Transmission Planning Code – Design Margin UpdateTransmission Workstream, 4th February 2010

Introduction

National Grid NTS has revised the Design Margin used in long term planning analysis following a review by Advantica (now GL Industrial Services UK Ltd.)

Intention is to consult on the required changes to the Transmission Planning Code in February 2010

Following slides describe the Design Margin and how it is used, to supplement the consultation process

Design Margin – Brief Overview (1)

National Grid NTS must be able to maintain sufficient pressure for gas leaving the NTS, to ensure safety and security of supplies to downstream parties (DNOs, directly connected customers)

Planning and development of the system is undertaken over a ten year timescale but must consider uncertainties present ahead of and within the gas flow day such as

NTS supply levels, flow profiles, distribution and supply availability

NTS demand levels, flow profiles and distribution

Plant availability

Design Margin is applied as a % uplift to forecast flows within long term planning analysis to account for uncertainties or unplanned events

Key assumption is that Operating Margins gas is available within 2 hours of the event occurring and will be used to support the system from this time onwards

Design Margin – Brief Overview (2)

Design Margin in use since 1986, reviewed in 2000 and 2008

Level of design margin required is calculated through statistical modelling of within day events and network analysis to verify proposed design margin

Applied automatically within planning software to increase the flows used within network models

Effect is to slightly exaggerate the pressure drops across the system

Provides a small degree of pressure cover across the system for a limited time until Operating Margins gas can be deployed

If system is constrained, or only just meets required pressure and flow obligations, introducing a Design Margin may signal that a reinforcement is required

Design Margin Review 2008

National Grid NTS recently commissioned a review from Advantica to consider

Previous flow margin studies undertaken in 1986 and 2000

Statistical analysis of within day variations over a 2 year period

Network modelling to confirm level of design margin required

Rationale behind the design margin and the operating margin, and whether these could overlap

Main conclusion was transmission component no longer required due to

Development of contractual/market-based commercial regime

Increased use of supply and demand scenarios to understand sensitivity away from Base Case flows

Additional pressure cover at system extremities was recommended

Study confirmed that the design margin and operating margin are used for different purposes and therefore do not overlap

Design Margin Components

Transmission Component

Typical lead-times for NTS investment projects are 3-4 years from inception

Peak day supply and demand forecasts may change in this period

NTS must be maintained to the 1-in-20 peak day security standard and to ensure gas is delivered at the required pressures to downstream parties

Transmission component intended to counteract annual plan changes due to

Geographical redistribution of supply between entry points and demand between exit points

Demand variation due to changes in economic assumptions

Supply level variations

Uncertainty in model pipe efficiency parameters

Original requirement for transmission component driven by uncertainty around central case planning assumptions

Increased use of scenario planning has reduced the requirement to model uncertainty by use of a fixed uplift factor

Transient Component

Long-term planning analysis is conducted assuming 100% plant availability including upstream infrastructure

Plant failures and supply outages may occur in reality

Daily balancing regime means that instantaneous/cumulative NTS supply and demand levels are often not balanced within day

Total supplies may lag behind demand especially if back-loading is present

Linepack (NTS gas stock) may be depleted in some areas and increased in other areas as a result

Transient component allows for within-day effects (and is still required)

LDZ demand forecast error

Offshore supply outages leading to temporary reductions in supply

Supply (re)nominations requiring NTS reconfigurations

Compressor trips

Design Margin - prior to 2008 review

Analysis type Demand ConditionTransmission

componentTransient

componentDesign Margin

Long term planning analysis - steady

state

1 in 20 peak day 3% 2% 5%

“Severe” demand conditions

3% 2% 5%

“Average” demand conditions

1% 2% 3%

Long term planning analysis - transient

All 0% 2% 2%

Operational analysis - transient

All 0% 0% 0%

Varies from 0-5% depending on type of analysis undertaken

Design Margin – recommendations from 2008 review

Analysis type Demand ConditionTransmission

componentTransient

componentDesign Margin

Long term planning analysis - steady

state

1 in 20 peak day 0% 2% 2%

“Severe” demand conditions

0% 2% 2%

“Average” demand conditions

0% 2% 2%

Long term planning analysis - transient

All 0% 2% 2%

Operational analysis - transient

All 0% 0% 0%

Review concluded Transmission component not required due to

Development of contractual/market-based commercial regime

Increased scenario analysis to understand sensitivities around TBE Base Case flows

Additional pressure cover for sensitive geographic locations required

Pressure Cover

Use of a Design Margin inherently provides pressure cover for operational circumstances as pressure drops are slightly exaggerated in long term planning analysis

Extremities of the system may still be sensitive to unplanned plant failures e.g. where they are immediately downstream of a compressor station

Additional pressure cover can be calculated by simulating the compressor trip at different times during the gas day to determine

The pressure cover implied by the Design Margin

The pressure decay immediately after the compressor trip, up to two hours after the trip

Additional pressure cover is applied as an increment to the Assured Offtake Pressures at the extremity to ensure minimum pressures are not breached within two hours of the compressor trip

Two system extremity points are currently considered

Transmission Planning Code

National Grid NTS has a licence obligation to maintain a Transmission Planning Code to describe how it undertakes the planning and development of the network

Current version available at: http://www.nationalgrid.com/uk/Gas/TYS/TPC/

During development of the planning code, National Grid discussed its intention to revise the value of the Design Margin used within its long term planning models

Presentations available on Joint Office website (TPC Workshop 3 on 5 June 2008)

Design Margin revised from 5% to 2% following a review by Advantica (GL) in 2008

Design Margin described with National Grid’s Safety Case for the NTS and therefore required discussion with the HSE before modification

Transmission Planning Code – Required Updates

Modifications to Safety Case are now complete and National Grid will be launching a consultation in February 2010 to update the Transmission Planning Code as required by its NTS Licence

Further consultations are planned in 2010 to reflect changes in the Exit regime and Planning legislation

Please contact Chandima Dutton on 01926 653231 or chandima.dutton@uk.ngrid.com if you wish to discuss any aspects of the Transmission Planning Code

Design Margin will be kept under review

Design Margin value will be stated in Transmission Planning Code rather than the Safety Case

Changes will still require discussion with HSE

Appendix: Example

Example: Design Margin impact after a compressor trip

Compressor 1

Compressor 2

Supply A

Supply B

Demand C

Operating Margins gas in store, availability within 2

hours

Example assumes that only one supply/demand scenario used for long term planning

In reality various sensitivities around the TBE Base are considered to ensure that a range of flow patterns and flow levels are modelled

Explicit modelling of sensitivities and obligations reduces level of design margin required

Case 1: No Design Margin appliedSystem meets min pressure requirements

Pressure drop calculated across system = 30 bar(g)

Min pressure at Demand C just satisfied

Long term planning analysis identifies no reinforcement requirement

design flow = 40 mscmd

design flow = 60 mscmd

min pressure = 40 bar(g)

supply pressure = 70 bar(g)

modelled pressure = 40 bar(g)

Supply A

Supply B

Demand C

Long Term Planning

Long Term Planning

Case 1: No Design Margin appliedWithin-day compressor trip

Compressor trip causes extremity pressure to fall below min pressure before operating margins gas can be brought online

Operating Margins gas in store, 2 hours

availability

system pressure = 40 bar(g) before trip decays

rapidly after trip

flow = 40 mscmd

flow = 60 mscmd

supply pressure = 70 bar(g)

Supply A

Supply B

Demand C

min pressure = 40 bar(g)

OperationsOperations

Case 2: Design Margin applied e.g. 5% used in planning analysis

Pressure drop calculated across system = 31.5 bar(g)

Min pressure at Demand C not satisfied

Long term planning analysis identifies that reinforcement is required

supply pressure = 70 bar(g)

design flow = 40 x 1.05 mscmd

design flow = 60 x 1.05 mscmd

Supply A

Supply B

Demand C

modelled pressure = 38.5 bar(g)

min pressure = 40 bar(g)

Long Term Planning

Long Term Planning

Case 2: Design Margin appliedSystem reinforced to meet min pressure requirements

Pressure drop across system = 30 bar(g) after reinforcement

System meets required pressure at extremity

Pressure cover of 1.5 bar(g) at Demand C has been provided by Design Margin for operational use

Reinforcement

design flow = 40 x 1.05 mscmd

design flow = 60 x 1.05 mscmd

Supply A

Supply B

Demand C

modelled pressure = 40 bar(g)

min pressure = 40 bar(g)

Long Term Planning

Long Term Planning

Case 2: Design Margin appliedWithin-day compressor trip

Extremity pressure affected by compressor trip, however pressure cover of 1.5 bar(g) is used to absorb impact of trip for two hours

After 2 hours, Operating Margins gas assumed to be available

Supply A

Supply B

Demand C

Operating Margins gas in store, 2 hours

availability

system pressure = 41.5 bar(g) before trip and

decays rapidly after trip

flow = 40 mscmd

flow = 60 mscmd

min pressure = 40 bar(g)

OperationsOperations