Distribution Transformer Losses Evaluation under Non-Linear Load

Transcription

1 Distribution Transformer Losses Evaluation under Non-Linear Load *Dalila M.S., Kalid M. N. and Md Sa M. Centre of Electrical Energy System, Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 830 Skudai Joor, Malaysia. Abstract effect on power system component and load are one of te important considerations wen evaluating te impact of armonics. Transformers are one of te component and usually te interface between te supply and most nonlinear load. Wit te increasing use of nonlinear load te armonic problem become worse. Te increased losses in te transformer due to te armonic distortion can cause excessive winding loss and abnormal temperature rise. Tis paper presents te analysis and evaluation of distribution transformer losses under nonlinear load. In tis study te armonic data logging was conducted in te commercial building and te losses caused by armonic in distribution transformer were calculated. Te life of te transformer also can be estimated from te simulation results. Te results sow tat an increase in te current armonic distortion will increase te transformer losses and ence decreased it life expectancy. Keywords armonic distortion, transformer losses, real life of transformer, armonic loss factor I. INTRODUCTION in power system as increased substantially due to te increasing of non linear load in recent years. Transformers are usually designed for utilizing at te rated frequency and linear load. Nowadays wit te present of nonlinear load, transformer leads to iger losses and reduction of te useful life. Te increased losses due to armonic distortion can cause excessive winding loss and abnormal temperature rise. If te transformer cannot be operated up to its standard lifetime, tere will be an economic loss. Te measurement of a transformer s losses and calculation of its efficiency is applied in te power and distribution transformer. Tree metods of estimating armonic load content are; te crest factor, armonic factor (percent total armonic distortion- %THD) and K- Factor. Te first two metods are most common metods of armonic estimation but limited because te armonic frequencies are not considered, and a K-factor is te most complex []. Increased in armonic distortion component of a transformer will result in additional eating losses, sorten insulation lifetime, iger temperature and insulation stress, reduce power factor, lower productivity and capacity and lack of performance of te system [5].To prevent tese problems, rated capacity of transformer supplying non-linear load must be reduced []. Manufacturer of distribution transformer ave developed a rating system called K-Factor, a design tat is capable of witstanding te effects of armonic load currents [3]. Transformers are one of te component and usually te interface between te supply and most nonlinear loads. voltage increase losses in its magnetic core wile armonic currents increased losses in its winding and structure [6]. In general, armonics losses occur from increased eat dissipation in te windings and skin effect bot are a function of te square of te rms current, as well as from eddy currents and core losses [7]. Tis extra eat can ave a significant impact in reducing te operating life of te transformer insulation. Te increased of eddy current losses tat produced by a non-sinusoidal load current can cause abnormal temperature rise and ence excessive winding losses. Terefore te influence of te current armonics is more important. Many works ave been developed about te armonic effect on power transformer loss of life [4,8- ]. However, tese works did not take into consideration te standards of armonics. In tis study, armonic data at distribution transformer of te commercial building ave been measured. Tis study will investigate on te transformer loss of life based on te Malaysian standard on armonics [5]. Te organization of tis paper is as follows: Section II gives a definition of armonic, section III presents transformer losses, section IV sows te calculation of armonic loss factor, section V describe transformer s loss of life calculation, section VI explain on armonic data logging, section VII gives te result and analysis and finally tis paper was conclude in section VIII. II. HARMONIC DEFINITION A armonic component in an AC power system is defined as a sinusoidal component of a periodic waveform tat as a frequency equal to an integer multiple of te fundamental frequency of te system. [7]. Te major source of armonics is from te tree categories of equipment, wic are power system equipment, industrial loads and residential loads. currents are generated to a small extent and low distortion level by generation, transmission and distribution equipments and to a larger extent by te industrial and domestic loads.

2 IEEE 59:99 [6] also identifies te major source of armonics in power system. Te armonic sources describe in tis standard include power converters, arc furnaces, static VAR compensator, inverters of dispersed generation, electronic pase control of power, switced mode power supplies and pulse wide modulated drives. Tis standard concludes wit recommendation for evaluating new armonic source by measurement and detailed modeling and simulation studies. It provides several examples to illustrate ow tis recommendation can be implemented effectively in several practical systems. III. TRANSFORMER LOSSES Transformers are designed to deliver te required power to te connected loads wit minimum losses at fundamental frequency. Transformer losses are generally classified into no load losses and load losses as sown in eqn. [, 3, 8]. + () T T NL LL NL LL total loss, watt, no load loss, watt, load loss, watt Te no load loss or excitation loss are te losses due to te voltage excitation of te core and magnetic ysteresis and eddy currents. Te load loss or impedance loss is subdivided into I R loss and stray loss caused by electromagnetic flux in te winding, core, core clamps, magnetic sield, enclosure or tanks walls, etc [3,8]. Tus, te total stray loss is subdivided into winding stray loss and stray loss in components oter tan te windings ( OSL ). Te windings 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 stated as follows: + + () LL I R EC OSL I R EC OSL loss in te winding eddy current loss oter stray loss Te I R losses are calculated as in eqn. 5, [ rated R I rated ] K I + R I R rated (5) Were K.0 for single-pase transformers.5 for tree-pase transformers LL rated + I R rated (6) TSL rated EC rated + I I R R TSL TSL rated (7) OSL rated current in primary side of transformer, current in secondary side of transformer primary dc resistance secondary dc resistance total stray loss Based on te IEEE Standard [3] for te oil type transformer, te eddy current loss is assumed to be about 0.33 of te total stray losses TSL rated (8) EC rated OSL rated (9) TSL rated EC rated IV HARMONIC LOSS FACTOR loss factor, F HL is a key indicator of te current armonic impact on te winding eddy loss and oter stray loss. Te armonic loss factor is normalized to eiter te fundamental or te rms current. F HL for winding eddy current is te ratio of te total eddy current losses due to te armonics, to te eddy current losses at te power frequency. Te F HL-STR is te ratio of te oter stray loss due to te armonic to te oter stray loss at power frequency. Te eddy current loss is increased by a factor of F HL and te oter stray loss are increased by a factor of F HL-STR in te presence of armonics. Te transformer load losses in non-sinusoidal condition as sown in eqn. (0); + F F (0) LL I R rated HL EC rated + HL STR OSL rated Tis factor is calculated by te following equations [3] Te rated losses of te transformer can be calculated using te data provided. Initially, te rated current at te primary and secondary sides are calculated as follows; S( kva) I rated 3V S( kva) I rated 3V (3) (4) F F HL HL STR max max max I I max I I 0.8 () ()

3 F HL F HL-STR armonic factor for eddy current loss, armonic factor for oter stray loss Te per unit load losses and rated per unit eddy current loss is given by te expression below [3] max I ( ) LL pu LL rated pu (3) I EC ( EC rated ) ( I rated ) R.8 rated pu (4).5 max I pu armonic order maximum armonic order rms current of fundamental component per unit quantities V. TRANSFORMER S LOSS OF LIFE CALCULATION Te transformer loss of life estimation is based on te deterioration rate acieved by insulating materials [8, 0, and ]. About 50% of a transformer loss of life is caused by termal stresses wic are produced by te non-linear load current [0]. Te ottest spot winding temperature is calculated as follows [8];. (5). (6) Te ot spot temperature is; (7) θ TO oil temperature rise, θ W winding temperature rise, θ A ambient temperature θ g ottest spot conductor rise over top oil temperature, θ H ot spot temperature Te relative aging factor, te loss of life and real life of a transformer can be expressed in te following manner [4]; F exp (8) AA 383 θh + 73 FAA t 00 % LOL normal_ insulation_ life (9) Life 8 H + ( pu) e θ (0) Real life Life (pu) x normal insulation life or () Real life normal insulation life(year)/ F AA () F AA relative aging factor %LOL lost of life in percent t given time period VI. HARMONIC DATA LOGGING Te power quality analyzer is used to log te armonic data. Te measurements were taken for two different buildings. Te first case is at te faculty and te second case being te lecture all building. Te type of loads connected includes personal computers, fluorescent lamps, air conditioners, printers, potocopy macines LCD projectors and oters. For bot cases te data was logged for one week wit intervals of five-minute. Optical cable for USB and ower Log software was used to transfer te data stored in te instrument to te computer. Te logger was set to measure quantities suc as frequency, voltage, current, total armonic distortion, active power, reactive power, apparent power, voltage armonic, current armonic, true power factor and displacement power factor. Table I sows te specifications of te distribution transformers and Table II sows armonic current of te transformer load wic are compared to te standards. Te total armonic distortion for Case and Case are 37.05% and 8.36% respectively. Te daily curve or load per unit of te transformer is sown in Fig.. Te usage of te loads in te building are based on te duration of working ours at te buildings wic is from 8.00 am to 0.30 pm for te library and for te office and lecture all building te operating ours is from 8.00 am to 5.00 pm. Te maximum per unit load acieved by te transformers are approximately 0.4 per unit and 0.7 per unit for Case and Case respectively. TABLE I: THE CHARACTERISTICS OF DISTRIBUTION TRANSFORMER CASE CASE No Load Losses 000 W 600 W Load Losses 5000 W 4570 W ower (kva) rimary Voltage 000 V 000 V Secondary Voltage 433 V 433 V Winding Temperature Rise 65 C 65 C Ambient Temperature 30 C 30 C Normal Life Insulation 0.55 years 0.55 years

4 Figure : Daily load cycle of te transformer TABLE II: HARMONIC DISTORTION FOR A TRANSFORMER LOAD COMARE WITH THE STANDARD No. current (A) (Case I) Maximum Limit of MS 555:00(IEC TR :998) current (A) (Case II) Maximum Limit of MS 555:00(IEC TR :998) TABLE III: THE BREAKDOWN OF LOSSES IN DISTRIBUTION TRANSFORMER Type of loss Rated losses(w) Load losses (W) multiplier Corrected losses (W) No load I R Eddy current Oter stray Total losses V. RESULTS AND ANALYSIS For Case as in Table II, te armonic loss factor for eddy current loss and armonic factor for oter stray loss are calculated using eqn. and eqn.. Hence, F HL.965, F HL-STR.6 In tis case, te transformer loading is considered 0.5 per unit. Te total load loss of te transformer being used in eqn. (3) is, LL pu pu Te load losses are also calculated considering te actual load and te effects of armonic. Te total eddy current loss and oter stray loss are also determined using armonic loss factor for eddy current loss and armonic factor for oter stray losses. Te breakdown of te losses is summarized in Table III. Te result sows tat te armonic load current can be detrimental on te power transformer, wic is about 40.55% losses increased wit armonic present. Wit 0.5 per unit loading, te aging acceleration factor is less tan. Tis means tat te transformer life will not be less tan its normal life. However, wit te same armonic levels, 30 C ambient temperature and references temperature of 0 C, if te load is increased to 0.8 per unit, te aging acceleration factor will be.7 and te real life of tis transformer decreased from 0.55 to about.95 years as sown in Fig.. Fig. also sows tat for te total armonic distortion 37.05% te transformer will ave a normal life wit loading smaller tan 0.78 per unit, because its aging factor is less tan. For Case, from te simulation result, Fig.3 sows tat te transformer still as its normal life for te iger load but if te load exceeds 98%, te transformer s real life will decrease dramatically. Wen te armonic limit are applied for bot cases, Fig.4 and Fig.5 sow te curve of transformer s real life reduction versus per unit loading, for Case te

5 5 5 X: 0.77 Y: 0.55 X: 0.84 Y: X: 0.5 Y: X: 0.5 Y: 0.55 Real Life (year) 5 0 X: 0.8 Y:.95 Real Life (year) 5 0 X: 0.88 Y: X: 0.88 Y: X: 0.94 Y: Load(pu) Figure : Transformer life versus loading (Case I-THD 37.05%) Load(pu) Figure 4: Transformer life versus loading (Case I- Max Limit) Figure 3: Transformer life versus loading (Case II-THD 8.36%) Figure 5: Transformer life versus loading (Case II-Max Limit) limit of per unit load to maintain its normal life increased from 0.77 per unit to 0.84 per unit. However te maximum limit was decreased in Case because of te measured armonic data are lower tan te maximum limit. VI. CONCLUSION Te simulation result of armonic effect on transformer sows tat iger te loads, te lower will be te life of transformer due to current armonics generated by te electrical devices. Higer THD also lower te life of transformer, due to te increase of transformer losses and ot spot temperature. In Case I, te transformer will not be less tan its normal life until te loading exceed 0.77 per unit. For Case, te transformer will maintain its real life until te loading is 0.98, it is sufficient to apply te standard only in Case I. Te armonic standard sows tat armonic current need to be monitored. Tese maximum limits of MS 555:00 [5] wic correspond to IEC TR :998 Standard sould be used as a guide for bot customer and suppliers facing te igest total armonic distortion to protect te power system equipment tat could be affected by ig armonic currents and raise te quality of power supply. ACKNOWLEDGMENT Te autors gratefully acknowledge te Ministry of Higer Education (MoHE) for te financial support of tis project. REFERENCES [] Massey, G.W. (994). Estimation Metods For ower System Effects On ower Distribution Transformers. IEEE Transactions on Industry Applications. Marc- April. Kansas City, MO: IEEE, [] Sarifian, M.B.B., Faiz, J., Fakeri, S.A. and Zraatparvar, A. (003). Derating of Distribution Transformers for Non-Sinusoidal Load Currents Using Finite Element Metod. ICECS 003. roceedings of te 003 0t IEEE International Conference on Electronics, Circuits and Systems. 4-7 December. Iran: IEEE, [3] Jayasinge, N.R., Lucas, J.R. and erera, K.B.I.M. (003). ower System Effects on Distribution Transformers and New Design Considerations for K Factor Transformers. IEEE Sri Langka Annual Sessions. September 003. Sri Lanka: IEEE, [4] Delaiba, A.C., de Oliveira, J.C., Vilaca, A.L.A. and Cardoso, J.R. (996). Te Effect of on ower Transformer Loss of Life. roceedings of te 38t Midwest Symposium on Circuits and Systems.3-6 August. Rio de Janeiro: IEEE,

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