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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) Analysis of Distribution Transformer Performance under Non-linear Balanced Load Conditions and Its Remedial Measures Sanjay A. Deokar1, Laxman M. Waghmare2 1 Dnyanganga College of Engineering and Research, Pune University, Pune-411041 2 S.G.G.S., Institute of Engineering and Technology, Nanded- 431606 1 s_deokar2@rediffmail.com 2 lmwaghmare@yahoo.com 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 Kfactor, 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. 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 nonlinear 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 it’s actual response to 152 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) 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 nonlinear 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 nonlinear 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. 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, (1) PTOTAL PCORE PLOAD L Where, PTOTAL Total loss, PCORE Core or No load loss and PLOADL Load loss The total load loss can be given as, PLOAD L I 2 RDC PWEC L POSL L (2) Where, PWEC L is the winding eddy current loss and POSL L 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], PTotal STR P WEC L POSL L PLOAD L P POSL L PTotal STR PWEC L (3) (4) The winding eddy current loss can be calculated using the following expression, (5) PWEC L 0.33 P Total STR Losses during non- linear loading of a distribution transformer In modern power systems, the total harmonic voltage distortion (THD v ) 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 following expression, (6) PLOAD L PCU PWEC L POSL L The harmonic component of load current increases the r.m.s. value of the load current and hence PCU I 2 R loss will be increased accordingly. PWEC L is the winding eddy current loss due to the non-sinusoidal load current. It can be given as follows, 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. PWEC L PWEC R 153 I h h h 1 I RT h m ax 2 2 (7) International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) Where, The distribution transformer must be de-rated under non-sinusoidal load conditions [1].The transformers derating can be performed using following methods: a) Direct loss measurement. b) Using K-Factor and c) Based PWEC R is the rated eddy current loss under full load conditions, h is the harmonic order, I h is the r.m.s. current at harmonic order h and I RT is the rated on harmonic loss factor ( FHL ) . 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. POSL L are the stray losses in the structural parts due A. Distribution Transformer De-rating Based on KFactor 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, to non-sinusoidal current. It can be calculated by the following expression, POSL L POSL R I h h h 1 IR h m ax 0.8 2 (8) Where, POSL R are the structural part stray losses under rated conditions. The factor 0.8 is accepted by IEEE after manufacturer’s 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 following expression, PLOSD L PCU POSL R h max h max h 1 2 h max I PWEC R h h 1 I R I h K I h 1 2 2 h 2 h 2 (10) 2 h 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, 2 I h 0.8 h R I h 1 Ih IR hm ax (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. I max 1 PECL R (I R ) 1 kPEC R (11) I R the fundamental rms current under is rated load conditions, PECL R is the eddy current loss to rated Where I 2 R loss in which I is the total rms current. The reduction in apparent power is given by the following expression, PKVA Re duction 1 ( 154 V Non linear Rms p.u . ) I m ax V Rated Rms (12) International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) Where, IV. MODELING AND SIMULATION OF DISTRIBUTION TRANSFORMER V Non linear Rms the total rms is value of the secondary 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. voltage including harmonics and V Rated Rms 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 FHL Factor can be defined using following expression, FHL AT FULL LOAD 2 Ih 2 h h 1 I 1 2 h m ax Ih h 1 I 1 h m ax TABLE I ALL PARAMETERS AND LOSSES OF 200KVA TRANSFORMER WORKING Parameters (13) The stray loss harmonic factor can be given as, FHL STRAY 2 I h 0. 8 h h 1 I 1 2 h m ax Ih h 1 I 1 h m ax Hence, the relation between K-factor and as follows, h m ax 2 Ih K h 1 2 I R F HL (14) FHL is given (15) Therefore, the maximum amount of R.M.S. harmonic load current that the transformer can deliver is given as, I max PLOAD L (16) 1 [ FHL PECL ] [ FHL STRAY POSL ] 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 14.75Ω R2 0.0062Ω L1 0.003H L2 0.067mH Rc 728 kΩ Lm 32105H A transformer is tested characteristics at full loads. Under harmonic load condition, the new load loss can be calculated by the following expression, PLOAD LNEW I 2 [1 FHL PECL R FHL STRAY POSL ] (17) The reduction in the apparent power rating is given by the equation (12). for the following load 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. 155 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) 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. 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 35 th 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. Supply 11kV, f rom power utility 11/0.433kV 200kVA R1=14.75ohm L1=0.003H R2=0.0062ohm L2=0.067H Rm=728kohm Lm=32105H X/R=2.5 BUS BAR 400 300 Linear Load Full load 200kVA p.f .=0.8lagging S2 S3 Non-Linear Capacitor Load bank %THDi=28 67.41kVAR S4 200 'a' phase current(A) S1 Passiv e Filters,5th, 30kVAR/Phase 7th and 15th onwards20kVAR/ phase 100 0 -100 -200 -300 -400 0 0.01 0.02 0.03 0.04 0.05 Time (sec) 0.06 0.07 0.08 0.09 0.1 (a) 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. 400 Amplitude of current (A) 350 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. 300 250 200 150 100 50 0 0 5 10 15 20 25 30 35 40 Harmonic order (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. 156 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) TABLE II VI. A CASE OF 200KVA DISTRIBUTION TRANSFORMER FEEDING NON-LINEAR LAD WITH P.F.IMPROVEMENT SIMULATION RESULTS OF 200KVATRANSFORMER TESTED AT DIFFERENT LOAD CHARACTERISTICS 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 Base case Load Characteris tics (Total Linear nature of Load) Base Case+ Capacitor Banks for P.F. improvem ents Total NonLinear NonLoad linear without Load+ ....P.F…… P.F. ….... Capacit improveme or. nt 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 <5% <5% 28.09% 35.52% Imax Rated Capacit y Rated Capacity 221.78 Amp 155.13 Amp K-rating No Derating No Derating 7.057 19.7 % Efficiency 97.73% 98.24% 96.60% 92.80% Total Power Factor 0.8 lagging 0.95 lagging 0.77 lagging 0.942 lagging 0.0% 17.1% 41.87% 600 'a' phase current(A) 400 200 0 -200 -400 -600 0 0.02 0.04 0.06 Time (sec) 0.08 0.1 (a) 400 350 Amplitude of current (A) 300 250 200 % kVA Capacity reduction 150 100 50 0 0 5 10 15 20 25 30 35 40 Harmonic order (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. 157 0.0% International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) VII. POWER FACTOR UNDER NON-LINEAR LOAD ENVIRONMENT 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. 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 following expression, hmax THDV V h2 VIII. MITIGATION MEASURES 2 h 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 can’t 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 highorder 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 reactance’s 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 100 V1 OR (18) hmax THDI I h2 I1 2 h 100 Hence, correct form of true power factor under linear and non-linear load environments is given by the following expression, TRUEPF PAvg V fun I fun 1 1 (THDV 100) 2 (19) 1 1 (THDI 100) 2 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, TRUE PF Pfun V fun I fun 1 1 (THD I 100 ) 2 (20) Displaceme nt PF Distortion PF Where, Pfun, Vfun and Ifun are the fundamental power, voltage and currents. 158 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) 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. 400 350 Amplitude of Current (Amp) 300 250 200 150 100 50 0 0 5 10 15 20 25 30 35 40 Harmonic order TABLE III (b) SIMULATION RESULTS OF 200KVA TRANSFORMER TESTEDWHEN PASSIVE FILTERS ARE TESTED Load Characteristics 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. Non-linear Loads+ Passive Filters B. Transformer De-rating using K-Factor and FHL Total Load losses 3143 Watts THDi 1.1432% Imax 264.00 Amp K-rating 1.1432 Factor If the filters are not installed then transformer derating 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. % Efficiency 98.37% Total Power Factor 0.95 lagging 2) Core design flux density should be minimum to protect against voltage distortion. Reduction in kVA capacity 0.97% 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. 400 300 a phase Current (Amp) 200 4) Heavier conductors and transposition of winding conductor to reduce magnetic losses. 100 0 -100 5) Electrostatic shielding between primary and secondary winding to reduce eddy current losses and heating. -200 -300 -400 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Time (sec) 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. (a) 159 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) TABLE IV 1 TRANASFORMER K-RATINGS Type of Loads K-factor Incandescent lighting Electric resistance heating, Motors, Control transformers without solid state controllers. K-1 0.9 Total pf 0.8 0.7 0.6 Electric discharge lighting UPS, Induction heating equipment, Welders, PLC’s. K-4 Telecommunication Equipments, UPS without filtering, General health care and classrooms of schools, Various testing equipments. K-13 Mainframe computer loads, Moters with VFD’s, 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 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 derating 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. 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 Kfactor and THDi is shown in Fig.5, as given below. 50 %THDi 20 40 60 80 100 120 140 160180 %THDi 40 IX. CONCLUSIONS 30 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 Kfactor 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 can’t be operated at its full KVA capacity, and hence it would require de-rating. 20 10 1 4 13 20 K-FACTOR 40 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 doesn’t helpful for improving total power factor but can improve displacement power factor. This relation is plotted in Fig 6. 160 International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, Volume 1, Issue 2, December 2011) [12] “IEEE Recommended Practices & Requirements for Harmonic 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. Control in Electrical Power Systems,” IEEE Std 519-1992. 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