eds 08-0119 guidance for the application of ena er...

Document Number: EDS 08-0119

Version: 3.0

Date: 22/09/2014

© UK Power Networks 2014 All rights reserved 1 of 28

TH

IS IS

AN

UN

CO

NT

RO

LL

ED

DO

CU

ME

NT

, T

HE

RE

AD

ER

MU

ST

CO

NF

IRM

IT

S V

AL

IDIT

Y B

EF

OR

E U

SE

ENGINEERING DESIGN STANDARD

EDS 08-0119

GUIDANCE FOR THE APPLICATION OF ENA ER P2/6 SECURITY OF SUPPLY

Network(s): EPN, LPN, SPN

Summary: This standard provides guidance on the application of Engineering Recommendation P2/6 published by the Electricity Networks Association (ENA); P2/6 is a guidance document on system planning and network capacity requirements and details the minimum standards for the security of supply. The concepts and requirements relating to the security contribution of distributed generation and demand side response (DSR) are also included.

Owner: Allan Boardman Date: 22/09/2014

Approved By: Steve Mockford Approved Date: 23/09/2014

This document forms part of the Company’s Integrated Business System and its requirements are mandatory throughout UK Power Networks. Departure from these requirements may only be taken with the written approval of the Director of Asset Management. If you have any queries about this document please contact the author or owner of the current issue.

Circulation

UK Power Networks External

All UK Power Networks G81 Website

Asset Management Contractors

Capital Programme ICPs/IDNOs

Connections Meter Operators

HSS&TT

Network Operations

UK Power Networks Services

Other

Guidance for the Application of ENA ER P2/6 Security of Supply


Version: 3.0

Date: 22/09/2014


Revision Record

Version 3.0 Review Date 10/09/2019

Date 10/09/2014 Author Stephen Tucker

Document reviewed.

Structure revised and EDP 08-0107/EDP 08-0108 integrated into document. Document updated to include the use of demand side response from ENA ETR 130-1

Version 2.2 Review Date 02/03/2014

Date 15/09/2011 Author John Lowe

Document reclassified to EDS

Version 2.1 Review Date

Date 01/03/2011 Author Pete Lawson

Updated Section 7.1 and 7.2 CHLDZ boundary map and main substations list


Date 24/1/2011 Author Pete Lawson

Document revised – notes added to tables and reference documents updated. London documents E2/1/1 and E2/1/1/1 integrated into this document


Date 20/01/2007 Author Matt Freeman

New document



Version: 3.0

Date: 22/09/2014


Contents

1 Introduction ............................................................................................................. 5

2 Scope ....................................................................................................................... 5

3 Glossary and Abbreviations ................................................................................... 6

4 Security of Supply ................................................................................................... 7

4.1 Class of Supply ......................................................................................................... 7

4.2 Normal Level (for all Regions except the London CHLDZ) ......................................... 8

4.2.1 Normal Level Overview ............................................................................................. 8

4.2.2 Class A – Up to 1MW ................................................................................................ 8

4.2.3 Class B – Over 1MW up to 12MW ............................................................................. 8

4.2.4 Class C – Over 12MW up to 60MW ........................................................................... 9

4.2.5 Class D – Over 60MW up to 300MW ......................................................................... 9

4.2.6 Class E – Over 300MW up to 1500MW ..................................................................... 9

4.2.7 Class F – Over 1500MW ........................................................................................... 9

4.3 Enhanced Level (London CHLDZ) ........................................................................... 10

4.3.1 Enhanced Level Overview ....................................................................................... 10

4.3.2 Class A – Up to 1MW .............................................................................................. 11

4.3.3 Class B – Over 1MW up to 20MW ........................................................................... 11

4.3.4 Class C and D – Over 20MW up to 300MW ............................................................ 12

4.3.5 Class E – Over 300MW up to 1500MW ................................................................... 12

4.3.6 Class F – Over 1500MW ......................................................................................... 12

5 P2/6 Compliance Process ..................................................................................... 13

5.1 P2/6 Definitions ....................................................................................................... 13

5.1.1 Defining a ‘Group’ .................................................................................................... 13

5.1.2 Time at Risk ............................................................................................................ 13

5.1.3 Demand at Risk ....................................................................................................... 13

5.2 Considerations when Looking at DG Contributions .................................................. 14

5.2.1 General ................................................................................................................... 14

5.2.2 Commercial Availability ........................................................................................... 14

5.2.3 Technical Availability ............................................................................................... 14

5.2.4 Fuel Source Availability ........................................................................................... 15

5.2.5 Common Mode Failure ............................................................................................ 15

6 P2/6 Assessment Process .................................................................................... 15

6.1 Flow Diagram .......................................................................................................... 15

6.2 Determining Group Demand and De-minimis Test 1 ................................................ 17

6.3 Assessing Network Capacity ................................................................................... 19



Version: 3.0

Date: 22/09/2014


6.4 DG Information ........................................................................................................ 19

6.5 DG Contribution and De-minimis Test 2 .................................................................. 19

6.6 Ride Through Capability .......................................................................................... 20

6.7 Determine DG Contribution ..................................................................................... 20

6.7.1 F Factor ................................................................................................................... 20

6.7.2 Non-intermittent Generation .................................................................................... 20

6.7.3 Non-intermittent Generation .................................................................................... 20

6.8 DG Dominance and Common Mode Failure ............................................................ 21

6.9 Examples ................................................................................................................ 21

7 Other Considerations ............................................................................................ 22

7.1 Demand Side Response .......................................................................................... 22

7.2 Load Transfers ........................................................................................................ 22

7.3 London Networks .................................................................................................... 22

7.4 Load Shedding ........................................................................................................ 22

7.5 Derogation ............................................................................................................... 23

8 References ............................................................................................................. 24

8.1 Internal Standards ................................................................................................... 24

8.2 External Standards .................................................................................................. 24

Appendix A – London Central High Load Density Zone ................................................. 25

Appendix B – P2/6 Tables................................................................................................. 26

Tables

Table 4-1 – Normal Security of Supply Levels (all regions except the London CHLDZ)......... 8

Table 4-2 – Enhanced Security of Supply Level (London CHLDZ) ...................................... 11

Figures

Figure 6-1 – P2/6 Assessment Process .............................................................................. 16

Figure 6-2 – Group Demand Example ................................................................................. 18

Figure 6-3 – Group Demand Example with DSR Operating ................................................ 18

Figure A-1 – London Central High Load Density Zone ........................................................ 25



Version: 3.0

Date: 22/09/2014


1 Introduction

This standard provides guidance on the application of Engineering Recommendation P2/6 published by the Electricity Networks Association (ENA); ENA ER P2/6 is a guidance document on system planning and network capacity requirements and details the minimum standards for the security of supply. The concepts and requirements relating to the security contribution of distributed generation (DG) and demand side response (DSR) are also included.

EDP 08-0107 and EDP 08-0108 have been integrated into this document and these documents will be withdrawn.

All Distribution Network Operators have obligations under their distribution licence to 'plan and develop the [licensee’s] distribution system in accordance with a standard not less than that set out in Engineering Recommendation P2/6 of the Energy Networks Association'.

A further obligation under Licence Condition 9 is to comply with the Distribution Code which is designed so as 'to permit the development, maintenance and operation of an efficient, coordinated and economical system for the distribution of electricity'.

DPC 4.2.1 of the Distribution Code states that 'DNOs shall plan and develop their DNOs Distribution Systems to a standard not less than that set out in DGD Annex 1 Item 5, Engineering Recommendation P2/6 – ‘Security of Supply’ or such other standard of planning as DNO’s may, with the approval of the Authority, adopt from time to time'.

Estimating future demand allows system reinforcement needs to be identified. With an increasing number of network in-feeds coming from distributed generation, demand assessment has and will continue to become more involved. DG contributions can mitigate or delay the need for reinforcement in certain circumstances.

The objective is to ensure that UK Power Networks is maximising asset utilisation and minimising load-related expenditure, while at the same time managing network design security risk on its public networks as necessary to meet its Licence and Distribution Code obligations; and ensuring that security risk does not adversely affect customer interruptions (CI) and customer minutes lost (CML) targets.

EHV networks designed in accordance with EDS 08-0145, HV networks designed in accordance with EDS 08-0109 and LV networks designed in accordance EDS 08-0136 will generally be P2/6 compliant.

The appraisal methodology detailed in this standard will provide a robust assessment of risk on a site-specific basis and thus permit the effective prioritisation of reinforcement schemes to address out-of-firm and possible P2/6 non-compliant situations.

2 Scope

This standard applies to the security of supply as defined in ENA ER P2/6 and its application to UK Power Networks distribution networks.

This standard details how UK Power Networks will meet the obligations outlined above, how its processes will be applied to assess compliance at grid and primary substation sites, and how the effects of load growth will be analysed to develop reinforcement schemes to address out-of-firm and possible P2/6 non-compliant situations.



Version: 3.0

Date: 22/09/2014


3 Glossary and Abbreviations

Term Definition

CHLDZ

The Central High Load Density Zone within the London network where the security of supply has developed with an enhanced level to the normal level. Refer to Section 4.3

Circuit Part of an electricity system between two or more circuit-breakers, switches or fuses. It may include transformers, reactors, cables and overhead lines. Busbars are not included in the term circuit and are considered on their merits

CI Customer Interruption (number of occurrences in which a customer is off supply for more than 3 minutes)

CML Customer Minutes Lost (number of minutes in which a customer is off supply after the first 3 minutes)

Declared Net Capability (DNC)

The declared gross capacity of an individual generator, or the aggregate distributed generation in a load group, less any normal parasitic power consumption attributable to the respective plant’s auxiliary supplies

Demand Side Response (DSR)

Demand normally imported from the distribution network to a customer’s premises that is controlled in response to an instruction issued as part of an agreed demand side management arrangement with the DNO

Distributed Generation (DG)

Generating plant connected to the distribution network

Distribution Network Operator (DNO)

An organisation that owns and/or operates a distribution network

Firm Capacity A maximum power requirement that meets or exceeds the requirements of ENA ER P2/6

F factor Security contribution for individual DG specified as a percentage

First Circuit Outage Fault or an arranged circuit outage (n-1)

Second Circuit Outage A fault following an arranged circuit outage (n-2)

Group Demand An estimate of the maximum demand of the group, comprising the Measured Demand attributable to network in-feeds (but excluding generators) plus any Latent Demand identified within the group (if present). See Section 5 for more information

HV Feeder Group A group of interconnected HV feeders that are arranged with open points between them to provide n-1 support for feeders within the group. Note the LV network supplied from a HV feeder group may also have interconnection or normally open points

Intermittent Generation

Generation plant where the energy source of the prime mover cannot be made available on demand (e.g. wind)



Version: 3.0

Date: 22/09/2014


Term Definition

Latent Demand The demand that would appear as an increase in Measured Demand if all distributed generation within the group were not producing any contribution and any demand side response was inactive.

Therefore Latent Demand is the generator export plus any site demand masked by the generator (excluding generator auxiliary supplies) plus any demand side response that is enabled, at the time of measured Maximum Demand.

Note: DSR may be considered as either a reduction in network demand or an increase in network capacity in line with ENA ETR 130-1; UK Power Networks considers DSR an increase in network capacity.

Measured Demand The demand attributable to all network in-feeds (excluding generation) in the Group under assessment

Non-Intermittent Generation

Generation where the energy source of the prime mover can be made available on demand

Persistence (Tm) The minimum time for which an Intermittent generation source is expected to be capable of continuously contributing to securing the Group Demand

4 Security of Supply

4.1 Class of Supply

Table 1 in P2/6 details the normal levels of security of supply requirements for different classes of supply. This is reproduced in Table 4-1 and has been extended in Table 4-2 to include the enhanced level of security of supply within the Central High Load Density Zone (CHLDZ) in London.

Each class of supply refers to ranges of Group Demand. The Group Demand may now include Latent Demand that was not assessed previously and reassessment under P2/6 could result in the Group Demand being higher. The effects of this are described in Section 4.2.5.

The classes of supply are defined in MW but due regard should be paid to power factor when assessing plant capabilities.



Version: 3.0

Date: 22/09/2014


4.2 Normal Level (for all Regions except the London CHLDZ)

4.2.1 Normal Level Overview

Table 4-1 provides an overview of the normal level of security of supply and should be read in conjunction with the notes in the sections that follow.

Table 4-1 – Normal Security of Supply Levels (all regions except the London CHLDZ)

Class of

Supply

Group Demand Range

Minimum Demand to be Met After

First Circuit Outage (n-1) Second Circuit Outage (n-2)

A Up to 1MW

In repair time: Group Demand Nil

B Over 1MW

and up to 12MW

(a) Within 3 hours: Group Demand minus 1MW

(b) In repair time: Group Demand

Nil

C Over 12MW

and up to 60MW

(a) Within 15 minutes: Smaller of Group Demand minus 12MW and 2/3 Group Demand

(b) Within 3 hours: Group Demand

Nil

D Over 60MW

and up to 300MW

(a) Within 60 seconds: Group Demand minus 20MW (automatically disconnected)

(b) Within 3 hours: Group Demand

(c) Within 3 hours (for Group Demand greater than 100MW): Smaller of Group Demand minus 100MW and 1/3 Group Demand

(d) Within time to restore arranged outage: Group Demand

E Over 300MW

and up to 1500MW

(a) Within 60 seconds: Group Demand (b) Within 60 seconds: All customers at 2/3 Group Demand

(c) Within time to restore arranged outage: Group Demand

F Over 1500MW

In accordance with the relevant transmission company licence security standard

4.2.2 Class A – Up to 1MW

The group range may be extended in circumstances where the demand is satisfied from a single 1000kVA transformer to allow for its emergency cyclic rating. For distribution transformer ratings refer to EDS 08-0115.

4.2.3 Class B – Over 1MW up to 12MW

Load groups in Class B are likely be associated with small primary substations or groups of HV feeders. For the loss of a single circuit an alternative supply shall be available.



Version: 3.0

Date: 22/09/2014


4.2.4 Class C – Over 12MW up to 60MW

Class C supplies will normally be supplied by:

At least two normally closed circuits or

One circuit, with automation in place to switch to an alternative circuit if required.

4.2.5 Class D – Over 60MW up to 300MW

Class D supplies could be grid sites, main substation sites, or areas of interconnected network. At the 100MW threshold, the requirements become more onerous.

This presents a potential problem for planners because of the way Group Demand is defined. If there is significant generation in the group, Latent Demand may increase the Group Demand figure significantly from a previous year’s assessment, pushing it above the 100MW threshold and introducing the second circuit outage criteria. The assessment of Latent Demand could also create transitions between Class B and C and Class C and D, but in these cases the second circuit outage considerations would not need to be considered.

If Group Demand is more than 100MW, for a second circuit outage the smaller of Group Demand minus 100MW and 1/3 of Group Demand should be reconnected within three hours and the remainder within the time to restore the arranged outage.

4.2.6 Class E – Over 300MW up to 1500MW

Class E applies to the in-feeds to the distribution system but not to systems regarded as part of the interconnected super grid (Class F should be applied to the super grid).

P2/6 states that all customers at 2/3 Group Demand should be restored immediately. This is a somewhat dated concept and is likely to be reviewed in the next revision to P2/6.

The majority of substations experience peak demand in winter; as a result of which, maintenance is generally undertaken during the summer months. P2/6 assumes that the demand on the system during the summer is 2/3 of the winter maximum (or less) and that if a fault occurred during a planned outage, it would be reasonable to ensure the network was capable of maintaining supplies to all customers at this level of demand.

Nowadays, the 2/3 criterion is in some cases not valid; for example, a number of London’s substations experience summer peaks. As a result, maintenance outages are more difficult to plan and work may need to be undertaken when the demand on the system is much higher than 2/3 of the Group Demand. Meeting the Class E second circuit outage requirements in these circumstances will require greater resilience than indicated by applying the 2/3 winter demand criterion and this may promote reinforcement investment.

4.2.7 Class F – Over 1500MW

Class F is generally only applicable to National Grid.



Version: 3.0

Date: 22/09/2014


4.3 Enhanced Level (London CHLDZ)

4.3.1 Enhanced Level Overview

Refer to Appendix A for further details of the extent of the CHLDZ.

The distribution system within the CHLDZ will continue to be planned on the principle that a first circuit outage at HV or above should not result in a loss of supply, unless it is more economical to accept a loss not exceeding 60 seconds duration. In the area having the enhanced level of security of supply alternative network designs will be applied progressively, as and when it is economic to do so. Existing interconnected networks in the enhanced level area should not have their level of interconnection reduced solely to conform to the security of supply standard; for further details refer to EDS 08-0111 (System 8 Design).

The case of a Group Demand which is located partially in the CHLDZ and partially in the normal level area will require special consideration of the security level to be adopted at the primary substation and higher levels of the system. The class of supply will be determined by the total demand; the choice between normal and CHLDZ level will usually depend on the proportions of the demand within the two areas. Specific reliability cost/benefit studies may be necessary to determine the optimum choice.

In the CHLDZ area particular care shall be taken to ensure that the method of affording a supply to a customer (particularly at HV) is such that the appropriate level of security is maintained on the associated network. This may necessitate higher expenditure than would be the case in the normal level area, e.g. to ensure correct operation of the interconnected LV network. Notwithstanding this, the principle of a single circuit supply to the customer is still applicable.

The design of the LV and HV networks in the CHLDZ area (LV Design System 4) is based on the provision of sufficient LV interconnection at or between secondary distribution substations for the loss of a single HV feeder not to cause a loss of supply to the general LV network.

Table 4-2 provides an overview of the enhanced level of security of supply applicable to the CHLDZ and should be read in conjunction with the notes in the sections that follow.



Version: 3.0

Date: 22/09/2014


Table 4-2 – Enhanced Security of Supply Level (London CHLDZ)

Class of

Supply

Group Demand Range

Minimum Demand to be Met After

First Circuit Outage (n-1) Second Circuit Outage (n-2)

A Up to 1MW

In repair time: Group Demand Nil

B Over 1MW

and up to 20MW

(a) Within 3 hours: Group Demand minus 1MW

(b) In repair time: Group Demand

Nil

C Over 20MW

and up to 300MW

(a) Within 60 seconds: Group Demand (b) Within 3 hours: All customers at 1/2 Group Demand


D

E Over 300MW

and up to 1500MW

(a) Within 60 seconds: Group Demand (b) Within 60 seconds: All customers at 2/3 Group Demand


F Over 1500MW

In accordance with the relevant transmission company licence security standard

4.3.2 Class A – Up to 1MW

The group range may be extended in circumstances where the demand is met from a single 1000kVA transformer to allow for its emergency cyclic rating. For distribution transformer ratings refer to EDS 08-0115 (as per the normal level).

4.3.3 Class B – Over 1MW up to 20MW

The range of Group Demand has been extended up to 20MW, compared with 12MW in the normal level, to broadly correspond with the demand supplied by an HV feeder group. The restoration criteria are such that the restoration of a Group Demand between 12 and 20MW will be more rapid than in the normal level area even though it is included in a lower class of supply. At least two, and usually more, circuits will be required to meet the first circuit outage criterion. Through the interconnection of the LV network there should normally be no loss of supply for a first HV circuit outage; though a 60 second restoration by automatic switching is permissible. Any isolated areas of LV network that lose supply should be restored within three hours by manual switching. An arranged circuit outage should not result in interruption of supplies.

A low voltage fault on an interconnected network can lead to a loss of supply within this range of Group Demand. Such a loss should be restricted to a maximum of 3MW of Group Demand and supply restored to all but the faulty section (which may be up to 1MW of demand) within three hours by manual switching. The full Group Demand will be restored in the repair time of the LV fault.

There is no requirement to make design provision for a second circuit outage, but, by the nature of the network design, it will often be possible to achieve some restoration before the outage is restored or the fault is repaired.



Version: 3.0

Date: 22/09/2014


4.3.4 Class C and D – Over 20MW up to 300MW

These two classes of supply have been combined in the CHLDZ with more stringent restoration criteria than Class D in the Normal Level area. The restoration criterion for a first circuit outage is the same as for Class B; normally no loss of supply should occur but 60 second automatic switching restoration is permissible. An arranged circuit outage shall not result in any interruption of supplies.

Design provision shall be made for partial restoration of the Group Demand following a second EHV circuit outage, thus a minimum of three circuits shall be provided (though they need not all be EHV circuits). This restoration would normally be by manual switching, with the full Group Demand met on restoration of the arranged outage.

In the case of the larger HV feeder groups that fall into this class of supply specific provision need not be made to meet the second circuit outage criteria, but the existence of at least 4 feeders will generally mean that the criteria can be satisfied.

4.3.5 Class E – Over 300MW up to 1500MW

The restoration criterion for this class in the CHLDZ is identical to that in the Normal Level area. There should normally be no loss of supply following a first circuit outage, but consideration can be given to the automatic disconnection and reconnection to another source within 60 seconds of up to 60MW of demand if this leads to significant economies. Such a design provision should not restrict the period during which maintenance can be scheduled.

The design provision for a second circuit outage requires at least three circuits to be available and is based on the assumption that the demand will be below 67% of Group Demand for a sufficient period to allow reasonable scheduling of normal maintenance. Where this is not the case special consideration will be needed.

4.3.6 Class F – Over 1500MW

Class F is generally only applicable to National Grid (as per the normal level).



Version: 3.0

Date: 22/09/2014


5 P2/6 Compliance Process

5.1 P2/6 Definitions

5.1.1 Defining a ‘Group’

A load group can be interpreted in many ways. A group may relate to a feeder, a number of feeders, a substation, or an area of network, and will depend on how the local network infrastructure has been designed and the running arrangements.

5.1.2 Time at Risk

Where Group Demand data indicates that a substation has exceeded its firm capacity, it is important to know over what period of the year this condition is likely to prevail. Two substations of equal n-1 transformer capacity could exhibit the same maximum demand, indicating a potential risk of P2/6 non-compliance under n-1 conditions. However, further analysis of the annual demand profile may show that the substations are at risk for significantly different periods because the load profiles differ.

When a transformer trips, or loses its primary supply, the remaining transformer(s) at the substation may be subject to a loading where a risk of thermal trip exists. The calculation of the seasonal emergency cyclic rating and the time-to-trip are necessary to assess the practicability of taking mitigating action.

5.1.3 Demand at Risk

P2/6 is structured around demand criteria. It follows that, when prioritising reinforcement to ensure continuing P2/6 compliance, it is necessary to consider the demand at risk, which as highlighted in P2/6 and its supporting documentation, could include Latent Demand.

It should be noted that a single customer does not constitute a group for the purposes of P2/6. In the case of a high-demand customer connected to a demand group requiring second circuit outage capability, the treatment of that customer’s demand when carrying out the P2/6 assessment could be material in determining compliance. Each case will need to be considered in relation to how the customer’s contractual security arrangements impact on the assessment.

For example at a number of Network Rail sites the contracted firm capacity is in excess of the normal maximum demand. Where an arrangement exists, with demand transferable between supply points, or transferable from generators embedded in the customer’s network, it is necessary to apply their contracted firm capacity when calculating security margins for first circuit outages.

While P2/6 does not make direct reference to customer numbers, it is also prudent to assess the potential impact on customer interruptions and customer minutes lost.



Version: 3.0

Date: 22/09/2014


5.2 Considerations when Looking at DG Contributions

5.2.1 General

There are two categories of generation described in P2/6 these being ‘intermittent generation’ and ‘non-intermittent generation’ (refer to Section 3 for definitions). Intermittent generation (e.g. wind farms) is, as would be expected, statistically less reliable.

DG contained within a Class A group should not be considered. For the loss of a network in-feed with no alternative supplies other than DG, the section of network could become islanded. Given that the DNO remains responsible for meeting statutory frequency and voltage characteristics when part of the network is islanded, it is recommended that only DG

in Class B groups and above should be considered1. Before allowing any contribution to

network security from DG that is capable of operating in island mode, there are a number of other considerations that need to be taken into account. Further guidance can be found in ER G75/1 (in particular Section 8).

For Class B supplies and above the contribution of DG should be considered in cases where network capacity alone cannot meet P2/6 and where the de-minimis’ test are satisfied.

When considering a contribution to security from DG, due regard shall be paid to the commercial, technical and fuel source availability. The sum of these three elements gives the overall average availability. An overview of these aspects can be found in Sections 5.2.2, 5.2.3 and 5.2.4. When carrying out more complex assessments, it may be necessary to seek guidance from the following documents; ER G59/3-1 and EDS 08-0051.

In relation to the contribution of DG to security it is important to consider the following quote from ETR 130:

‘In assessing any contribution from DG on the network, it is important to balance the effort required to obtain accurate availability data with the risks to loss of supplies from using inaccurate data.’

5.2.2 Commercial Availability

The generator may choose (for financial reasons) to run their plant at specific times which are of benefit to them. If a contribution to security is considered to be necessary it may be possible to enter into a contract with the generator, requiring them to be available on request. Such an arrangement is unfeasible for wind farms because of the intermittent availability of the fuel source, but could be considered for other types of generation. Commercial contracts between UK Power Networks and a generator for the purposes of P2/6 security have not been investigated and lie outside the scope of this document. An arrangement like this would be subject to detailed cost/risk/benefit studies to determine whether it is a more pragmatic approach to take as opposed to the reinforcement option.

5.2.3 Technical Availability

Technical availability is limited by planned and unplanned outages. Modern DG has a high technical availability. It could be argued that for new installations, technical availability is, on occasions, significantly reduced due to initial teething problems. A DG plant with more than one generator set will have higher statistical technical availability (i.e. a two-set plant can continue to generate if one set is out for maintenance).

1 This may change at a future date, as strategies are adopted to cater for higher penetration of micro generation.



Version: 3.0

Date: 22/09/2014


5.2.4 Fuel Source Availability

A reduction in fuel source availability results in a lower than expected output from a generator. A prime example of this would be a wind farm which in its favour has the benefit of modern reliable technologies from a technical and commercial aspect, but is restricted by the wind intermittency.

5.2.5 Common Mode Failure

DG is more likely to be subject to certain forms of common mode failure than centralised commercial generating plant (power stations). Due regard should be paid to:

The ability of a DG to remain stable during system disturbances.

Possible failure of a common fuel supply feeding a number of generator sets, resulting in failure of all of them.

Each type of DG subject to common mode failure should be aggregated, tested for dominance and capped using the method described in Section 6.8.

6 P2/6 Assessment Process

6.1 Flow Diagram

The flow diagram in Figure 6-1 outlines the approach for assessing P2/6 compliance including the contribution to security from DG and DSR.

Qualifying DG can be assessed for its contribution to security using a number of methods detailed in P2/6 and its supporting documents ENA ETR 130 and ENA ETR 131.



Version: 3.0

Date: 22/09/2014


Collect DG information

Non P/2 Compliant

DG contribution not sufficient

to secure Group Demand,

consider other options

including DSR

P2/6 class B-F?No security contribution

allowed for Class A

Assess DG ride-through

capability

Determine DG contribution

(F factors)

Aggregate all allowable DG

contributions

DG

sufficient to secure group

demand?

P/2 Compliant

Group Demand secured with

DG contribution

Identify individual DG with

DNC > 5% of Group Demand

Determine the measured

demand

Determine the DNC of each

DG

Aggregated

DG DNC > 5% of measured

demand?

No Group Demand = Measured

Demand + Latent Demand

YesGroup Demand = Measured

Demand

Determine the P2/6

class of supply

No

Yes

YesNo

Network

capacity including transfer capacity

sufficient to secure Group

Demand?

Yes

No

EDS 08-0119 Section, Table #

Section 6.5

Section 6.7

Determine network capacity

Section 6.3

Table 4-1/4-2

Check for DG dominance

and common mode failure

Section 6.4

Section 6.2

Section 6.6

Section 6.8

Section 7

Figure 6-1 – P2/6 Assessment Process



Version: 3.0

Date: 22/09/2014


6.2 Determining Group Demand and De-minimis Test 1

In order to determine the Group Demand for the purposes of a P2/6 assessment, a ‘de-minimis’ test is applied. For this test there is a need to:

Identify all significant generation2 connected to the group under study and aggregate its declared net capability (DNC).

Determine the simultaneous maximum Measured Demand, as the sum of all network in-feeds (i.e. excluding generator in-feeds).

If the aggregated DNC of all the generators exceeds 5% of the simultaneous maximum Measured Demand, the Group Demand is determined by taking the maximum Measured Demand3 and adding the summated Latent Demand masked by the generation. If the de-minimis criterion is not satisfied, the Group Demand is taken as the maximum Measured Demand.

For example Figure 6-2 shows a Class C group with 20MVA of transformer capacity and a single 5MW, 11kV generator.

The maximum Measured Demand for the year (aggregated from readings taken at the LV tails of the transformers) is 12MW without no DSR operating.

The aggregate DNC of all generation within the group is 5MW.

The total generation capacity accounts for more than 5% of the maximum Measured Demand, therefore the Latent Demand must be accounted for in the Group Demand figure.

The Latent Demand is the demand that would appear as an increase in Measured Demand if the generator was offline. At time of maximum Measured Demand, the generator supports 3MW of network load and 1MW of site load, all of which would need to be supported if the generator became unavailable. The Latent Demand is therefore 4MW.

Therefore the Group Demand = 12MW + 4MW = 16MW.

Demand side response can be considered as an increase in system capacity; therefore there is a need to consider the extent to which the Measured Demand should be increased to reflect the demand that has been suppressed by the DSR in order to establish the Group Demand that needs to be secured. Consider the above example with 2MW of DSR as shown in Figure 6-3.

If the DSR is operating at the time of the maximum Measured Demand it would reduce to 10MW and the Latent Demand would need to be increased by 2MW.

Therefore the Group Demand = 10MW + (4MW + 2MW) = 16MW.

2 Micro-CHP, micro-wind and photovoltaic sources should not be included unless considered significant.

3 Where possible, DG output profiles should be used to determine the likely output at the time of maximum

demand on the network. Where data is not available engineering judgement should be made, and decisions documented.



Version: 3.0

Date: 22/09/2014


10MVA10MVA

33kV

11kV

15MW

Network Load

12MW Max Measured Demand

5MW Generator operating at

80% of DNC (i.e. 4MW) at the

time of Measured Demand

(3MW Export + 1MW Site Load)

Transfer

Capacity

NOP

Group Demand = 12MW + 4MW = 16MW

Figure 6-2 – Group Demand Example4

10MVA10MVA

33kV

11kV

15MW

Network Load

including

2MW DSR

5MW Generator operating at

80% of DNC (i.e. 4MW) at the

time of Measured Demand

(3MW Export + 1MW Site Load)

Transfer

Capacity

NOP

Group Demand = 10MW + 4MW + 2MW = 16MW

10MW Max Measured Demand

(DSR Operating)

Figure 6-3 – Group Demand Example with DSR Operating

4 Based on ENA ETR 130 Section 9.



Version: 3.0

Date: 22/09/2014


6.3 Assessing Network Capacity

The capability of a network to meet a Group Demand after first circuit and second circuit outages should be assessed as:

The appropriate cyclic rating of the remaining transmission or distribution circuits which normally supply the Group Demand, following outage of the most critical circuit(s).

The transfer capacity which can be made available from alternative sources.

Available demand side response contracts.

The available network capacity should be assessed by determining the following:

Cyclic rating of each infeed circuit for the time of year when the Group Demand occurs.

Capacity of the network under normal running arrangements using the cyclic ratings.

Capacity of the network under first circuit conditions (classes B to E).

Capacity of the network under second circuit conditions (classes D and E).

Transfer capacity and the time it can be made available (classes B to E).

6.4 DG Information

Information on DG should be gathered from the company DG databases as follows:

Fuel Source (essential).

Number of units (essential). Note: Intermittent generation counts as one unit.

Declared net capability (DNC) (essential).

Commercial availability, technical availability and operating regimes (preferable).

Half-hourly output data (desirable, particularly for the larger studies).

6.5 DG Contribution and De-minimis Test 2

If the Group Demand is unable to be declared as ‘secure’ having assessed the transformer, circuit and transfer capabilities, there may be a case to consider the contribution of generation to security.

The first de-minimis test, as described in Section 6.2, will have determined whether there is sufficient generation to consider in a P2/6 assessment. If there is sufficient generation, a second de-minimis test is required to filter out all generation which is not deemed to be of sufficient capacity to be considered for contribution to security.

For this second de-minimis test there is a need to review the DG previously identified within

the group and ignore all generation units5 with a DNC below 5% of Group Demand, subject

to a minimum of 100kW.

5 For non-intermittent generation, a unit can refer to an individual gen-set. With intermittent generation, this is not

the case. For example a wind farm should be treated as one unit, regardless of the number of turbines.



Version: 3.0

Date: 22/09/2014


6.6 Ride Through Capability

Some generation may possess fault ride-through capability whereas in other cases a fault event will cause the generators to break their parallel with the network. Fault ride-through is an important consideration for larger plant where it is essential that the generator remains stable for system disturbances. This will generally apply to generators above 5MW connected at 33kV and 132kV. In these cases the generator’s ride-through capability shall be determined and it may also be necessary to conduct transient stability studies to establish that the generator will remain stable. This is covered in more detail in EDS 08-0051.

Smaller generating plant units will generally intentionally disconnect from the network when a system disturbance, such as a fault, occurs. Unless there is evidence to the contrary it should be assumed that these smaller generators will trip and, in the majority of cases, resynchronise once the network re-stabilises. If this generation is to be relied upon for security of supply support it will be necessary to establish the resynchronisation delay. The thermal performance of the remaining transformer(s) at the source substation(s) can then be assessed to determine whether the delay in resynchronisation and reconnection of the generation capacity can be tolerated in terms of the risk of transformer thermal trip. It is important to recognise that the system transformers may still be performing emergency duty even after the generation capacity is re-established, so it will be necessary to conduct a ‘second stage’ thermal analysis to determine whether risk still exists even after the generation has returned.

6.7 Determine DG Contribution

6.7.1 F Factor

The F factor is the proportion of the DNC of a generator that can be used to contribute to P2/6 security. Therefore for each generator that has passed the de-minimis criterion there is a need to establish the 15 minute, 3 hour and continuous contribution capability for first circuit outage conditions (and second circuit outage for groups greater than 100MW).

The F factors should ideally be determined using half-hourly data from PI. Where sufficient data is not available the F factors can be determined as detailed in Sections 6.7.2 and 6.7.3.

6.7.2 Non-intermittent Generation

The contribution from non-intermittent generation is based on the number of units available and it is assumed that the generator will be capable of a continuous F factor.

The F factor for non-intermittent generation can be determined from P2/6 Table 2-1 (Appendix B) for differing numbers of units and applied using Table 2 (Appendix B).

6.7.3 Non-intermittent Generation

The contribution from intermittent generation sources, such as wind farms, needs to be considered carefully. The allowable contribution is only a small proportion of the theoretical output value and this contribution decreases as the required period of continuous operation (persistence) increases. Intermittent generation may be useful for a short time (15 minutes or three hours) contribution while other switching on the network takes place. However, for more prolonged outages extending into days, rather than hours, the statistical reliability will not be sufficiently high to secure any Group Demand figure and should therefore be excluded.



Version: 3.0

Date: 22/09/2014


The persistence for intermittent generation can be determined from P2/6 Table 2-4 (Appendix B) and in can then be used to determine the F factor from P2/6 Table 2-2 (Appendix B) and applied using Table 2 (Appendix B).

6.8 DG Dominance and Common Mode Failure

When calculating the F factor for a DG, ensure that the DG dominance and common mode failure (Section 5.2.5) criteria have been met, whilst considering any other site specific limiting factors that may exist.

The loss of any DG contribution should never have a greater impact on system security than the most onerous network circuit outage(s). P2/6 guidelines specifically refer to first circuit outage and second circuit outage as circuit outages rather than generator outages. As a result, using the equivalence criteria as detailed in P2/6 Table 2-3 (Appendix B), no DG unit or group of DG units should be dominant under first circuit outage or second circuit outage conditions.

If the following conditions are not satisfied then the contribution (F factor) from DG should be capped as follows:

The cyclic rating of the largest circuit should be greater than the F factor corrected DNC of the first circuit outage DG equivalent derived from P2/6 Table 2-3 (Appendix B).

The cyclic rating of the two largest circuits should be greater than F factor corrected DNC of the second circuit outage DG equivalent derived from P2/6 Table 2-3 (Appendix B).

6.9 Examples

Section 9 of ENA ETR 130 contains several worked examples to illustrate the process.



Version: 3.0

Date: 22/09/2014


7 Other Considerations

7.1 Demand Side Response

Demand side response can be considered as an increase in system capacity and therefore used for P2/6 compliance. In order to determine the effective security contribution from DSR, an assessment is needed of the magnitude and longevity of the demand reduction which is likely to be delivered by the DSR arrangements in place at the time when the intervention would be needed to meet the security requirements of P2/6.

In each case the assessment should be formally recorded as part of the overall compliance assessment.

7.2 Load Transfers

Load transfers can be used either as a permanent means of moving demand from one substation to another, thus removing an out-of-firm condition, or as a temporary mitigation measure under outage conditions.

Transferring load to another substation is often necessary to meet second circuit outage requirements. In such cases the load is transferred prior to taking the maintenance outage (first circuit outage).

Load can also be transferred subsequent to a first circuit outage provided the transfer can be affected within the restoration period allowed by P2/6. Where the transfer is required to reduce transformer loading rather than restore supply it shall be achievable before the transformer(s) reach a winding temperature trip condition.

7.3 London Networks

In London a form of load transfer termed a ‘swinger’ group is often configured. This comprises a group of HV feeders connected to two separate substations. This allows the whole group to be transferred as a single entity (using busbar switching) which is necessary where the underlying LV network is interconnected. The disadvantage is that this means an all-or-nothing transfer, which for large groups can impose its own constraints.

For HV in London reference should be made to the System 8 network design parameters (EDS 08-0111) which provides guidelines for the restructuring of the HV/LV network in central London to remove full LV interconnection and establish simple LV interconnection with the option of applying automation to the 11kV system.

For LV in London reference should be made to the System 4 design guidelines EDS 08-0140.

7.4 Load Shedding

P2/6 allows some load shedding although this often conflicts with the Company’s need to achieve CI and CML targets. It also impacts on UK Power Networks reputation. In reality, therefore, while meeting P2/6 is obligatory under the terms of our Licences, security of supply assessments shall in addition take account of the particular circumstances of each case, and there will on occasions be a need to design the network to exceed the requirements of P2/6.



Version: 3.0

Date: 22/09/2014


7.5 Derogation

It should be noted that P2/6 is not applicable to individual end customers (it applies to Group Demand) so specific solutions may be offered to meet an individual customer’s requirements and written into the supply agreement.

The connection of a new or additional load shall not adversely affect the performance of the existing network or the security of supply provided to existing customers to levels below P2/6 minimum standard. A group of customers that include a single large customer shall however comply with P2/6. EDS 08-0109 gives guidance to be followed when connecting larger loads to the distribution network.

For many customers a single circuit arrangement is unacceptable and various forms of enhanced supply arrangements are negotiable as described in EDS 08-0141.

There will be occasions where technical compliance with P2/6 is not achieved. A typical example is where an out-of-firm condition will be addressed within a reasonably short time by planned reinforcement elsewhere on the network. In these circumstances investing in reinforcement at the site that has not met P2/6 would be an inefficient use of investment capital.

Such a decision not to address a P2/6 non-compliance needs to be regularised by seeking formal derogation from Ofgem. It will, of course, be necessary to present a sound case to Ofgem citing the reasons for seeking derogation. The derogation request should be formally submitted to Ofgem.



Version: 3.0

Date: 22/09/2014


8 References

8.1 Internal Standards

EDS 08-0051 Guidance on 6.6kV-33kV New Demand and Generation Connections

EDS 08-0109 11KV/6.6KV Secondary Distribution Network Design

EDS 08-0111 System 8 Network Design Guidelines

EDS 08-0115 Loading of Secondary Distribution Transformers

EDS 08-0136 LV Network Design

EDS 08-0140 System 4 Network Design Guidelines

EDS 08-0141 Customer HV Supplies

EDS 08-0145 EHV Network Design

8.2 External Standards

ENA ER P2/6 Security of Supply

ENA ER G59/3-1 Recommendations for the Connection of Embedded Generation Plant To The Public Electricity Suppliers’ Distribution Systems

ENA ETR130-1 Application guide for Assessing the Capacity of Networks Containing Distributed Generation

ENA ETR131 Analysis Package for Assessing Generation Security Capability – Users Guide

ACE 51 Report on the Application of Engineering Recommendation P2/5 Security of Supply



Version: 3.0

Date: 22/09/2014


Appendix A – London Central High Load Density Zone

The extent of the London Central High Load Density Zone (CHLDZ) and the substations serving it is shown below. For more detail on boundary interfaces refer to the HV and LV operational diagrams.

London Central High Load Density Zone

Figure A-1 – London Central High Load Density Zone

Fulham Palace Road C

Old Brompton Road

Victoria Gardens

Aberdeen Place A & B

Bloomfield Place

Carnaby Street

Duke Street

Hyde Park Estate A

Hyde Park Estate B

Imperial College

Moreton Street B

Moscow Road

Back Hill

Fisher Street

St.Pancras A

Kingsway

Shorts Gardens

City Road

Finsbury Market A, D and E

Hearn Street

Paternoster

Seacoal Lane

Leicester Square

Devonshire Square

Ebury Bridge

Whiston Rd

Beech St A & B

Longford St

Osborn Street

Bankside C



Version: 3.0

Date: 22/09/2014


Appendix B – P2/6 Tables



Version: 3.0

Date: 22/09/2014


eds 08-0119 guidance for the application of ena er...

Documents