load flow methods

8/16/2019 load flow methods

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CHAPTER 1

LOAD FLOW ANALYSIS

1.1 Distribution system

An electric power distribution system is the final stage in the delivery of electric

power; it carries electricity from the transmission system to individual consumers.

Distribution substations connect to the transmission system and lower the

transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use

of transformers. Primary distribution lines carry this medium voltage power

to distribution transformers located near the customer's premises. Distribution

transformers again lower the voltage to the utilization voltage of household appliances

and typically feed several customers through secondary distribution lines at this

voltage. Commercial and residential customers are connected to the secondary

distribution lines through service drops. Customers demanding a much larger amount

of power may be connected directly to the primary distribution level or the sub

transmission level.

1.1.1 Primary Distribution system

Primary distribution systems are at voltages 22kV or 11 kV. Only large consumers are

fed directly from distribution voltages; most utility customers are connected to a

transformer, which reduces the distribution voltage to the low voltage used by lighting

and interior wiring systems. Voltage varies according to its role in the supply and

distribution system. According to international standards, there are initially two

voltage groups: low voltage (LV): up to and including 1,000 V AC (1,500 V DC) and

high voltage (HV): above 1 kV AC (or 1.5 kV DC).

Primary distribution system is shown in Figure 1.1

Figure 1.1 Primary Distribution systems


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1.1.2 Secondary distribution system

Electricity is delivered at a frequency of either 50 or 60 Hz, depending on the region.

It is delivered to domestic customers as single-phase electric power. Seen in

an oscilloscope, the domestic power supply in North America would look like a sinewave, oscillating between -170 volts and 170 volts, giving an effective voltage of 120

volts. Three-phase power is more efficient in terms of power delivered per cable used,

and is more suited to running large electric motors. Some large European appliances

may be powered by three-phase power, such as electric stoves and clothes dryers.

Secondary Distribution system is shown in Figure 1.2

Figure 1.2 Secondary Distribution system

A ground connection is normally provided for the customer's system as well as for the

equipment owned by the utility. The purpose of connecting the customer's system to

ground is to limit the voltage that may develop if high voltage conductors fall down

onto lower-voltage conductors which are usually mounted lower to the ground, or if afailure occurs within a distribution transformer. Earthing systems can be TT, TN-S,

TN-C-S or TN-C.

Distribution networks are divided into two types, radial or network.

1.1.3 Radial Distribution System

In early days of electrical power distribution system, different feeders were radially

come out from the substation and connected to the primary of distribution transformer

directly.


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usually of non-statistical type due to practical difficulties in data acquisition in large

and complex distribution systems.

Some inherent characteristics of electric distribution systems are (i) Radial or weakly

meshed structure (ii) unbalanced operation and unbalanced distributed loads (iii) large

number of buses and branches (iv) It has wide range of resistance and reactance

values (v) Distribution system has multiphase operation.

The Newton Raphson and the fast decoupled power flow solution techniques and a

host of their derivatives have efficiently solved ‘well behaved’ power systems for

more than two decades. However, the shortcomings have been encountered when

there algorithms are generally implemented and applied to ill-conditioned and poorly

initialized power system. The Gauss Siedel power flow technique, another classical

power flow method, although very robust, has shown to be extremely inefficient in

solving large power systems.

Distribution networks, due to their wide ranging resistance and reactance values and

radial structure, fall into the ill conditioned power systems for the generic Newton

Raphson and fast decoupled power flow algorithms. Therefore, the modification in

the load flow method is necessary for solving the distribution systems.

1.2 Literature review

A brief literature review on the distribution system aspects including its load flow is

represented herewith.

G. X. Luo and A. Semlyen sun et al. [1] presented a fast and efficient method for

obtaining load flow solutions of weakly meshed distribution system by three new

features. First by using powers as variables in the solution instead of complex

currents. Second by applying tree labelling technique of net flow programming to

labeling the radial network. Third finding in each step the CPU time required for

obtaining the break point voltages by single sweeps.

T. Ramana, V. Ganesh and S. Sivanagaraju et al. [2] proposed a method which solves

the distribution load flow directly using single dimension vectors and an efficient

method which identify the lines and number of lines available beyond that particular


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line. The proposed method is effective convergence approach which is simple, fast

and efficient.

T. Sathiyanarayan and M. Sydulu et al. [3] presented insights to three different load

flow methods i.e.., primitive impedance distribution based distribution load flow,

Current Injection Method and Single Matrix Model to study the distribution load flow

and made a Comparision study with respect to voltages, Phase angles and

Computational time .

N. Makwana Nirbhaykumar et al. [4] presented a review of the various computational

methods suited for the analysis of weakly meshed distribution system and discussed

analytical bases, Computational requirements and comparative numerical

performance.

Sivanagaraju, et al. [5] described a distinctive load flow solution technique for weakly

meshed distribution systems using branch injection branch current matrix

(BIBC).which is obtain by applying Kirchhoff’s current law. Bus voltages are found

by forward sweep of the network.

S. Gosh and D. Das et al. [6] proposed a new load flow technique for solving radial

distribution network which is efficient and has good and fast convergence

characteristics that involves evaluation of simple algebraic expression of receiving

end voltages.

M. H. Haque et al. [7] proposed for the analysis of both radial and mesh networks. A

mesh network is converted to a radial network by breaking the loops through adding

some dummy buses. The power injections at the loop break points (LBP) in the

equivalent radial network are computed through a reduced order node impedance

matrix. Unlike other methods, the shunt admittances are considered in the proposed

load flow algorithm and the effect of load admittances is also incorporated in the

calculation of power injections at the LBPs.

D. Das, H. S. Nagi, and D. P. Kothari et al. [8] proposed a load-flow method using

sequential numbering scheme. A number of coding is to be supplied when the lateral

and sub laterals exist. For large system this increases the complexity of the

computation.


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V. V. S. N Murthy, B. Ravi Teja, Ashwani Kumar et.al [9] proposed a new

development to load flow analysis which does not require formation of BIBC, BCBV

& identification of nodes beyond each node. He proposed load flow algorithm based

on forward and backward sweep and is done in two ways, one is current bias and the

other is power bias

1.3 Objective of work

Objective of the present work is to study and implement the load flow solutions by

using various methods for both radial and weakly meshed distribution networks. And

too use a method that can be applied to both systems to get an effective convergence

approach which is not only simple and fast but also is efficient from time perspective

and needs very less memory for any size of the distribution system.

1.4 Organization of project

The work carried out in this project has been summarized in four Chapters. The

Chapter 1 highlights the brief introduction, requirements of distribution system and

summary of work carried out by various researchers, and the outline of the project.

The Chapter 2 explains load flow technique using BIBC matrix, Primitive Impedance

based distribution load flows and BIBC & BCBV matrix for radial distribution

network. The Chapter 3 deals with load flow analysis for meshed distribution system

using BIBC & LILC method and finally the fourth chapter deals with results and

discussion pertaining to two test cases, namely 33 bus and 69 bus distribution system.

The conclusions are detailed in Chapter 5.

load flow methods

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