Reading is required for a particular Settlement Day to carry out Gross Volume Correction and an actual Meter reading is not available, a realistic reading can be deemed for that Settlement Day using a valid Meter register reading (before or after the Realistic Reading date) and a realistic EAC (i.e. a previous valid EAC or, if one is not available, an initial (class average) EAC).

Applying GVC

In the example below (Figure 1), an erroneous meter reading has been treated as valid, but a subsequent reading has shown that it was invalid. The correct rate of consumption is illustrated by the green dashed line. AA 1 is valid, AA 2 is erroneously large and AA 3 is a lower value which compensates for the error in AA 2. You cannot withdraw the erroneous reading because the Meter Advance Period of associated AA includes dates for which RF Runs have taken place. In order to withdraw the invalid reading, you need to apply GVC.

Figure 1 – Part-crystallised settlement error

complex image of process

To apply GVC you will to need to carry out the following steps:

    • choose a date for the Error Freezing Reading. This should be within the “RF Window‟ (see GVC Terms);

    • use the EAC/AA system to calculate a Deemed Meter Advance (DMA) for the Meter Advance Period between Valid Read 1 and the date of the Error Freezing Reading. Use AA 2 (from Figure 1) to calculate the DMA;

    • add the DMA to Valid Read 1 to give an Error Freezing Reading;

    • use the EAC/AA calculator to calculate AA 4 (from Valid Read 1 to Error Freezing Reading), AA 5 (from Error Freezing Reading to Valid Read 2) and EAC 2;

    • remove the Erroneous Reading, AA 2, AA 3 and EAC 1 (from Figure 1)

    • replace with the Error Freezing Reading, AA 4, AA 5 and EAC 2 (from Figure 2)

    • send a revised D0019 flow to the NHHDA

Please note that these are logical steps. The actual sequence of steps will depend on how your NHHDC system operates.

Figure 2 – Application of GVC

complex image of process

The dashed red line shows how the error has reduced.

In this example, a lower AA compensates for an erroneously large AA to give the correct gross volume. The opposite can also occur – i.e. where a large positive AA is used to compensate for a negative (or lower value) AA – but this is less common. Large negative errors tend to arise only as a result of failing to identify a meter rollover or where GVC has been applied previously.

Applying GVC using an ‘Error Correcting Reading’

In the previous example, the lower AA between the Error Freezing Reading and the valid actual Reading compensates for the error that has already crystallised. If the valid Actual Reading is recent, this has the effect of compensating for the error over a period of, perhaps, 12 to 13 months. You can compensate for the error over a shorter period by deeming an Error Correcting Reading at a date earlier than that of the valid actual reading. This is shown in Figure 3 below. Instead of having one AA between the Error Freezing Reading and Valid Read 2, this results in two AAs – one between the Error Freezing Reading and the Error Correcting Reading and the other between the Error Correcting Reading and Valid Read 2.

If you use an Error Correcting Reading this has the advantage that you are putting the consumption profile ‘back on track’ at an earlier date. You will need an additional deemed reading and will have to process an additional AA, so it is more complicated. Because you are compensating for the original crystallised error over a shorter period the AA will be lower than if you compensate over the full Fluid Period (and may be negative, as in the example below). If the period between the Error Freezing Reading and the Error Correcting Reading is short, this can result in excessive negative values. BSCP504 recommends that this period should be at least 60 working days, although this may not always be possible.

Figure 3 – using an Error Correcting Reading

complex image of process

Applying GVC using a ‘Realistic Reading’

In the previous example, there is a valid reading with a later date than the erroneous reading to be withdrawn. In some cases, you may need to apply GVC to bring the consumption history “back on track‟ without the benefit of a later reading. Or you may have one or more later readings that are failing validation because they are inconsistent with previous readings, which have been passed as valid, but have now been shown to be invalid.

Under these circumstances you can deem a Realistic Reading. The process is the same as in Figures 1 and 2, with the difference that Valid Read 2 is a deemed reading rather than an actual reading.

To apply GVC you will need to carry out the following steps:

    • use the EAC/AA system to calculate a Deemed Meter Advance (DMA) for the period between Valid Read 1 (in Figure 1) and a recent date. You should select the latest valid AA or EAC to use in the DMA calculation;

    • add the DMA to Valid Read 1 to create the ‘Realistic Reading’;

    • follow the remaining steps in Figures 1 and 2, using the deemed Realistic Reading in place of Valid Read 2.

Please note that, although BSCP504 allows you to use a deemed Realistic Reading, you should use this technique very sparingly, if at all. Where possible, you should limit the use of GVC to those situations where you can prove or strongly justify that a consumption error has occurred using at least one subsequent reading.

Use of realistic forward EACs

Applying GVC to compensate for an earlier excessively large AA, can result in a negative AA. Depending on the size of the negative AA and the duration of the Meter Advance Period, the associated forward-looking EAC can also be negative or much lower than the likely rate of consumption for the Metering System.

Conversely, though less frequently, an AA can be too high, as a result of compensating for an earlier negative AA. This can result in a forward EAC that is too high.

If you can demonstrate that the forward EAC is inconsistent with normal demand (or generation) for that Metering System and is likely to lead to failure to validate subsequent readings, you can replace it with a realistic EAC.

Is it mandatory to replace unrepresentative forward EACs?

The EAC/AA system automatically replaces negative forward EACs with a class average value. So the replacement of negative EACs is mandatory.

You can optionally overwrite the class average EAC value with a Metering System specific value. This is optional.

You can also (manually) replace an unrepresentative positive EAC by either a class average EAC or a Metering System specific value. This too is optional.

If you have later readings that would allow you to bring the EAC back into line with previous valid demand trends (by calculating a further AA), then there is no need to replace the EAC with a realistic value.

When you replace an EAC you should maintain an audit trail, identifying how you derived the replacement EAC.

GVC tends to be used in cases where a Metering System has a poor quality meter reading history. This means that it will not always be possible to determine a representative EAC for the Metering System. As such the optional replacement of unrepresentative EACs is unlikely to be used on a regular basis, if at all.

Does the replacing of negative EACs only apply to GVC?

No. The EAC/AA system automatically replaces all negative EACs, whether created by GVC or not. The system has no way of determining whether AA/EAC calculation requests are the result of GVC.

The optional processes to change the class average to a more representative value and to amend unrepresentative positive EACs only apply as part of GVC.

When to correct ‘part-crystallised’ errors

Figure 4

complex image of process

Figure 4 above shows a reading which is out of line with earlier and later readings. This reading cannot be withdrawn because the associated Meter Advance Period spans the date of the latest RF Run. The erroneous reading results in corrective (lower) AA, which compensates for the previous AA which was too large. The gross volume of energy settled between Valid Read 1 and Valid Read 2 will be correct, whether GVC is applied or not. Unless the intermediate reading is causing later valid readings to fail validation, there is little benefit in applying GVC.

Figure 5

complex image of process

Figure 5 above shows the same situation but the error value is greater. Even if GVC is not applied, the gross volume of energy between Valid Read 1 and Valid Read 2 will be correct. But by applying GVC, the misallocation of energy will be greatly reduced.

Figure 6

complex image of process

Figure 6 shows how GVC is applied in the situation shown in Figure 5. The correct volume of energy is indicated by the green dashed line. The allocation of energy after applying GVC (as shown by the black lines) is closer to the correct allocation than had GVC not been applied (as shown by the red dashed line).

Figure 7

complex image of process

In Figures 5 and 6, there is a long time to go before the RF Run for the date of the erroneous reading. Figure 7 shows the effect of applying GVC where erroneous Meter Advance spans a period for which most Settlement Days have already been subject to a RF Run. Whilst GVC will reduce the misallocation of energy (as shown by the area between the black line and red dashed line), the reduction is much less than in Figure 6.

Before using GVC to correct a part-crystallised error, you need to consider a) the volume of the error and b) the duration of the ‘Fluid Period’ over which the error can be corrected. Where possible you should try to avoid using GVC to compensate for errors over periods of less than 60 working days.

GVC and Trading Disputes

Figure 8 – where a Metering System is subject to an upheld Trading Dispute and a PFSR is scheduled for the relevant GSP Group

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Where you have identified an erroneous Meter Advance, you can withdraw the associated AA, EAC and (where applicable) the associated reading. But only if none of the Settlement Dates in the Meter Advance Period have been subject to a PFSR. In Figure 8 above, you can withdraw AAs 3, 4 and 5. You cannot withdraw AA 1 because it is fully crystallised or AA 2 because it is part crystallised.

PLEASE NOTE that you cannot apply GVC to withdraw AA 2. You can only apply GVC at the RF Run.

You can only use the PFSR to amend data that is subject to an authorised Trading Dispute. In the case of the current erroneously large EAC/AA dispute, this only includes AA/EAC values which exceed Elexon’s monitoring thresholds. You can only withdraw AA 3 (in this example) if the AA is included in Elexon’s ‘instance report’ and a PFSR is scheduled for the GSP Group and date range in question. Similarly, you can only withdraw AA 4 without using GVC, if the AA is included in Elexon’s ‘instance report’ and a PFSR is scheduled for the GSP Group and date range in question.

Figure 9 – when a Metering System is not subject to a Trading Dispute or no PFSR is scheduled for the relevant GSP Group

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In Figure 9 above, you can withdraw AA 5 because none of the Settlement Dates in the Meter Advance Period have been subject to a RF Run. You cannot withdraw AAs 1 to 3 because they are fully crystallised at RF. If you want to withdraw AA 4, you can do so by applying GVC.

GVC and Change of Supplier

How do we apply GVC where the error spans a Change of Supplier?

As a Supplier, you cannot compensate within your Registration period for an error which occurred in a previous Supplier’s Registration period. You can only correct energy across Supplier registrations by withdrawing and replacing the Change of Supplier (CoS) reading. You can only do this with agreement of both Suppliers via the disputed Change of Supplier reading process. You can only use GVC to correct error within each of the respective Supplier’s registrations, i.e. without a change to the CoS reading.

Can we apply GVC where the error relates to the Meter Advance Period beginning on the Supply Start Date or ending on the day before the Supply Start Date?

Yes you can. You can apply GVC to correct errors which do not impact the Change of Supplier reading. For example, if the NHHDC has incorrectly calculated the first or last AA of a Supplier Registration, because a Meter rollover has not been identified (or has been incorrectly assumed), the NHHDC can correct the AA using GVC (subject to it not having fully crystallised at RF), because the Change of Supplier reading would not need to be replaced or withdrawn.

Can we amend or withdraw a Change of Supplier reading after 12 months?

Suppliers have up to 12 months from the Supply Start Date to initiate the disputed CoS reading process as defined in MRA Agreed Procedure MAP081. The exception is where a Metering System is subject to an authorised Trading Dispute. In this case, agreement on a revised Change of Supplier reading needs to be reached in time for it to be processed in the PFSR. In either case, both Suppliers need to reach agreement on the revised CoS Reading.

Can we use GVC across Registration periods if we are the Supplier for both Registrations (i.e. an internal migration between Supplier Ids)?

BSCP504 makes no distinction, but agreement of a revised CoS reading will clearly be straightforward where both Supplier Ids are owned by the same organisation.

Audit and Record Keeping

NHHDCs must be able to provide Suppliers, Elexon or the BSC Auditor with an audit record of every instance of GVC and every dummy meter exchange carried out as set out in BSCP504 Section 4.14.6. There are a number of exceptions where NHHDCs do not need to retain records and these are also described in BSCP504 Section 4.14.6, along with the content and format of the audit records. If their systems allow it, NHHDCs can generate the audit records on request. Otherwise, NHHDCs should maintain the audit records as and when they carry out an applicable GVC or dummy meter exchange. It is unlikely that Elexon or the BSC Auditor will request audit records more than once a year, unless the audit records give cause for concern. BSCP504 Section 4.14.6 also provides details about how to calculate the energy volumes associated with the GVC instances in the audit records.

Other Frequently Asked Questions

Can we use GVC to correct errors on old Meters?

You can use GVC to correct a part-crystallised error on the old Meter (i.e. where the Meter Advance Period associated with the Final Read for the Meter isn’t yet fully crystallised).

You cannot use GVC on the new meter to compensate for an error which occurred on the old Meter.

The same rules apply on change of Standard Settlement Configuration.

Can we use GVC for disconnected Metering Systems?

You can use GVC to correct a part-crystallised error (i.e. for Settlement Days before the disconnection date which haven’t yet been subject to a RF Run).

The principle of applying GVC where there is an ongoing impact on NHHDC validation clearly doesn’t apply in the case of a disconnected Metering System

The same rules apply on a change of Measurement Class from NHH to HH.

Can we use GVC to correct Meter rollover errors?

There are two types of Meter rollover error. Failing to identify that a Meter rollover has occurred will result in an incorrect negative AA. Assuming there has been a Meter rollover when there hasn't, will result in an incorrect positive AA. In both cases the Meter Advance and associated AA/EAC will be incorrect, but the Meter readings will be correct. A Meter rollover shouldn’t impact future validation, assuming that any negative forward EAC is replaced, so you shouldn’t need to use GVC for this purpose. But you can use GVC to correct a partially crystallised Meter rollover error.

Is there a consumption threshold for applying GVC?

There is no consumption threshold for applying GVC. The main purposes of GVC are to correct part crystallised errors and to allow NHHDCs to validate correct reads in the fluid period. You may need to use GVC at lower consumption volumes in order to allow you to validate subsequent readings, but this is more likely to be a problem at higher consumption volumes. So although there is no consumption threshold, you are likely to only use GVC for higher volume errors.

How far back can we use GVC?

The earliest error that you can apply GVC to is the one that spans the date of the latest RF Run. In the isolated cases where you use a Realistic Reading to apply GVC, you will need to deem a reading using a historic valid AA or EAC. As the Meter may not have been read for a long time, there is no absolute time limit on the age of the AA or EAC used to deem a Realistic Reading.

Would you expect NHHDC to be carrying out GVC in large numbers?

No. Ideally you should address erroneously large AA/EAC values as they arise and before any Settlement Days within the Meter Advance Period have crystallised at RF Runs. This is not always possible for Meter Advance Periods of longer durations (i.e. close to 14 months). The more frequently Meters are read, the less likelihood of GVC being required.

Who is responsible for carrying out GVC following a change of NHHDC?

On change of NHHDC, the new NHHDC takes full responsibility for all Metering System data for all Settlement Days within the appointing Supplier’s latest Registration. It is therefore the latest appointed Agent that is responsible for applying GVC to correct any errors that exist within this period.

How do you apply GVC when the error spans a change of NHHDC?

GVC is applied in the normal way by the latest NHHDC within the Supplier Registration.

How does the new NHHDC apply GVC without a complete reading history?

In order to apply GVC you need to be in full possession of all the data required to so correctly. You should not attempt to apply GVC based on assumptions about the consumption settled by a previous NHHDC.

Can GVC always be applied?

In some cases you may not have enough history to carry out GVC. There may also be situations where applying compensation will introduce further error. In these cases, the simple answer is not to apply GVC. As a NHHDC this may leave you with an ongoing validation problem. As an action of last resort, you can take such steps as are needed to address this problem, without ensuring that the gross volume is correct. This will have the effect of ‘writing off’ historic error, but avoiding future error. Where you take such actions, you should apply a robust and auditable process.

An example of alternative techniques to GVC is what is commonly known as a ‘dummy meter exchange’. This involves using Initial and Final Meter readings to effectively re-start consumption histories even though no actual, physical change of Meter has taken place.

Does NHHDA or NHHDC archiving affect GVC?

NHHDA archiving should have no impact on GVC as data is archived only after it has passed through the PFSR. Provided the NHHDC has a complete consumption history, enough information is available to carry out GVC. You will need a minimum of the latest valid reading and the latest valid EAC to apply GVC. As these are effectively the ‘current’ values, NHHDCs shouldn’t archive them.

Need more information?

For more information please contact the BSC Service Desk or call 0370 010 6950.

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1 “The Procedure for Agreement of Change of Supplier Readings and Resolution of Disputed Change of Supplier Readings” available at: