Economic Operation of Power Systems

Main Objective of ESI Main objectives of electricity Supply Industry (ESI) Minimise cost (maximum efficiency) Reliability (Continuity of the supply) Security of Supply Sustainability A compromise is sought which must not jeopardise the safety to personnel and equipment.

Actions Required Long-term load forecasts Network development Short-term load forecasts Power system operation and control Voltage regulator (AVR) – Control of Q and V Turbine Speed governors – Control of P and f Optimisation of power system operation

Load Characteristics In order to make the best utilisation of the power system and available resources, knowledge on loading condition and characteristics of the individual loads are essential. Loads may be characterised in regard to Size (Watts to MW) Symmetry (single or three-phase) Load constancy (with respect to time, frequency, voltage) Use cycle (regular or random) Symmetry between three phases can be achieved by intentional distribution of single-phase loads

Load Curve Source: www.nationalgrid.co.uk

Load Factor Load curve is the plot of variation of load demand with respect to time. Peak or maximum demand is defined as the maximum load (occurring in the time under consideration). Load factor is defined as the ratio of average load during a period to maximum load during the same period.

Load Factor You can define daily Load factor or annual load factor of the plant using this definition.

Diversity Factor Diversity factor applies to a group of loads whose individual maximum demand do not all occur at the same time. Industrial, commercial and domestic loads The system maximum demand will always be less than the sum of the maximum demand of all the loads. It is the parameter which provides the diversification in the load and is used to decide the installed capacity of the power system. Diversity factor is the ratio of (sum of maximum demand of individual consumers) to (maximum demand on system) Diversity factor is greater than unity.

Utilisation Factor Utilisation factor shows how well the plant is being used. Utilisation factor is the ratio of maximum demand to installed capacity. Installed capacity or effective plant rating is the total rated capacity of the plant. The difference between the maximum demand and the effective plant rating is known as generating plant margin. Normally given as a percentage of the estimated maximum demand.

Reserve Capacity To provide best reliability Failure of one generating unit should not affect the power supply to consumers and / or power quality This means that additional generating capacity of at least the value of the largest set operating must be available instantaneously. Known as spinning reserve Hot standby Its output is available in a short time (about 30 minutes)

Demand Side Management (DSM) Objective: encourage the consumer to use less energy during peak hours so load curves lose peak points. How: Shift demand from peak times i.e. to night time and or weekends. Incentive: change of tariff or use Smart metering to advise on best tariff rate and times.

Optimisation Method Economic Load Dispatch or Generation Scheduling Minimising transmission losses Computers with optimal power-flow programs are used

Optimisation of Power System Operation To optimise the performance of a power system, a system control strategy is established on a daily, monthly and yearly basis. The result is the preparation of generation schedules, line usage, etc. Factors to be considered: Fixed cost of supply e.g. interest, fixed salaries and depreciation Variable cost of supply e.g. fuel Transmission losses Transportation of fuel Obviously, plant rating and stability limits must be observed.

Economic Dispatch In this section of the lecture we will look at how we can optimise generator loading so to achieve ‘Economic Dispatch’

Variable Costs Although fixed element of the generation costs affects the overall cost of production, it is not normally considered when allocating generation to meet demand. The allocation is based entirely on the variable costs. This is obtained by considering fuel costs, generation efficiency, maintenance cost etc.

Heat Rate Characteristics This is the thermal input – electrical output characteristics of a turbine-generator set. The cost characteristics of thermal power stations depend primarily on whether turbines used are single or multiple valve units. Single valve units are preferred in the UK while multiple valves are preferred in the USA.

Heat Rate Characteristics Maximum fuel efficiency occurs at the point where the slope of the line from the origin to a point on the curve is tangent to the curve e.g. 0.8 pu

Economical Division For economical division of load between units, the incremental fuel costs for all contributing turbine-generator sets should be the same. If two sets having different fuel costs are sharing a load, the total cost can be reduced by transferring load from the set having higher incremental cost to the set with the lower incremental fuel cost. This decreases the incremental cost of the former and increases that of the latter. The limit is reached when the two incremental costs are equal and the total cost is minimum.

Exercise 1 A power plant consists of two generators that have the following fuel costs (F1 and F2) in £/hour: Where P1 and P2 are in MW. The plant supplies a total load of 150 MW. By neglecting the losses: Determine the power supplied by each generator if these generators are to be operated in a mode of optimal dispatch. The savings in fuel cost (in £/hour) when the plant is operated in the optimal economic dispatch mode compared with equal load sharing.

Example 2 Installed capacity of the generators is 150MW. The generator supplied power to an industrial load having maximum demand of 140MW. If the capacity utilization factor of the generator is 70%. Determine Reverse Capacity in MW Reverse margin in % Provide your comment on adequancy of reserve margin. Load factor in % Annul energy in KWh If load factor is to be improved by 10%, recommend one method that can be used for this purpose.1

Example 3 The annual peak load on 30MV power system is 25MW. The power station supply load having maximum demand of 10MW,8.5MW, 5MW and 4.5MW. The annul load factor is 45%. Average Load? Energy supply per year? Diversity factor? Capacity Factor?

References further reading Weedy, B.M. and Cory B.J., “Electric Power Systems”, Wiley publication.