Transmission Capacity to Accommodate a Mixed Background of Generation Keith Bell and Dusko Nedic University of Strathclyde/TNEI Services Ltd. August 2007.

Transmission Capacity to Accommodate a

Mixed Background of Generation

Keith Bell and Dusko NedicUniversity of Strathclyde/TNEI Services Ltd.

August 2007

The purpose of transmission

• To provide energy transport from sources (generators) to consumers (loads) with an acceptable reliability.

• To pool resources and reserves so that security of supply is achieved.

• To obtain benefits of economic operation such that cost of energy to all consumers is a minimum at all times.

• To enable the electrical energy wholesale market and promote competition.

The power system will have ‘failed’ if demand for electricity is not met

• not enough generation available on the system as a whole to meet demand (a ‘single bus’ failure)

• insufficient available generation is utilisable due to network restrictions– might be a ‘main interconnected system’ issue– might be a local network connection issue

Is it reasonable to expect there never to be a failure?

When is risk of failing to meet demand highest?– when demand is highest

Power system failure

How much main system capacity at time of peak demand?

What is a ‘reasonable’ level of failure?

• The present MITS design criteria do not guarantee sufficient capability to meet demand– some failure to meet demand will sometimes occur– constraint of generation also sometimes arises

• The present criteria have been regarded for many years as acceptable– what is the level of ‘failure’ implied now by a MITS built in

accordance with the present MITS design criteria?– ‘benchmark’ studies have been done to quantify this

• What level of failure will a changed generation background imply for any given level of MITS capacity?– Can a level of capacity sufficient to satisfy a given level of

failure be identified?• Yes!

Required boundary transfer

‘Single bus LOLP’ and ‘plant margin’

Generation group

Main systemGenerationconnection

Minimumimportrequirementinto B = DB - GB

Inter-area transfer

A

B

B1

Suppose there is sufficient generation available on

the system as a whole to meet demand

GBDB

At a particular time,demand in area B is DB

and the total available generation

in the area is GB

For demand DB to be met, boundary B1 must be capable of transferring

at least DB - GB

Variation of plant margin

16.00%

18.00%

20.00%

22.00%

24.00%

26.00%

28.00%

30.00%

32.00%

1990

/91

1991

/2

1992

/3

1993

/4

1994

/5

1995

/6

1996

/7

1997

/8

1998

/9

1999

/00

2000

/01

2001

/02

2002

/03

2003

/04

2004

/05

Year

Pla

nt

mar

gin

Required boundary transfer capability

• Over many cases of total area demand and available generation, what should the boundary import capability be in order to satisfy some given risk to demand in the area?– Present GB SQSS specifies secure capability

• no bad things in an N-1 situation, or• no bad things in an N-2 situation

– Quantify ‘risk’ on any one boundary due to uncertainty in demand and the available generation as

• the ‘demand reduction probability’ (DRP), or• the ‘demand at risk’ (DAR)

A

B

B1

GBDB

A

B

B1A

B

B1

GBDB

A

B

B1

GB DB

A

B

B1A

B

B1

GB DB

A

B

B1

GBDB

A

B

B1A

B

B1

GBDB

Use of Monte Carlo simulation

Data fromtransmissionlicensees

Model from Bath and Garrad Hassan

Find the deficit (or surplus) of available generation in an area relative to demand in the area

• Use of simulation permits frequency distribution to be found– Weather variation of ‘consumer’ demand

• sampled from lognormal distribution

– Operation of embedded generation• sampled from normal distribution

– Available ‘large scale’ generation and interconnection• bernoulli trials

– Available hydro power• sampled from lognormal distributions

– Available wind power• based on multivariate autoregression model of wind speed• 17 years of winter wind speeds, spatial correlations respected• conversion to hub height wind speed and available power

Comparison of benchmarks: DRP

0

5

10

15

20

25

30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Boundaries

N-2

DR

P

DRP-TNEI DRP-Strathclyde Uni

4%

7%

11%

0

1

2

3

4

5

6

7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Boundaries

N-1

DR

P(%

)

DRP-TNEI DRP-Strathclyde Uni

2.5%

1.125%

0.25%

N-1: DRP due to transmission = 2.5%N-2: DRP due to transmission = 11%

Results from simulation of 2005 scenarioChanges between simulations• Assumed nuclear availability• Assumed demand uncertainty• Definition of B5

Initial simulationsRevised simulations

N-2 Demand At Risk - MW

0

10

20

30

40

50

60

70

80

90

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Boundaries

Ris

k (M

W)

Comparison of benchmarks: DARN-1 Demand At Risk - MW

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Boundaries

Ris

k (M

W)

N-1: average DAR = 7 MWN-2: average DAR = 36 MW

Results from simulation of 2005 scenario

Characterisation of required import capability to meet demand

REQUIRED TRANSFER FOR N-1 CONDITION AND A TOTAL GENERATION CAPACITY OF ~17,000 MW

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

-0.15 0 0.15 0.3 0.45 0.6 0.75 0.9

Area ACS peak demand- Total Generation Capacity (pu)

Req

uir

ed T

ran

sfer

Cap

abil

ity

(pu

)

WP=0% WP=15% WP=30% WP=45% WP=60% WP=75% Series7

Increasing wind penetration in area

Characterisation of required import capability to meet demand

Interpretation

• The capability of the system to import power from an area A to meet demand in another area B may be interpreted as– the extent to which demand in area B depends on

generation in area A• If generation in area B is unreliable, demand in area B will be

more dependent on generation in area A• Demand in a large area B with a lot of generation tends not

to depend on generation in another smaller area A– Required transfer capability from A to B to meet demand

in B is likely to be small

Use of the characterisation in a spreadsheet

Input area data:1. Demand at ACS peak2. Total Thermal and Hydro Capacity3. Wind Capacity4. Interconnection

Input system parameters:1. ACS peak demand2. (Optional) ‘Effective plant margin’

or ‘Beta coefficient’

Calculate:• Total generation capacity in area• Proportion of area generation that is wind• Difference between ACS demand in area at peak

and total generation capacity in area

Interpolate between lines in 3d characterisation• Difference between area demand and generation capacity• Wind penetration in area

Output data:Required boundary capability (MW)• N-1• N-2

The purpose of transmission

• To provide energy transport from sources (generators) to consumers (loads) with an acceptable reliability.

• To pool resources and reserves so that security of supply is achieved.

• To obtain benefits of economic operation such that cost of energy to all consumers is a minimum at all times.

• To enable the electrical energy wholesale market and promote competition.

The transmission licensees have a licence condition tofacilitate competition

Required boundary transfer to facilitate competition

Generation group

Main systemGenerationconnection

Inter-area transfer

A

B

B1

Suppose there is a surplus of generation available on the system as a whole relative to demand

GB DB

At a particular time,demand in area B is DB

and the total available generation

in the area is GB

GA DA

At the same time,demand in area A is DA

and the total available generation

in the area is GA

Which generation willthe market ‘prefer’ to use?

Boundarytransfercould beA to B orB to A.By howmuch?

Required boundary transfer to facilitate competition

Generation group

Main systemGenerationconnection

Inter-area transfer

A

B

B1

GB DB

GA DA

Which generation willthe market ‘prefer’ to use?

Depends on whichgeneration is most

‘competitive’

Transmission licensee’srole is to facilitatecompetition• Give equal opportunity

to the availablegeneration

• Don’t restrict anyavailable generationmore than any other

Required boundary transfer to facilitate competition

Generation group

Main systemGenerationconnection

Inter-area transfer

A

B

B1

GB DB

GA DA

Don’t restrict any available generation more than any other

In order to balancethe system in thissituation and giveequal treatment to

all available generation, one could scale back all

available generationby the same factor

GBDB

GA DA

The ‘scaled transfer’in this situation is

representative of themarket facilitation

requirement

Required boundary transfer to facilitate competition

• Benchmark the present SQSS MITS required capability in terms of the percentage of ‘scaled transfers’ that are facilitated– The ‘planned transfer’ is the median of these transfers

• ‘Planned transfer’ plus (half) interconnection allowance is something more than the median

• As noted last time, present A factors are quoted on a premise of facilitating a certain percentage of transfers at peak

• How do ‘scaled transfers’ vary with– system wind penetration?– split of wind generation capacity either side of a boundary?

• Next stage of TNEI/Strathclyde analysis aims to characterise the variation of ‘scaled transfer’ with wind penetration and location

Required boundary transfer to facilitate competition: comparison with current practice

• From the Seven Year Statement,– “The [A factor] values are chosen in order that the

'required transfer capability' , which is simply the sum of the 'planned transfer' and the appropriate 'interconnection allowance', will represent approximately the same percentile of the actual distribution of power transfers at time of peak demand whether the background includes wind or hydro generation or not.”

Required boundary transfer to facilitate competition

• As in previous studies, the analysis will– be by means of a Monte Carlo simulation– take into account the relative availability of different types

of generation and correlations between individual power sources

• A boundary transfer capability based on enabling a certain percentage of ‘scaled transfers’ found from a simulation will– facilitate competition (in a manner similar to that achieved

by the present SQSS) proportional to a generator’s ability to exploit it

• the less available generation in an exporting area will tend to give less ‘push’ to the ‘scaled transfers’

• a high percentile of ‘scaled transfer’ may still represent conditions with significant wind output

A 3-layered design criterion?

• The minimum secure transfer capability on a boundary should be the maximum of– that required for the risk of demand reduction to be no

higher than a benchmark value– that which, based on ‘scaled transfers’, doesn’t restrict

generators’ access to the market more often than x times out of 100 proportional to its ability to exploit access

– that required for minimisation of the total cost of transmission

• cost of transmission infrastructure + cost of constraints + cost of unreliability

• guidance in SQSS on strongest influences on cost of constraints• specification in SQSS not only of necessary considerations but

also those that are sufficient

Other work

• Background work at Strathclyde is investigating the economics of generation group connection capacity– dependency on relative sizes in the group of

• demand• thermal generation• hydro generation• wind generation

Conclusion

• Work by Strathclyde and TNEI building on previous work at Bath has seen the development of– a (relatively) simple to apply characterisation of required

boundary transfer capability• based on a large number of detailed simulations covering a very

wide range of possible future scenarios• respects spatial correlations between available wind power• based on 17 years of wind speed data• respects spatial correlations in demand• is substantially decoupled from whole system ‘plant margin’• may be based on ‘demand reduction probability’ or ‘demand at risk’

• Further work ongoing to seek similar characterisation of facilitation of equal generation access at time of system peak demand