Analysis of High Neutral Currents and Harmonic Impacts on Losses and Capacities of Distribution Transformers

Transcription

1 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. Analysis of Hig Neutral Currents and Harmonic Impacts on Losses and Capacities of Distribution Transformers George Eduful, Member, IAENG and Kingsford J.A. Atanga Abstract In tis study, tecnical loss evaluation of distribution transformers in te Electricity Company of Gana (ECG) distribution system is considered. Te purpose is to estimate level and severity of losses in distribution transformers and sow tat system losses can significantly be reduced by reducing losses in distribution transformers. Te focus is owever on armonic related losses. Te study analyses armonic contents of 48 distribution transformers of total capacity of 74 MVA. Te number of transformers investigated represents.6% of te total number of distribution transformers in te ECG distribution system. Te evaluation of te losses was based on ANSI/IEEE C Impact of armonics on capacities of te transformers was also examined. Results sow tat armonic currents, especially zero sequence armonics, can increase losses in distribution transformer to over 00%. Te cost of energy per year resulting from te armonic loss was found to be GHc,0,395./year (US$ 555,098.78/year). It was also found tat te armonic effect de-rates te total capacity of te transformers by 7%. In oter words, to ensure tat te transformers are working witin teir economic life span, teir total capacity sould be reduced by 0 MVA. Index Terms Harmonics, Triplen Harmonics, Distribution Transformers, Transformer Losses, Neutral Currents S I. INTRODUCTION YSTEM losses in power distribution networks represent a major cost in te delivery of electric energy. Magnitude of te losses varies depending on te country and te state of te networks. Generally, system loss of around 6 to 8% is internationally accepted []. However, in developing countries, 0 to more tan 40% system losses ave been recorded [, 3]. Te problem is not different in Gana: te president of te republic, in an address at function of te 04 Gana Journalists Awards, put Gana s system losses at 30% [4]. In te distribution system of te Electricity Company of Gana (ECG), te losses are estimated at 3.54% (as at June, 05) [5]. Te problem of system losses continues to treaten te financial sustainability and operational efficiency of utility companies. As a result, effort to reduce losses in Gana George Eduful is wit te Electricity Company of Gana, Accra, Box AN 578, Accra-Nort, Gana. Telepone Number : ; e- mail: eduful@ieee.org Kingsford J. A. Atanga is wit te Electricity Company of Gana, Accra, Box AN 578, Accra-Nort, Gana. Telepone Number : ; katanga3@gmail.com distribution system is a topmost priority. Accordingly, te ECG, in its budget for 06, targets to reduce its system losses to less tan 0% by end of 06 [5]. System losses are made of commercial and tecnical losses. Wilst commercial losses relate to losses due to power teft and errors in billing systems, tecnical losses are inerent in te pysical delivery of electric energy: it includes conductor and transformer losses. Terefore, to effectively deal wit system losses, te effort sould be focused on tese two areas. In tis study, tecnical loss evaluation of distribution transformers in te ECG distribution system is considered. Te purpose is to estimate level of losses in distribution transformers and sow tat system losses can significantly be reduced by reducing losses in distribution transformers. Te focus is owever on armonic related losses and tis is due to te presence of ig neutral currents in most of te transformers. Hig neutral currents in distribution transformers normally result from two situations. Te first, and most common, is one were tere are simply eavily unbalanced loads. Tis situation is usually easy to explain and remedy: loads need to be evenly distributed across te pases to reduce te neutral current. However, in armonic environment te situation is different. Wilst positive and negative sequence components of armonic current add to zero at neutral point, zero sequence armonics do not. Te zero sequence components are additive at te neutral and are te reason for ig neutral current, even toug te loads may be perfectly balanced [6]. Excessive neutral current usually results in ig power losses in distribution transformers. Hig neutral current can also generate potential difference between neutral and eart. Te acceptable neutral to ground voltage is less tan V. However, neutral voltage of more tan 7 V as been found [7]. Hig neutral voltage can cause system lockups, communication errors and operational problems [7]. Te study analyses armonic contents of 48 distribution transformers of mainly 500 kva capacities. Te evaluation of te armonic losses was based on te IEEE Standard guidelines. Impact of armonics on capacities of te distribution transformers was also assessed. Results sow tat armonic currents, especially zero sequence armonic currents, can increase losses in distribution transformers to over 00%. Te cost of energy per year resulting from te armonic loss was found to be GH,0,395./year (US$ ISBN: ISSN: (rint); ISSN: (Online) WCE 06

2 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. 555,098.78/year). Harmonic effect on te capacities of te transformers caused a total capacity loss of about 0 MVA. II. HARMONIC THEORY Harmonics are sinusoidal voltages and currents wit frequencies tat are integral multiplies of te fundamental frequency tat is 50 or 60 Hz in a typical power system. Matematically, armonic currents can be expressed as follows: i DC n I I sin f () were ᵟ is te pase angle of te armonic current and IDC is te direct component of te current (does not exist always). Te equation of te armonic voltages as te same form, but te current is replaced wit voltage. A. Harmonic Distortions and teir Measurements Total Harmonic Distortion (THD) is te most common index used to quantify armonic distortion in power systems [8]. Te THD value is te effective value of all te armonics current or voltage added togeter, compared wit te value of te fundamental voltage or current. Altoug many of today's test and measurement instruments can provide THD values, it is still important to understand te calculation tat derives THD. Te basic equation is as follows [9] : transformers ranged between 00 to 500 kva at /0.433kV voltage level. Load on te transformers and armonic contents were measured using te Megger ower Analyzer, model A9 lus. Te measurement spanned for ours at every distribution transformer station. Load loss and winding resistance of te transformers were measured at a transformer worksop of te Electricity Company of Gana. A Tree ase recision Analyzer wit model number VA-60080, manufactured by Veer Electrical Instrument, was used for te load loss measurement. In te case of te winding resistance, te Megger instrument wit te model number MT00 was used. Te measured values were all corrected to a reference temperature of 75 degree Celsius. To determine te cause of te ig neutral currents, te load distribution on te transformers and te respective armonic contents were analysed. Based on te level of loading and te respective armonic contents measured, power losses in te transformers were evaluated using te metod prescribed in te ANSI/IEEE C Total cost of energy arising from te power losses was ten estimated for a period of year. In te cost estimate of te energy loss, load factor of eac transformer was considered. Fig sows a sample of ow te load factors were calculated. U rms THD U U () U Urms were THD U represents voltage or current total armonic distortion (alternatively represented as THD V and THD I respectively) and Urms is te rms fundamental voltage or current. Alternatively, rms voltage or current can be represented in terms of total armonic distortion as given below. U rms. U rms U rms B. Effects of Harmonic Distortion THDU Harmonics ave a number of undesirable effects on power system components and loads. Te effects can be overeating of transformers, cables, motors, generators and capacitors connected to te same power supply wit te devices generating te armonics. Electronic displays and ligting may flicker, circuit breakers can trip, computers may fail and metering can give false readings [0]. According to [] wen te level of distortion in te supply voltage approaces 0%, te duration of te service life of equipment significantly reduces. III. METHODOLOGY One undred and forty-eigt (48) distribution transformers wit ig neutral currents were identified and purposively selected for te study. Capacities of te (3) Fig : A sample of ow load factors were determined A. Analysis of Hig Neutral Current in Typical Distribution Transformers In tis section, armonic and sinusoidal currents (fundamental) in a typical transformer neutral conductor are examined. Even toug te focus of te study is on armonic loss evaluation in distribution transformers, it is important to understand ow fundamental and armonic current contribute to ig neutral current. Te section also gives an overview of ow unbalanced load can result in significant power losses. Figure sows a sample of a tree pase load profile of a 500 kva transformer. Te current in te transformer neutral is also sown. Ordinarily, it would be tougt tat te current in te neutral represents te vector sum of te tree pase loads. However, analysis of te neutral current sowed tat te current was composed of fundamental and ISBN: ISSN: (rint); ISSN: (Online) WCE 06

3 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. triplen armonic currents, see Fig 3. Triplen armonics are zero sequence armonics tat are in tird multiples of te fundamental. It is important to note te variations in bot te fundamental and te triplen armonic content. Wereas te coefficient of variation of te fundamental is 43%, te variation in te triplen armonics is 7.8%. As can be seen from (4) and (5), power losses under balance condition are limited only to te tree pases of te distribution lines. However, under unbalanced condition, current in te neutral results in additional losses. Te losses in te neutral can be very significant depending on te circuit lengt of te distribution system. Te low variation in te triplen armonics, as sown in Fig 3, is explained by te fact tat te triplen armonics in te neutral conductor are not affected by balanced or unbalanced load conditions. Te variation may be due to normal load canges experienced by distribution system. resence of te triplen armonics results in significant power losses and tis is considered in te next section. Fig : Tree ase Load rofile and Corresponding Neutral Current in a 500kVA transformer Fig 3: Fundamental and 3rd Harmonic Current in te Neutral Conductor Te ig fundamental current and its large variations can be explained by unbalanced load and load variations or canges in te tree pase supply. Hig fundamental current in te neutral is undesirable as it results in power losses. Te most effective way of reducing fundamental current in te neutral is to ensure tat te loads are almost balanced across te tree pase supply. As an example, (4) and (5) can be used to illustrate power losses in distribution lines in balanced and unbalanced load conditions. Balanced condition, line_ losses 3 R f I (4) Unbalanced condition, loss I I I3 R f I N RN (5) were Rf is resistance per pase of a distribution line in Ω/km; I, I, I3 are te tree pase currents drawn by te load; IN is te current in te neutral under unbalanced condition and RN is te resistance of te neutral conductor in Ω/km. Under balanced condition, I=I=I3. B. Transformers Losses in Harmonic Environment Generally, transformers are manufactured suc tat minimum losses occur in rated voltage, rated frequency and sinusoidal current. However, in armonic environment, te load current is no longer sinusoidal. Te distorted load currents, resulting from armonics, cause extra losses in transformers. According to [, 3], transformer losses are categorized as no-load loss; load loss; and total loss. Tis is sown in (6) below. (6) T NL were T is te total loss, NL is te No-load loss and is te Load-loss. No-load loss is due to te voltage excitation of te transformer core. In most power systems, te armonic distortion of te system voltage THDv is well below 5% and te magnitudes of te voltage armonics components are small compared wit te fundamental component. Terefore, armonic effects on no-load losses are often neglected. However, if THDv is not negligible, losses under distorted voltages can be calculated based on ANSI-C.7-90 standard wit (7) V ms M ec (7) Vrms Were Vrms and Vrms are 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. Load loss () is subdivided into IR (omic) loss resulting from load current and dc resistance of te windings in watts and total stray loss TSL caused by electromagnetic flux in te windings, core, core clamps, magnetic sield, enclosure or tank walls. Te total load loss can ten be stated as (8) I R Te total stray loss comprises winding eddy loss EC in watts and oter stray losses OSL in clamp, tanks etc in TSL ISBN: ISSN: (rint); ISSN: (Online) WCE 06

4 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. watts, given as TSL (9) EC OSL Substituting (9) into (8) gives (0) I R EC Transformer rated IR losses can be calculated as follows K I R I () rated OSL rated R I Rrated were K = for single pase transformer; K=.5 for tree pase transformer; I-rated is te rated transformer current at te primary side; I-rated is te rated transformer current at secondary side. R and R are te winding resistance of te transformer at te primary and te secondary side respectively. I I rated kva () rated 3 V kva 3 V (3) were kva is te rated power of te transformer; V and V are rated primary and secondary voltage of te transformer respectively. According to IEEE C57.0, for oil immersed transformer, eddy current loss can be estimated using te following relation (4) EC TSL In armonic environment, te eddy current loss is increased by a factor FHL and oter stray losses also increased by a factor FHL-STR. Accordingly, in te presence of armonics, (0) can be modified and expressed as F F (5) I R HL EC HLSTR were I I F HL (6) I I I 0.8 I F HLSTR (7) I I Were I is te armonic current (A) at te t armonic order and I is te fundamental current (A). OSL C. Transformer Capacity Evaluation in Harmonic Environment Overeating of transformers is one of te effects of armonics. Accordingly to ensure tat te transformer is working witin its economic lifespan, te rated power of te transformer needs to be de-rated in armonic environment. Tis means te imum permissible current of transformer in armonic load must be determined. Te equation tat applies to linear load conditions is: R pu pu pu (8) ECR OSL R Were, - R is rated load losses in per unit (pu); EC-R is rated eddy current loss in per unit (pu), OSL-R is rated oter stray loss in per unit (pu). Note tat te base current is rated current and base loss is te I R loss at rated current in watts. A general equation for calculating te transformer losses wen supplying a armonic load is given follows: R pu I pu F pu F pu (9) HL EC R HLSTR OSL R Accordingly, from (9) te imum permissible load current of transformer under armonic condition can be determined from (0) below. I R pu F pu F pu pu (0) HL EC R HLSTR OSL R IV. CASE STUDY IN ECG DISTRIBUTION SYSTEM As mentioned above, tis case involved evaluation of armonic losses in 48 distribution transformers. Te capacities of te transformers in armonic environment were also determined. In tis section, results and discussions of te studies are presented. Figure 4 below sows a sample of te armonic load measurement obtained from te power disturbance analyzer. As can be seen in Fig 4, te THD of 34.3% is mainly due to te 3rd armonic. ower losses resulting from tese ig armonic contents were evaluated using (0) and (9). ISBN: ISSN: (rint); ISSN: (Online) WCE 06

5 0.3 % Of Fund roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. Event: 7 Of 5 Duration: Cycles Event Triggered by Time Event Trigger Cycle Time: 0/0/000 :4:.763 Input: IHD Amps Cycle: Amps Cycle Waveform Total Harmonic Distortion 34.3 % Odd Contribution % Even Contribution 8.48 % RMS Of Fundamental 44.9 A RMS Of Fund + Harm 46.3 A K Factor 9.8 % O f % O f Harm Fund Amps Angle Harm Fund Amps Angle Fund Harmonic Fig 4: A sample of te armonic measurement Table - Losses Estimation at Full Load Rating of te Transformers For ease of computations, Microsoft Exel formatted wit te above formulae were used, refer to Table. From Table, te total load loss () at full load rating of te transformers witout armonic effect is = 63,693 Watts. Based on te rated load losses of te transformers, te average load losses of te transformers at estimated average load on te transformers were determined. Te results of te average load losses are sown in Table. From Table, te total average load loss (-A) of te transformers witout armonic effect is -A =55, 674 Watts In armonic environment, (5) applies. Accordingly, from Table 3, total load loss (+H) of te transformers in presence of te armonics is +H =,09, 806 Comparing -A and -H, it can be seen tat operating transformers in armonic environment can increase te load losses to about 00%. ISBN: ISSN: (rint); ISSN: (Online) WCE 06

6 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. Table - Losses calculation at Estimated Average Load on te Transformers witout Harmonics Table 3 : Losses Calculation at Estimated Average Load on te Transformer in te resence of Harmonics A. Cost of Harmonic Energy Loss Te cost of energy resulting from te total armonic power losses can be calculated as follows [4]: F B H A 8760 Were, F= loss of load factor; B=Cost of energy in Gana Cedis (GHȼ 0.67/kW) and 4x365=8760 rs in a year Loss of Load Factor for urban systems [4]: = 0.3 Load _ Factor 0.7 Load _ Factor From a simple spreadseet (see Table 4) calculation based on te above formulae, te annual cost of energy resulting from te armonic power losses is GHc,0,395./year (US$ 555,098.78/year) ISBN: ISSN: (rint); ISSN: (Online) WCE 06

7 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. Table 4 : Estimation of Cost of Energy Table 5: Estimation of Capacity Loss due to Harmonics ISBN: ISSN: (rint); ISSN: (Online) WCE 06

8 roceedings of te World Congress on Engineering 06 Vol I WCE 06, June 9 - July, 06, London, U.K. B. Harmonic Effect on Capacities of te Transformers Te imum allowable currents, in per unit, for decreasing te rated capacities of te transformers due to armonic effects were determined. Te de-rated values of te capacities are sown in Table 4. Te overall capacity loss of te transformers is also sown in Table 4. As can be seen, te total capacity loss of te transformers is approximately 0 MVA. It is important to note tat tis loss relates only to te 48 distribution transformers, representing.6% of total number of distribution transformers in ECG distribution network. Considering te current power crisis in te country, losing 0 MVA capacity to armonics can be very disturbing, especially wen solutions to armonic problems can be very simple and inexpensive. V. CONCLUSIONS Te study as analysed te contents of ig neutral currents and investigated te main effects of armonics on distribution transformers. Te study terefore, concludes as follows:. Triplen Harmonics are most often responsible for ig neutral currents in distribution transformers. Tey represent more tan 70% of te RMS current in te neutral conductor.. Variation of te fundamental current in te neutral conductor is larger compared wit variation of te triplen armonic current. Tis variation is mainly due to unbalanced load conditions and frequent load canges in te tree pase supply. [3]. Reducing Tecnical and Non Tecnical Losses in te ower Sector, 009. Background Study for te World Bank Group Energy Sector Strategy July 009 [4]. ECG must ceck 'waste' in te system. Online source, date access: 7 t August, 05. Available at CitiFm 7t Aug, 05. [5]. Te Managing Director of te Electricity Company of Gana lanning letter for 05 Budget. [6]. M. Davudi, S. Torabzad, B. Ojagi, 0: Analysis of Harmonics and Harmonic Mitigation Metods in Distribution Systems. Australian Journal of Basic and Applied Sciences, 5(): , 0. ISSN [7]. Nandita Dey and A.K.Cakraborty, 03: Neutral Current and Neutral Voltage in a Tree ase Four Wire Distribution System of a Tecnical Institution. International Journal of Computer Applications ( ) Volume 7 No.3, May 03 [8]. Browne, T.J. (006). Experience in te Application of IEC/TR to Harmonic Allocation in Transmission System. 4st International Conference on Large Voltage Electric ower System. aris, CIGRE. [9]. Caudari,. (04). Novel ways for ower System Harmonic Mitigation for Tidal power plant. Online source, date accessed: t February, 03. Avalaible at ijsae.in/ijsaeems/index.pp/ijsae/article/view/97 [0]. Mueller, A. (0). Switcing enomena in Medium Voltage Systems - Good Engineering ractice on te Application of Vacuum Circuit-Breakers. etroleum and Cemical Industry Conference Europe Conference roceedings (CIC EUROE). Rome: IEEE. []. Sankaran, C. (00). ower Quality. CRC ress C. Online source, date accessed: 3t November, 04. Available at ttps:// []. ANSI/IEEE C (R987), American National Standard Terminology for ower and Distribution Transformers. [3]. ANSI/IEEE C , American National Standard Test Code for Dry-Type Distribution and ower Transformers. [4]. ECG. (0). Electricity Company of Gana-Gana Energy Development & Access. Accra- lanning Manual : Electricity Company of Gana, Engineering Directorate 3. Wereas te fundamental current can be reduced by load balancing across tree pase supply, te triplen armonics cannot. To reduce te triplen armonic currents, armonic suppressing devices are required. 4. Harmonics can increase losses in distribution transformer to more tan 00%. Tis represents a major cost in te delivery of electric energy. Te study as sown tat a total cost of energy of GHc,0,395./year (US$ 555,098.78/year) can be wasted troug armonic effects. 5. Harmonic effect de-rates capacity of transformers. In te study, armonic effect reduced te total capacity of te transformers by 7%. In oter words, te total capacity of te transformers ad to be reduced by 0 MVA to ensure te transformers are working witin teir economic life span. REFERENCES []. Raúl Jiménez, Tomás Serebrisky and Jorge Mercado, 04: Sizing Electricity Losses in Transmission and Distribution Systems in Latin America and te Caribbean. ublised by Inter-American Development Bank. []. Gana Sustainable Energy for All Action lan, 0. Online source, date access: 7 t September, 05. Available at energycom.gov.g/files/se4a- GHANA%0ACTION%0LAN.pdf ISBN: ISSN: (rint); ISSN: (Online) WCE 06