SKEE2413Basic Power and Electric

Machines

Topic 5:Power System Loads

ADAPTED FROM TEACHING MODULE SEE 3423: BASIC POWER ENGINEERING BY PM FARIDAH MOHD TAHA, PM DR. AZHAR KHAIRUDDIN

OUTLINES

Introduction Types Of Loads Terms And Definitions Load Profile Or Load Curve Load Duration Curve Load Factor Diversity Factor Load Sharing Between Base Load And Peak Load Tariff Power Factor

OBJECTIVES

Student should be able to:

1. Explain the types of electrical loads.

2. Define and determine various terms and factors in power plant engineering.

3. Explain the nature of loads and how they are met.

4. Explain, determine and analyzed tariff and energy consumption and cost of consumers.

INTRODUCTION

In our daily life we use many form of energy such as mechanical, heat and light.

Electricity is special as it can easily be transform to these forms of energy.

This chapter discusses characteristics and ways of electricity being used and how utility meet these demands economically, effectively and satisfactorily.

TYPES OF LOADS

Residential load: This type of load includes domestic lights, power needed for domestic appliances such as radios, televisions, water heaters, refrigerators, rice cookers, fans, small motors etc.

Commercial load:It includes lighting and air-conditioning for shopping complexes, offices, hospitals and schools.

TYPES OF LOADSIndustrial load: It consists of various demands of industries. The main loads in the sector are three phase induction motors.

Public lighting and Municipal loads: Mostly consist of street and city/town lighting, power required for water supply and drainage purposes.

Agricultural load: Motors for irrigation and lighting.

Mining load: Motors for pumping etc.

TERMS AND DEFINITIONS

Connected load:

The connected load on any system, or part of a system, is the combined continuous rating of all the receiving apparatus on consumers’ premises, which is connected to the system, or part of the system, under consideration.

TERMS AND DEFINITIONS

Demand:

The demand of an installation or system is the load that drawn from the source of supply at the receiving terminals averaged over a suitable and specified interval of time.

Demand is expressed in kilowatts (kW), kilovolts-amperes(kVA), amperes (A), or other suitable units.

TERMS AND DEFINITIONS

Maximum or peak demand or load:

The maximum demand of an installation or system is the greatest of all demands that have occurred during a given period.

It is determined by measurement, according to specifications, over a prescribed interval of time.

TERMS AND DEFINITIONS

Base Load:

It is the unvarying load which occurs almost the whole day on the power station.

It is the lowest load to be supplied at any time.

Intermediate load:

Loads between the base and peak loads that have to be fed by the system.

LOAD PROFILE OR LOAD CURVE.

A load curve is a graphic record showing the power demands for every instant during a certain time interval.

Such a record may cover

i. one hour, in which case it would be an hourly load profile; ii. 24 hours, in which case it would be a daily load profile;iii. a month in which case it would be a monthly load profile;iv. a year (8760 hours), in which case it would be a yearly

load profile.

A load profile will vary according to customer type (typical examples include residential, commercial and industrial), temperature and holiday seasons.

LOAD PROFILE OR LOAD CURVE.

The following points are worth noting:

1. The area under the load curve represents the energy generated in the period considered.

2. The area under the curve divided by the total number of hours gives the average load on the power system.

3. The peak load indicated by the load curve represents the maximum demand of the power system.

4. The base load is the lowest demand registered.

LOAD PROFILE OR LOAD CURVE.

Load profiles could be an hourly, daily, monthly, or annually, depending on the nature of analysis carried out and the level of precision required. This load assessment reveals the power consuming behavior of the people occupying the building.

Villa load profile in Al Ain (Source: AADC).

Brazilian residential load profiles.

Source: Eletrobrás (2005).

Measured hourly electrical load profiles for Building A. (Commercial Load)

LOAD PROFILE OR LOAD CURVE.

Significance of load curve or load profile:

1. Give information about the incoming load and help to decide the installed capacity of the power station and to decide the economical sizes of various generating units.

2. Help to estimate the generating cost and to decide the operating schedule of the power station i.e., the sequence in which different units should be run.

LOAD DURATION CURVE.

Fig shows a typical daily load curve for a power station.

It may be observed that the maximum load on power station is 35 kW from 8 a.m. to 2 p.m.

Represents rearrangements of all the load elements of chronological load curve in order of descending magnitude.

LOAD DURATION CURVE.

The load curve that are plotted in descending order is the same figure to form the load duration curve

LOAD DURATION CURVE.

Points to be noted:

1. The area under the load duration curve represents the total energy delivered by the generating station.

2. Load duration curve gives a clear analysis of generating power economically.

3. Proper selection of base power plants and peak load power plants become easier.

LOAD FACTOR

Definition:

The load factor is the ratio of the average power to the maximum demand.

The load factor should be high; if it is unity, all the plant is being used over all of the period.

It varies with the type of load, being poor for lighting and high for industrial load (e.g. 100% percent for pumping stations).

LOAD FACTOR

LOAD FACTOR

LOAD FACTOR

DIVERSITY FACTOR

Defined as the sum of individual maximum demands of the consumers, divided by the maximum load on the system.

This factor measures the diversification of the load and is concerned with the installation of sufficient generating and transmission plant.

If all the demand occurred simultaneously, diversity factor would be unity, many more generators would have to be installed.

However, the factor is usually higher especially for domestic loads.

DIVERSITY FACTOR

Higher diversity factor could be obtained if consumers could be persuaded to take load as shown in case 1

DIVERSITY FACTORDIVERSITY FACTOR

LOAD SHARING BETWEEN BASE LOAD AND PEAK LOAD

In examining the load curve, it is noted that the load on the power station or power system is seldom constant.

The load may be considered into three parts:

base load : unvarying load which occurs almost all the time on the power station. peak load : various peak demands of the load over and above the base load. intermediate load : between the above two loads.

Deliver full power at all times. Nuclear and coal-fired stations are particularly well adapted to furnish base demand.

Thermal power station using gas or coal take from 4 to 8 hours to start up, while nuclear stations may take several days.

The economic characteristics of base load plants should be :i. supply power at high capital costsii. low costs of operation. iii. high load factors iv. very efficient.

Base Power plants

LOAD SHARING BETWEEN BASE LOAD AND PEAK LOAD

Intermediate power plants

• The stations can respond relatively quickly to changes in demand, usually by adding or removing one or more generating units.

• Hydropower stations are well suited for this purpose.

LOAD SHARING BETWEEN BASE LOAD AND PEAK LOAD

• Deliver power for brief intervals during the day. • Such stations must be put into service very quickly. • They are equipped with prime movers such as diesel engines,

gas turbines, compressed air motors, or pumped storage turbines that can be started up in a few minutes.

• The peak load plants should be inexpensive in starting and shutting down operations.

• The economic characteristics should be they supply power at low capital costs, although at high cost of operation

LOAD SHARING BETWEEN BASE LOAD AND PEAK LOAD

Peak generating stations

EXAMPLE 1

a) Plot the load profile.b) Plot load duration curve.c) Choose suitable generating units to supply the loadd) Determine the maximum demand, energy generated per day, average load and load factor

SOLUTION EXAMPLE 1

SOLUTION EXAMPLE 1

SOLUTION EXAMPLE 1

Maximum demand- from the load curve the maximum demand on the power station is 150

MW and occur during the period 14-18 hours.

SOLUTION EXAMPLE 1

Energy generated/ day = 45x6 + 136x6 +90x2 +150x4 + 75x6 = 270 + 810 +180 + 600 + 450 = 2310 MW

TARIFFS

• Power utility charges consumers for the use of their electricity.

• The rate at which electrical energy is supplied to a consumer is known as tariff.

• Each company has its own set of tariffs. The following items are chargeable:

(a) Usage charge: amount of electricity consumed in kWh,(b) Capacity charge: maximum demand in kW,(c) Reactive power charge: penalty for poor power factor.

TARIFFS

Charges are usually made on the monthly usage.

Some supply authorities encourage consumers to use high voltages by reducing tariffs charges for the kWh consumption while others do not.

Another incentive in the form of lower kWh charges during off-peak hours (usually from midnight to dawn) is given to consumers to encourage them to operate during these hours when the overall electricity consumption is low.

TARIFFS

The TNB new tariff implanted as from 1st June 2011 provides 16 group of consumers.

Tariff A is for residential, tariffs B, C1 and C2 are for commercial, and tariffs D, E1, E2, E3 are for industrial sector.

Special tariffs for mining, public lighting and agriculture are also available.

TNB TARIFF

EXAMPLE 2

In June 06, Madam Kamala consumed 300 kWh of electricity. Determine her electricity bill for the month.

SOLUTION EXAMPLE 2

EXAMPLE 3

SOLUTION EXAMPLE 3

POWER FACTOR

Power Factor is an index used to compute the efficiency level of electricity usage. The index is measured from 0 to 1. A higher index shows efficient usage of electricity and vice versa. Low power factor shortens the lifespan of electrical appliances and causes power system losses to TNB.

To understand power factor, we will start with the definition of some basic terms:-

KW: Working Power (also called Actual Power, Active Power or Real Power). It is the power that powers equipment and performs useful work.

KVAR: Reactive Power. It is the power which magnetic equipment such as transformers, motors and relays need to produce the magnetizing flux.

KVA: Apparent Power. It is the vectorial summation of KVAR and KW.

POWER FACTOR

Power Factor is the ratio of Working Power to Apparent Power.

Power Factor = KW / KVAFor efficient usage of electricity, power factor must

approach 1.0. Power factor that is less than 0.85 shows inefficient use of electricity.

Calculation for Power Factor = KWh _ √(KWh2 + KVARh2)

Causes of Low Power Factor

Low power factor is caused by inductive loads, which are sources of reactive power.

Examples of inductive loads are:- Transformers Induction motors High Intensity Discharge (HID) lighting

Inductive loads constitute a major portion of power consumed by commercial and industrial sectors.

How to Improve Your Power Factor

Customers are advised to follow these steps:- Install capacitors (KVAR Generators)

Capacitor Corrector Synchronous generators Synchronous motors

Minimize operations of idling or lightly loaded motors. Avoid operating equipment above its rated voltage. Replace standard motors as they burn out with energy

efficient motors.

Benefits of Improving Power Factor

Benefit 1: Reducing KW billing demand Low Power Factor requires high reactive power (KVAR) and

apparent power (KVA), which is the power that TNB supplies. Therefore, a facility’s low power factor forces TNB to increase its generation and transmission capacity in order to handle this extra demand.

By increasing power factor, customers use less KVAR. This results in less KW, which equates to RM savings for TNB.

Benefit 2: Eliminating power factor surcharge Utility companies all around the world charge customers an

additional surcharge when their power factor is less than 0.95. In fact, some utilities are not obliged to deliver electricity to their customers at any time the customer’s power factor falls below 0.85.

Thus, customer can avoid this additional surcharge by increasing power factor. In Malaysia, TNB is allowed through the Malaysian Grid Code and the Malaysian Electricity Distribution Code, to impose a surcharge to the customer if the power factor is <0.85 for customers receiving less than 132kV .

Benefits of Improving Power Factor

Benefit 3: Increased system capacity and reduced system losses in your electrical system

Low power factor causes power system losses in the customer’s electrical system. By improving power factor, these losses can be reduced. With the current rise in the cost of energy, increased facility efficiency is important. Moreover, with lower system losses, customers are able to add additional load in their electrical system.

Benefits of Improving Power Factor

Benefit 4: Increased voltage level in your electrical system, resulting in more efficient motors

As mentioned before, low power factor causes power system losses in customer’s electrical system. As power losses increase, customer may experience a voltage drop. Excessive voltage drops can cause overheating and premature failure of motors and other inductive equipment.

Therefore, by raising the power factor, customers can minimize these voltage drops along feeder cables and avoid related problems. Motors will run more efficiently, with a slight increase in capacity and starting torque.

Benefits of Improving Power Factor

Power Factor Surcharge

Power factor surcharge is imposed when your power factor is less than 0.90 (electricity supply 132kV and above) or less than 0.85 (electricity supply below 132 kV).

How it is Calculated

Power factor surcharge for customers with electricity supply below 132 kV is calculated as follows:-

1.5% surcharge of the current bill – for every 0.01 less than 0.85 power factor.

3% surcharge of the current bill – for every 0.01 less than 0.75 power factor.

Power Factor = 0.80Surcharge

= [(0.85 – 0.80) / 0.01] x 1.5% x RM2,000 = RM150

Example 4:-Current bill: RM2,000

Power Factor = 0.75Surcharge = [(0.85 – 0.75) / 0.01] x 1.5% x RM2,000

= RM300

Example 5:-Current bill: RM2,000

Power Factor = 0.60Surcharge = [((0.85 – 0.75) / 0.01) x 1.5% x RM2,000] + [((0.75

– 0.60) / 0.01) x 3% x RM2,000] = RM1200

Example 6:-Current bill: RM2,000