Combined Operation of Power Plants

Nishkam Dhiman

Asst Prof : Electrical and Electronics Engineering

Chitkara University, Punjab

Advantages of Combined operation of PlantsIf several power plants work together to meet the demand of the consumer then the system is known as “Interconnected System” Advantages:1. Greater reliability of supply to the consumers.2. When one of the stations fails to operate the consumers can be fed from the other stations, thus avoiding complete shut down.3. The overall cost of the energy per unit of an interconnected system is less.4. There is more effective use of transmission line facilities at higher voltage.5. Less capital investment required.6. Less expenses on supervision, operation and maintenance.7. Spinning reserve required is reduced.(Spinning reserve is any back-up energy production capacity which is can be made available to a transmission system within ten minutes' notice and can operate continuously for at least two hours once it is brought online.)

Load Division between power stations

• A load duration curve illustrates the variation of a certain load in a downward form such that the greatest load is plotted in the left and the smallest one in the right. On the time axis, the time duration for which each certain load continues during the day is given.

• When the load curve has a very high peak value, it is usually supplied for two or more power stations by interconnection.

• Total load my be divided into two parts.

A. Base Load B. Peak Load

Base load is supplied by one power station and other power station takes care of the peak load.

Requirements of a plant supplying “Base Load”

Hydroelectric Power Plants: • Operating cost is minimum• Maintenance cost is lower.• Initial cost is high• Cant be located near to load.

Steam Power Plants:• Capital cost is more• Can be located near to load center.• Maintenance requirement is slightly higher.

Diesel Power Plants:

Due to limited generating capacity, it is not used as base load plant.

Nuclear Power Plant: • Initial Cost is High• Limited availability of fuel, not favoured.• Whenever they are constructed they are used as base load plants due to constant

supply.

Hydroelectric Plants: can be considered as peak load plant.• It can be easily started.• No fuel required.

Nuclear Power Plants: they are not used as peak load plants.

Diesel Power Plants: can be used for peak load plants• Easily start from cold conditions.• Number of auxiliaries required is limited, so less

maintenance.• They have good overload capacity.• They have high thermal efficiency therefore less operating

cost than steam power plants.

Hydroelectric Plant in combination with Steam Power Plant.

• Hydro Electric plants can take up the load quickly and follow the peak variations much better than thermal plants.

• When the run-off is sufficient (in monsoons) Hydro plant is used as base load plant and thermal plant is used as peak load plant.

• Thermal plant is used as base load plant during the draught period and hydro plant works as peak load plant.

Hydrothermal Scheduling

• Hydrothermal scheduling is performed in order to find the optimum allocation of hydro energy so that the annual operating cost of a mixed hydro-thermal system is minimized.

Hydro Thermal Scheduling Methods

1. Constant Hydro generation: In this method the hydro generation is kept constant, throughout the operation period, at such a value as to use the desired amount of water. The remaining load is met by the steam plant.

Load on steam plant keep on varying from time to time.

2. Constant Steam Generation: In this method the steam generation is kept fixed throughout the operating period. The remaining load is supplied by hydro plants. The steam generation is kept fixed at such a value that hydro-generation would use the desired amount of water over the operating period. The hydro plant operate on varying schedules.

3. Maximum hydro efficiency method: In this method the hydro plants during peak load periods are operated at maximum efficiency.

During off – peak hours the hydro generation is scheduled so that desired amount of water is used over the operating period and hydro plant works near the point of maximum efficiency as possible. The remaining load is met by steam plant.