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Prof. Hani Obeid
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LOAD FLOW CALCULATIONS
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The load flow calculation is an important tool in theDesign and analysis of power system.
The important questions to be answered, given a specified
Load condition, are:
What are the line, transformer, and generator loads throughout
The system?
What are the voltages throughout the system?
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In answering these questions we can easily evaluate the
Proposed changes to an existing system, such as:
New generation sites
Projected load growth
New transmission line location Interconnection with other systems
Various load conditions, such as peak and off-peak
The impact of losing major components (lines,
transformers, or generators).
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Circuit Analysis Versus Load Flow Analysis
In two busbar system assume that generators power andVoltage are specified. Then:
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The equivalent circuit is shown in Fig, 6.5
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BUS CLASSIFICATIONS
Two out of four quantities are specified and the remaining
Two quantities are to be obtained through solutions ofEquations.
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The system buses are classified into three categories:
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BUS ADDMITTANCE MATRIX
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BUS LOADING EQUATIONS
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2
3
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From eqs. (3) and (4), we get:
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GAUSS-SEIDAL ITERATIVE METHOD
4-bus systemBus 1 is considered as slack bus, where voltage magnitude &
Its angle are known.
In this case, n=4 and slack bus s=1, from eq. (60 we can write:
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In Gauss-Seidal method , the iteration will take the following form:
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From Eq. (5), we get:
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Separating real & imaginary part of Eq. (8)
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For P-Q buses, the real & reactive powers and
Are known. Starting with initial values of the voltages, a set of voltage equations
Can be solved iteratively. For the voltage controlled buses , where
And are specified, first Eq. (10) is solved for:
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Where
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Computation of line flows and losses
Consider the line connecting buses I and k. the line and
Transformer at each end can be represented by a circuitWith series addmittance and two shunt addmittances
and
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When the taps of a transformer are set to its nominal ratio
The transformer is represented by a single series impedance.When the taps are off-nominal, adjustments have to be made
As follows.
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Fig. 6.10a
Equivalent circuit of transformer with off-nominal tap ratio.
Transformer series admittance on non-tap side.
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Fig. 6-10-b
The section to represent transformer with off-nominal ratio.
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Steps to solve power flow analysis
1. Represent the system by its one line diagram. The point of this
is to identify all the buses in the system.
2. Convert all quantities to per unit.
3. Draw the impedance diagram.
4. Obtain the Y-Matrix.
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