These slides present an introduction to load flow analysis for distribution system. Later the detail algorithm, matlab coding and application to IEEE radial distribution system will be subsequently provided.

1. Class-21: Load flow analysis of radial distribution
system
Prof. (Dr.)Prof. (Dr.) PravatPravat Kumar RoutKumar Rout
SubhasisSubhasis Panda (Research Scholar)Panda (Research Scholar)
Department of EEE, ITERDepartment of EEE, ITER
SikshaSiksha ‘O’‘O’ AnusandhanAnusandhan (Deemed to be University),(Deemed to be University),
Bhubaneswar,Bhubaneswar, OdishaOdisha, India, India
Course:Course: Distribution Generation and Smart GridDistribution Generation and Smart Grid

2. Introduction to Load Flow Studies
• Load flow studies are important for system planning and system operation.
• Load flow studies are used to assess system performance and operations under a given
condition.
• Power system is very complex and nonlinear dynamics.
• Effect of one parameter variation needs to be studied to know the variations of state
parameters like voltage, current, power etc. under steady state condition.

3. Continue…..
• Power flow (or load flow) analysis provides the steady state solution of a power network
for specific network conditions which include both network topology and load levels
• This is the basic tool for analysis, operation, and planning of distribution network
• The power solution gives the nodal voltages and phase angles and hence the power
injections at all buses and power flows through lines, cables and transformers
• The power flow can be formulated as a set of nonlinear algebraic equations and then a
suitable mathematical technique for the solution of the equations

4. Types of buses in power flow analysis
Slack bus (Quantities specified : node voltage magnitude and node voltage
angle)(Quantities to be obtained: Line active and reactive power )
PV bus (Quantities specified : line active power and node voltage magnitude)(Quantities to
be obtained: Line reactive power and node voltage angle )
PQ bus (Quantities specified : Line active and reactive power)(Quantities to be obtained:
Node voltage magnitude and node voltage angle )

5. Types of buses in power flow analysis
Bus type Quantities Specified Quantities to be obtained
Slack Bus |V|,θ P, Q
PV Bus P, |V| Q, θ
PQ Bus P,Q |V|,θ
In a power system, each busbar is associated with four quantities: the
magnitude of voltage (|V|) and its angle (θ), real power injection (P) and
reactive power injections (Q)

6. Challenges of load flow in smart grid
• Condition adaptiness of transmission and distribution to accommodate
load flows comprising renewable generation
• Self addictiveness to ensure proper coordination
• High impedance topology matching for distribution network with
randomness and uncertainty requiring intelligence analytical tools
• Since reverse power flow technique is possible, the use of FACTS devices to
power electronics building blocks is essential

7. Load flow techniques comparison
Old Load FlowTechnique
• Central Generation and Control
• Load flow by Kirchhoff’s law
• Power Generation according to demand
• Manual switching and trouble response
• Simulation and response tracking
Desired Load flowTechnique
• Central and distributed generation control
and distributed intelligence
• Load flow by electronics
• Controllable generation,
fluctuating/random sources and demand in
dynamic equilibrium
• Automatic response and predictive
avoidance
• Monitoring overload against bottlenecks

8. Distribution system load flow characteristics
• Distribution systems are radial or weakly meshed network structures
• High X/R ratios in the line impedances
• Single phase loads handled by the distribution load flow program
• DistributedGeneration, other renewable generation, and/or cogeneration power
supplies installed in relative proximity to load centers
• Distribution systems with many short line segments, most of which have low
impedance values

9. Classical methods of load flow
• Gauss Seidal
• Newton Raphson
• Fast Decoupled

10. Difficulties of distribution power flow analysis
• The network structure of a distribution system being radial or weekly
meshed
• The low X/R ratio in a distribution feeder
• Unbalanced loads
• A mix of short line segments with low impedance and long feeders with
high impedance
• Connection of distributed generators

11. Why the classical techniques fail in radial
distributed system ?
• Because of the lack of branch connections between a largest set of surrounding
buses
• It should be noted that the injection voltage correction propagates out to the
surrounding buses on the layer of neighboring buses for each iteration
• The GS method uses the bus impedance matrix to del with the branch currents
• Newton Raphson method fails in the distribution system due to low x/r ratio
because it may result singular jacobian matrix
• Poor convergence of Fast decoupled method with a high r/x ratio system

12. Load flow methods for distribution systems
• Forward Backward sweep methods solves branch current or load flow by using the
forward sweeping method
• Compute the nodal voltages using backward sweep approach
• Newton method uses power mismatches at the end of the feeders and laterals to
iteratively solve the nodal voltage
• Gauss method on the bus impedance matrix equation solves iteratively for the branch
currents
• The forward/backward method was designed specifically for radial or weakly meshed
distribution networks

13. Procedure for forward and backward Method
Step-1:Assume an initial nodal voltage magnitude and angle for each bus(a
nominal voltage is often used )
Step-2: Start from the root and move forward towards the feeder and lateral
ends while calculating
*
1
)(
)( 
 k
i
ik
i
V
S
I

14. Continue…….
• Step-3: Start from the feeder and lateral ends towards the root while
calculating
Where j is the adjacent down-stream bus i, and two bus bars are
connected by a branch having impedance of
k
ijij
k
i
k
i IZVV 1
ijZ

15. Continue……
• Step-4: Check the termination criterion through calculating the power
mismatch (or the voltage mismatch)
Where is the given threshold
• Step-5:The forward/backward sweep is repeated until the criterion set is
met
  
*k
i
k
ii
k
i IVSS


16. Problem on forward and backward method
For the network shown below, show two iterations of the forward/backward
method.
Z23 = 0.05+j0.07 pu Z34=0.05+j0.07 pu
All quanties are in per unit on 20 kV, 100 MVA base

17. Answer……

18. Answer……

19. • Rupa, J. M., & Ganesh, S. (2014). Power flow analysis for radial distribution system using
backward/forward sweep method. International Journal of Electrical, Computer, Electronics
andCommunication Engineering, 8(10), 1540-1544.
• Hatziargyriou, N., Asano, H., Iravani, R., & Marnay, C. (2007). Microgrids. IEEE power and
energy magazine, 5(4), 78-94.
• S.C.Tripathy, G.Durga Prasad, O.P.Malik and G.S.Hope, “Load Flow for Ill-
ConditionedPower Systems by a Newton like Method”, IEEETrans., PAS-101, October 1982,
pp.3648-365.
• G.X. LUO and A. Semlyen, “Efficient Load Flow For Large Weakly Meshed Net- works”,
IEEETransactions on Power Systems,Vol. 5, No. 4, November 1990, pp- 1309 to 1313
References....

20. • Why the classical techniques fail in radial distributed system?
• Describe the types of buses in power flow analysis?
• Describe the challenges of Load flow in Smart Grid ?
Questions?Questions?