Energy Savings with an Energy Star Compliant Harmonic Mitigating Transformer

Transcription

1 Energy Savings wit an Energy Star Compliant Harmonic Mitigating Transformer Tony Hoevenaars, P.Eng, Vice President Mirus International Inc. Te United States Environmental Protection Agency s Energy Star program as gained tremendous popularity since it s inception in 199. Te Energy Star label is now recognized by many as a sign of energy efficiency as it appears across more tan 30 different product areas suc as computer equipment and ouseold appliances. As society continues to work towards environmentally friendly and self sustaining energy solutions, tis trend will certainly continue. In 1998 Energy Star expanded its program to include Commercial and Industrial (C&I) transformers. A voluntary efficiency standard developed by te National Electrical Manufacturers Association (NEMA), known as NEMA TP , Guide for Determining Energy Efficiency for Distribution Transformers, was adopted for te program. States suc as Massacusetts, Wisconsin, Minnesota, California, New York and most recently Oregon and Hawaii, ave establised NEMA TP 1 in teir state minimum efficiency standards or now include TP 1 as a provision in teir commercial energy codes 1. It is important to note tat te NEMA TP-1 standard was based upon linear loading and is optimized for 35% load levels. Tis criteria as merit wen te load is, in fact, linear because surveys ave sown tat many distribution transformers in Nort America are only ligtly loaded. However, if te transformer is more eavily loaded and/or its load is primarily nonlinear, designing to optimal efficiencies at 35% linear load may actually result in iger losses and lower efficiencies. Since non-linear loads, wic include computers, variable frequency drives and oter power electronic equipment now constitute a very large component of today s load, simply meeting NEMA TP 1 and Energy Star compliance is often not a sufficient means of assuring optimal efficiency levels are met. Tis is because non-linear loads can very significantly increase armonic losses and NEMA TP 1 was not intended to address tese additional losses. As a result, transformers designed for non-linear loads, suc as K-rated and Harmonic Mitigating, are specifically exempted in NEMA TP 1. And since most loads today are nonlinear, tis means tat meeting TP 1 and Energy Star compliance, in itself, will not ensure tat optimal efficiency is acieved in many actual applications. To address tis, Mirus International as developed a line of Harmonic Mitigating Transformers (HMT s) tat ensure optimal efficiency levels are reaced wit eiter linear or non-linear loading and at ligtly loaded or eavily loaded levels. Tis is accomplised troug two principle strategies: (i) transformer windings wic are configured suc tat critical armonics are cancelled witin te transformer secondary and (ii) linear load efficiencies wic meet NEMA TP 1 levels, not only at 35% load, but in te full operating range from 35% to 65%. NEMA TP Transformer Efficiency Standard NEMA TP-1 defines minimum efficiency levels for transformers wit linear loads at 35% loading. Tis criteria was cosen based on surveys wic indicated tat te average loading on distribution transformers in Nort America is about 35%. Te efficiency limits vary by transformer size but are generally in te 98% range. In coosing 35% loading, NEMA TP-1 puts extra empasis on noload (core) losses rater tan load (copper) losses. Wit empasis on no-load losses, NEMA TP-1 does not adequately address armonic losses and terefore, specifically exempts transformers wic service non-linear loads. Te following are taken from its exemption list: c. Drives transformers, bot AC and DC d. All rectifier transformers and transformers designed for ig armonics g. Special impedance, regulation and armonic transformers Te reason tat transformers designed for ig armonics are exempted is tat armonics will MIRUS INTERNATIONAL INC invader cres., unit 1, mississauga, Ont., CANADA l5t k6_ CALL: TO MIRUS OR FAX: [ ]

2 Energy Savings wit an Energy Star Compliant HMT dramatically increase load losses (I R and eddy current) and ave very little effect on no-load losses. Terefore, NEMA TP-1 s empasis on noload losses can be counter productive wen supplying non-linear loads. To meet te efficiency limits, a manufacturer must optimize for lower noload losses, often at te expense of iger load losses. For example, one common way of reducing no-load losses is to reduce te flux density by adding more steel to te transformer s core. Wit a larger core, eac turn of te transformer s windings must cover a larger circumference. Te extra lengt of copper winding adds resistance wic increases I R load losses. Tis can significantly INCREASE losses and REDUCE efficiencies wen supplying non-linear loads at load levels above 35%. Wy Design for Peak Efficiency over a Load Range of 35% to 65%? NEMA TP 1 s empasis on linear load efficiency under ligtly loaded conditions is justified only if te power system is indeed ligtly loaded and consists primarily of linear loads. If te load is non-linear or te system is more eavily loaded, optimizing efficiency at 35% can prove to be an inferior design resulting in iger losses rater tan lower ones. Tis problem can be averted if te transformer is designed for optimum efficiency over a wider load range and if its windings are configured to reduce armonic losses. Figure 1 provides linear load efficiency curves for a standard 75 kva Energy Star compliant TP 1 delta-wye transformer and for a 75 kva Energy Star compliant Harmony-1E Harmonic Mitigating Transformer. Bot meet te % Efficiency 100% 99% 98% 97% 96% 95% 94% 93% 9% Linear Load Efficiency Comparison 75kVA Transformers % Loading Figure 1: Linear load efficiency comparison TP1 H1E NEMA TP 1 efficiency limit of 98% at 35% linear load but te efficiency of te standard unit drops off rapidly under more severe loading. By maintaining TP 1 efficiency over te entire range from 35% to 65%, te Harmony-1E is more efficient at all load levels above 35%. Tis improved performance is magnified wen te load is non-linear. In Figure, non-linear load efficiencies are sown using te same transformers but wit a K-9 load profile (Itd = 80%) wic is typical of computers and oter 10V power electronic equipment. Under tis loading, te Harmony-1E provides significantly more energy savings especially as te load increases on te transformer. % Efficiency 100% 99% 98% 97% 96% 95% 94% 93% 9% Nonlinear Load Efficiency Comparison 75kVA Transformers H1E % Loading Figure : Non-linear load efficiency comparison TP1 How Harmonics Increase Transformer Losses Harmonics generated by non-linear loads will dramatically increase te losses in a conventional delta-wye distribution transformer. Tese added losses increase te montly utility bill, indirectly add to environmental pollution and can sorten te transformer life by increasing its operating temperature. To address te overeating transformer problem, K-rated delta-wye transformers are now frequently used in non-linear load applications. Tese transformers are designed to witstand te additional eat generated by te armonic losses but will actually reduce tese losses only marginally. Harmonic Mitigating Transformers, on te oter and, substantially reduce armonic generated losses by using winding configurations tat promote armonic flux cancellation.

3 Energy Savings wit an Energy Star Compliant HMT 3 Transformer loss components include no load (P NL ) and load losses (P LL ). Te no load losses are transformer core losses. Tey are essentially independent of te load current and its armonic content. Furtermore, no load losses are affected only marginally by voltage armonic distortion and terefore, can usually be neglected wen determining te effect of armonics on transformer losses. Load losses owever, vary wit te square of te load current and are very significantly affected by armonic content. Load losses consist primarily of I R copper losses (P R ) and eddy current losses (P EC ). Harmonics increase tese losses in te following ways: 1. Copper Losses, I R Harmonic currents are influenced by a penomenon known as skin effect. Since tey are of iger frequency tan te fundamental current tey tend to flow primarily along te outer edge of a conductor. Tis reduces te effective cross sectional area of te conductor and increases its resistance. A iger resistance leads to iger I R losses. Proximity effect between adjacent conductors compounds tis problem by furter distorting te current distribution in te conductors. Figure 3: Skin effect in a conductor max PEC = PEC R 3 I = 1. Eddy Current Losses Stray electromagnetic fields will induce circulating currents in a transformer s windings, core and oter structural parts. Tese eddy currents produce losses wic increase substantially at te iger armonic frequencies. Te relationsip is as follows: P EC = P max EC R = 1 I Were: P EC = Total eddy current losses for non-linear load P EC-R = Eddy current losses at rated linear load I = Ratio of rms current at armonic to full load current of transformer = armonic # For linear loads, eddy currents are a fairly small component of te overall load losses (approx. 5%). Wit non-linear loads owever, tey become a muc more significant component, sometimes increasing by as muc as 15 to 0x. In addition to increasing conventional losses in a transformer, pase-to-neutral non-linear loads will also produce excessive primary winding circulating currents. Te 3 rd and oter odd multiples of te 3 rd armonic (referred to as triplens) are zero pase sequence in nature and as suc become trapped in te primary delta windings of conventional and K-rated transformers. I R and eddy current losses increase as tese currents circulate in te transformers primary windings. How HMT s Reduce Harmonic Losses Harmonic Mitigating Transformers save energy by reducing losses in te following ways: Figure 4: Eddy currents in te steel laminations of a transformer 3 1. Zero pase sequence armonic fluxes are cancelled by te transformers secondary windings. Tis prevents triplen armonic currents from being induced into te primary windings were tey would circulate. Consequently, primary side I R and eddy current losses are reduced.. Multiple output HMT s cancel te balanced portion of te 5 t, 7 t and oter armonics

4 Energy Savings wit an Energy Star Compliant HMT 4 witin teir secondary windings. Only residual, unbalanced portions of tese armonics will flow troug to te primary windings. Again I R and eddy current losses are reduced. 3. HMT s are designed to be igly efficient at 60Hz as well as at armonic frequencies. Energy Star compliant models meet NEMA TP-1 energy efficiency minimums at 35% loading. Tis is typically acieved by reducing core losses but not at te expense of iger copper losses. Energy Savings Comparison Figure 5 provides an example of te energy savings tat can be realized wen HMT s are used in lieu of conventional or K-rated transformers. A K-9 load profile, typical of a ig concentration of computer equipment (Itd = 80%), was selected for te analysis. Losses were calculated for various types of 75 kva transformers at varying load conditions. In te grap, Conv is a conventional delta-wye transformer, K-13 is a K-13 rated delta-wye and H1E is a Harmony-1E single output Energy Star compliant HMT. Calculating Transformer Losses under Non-Linear Loading 4 Calculating transformer losses under non-linear loading is a fairly complex process. Te following procedure is commonly followed: 1. Determine te core loss at fundamental (60 Hz) frequency - P NL.. Calculate I R losses in bot te primary and secondary windings - P R. a) Determine te AC resistance at te fundamental frequency for te specific wire size and material used in te primary and secondary windings. b) Determine te effective AC resistance due to skin effect at eac of te armonic frequencies. c) Calculate te I R losses for eac armonic at te load K-factor cosen and te percent loading of te transformer. d) Total all te I R losses 3. Calculate eddy current losses in bot primary and secondary windings P EC. a) Determine te eddy current loss at te fundamental frequency (P EC-1 ). Tis is typically 5% of te I R loss at te fundamental frequency. b) Calculate I for eac armonic at te load K-factor cosen and te percent loading of te transformer. c) Calculate total eddy current losses by te following formula, P EC = P 4. Total all loss components, P L = P NL + P R + P EC max EC 1 = 1 I Losses (W) Nonlinear Load Loss Comparison 75kVA Transformers Conv. K13 H1E % Loading Figure 5: 75 kva Transformer losses at various loading conditions wit non-linear K-9 load profile. Te cart sows ow energy savings become more and more substantial as a transformer s load increases. Tis is logical since it is te load losses wic are most affected by te armonic currents and tese are proportional to te square of te current (I R and I ). Figure 6 furter empasizes ow transformer efficiencies are affected by non-linear loading. It compares te performance of various types of transformers wit linear loading (K-1) and nonlinear loading (K-9). Te efficiencies of te conventional and K-13 transformer are muc lower wen tey are subjected to a load wit a K-9 profile, especially under te eavier loading conditions. Determining te amount of energy savings associated wit a reduction in armonic losses requires information on te Electric Utility rate and te load s operating profile. Tese parameters can vary quite substantially depending upon te location of te facility and te specific application.

5 Energy Savings wit an Energy Star Compliant HMT 5 Efficiency 100% 99% 98% 97% 96% 95% 94% 93% 9% 91% H-1E (linear) H-1E (nonlinear) K13 (linear) Conv. (linear) K13 (nonlinear) Conv. (nonlinear) Efficiency Comparison 75 kva Transformers 90% % Load Figure 6: Energy Efficiencies for various types of 75 kva transformers supplying linear (K-1) loads and non-linear (K-9) loads under varying load conditions. Table 1 sows te energy savings tat can be realized wen a Harmony-1E HMT is compared wit a typical K-13 transformer. As in te previous examples, te transformers are 75 kva and te non-linear load profile is tat of a typical K-9 load. Te monetary savings are based on te equipment operating 1 ours per day, 60 days per year at an average Utility rate of $0.07 per kwr and assumes tat additional cooling energy is required by te building s air conditioning system to remove te eat produced by te transformer losses. Te calculation used is sown below te table. Tis scenario could be typical of an office environment wit a ig concentration of computer loads and wit te transformer located in air conditioned space. Te requirement to cool te eat produced by te transformer s losses is typically 30% to 40% of te power in te losses (tus te 1.35 multiplier in calculation of $/yr Savings). Paybacks were calculated based on estimated transformer costs and would result in recovering te Harmony-1E premium many times over based on te transformer s life expectancy of 30 to 40 years. Table provides anoter example. In tis case, a lower armonic content K-4 load profile was used wit te equipment operating 4 rs/day, 365 days a year and te transformer located in air conditioned space. An example of suc a location migt be a Broadcasting Facility or Data Center. As can be seen, paybacks are even more attractive. Transformer % Losses (Watts) Annual Consumption Transformer Payback on Load NLL LL Total (kwrs) ($ / yr) Cost (Est.) HMT Premium 35% ,866 $365 K-13 50% ,478 $518 65% ,787 $736 $, % ,453 $1,555 35% ,05 $ yrs Harmony-1E 50% ,674 $53.9 yrs $3,530 65% ,606 $341.0 yrs 100% ,109 $ yrs Table 1: HMT energy savings and payback estimate comparing a 75 kva HMT to a K-13 transformer in a typical office environment wit a ig concentration of computer equipment Annual Consumption = (Total losses in kw) x (rs/day) x (days/yr) + (NL loss in kw) x (4 rs/day) x (365 days/yr)) $/yr Savings = (H1E Annual Consumption K13 Annual Consumption) x 1.35 x (rate in $/kwr) Transformer % Losses (Watts) Annual Consumption Transformer Payback on Load NLL LL Total (kwrs) ($ / yr) Cost (Est.) HMT Premium 35% ,381 $79 K-13 50% ,48 $1,180 65% ,381 $1,737 $, % ,681 $3,844 35% ,458 $41.1 yrs Harmony-1E 50% ,30 $ yrs $3,530 65% ,958 $ yrs 100% ,04 $1, yrs Table : HMT energy savings and payback estimate comparing a 75 kva HMT to a K-13 transformer in a typical Broadcasting Facility or Data Center

6 Energy Savings wit an Energy Star Compliant HMT 6 Summary In summary, te inerent ability of Harmonic Mitigating Transformers to cancel armonic currents witin teir windings can result in quantifiable energy savings wen compared wit te losses tat would exist if conventional or K- rated transformers were used. If we consider te average premium cost of an HMT over a K-13 transformer, te typical payback in energy savings is 1 to 4 years wen loading is expected to be in te 50% to 65% range. Tis, in itself, can be justification for te use of HMT s but wen consideration is also given to te power quality improvement tey provide by eliminating voltage distortion in te form of flat-topping, teir use becomes even more easily justified. For te most optimal energy efficiency design, Mirus Energy Star compliant Harmony-1E HMT meets NEMA TP-1 minimum efficiencies at not only 35% load but also across te entire operating range from 35% to 65%. In tis manner, energy savings can be assured not only at ligtly loaded conditions but also at more eavily loaded conditions weter te loads are armonic generating non-linear in nature or simply linear. References: 1. CEE Update, News for Stakeolders in CEE s Hig- Efficiency C&I Transformer Initiative, Marc 00. NEMA TP 1-00, Guide for Determining Energy Efficiency for Distribution Transformers, National Electrical Manufacturers Association, p 1 3. N. Moan, T. Undeland, W. Robbins, Power Electronics - Convertors, Applications and Design, Jon Wiley & Sons Inc., New York, 1995, pp T.S. Key, J.S La, Costs and Benefits of Harmonic Current Reduction for Switc-Mode Power Supplies in Commercial Office Building, IEEE Transactions on Industry Applications, Vol. 3, No. 5 Sept/Oct 1996, pp ANSI/IEEE C , Recommended Practice for Establising Transformer Capability Wen Supplying Nonsinusoidal Load Currents, American National Standards Institute