Loads

Any device that works with the electrical energy is called load.

loads are a main part of the distribution system which represents 40% of  the total cost of power system so it is very important to study the electrical loads , its types  and its own parameters as the aim of all electric power system is to provide all loads with its demands.

Loads represent the passive elements in the network.

Types of loads:

1- According to its application:

a- Industrial :

consumes the biggest percentage of power as compared to most of other loads . the next table shows that fact:

Product	cement	steel	paper	sugar
Kwh \ ton	112	230	500	220

b- Domestic .

c- Irrigation.

d- Lighting streets.

e- Commercial , etc.

2-According to its nature:

a- Resistive.

b- Inductive.

c- Capacitive.

3-According to its behavior:

a- Linear.

b- Non-linear.

load Characteristics:

An accurate representation of electric load in a power system is of interest for  the purpose of more detailed system behaviour studies by simulation.

The characteristics of loads can be static or dynamic .

1-Static Characteristics:

Is a relation between power, torque or current and the voltage (or frequency) determined at such slow variations of the operating condition that each of its point might be considered as corresponding to steady state .

For example , for p= p(v) and Q= Q(v) are defined as static  characteristics where:

p : Active power.

Q : Reactive power.

V : Voltage.

2- Dynamic Characteristics:

Is the same relation but defined for such fast variation of the operating condition that their rate of change has to be taken into account

 For example  P=p(v , F, dv/dt , df/dt )  where F: frequency.

Within certain voltage and frequency intervals , the active and reactive power  varies with the voltage and frequency according to the following exponential relations :

where:

       nominal voltage.

       nominal frequency

       nominal frequency.

       nominal reactive power

kv , kf :  exponential of reactive power dependence.           

mv, mf : exponential of active power dependence.

The following table indicate typical values of load parameters for some loads:

load	kv	Kf	mv	mf
Heater	2	0	0	0
Filament lamp	1.6	0	0	0
Induction motor:              - Half load              - Full load	0.2 0.1	1.5 2.8	1.6 0.6	-0.3 1.8
Induction furnace	1.9	-0.5	2.1	0

If the load does not depend on the voltage or the frequency , the load parameters (kv , kf , mv , mf ) will be zeros . If the load parameters are positive , the relation will be proportional relation.

-For  example if mv=1  so :

                                  

So the relation will be straight line with a slope of:

- As increasing the value of mv the relation will be curve with different shapes as shown :

- As increasing the value of mv the load is high dependence of voltage>

- At induction motor   Tα v^2  so  Pα v^ 2

- The general form is :

                                      

- There are another forms for these relations such as:

-The same concepts are applied to the reactive power.

Load Modeling:

The load can be represented by one of the following models:

1)Constant Impedance :

where Z = constant and I,V, P are varied,

- This model is used in control applications.

 

2)Constant Current:

where P, Q, Z are variable and Current is constant,

- This model is used in protection applications.

3)Constant Power:

where P, Q are constant and I, Z are variable, -  This model is used in power flow application.

Some parameters related to loads :

Demand Factor = (maximum demand / connected loads) and is always <= 1

Maximum Demand : is the maximum loads that may be switched at once.

Connected Loads : total loads that are already exist.

Load Factor = average power / maximum power .

Utilization Factor = maximum power / capacity .

Capacity Factor = average power / capacity . 

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Ahmad Atiia

Hi. I’m Designer of Engineering Topics Blog. I’m Electrical Engineer And Blogger Specializing In Electrical Engineering Topics. I’m Creative.I’m Working Now As Maintenance Head Section In An Industrial Company.

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