ijetae_1211_27

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011)

152

Analysis of Distribution Transformer Performance under

Non-linear Balanced Load Conditions and Its Remedial Measures

Sanjay A. Deokar1, Laxman M. Waghmare

2

1 Dnyanganga College of Engineering and Research, Pune University, Pune-411041

2S.G.G.S., Institute of Engineering and Technology, Nanded- 431606

[email protected]

[email protected]

Abstract In recent years there has been very extensive use of power electronic devices, which result in harmonic

proliferation in the power distribution system. In this paper,

as per IEEE C 57.110 standards, procedure to calculate total

loss in the distribution transformer under non-linear

distortion environment is proposed. The power factor

capacitor performance under non-linear load conditions is

also analyzed. The relation of total current harmonic

distortion in the distribution system with load power factor,

transformer losses, efficiency and maximum current

delivered is also analyzed. The mitigation methods are

proposed to minimize the non-linear load impact on the

distribution transformer performance. Instead of K-factor

transformer approach, a passive harmonic filter method is

developed based on higher savings in energy losses. The

simulation studies, are performed using Math works

MATLAB 7.0.1 for distribution system at 11/0.440 kV, 200

kVA distribution transformer under non-linear balanced

load conditions. It is observed that the power factor

capacitor bank acts as a source of harmonic under the non-

linear load conditions in the presence of passive filters.

Keywords Harmonic Proliferation, k-Factor, Non-linear Load, Power Factor, Mitigation.

I. INTRODUCTION

The transformers are designed and manufactured to be

used for non-linear load, at rated frequency and balanced

supply voltage. The present design trend in electrical load

devices is to increase energy efficiency with solid-state

electronics. One of the major drawbacks of this trend is

the injection of harmonics into the power systems. Almost

all the utilities have expressed concern about

overheating of oil immersed distribution transformers,

which supply the non-linear loads. A transformer thermal

response to sinusoidal loads is properly evaluated at the

transformer design stage, but its actual response to

non-linear loads should be estimated after proper

evaluation of present load conditions[1].The increasing

usage of non-linear loads on electrical power systems is

causing greater concern for the possible loss of

transformer life. Manufacturers of distribution

transformers have developed a rating system called K-

factor, a design which is capable of withstanding the

effects of harmonic load currents. An application of this

rating system to specify a transformer for a particular

environment requires knowledge of the fundamental &

harmonic load currents predicted. In almost all the cases,

the field measurements are required to diagnose problems

at a specific location, by analyzing load currents.

Electrical insulation used in distribution

transformers gets degraded when it is subjected to the

thermal, electrical, environmental, mechanical and

combined stresses during its operation. Electrical

stresses are caused by voltage gradient. The average life

expectancy of a transformer is decided by the average life

of insulating materials. The steady-state power quality

problem like harmonics and variation in frequency are

responsible for accelerated aging of its insulating

material. A transformer designed without considering all

these issues will result into premature failure. In [2], a

different method to calculate the impact of non-linear

loads has been discussed. It also gives an overview of

impact of nonlinear load on the distribution

transformer winding losses. The standard K-factor

transformer ratings and typical loads as well as its design

guidelines are given in [3]. In [4], measurement methods

for reactive power demand under non-linear loads have

been presented.

In [5], on line monitoring of all losses of both single

and three-phase transformers has been investigated under

a different percentage of load conditions.



153

Harmonics and its impact on the power factor with

their relation have been investigated in [6]. It also

explains the important to the true power factor compared

with displacement power factor under non-linear load.

The transformer de-rating methods during non-linear load

supply conditions are given in [7].In [8], transformer

modeling under the non-linear load conditions is

investigated and tested under non-linear load conditions.

The measurement of the losses for estimation of the

transformer de-rating and harmonic loss factor

comparison has been discussed in [9]. The measurement

of eddy current loss coefficient and de-rating of single

phase transformers as well as comparison with K-factor

has been presented in [10]. The transformer design and

application considerations for non-sinusoidal load

currents has been discussed in [11].The impact of non-

linear loads on temperature rise of small oil filled

distribution transformers has been analyzed in [13]. A dry

type distribution transformer specifications and

calculations of winding temperatures in distribution

transformers under harmonic load conditions have been

elaborated in [14], [15].Considering all these issues it is

necessary to study and analyze the various effects of non-

linear load on distribution transformers. The power factor

during linear load condition is called displacement power

factor and during non-linear load condition, it is called

distorted power factor. If harmonic currents are

introduced in the system, true or total power factor is

always less than the displacement power factor. In this

paper, a case study of 200kVA, 11kV/440V, 3-phase

distribution transformer with balanced load nature is

simulated using Math works MATLAB-7.0.1 for

analyzing the impacts of non-linear loads. The relation

between current harmonics in the distribution system and

losses, efficiency, maximum current delivered, apparent

power capacity of the distribution transformer has been

analyzed and presented. The impact of ordinary power

factor capacitor bank on total current harmonic distortion

is also analyzed. A mitigation measures like K-rated

transformers and application of passive filters are

presented and results are compared with harmonic content

base case. From this comparison, instead of K-factor

transformer, passive filter method is recommended.

II. LOSSES DURING NON-LINEAR LOADING OF DISTRIBUTION TRANSFORMER

An easy way to comply with the conference paper

formatting requirements is to use this document as a

template and simply type your text into it.

As per ANSI/IEEE C57.110-1986[7],[12], the

transformer losses are mainly no-load loss (excitation

loss); load loss (impedance loss); and total loss. This can

be written by using following expression,

LLOADCORETOTAL PPP (1)

Where,

TOTALP Total loss, COREP Core or No load loss and

LLOADP Load loss

The total load loss can be given as,

LOSLLWECDCLLOAD PPRIP 2 (2)

Where,

LWECP is the winding eddy current loss and LOSLP is

the other stray loss.

Total stray losses include winding eddy current losses and

structural part stray losses. These are given by the

following expressions [4],

PPPPP LLOADLOSLLWECSTRTotal (3)

LWECSTRTotalLOSL PPP (4)

The winding eddy current loss can be calculated using the

following expression,

STRTotalLWEC PP 33.0 (5)

Losses during non- linear loading of a distribution

transformer

In modern power systems, the total harmonic voltage

distortion )( vTHD is normally below 5% and the

magnitudes of the voltage harmonic components are small

compared to fundamental components(2% to

3%).Therefore voltage harmonics effects are neglected.

The current harmonics are more significant. These

harmonic load current components cause additional losses

in the winding and other structural parts. Hence total load

losses under harmonics load condition can be given by the


LOSLLWECCULLOAD PPPP (6)

The harmonic component of load current increases the

r.m.s. value of the load current and hence RIPCU2

loss will be increased accordingly. LWECP is the winding

eddy current loss due to the non-sinusoidal load current. It

can be given as follows, 2

max

1

2

RT

hh

h

RWECLWECI

IhPP (7)



154

Where,

RWECP is the rated eddy current loss under full load

conditions, h is the harmonic order, hI is the r.m.s.

current at harmonic order h and RTI is the rated

fundamental current at full load conditions and rated

frequency. The increased winding eddy current losses

produced by a non-sinusoidal load current can cause

excessive winding losses and hence abnormal temperature

rise.LOSLP are the stray losses in the structural parts due

to non-sinusoidal current. It can be calculated by the


2max

1

8.0

R

hh

h

ROSLLOSLI

IhPP (8)

Where,

ROSLP are the structural part stray losses under rated

conditions. The factor 0.8 is accepted by IEEE after

manufacturers verification. For oil filled transformers, these stray losses increase the oil temperature and thus the

hot spot temperature. Total load losses in both oil cooled

and dry type transformer under non-sinusoidal load

condition with current harmonics are calculated by the


8.0

2max

1

2

2max

1

2max

1

hI

IP

hI

IP

I

IPP

h

h R

h

ROSL

h

h R

h

RWEC

R

hh

h

CULLOSD

(9)

III. DE-RATING OF DISTRIBUTION TRANSFORMER

According to the IEEE dictionary, de-rating is defined

as "the intentional reduction of the stress/strength ratio

(e.g., real or apparent power) in the application of an item

(e.g., transformer), usually for the purpose of reducing the

occurrence of stress-related failure (e.g., reduction of

lifetime of transformer due to increased temperature

beyond the rated temperature)."Harmonic currents and

voltages result in harmonic losses increasing the

temperature rise. This rise beyond its rated value results in

a reduction of lifetime.

The distribution transformer must be de-rated under

non-sinusoidal load conditions [1].The transformers de-

rating can be performed using following methods: a)

Direct loss measurement. b) Using K-Factor and c) Based

on harmonic loss factor )( HLF .

A. Distribution Transformer De-rating Based on K-Factor

The impact of nonlinear loads on distribution

transformers greatly depends on the nature and the

harmonic spectrum caused by the nonlinear load, which is

not considered by the manufacturers. The IEEE standard.

C57.110-1998[7] introduced a term called the K-factor for

rating a transformer as per their capability to handle load

currents with significant harmonic contents .It is an

alternate technique for transformer de-rating which

considers load characteristics. It is a rating optionally

applied to a transformer indicating its suitability for use

with loads that draw non-sinusoidal currents. It is an

index that determines the changes in conventional

transformers must undergo so that they can dissipate heat

due to additional iron and copper losses because of

harmonic currents at rated power. Hence the K-factor can

be written as,

2

1

22max

h

h

h

h

I

hI

K

(10)

This K-factor is only an indicative value. The main

objective is to design and manufacture an oil filled

distribution transformer which can operate for a specific

K-factor value without loosing its expected life span.

Therefore, the maximum amount of R.M.S. harmonic

load current that the transformer can deliver is given by

the following expression,

)(1

1max R

REC

RECL IkP

PI

(11)

Where RI the fundamental rms current under is rated

load conditions, RECLP is the eddy current loss to rated

RI 2 loss in which I is the total rms current. The reduction in apparent power is given by the


..

maxRe )(1up

RmsRated

RmslinearNon

ductionKVA IV

VP

(12)



155

Where,

RmslinearNonV the total rms is value of the secondary

voltage including harmonics and RmsRatedV is the rated

R.M.S. value of the secondary winding without

harmonics.

B. Distribution Transformer De-rating Based on FHL Factor

As per IEEE Std. C57.110/D7-1998[7], this represents

an alternative approach for assessing transformer

capability supplying non-linear loads. Hence HLF Factor

can be defined using following expression,

max

1

2

1

max

1

2

2

1

h

h

h

h

h

h

HL

I

I

hI

I

F (13)

The stray loss harmonic factor can be given as,

max

1

2

1

max

1

8.0

2

1

h

h

h

h

h

h

STRAYHL

I

I

hI

I

F (14)

Hence, the relation between K-factor and HLF is given

as follows,

HL

R

h

h

h

FI

I

K

2

max

1

2

(15)

Therefore, the maximum amount of R.M.S. harmonic

load current that the transformer can deliver is given as,

][][1max

OSLSTRAYHLECLHL

LLOAD

PFPF

PI

(16)

Under harmonic load condition, the new load loss can

be calculated by the following expression,

]1[2 OSLSTRAYHLRECLHLLNEWLOAD PFPFIP (17)

The reduction in the apparent power rating is given by

the equation (12).

IV. MODELING AND SIMULATION OF DISTRIBUTION TRANSFORMER

A 200kVA three-phase distribution transformer is

modeled and simulated using Matlab-7.01 for different

load characteristics. All parameters when the transformer

is tested at balanced linear full load were taken from

Maharashtra State Electricity Distribution Company

Limited (MSEDCL) manual. All these parameters are

given in Table I.

TABLE I

ALL PARAMETERS AND LOSSES OF 200KVA TRANSFORMER WORKING

AT FULL LOAD

Parameters Rating

KVA Rating 200 KVA

Voltage Range 11KV/440V

I1 10.5A

I2 266.7A

No load iron loss 500W

Full Load Cu Loss at

750C

3000W

R1 14.75

R2 0.0062

L1 0.003H

L2 0.067mH

Rc 728 k

Lm 32105H

A transformer is tested for the following load

characteristics at full loads.

A. Base Case of 200kVA Distribution Transformer with

Linear Nature of Load at 0.8 P.F.

In this case 200kVA distribution, transformer is

loaded at its full capacity with non-linear load.



156

The single-line diagram of the simulated power system

is shown in Fig.1 (a).Both primary and secondary full

load currents, current THD, and total losses are

calculated. It is matching with standard full load test data

of the 200kVA distribution transformers with 15%

tolerance given by distribution Company. Efficiency of

the transformer under this case is 98.12% at 0.8 lagging

power factor and current harmonics are below the IEEE

standard.

Linear Load

Full load 200kVA

p.f .=0.8lagging

Capacitor

bank

67.41kVAR

Non-Linear

Load

%THDi=28

Passiv e

Filters,5th,

30kVAR/Phase

7th and 15th

onwards20kVAR/

phase

11/0.433kV

200kVA

R1=14.75ohm L

1=0.003H

R2=0.0062ohm L

2=0.067H

Rm

=728kohm Lm

=32105H

Supply 11kV,

f rom power utility

S1 S4S3S2

X/R=2.5

BUS BAR

Fig.1. A distribution transformer feeding a linear/non-linear full

load; (a) The linear nature of load at 0.8 lagging p.f.; (b) The linear

nature of load with 0.95 p.f. improvement using capacitor bank; (c)

Non-linear nature of load without p.f. improvement with % THD

=28%.; (d) The Non-linear nature of load with p.f. improvement (e)

Non-linear nature of load with passive harmonic filters for p.f

improvement and %THD mitigation.

B. Base Case of 200kVA Distribution Transformer

Feeding Linear Nature of Load with P.F. Improvement at

0.95

In this case, a transformer performance is checked

when a capacitor bank of 67 47kVAR. is installed at the

point of common coupling (PCC ) to improve power

factor of 0.95 lagging without changing load nature.

It is observed that the losses are reduced and hence

efficiency is also improved about 98.24%.For the same

load, current to be supplied by a transformer is reduced by

16%. This arrangement is simulated in matlab-7.01 as

shown in Fig.1 (b), and results are shown in Table I.

V. A CASE OF 200KVA DISTRIBUTION TRANSFORMER FEEDING NON-LINEAR NATURE OF LOAD WITHOUT P.F.

IMPROVEMENT

In this case transformer, performance is checked

without power factor improvement for non-linear load

only in which load is adjusted at THDi=28.09% up to 35th

harmonics level. From the simulation, it is observed that

the losses are increased drastically, which results in

efficiency at 96.60%. The transformer maximum current

delivery capacity is reduced by 15% as compared to

secondary full load current. The voltage profile is also

reduced due to increased voltage drop in distribution

lines. This arrangement is simulated and is shown in

Fig.1(c). The current spectrum and its harmonic current

level of a single phase are shown in Fig.2 (a) and (b)

respectively. The results are shown in Table 2.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-400

-300

-200

-100

0

100

200

300

400

Time (sec)

'a'

ph

as

e c

urr

en

t(A

)

(a)

0 5 10 15 20 25 30 35 400

50

100

150

200

250

300

350

400

Harmonic order

Am

pli

tud

e o

f c

urr

en

t (A

)

(b)

Fig.2. (a) Harmonic current spectrum of phase A ; (b)Current

harmonic bar chart of phase A at non-linear full load without

power factor improvement.



157

VI. A CASE OF 200KVA DISTRIBUTION TRANSFORMER FEEDING NON-LINEAR LAD WITH P.F.IMPROVEMENT

In this case transformer, performance is checked with

power factor improvement capacitor and non-linear load

nature at full load. From the simulation results it is

observed that the THDi is increased to 35.42%.It is also

observed that the current harmonics are increased in each

level compared to previous case. The losses are increased

drastically results in efficiency reduction at 92.80%. For

the same load distribution, line is overloaded by 4.5% and

the load carrying capability of a transformer is reduced by

30% compared to previous case. The transformer

maximum current delivery capacity is reduced by 44% as

compared to secondary full load current. The voltage

profile is also disturbed due to increased voltage drop in

distribution lines results in reduction in apparent power

capacity. Here under non-sinusoidal load condition,

power factor improvement is impossible with simple

capacitor banks only. An ordinary capacitor bank also

acts as a source of harmonics as current THD is increased.

This arrangement is simulated and is shown in Fig.1(d).

The current spectrum and harmonics level is shown in

Fig.3(a)&(b) respectively.The results are shown in Table2

0 0.02 0.04 0.06 0.08 0.1-600

-400

-200

0

200

400

600

Time (sec)

'a' p

hase

cur

rent

(A)

(a)

0 5 10 15 20 25 30 35 400

50

100

150

200

250

300

350

400

Harmonic order

Am

plit

ude o

f curr

ent (A

)

(b)

Fig.3. (a) Harmonic current spectrum of phase A when distribution

transformer is feeding non-linear full load with capacitor bank for

p.f. improvement up to 0.95 lagging. (b) Current harmonic level bar

chart of phase A when base case feeding non-linear full load with

capacitor bank.

TABLE II

SIMULATION RESULTS OF 200KVATRANSFORMER TESTED AT

DIFFERENT LOAD CHARACTERISTICS

Load

Characteris

tics

Base

case

(Total

Linear

nature

of

Load)

Base

Case+

Capacitor

Banks for

P.F.

improvem

ents

Total Non-

Linear

Load

without

....P.F

....

improveme

nt

Non-

linear

Load+

P.F.

Capacit

or.

I1 10.3Am

p

8.67Amp 10.3Amp 9.38

Amp

I2 266.6A

mp

218.4Amp 261.5Amp 273.5

Amp

Total Load

Losses

3714.9

Watts

2852.9

Watts

5622.5

Watts

12347

Watts

THDi



158

VII. POWER FACTOR UNDER NON-LINEAR LOAD ENVIRONMENT

Under the harmonic load conditions, total harmonic

distortion or distortion factor is used for its level

measurement. It is the ratio of the rms value of the

harmonics (voltage or current) above fundamental to the

rms value of the fundamental. It can be given by the


100

100

1

2

2

1

2

2

max

max

I

I

THD

OR

V

V

THD

h

h

h

I

h

h

h

V

(18)

Hence, correct form of true power factor under linear

and non-linear load environments is given by the


2

2

)100(1

1

)100(1

1

I

Vfunfun

Avg

PF

THD

THDIV

PTRUE

(19)

Normally in most of harmonic load cases, average

power variations are negligible and voltage total harmonic

distortion is also less than 5%, hence it is also

neglected[1],[6].By considering these assumptions the

approximate equation for true power factor is given as,

PFPF

Ifunfun

fun

PF

DistortionntDisplaceme

THDIV

PTRUE

2)100(1

1

(20)

Where,

Pfun, Vfun and Ifun are the fundamental power, voltage

and currents.

Since displacement power factor is always less than

unity, hence true power factor is always less than the

distorted power factor. The true power factor variation

under different non-linear load conditions is depicted

Table II and Table III respectively. It is seen that the

harmonic loads, especially current harmonic content has a

significant impact on the true power factor and the

transformer efficiency. The true or total power factor

variations with current total harmonic distortions are

plotted in Fig.6.

VIII. MITIGATION MEASURES

A. Harmonic filter design-A shunt passive filters

It can be seen that the shunt capacitor acts as a source

of harmonics when load nature is non-linear. With

incorporation of the power factor capacitor, total current

harmonic distortion level is increased from 28.09% to

35.52%. It is important to note that just by adding a shunt

capacitor poor distortion power factor cant be compensated. The displacement power factor can be

improved with shunt capacitors. Here existing power

factor capacitor is removed, and it is converted into

harmonic passive filter. A single tuned band pass passive

filter for 5th

and 7th

harmonic level and high-pass filter

from 15th

harmonic onwards are designed and simulated

for 28.09 % of current THD. Harmonic filters are

designed to be capacitive at fundamental frequency, so

that they are also used for producing reactive power

required by non-linear loads and for power factor

correction. High-pass filters, which are used to filter high-

order harmonics and cover a wide range of frequencies. A

shunt filter is said to be tuned to the frequency which

makes its inductive and capacitive reactances equal. Three-phase harmonic filter are shunt elements that are

used in power systems for decreasing both current and

voltage distortion as well as for power factor correction.

The high-pass filter is a single-tuned filter where the L

and R elements are connected in parallel instead of series.

This connection results in a wide-band filter having

impedance at high frequencies limited by the resistance R.

The quality factor is adjusted according to the harmonic

order which determines the sharpness of tuning. It is

observed that the harmonic filters reduce the THD of the

current injected in the system from 28.09% to 4.8% which

is bellow IEEE standard [9]. The total 70 KVAR is

adjusted as per the following configuration: 30 KVAR

low-pass filter tuned to the 5th

harmonic with quality

factor of 2 and 20 KVAR low-pass filter tuned to the 7th

harmonic with quality factor of 20 as well as 20 KVAR



159

high-pass filter tuned to 15th

harmonics onward. The total

load losses are reduced by 44% compared to 28.09% of

THDi case and 74.5% compared to 35.52% of THDi case.

The corresponding simulation results are shown in Table

3. From the table, it observed that the transformer

maximum current delivery capacity is close to be rated

current capacity and efficiency at full load is 98.37%. The

current spectrum and current harmonic bar chart of phase

A is shown in Fig. 4 (a) and (b) respectively.

TABLE III

SIMULATION RESULTS OF 200KVA TRANSFORMER TESTEDWHEN

PASSIVE FILTERS ARE TESTED

Load Characteristics Non-linear Loads+

Passive Filters

Total Load losses 3143 Watts

THDi 1.1432%

Imax 264.00 Amp

K-rating 1.1432

% Efficiency 98.37%

Total Power Factor 0.95 lagging

Reduction in kVA capacity 0.97%

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-400

-300

-200

-100

0

100

200

300

400

Time (sec)

a p

ha

se

Cu

rre

nt

(Am

p)

(a)

0 5 10 15 20 25 30 35 400

50

100

150

200

250

300

350

400

Harmonic order

Am

plitu

de

of

Cu

rre

nt

(Am

p)

(b)

Fig.4. (a) Harmonic current spectrum of phase A; (b) Current

harmonic level bar chart of phase A at non-linear full load with

passive filters.

B. Transformer De-rating using K-Factor and FHL

Factor

If the filters are not installed then transformer de-

rating using K-factor and FHL factor can be implemented.

Some of the changes in the design of K-rated transformers

are given below:

1) Optimum increase in the delta connected primary

winding conductor size which can tolerate the circulating

triplen harmonics.

2) Core design flux density should be minimum to

protect against voltage distortion.

3) Multiple and transposed secondary winding

conductor to reduce resistance to avoid heating due to

skin effect from high frequency currents. These design

factors can improve the thermal dissipation to minimize

the additional losses.

4) Heavier conductors and transposition of winding

conductor to reduce magnetic losses.

5) Electrostatic shielding between primary and

secondary winding to reduce eddy current losses and

heating.

60 Double sized neutral conductor to protect against

triplen harmonics.

As per reference [2], the standard K-factor transformer

ratings for specific loads are given in Table IV.



160

TABLE IV

TRANASFORMER K-RATINGS

Type of Loads K-factor

Incandescent lighting Electric resistance heating,

Motors, Control transformers without solid state

controllers.

K-1

Electric discharge lighting UPS, Induction heating

equipment, Welders, PLCs.

K-4

Telecommunication Equipments, UPS without

filtering, General health care and classrooms of

schools, Various testing equipments.

K-13

Mainframe computer loads, Moters with VFDs,

Health care equipments in critical care areas and

operating rooms of hospitals.

K-20

Multi-wire receptacle circuits in industrial

,medical, educational laboratories etc.

K-30

Loads producing high order harmonics K-40

The calculations shown in Table I and Table II, it can

be observed that the K-factor rating increases with total

current harmonic distortion. The relation between K-

factor and THDi is shown in Fig.5, as given below.

%THDi

K-FACTOR

4 13 20 401

10

20

30

40

50

Fig.5. Relation between K-Factor and THDi.

Power factor capacitor contributes for increase in total

current harmonic distortion level with non-linear load. It

alone doesnt helpful for improving total power factor but can improve displacement power factor. This relation is

plotted in Fig 6.

Total pf

%THDi20 40 60

1

80 180160140120100

0.9

0.8

0.7

0.6

0.5

0

Fig.6. Relation between total power factor and THDi.

The maximum current delivered by the transformer is

inversely proportional to the total current harmonic

distortion. Transformer KVA capacity also reduced with

current harmonic level. When total current harmonic

distortion level is 28.09%, K-13 rating and for

THDi=35.52%, K-20 rating transformer is recommended.

When passive filters are used, the K-factor is reduced to

K-1.Other mitigation measures suggested are given

below:

1) If the filters are not installed then transformer de-

rating using K-factor and FHL factor can be implemented.

2) Use energy efficient transformers to control

temperature rise and losses. It will extend the life of

transformer.

3) Design of proper sizing of distribution transformer

neutral conductor.

4) Use of Star-delta connected transformer to block

triplen harmonics.

IX. CONCLUSIONS

A three -phase distribution transformer was simulated

for critical analysis under balanced non-linear load. It was

shown that the THDi has a significant impact on the

transformer efficiency as compared with linear nature of

the load. It is observed that power factor, KVA capacity

and transformer efficiency decreases with non-linear load.

It is also shown that the power factor capacitors act as a

source of harmonics during non-linear loading. The K-

factor de-rating of the distribution transformer increases

with an increase in % THDi. If the load THDi is increased

in such a way that the load K-factor greater than the rated

K-factor, then the transformer cant be operated at its full KVA capacity, and hence it would require de-rating.



161

From this analysis, it is concluded that as compared to

K-factor transformer, a passive filter technique is

effective for harmonic mitigation and power factor

improvement. When ever the passive filter is used the

transformer apparent power capacity and distribution line

loading capability can be improved for the same nature of

load. With the implementation of passive filters, there is

significant reduction in the energy losses. In case of

unbalanced non-linear load, an active filter can be used to

improve the power system performance.

REFERENCES

[1] E.F Fuchs, A. Mohammad, S.Masoum Power Quality in Power

Systems and Electrical Machines, Elsevier Inc, ISBN 978-0-12-369536-9, March- 2008.

[2] G.W.Massey, Estimation methods for power system harmonic effects on power distribution transformers, IEEE Trans.Indistrial Application,vol.30,no.2,pp.485-489,Mar/Apr.1994.`

[3] Underwriters Laboratories, UL-1561, Proposed requirements and proposed effective dates for the first edition of the standard for dry

type general purpose and power transformers, Santa Clara , CA, 1991.

[4] E.F Fuchs, D. Lin, and J Martynaitis, Measurement of three-phase transformer derating and reactive power demand under nonlinear

loading conditions, IEEE Transactions on Power Delivery, vol. 21, no.2, pp. 665-672, 2006.

[5] D.Lin, E.F Fuchs, Real-time monitoring of iron-core and copper losses of three-phase transformers under non-sinusoidal operation, IEEE Transactions on Power Delivery, vol. 21, no.3, pp. 1333-1341, July 2006.

[6] W.M.Grady, Robert J. Gilleskie, Harmonics and how they are relate to power factor, Proceedings of the EPRI power quality issues & opportunities conference (PQA-93), San Diego, CA,

November-1993.

[7] IEEE Standard, Recommended practice for establishing transformer capability when supplying non-sinusoidal load

currents, ANSI/IEEE C57.110, February 1998.

[8] J Pedra, F Corcoles, L Sainz, Harmonic nonlinear transformer modeling, IEEE Transactions on Power Delivery, vol. 19, no. 2, pp. 884--890, 2004.

[9] E.F Fuchs, D Yildirim, Measured transformer derating and comparisons with harmonic loss factor FHL approach, IEEE Transaction on Power Delivery, vol. 15, no. 1, pp.186-191,

January-2000.

[10] E.F.Fuchs, D.Yildirim, W.M. Grady ,Measurement of eddy current loss coefficient PEC-R ,derating of single phase

transformers, and comparison with K-factor approach, IEEE Transaction on Power Delivery, vol. 15, no. 1,pp-148-154,

January-2000.

[11] W.P.Linden,Transformer Design and Application Considerations for non-sinusoidal Load Currents, IEEE Transactions on Industry Applications, vol.32, no. 2, pp 633-645, May/June 1996.

[12] IEEE Recommended Practices & Requirements for Harmonic Control in Electrical Power Systems, IEEE Std 519-1992.

[13] A.W Galli, M.D Cox, Temperature rise of small oil filled distribution transformers supplying non-sinusoidal load currents, IEEE transactions on Power Delivery, vol.11, no.1,PP 283-291,

January 1996.

[14] Isadoro Kerzenbaum , Alexander Mazur, Mahendra Mistry, Jerome Frank Specifying Dry type Distribution Transformers for Solid-State Applications, IEEE Transactions on Industry Application, vol.27, no.1, pp. 173-178, January/ February 1991.

[15] M.D.Hwang,, W.M.Grady , H.W.Sanders Jr., Calculation of winding temperatures in distribution transformers subjected to harmonic currents, IEEE Transactions on Power Delivery,vol.3, no.3, pp. 1074-1079, July 1988.

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