Accuracy of CV determination systems for calculation of FWACV

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Accuracy of CV determination systems for
calculation of FWACV

• Dave Lander

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Overview

• Based on work previously carried out October 2006
• Examines how consumers gas bills are estimated
• Examines how the accuracy of all of the inputs
  into the calculation affects the overall accuracy
  of the gas bill
• Poses questions about
• fairness
• the appropriate level of acuracy

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Introductory concepts error, uncertainty, bias...

• Uncertainty
• "Parameter that characterises the spread of
  values that could reasonably be attributed to the
  measurand."
• Range and an associated probability
• Error
• Measured result minus a true value
• Bias
• Mean value of a distribution of errors.
• Associated with an agreed set of conditions (each
  showing an error)

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The Charging Area CV

• Charging area CV is calculated as the Flow
  weighted average CV
• Subject to a 1 MJ/m3 cap
• Uncertainty in FWACV arises from
• Uncertainty in measurement of CVs and flows
• Variation in the CV of the sources of gas

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The Charging Area CV

• Consumer A receives high CV gas all the time
• For him the FWACV is biased
• Consumer B receives low CV gas all the time
• For him the FWACV is biased
• FWACV delivers zero bias in charging area energy
• CV cap limits the exposure of consumer B

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The Consumers Energy Bill

• Energy quantity of gas x representative
  calorific value
• Quantity is expressed as volume at reference
  conditions
• Consumer
• actual metered volume x conversion factor
• conversion factor is provided in the Regulations
• Representative calorific value represents the CV
  of the gas seen by the consumer
• Consumer
• average of charging area CVs over the billing
  period
• determined through use of approved CVDDs

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Sources of Error, bias and Uncertainty

• FWACV
• Daily volumes at Network Offtakes
• Error, bias in daily volumes
• CVs at Network Offtakes
• Error, bias in CVs
• Actual gas quality received
• Variation in gas quality
• Location uncertainty
• Quantity of gas
• Error, bias in domestic meter
• Error, bias in conversion factor

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Estimating error, bias and uncertainty

• Principles suggested by Marcogaz Energy
  Measurement Working Group
• Provides guidance on implementation of OIML
  Recommendation Gas Metering
• Estimates errors and bias in each component of
  measurement, which are then combined
  arithmetically to provide and overall bias in
  energy measurement
• Estimates uncertainties in bias for each source,
  which are then combined in quadrature to provide
  an overall uncertainty in bias.
• Sources measurement instrumentation fixed
  factors representative CV calculation

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Estimating error, bias and uncertainty

• Domestic meter bias and uncertainty
• Fixed factor bias and uncertainty
• Compare with average and variance in pressure,
  temperature, altitude
• Matrix of FWACV scenarios
• Uncertainty in CV determination at NTS Offtakes
• 0.125, 0.25, 0.5 (i.e. 0.05, 0.10, 0.20 MJ/m3)
• Uncertainty in NTS offtake metering
• 1, 4

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Results Consumers energy bills

• Current situation
• MPE in CV determination is 0.25
• MPE in Offtake volume metering is 1
• Overall bias is close to zero (-0.081), because
• Daily CVs and volumes, and hence FWACV, assumed
  to be unbiased
• Small bias arises from assumptions in fixed
  factor in the Regulations
• Expanded uncertainty in bias is 5.8
• 61 of variance arises from temperature variation
• 25 of variance arises from CV variation (i.e. 1
  MJ/m3 cap)
• 9 of variance arises from domestic meter
• 0.06 of variance arises from FWACV uncertainty

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Results Consumers energy bills

• Current situation
• MPE in CV determination is 0.25 0.5
• MPE in Offtake volume metering is 1
• Overall bias is close to zero (-0.081), because
• Daily CVs and volumes, and hence FWACV, assumed
  to be unbiased
• Small bias arises from assumptions in fixed
  factor in the Regulations
• Expanded uncertainty in bias is 5.817 5.822
• 61 of variance arises from temperature variation
• 25 of variance arises from CV variation (i.e. 1
  MJ/m3 cap)
• 9 of variance arises from domestic meter
• 0.06 of variance arises from FWACV uncertainty
  0.22

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Points for discussion

• Overall, consumer billing is largely unbiased,
  provided assumptions about CV measurement and
  domestic and offtake metering are appropriate.
  (This can be part of a specification.)
• Some consumers experience bias and are under- or
  over-billed, largely because of temperature CV
  variation.
• This is as fair as the current system can get
  suppliers and gas transporters dont gain. The
  cap limits the exposure of the worst affected
  (although arguably at the expense of bias in LDZ
  energy).
• Doubling the uncertainty in CV determination at
  NTS Offtakes has little impact.
• Uncertainty in CV determination at small entry
  points is unlikely to have significant impact
  (although yet to be modelled).
• Cheap and cheerful CV measurement in Smart
  meters?