ABSTRACT

The transmission network may be classified into three types, they arc: -

1) EHV transmission network

2) Subtransmission network

3) Distribution network

Usually the networks operated at 132kv and above voltages are known

as EHV transmission network. 33kv and below voltages mostly in the form of radial

arc Sub transmission network. The laterals extended to the individual loads arc

known as Distribution network.

The objective of the power companies is to give quality of power to the

customers i.e., at defined voltages and reliable supply. To achieve this, load 80w

studies at different points of network is essential. The focus of this project is eo. 33kv

network i.e., sub-transmission network. Load flow is a study that gives voltages &

power flow at different nodes / locations of a network.

1

NOMENCLATURE

Pj= Active power of load incident at node j

Pj,= Active power flow in section with node end j

Qj= Reactive power of load incident at node j

Qk= Reactive power flow in section with node end .i

Plj= Active power loss in section with node end j

Qlj= Reactive power loss in section with node end j

Ej= Voltage in KV at node j

OLDLOS = Old Loss

AVB = Automatic Voltage Boosters ITER =

Iteration

TLOS = Total Loss

MAXIT = Maximum Iteration

ABS = Absolute

EPSL = Epselon

SHC = Shunt Capacitor

TOTP = Total Real Power TOTQ

= Total ReactivePower TOTC =

Total Capacitance

Pj= Active power of load incident at node j

Pi,= Active power flow in section with node end j

Qj= Reactive power of load incident at node j

Qk= Reactive power flow in section with node end j

Plj= Active power loss in section with node end j

Qlj= Reactive power loss in section with node end j

Ej= Voltage in KV at node .i

Kp = Constant Power

Kc = Constant Current

2

Ki = Constant Impedance Component Of Load

ELF = Equivalent hours loss factor

LDF = Load Factor

P, = Demand on the feeder at the substation

GJL = Gajwel

GRR = Gowraram

MUL = Mulugu

MNH = Manoharabad

KLKL = Kallakal

MDCL = Medchal

ALBD = Aliahad

KSVR ' = Kesavaram

BMOR = Bandamadararn

TM K = Tumk i Bollaram

VTM = Vantimamidi

VD = Voltage drop

CABC = Cable Capacitor

TR = Transformer

PL = Power loss

3

LIST OF FIGURES

Figure No. Description Page No.

4

2.1 An illustration of voltage spreads 13 occuring at the utilization point

2.2 One -line diagram of a typical 16

Residential feeder

2.3 One line diagram of Rural feeder 18

2.4 I I kv Feeder 22

3.1 Distribution Feeder 29

3.2 Vector diagram 30

3.3 Flow chart for load flow module for balance 32

3.4 Flow chart for network parameter module 33 3.5

Flow chart for demand module 34

3.6 The load voltage characteristic 37

4.1 Block diagram of distribution 47

4.2 Single line diagram of load flow analysis 49

4.3 Frame with scroll bar displaying results 54

4.4 Dialogue box for data entry 55

4.5 Hardware for power distribution studies 56

LIST OF FIGURES

Figure No. Description Page No.

5

LIST OF TABLES

Table No. Description Page No.

2.1 Typical voltage drop allocations for rural feeder components

19

2.2

2.3 ,

3.1

3.2

3.3

5.1

5.1

Supply voltage to the transformer 20 which serve the plant

Voltage Regulation Of Conductors 24

Summary of the results of load factor 39

Value of RLC for different loading patterns 41

Voltage and percentage loss data 42

Feeder Data 58

Voltage regulation and Power loss data 61

CHAPTER 1

6

INTRODUCTION

Power Flow studies are also known as load flow studies. The principal information obtained from these studies is the magnitude and phase angle of the voltage at each bus and real and reactive power flowing in each lin. There by we can calculate voltage regulation of each feeder, power flow in all branches, feeder circuits, losses in each branch and total system power losses. The importance of these studies is planning the future explanation of power system as well as in determining the best operation of existing system. It is an important tool involving numerical analysis applied to power system. The conventional method of calculating voltage regulation is simple which is based on KVA KM loading of conductors.

Also, the conventional method to evaluate the demand loss (or) peak power loss on a distribution feeder is based on the use of loss constants. The conventional technique is simple but may not result in correct computation of loss for the following reasons:

The loss constants are obtained on the assumptions that6 voltage at all buses along the length of feeder is the voltage.

The drop in voltage of the feeder from source to tail end and the consequent increase in loss are not considered.

The demand loss in each feeder segment increases the power flow in all preceding feeder segments up to the source. The effect of such loss is not considered in the conventional technique.

Hence there is a need for more accurate techniques such as “DISBUT” used in ‘APTRANCO’. The power flow techniques such as NEWTON-Rap son method, fast decoupled load flow method etc. are used to solve well behaved power systems effectively. But, when these techniques are applied to ill-conditioned (or) poorly initialized power systems, have led to many short comings. Whereas the ‘DISBUT’ package for calculating voltages, current, active and reactive power flowing in each line, there by voltage regulation and power loss of feeders at different voltage levels(i.e.., 33kv, 11kv etc). As a case study, the voltage regulation and power loss of the following 33kv feeders where considered in annex-2.

1) Gajwel………… Gouraram2) Gajwel………… Mulugu3) Medchel………. Alliabad4) Medchel………. Bandamandaram5) Manoharabad…. Kallakal6) Medchel……….Kesavaram

The results were analyzed and the solutions suggested for better network operation.

7

LOAD FLOW STUDIES ON SUBSTATIONS

The primary objective of the system voltage control is to provide power to each user at specified voltage that is directed by regular. Ideally the output of most power supplies should be constant voltage .unfortunately this is difficult to achieve. Voltage drop exist in each part of the power system from source to consumer service drop .it also occurs in interior wiring system voltage regulations essentially no more than maintaining the voltage at the consumer service entrance with in permissible limits by the use of voltage control equipment.

In general the voltage drop is the difference between the voltages at transmitting and receiving ends of a feeder, main or service. There are two factors that can cause the output voltage to change. First the ac line voltage is not constant. The second factor that can change the dc output voltage is a change in the load resistance. Many circuits are design to operate with a particular supply voltage. When the supply voltage changes the operation of circuit may be effected. Consequently some type of equipments must have power supplies that produce the same output voltage regardless of changes in the load resistance or changes in the ac line voltages

2.1 VOLTAGE SPREAD:

It is a difference between maximum and minimum voltage at a particular point in the distribution system. It will vary n magnitude depending upon the particular location within the system where the spread is measured. An illustration of the voltage spreads occurring at the utilization point is shown in figure below. Consumer A, which is the first consumer served by the feeder, has a voltage spread of a just 1 volt when going from light load (123v) to heavy load (122v) conditions. Consumer B which is the last consumer served by the feeder has a voltage spread of seven volts: 111v at the heavy load conditions A or B for the load conditions between the maximum and minimum values of respective voltage spread at utilization point of any other consumer on the same feeder would have a voltage spread with somewhere between one and seven volts, depending upon the location.

8

fig 2.1 An illustration of voltage spread occurring at utilization point

9

2.1.1 EFFECT OF VOLTAGE SPREAD ON UTILIZATION EQUIPMENT:

When ever the voltage applied the terminals of utilization device varies from the rated or name plate voltage of the device varies from the rated or name plate voltage of the device, performance charesticand equipment life will also change. The extent of change will also monitor serious depending upon the device, how it is applied and how much the terminal voltage deviates from the nameplate rating. Voltage drop will result in huge losses in network.

2.2 VOLTAGE ZONES;

For each voltage level the total operating range has been divided into three zones.

1. FAVOURABLE ZONE;

This zone contusions the majority of the existing operating voltages. It should be designed in such a way that most of their operating voltages lie within the zone. The equipment should be designed and rated so as to give completely adequate and efficient performance throughout the zone, but it should be adequate and satisfactory.

2. TOLERABLE ZONE;

This zone includes operating voltages slightly above and below the favorable zone. The zone is necessary because from practical field conditions voltages slightly outside of the favorable zone often results. Equipment should give fairly satisfactory operation throughout tolerance zone, although at the low and high ends the operating characteristics may not be as good as obtained throughout the favorable zone.

3. EXTREME ZONE;

This zone doesn’t have any set boundary limits but it normally extends two or three percent above and below the tolerance zone. Above and below the tolerable zone, should be temporary. That is, they should occur only during emergency periods such as during fault conditions or as a temporary measure during periods of construction.

2.3VOLTAGE DROP IN SYSTEM COMPONENTS;

The system component voltage drop will be discussed only for the various types of feeder’s form the location on the feeder of the first consumer served to the last.

2.3.1 RESEDENTIAL FEEDERS;

10

The voltage at the point of utilization when keeping with in the favorable zone can be 110 to 125 volts. The logical primary feeder designed to permit maximum loading and area coverage is to permit the first consume electrically nearest to the source to have maximum voltage of 125 volts during maximum load conditions. The most remote consumer electrically from the source to have the minimum permissible voltage of 110 volts, The average voltage drop for residential interior wiring during maximum load conditions is approximately three volts; hence to have the utilization voltage no lower than 11-0 volts the voltage at the consumers service entrance or meter socket must be 113volts or above. The feeder components of a residential feeder are shown in one line diagram below.

Studies of residential feeder design have shown that at a definite amount of voltage drop can be allocated to each component for maximum economy.

SERVICE DROP;

The voltage drop most generally found for service drops during heavy load conditions is 1volt.

11

SECONDARY LINE;

Secondary conductors when installed generally have a voltage drop of approximately two to two and half volts, and as load increases the voltage drop is permitted to increase to three or three and half volts. /when the voltage drop reaches the upper limit another distribution transformer is added between the existing and the secondary line is split between the new and existing units. Such a procedure reduces the secondary voltage drop to less than 1 volt.

DISTRIBUTION TRANSFORMER;

At the time of installation in a developed residential area,. The transformer loading during peak periods is generally 80 to 100 percent. For the average distribution transformer rating this represents a voltage drop of 1.75 to 2.5 volts the transformer remains in service until the peak load increases to about 140 to 160 percent. This represents a voltage drop of 3.25 to 4 volts.. The amount of voltage drop al.lo0cated to the distribution transformer out of the permissible 12 volts spread is generally 3 volts.

PRIMARY FEEDERS INCLUDING LATERALS;

The voltage drop allocated to the primary portion of the residential feeder i9s 3 volts, on a 120-volts base and is as measured from, the primary terminals of the first distribution transformer on the feeder to the last ot most remote transformer electrically. /where single phase laterals are tapped off the three phase main, they generally have a voltage drop from one to three volts, with the last lateral having about one volt drop, and the lateral tapped off near the first distribution transformer on the feeder three volts.

2.3.2 RURAL FEEDERS;

Rural feeders differ somewhat from residential feeders. There are no secondary’s as a rile9i.e run and owned by the power company), because of the distance between consumers. Each consumer has his own distribution transformer. The distribution transformer ratings are smaller, and since the transformer pole is centrally located between all the farm buildings requiring electric power the service drops are longer than for a residential customer.

12

Fig 2.3 one-line diagram of rural feeder

A one line diagram of a typical rural feeder is shown in fig/. It is much longer than a residential feeder, often 5 to 10 times longer.

Below table shows typical voltage drop allocations for rural feeder components. The table values keep the service voltage within the favorable zone. Even with the increased primary line drop as compared to the residential feeder, it is often necessary to add some supplementary voltage boost out on the feeder.

13

FEEDER COMPONENTS

RESIDENCIAL FEEDERS

RURAL FEEDER

MAXIMUM LOAD CONDITION

MINIMUM LOAD CONDITION

MAXIMUM LOAD CONDITION

MINIMUM LOAD CONDITION

Primary feeder from first distribution transformer to last distribution transformer

3.5 1.0 6 2.0

Distribution transformer

3 1.0 3 1.0

Secondary line 3.5 1.0 … …

Service drop 1 0.3 2 1.0

Total 11.0VOLTS 3.3 VOLTS 11.0VOLTS 4.0 VOLTS

TABULAR COLUMN 2.1

14

2.3.3INDUSTRIAL FEEDERS;

Industrial feeders are relatively short feeders and serve anywhere from one to several consumers. They are similar to rural feeders, in that there are generally no secondaries, as each consumer has his own transformer,

There are no recommended allocations of voltage drop for industrial feeder components. Each industrial consumer on a feeder should have the supply voltage to the transformer or transformers which serve the plant fall within the zone shown in columns 2 3 in the below table. The voltage spread for the [primary supply should be four percent and should fall within the recommended voltage zone.

NOMINAL SYSTEM VOLTAGE

ZONES OF VOLTAGES OF PRINARY ON TRANSFORMERS

PRIMARY VOLTAGES

SPREAD

COLUMN 1

VOLTS

COLUMN 2

MINIMUMVOLTS

COLUMN 3

MAXIMUM VOLTS

COLUMN 4

PERCENT OF COTUMN 1

COLUMN 5

VOLTS

2400

4610

4800

6900

11500

13300

2130

3680

4260

6100

10200

12200

2520

4360

5040

7250

12100

14500

4%

4%

4%

4%

4%

4%

100

170

190

280

460

550

TABLE 2.2

15

2.4VOLTAGE REGULATION

Owing to the variations in the current flow through a transmission line, there is variation in the voltage drip in the line. Thus the receiving changes with changing load. It is necessary under electricity rules to maintain the voltage at the receiving ends of O.H line within permissible limits as given below:

Declared voltage of supply to consumers

Not greater than 250 volts +6%

Medium voltage not great than -650 volts

H.V not greater than 33,000 +6% volts

Extra higf voltage above 33,000 ±12.5 volts

Procedure :

The vector diagram for known receiving end voltage conditions, for lagging PF is indicated below. The voltage drop per phase is given by equation.

1(R Cos f +X Sin f) for lagging power factors

The voltage regulation is usually consider as the percentage drop with reference to the receiving end voltage.

Percentage regulation = 100(Es-Er)

Er

Where,

Es = sending voltage

Er = receiving end voltage

16

Sample calculations:

Let consider an 11 KV feeder enumerating from a 33/11 KV S/S with 7/2. 59 mm ACSR for the main feeder and 7/2.11 mm ACSR for tab lines with the connected distribution transformers and distances as indicated below:

Total connected transformer capacity on the 11KV line is :

1) 10100 = 1000 KVA

2) 6*33 = 378 KVA

3) 5*25 = 125 KVA

…………………

Total 1503KVA

17

…………………

Total calculating the voltage regulation of the main feeder, it is assumed that the loads on the tab lines are concentrated at the point of tapping and taking moments about the section we have.

OA = 1503 * 1 = 1503 .0

AB = 1403 * 1.5 = 2104.5

BH = 1215 * 1.5 = 1822.5

HC = 1115 *. 5 = 557.5

CI = 927 * 1 = 927.5

IO = 764 * 1 = 764.0

OR = 601 * 2 = 1202

RE = 413 * 2 = 826

EF = 288 * 2.5 = 720

FG = 100 * 1 = 100

………………………

TOTAL KVA KM 10526.5

………………………..

% regulation = total KVA KM *Regulation per 100 * KVA KIM / 100 * DF

Assuming factor of 2.5, regulation constant for 7/2.59 ACSR at 0.8 power factor is 0.08648 from table 2.

% Regulation = 10526.5 * 0.08648 / 100* 2.5 = 3.64%

Similarly the regulation of the tap also calculated.

Tet us consider the furthest tap lines and find out the regulation at M taking moments in KV KM for the main feeders with 7/2.59 mm ACSR; we have

18

1) 1503 * 1 = 1503.0

2) 1403 * 1.5 = 2104.5

3) 1215 *1.5 = 557.5

4) 927 * 1.0 = 927.0

5) 927 * 1.0 = 927.0

6) 764 * 1.0 = 764.0

7) 601 * 2.0 = 1202.0

8) 413 * 2.0 = 826.0

9) 288 * 2.5 = 720.0

……………………….

Total KVA KM 1110, 426.5

……………………..

For tab FM with 7/2.11 mm ACSR we have

1) 188 * 1.0 = 188

2) 88 * 0.5 = 44

3) 25 * 0.5 = 12.5

………………….

Total KVA KM 224.5

………………….

1) % Regulation on 11 KV main feeder = 10426.5 * 0.8648 / 100 * 2.5

2) % Regulation on 11 KV tap line = 244 * 0.12115 / 100 * 25

= 0.118%

% Regulation at point M of tap line = 3.6.1 + 0.118 = 3.728%

19

VOLTAGE REGULATION OF CONDUCTORS

DETAILS OF THE CONDUCTORS PRECENTAGE REGULATION PER100 KVA PER KM

415 11 kv 33 kv

7/2.0 0 (SQUIRRAL) 20 Sq.mm

7/2.0 0 (SEASEL) 30 Sq.mm

7/2.0 0 (RABBIT) 20 Sq.mm

7/2.0 0 (DOG) 100 Sq.mm

83.75

59.4

40.09

-

0.1211

0.08062

0.05853

0.03294

-

-

0.0064

0.00394

TABLE 2.3

20

2.5METHODS OF IMPROVING VOLTAGE REGULATION;

There are several methods of improving voltage regulation throughout the distribution system. The various methods of improving voltage regulation through a distribution system are listed below. Each method has its own characteristics concerning amount of voltage improvement, cost per volt improvement and flexibility.

1. Use of generator voltage regulates.

2. Application of voltage regulation equipment in the distribution substations.

3. Application of capacitors in the distributions substations.

4. Balancing of the loads on the primary feeders.

5. Increasing of feeder’s conductor size.

6. Changing of feeder sections from single phase to multi phase.

7. Transferring of loads to new feeders.

8. Installing of new substations and primary feeders.

9. Increasing of primary voltage level.

In order to evaluate the performance of a power distributions network and to examine the effectiveness of proposed alterations to system in the planning stage. It is essential that a load flow analysis of the network is carried out. The load flow studies are normally carried out to determine:

The flow of active and reactive power in network branches/

No circuits are overloaded, and the bus bar voltages are within acceptable limits/

Effect of additions or alterations on a system such as voltage regulators, shunt and series capacitors.

Effect of loss of circuits under emergency conditions.

Optimum system loading conditions.

Optimum system loss/The load flow in distribution network is carried out for simple radial networks in this project, which is largely self evident.

21

CHAPTER 3

:

22

RADIAL LOAD FLOW ALGORITHM

The structure of distribution network is simple, as the power flow is in directional. but the load flow analysis of distribution network is complicated, due to the following special characteristics of distribution feeders.

The distribution feeders have multiple toppings along the length of the feeder forming laterals and sub laterals.

The conductor sizes vary long the length of feeder.

The coincident demands at various points along the length of the feeder at the time of system peak is neither monitored nor recorded.

The influence of voltage variations on load characteristics is significant and has to be considered for assessment of losses.

The distribution feeders, in developing countries, are ill condition i.e., heavy loaded with high voltage drop. The load flow techniques which give more accurate results with assured convergence are required.

Therefore a load flow algorithm, which is capable of taking into account load modeling, load allocation, un balanced loads, un symmetrical networks and also exploits of the radial structure of distribution feeder to obtain convergence , even for ill conditioned feeders is necessary. The proposed algorithm first developed for balanced network , considering single phase representation and is later extended to un balanced network. The algorithm is based on basic equations used to compute the receiving end voltage and sending and power are specified as descript below.

23

Consider a branch (k+1, k) of distribution feeder shown above. The active and reactive power flowa in the branch are given by the exppressipon:

Pk= ΣKJ=1 Pj * Ej + Σk

j=1 plj ……….. 3.1c

Qk= ΣKJ=1 qj Ej + Σk

j=1 qlj ……….. 3.2

Pj = active power of load incident at node j

Pk = active power flow in section with node end j

Qj = Reactive power of load incident at node j

Qk = active power flow in section with node end j

24

Plj = active power loss in section with node end j

Qlj = Reactive power loss in section with node end j

Ej = Voltage in KV at node j

The vector diagram of the branch is shown below.

Fig 3.2

E2 k+1 = (Ek cosθ + IkRk )2 + (Ek sin θ + IkRk )2

= E2k +2(Ek cosθ * Rk + EkIk sinθ * Xk) + (R2

k + X2K)

= E2k + 2(PkRk + QkXk) + (P2

k + Q2k) (R2

k + X2k)/E2

k )

= E4k + E2

k [2(PkRk + QkXk) - E2k+1] + (P2

k + Q2k)(R2

k + X2)=0……3.3

25

It can be seen that the equation is presented in terms of the voltage magnitude,

Only avoiding the complex quantities due to the elimination of voltage phase angle

From the equations. The simplifies the solution to the problem. The P & Q in the

Equations are total load fed by node ‘k’ which comprises the load incident at the node and all other loads fed through it, including losses. In other words P&Q is the load of the wquivalent network connected to node ‘k’. The load is estimated taking into account the effects of load modeling and load allocation described later.

The active and reactive losses are computed as follows:

LPk = Rk * [(P2K + Q2

K )/E2K] …….3.4

LQK = XK *[(P2K + Q2

K)/E2K …..3.5]

The solution is obtained through an iterative which comprises the losses at the end of each iteration and computes the power flows in each feeder segment and voltage bus. The convergence criteria proposed to be adopted for the solution is that the absolute value of difference between Active power Losses in the current and the preceding iteration is less than the prescribed error limit. The flow chart for load flow algorithm for balanced loads indication the important steps is shown in fig.3.3

The algorithm in turn calls for two sub-modules. The module ‘PARAMETERS’

Computes resistance, reactance ad shunt capacitance of different elements of distribution system like feeder segments, transformers, regulators, series capacitors etc. The charging capacitance of overhead line is negligible but capacitance of effect of underground cables is significant. Therefore, the charging capacitance is shown as equivalent shunt capacitor at the end of each deader segment. The flow chart of this module indicating these is shown in fig:3.4

26

1. Read network data & load data

2. Call parameter module

3. Intialise E= 1.0 FOR j =1,2….n, OLDLOS=0.0

4. Call demand module

5. Select selection ‘J’

6. Conpute ‘BB’ = (PKRK+ QKXK-E2K+1)

7. IS BB ≥ 0 if yes goto 8 in no goto 2.5

8. Compute ERj using Eq 3.3

9. Is AVB or TR located at end node

If ‘yes’ go to 10. If ‘NO’ goto 12.

10. As ABS (ERj-vest) ≤ step. If ‘YES’ goto 12.

If ‘NO’ goto 11.

11. Adjust tap, goto 10

12. Calculate PTOSJ , QLOSJ

13. TLOS = TLOS +PLOSJ

14. Is J ≤ NS. If ‘YES’ goto 15. If ‘No’ goto 16.

15. J=J+1 goto 5.

16. ITER +ITER+1

17. IS [(TLOS-OLDLOS)/TLOS]≤ EPS if ‘TES’ go to 23. If ‘NO’ GO TO 18.

18. OLDLOS = TLOS

19. Is ITER≤ MANIT. If ‘YES’ GO TO 21.if ‘NO’ goto 4.

20. Is ABS(RP-CP) ≤ EPSL if ‘YES’ goto 21. If ‘NO’ goto 4

21. Is ABS (RQ-CQ) ≤ EPSL . if ‘YES’ goto 22. If ‘NO’ goto 4.

27

22. Load flow converged . print results.

23. Print message load flow convergence failed and convergence linit.

24. Is the maximum number of inerations increased

If ‘yes’ goto 4. If no goto STOP.

25. Print message that load flow convergence failed due to votage collapse and stop.

28

1. Intialise J=1

2. Select section ‘J’

3. Is it transformer. If ‘YES’ goto 4.

a. If ‘NO’ goto 5.

4. 4.XJ = XJ-SCXJ goto 12.

5. compute R&X of feeder segment

6. is AVB located at end load.

If ‘YES’ goto 7. If ‘NO’ goto 10.

7. XJ = XJ- SCXJ

8. Is series capacitors located at n node.

If ‘YES’ goto 9. If ‘NO’ goto 10.

9. XJ = XJ - SCXJ

10. is it cable section . if ‘YES’ goto 11.

If ‘NO’ goto 12.

11. SHCJ = CABJ

12. Is J≤ NS . if ‘YES’ goto 13. If ‘NO’ return.

13. J = J+1 goto 2.

29

Fig 3.3 LOAD FLOW MODULE FOR

The module ‘DEMAND’ considers load modeling and load allocation and evaluates the active and reactive power flow in each feeder segment, considering loads incident at buses. The co-generation and dispersed generating units connected to the distribution network in parallel operating are common today. The flow chart for this module indicating the important steps is in figure..

30

LOAD ALLOCATION:

The simultaneous maximum demand at each distribution transformer is necessary for load flow analysis of primary distribution feeder. Unfortunately, neither peak demand nor the simultaneous demand at each transformer is recorded or monitored. The procedure described below is proposed to be adopted to estimate the same.

PEAK DEMAND AT EACH DISTRIBUTION TRANSFORMER:

The peak demand on each distribution transformer is computed in two ways. In the first method, the customers on a transformer are classified into a set of classes, based on the pattern of energy usage. The demand of each class of customer at the time of the system peak is obtained from the daily load curve for the customer class or by considering the product of average connected load and the peak contributing factor of that customer classes. The peak demand on the transformer is the summation of products of number of customer and the peak demand of the different customer classes, incident on the transformer. Other method is based on the transformer load management (TLM) techniques. Under TLM the energy sales to the customers on each distribution transformer is obtained from the billing records and then is converted into de4mand using regression techniques. The regression coefficients are evaluated through a sample study of the demand and energy sales of typical distribution transformers.

31

SIMULTANEOUS PEAK DEMAND:

The data monitored or recorded is KW 7 KVAR or KVA 7 PF on the feeder at the substation bus. This demand is allocated among the distribution transformers on the feeders on prorate basis, considering the peak demand of each transformer and peak coincident factor of transformer, if available.

LOAD MODELLING:

The influence of voltage variation on load characteristics is significant and it is desirable to consider it for accurate load flow analysis. The loads can be broadly classified into three categories, VIZ., constant current, constant impedance and constant power. Based on the influence of voltage variations on loads. The load voltage characteristics of the above three types of loads are shown in the figure below and are represented by an equation, as follows:

P/Pn = (V/Vn)k ……… 3.6

Where,

P = Power at voltage

V = Voltage at bus

Vn = Nominal rated voltage

Pn = Nominal active or reactive power of load

K = an exponent describing the voltage sensitivity of the load.

32

For constant power loads K=0; for constant current loads K=1; and for constant impedance loads K=2; in a graph of P/Pn against V/Vn, K is the slope of the curve at the operational condition. Most of the loads in the practical distribution networks are the composite loads that are assumed to be a combination of constant current, constant impedance and constant power. The load voltage characteristics of practical loads are represented by an equation as shown below.

Where Kp, Kc, Ki indicate proportion of constant power, constant current and constant impedance component of load and (Kp+Kc+Ki=1.0).

EMPIRICAL LOSS FORMULA:

The most popular and widely used method for estimation of energy losses is by the use of a empirical loss formula. The empirical loss formula for loss factor in terms of load factor was suggested by F.H Bulker and C.A. WOODROW and additional work was reported by H.F.HOEBEL. The formula suggested was

33

P = Pn(Kp * Kc + Ki * V2)……… 3.7

Where, KP, KC Ki indicate proportion of constant power, constant current and constant impedance component of load and (Kp+Kc+Ki=1.0).

EMPIRICAL LOSS FORMULA:

The most popular and widely used method for estimation of energy losses is by the use of a empirical loss formula. the empirical loss formula for loss factor in terms of load factor was suggested by F.H. buller and C.A Woodrow and addition work was reported by H.F Hoebol.

The formula suggested was

ELF = (LDF)2 (1-A) + (LDF) A ………… 3.8

Where,

ELF = Equivalent hours loss factor

LDF = Load Factor

A = constant coefficient, whose value varies between 0.3 to 0.15 For the utilites

Another empirical formula for estimation of loss factor suggested by EPRI is given below

ELF = LFα ………………..3.

34

Where α is constant and value varies between 1.7 to 1.9 for most of the utilities. The coefficients are determined for each utility by carrying out sample studies on the feeders. Unfortunately no such studies have been conducted on Indian utilities so far. The value of the constant in empirical formula is taken as 0.2 based on UK practice, even though the load conditions in India are totally different from UK. Hence the loss factor coefficients are evaluated for a large number of feeders of A.P system. A summary of the results are presented in below table.

S.NO NO. of

Feeders

Load

Factor

Loess

Factor

ALOS Alpha

1 22 0.3 – 0.4 0.193 0.250 1.64

2 43 0.4 – 0.5 0.260 0.191 1.74

3 33 0.5 – 0.6 0.357 0.180 1.77

Total 98

Average 0.278 0.200 1.73

Table 3.1

On review of table, it could be seen that the average value of co-efficient of equivalent hours loss factor varies between 0.18 to 0.25 for Indian utility and average value is 0.2. The exponential coefficient for equivalent hour’s lo9ss factor varies break when 1.64 to 1.77 with the average value of 1.73.

RELARTIONSHIP BETWEEN PERCENTAGES OF VOLTAGE DROP & PERCENTAGE OF POWER LOSSES:

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The percentage of loss (PL%) and the percentage of voltage drop (VD%) of a distribution feeder are interrelated, but no empirical formula relating these has been derived, so far. The relationship is of great practical significance as the voltage drop of feeder can be easily measured by recording voltage at substation end and tail end; whereas it is difficult to measure the power loss and it can only be estimated through load flow analysis of the feeder. A mathematical expression relating the two parameters is developed as shown below

VD % = ∑I=1n (PiRi/Fi)* KR…………….. 3.10

Where, KR = [cosθ + (x/r) sinθ] * (0.1/FV)

PL % = ∑i=1n (P2

iRi/Fi) * KR………… 3.11

Neglecting variation of voltage along the lenth of feeder i.e assuming Ei = FV

PL % / VD % = RLC/ KVL…………….3.12

Where RLC = {∑i=1n p2

i * Ri / Ps ∑ni=1 PiRi}

KVL = [ cosθ + (x/r) sinθ] cosθ …………………3.13

Ps = Demand on the feeder at the substation

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The term ‘KVL’ is a constant based on the feeder configuration, power factor and the conductor size. The term ‘RLC’ factor for four different types of loading is computed and is as shown in the below table to get an idea of the variation in the value of RLC.

Values of RLC for different loading patterns

SI.

NO

LOADING PATTERN VALUE OF RLC

1. Concentrated load 1.00

2. Uniform distribution of equal loads with equidistant section

0.67

3. Non uniform distribution of equal loads

With section length increasing from source

To load

0.50

4. Non uniform distribution of equal loads

With section lengths increasing from load end to source

0.75

Table 3.2

The loading of the practical distribution feeders does not follow any specific pattern. Therefore, the value of RLC for 500 practical feeders of AP system is computed. A summary of the results is in below table.

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Voltage and Percentage loss data

SI.

No.

No. of

feeders

Percentage

Of feeders

Value of RLC

1 141 33 0.6 – 0.7

2 202 47 0.7 – 0.8

3 88 20 0.8 – 0.9

Total 431 100

Average 0.74

Table 3.3

On review of table it could be seen that the value of RLC for practical Distribution feeders in Indian utility varies between 0.7 and 0.8 for majority of the feeders and the total range of variation is 0.6 to 0.9. The average value of RLC is 0.7.

ASSESSMENT OF COMMERCIAL LOSSES:

Commercial losses are further classified in to the following six categories considering all the possible sources of energy losses. The procedure for estimation is also presented.

DECALIBRATION OF ENERGY METER DUE TO NORMAL WEAR OR DUE TO WRONG CALIBRATIN OF UTILLITY:

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The loss on this account is determined by testing the meters of a sample of customers. As ample of the order of 0.5% of total customers is considered is nominal current. The losses are estimated based on sample testing and current energy consumption of consumers.

DAMAGED METER:

A meter is considered to be damaged when it does not measure energy because of blocked rotor. Etc. The meter is normally detected to be blocked by two identical meter readings. The losses

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