Variable Load on Power Stations, Lecture notes of Electrical Engineering

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Variable load on Power Stations

The load on a power station varies from time to time due to uncertain demands of the consumers and is known as variable load on the station.

Variable Load on Power Station

• Load/demand vary from one customer to

other

• Different consumers require different (small or

large block of power) power in accordance

with their demands at different times

• Ideal Load on the station would be one of

constant magnitude and steady duration

• Steady load is never realized in practice

Introduction

Why do Load vary?

• Consumer activity (daily, monthly, yearly)

• Weather changes (rain, snowfall, wind, etc.)

• Seasonal variations (winter, summer)

• Large industries (shutdown/coming online,

time of year)

Problems of Variable Load on

Power Stations

What is the result of load variations?

• Load on power station is never constant

Why variations of Load is a problem for power engineer?

• Max efficiency of Power Plant occurs when it is operating close to rated capacity
• Power cannot be stored, so, it has to be produced when needed. Output from Power station have to constantly change w.r.t time
• Varying Power output of some Power stations is not possible or recommended (Hydro/Nuclear)
• Brings difficulty in balancing supply and demand

Effects of Variable Load

Need of additional equipment.

• To produce variable power, correspondingly the supply

of raw materials to the plant will be required to vary

• E.g. in steam station, if demand on plant increases,

steam value has to be opened, more water flow to boiler to produce more steam, more coal and air for combustion, etc.

• Much equipment is needed just to adjust the rates of

supply of raw materials

• Equipment needed to control the system and maintain

all parameters within permissible ranges.

Effects of Variable Load

Increase in production cost

• Alternator has maximum efficiency near its rated capacity
• Single alternator will have poor efficiency during periods of light loads
• Number of alternators of different capacities are installed so that most of the alternators can be operated at nearly full load
• Increase in number of generating units increases initial cost/kW and floor area required
• Leads to increase in production costs

Importance of Load Curve

Readily available info

• Variations of load on the power station during different hours of the day.
• The area under the daily load curve gives the number of units generated in the day.
• Highest point on the daily load curve represents the maximum demand on the station

Importance of Load Curve

Average load =

Area (in kWh) under daily load curve 24 hours

Load Factor =

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐿𝑜𝑎𝑑 × 24 𝑀𝑎𝑥 𝐷𝑒𝑚𝑎𝑛𝑑 × 24

Load Factor =

Area (in kWh) under daily load curve Total area of rectangle in which the load curve is contained

Types of Load

Base Load

• The unvarying load which occurs almost the whole day

on the station is known as base load

• Have to be provided constantly throughout the day

Peak Load

• The various peak demands of

load over and above the base load of the station is known as peak load

• Demands/load appearing above

base load for a certain time period (small, can be for all day)

Meeting the Load

• More efficient plant is used to supply the base load
• Less efficient plant is used to supply the peak loads

Example: Interconnection of steam and hydro plants as

Base load and Peak load power station during summers

and winters.

Important Points in the Selection of

Units

• Have spare generating unit so that repairs and

overhauling of the working units can be done

• Selection of large number of smaller capacity

units should be avoided in order to fit the load curve very accurately, as it increases cost per kW (due to more floor area and maintenance etc.)

IMP Terms and Factors in Power Plant

Engineering

Connected Load

• Sum of continuous ratings of all the equipment's connected to supply system
• Sum of the connected loads of all the consumers, is the connected load to the power station

Maximum Demand

• Greatest demand of load on the power station during a given period.
• Helps in determining the installed capacity of the station
• Generally less than the connected load

IMP Terms and Factors in Power Plant

Engineering

Average Load

The average of loads occurring on the power station

in a given period ( day or month or year ) is known as

average load

• 𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐿𝑜𝑎𝑑 =

𝑁𝑜 𝑜𝑓 𝑢𝑛𝑖𝑡𝑠 𝑘𝑊ℎ 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑖𝑛 𝑎 𝑑𝑎𝑦 24 ℎ𝑜𝑢𝑟𝑠

• 𝑀𝑜𝑛𝑡ℎ𝑙𝑦 𝐴𝑣𝑔 𝐿𝑜𝑎𝑑 =

𝑁𝑜 𝑜𝑓 𝑢𝑛𝑖𝑡𝑠 𝑘𝑊ℎ 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑖𝑛 𝑎 𝑚𝑜𝑛𝑡ℎ 𝑁𝑜 𝑜𝑓 ℎ𝑜𝑢𝑟𝑠 𝑖𝑛 𝑎 𝑚𝑜𝑛𝑡ℎ

• 𝑌𝑒𝑎𝑟𝑙𝑦𝐴𝑣𝑔 𝐿𝑜𝑎𝑑 =

𝑁𝑜 𝑜𝑓 𝑢𝑛𝑖𝑡𝑠 𝑘𝑊ℎ 𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑖𝑛 𝑎 𝑦𝑒𝑎𝑟 8760 ℎ𝑜𝑢𝑟𝑠

IMP Terms and Factors in Power Plant

Engineering

Load Factor:

𝐿𝑜𝑎𝑑 𝐹𝑎𝑐𝑡𝑜𝑟 =

𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐿𝑜𝑎𝑑 × 𝑇 𝑀𝑎𝑥 𝐷𝑒𝑚𝑎𝑛𝑑 × 𝑇

=

𝑈𝑛𝑖𝑡𝑠 𝑖𝑛 𝑇 ℎ𝑜𝑢𝑟𝑠 𝑀𝑎𝑥 𝐷𝑒𝑚𝑎𝑛𝑑 𝑖𝑛 𝑇 ℎ𝑜𝑢𝑟𝑠

• Always less than 1, as average load is always less than Max demand
• If LF is less, it means more difference b/t average load and peak load, which shows that Power output has to change more often and plant is not utilized much w.r.t max demand appearing on it, which leads to lesser efficiency and more cost
• Plays key role in determining the overall cost per unit generated.
• Higher the Load Factor, lesser will be the cost per unit generated