Load Flow Study

IntroductionLoad flow studies or Power flow studies is the

analysis of a power system in normal steady state condition.

Load flow studies basically comprises of the determination of

VoltageCurrentActive PowerReactive Power

Importance Generation supplies demand(Load) plus

losses.Bus voltage magnitude remain close to rated

value.Generation operates within specified real and

reactive power limits.Transmission line and transformer are not

overloaded.

Need of Load flow studyDesigning a power system.Planning a power system.Expansion of power system.Providing guide lines for optimum operation

of power system.Providing guide lines for various power

system studies.

Bus ClassificationA bus is a node at which many Transmission

lines, Loads Generators are connected.It is not necessary that all of them be

connected to every bus.Bus is indicated by vertical line at which no. of

components are connected.In load flow study two out of four quantities

specified and other two quantities are to be determined by load flow equation.

Depending upon that bus are classified.

Flow chart

Load bus or PQ BusA buss at which the Active power and

reactive power are specified. Magnitude(V) and phase angle(δ) of the

voltage will be calculated. This type of busses are most common,

comprising almost 80% of all the busses in given power system.

Generator bus or P-V busA bus at which the magnitude(V) of the

voltage and active power(P) is defined.Reactive power(Q) and Phase angle(δ) are to

be determined through load flow equation.It is also known as P-V bus.This bus is always connected to generator.This type of bus is comprises about 10% of all

the buses in power system.

Slack BusVoltage magnitude(V) and voltage phase

angle(δ) are specified and real(P) and reactive(Q) power are to be obtained.

Normally there is only one bus of this type is given in power system.

One generator bus is selected as the reference bus.

In slack bus voltage angle and magnitude is normally considered 1+j0 p.u.

Bus Classification table

Static methodThe following variables are associated with

each bus:Magnitude of voltage(V)Phase angle of voltage(δ)Active power(P)Reactive power(Q)The load flow problem can solved with the

help of load flow equation(Static load flow equation).

ContinueThe bus admittance matrix is given by:

In general the equation for bus-1 can be written as:

Y11V1+Y12V2+Y13V3=I1

For bus-2 and bus-3 we can write: Y21V1+Y22V2+Y 23 V3=I2 Y31V1+Y32V2+Y 33 V3=I3

ContinueSo Ii=∑ Yik Vk where i,k=1,2,…,n So complex power is denoted as

ContinueIn polar form we can write

The equation is written as:

Real and reactive power expressed as:

Approximate methodA simple and approximate solution can be

made by following assumption:1. Small line resistance are neglected which

means active power loss in line is zero i.e. θik ~ 90 ˚

2. Voltage magnitude at various must be within limits.

3. Active and reactive generator power at different buses must be within the limits.

Continue4. Total power generation must be equal to load

plus losses.5. The system stability consideration impose a

limit on maximum values with δ.6. All buses other than slack bus are PV buses.

i.e. voltage magnitude at all the buses, Including the slack bus, are specified.

7. The angle δi so small that (sin(δi))= δi.

Continuewith the above assumption the above

equation can be written as:

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