Power Generation/Variable Load

Review: Lesson 6

The previous Lesson was about Gas turbine Power station. The student/User is expected to remember the following from the lesson 6.

• Basics:
• Arrangements:
• Advantages & Disadvantages:

Definition
components that make up a gas turbine power station.
How this station measures up

Preview: Lesson 6

This Lesson is about Variable loads on Power stations. The student/User is expected to understand the following at the end of the lesson.

• Definitions
• Load Curves
• Types of loads on a power station

• Selecting generation units for a power station
• Base & Peak loads
• Methods of meeting load

To begin this lesson we look at definitions that will be used. learn these well.

Maximum demand ( MD ): The greatest demand of load on the power station during a given period. The highest peak on the power station load curve.	Demand Factor ( Dem.F ): Ratio of maximum demand to connected load. this is usually less than 1. $Dem.F={\frac {MD}{Con_{.}Load}}$	Average load ( MD ): This is the average of loads occuring on the power station in a given period.
Daily average load: Average of loads occuring on a power station in 1 day (24Hrs). $={\frac {Total_{.}of_{.}units(in_{.}kWh)}{24Hrs}}$	Monthly average load: Average of loads occuring on a power station in 1 month (24Hrs x No. of days). $={\frac {Total_{.}of_{.}units(in_{.}kWh)}{24Hrs\times days}}$	Yearly average load: Average of loads occuring on a power station in 1 year (8760Hrs). $={\frac {\sum Total_{.}of_{.}units(in_{.}kWh)}{8760Hrs}}$
Load factor ( LF ): The ratio of average load to maximum demand. typical ≈ less than 1. This is the measure of the effective use of the power station. $LF={\frac {Load_{ave}}{MD}}={\frac {Output_{Annual}(in_{.}kWh)}{(Capacity_{Installed}\times 8760Hrs)}}$ High LF = Low MD = Low plant capacity = Low cost per unit generated.	Diversity factor ( DF ): The ratio of the sum of all individual maximum demands on the power station to the Maximum demand on the station. Consumer maximum demands donot occur at the same time thus maximum demand on power station will always be less than the sum of individual demands. $DF={\frac {MD_{individual}}{MD_{On_{.}station}}}$ High DF = Low MD = Low plant capacity = Low investment capital required.	Plant capacity factor ( PCF ): The ratio of actual energy produced to the maximum possible energy that could have been produced on a given period. $PCF={\frac {kWh_{perannum}}{Plant_{.}Capacity\times 8760Hrs}}$ This indicates the reserve capacity of a plant. From these we can see that: $Units_{.}generated=MD\times LF\times 8760Hrs$

Objectives of a Power station:

The power station is constructed, comissioned and operated to supply required power to consumers with generators running at rated capacity for maximum efficiency.

we saw in lesson one that the fundamental problem in generation, transmission and distribution of electrical energy is the fact that electrical energy can not be stored. It must be generated, transmitted and distributed as and when needed.

This lesson looks at problems associated with variable loads on power stations, and discusses the complexities met in deciding the make, size and capacity of Generators (Generating units) that must be installed in a power plant to successfully meet these varying energy demands on a day to day basis.

Variable load:

The load on a power station varies from time to time due to uncertain demands of consumers. Energy demand of one consumer at any given time is distinct/differs from the energy demand of another consumer. This results in the total demand on the power station to vary over a given period of time and may necesitate the following:

• Additional equipment/Generating units to meet demand

• Increase in production cost to recuperate use of more material/equipment

In order to study the pattern and effect of the varying load, station engineers use load curves.

A load curve is a graph showing the variation of load on the power station with respect to time. the following load curves are used in power stations:

• Daily load curve: -- Load variations captured during the day ( 24Hrs ), recorded either half-hourly or hourly.

• Monthly load curve: -- Load variations captured during the month at different times of the day plotted aginst No. of days.

• Yearly load curve: -- Load variations captured during the Year, this is derived from monthly load curves of a particular year.

Information obtained from load curves:

• Area under load curve = Units generated

• Highest point of the curve = MD

• (Area under curve) ÷ (by total hours) = Average load

• (Area under load curve) ÷ (Area of rectangle containing load curve) = LF

• Helps to select size & number of generating units.

• Helps to create operating schedule of the power plant.

Selecting generating units:

The following must be considered when selecting the number and size of Generating units (Generators):

• Number and size of units to aproximately fit the annual load curve.

• Units to be of different capacities to meet load requirements.

• Atleast 15 - 20% of extra capacity for future expansion should be allowed for.

• Spare generating capacity must be allowed for to cater for repairs and overhauling of working units without affecting supply of minimum demand.

• Avoid selecting smaller units to closely fit load curve.

Meeting Load:

The best method to meet load requirements on power station is to Interconnect two different power stations in paralell as follows:

• More efficient Plant → Carries Base load( The unvarying load on the load curve ).

-- Generally thermal & Nuclear power stations.

• Less efficient Plant → Carries Peak load ( Various load peak demands on the load curve ).

-- Generally Hydro, Pumped storage & gas turbine power stations.

Careful study of load curves must be undertaken before deciding which type of station will be used for what purpose as this is greatly dependant on enviromental issues and availability of fuel used by a particular power station.

The power grid is constructed by connecting several generating stations together in parallel. This method has helped solve most transmission and distribution problems facing power engineers. Below are the advantages of using a power grid:

• Economical operation:

Sharing of load among stations allows for more efficient stations to work constantly at high load factors and less efficient stations to be used for peak supply only.

• Increased diversity factor:

Different stations have different load curves thus the total maximum demand of the system is decreased, thus effectively increasing the diversity factor of the system.

• Reduces Plant Reserve Capacity:

The stand-by capacity required of individual plants is reduced when they are interconnected in a grid.

• Increased reliability:

If major breakdown occurs on one station, supply is maintained by other stations.

• Exchange of peak loads:

Excess load can be shared from highly stressed plants to plant with lower peak loads ( Identified from load curves ).

• Older plants can still be used:

Older plants which are less efficient can still be used to carry peak loads of short durations.

References

• This resource is prepared from Lecture notes by Thuvack.
• V.K Mehta & Rohit Mehta :- Principles of Power systems (1st ed.). S.CHAND .ISBN 81-219-2496-0