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ISG127/06

Metering Dispensation – D/358 extension

Meeting Name Imbalance Settlement Group (ISG)

Meeting Date 23 August 2011

Purpose of paper For Decision

Summary

This paper presents an application from National Grid Interconnectors Ltd (NGIL) for an extension to the temporary Metering Dispensation, D/358. D/358 was granted by the ISG

in December 2009 on condition that NGIL collected and presented data to the ISG on the operation of the new optical current transformers they were proposing to use for the

Anglo-French interconnector at Sellindge. Due to unexpected delays in commissioning the circuit, NGIL have not been able to collect the required data and are therefore seeking an

extension to D/358 of 16 months so that they can commission the circuit and collect the

requested data before presenting this to the ISG 4 months before the requested extension expiry date. The ISG is invited to approve an extension to D/358 a temporary basis until

April 2013.

1. BSC Requirements

1.1 Section L3.2 of the Balancing and Settlement Code (BSC) requires that all Metering Equipment comply

with the requirements set out in the relevant Code of Practice (CoP) at the time of the Metering System’s

first registration for Settlement or be the subject of, and comply with, a Metering Dispensation.

1.2 Section L3.4 allows a Registrant of a Metering System to apply for a Metering Dispensation if, for financial

or practical reasons, Metering Equipment will not or does not comply with some or all the requirements of

a Code of Practice.

1.3 The process for applying for a Metering Dispensation is set out in BSCP321.

2. Original Application - D/358

2.1 In October 2009 we received an application from National Grid Interconnectors Limited (NGIL) for a

lifetime Metering Dispensation from CoP1.

2.2 As part of an upgrade to replace the existing DC Control and Protection Equipment and thyristor valves at

the Sellindge Convertor Station, NGIL proposed to replace the existing DC current transformers (CTs)

with optical CTs instead of wire wound CTs as required by CoP1. The anticipated commission date for the

upgraded circuit at the time was Spring 2010.

2.3 We presented the application to the ISG at their meeting in November 2009 and the ISG deferred a

decision until their December meeting as they had some questions about the application. NGIL provided

1 ‘Metering Dispensations’

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responses to these questions (Attachment A). The responses (including supporting information) were

presented to the Metering Dispensation Review Group (MDRG) members (at that time).

2.4 At the December meeting the ISG granted a temporary Metering Dispensation, D/358, until December

2011 on condition that the applicant provided ‘clear and comprehensive evidence that the optical CTs are

performing as intended and should include results from calibration tests’. The ISG asked that this

information was provided to the ISG four months before the Metering Dispensation expiry date, i.e. in

August 2011.

3. Application for an Extension to D/358

3.1 On 3 August 2011 we received an application (Attachment B) from NGIL for an extension to Metering

Dispensation D/358 because the circuit is yet to be commissioned and therefore no data has been

collected on the operation of the optical CTs.

3.2 NGIL are seeking a further 16 month extension to D/358 until April 2013 so that the circuit can be

commissioned in September 2011 and the requested data collected. NGIL propose to present the data to

the ISG four months before the requested expiry date, i.e. in December 2012.

4. MDRG and Transmission Company Comments

4.1 The Metering Dispensation application was circulated to the MDRG and the Transmission Company for

comment on 3 August 2011 for 5 Working Days (WD).

4.2 Two MDRG members responded within the shortened timeframe (normally 10WD). One MDRG member

supported the Metering Dispensation application but questioned whether any new information on the

accuracy/reliability of optical CTs had emerged in the last two years which would provide sufficient

evidence to support a change to the CoP allowing the use of optical CTs. We responded that we were not

aware of any. The other MDRG member (who had not seen Attachment A and supporting information)

could not offer much technical advice on the operation of optical CTs themselves and said it would be

difficult to support the Metering Dispensation application on the information provided so far. We have

since provided this additional information and are awaiting a response from this MDRG member -

unfortunately their response may have to be presented verbally at the ISG meeting itself due to their

non-availability at the time of writing this paper. Other MDRG members were re-contacted on 10 August

2011 for any comments that we could also present verbally at the ISG too.

4.3 The Transmission Company fully supports the application.

5. ELEXON’s View

5.1 We originally supported Metering Dispensation D/358 on the following grounds and these still stand:

Accuracy - The optical CTs meet the accuracy requirements for CoP1 wound CTs;

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Redundancy – Separate dedicated optical CT channels will be provided for Main and Check

Settlement metering;

Contingency - The optical CTs will also be critical to the availability and reliability of the DC link

itself; and

Reliability - Calculations performed to a known standard (the Telcordia SR332 Issue 1 standard)

produces figures that predict a Mean Time Before Failure for the CTs of 200 years.

5.2 We support the extension to Metering Dispensation D/358 on a temporary basis on the grounds that

NGIL have not yet commissioned the circuit and have therefore been unable to collect the required data

on the operation of the optical CTs. Allowing for an extension will give the ISG the data it requested and

so enable it to consider the original request for a lifetime Metering Dispensation, in light of this data, 4

months before the requested extension is due to expire.

6. Recommendations

6.1 We invite you to:

a) NOTE the responses from the MDRG members and the Transmission Company;

b) NOTE ELEXON’s recommendation to support the application; and

c) APPROVE an extension to Metering Dispensation D/358 on a temporary basis until April 2013.

Attachments:

Attachment A – Responses from NGIL to ISG’s questions

Attachment B – D/358 Metering Dispensation extension application

For more information, please contact:

Mike Smith, Metering Analyst

mike.smith@elexon.co.uk 020 7380 4033

National Grid Interconnectors Limited 8th

December 2009

Response to BSCP32/4.4 Reference D/358

Question: The ISG would like independent evidence that the Optical CTs have the same

(or greater) accuracy and reliability than wound CTs.

Response

The NXTPhase optical current transformer (NXCT) is leading edge technology offering

highly accurate and reliable performance based on trusted applications developed for the

aeronautic and space industry. The sensors utilise optical measuring technology used in

safety critical applications (gyro stabilizers) where product reliability and performance is

essential in order to meet stringent aviation qualification standards.

This technology has been adapted to meet power industry demands for safer and lighter

designs of CTs, and solutions which could be retrofitted to provide revenue metering [1].

This specific technology was developed based on its pedigree of accuracy and reliability

in harsh operational environments. AREVA have recently acquired NXTPhase (2009) as

a strategic investment to establish world leadership in the optical and non conventional

instrument transformer market.

The measurement performance has been demonstrated to be equivalent or better than the

equivalent conventional wound technology [2]. The performance exceeds IEC Class 0.2S

& IEEE Class 0.3 accuracy requirements to address stringent revenue metering

requirements. Reference [2] is the test performance record for a 345kV NXTPhase CT

carried out in the Powertech Labs, Canada, which is an independent accredited testing

facility.

The technology is in the early stages of implementation. Operational stability has been

monitored at a number of utilities to provide confidence that performance in a harsh

substation environment is equivalent to conventional equipment [3]. More specifically,

the technology has been installed by AREVA in the Hydro Quebec SVC/De-Icer Project

in Canada for DC measurement. The unit has been operational since 2007 [4].

References

[1] IEEE PES T&D Meeting 2006, Applications of High-Voltage Fiber Optic

Current Sensors, Farnoosh Rahmatian, Member, IEEE-PES, and James N. Blake

[2] NXCT 345 kV Test Performance Record for LAPEM, PowerTech Labs, Canada, 2005

[3] Measurement stability over time and temperature, NXTPhase application note,

D00043R00.00, 2004

[4] Power Electronics based transmission line de-icing system, Davidson et al, IEE 8th

International conference on AC-DC Power Transmission, London, 2006.

Question: They would also like to know whether there is an independent power supply to

the AREVA measurements cubicle.

Response

The Areva Measurements Cubicle is supplied by two 110V battery systems labelled

“Station Battery A” and “Station Battery B” in the “Measurements Cubicle” figure, page

2 of 4 ISG106/03 – Attachment B refers. Each battery system is 100% rated providing

full redundancy. Similarly, the supplies for the optical CTs are sourced from 100%

redundant dc/dc convertors thus ensuring that any single failure in the dc supply system

will not cause a loss of supply to the optical CTs.

National Grid Interconnectors Limited 8th

December 2009

Response to BSCP32/4.4 Reference D/358

Question: The diagram also suggests that the CTs are not dedicated to Settlement, if this

is the case then they must be included in the Dispensation.

Response

Duplicate optical CT systems are provided to supply Lane 1/Main Metering and Lane

2/Check Metering respectively.

In all cases dc measurement outputs are buffered and segregated from each other via

analogue isolation amplifiers, as shown in the diagram (page 3 of 4 ISG106/03 –

Attachment B refers) and PCB5008 circuit boards contain analogue isolation amplifiers

for the same purpose.

The magnitude and accuracy of each output is therefore independent of the burden

attached to it and is unaffected by any discrepancy in any other output.

For example, a fault in Lane 1 (HVDC control equipment) would not affect the

measurement to the Main Settlement Metering equipment. Similarly a fault in the Check

Settlement Metering equipment would not affect the measurement to Lane 2 control.

Lane 1 and Lane 2 measurements are continuously and automatically checked for

discrepancies so that a measurement fault on one of the Lanes (e.g. a failure of the CT

Electronics unit DCT-B2) would be immediately alarmed to the Operator.

[End]

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SELLINDGE METERING DISPENSATION APPLICATION BACKGROUND INFORMATION

Background

The tariff metering installed at Sellindge Convertor Station for the energy transfers over the Anglo-French 2 GW Interconnection measures on the DC side and has been in service since the link was first commissioned (~ 1985).

The tariff point or trading boundary for the link is the mid-channel point (on the cable link between the convertor stations).

The DC metering system uses resistive voltage dividers (270 kV to 5 V) and DC current transformers (1850 A to 5 V) using the transductor principal (Secondary windings on magnetic cores placed on a bar primary winding and using a two phase excitation supply with series rectifier fed burden.)

The existing transductor type DC CTs are used for all the C& P functions and the tariff metering.

To achieve a high level of integrity the metering system uses a Main and Check or dual system throughout.

Since the metering was in place before the creation of the ‘Electricity Market’ it was initially adopted as one of the original ‘Alpha’ Codes of Practice (Metering code I).

Present Situation

The Direct Current Control & Protection (C&P) equipment and the Convertor Thyristor Valves are being replaced to maintain availability and enhance reliability of the link.

The existing DC Current Transformers are to be asset replaced in situ because these are considered near end of life and there will be advantage in interfacing with the C&P equipment.

The project is planned to be completed in two phases during Bipole outages in May 2010 and April 2011.

The way the DC Link operates is also to be slightly changed and there are consequently benefits in using new optical based CTs which offer improved performance over the existing units.

The CTs supplying the metering on each pole (Convertor Station contains two Bi-Poles) are situated on the Bi-Pole neutral side in a common assembly / physical location with the CTs used for the C&P functions.

It is proposed that the Main and Check metering on each pole each receive a dedicated output from optical CT sensing head on the pole neutral connections.

With the changes to the thyristor valves and control equipment the loss factor for the metering will change from 1.17% to 1.13%.

Dispensation Application Code of Practice 1 Issue 2 clause 5.1 ‘Measurement Transformers’ requires that all measurement transformers to be of a wound construction. Optical based CTs will be installed at Sellindge to asset replace the existing CTs.

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Optical Current Transformers

The units have an improved accuracy specification when compared to the existing CTs and have a specification meeting IEC class 0.2S accuracy maintained over a range from 0.2 % to 150 % of rated current (exceeds IEC 60044-1 range). This satisfies the COP 1 requirement.

Duplication principals of COP1 are maintained because the proposed scheme for Sellindge provides separate dedicated CT outputs for the Main and Check metering. By ensuring that there is no common failure mode for the main and check the metering data availability should be maintained at a high level. The two attached diagrams show a) the duplication arrangement from the sensing heads through to the Main and Check Meters and b) power supply duplication from station battery systems to psu voltages for electronics. Alarm contacts from the CT electronics modules will also improve the availability.

The availability and reliability of the overall Link will continue to be critically dependent on these CTs which feed the C&P as well as the metering. Calculated MTBF data for each complete CT channel using a 40 deg C basis has produced figures of 200 years (uses calculations based on Telcordia SR332 Issue 1 standard).

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