Combined operation of power plants, steam and gas, steam and diesel, nuclear and steam, base load plants, peak load plants.

1. Combined Operation
of Power Plants
Nishkam Dhiman
Asst Prof : Electrical and Electronics
Engineering
Chitkara University, Punjab

2. Advantages of Combined operation of
Plants
If 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.)

3. 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.

4. • 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.

5. Requirements of a plant supplying
“Base Load”

6. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.