Transmission Capacity to Accommodate a Mixed Background of Generation Keith Bell and Dusko Nedic...
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Transcript of Transmission Capacity to Accommodate a Mixed Background of Generation Keith Bell and Dusko Nedic...
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