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ETSU PROJECT K/EL/00287

NETWORK SECURITY STANDARDS

WITH INCREASING LEVELS

OF EMBEDDED GENERATION

Final Report 10 August 2002

Professors Ron Allan and Goran Strbac

Manchester Centre for Electrical Energy

Department of Electrical Engineering and Electronics

UMISTManchester M60 1 QD

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ETSU Project K/EL/00287 Final Report

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CONTENTS

PREFACE TERMS OF REFERENCE

1. Programme of work

2. Constraints and scope of project

3. Status of report and material

EXECUTIVE SUMMARY

MAIN REPORT

Part I Introduction

1. Background

1.1. DTI/Ofgem targets and reports

1.2. Aims and objectives of these studies

1.3. Structure of report

Part II Task A

2. Security and its achievement in distribution systems

2.1. General aspects

2.2. Engineering Recommendation P2/5

2.3. Effective generation2.4. Present-day position regarding P2/5

2.5. Guaranteed and overall standards relationship with P2/5

2.6. Information and incentives project (IIP)

2.7. Future development of security standard

2.8. Recent DTI/Ofgem developments

2.9. Concluding comments

Part III Task B

3. Concepts for evaluating security

3.1. Basic approaches3.2. Levels of evaluation

3.2.1. Level 1 data

3.2.2. Level 2 models

3.2.3. Level 3 results

3.2.4. Summary of staged developments

4. Security considerations

4.1. Review of P2/5 as a security standard

4.2. Assessment of effective generation

4.3. Direct application of P2/5

5. Deterministic studies using an updated P2/5

5.1. Introduction5.2. Effect of availability

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5.3. Effect of number of units

5.4. Effect of technology of units

5.5. Effect of common energy source

5.6. Effect of location

Part IV Task C

6. Introductory remarks

7. Formulating and extending the P2/5 approach

7.1. Introductory comments

7.2. Systems studied

7.3. Capacity outage probability tables

7.4. Expected load lost (ELL)

7.4.1. Constant load

7.4.2. Variable load

7.5. Expected energy no supplied

7.5.1. Constant load7.5.2. Variable load

7.6. Sensitivity study for variable load curves

7.7. Concluding comments

7.8. Implementation

8. Alternative approaches

8.1. Introduction

8.2. Expected demand that cannot be supported

8.2.1. Base case studies

8.2.2. Effect of unit availability

8.2.3. Effect of load model

8.2.4. Comments

8.3. Generating capacity with specified availability

8.3.1. Base case studies

8.3.2. Capacity that can be relied on

8.4. Effect of network

8.4.1. Systems studied

8.4.2. Expected load lost or not supported

8.4.3. Ability to support demand for proportions of time

8.5. Implementation

8.5.1. Expected load lost or supported

8.5.2. Capacity that can be relied on9. Extending P2/5 concepts to frequency and duration

9.1. Considerations

9.2. Suggested form of a frequency criterion

9.3. Application of the approach

9.3.1. Class A

9.3.2. Class B

9.3.3. Class C

9.3.4. Other supply classes

9.4. Effect of load duration curve

9.5. Benefit of embedded generation

9.6. Implementation10. An approach compatible with the Information and Incentives Project

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10.1. Background and concepts

10.2. Systems studied

10.3. Customer minutes lost

10.3.1. Base analysis

10.3.2. Ability of generation to substitute circuits

10.3.3. Credit for reduction in CML10.4. Customer interruptions and customer minutes lost

10.5. Overall comments

10.6. Implementation

11. Assessment of intermittent sources and correlation

11.1. Generator considerations

11.2. Load and correlation considerations

11.3. Modelling and assessment approach

11.4. Application

11.4.1. Wind plant

11.4.2. CHP plant

Part V Task D

12. Commercial arrangements for rewarding generators

12.1. Introduction

12.2. Form of payment to generators

12.2.1. Option fee

12.2.2. Exercise fee

12.3. Funding arrangements

12.3.1. Payments facilitated through network regulatory asset base

12.3.2. Payments facilitated through network operating expenditure

12.3.3. Payments facilitated through distribution use of system charges

12.3.4. Payments facilitated through IIP incentive scheme

12.4. Transparency and tendering process

12.5. Concluding remarks

Part VI Task E

13. General conclusions

13.1. Summary

13.2. Governance

14. Recommendations

Part VII - Appendices

A1. Capacity outage probability tables

A2. Expected load lost

A3. ELL with a load duration curve

A4. Frequency and duration indices

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PREFACE

Terms of Reference

1. Programme of Work

The agreed programme of work had the following five main tasks:-

Task A - To identify the issues including the strengths and weaknesses of P2/5

particularly those associated with the effect on embedded generation and the

impact on financial and technical risk

Task B - To determine how the present Table 2 in P2/5 can be amended to

accommodate present types of embedded generation

Task C - To determine how P2/5 could be more extensively amended in the longer term

Task D - To suggest proposals for a framework for modelling the trading of security and

corresponding contractual arrangements

Task E - To develop conclusions and recommendations to provide focus for further

discussion including Governance and future involvement by all relevant parties

2. Constraints and Scope of Project

At the first project meeting the following constraints and scope were confirmed:-

the output of the project would be in the public domain

the project is a discrete piece of work and would not form part of the workstreams

being created by the DTI/Ofgem Distributed Generation Coordination Group

(DGCG) and the Technical Steering Group (TSG). However it was hoped and

anticipated that the output could provide input to, and could impact on the future

ideas of, the DGCG and the TSG

the project was not expected to provide answers to all questions, but it should

raise issues in order to provide an understanding of the issues and the various

possible ways forward. Also it was not expected to solve all these ways, but to

identify them and to discuss the merits and demerits, the implications on the

various parties, and the possible consequences

the interim report due at the end of April was expected to address Tasks A and B

only.

3. Status of Final Report and Material

This report is the Final Report of this project and therefore all the content, including

concepts and ideas, results, discussions and conclusions are the definitive findings of

our studies.

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EXECUTIVE SUMMARY

The overall aims of this project were to improve the understanding of security and the

contribution that embedded generation can make, and to propose improved methods forassessing security and the embedded generation contribution.

This report has identified and discussed weaknesses and strengths of the present

security standard P2/5. These include the inability to take into consideration the effect of unit

availability, number of generating units, availability of primary energy source, features of

modern generating plant and the impact of network reliability performance. A number of

small examples are included to expose these weaknesses. It is important to stress that, by

using relatively simplistic extensions to P2/5, these parameters could be taken into account,

although the underlying weaknesses of P2/5 would not be resolved. Instead, it would still

remain a deterministic standard with all the existing demerits remaining.

A number of alternative approaches to designing new security standards, with various

degrees of complexity, accuracy and ability to take the inherent probabilistic nature of system

security into account, were developed and discussed. These include approaches based on:

(i) extending the P2/5 approach using expected load lost (ELL) and expected

energy not supplied (EENS) as criteria

(ii) expected demand that cannot be supported

(iii) generating capacity that can be relied on for proportions of time

(iv) frequency and duration criteria

(v) customer interruptions (CIs) and customer minutes lost (CMLs).

These approaches were conceptually developed, their application illustrated on simple

examples and the implementation procedure outlined. For any given situation, the

contribution that a generator makes to system security, and consequently the monetary value

of such contribution, will be driven by the approach on which the security standards are based

and the criterion chosen would have a significant impact on the value of the generator

contribution to network security.

Furthermore, a number of issues associated with the development of commercial

arrangements for rewarding generators for their contribution to network security were

identified and discussed. These include the form of payment (option and exercise fee),

alternative funding arrangements, allocation of risk, transparency and tendering for networksecurity support.

In designing new security standards, the question as to how the potential application

of each of these approaches would affect customers, generators and DNOs, would require

appropriate reliability assessments to be performed (technical aspect) and the framework for

commercial arrangements formulated (commercial aspect). The final choice of the approach

would also be influenced by the overall regulatory and commercial environment within which

network operators are being regulated and incentivised together with commercial

arrangements for connecting to and using distribution networks. Therefore, the update of the

present security standards should be conducted against a well-defined context in order to

ensure consistency.

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PART I - INTRODUCTION

1. Background

1.1. DTI/Ofgem Targets and Reports

The Government has already set targets for renewable energy and CHP plant

to be achieved by 2010, and have recently issued a consultation document suggesting

targets for 20201. Because of their individual relative size, this type of plant is

expected to be connected into the distribution network, which imposes many technical

and commercial challenges. In addition, the new regulatory structure requires

distribution network operators (DNOs) to facilitate competition in generation and

supply. It follows that DNOs are entering an era in which their modus operandi is

rapidly changing with significantly different technical and commercial pressures

placed upon them.

This background led the DTI and OFGEM to set up a joint Working Group 2 to

review all the relevant issues. This reported in January 2001. It is not appropriate to

review or summarise the WG Report in the present report, but it is appropriate to

indicate the connection between them. The WG Report made two overall

recommendations. Recommendation 1 stated that Ofgem should review the structure

of regulatory incentives on DNOs in the light of the new statutory duty on DNOs to

facilitate competition. One of the activities identified under this recommendation

was:-

Review and prepare guidance that will allow DNOs to intepret design and

operational codes in such a way as to allow the contribution of embedded generation

to network performance to be fully taken into account. A review of the codesthemselves and of the governance arrangements for distribution networks should

follow.

One such code is Engineering Recommendation ER P2/53 dealing with

Security of Supply and instituted by the then Electricity Council in 1978.

Since the WG Report was issued, ETSU has created small scale projects

addressing several of the issues raised by the WG. UMIST was commissioned to carry

out one of these projects, and this is the subject of this report.

1.2. Aims and Objectives of these Studies

It is useful at this stage to state the aims and objectives proposed by UMIST

and which were subsequently agreed with ETSU, the project sponsor.

The overall aims were agreed as:-

to improve the understanding of security and the contribution that embedded

generation can make

1 The Energy Review. Performance and Innovation Unit, Cabinet Office,www.piu.gov.uk/2002/energy/report/index.htm2 Joint Government Industry Working Group on Embedded Generation, Report into Network Access Issues.

Department of Trade and Industry, January 20013Engineering Recommendation P2/5, Security of Supply. The Electricity Council, October 1978. (now the

Electricity Association)

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to propose improved methods for assessing security and the EG contribution.

The objectives were agreed as:-

to review the present issues regarding security, the present strengths and

weaknesses with P2/5, and why these limit the contribution that EG iscurrently allowed to make

to identify short term procedures and measures so that Table 2 of P2/5 could

be updated very simply in order to recognise modern types of generation

plant including renewables

to identify longer term procedures and measures that would allow security

standards to treat all players equitably, and allow improved design and

operating standards to be introduced

to identify models for establishing a market in security in order to enhance

trading opportunities

to support the proposals by small-scale system studies, so the significanceand scale of the problem and issues are clearly apparent.

These general aims and objectives led to the specific Tasks stated in the

Preface, and repeated here for completeness. Five main tasks were agreed:-

Task A - To identify the issues including the strengths and weaknesses of P2/5

particularly those associated with the effect on embedded generation and

the impact on financial and technical risk

Task B - To determine how the present Table 2 in P2/5 can be amended to

accommodate present types of embedded generation

Task C - To determine how P2/5 could be more extensively amended in the longerterm

Task D - To suggest proposals for a framework for modelling the trading of

security and corresponding contractual arrangements

Task E - To develop conclusions and recommendations to provide focus for

further discussion including Governance and future involvement by all

relevant parties

1.3. Structure of Report

This report is divided into separate parts, each dealing with one of the mainTasks set out above. This part addresses the background and general objectives of the

project. Part II considers Task A, that is the general aspects of security, the historical

developments, how it is presently achieved by DNOs taking into account the

obligations set by the Regulator and Statutes and the wishes of customers, and the

trends currently in place for the future. Part III considers Task B, that is a

consideration of the approaches by which Table 2 of P2/5 could be amended in the

short term. Part IV considers Task C, which addresses a range of alternative

approaches, including discussion of their merits and demerits and suggestions for the

implementation of each approach. Part V deals with Task D and considers alternative

approaches for creating a commercial framework to reward embedded generators for

their contribution to network security. Finally Part VI summarises the information andsuggests the next steps needed to create updated security standards.

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PART II - TASK A

To identify the issues including the strengths and

weaknesses of P2/5 particularly those associated with the

effect on embedded generation and the impact on financial

and technical risk

2. Security and its Achievement in Distribution Systems

2.1. General Aspects

The term security can mean many different things depending on the context,

whether it is an operational situation or planning situation, and the viewpoint of the

utility or user of the system, including suppliers and end-customers. In the case of

distribution systems, it generally refers to the ability to maintain a connection to theend-customers and to supply them with the power level and energy demanded. This

appears to be synonymous with the term reliability, but care is needed in this

interpretation.

The term reliability is nowadays very specific in its meaning, and is

accepted to be defined by a set of probabilistic indices even if only expected (average

or mean) values are reported or predicted. Reported indices include frequency of

interruptions, duration of interruptions, annual unavailability, and load and energy not

supplied. In addition there is a growing interest in determining worth of supply and

consequently indices representing cost of interruptions perceived by both customers

and utilities are rapidly gaining importance. These indices include information

regarding the likelihood of interruptions (frequency and duration) as well asinformation regarding the severity of the interruptions (load and energy not supplied).

A combination of likelihood and severity provides a true measure of risk. If only

average values are reported or predicted, it is important to recognise that these are not

deterministic values, should not be treated as such, and that they are associated with

probability distributions, the shape of which may be unknown.

It can be argued that the term security should do likewise. However this is

not usually the case although the situation is changing and gradually moving in this

direction. Instead, security is often measured by deterministic indices that may

include the severity of situations but ignore the likelihood. Examples are percentage

reserve used in spinning reserve assessment, and the n-1 or n-D criteria used in

transmission operation and planning. The current security criteria used in distribution

planning and operation generally have similar deterministic characteristics, including

the current P2/5 security standard with which all DNOs are obliged to conform. This

is a potential weakness, which is partly, but not solely, the cause of embedded

generation not being given any or much credit for security contributions. These

aspects are discussed in more detail in the following section of this report.

2.2. Engineering Recommendation P2/5

Security is, and always has been, of high priority to the electricity supply

industry (ESI). Therefore effort has always been made at both the planning andoperational stages to ensure that security is as high as possible commensurate with the

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cost of achieving it. Because of the need to balance cost against the benefit, different

distribution systems inherently possess different levels of security: the security

generally improves with increasing voltage level, and with increasing levels of

underground cables compared with overhead lines. Therefore transmission is

generally more secure than distribution, and urban systems more secure than rural

ones. In order to ensure that all customers have a minimum level of security,guidelines and standards exist.

Prior to privatisation of the ESI, the then Electricity Council was responsible

for setting and maintaining a range of common technical and economic requirements,

one of which was to determine and set appropriate levels of security. In order to create

consistency between all distribution companies existing at that time, the Electricity

Council formulated Engineering Recommendation P2/54 in 1978. This is intended to

be a guide to system planning and design: it is not an operational standard or guide.

Although departures from P2/5 recommendations are permitted, these have to be

justified using detailed risk and economic studies. Whether any such studies have

been done remains unknown although anecdotal information suggests that these are

likely to be few in number. Primarily P2/5 consists of two tables. Table 1 specifies themaximum reconnection times following pre-specified events leading to an

interruption. This time is dependent on the Group Demand affected by the

interruption. Table 2 specifies the security contribution that could be credited to any

embedded generation. P2/5 permits this contribution to be added to the cyclic rating

of the remaining circuits and the transfer capacity of alternative sources in deciding

whether the Group Demand could be satisfied in accordance with the values quoted in

Table 1. Although intended to create consistency, P2/5 can not do so for several

reasons, the most significant being the following:-

it specifies duration of interruptions but does not specify frequency.

Anecdotal information suggests that frequency was intended to be part of

the standard at one stage of development but that this was subsequently

deleted. Some in the industry have argued that frequency is considered

implicitly in some instances because P2/5 states that Group Demands

greater than 60MW must be reinstated immediately, implying that,

although a circuit outage has occurred, an interruption is not seen by the

end-customer. However, immediately is defined, as loss of supply

should not exceed one minute. Consequently the interruption is seen and

customers do experience loss of supply, even if the distribution companies

do not, and need not, count it.

it is treated as a deterministic standard and does not account for thestochastic variations of interruptions. This is frequently misunderstood and

some in the industry believe and stress that it is a probabilistic standard:

this is implied in P2/5 itself. There is no doubt that the Electricity Council

wished to take into account the probabilistic nature of system failures in a

more formal way than existing prior to P2/5 and did in fact use reliability

evaluation techniques and computer programs (including UMISTs

RELNET5 program) in deciding the values to specify in Tables 1 and 2.

However, although these values may be based on reliability evaluations,

they are used deterministically without recognising the stochastic nature of

4

Engineering Recommendation P2/5, Security of Supply. The Electricity Council, October 1978. (now theElectricity Association)5

RELNET, Reliability Evaluation of Electrical Networks. UMIST

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system behaviour. For example, the values are equally applicable to

Hydro-Electric which operates in a very adverse environment and with a

very low customer density, and to London Electricity which operates in a

very favourable environment and with a very high customer density.

one of the weaknesses associated with the deterministic nature of P2/5 is

the basis of the values quoted in Table 2. This table implies that, in mostcases, 67% of the installed generating capacity can be considered as the

effective capacity and this can be considered as the contribution which

should be credited to the embedded generation. Very little justification for

this is provided in P2/5. As this is at the heart of the debate regarding the

contribution that embedded generation can make to security, this is

discussed in more detail in the next section.

Before considering this concept of effective generation in more detail, it is

worth re-iterating that P2/5 is a design standard, not an operational one. The

significance of this is very important. A design standard simply implies that, if the

design conforms with a specified set of conditions, then the designer has satisfied thestandard. This does not mean that these conditions will be satisfied under all

operational circumstances. If violations occur during the operational phase then

perhaps one or more operational standards will not be satisfied, but this does not mean

the design standard has been violated. Violations could mean that the embedded

generation was insufficient at the time, i.e. the output was less that 67%. However it

could also mean that insufficient network capacity was available at that time due to

network outages being greater than that considered creditable. It follows therefore that

a design standard is intended to minimise these violations, not necessarily to eliminate

them.

This does not mean that changes to the concept of design standards,

particularly P2/5, are not warranted, quite the opposite, but it is important in putting

the background and perspective of P2/5 into proper context.

2.3. Effective Generation

In the 1970s when P2/5 was developed, considerable amount of local or

embedded generation existed in the distribution networks, this generation generally

being that left over from the pre-nationalised industry. The main question at that time

was whether this local generation had value in the system or should be neglected and

declared obsolete. This provided the background logic for creating Table 2 of P2/5.However the reasoning and how the values were determined is not included in P2/5

and little other documentation now seems to exist. Part of the reasoning is contained

in an application report ACE Report 516 with reliability cost assessments provided in

ACE Report 677.

ACE Report 51 defines the effective generation contribution as the

transmission circuit capacity which, when substituted for the generating plant in

various generation/transmission systems, results in the same reliability of supply from

each of these systems. No explanation is given of how this was done, although a

6 ACE Report No. 51, Report on the Application of Engineering Recommendation P2/5, Security of Supply.

The Electricity Council, May 1979. (now the Electricity Association)7ACE Report No. 67, Report on Reliability Investment in Radial H.V. Distribution Systems with Overhead

Lines. The Electricity Council, March 1979. (now the Electricity Association)

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summary of the outcomes is provided. The report states that the availability of

generator units was assumed to be 86% and the reliability of the circuits with which

they are compared were considered to be perfectly reliable. The outcomes are quoted

as the ratio of effective output to maximum output of generators. Depending on the

set size, these ratios varied between 0.4 and 0.9. From these ratios the report

concludes that an average ratio of 0.67 could be used, this being the value quoted inTable 2 of P2/5.

Although this single value may be convenient for deterministically assessing

system security, it does not take into account the fact that the ratios themselves are

clearly variables, the availability of units can vary significantly, and lines and

transformers with which they are compared are not perfectly reliable. Also it neglects

the fact that a mean value is only the long-run average and this value may never be

achievable in its own right. For example, a single unit either is capable of delivering

full capacity or nothing, the average is only the weighted value of these two actual

achievable levels.

It follows therefore that the 67% is simply a level deemed to be the

contribution that can be associated with the embedded generation. It does not meanthat this capacity level will always be available but can be considered to be available

sufficiently frequently that violations of network reliability happen infrequently. It is

therefore a value that can be added to the cyclic rating of the remaining transformers,

which themselves may not always be available, or the actual capacity may be reduced

due to adverse environmental conditions.

However there is a tacit assumption in P2/5 that 67% of installed capacity can

be relied on at all times. This is illusory. It also provides comfort that irrespective of

when the original circuit outage occurs, the embedded generation will be able to

supply 67% of its installed capacity. In reality this is not possible since it could only

provide the capacity of available units, which could range from 100% of installed

capacity to nothing at all. It may therefore not be possible to provide the capacity

required to satisfy the requirements of Table 1 in P2/5.

This aspect of effective generation is considered in more detail in Section 4.2

during the discussion of Task B.

2.4. Present-day Position Regarding P2/5

P2/5 was developed prior to privatisation and the restructuring in 1990 of the

ESI. It therefore really reflects the industry structure of the 1970s, and not that of

today and certainly not that associated with the underlying market concepts. Despite

this, P2/5 was incorporated without revision into the statutes dealing with theprivatisation of the industry and is now an inherent part of the licence of each DNO.

The consequence is that it must be adhered to in all situations. This leads to several

problems and inconsistencies in addition to those outlined above. These additional

ones are briefly discussed below:-

the types of generating units existing in the 1970s are different to those of

today. P2/5 recognises two main types of plant, steam and gas turbines.

Embedded steam plants operating on 1, 2 and 3 shifts no longer exist.

Modern gas turbines should be more reliable. No consideration is given to

renewables (wind, solar, water, etc), nor to CHP. Consequently the

assumption of generating units having an availability of 0.86 is no longerjustified, gas turbines are likely to be better (may be up to about 0.95), and

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some renewables such as wind are likely to be much worse (perhaps less

than 0.40). This means that the 0.67 factor used in Table 2 of P2/5 is likely

to be inappropriate for reasons additional to those discussed above.

the operation of generating capacity is no longer centrally controlled and

dispatched as in the 1970s. Consequently, even if embedded generation

exists in the distribution network suffering an interruption and this couldprovide energy to disconnected customers, there is no obligation on the

generator to provide it. Therefore, since security is the sole responsibility

of the DNOs, it is reasonable for DNOs to argue why should they rely on

embedded generators which have no need to provide the cover unless they

so wish.

the Regulator determines the mechanism by which DNOs achieve their

revenue stream during the price control review; this being done on a five-

yearly cycle. Presently the current regulation process is one effectively

based on value of assets, i.e. asset-based regulation. This process seems to

financially encourage DNOs to increase the size of their asset base. DNOs

are not incentivised to purchase security from some other supplier.

2.5. Guaranteed and Overall Standards - Relationship with P2/5

Since privatisation, the Regulator (Ofgem8, previously Offer9) has established

two sets of operational standards, the Guaranteed Standards (GS) and the Overall

Standards (OS). Guaranteed Standards set service levels that must be met in each

individual case. If the DNO fails to provide the level of service specified, it must

make a penalty payment to the customer affected. Overall Standards cover areas of

service where it is not appropriate to give individual guarantees, but where customers

in general have a right to expect from DNOs predetermined minimum levels of

service. No penalty payments are made if these levels are not achieved.

Presently the main GS from a security point of view is GS2 relating to

restoration of supply. To ensure that inconvenience to customers is kept to a

minimum, this GS requires companies to restore the supply within 18 (previously 24)

hours of the company becoming aware of a fault on the distribution system.

Additional requirements are being developed under the Information and Incentives

Project (see Section 2.6).

There is debate within the industry that centres on the relationship between

P2/5 and these operational standards, and on the need for P2/5 now that DNOs are

subject to the increasingly stringent requirements of these GS and OS. It has been

argued in some quarters that P2/5 is no longer relevant or needed and has beensuperseded by these operational standards. This however ignores the fact that there is

a fundamental difference between them. P2/5 is a design standard and the relevant GS

and OS are operational standards. P2/5 therefore must be satisfied at the design and

planning stages and ensures that all customers receive a minimum level of security.

The ability to satisfy the relevant GS and OS however is only known after the system

is in operation, an operational period that may be quite significant before the

information becomes available. Consequently P2/5, or more specifically a standard

similar to it but may be one that more truly reflects modern plant, operation and

8Office of Gas and Electricity Markets

9Office of Electricity Regulation

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structural aspects, is required if DNOs are to remain committed to obligations set by

the Regulator.

2.6. Information and Incentives Project (IIP)

Ofgem are currently discussing a series of proposals with the industry thatcome within a project, which the Regulator has defined as the Information and

Incentives Project (IIP). The basis of the project and its possible impact on P2/5 and

any future enhancement of this security standard is worthy of reflection.

All DNOs are required to report certain performance figures annually to the

Regulator. These enable Ofgem to publish the annual Security (number of customer

interruptions, CI) and Availability (number of customer minutes lost, CML)

performance figures for each DNO. Although published and DNOs compared, these

performance figures do not relate to any GS, and therefore are not associated with any

penalty payments. Also they are not directly linked to the price control. The only

security related GS is the time for reconnection of supply following an interruption.

Through the IIP, the Regulator is proposing to increase the amount of information thata DNO must provide on an annual basis, and to extend the use of this information.

As discussed in Section 2.4, the current regulation process is one based on the

value of assets, i.e. asset-based regulation. This concept is being reviewed within the

IIP with the intention of not only extending the information that must be provided, but

also linking the network performance to the price control and thus moving towards a

regulation process based on performance, i.e. performance-based regulation.

Also as part of this IIP, Ofgem is intending to extend GS and OS to include

interruption frequencies, with a GS requirement that no customer should experience

more than a specified number of interruptions, a value that is not yet finalised. This

would mean that P2/5 would have less requirements than the GS, a situation which

itself may mean there is a need to review and revise P2/5.

2.7. Future Development of Security Standard

It is evident that, although P2/5 exists and is an integral part of the licence

under which each DNO operates, it has many weaknesses. This is generally

recognised within the industry. One of the most recent occasions when this problem

was addressed and exhaustively discussed by all players in the industry (DTI, Ofgem,

DNOs, generators, suppliers, customers), was by the DTI/Ofgem Working Group on

Embedded Generation Network Issues10. It would not be appropriate to reproduce or

specifically summarise the EGWG Report, but the Main Report and Annex 2 containseveral important concerns and issues which relate to the present dilemmas and

suggestions for the future direction of security standards, particularly with respect to

embedded generation. The following brief points are gleaned from this report, not to

provide a comprehensive summary, but to indicate the thoughts within the minds of

those participating in the discussions:-

both DNOs and generators agree that the current regulatory framework

inhibits DNOs taking account of the contribution that embedded

generation could make to security.

10Joint Government Industry Working Group on Embedded Generation, Report into Network Access Issues.

Department of Trade and Industry, January 2001

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both DNOs and generators agree that there could be material benefits from

the contribution that could be made by embedded generation, that this

contribution to security has value to DNOs and that this value should be

rewarded.

both DNOs and generators agree that there is a need to review P2/5

particularly regarding Table 2. One point of consensus is that a guidancenote should be agreed to supplement the treatment of the Effective

contribution of generation to network capacity paragraphs in Section 3 of

P2/5.

embedded generators consider that any review should move towards a

more probabilistic approach.

DNOs consider that any review of P2/5 should take into account the

diverse nature of current embedded generation as well as the obligation

and risks that DNOs carry for maintaining security.

DNOs consider that any method of assessing and valuing security

contributions must be transparent.

It follows from the above points that the industry is now willing to review

P2/5, particularly with respect to embedded generation. However there remains a

hesitance to establish a major review of the bulk of P2/5, despite acceptance that a

number of weaknesses exist. One reason for this, which is understandable, is that the

time required for conducting a major review and obtaining universal agreement could

be substantial. On the other hand a short review only of Table 2 and its implications

could be achieved very much more quickly. The EGWG Report recognises this

dilemma and suggests that the review of Table 2 should then be associated with an

early deliverable, with the full review and more extensive studies dealt with

subsequently. On the basis of current knowledge, the future of P2/5 can be anticipated

as follows:

in the immediate future, no changes are likely to occur because of the need

to obtain the agreement of all players in the industry and the difficulties of

achieving this. This time-scale is likely to be at least 1-2 years

in the intermediate future, the likely change is an enhancement of Table 2

of P2/5. This would logically centre on extending the types of generating

units considered and modifying the effective contributions that could be

associated with each of these. The likelihood is that the industry players

would wish these to be based on those underpinning the values in the

current P2/5 - the debate however may centre on the difficulty of knowingexactly or even approximately how this was done. Although the EGWG

anticipated that a target date of January 2003 would be possible, this is

likely to slip

in the more long-term, a view is that a more extensive review of the basis

of P2/5 is required. This would be two-part. One part would be to review

the underlying concepts of P2/5 with the intention of replacing Table 1 and

enhancing the criteria. The other part would be to review the method of

assessing the benefit that embedded generation could make to security.

Both parts would need to address the probabilistic issues. A possible date

for achieving this remains difficult to predict because it requires an

agreement to proceed, and an agreement of the approach to be used. It isnot likely to happen within the next 3-5 years.

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2.8. Recent DTI/Ofgem Developments

Since the EGWG Report was published, several developments have taken

place at both the DTI and Ofgem. These include a Consultation Document11 issued by

Ofgem and a joint DTI/Ofgem Workshop12

organised by ETSU and UMIST.The Consultation Document in particular is of considerable interest since it

outlined and summarised the current views and responses of Ofgem to the outcomes

and proposals of the EGWG Report. The document covers many issues relating to the

future pricing and incentivising of DNOs with respect to embedded generation, but

only the points specific to the thrust of this report are highlighted. The following

statements, extracted from the Consultation Document, are relevant and are worth

noting:-

Replacing investment in a relatively reliable network with investment in

generating plant that may be less reliable could constitute a disbenefit

During the period July 2001 to January 2002, we plan to participate inwork on: consideration of changes that might usefully be made to

Engineering Recommendation P2/5

In the current framework, DNOs are probably right to argue that there is

no profit for them in connecting embedded generation

DNOs can argue that existing and planned technical requirements act as

disincentives to the connection of embedded generation

DNOs are required, by their licences, to design networks in accordance

with Engineering Recommendation P2/5. Reliance on embedded

generation rather than on network reinforcement could breach P2/5

In the longer term, it might be possible to review P2/5. Ofgem will be

discussing the possibilities with other interested parties

Ofgem notes that meeting IIP requirements will be a major concern for

DNOs when considering embedded generation

There are two specific implications that can be extracted from these

statements. Firstly, Ofgem seems to recognise the difficulties that DNOs have with

regard to embedded generation in the current climate and regulatory regime.

Secondly, the time-scale by which changes to P2/5 may be made seem to be long term

rather than short term, thus no immediate revisions to any part of P2/5 are imminent.

The purpose of the Workshop was to respond to the proposals made by the

EGWG, to identify possible work programmes, and to establish priorities and timeschedules. It was not intended to propose changes to the proposals, nor did it do so.

However it reflected on these in light of the Ofgem Consultation Document.

11

Ofgem, Embedded Generation: Price Controls, Incentives and Connection Charging. A PreliminaryConsultation Document, September 200112

DTI/Ofgem Joint Workshop on Embedded Generation, Stratford-on-Avon, 1-2 October 2001

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2.9. Concluding Comments

At this point in time, the following aspects and issues are applicable regarding

security and P2/5:-

P2/5 as published in 1978 remains in force DNOs are compelled to comply with this version of P2/5 under their licence

DNOs must design to the maximum interruption times specified in Table 1

DNOs may, if they wish and can agree conditions, give some credit to the

contribution that embedded generation could make to security

this contribution would need to be based on the capacities specified in Table 2

there is a mood within the industry for a review of Table 2, but no decision as

to how has yet been proposed

there is less inclination in the industry for a more radical review of P2/5

the implication of recent Ofgem consultations is that no immediate changes

are likely to be made to P2/5 in our opinion, this could lead to the following concerns:

simple changes to the values in Table 2 leaves the underlying problem that

the values in this table are only average values

because no-one can guarantee that these average values would be available

as actual values at all times, DNOs would still be inclined to ignore this

possible contribution

the weaknesses existing in the bulk of P2/5 would remain, including no

association with frequency of interruptions and no recognition that systems

behave stochastically.

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PART III - TASK B

To determine how the present Table 2 in P2/5

can be amended to accommodate present typesof embedded generation

3. Concepts for Evaluating Security

3.1. Basic Approaches

In order to consider the principles underpinning P2/5 and to consideralternative approaches, it is first necessary to define the principle of the modelling

procedure. This is shown schematically in Figure 1.

data 1 data 2 data 3 . etc

model 1 model 2 model 3 . etc

result 1 result 2 result 3 . etc

level 2

level 1

level 3

Figure 1 - Hierarchical levels in the modelling procedure of security contribution

This figure illustrates the hierarchical structure of the modelling procedure.

The principle is that a set of input data is selected from known or unknown

information (level 1), which is then processed using some defined model that reflects

system behaviour (level 2), into a set of output data generally called results (level 3).

It is important to recognise that these three levels apply equally to every type of

system analysis, including security assessment, and is nothing more than data

processing albeit a complex process. The essential problems in such processing is

whether the data is known or known with sufficient certainty, whether a processing

model can be developed with sufficient accuracy to reflect all system effects, and

whether the results and assumptions embedded within them are fully recognised by

decision makers and therefore used appropriately. One or more of these steps are

frequently performed using significant approximations and simplifications. These are

discussed in the following sections with respect to security assessment.

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3.2. Levels of Evaluation

3.2.1. Level 1 - Data

Level 1 could be expected to include such data as the number of units, the type

of units, the availability of units, the system demand levels, and similar aspects. Sincesome of these are stochastic in nature, e.g. unit availability and system demand, it

could also be expected that the probabilistic nature of these should be specified,

perhaps in terms of average values, variances or probability distributions.

In P2/5 however, the unit types are limited to conventional steam plant and gas

turbines, and the availability of each set is fixed as 86%. Conventional steam plant no

longer exists in distribution systems, and a single value of unit availability does not

recognise the considerable variation that exists with modern generating plant. These

are two of the main weaknesses of P2/5, as discussed in Section 2.4, and are likely to

be two of the most significant reasons why DNOs and others remain reluctant to use

P2/5 for assessing the contribution that could be made by embedded generation.

In reality it can be argued that gas turbines do form part of modern embeddedgeneration and, since P2/5 does not include any restrictions or assumptions about such

plant, the values included in P2/5 applicable to gas turbines could be used by DNOs

without violating any of their licence conditions. The only logical reason why such

applications are not done is because of uncertainty whether this argument is legally

valid and because DNOs remain responsible for ensuring the required security levels,

not a third party such as an embedded generator.

The present studies address these weaknesses and take into account the

stochastic variations that exist in the system attributes. These extensions however are

included sequentially, rather than including them all simultaneously. The attributes to

be considered are: unit availability, number of units, technology (gas, wind, etc),

common source, and location.

3.2.2. Level 2 - Models

Modelling the process is the most complex part of the procedure and the most

frequent cause of inappropriate results. In order to make the process tractable, or as

easy and as straightforward as possible, simplifications and approximations are often

made. Sometimes these have to be done because the exact system behaviour is not

known or cannot be modelled. Such assumptions must be quantified and the impact of

these should, if possible, be assessed by sensitivity studies. However simplifications

and assumptions are also made in order to simply reduce the complexity of theevaluations. Downsizing of companies have resulted in some skill levels, particularly

those that are used infrequently, being reduced or even eliminated. Therefore the

ability for companies to perform complex analyses may no longer exist in certain

areas. This is enhanced by companies expecting employees to be more generalist

rather than specialist. Two of the most simplistic approaches are the use of look-up

tables and the use of spread-sheet assessments.

Look-up tables only require knowledge of some specified input data or

information, and the result is found directly from the table. P2/5 clearly comes into

this category. There is no suggestion that the look-up values quoted in P2/5 were

not deduced by extensive evaluations, nor that the values are not appropriate for the

set of input data and information specified. Extensive studies were certainly done bythe Electricity Council in the 1970s, although the procedures used have become lost

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with the passage of time. The weakness however is that the values then become

rigidly fixed in time and are used deterministically.

This is the present position with P2/5. It is fully understandable why such a

position has been reached. Firstly, it is easy to demonstrate that a deterministic

standard has or has not been satisfied. Secondly, uncertainty about the impact of a

new set of standards deters the ready acceptance of such developments.These studies are staged in the usage of additional modelling:-

Task B retains the methodology used in the original development of P2/5,

at least as far as it is possible to decipher this. Therefore this addresses the

problem deterministically and provides an approach for producing a

suitable set of values, these values then being treated deterministically. It

must be recognised that these values retain all the erroneous characteristics

that the values in P2/5 possess. The essential difference between updated

values and the previous ones is that the updated ones would reflect

present-day attributes. The models used to obtain these values are based on

probabilistic techniques and the values are therefore underpinned by

consistent probability theory. However the results are then tabulated in theform of look-up tables. This Task is considered in this part of the report.

Task C will extend the security assessment into a more substantial

probabilistic domain with the intention of creating a more objective

consideration of the contributions that embedded generation could make to

security. Because of its stochastic nature, these considerations will be

based more soundly on probabilistic approaches. The framework of

assessment is likely to be more complex. One framework could structure

the methodology on spread-sheet evaluations in which the elements are

evaluated using probability theory and equations. Consequently, no

predefined values would exist. Instead the user would identify appropriateattributes, choose these from options or input their values, and the spread-

sheet elements would calculate the appropriate result. The second

framework could structure the methodology on a sequence of theoretical

equations and techniques, probably defined in terms of an algorithm. The

benefit of this approach is that it would be completely flexible, and could

be structured to reflect any system configuration, plant operation scheme

and commercial agreement. This Task however is beyond the scope of this

Interim Report, but is discussed in a little more detail in Section 7.

3.2.3. Level 3 - Results

The values of the output results clearly reflect the input data and the

evaluation approach and theory. The range of results reflect the need within the

decision making process. The output results quoted in Table 2 of P2/5 are the

percentage of capacity that can be credited to the embedded generation. This is very

restrictive, but evidently was sufficient for the purposes proposed for P2/5 in the

1970s. Task B in these studies retains this restriction in order to be compatible with

the constraints and structure of P2/5. However Task C extends this concept

conceptually to a range of possible values including the possibility of addressing

frequency of interruptions (i.e. Security or CIs) and duration of interruptions (i.e.

Availability or CMLs). This would make any derivative of P2/5 more compatible with

Ofgems IIP.

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3.2.4. Summary of Staged Developments

The above discussion can be summarised as illustrated in Table 1.

Table 1 - Summary of the scope in Task B and Task C

Task Input data Model Results

B to reflect present-

day plant and

attributes

using P2/5 approach P2/5 format and scope

presented as look-up table

C to reflect present-

day plant and

attributes

using probability

theory, models and

techniques

extended set of output results and

scope that could be presented as

Look-up table

spread-sheet format

algorithmic theory

4. Security Considerations

4.1. Review of P2/5 as a Security Standard

The underlying concept of security is that there should be sufficient

connections and capacity in the system such that, when an outage event occurs,

customers are able to continue receiving a supply or have it restored within anacceptable time period. The minimum level of security for different Group Demands

are those specified in Table 1 of P2/5, and these must be achievable by licence and

statute. There are various ways in which these levels can be achieved:-

increasing the capacity of existing circuits

constructing additional circuits

using embedded generation to replace the capacity of circuits on outage.

All three options are permitted by P2/5. The amount can easily be determined

through power flow studies in the case of the first two options, i.e. those associated

with circuit capacity and, in principle, from a simple comparison with Table 2 in thecase of the third option. The latter is the focus of these studies and therefore only this

is addressed in this report.

This is a relatively straightforward consideration using P2/5 because the only

data required is the type of units and the capacities of each unit. P2/5 has an identified

Type of Generation defined as Gas Turbine Units. These are not qualified in any way

and a simple reading and interpretation of P2/5 would suggest that the values quoted

in P2/5 could be used for any gas turbines. The debate for and against this are:-

For - since the type of gas turbine is not qualified in P2/5 and the licence

permits Table 2 to be used, then the licence will be satisfied if the values

given in P2/5 are adhered to without amendment

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Against - although there may not be any qualifications given for gas

turbines in P2/5, it is only reasonable to note that P2/5 was written in the

1970s and modern gas turbines are likely to be different to those available

at that time. Therefore it is unreasonable to continue using values

determined in the 1970s for units being used in the 2000s

The dilemma in this debate is that both points are true, although seemingly

diametrically opposite. The distinction is solely whether one adheres rigidly to P2/5

and the licence requirements, or whether one adds the qualification, is this adherence

now reasonable?

This section however is concerned only with reviewing the outcome of P2/5 if

it is used without modification. In this case, the security levels must conform with the

requirements of P2/5.

Table 1 of P2/5 specifies the minimum Demand that must be met after certain

specified circuit outages. This level is dependent on the Group Demand or Class of

Supply. The amount of demand that can be supplied depends on the available circuit

capacity and the amount of generation capacity that can be credited. In the latter case,this can be assessed from Table 2 of P2/5. This table indicates that the effective

contribution of most units is 67% of its declared net capability, and that this

contribution should be restricted to supplying that part of the demand which is not

required to be supplied immediately following the first circuit outage.

It is therefore necessary to determine the net capability of the embedded

generation, i.e. the capacity that could be provided to the network in terms of security,

which would then be weighted by the specified 67%. In many cases net capability will

be the rating of the units, in which case the only required knowledge is the capacity

and number of units. If restrictions are set by the generator owner or operator then, in

addition, knowledge of the export agreements would be needed. The main problem is

with CHP plant since the primary purpose of such plant is to supply local heat,

electrical energy being a by-product. Because of this, the available electrical energy

will be dependent on the amount of steam required at any time, and this may be less

than that needed to produce the required electrical capacity levels. This problem is

discussed separately later but briefly the effect could be dealt with by considering

variation in the available capacity levels and therefore in the value of unit capability.

4.2. Assessment of Effective Generation

As discussed in Section 2.3, P2/5 was developed when the embedded

generation that existed in the distribution networks was generally that left over fromthe pre-nationalised industry, and the main question was whether this local generation

should be neglected and declared obsolete. Table 2 of P2/5 assisted in this decision

making. However the only reasoning of how the values were determined is contained

in the application report ACE Report 5113 with reliability cost assessments provided

in ACE Report 6714.

Since ACE Report 51 seems to be the only definitive document presently

available that quantifies the reasoning behind the values specified in P2/5, it is

appropriate to consider this report in some detail. The most significant part appears to

13 ACE Report No. 51, Report on the Application of Engineering Recommendation P2/5, Security of Supply.

The Electricity Council, May 1979. (now the Electricity Association)14ACE Report No. 67, Report on Reliability Investment in Radial H.V. Distribution Systems with Overhead

Lines. The Electricity Council, March 1979. (now the Electricity Association)

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be Appendix A3 of ACE Report 51 headed Comparison of Generation and

Transmission/Distribution Firm Capacity, and it is therefore worthy to quote the

relevant paragraphs. These are:-

The effective generation contribution was estimated by determining the

transmission circuit capacities which, when substituted for the generating plant in

various generation/transmission systems, would result in the same reliability ofsupply from each of these systems. This substituted capacity was considered as the

effective generation contribution; the ratio of effective output to maximum output

was determined in each case. For the various combinations of generators and

transmission circuits examined, the ratio of effective output to maximum output of

generators was not constant, but varied mainly as a function of the ratio of generator

unit size to the transmission circuit capacity. Thus, where the set size was about one

quarter of the circuit size the ratio ranged between 0.8 and 0.9; with set sizes of one

half the circuit size the ratio ranged between 0.7 and 0.8, and with set sizes equal to

the circuit size the ratio ranged between 0.4 and 0.5. It should be noted that

throughout these studies a winter-time average availability of generation of 86% was

assumed.Based on an examination of networks with local generation, it was decided

that for the purpose of developing Table 2 of P2/5 a factor of two-thirds for the ratio

of effective output to maximum output could be adopted.

The following comments and critical assessments can be made:-

the effective generation contribution is defined as the transmission circuit capacity

which, when substituted for the generating plant, results in the same reliability of

supply.

no explanation is given of how this was done, what the combinations of

generation and transmission were, or what the same reliability of supply

means. Presumably the transmission capacity and generating plant capacity are

the total capacities in each case, in which case the total generating capacity

will be greater, perhaps much greater, than the total circuit capacity. Although

the reliability parameter is not specified, it was possibly expected energy not

supplied since this was one output parameter of the REFOS program15 stated

to be used in the studies.

several studies have been performed during this project but it has not been

possible to replicate the values given in ACE Report 51. The main problem is

that the reliability of supply (e.g. expected energy not supplied) depends on

whether a load duration curve or a fixed loading level is used, and the value ofpeak load compared with the installed generation capacity. This information is

not provided.

a solution to this problem requires extensive system studies, which are out of

range of the scope of this project. However they could be done given

appropriate time and resources.

instead the basic concept of P2/5 is used in this project in order to establish the

important issues and to identify appropriate approaches and procedures

15

REFOS: CEGB CS/C/P205. This program is said to perform a risk evaluation of parallel connectedtransmission circuits and apparently calculates the expected annual outage time for which the demand at asupply point will exceed the available capacity, and the energy lost

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one of the most significant points is that the values of 0.4 to 0.9 are the ratios

between the effective generation (equivalent to the circuit capacity) and the total

generation capacity. This provides the following information for the case when a

transmission line is used in comparison with a group of generators:-

assume all generating units in the group are identical and of equal size in each

of the following cases, and let:set size = SS

circuit capacity = CC

total generation = TG

ratio of effective generation to maximum generation = R

number of generating units = n

then:

R = CC/TG = CC/n.SS

when SS equals CC, R = 0.4 - 0.5. This implies that the number of generating

units lies between 2 and 2.5, on average.

when SS equals one half of CC, R = 0.7 - 0.8. This implies that the number ofgenerating units lies between 2.5 and 2.9, on average.

when SS equals one quarter of CC, R = 0.8 - 0.9. This implies that the number

of generating units lies between 4.5 and 5, on average.

the value of two-thirds (0.67) is stated to be the average of the range of ratios

between 0.4 and 0.9. However, how this average was calculated is not explained.

Although the single value of 0.67 may be convenient for deterministically

assessing system security, it does not take into account the fact that the ratios

themselves are clearly variables, the number of units varies, the availability of

units can vary significantly, and lines and transformers with which they are

compared are not perfectly reliable. Also it neglects the fact that a mean value isonly the long-run average and this value may never be achievable in its own right.

For example, a single unit either is capable of delivering full capacity or nothing,

the average is only the weighted value of these two actual achievable levels.

there is a tacit assumption in P2/5 that 67% of installed capacity can be relied on

at all times. This is illusory. It also provides comfort that irrespective of when

the original circuit outage occurs, the embedded generation will be able to supply

67% of its installed capacity. In reality this is not possible since it could only

provide the capacity of available units, which could range from 100% of installed

capacity to nothing at all. It may therefore not be possible to provide the capacity

required to satisfy the requirements of Table 1 in P2/5. The significance and a

balanced perspective of this is discussed in more detail in Sections 2.2 and 2.3.

4.3. Direct Application of P2/5

Direct application of P2/5 is straightforward. All that is required is to identify

the types of units in the system, assume these conform with P2/5 and have a

corresponding effective contribution of 67%.

Example 1.1 Consider four units each of 60MW capacity

Total Effective Contribution = (4 x 60) x 0.67= 160.8 MW

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Example 1.2 Consider 30 units each of 1MW capacity

Total Effective Contribution = (30 x 1) x 0.67

= 20.1 MW

Example 1.3 Consider a single unit of 60MW capacity and 30 units each of 1MWcapacity

Total Effective Contribution = (1 x 60 + 30 x 1) x 0.67

= 60.3 MW

Note: a) These examples assume that all the units considered conform with P2/5.

5. Deterministic Studies Using an Updated P2/5

5.1. Introduction

In order to understand how P2/5 might be updated, the current restrictions and

assumptions must be recognised. The assumptions underpinning P2/5 are the

following:-

P2/5 assumes that units have an availability of 0.86. {It is likely that many

modern units, e.g. gas turbines, have availabilities greater than 0.86, and

other units, e.g. wind turbines, have availabilities much less than 0.86.

How much greater or less is difficult to assess, as data concerning these

values are often difficult to obtain. The main current reason is that most

generating companies treat this information as commercially confidential.

Therefore all values used in these studies are estimated and it has proved

useful to perform a series of sensitivity studies and to observe the effect of

changes in the values of the availabilities - It will be essential for

transparency of data to be established}.

gas turbines in the 1970s were usually installed for security or system

support purposes. They were controlled centrally and were available on

demand. {Gas turbines and other units in the 2000s are owned by private

generating companies who operate them according to commercial

principles. They are frequently operated as CHP plant, when their primary

purpose is to supply heat and energy to local sites, with external system

support being of secondary importance. Therefore they may not beavailable for system security and support on demand unless a commercial

agreement has been established between the generating company and the

relevant DNO in advance. This is rarely, if ever, done at present - It will

be essential for commercial agreements to be reached}

the capability of gas turbines in the 1970s was usually equal to the rated

capacity since their primary purpose was to provide system support. {The

capability of gas turbines and other units to support the system in the

2000s may be less, or much less, than the rating if only part of the

capability is available on demand because of the need to maintain

capacity for their primary function of CHP plant - It will be essential for

available capacity to be declared and provided when required}

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These considerations imply the following:-

the availability of modern units may be greater than those of the 1970s and

therefore their effective contribution would also be greater

the availability of modern units may be less, even much less, than those of

the 1970s and therefore their effective contribution would be less the net capability of modern units may be less as a percentage of installed

capacity due to operational practices and therefore their effective

contribution could also be less,

and that the following attributes are not considered explicitly or in most

instances not even implicitly:-

the effect of unit availability

the effect of number of units

the effect of a wide range of unit technologies

the effect of common energy source

the effect of location of units

All of these aspects need to be taken into account. This requires knowledge of

the relationship, if one exists, between the unit availability and the effective capacity

as deduced by the Electricity Council in the 1970s. Since the methodology is not, to

the authors knowledge, described in any public document, this is virtually impossible

to determine. Instead various approaches that seem compatible with the outcome of

P2/5, have been tried and several assumptions have been made. These are described in

the following application studies. The attributes are studied sequentially as follows:-

the effect of unit availability

the effect of number of unitsthe effect of technology of units

the effect of common energy source

the effect of location of units.

It is important to note at this point that the following considerations and

examples have significant assumptions and approximations embedded within them.

These are clearly indicated within the text. The results are therefore not intended to be

exhaustive nor firm proposals, only indicative of the variations that may exist and the

approaches that may be used. Consequently they illustrate the underlying principles

and the impact that may be observed if the input values and modelling procedures

used to produce them more truly reflect the parameters of modern generating plantand operational strategies.

In order to produce more precise results, agreement is required between all the

players on the values of relevant input data and on the relationship between them. In

addition, a more exhaustive set of studies is needed than is possible in this short

exploratory study.

5.2. Effect of Availability

P2/5 assumes all units have an availability of 86% and that such units make an

effective contribution of 67% of declared net capability. The first step in updating

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P2/5 is to allow flexibility in the value of availability. In order to deduce a possible,

but simplistic relationship, the following corresponding values are known or can be

assumed. A unit availability of 86% corresponds to an effective capacity of 67%, as

given in P2/5. It seems reasonable to assume that unit availabilities of zero and 100%

correspond to effective capacities of zero and 100% respectively. From a plot of these

three points, one can interpolate the values of effective capacities that correspond to arange of unit availabilities. These are shown in Table 2.

Table 2 - Effective generation contribution as function of unit availability

unit availability

%

effective contribution

%

25

35

40

5060

70

80

86 - P2/5

90

95

99

12

17

20

2836

46

58

67

75

85

98

It is evident, as would be expected, that the effective contribution of the

embedded generation increases as the unit availability increases. The main questionsthat follow are:

what value of availability is reasonable?

is the assumed relationship reasonable?

is the approach used (as per P2/5) reasonable?

There are no simple answers to these questions but the following comments

are pertinent:-

the limited data that exists concerning unit availabilities of gas and steam

turbines suggests that a value in the vicinity of 0.90 - 0.95 could be consideredreasonable

since the approach used in P2/5 is not described in any public document, it is

not possible to be prescriptive as to whether the relationship between

availability and effective capacity given in Table 2 complies with the approach

used in the 1970s. However it is not likely to be greatly distant from the

reality. Therefore the capacity credit that appears to be implicit from these

results is probably fairly consistent with the approach, and that the values

given are indicative of the improvements gained from modern types of units.

A more important and dubious point is whether the approach itself is

reasonable - the third question listed above.

the approach used in this section is essentially the same as that used in P2/5. Ittherefore has all the weaknesses associated with the underlying concepts of the

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P2/5 approach. Most of these have been discussed before but need to be

highlighted once again:

it assumes that a constant value of effective generation can be used

irrespective of number of units and availability of units. This is known

to be incorrect

it tacitly assumes that this effective generation (credit value) isconstant at all times and neglects the fact that units can be in one of

several states, including complete failure

the credit value is only the long-run average value, and in some cases

can not itself be achieved. For example, a single unit is either up and

working or down and not working: it is never in the average output

state

these assumptions can give a false sense of security. They imply that a

demand of this level can be restored at all times irrespective of any

other system situation, whereas in reality a demand greater than this

level may be restored but at other times a demand maybe much less

than this is possible. The only other alternative is the use of anapproach based on probabilistic assessment.

Application of this enhancement to P2/5 is straightforward. All that is required

is to identify the types of units in the system, select an appropriate value of

availability, and then to look up the corresponding effective contribution.

Example 2.1 Consider four units each of 60MW capacity and availability of 90%


= 180 MW

Example 2.2 Consider 30 units each of 1MW capacity and availability of 40%


= 6 MW

Example 2.3 Consider a single unit of 60MW capacity and availability of 90% and

30 units each of 1MW capacity and availability of 40%

Total Effective Contribution = (1 x 60) x 0.75 + (30 x 1) x 0.20

= 51 MW

Notes: a) These examples assume that the effective contribution is only affected by

the unit availability and not by the number of units.b) The effective contribution of Example 2.1 is greater than that of Example

1.1, but that of the other two examples move in the opposite direction,

reflecting the relative changes in the percentage credit with unit availability.

c) These results are illustrative only and would need more extensive studies

in order to confirm actual and appropriate values

5.3. Effect of Number of Units

P2/5 assumes that each unit of a group contributes 67% of its capacity

irrespective of the number of units in the group. In fact, as discussed in Section 4.2,ACE Report 51 concludes this value to be an average value only and that, depending

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on the set size, the actual value varies between 0.4 and 0.9. Also as illustrated in

Section 4.2, these ratios depend on the number of units, although ACE Report 51 is

not explicit, or even implicit, in this respect. In order to reflect this effect, it is

necessary to consider the outcome of the ACE Report 51 studies in a little more detail.

The results included in Section 4.2 imply that, if all units of a group are

identical, which may not have been the case in the development of P2/5, and each hasan availability of 86%, then:-

effective contribution = 0.4 - 0.5 if number of units is between 2 and 2.5

= 0.7 - 0.8 if number of units is between 2.5 and 2.9

= 0.8 - 0.9 if number of units is between 4.5 and 5.

Using these results, a plot of effective generation contribution as a function of

number of units can be plotted. This proved to be a difficult task because of the

limited information and the significant scatter in the values. This indicates the need

for a more extensive set of studies and a known and consistent approach for

determining the same reliability of supply. However the results are sufficient toillustrate the effect of number of units and to demonstrate a possible approach. On the

basis of these results, the Effective Generation ratios for units having an availability

of 86% are shown in Table 3. If similar characteristics are constructed for effective

contribution as a function of number of units but for different unit availabilities, then

the effect of number and availability can be observed. Such plots were drawn, from

which the results shown in Table 3 for unit availabilities of 40% and 90% were

deduced.

Although no categorical assurance can be given to the preciseness of the

values shown in Table 3, the following exhibited trends in the values are certainly

expected:-

the percentage contribution will increase as the number of units is

increased because a single unit failure will have less relative impact

the percentage contribution will increase as the availability of units

increases because of the reduced number of failures

the percentage contribution will never reach 100% but only asymptote to

100% as the number of units approaches infinity. Therefore the greatest

variation occurs with a relatively small number of units, less than 3 - 4, and

changes very little when the number exceeds 5 - 8 units. This effect is

clearly beneficial when wind farms with relatively large number of

turbines are considered because any change in actual number of units will

have little effect on the effective contribution.

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Table 3 - Effective generation contributions as a function of number

of units and unit availability

effective generation contribution (%)

for unit availabilities of

number of

units

86% 40% 90%

5

15

20e

30

35

40

50

54

58

35

58

75e

84

87

90

94

96

97

1

2

P2/5

3

4

5

10

20

30

25

50

67

70

80

85

93

95

96e = estimated

The information given in Table 3 is used to deduce results for the following

examples. Examples 3.1 - 3.3 assume that each unit has an availability of 0.86, and

Examples 4.1 - 4.3 assume that the units have the availabilities defined in the

example.

Example 3.1 Consider four units each of 60MW capacity and availability of 86%Total Effective Contribution = (4 x 60) x 0.80

= 192 MW



= 28.8 MW

Example 3.3 Consider a single unit of 60MW capacity and availability of 86% and

30 units each of 1MW capacity and availability of 86%


= 43.8 MW

Example 4.1 Consider four units each of 60MW capacity and availability of 90%


= 208.8 MW



= 17.4 MW

Example 4.3 Consider a single unit of 60MW capacity and availability of 90% and30 units each of 1MW capacity and availability of 40%

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= 38.4 MW

Notes: a) These examples assume that the effective contribution is affected by the

unit availability and by the number of units using the information provided in

Table 3. The previous assumptions underpinning these values should benoted.

b) The effective contributions vary significantly depending on the parameters

included. These clearly demonstrate the need for including all relevant

parameters and attributes if appropriate credit is to be given to the

contribution that embedded generation can make to security.

c) The benefit of a large number of units in Examples 3.2 and 4.2 partly

overcomes the reduced contribution made by these units due to a low value

of unit availability.

d) These results are illustrative only and would need more extensive studies

in order to confirm suitable values

5.4. Effect of Technology of Units

This effect is implicitly

dg contribution to network security_r1.pdf

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