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 [email protected] 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
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