Effect of Harmonics on Distribution Transformer Losses and Capacity

Transcription

1 nternational Journal of Engineering Tecnology Science and Researc SSN June 017 Effect of Harmonics on Distribution Transformer Losses and Capacity Jaspreet Sing Sanjeev Sing Amanpreet Sing unjab State ower Corporation Ltd, EE Department unjab Tecnical University Moga, unjab, ndia SLET Longowal, ndia Jalandar Abstract Te nonlinear loads are increasing day by day due to te use of controlled power electronic devices, te armonics produced by te nonlinear load affecting te power system. Transformer is basic component of power system and designed to work on linear load. Harmonics produced by nonlinear load increases losses, eating, ageing of insulation, reduces capacity and results in premature failure of transformer. n tis paper effect of nonlinear load is studied as per EEE standard to calculate derating of transformer due to armonics. Case study of two transformers of unjab State ower Corporation (SCL) presented for calculating capacity reduction under armonic load after measuring te power quality data. Transformer loss of life are also evaluated under nonlinear load. Keywords Transformer losses, eddy current loss, stray loss armonic distortion, power quality (Q).. NTRODUCTON n ower distribution system, distribution transformer is most important apparatus. t is designed to operate on power frequency wit linear load. Te caracteristics of te electric loads ave canged dramatically from linear to nonlinear wit te proliferation of new solid state controlled devices and sensitive computer type equipment. Te Electric ower Researc nstitute (ER) gives a roug estimation tat in 199, 15 to 0% of te total electric utility load was nonlinear and tis trend in rising and is expected to reac 50 to 70% in te year 00. Te rapid cange in te electric load profile from being mainly a linear type to greatly nonlinear, as created continued power quality problems wic are difficult to detect and is in general complex. ower quality is defined as any problem manifested in voltage, current or frequency deviation wic results in failure or malfunction of customer equipment [1]. Te most important contributor to power quality problems is te customers (or end-user electric loads) use of sensitive type nonlinear load in all sectors (ndustrial, Commercial and Residential). A major power quality concern is armonics distortion wic is caused by non-linearity of customers loads []. Harmonics are currents or voltage wit frequencies tat are integer multiples of fundamental power frequency. Nonlinear loads draw current in ig amplitude sort pulses, wic creates distortion in current and voltage wave sape wic is measured in term of total armonics distortion (THD) [3]. Total armonics distortion of current is te contribution of all te armonic frequency currents to te fundamental. Harmonics generated causes additional eating in transformer components wic results in iger losses, degradation to transformer insulation wic decreases te useful life of transformer and premature failure of transformer. Harmonics distortion increases bot no load and full load losses. ncrease in eddy current losses causes rise in temperature of transformer wic results in premature failure of transformer. Harmonics also causes derating in transformers capacity and may need to be derated to as muc as 50% capacity wen feeding loads wit igly distorted current waveform. Loads wit igly distorted current waveforms ave a very poor power factor and due to tis, tey use excessive power system capacity and causes overloading [4]. unjab State ower Corporation (SCL) is a utility responsible for Generation and Distribution of ower in te state of unjab in ndia. t as a very large consumer base consisting of domestic, commercial, industrial and agriculture loads divided in four zones. t as large number of distribution transformers feeding te consumers. Te 48 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

2 nternational Journal of Engineering Tecnology Science and Researc SSN June 017 transformer failure rate is above 15% wic is a uge loss to te organization. ower utilities are facing problems of transformer failure due to capacity degradation because of armonics produced by increasing nonlinear loads. n tis paper two failed transformer of city based subdivision of SCL are selected supplying T offices, sopping complexes and education institutes supplying nonlinear load to find out effect of armonics on losses and capacity of transformer. Harmonic data of transformers measured. Tis study will investigate on transformer losses and loss of life due to armonics.. TRANSFORMER LOSSES Transformers are designed to deliver te required power wit maximum efficiency and minimum losses. Transformer losses are classified into no load losses and load losses [5] T = NL + LL (1) NL is no load losses or excitation loss due to induced voltage in core and due to magnetic ysteresis and eddy currents. LL is te load loss and consists of R loss and stray loss caused by electromagnetic field in te windings, core, clamp, magnetic seets, enclosures or tank walls etc. R is calculated by measuring te dc resistance of winding and multiplying it wit square of te load current. Te stray losses can be furter divided into winding stray loss and stray loss in components oter tan winding ( OSL). Te winding stray loss includes winding conductor strand loss and loss due to circulating currents between strands or parallel winding circuits. Te total load loss can be expressed as: LL = R + EC + OSL () Te total stray losses are determined by subtracting R from te load losses measured during te impedance test. TSL = LL - R (3) A. Eddy current losses in winding Tis loss is due to time variable electromagnetic field across windings. Tere are two effects tat can cause increase in eddy current loss in windings, skin effect and proximity effect. n transformers, internal windings adjacent to core ave more eddy current loss, in comparison to external windings. Tis is due to ig electromagnetic intensity near te core tat covers tese windings. Te winding eddy current loss in te power frequency spectrum tends to be proportional to te square of te load current and te square of frequency, wic are due to bot te skin effect and proximity effect [6]. EC α f (4) Te impact of lower order armonics on te skin effect is negligible in te transformer winding. A portion of stray loss is taken to be eddy current loss. For oil type transformer te winding eddy loss is assumed to be: EC = 0.33 TSL (5) For dry type transformer te winding eddy loss is assumed to be: EC = 0.67 TSL (6) OSL = TSL - EC (7) Te division of eddy current loss and oter stray loss is assumed to as follows [6] a) 60% in low voltage winding and 40% in te ig voltage winding for all transformers aving a maximum current rating of less tan 1000A ( regardless of turn ratio). b) 60% in low voltage winding and 40% in te ig voltage winding for all transformers aving a turn ratio of 4:1 or less. c) 70% in low voltage winding and 30% in te ig voltage winding for all transformers aving a turn ratio greater tan 4:1 and also aving one or more winding wit a maximum cooled current rating greater tan 1000 A. a. roximity Effect n transformer HV winding produce a flux density due to carging current. Te LV winding and core cuts te flux density. Te flux density tat cuts te LV winding induces an emf tat produces circulating or eddy currents. Tis effect is called proximity effect, wic is caused by a current carrying conductor, or magnetic field tat induce eddy currents in oter conductor or magnetic fields. Tese eddy currents will dissipate power, EC, and contribute to te electrical loss in te windings in addition to tose caused by normal dc losses [7]. Te proximity loss can be expressed as: E = N nd 18l 4 Gr (8) 49 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

3 nternational Journal of Engineering Tecnology Science and Researc SSN June 017 Were n is number of strands, d is te strand diameter, is te maximum current. G r is proximity effect factor and by considering δ = 1 f d / δ decreases to unity ten G r 1. Eddy current loss may also be calculate from te equation too. TSL = LL_R [(R 1 1-R + R -R)] (9) Te winding eddy current loss is ten calculated by using assumption (5). B. Oter stray losses in Transformer Eac metallic conductor by electromagnetic flux experiences an internally induced voltage tat causes eddy currents to flow in tat ferromagnetic material. Te eddy current produce losses tat are dissipated in te form of eat, producing an additional temperature rise in te metallic parts over its surroundings. Te eddy current losses outside te windings are oter stray losses. Te oter stray losses in core, clamp and structural parts will increase at a rate proportional to te square of te load current but not a rate proportional to te square of te frequency as in eddy current winding losses [6]. Experimental results to find out cange of oter stray losses at low frequency sown tat te ac resistance of te oter stray losses at low frequencies (0-360Hz) is equal to [7]. f R OSL = 1.9 m Ω f1 (10) And at ig frequencies (40-100Hz) te resistance is R OSL = f f m Ω (11) 1 Tus tis loss is proportional to square of te load current and te frequency of to te power of. Given equation can be used to calculate te oter stray losses. OSL = TSL - EC (1),. EFFECT OF HARMONCS ON TRANSFORMER A. Effect of Voltage Harmonics According to Faraday s law te terminal voltage determines te transformer flux level d N dt v (t) (13) Transferring tis equation in frequency domain sows te relation between te voltage armonics and te flux components: Nj (ω) φ = V (14) Tis equation sows tat te flux magnitude is proportional to te voltage armonics and inversely proportional to armonic order. Furtermore witin most power systems te armonic distortion of te system voltage THD is well below 5% and te magnitude of voltage armonics components are small compared to te fundamental component, rarely exceeding a level of -3%. Terefore neglecting te effect of armonic voltage and considering te no load losses caused by te fundamental voltage component will only give rise to insignificant error. f THD V is not negligible, losses under distorted voltage can be calculated based on ANS-C standard V rms M ec (15) Vrms Were V rms and V rms are te rms values of distorted and sinusoidal voltages, M and are no load losses under distorted and sinusoidal voltages, and ec are ysteresis and eddy current losses, respectively [8] B. Effect of Current Harmonics n most power systems, current armonics of significance. Tese armonics current components cause additional losses in te windings and oter structural parts. a. Effect of Harmonics on dc losses f te rms value of te load current is increased due to armonic components, ten tese losses will increase wit te square of te current. max Rdc Rdc A, rms (16) 1 50 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

4 nternational Journal of Engineering Tecnology Science and Researc SSN June 017 a. Effect of armonics on eddy current losses Te eddy current losses generated by te electromagnetic flux are assumed to vary wit te square of te rms current and te square of frequency: max ec ecr (17) 1 R Te armonic loss factor for winding eddy currents is derived as: max max F HL (18) max max To get te true value of eddy current losses under armonic loads, eddy current losses in winding must be multiply wit armonic losses factor (F HL) wen te transformer supplying nonlinear load. b. Effect of armonics on oter stray losses Te oter stray losses assumed to vary wit te square of te rms load current and te armonic frequency to te power of : max OSL OSL R (19) 1 R Te armonic loss factor for oter stray losses are also expressed in a form as for te winding eddy currents: F HLSTR OSL OSLR max 1 max 1 max 1 max 1 (0) 1 To obtain te true value of oter stray losses, it must be multiplied by armonic loss factor (F HL-STR) wile supplying nonlinear load. 1 V. ROCEDURE FOR EVALUATON OF LOSSES AND CAACTY OF TRANSFORMER SULLYNG NON LNEAR LOAD Te equation tat applies to linear load condition is [6]: LL R Were EC R pu 1 (1) LL R ECR OSL rated load losses, 1 is dc losses is, is rated winding eddy current loss, OSL rated oter stray losses at rated current. As te effect of te armonic on losses of transformer evaluated in previous sections, a general equation for calculation of losses wen transformer supplying a armonic load can be defined as: LL [1 FHL ec R( pu) F ] () HLSTR OSLR Te permissible transformer current is expressed as max (pu) = 1 F F HL ecr LLR HLSTR OSLR 3) From te above mentioned equation, te permissible current and derating of te transformer can be determined. V. TRANSFORMER LOSS OF LFE CALCUTATON Harmonic losses occur in te form of increased eat dissipation in te winding and skin effect. Bot are a function of te square of te root mean square current. Tis extra eat ave a significant impact in reducing te operating life on te insulation of a transformer. Te estimation transformer loss of life is based te deterioration rate acieved by insulating materials. Te transformer loss of life is based on te deterioration rate acieved by insulating materials. About 50% of a transformer loss of life is caused by termal stress wic is produced by te nonlinear load [9]. Te top oil temperature rise is calculated as follows [6] LLC NL TO TO (4) RATED LLrated NL Te ottest spot winding temperature is calculated as follows is ( 51 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

5 nternational Journal of Engineering Tecnology Science and Researc SSN June FHL g w g R 1 Te ot spot temperature is H Were, TO g A ECR EC R (5) (6) TO = oil temperature rise w = winding temperature rise A= ambient temperature g = ottest spot conductor rise over top oil temperature H = ot spot temperature Te relative aging factor, te loss of life and real life of a transformer can be expressed as [10] F AA exp 383 H 73 (7) FAA t 100 % LOL normal_ insulation_ life (8) H 73 ( Life pu) e (9) Real life = Life (pu) normal insulation life or (30) Real life = normal insulation life (years)/f AA (31) F AA =relative ageing factor %LOL = loss of life in percent t = time period. V. OWER QUALTY MEASUREMENT Transformer is a static macine wit very ig efficiency and rugged construction. Te rate of failure of distribution transformers in ndia is iger (1-17%) as compared to developed countries (- 3%). Tis ig failure rate is cause of concern to all te Distribution Companies (Discoms) in te country. Every year, nearly 00 Crore of ndian Rupees (NR) are spent by te Discoms for repair and replacement of distribution transformers. [11]. LL Transformer failure analysis reveals tat insulation failure and line surges are te major cause of te failure of transformer. Transformers are also failing due to manufacturing defects, overloading, improper maintenance, and moisture and oil contamination. 16% transformers failed due to unknown reason, out of wic some of transformers may be failed due to power quality problems [1]. To evaluate te effect of armonics on transformer two no of transformers T1and T, 100 KVA and 00 KVA respectively given in table, of city based sub division of SCL failed due to over loading wile supplying load below teir capacity. 100 KVA transformer supplying educational institute and 00KVA transformer nstalled at city center supplying sopping complexes and T offices. n bot te cases load connected consists of personal computers, laptops, LCD projectors, printers, CFL lamps, air conditioners, inverters, US. Bot te failed transformers are replaced wit loads of iger capacity. Table : Distribution Transformer arameters T1 T Capacity (KVA) Voltage (V) 11000/43 3 No Load Losses 60 Load Losses (A) 5.5 (A) Connected Load (%) (Maximum Demand) Winding Temperature Rise Ambient Temperature o C 65 o C 40 o C 40 o C 11000/433 Harmonics data is logged using power quality analyzer in bot te cases for current armonics, voltage armonics, active power, reactive power, voltage, current, frequency, power factor for one week. 5 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

6 nternational Journal of Engineering Tecnology Science and Researc SSN June 017 V. CALCULATON OF TRANSFORMER CAACTY AND LOSSES UNDER HARMONC LOADS Tis section will perform calculations losses of and capacity under armonics load. Te parameters of 100 KVA and 00 KVA transformer is given table. A. 100 KVA Transformer a. Under linear load By using equation (), (3), (5) and (1), Omic loss, total stray loss, eddy current loss and oter stray losses are DC=58W, TSL=,115W, EC = 37.95W, OSL = 8.05W b. Under armonic load To calculate losses and capacity under armonic load te armonic load measurements for 100KVA transformer are given in table. Using equation (16) te Omic losses is dc = W Using equation (17) and (18) te eddy current loss and armonic loss factor are EC = 40.57W F HL = 5.99 Harmonic order TABLE. HARMONC LOAD SECFCATONS Magnitude To obtain te true value of eddy current loss it must be multiplied by armonic loss factor for winding eddy current EC = = 43.01W By using equation (19) and (0), oter stray losses and armonic loss factor for oter stray loss are OSL = 87.71W F HL-STR = 1.40 To obtain te true value of oter stray loss it must be multiplied by armonic loss factor for oter stray loss (F HL-STR) OSL = = W Te calculated transformer losses under linear and nonlinear load are given in table. TABLE. 100 KVA TRANSFORMER LOSSES UNDER LNEAR AND NONLNEAR LOAD. Type of losses No Load Rate d losse s (W) Losses under armonic s load Harmonic s losses factor Correcte d losses under armonic load (w) DC Windin g Eddy Current Oter Stray Total By using equations (1), () and (3) transformer capability under nonlinear load is L L (pu) = 1.06 max (pu) = max = = 97.84A Equivalent KVA = = KVA B. 00 KVA transformer a. Under linear load By using equation (), (3), (5) and (1), Omic loss, total stray loss, eddy current loss and oter stray losses are DC=196.84W, TSL=537W, EC = 177.1W, OSL = 94W b. Under armonic load To calculate losses and capacity under armonic load te armonic load measurements for 00KVA transformer are given in table TABLE. HARMONC LOAD SECFCATONS Harmonic order Magnitude Using equation (16) te Omic losses is dc = 14.44W. 53 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

7 nternational Journal of Engineering Tecnology Science and Researc SSN June 017 Using equation (17) and (18) te eddy current loss and armonic loss factor are EC = W F HL = 6.36 To obtain te true value of eddy current loss it must be multiplied by armonic loss factor for winding eddy current (F HL) EC = = W By using equation (19) and (0), oter stray losses and armonic loss factor for oter stray loss are OSL = W F HL-STR = 1.35 To obtain te true value of oter stray loss it must be multiplied by armonic loss factor for oter stray loss (F HL-STR) OSL = = W Te calculated transformer losses under linear and nonlinear load are given in table V. TABLE V. 00KVA TRANSFORMER LOSSES UNDER LNEAR AND NONLNEAR LOAD. Type of losses Rated losses (W) Losses under armonics load Harmo nics losses factor No Load Correcte d losses under armoni c load (w) DC Winding Eddy Current Oter Stray Total By using equations (1), () and (3) transformer capability under nonlinear load is LL (pu) =.917 max (pu) = max = = A Equivalent KVA = = 140. KVA V. TRANSFORMER LOSS OF LFE Te effect of increased losses in te form of te extra eat can be calculated form equations (4) - (31) as: a. 100 kva Transformer FAA = 1.69 %LOL = 43% Real life =1.16 years Normal life = 0.55 years b. 00 KVA Transformer F AA = 1.8 %LOL = 44.43% Real life =1.16 years Normal life = 0.55 years X. RESULTS AND DSCUSSONS Te transformer capacity and losses data is calculated in te previous section for 100KVA and 00KVA transformers is given in table and V respectively. For comparison purpose te important parameters are sown in table V. Table V. Summary of important parameters Conclusion 100KVA 00KVA Loss under linear load (W) Loss under nonlinear load (W) ercentage increase of losses (W) Capacity under armonic load (KVA) ercentage in capacity Relative Factor decrease ageing Loss of life in percentage (%LOL) Table V sows transformers losses under armonic load increase about 37% and 44%. Tis increase in total losses results from significant increase in eddy current losses in winding wic furter increases te eat generation in winding and reduces te capacity of te transformer or derated te transformer. Te 100KVA and 00 KVA transformer capacity is reduced to 73KVA and 54 Jaspreet Sing, Sanjeev Sing, Amanpreet Sing

8 nternational Journal of Engineering Tecnology Science and Researc SSN June KVA, wic means a loss of 6.5% and 9.90% loss of capacity respectively. Tis ampers te load carrying capacity of transformer and severely over loads te transformer. f te load connected to transformer is more tan te derated capacity of transformer te transformer can eventually fail. n tis case bot te transformers are carrying a load (maximum demand) of 90% and 88.5% wic are more tan te derated capacity of transformers and it can be te cause of failure of bot te transformers because bot are failed due to over loading wit all tree pase windings are found to be burnt in tear down analysis. Te transformers were installed in 003 and 004 and failed in 015 Furter from loss of life calculations and relative ageing factor te percentage loss of life is found to be about 40% and 44%, wic means te transformers are failed after about 1 years and 11years respectively. So bot te transformers failed prematurely after instaltion before completing te normal life of 0.55 years. V. CONCLUSON n tis paper effect of nonlinear load on te transformer capacity and losses is studied as per (EE standard C57-110) as been studied for derating purpose effect of armonics on load losses,eddy current losses and oter stray losses ave been computed in order to calculate equivalent KVA capacity of transformer wile supplying nonlinear load. t sows tat losses increase in armonic load wic decreases te capacity of transformer wic results in overloading of transformer and reduction in useful life of transformer. Case study of 100KVA and 00 KVA transformers presented to derating purpose to find out cause of failure and concluded tat as bot were eavily overloaded, so tey are failed due to capacity reduction due to armonics. To ceck te ageing of transformer furter loss relaive ageing factor and percentage loss of life is calculated and find out te transformers are failed about 1years and 11 years respectively before completing is normal life wic justified te transformers failure due to capacity reduction due to armonic load. V. ACKNOWLEDGMENT Autors are tankful to management of SCL for permitting tem to use transformer and power quality data for writing tis paper. REFERENCES [1] Sing, R. and Sing, A, Causes of failure of distribution transformers in ndia, in roc. 9t nternational Conference on Environment and Electrical Engineering (EEEC), rague, Czec Republic,010. [] Haren Sa, Harmonics- A power quality problem, ndustry watc Electrical and Electronics, September October 005, pp [3] Victor A. Ramos Jr., Treating armonics in electrical distribution ssystems, Computerower and Consulting, Jan [4] William H. Bartley, HSB, Analysis of Transformer Failures, roceedings of te Tirty Six Annual Conference, Stockolm, [5] EEE standrad , EEE recmmonded ractices and requirements for Harmonic control in Electrical ower System. [6] EEE standrad C , EEE recommended ractice for Establising Transformer Capability wen Supplying Non Sinusoidal Currents. [7] S.B.Sadati, H.Yousefi, B.Darvisi, A.Taani, Comparison of Distribution Transformer Losses and Capacity under Linear and Harmonic Loads, nd EEE nternational Conference on ower and Energy (ECon 08), December 1-3, 008, Joor Baaru, Malaysia. [8] D.S. Takac, Distribution Transformer no Load Losses EEETrans on AS, Vol. 104, No. 1, 1985, pp [9] Samesima, M..et. al., Analysis of transformer loss of lifedriving non linear industrial load by te finite element approac, EEE ndustry Application Conference, 8-1 Oct.1995, Orlando. [10] EEE Guide for loading Mineral Oil mmersed Transformers, EEE C [11] Sing, R. and Sing, A, Causes of failure of distribution transformers in ndia, in roc. 9t nternational Conference on Environment and Electrical Engineering (EEEC), rague, Czec Republic,010. [1] J. Sing and S. Sing,"Transformer Failure Analysis:Reasons and Metods," nternational Journal of Engineering Researc & Tecnology (JERT) Volume 4, ssur 15.ACMEE-016 Conference roceedings. [13] D.M. Said and K. M. Nor, Simulation of te impact of armonics on distribution transformers, nd EEE conference on power and energy, december 1-3, 008, Joar Baaru, Malasya. [14] EEE Guide for Failure nvestigation, Documentation and Analysis for ower Transformers and Sunt Reactors, EEE Standard C Jaspreet Sing, Sanjeev Sing, Amanpreet Sing