Transmission Capacity to Accommodate a Mixed Background of Generation Keith Bell and Dusko Nedic University 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 system Generation connection Minimum import requirement into B = D B - G B Inter-area transfer A B B1 Suppose there is sufficient generation available on the system as a whole to meet demand GBGB DBDB At a particular time, demand in area B is D B and the total available generation in the area is G B For demand D B to be met, boundary B1 must be capable of transferring at least D B - G B

Variation of plant margin

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)

Use of Monte Carlo simulation Data from transmission licensees 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 N-1: DRP due to transmission = 2.5% N-2: DRP due to transmission = 11% Results from simulation of 2005 scenario Changes between simulations Assumed nuclear availability Assumed demand uncertainty Definition of B5 Initial simulations Revised simulations

Comparison of benchmarks: DAR N-1: average DAR = 7 MW N-2: average DAR = 36 MW Results from simulation of 2005 scenario

Characterisation of required import capability to meet demand 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 peak 2.Total Thermal and Hydro Capacity 3.Wind Capacity 4.Interconnection Input system parameters: 1.ACS peak demand 2.(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 to facilitate competition

Required boundary transfer to facilitate competition Generation group Main system Generation connection Inter-area transfer A B B1 Suppose there is a surplus of generation available on the system as a whole relative to demand GBGB DBDB At a particular time, demand in area B is D B and the total available generation in the area is G B GAGA DADA At the same time, demand in area A is D A and the total available generation in the area is G A Which generation will the market prefer to use? Boundary transfer could be A to B or B to A. By how much?

Required boundary transfer to facilitate competition Generation group Main system Generation connection Inter-area transfer A B B1 GBGB DBDB GAGA DADA Which generation will the market prefer to use? Depends on which generation is most competitive Transmission licensees role is to facilitate competition Give equal opportunity to the available generation Dont restrict any available generation more than any other

Required boundary transfer to facilitate competition Generation group Main system Generation connection Inter-area transfer A B B1 GBGB DBDB GAGA DADA Dont restrict any available generation more than any other In order to balance the system in this situation and give equal treatment to all available generation, one could scale back all available generation by the same factor GBGB DBDB GAGA DADA The scaled transfer in this situation is representative of the market 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 generators 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, doesnt 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