HARMONIC EFFECTS ON DISTRIBUTION TRANSFORMERS AND NEW DESIGN CONSIDERATIONS FOR K-FACTOR TRANSFORMERS A Thesis presented to the Department of Electrical Engineering University of Moratuwa - Sri Lanka In Partial Fulfillment of the requirement for the Degree Master of Engineering in Electrical Engineering by N.R. JAYASINGHE Thesis Supervisors Prof. J R Lucas K.B.I.M Perera Department of Electrical Engineering Faculty of Engineering University of Moratuwa Sri Lanka December 2003 7 9 5 6 6 Abstract This paper presents the effects of harmonic distortion of load current & voltages on distribution transformers, the standard ways of calculating the harmonic effects & design & development of K Factor transformer, which can operate under a specific harmonic environment. The usage of non-linear loads on power systems has increased the awareness of the potential reduction of a transformer's life due to increased heat losses. The performance analysis of transformers in a harmonic environment requires knowledge of the load mix, details of the load current harmonic content & total THD. The additional heating experienced by a transformer depends on the harmonic content of the load current & the design principals of the transformer. Both No load & Load losses are effected by the presence of harmonics in load currents. But the variation in load losses contributes more to excessive heat generation in distribution transformer. Increment in no load losses in a distribution transformer due to harmonics is less compared to the load loss but it has a significant contribution to the capitalization cost when operating in longer term. The load loss components get affected by the harmonic current loading are the I2R loss, winding eddy current loss & the other stray losses. The methods of predicting extra losses are described in this thesis and standard ways of de-rating transformers are also discussed. The K-FACTOR method is an approximation of the total stray loss heating effect, including the fundamental and harmonic contributions & finally new design techniques for K-F ACTOR transformers are discussed. In designing of K-F ACTOR transformers different design techniques like parallel conductor arrangement for windings, lower flux density & introduction of static shields are discussed & the estimated results are compared with actual implemented results. DECLARATION I hereby declare that this submission is my own work and that, to the best of my knowledge and behalf, it contains no material previously published or written by another person nor material, which to substantial extent, has been accepted for the award of any other academic qualification of an univer~ty or institute of higher learning except where acknowledgement is made in text. December 2003 ~ Professor J.R. Lucas Project Supervisor December 2003 - ~9 K.B.~~ra Project Supervisor ~ December 2003 Acknowledgement I express my sincere gratitude to Prof. Rohan Lucas and Mr. Manjula Perera For all the encouragement, guidance and support given throughout to make this task a success. I also would like to thank Mr. Kosala Gunawardana, Factory Manager, Lanka Transformers Ltd, Transformer plant and Dr. Sarath Perera and Dr. Lalith Wickramaratne for the support given in numerous ways. Finally A big thank goes to my wife Chamari for finding me free time to do the research and baby Rumeth for being a quite and nice during that period. l .. --. CONTENTS PAGE 1. CHAPTER 1- INTRODUCTION 1 1.1 Non linear loads and transformer overheating 1 1.2 General Limitations and effects of transformer overheating 2 1.2.1 Short term risks 3 1.2.2 Long term risks 1.3 Effects of non sinusoidal voltages and currents 1.4 Transformer harmonic concerns 1.5 Review of transformer losses 1.6 Harmonic effects on transformer losses .1 4 4 4 5 6 2. CHAPTER 2- METHODS O F HARMONIC EFFECTS EVALUATION 8 2.1 Methods of calculation 8 2.1.1 Basic design data method 9 2.2 Harmonic loss factors 11 2.3 Calculation of temperatures • 14 2.4 Calculation of de-rating fac.tor 15 3. CHAPTER 3- INTRODUCTION TO K-FACfOR TRANSFORMERS 17 3.1 Introduction to the concept of K-factor 17 3.2 New design considerations forK-factor transformers 18 3.2.1 Design techniques 18 3.3 Forward design techn ique 19 3.4 Review on winding eddy current losses 20 3.5 Considerations for transformer windings ~ 21 3.6 Estimation of losses under non linear conditions 22 3.7 Stray losses in transformers 23 3.8 Study of stray loss variation with the transformer capacity 26 3.9 Stray loss control 27 II 3.10 Effect on insulation class 3.11 Impact on neutral 3.12 Usage of electrostatic grounding shields 4. CHAITER 4- RESULTS 4.1 Case study I 4.2 Case study II 5. CHAPTER 5- CONCLUSION Bibliography ~ Ill j --- 29 30 30 32 32 36 40 42 LIST OF SYMBOLS 8a =Ambient temperature (oC) 8g =Hottest-spot conductor rise over top-oil temperature (oC) Og-R =Hottest-spot conductor rise over top-oil temperature under rated conditions (oC) 081 =Hottest-spot HV conductor rise over lop-oil temperature (oC) Op,l-R =Hottest-spot HV conductor rise over top-oil temperature under rated conditions oC) eh =Ultimate (steady state) hot spot temperature (oC) 01o =Top-oil rise over ambient temperature (oC) 0 11.>-R =Top-oil rise over ambient temperature under rated conditions (oC) $2 = Power factor angle ~eo, =Initial top oil temperature rise ~eon =Top oil temp. rise at end of nth interval ~Oo(n· l) =Top oil temp. rise at end of (n-l)th interval ~8or =Top oil rise at rated current ~8ot =Top oil temp. rise after time t . 1 ~eou =Ultimate top oil temp. rise corresponding to load during time t ~Ooun =Ultimate top oil temp. rise in nth interval 68our =Ultimate top oil temp. rise corresponding to rated current A8tct =Temperature difference between hot spot and top oil ~Otct r =Temperature difference between hot spot and top oil at rated current p = Number of wound legs ... a ,aJ =Thickness of conductor in radial direction Bm = Peak Flux density .. c" =Axial saray loss constant for the winding material a t 75 OC FHL =Harmonic loss factor for winding eddy currents FHl.-s rR =Harmonic loss factor for other stray losses F 1R =Harmonic loss factor for winding I2R loss f =Frequency /,, = Fill factor in axial direction h = Harmonic order = RMS load current lt = RMS fundamental load current (ampere) lh = RMS current at harmonic "h" (ampere) ~ JR = RMS fundamental current under rated frequency and rated load conditions (ampere) I H-R =High voltage (HV) rms fundamental line current under rated frequency and rated load conditions (amperes) IV IL-R =Low voltage (LV) rms fundamental line current under rated frequency and rated load conditions (amperes) Ir =Sum of HV and LV line currentc; K h = hysterisis constant K e =eddy current constant K r = Form factor L = Loss of Life in per unit days N2 =No_ of turns in LV coil n,..,d =Number of layers in radial direction P = PR loss portion of the load loss (watts) P~:e. =Winding eddy-current loss (watts) PEC-R =Winding eddy-current loss under rated conditions (watts) PFC-o =Winding eddy-current loss at lhe measured current and the/ower frequency ~atts) - PK = nominal load losses PLL =Load loss (watts) Pu,-R =Load loss under rated condition (watts) PNJ =No load loss (watts) Po = idle losses Posr =Other stray loss (watts) Posr-R =Other stray loss under rated conditions (watts) PrsL-R =Total stray loss under rated conditions (watts) Pv =Losses at actual loading LR = Loss ratio = Load Joss at rated current -- -o load loss R =DC resistance (ohms) Rt =DC resistance measured between two HV terminals (ohms) R2 =DC resistance measured between two LV terminals (ohms) S = Rating of the transformer =time interval of application of specific load b- t, = period under consideration T Tp =Peak duration 'to =Oil time constant Vr v,.. =% resistance voltage at full load =%leakage reactance voltage at full load VR =% voltage regulation x =Oil exponent y = Winding exponent Z = Impedance voltage v ~