Electrical insulations are one of the basic parts of electrical machinery in any sizes and characteristics. Focusing on insulating, studies on the operation of industrial-electrical machinery came to the fact that the most important part of a machine is the Stator. This fact reveals the requirement for inspection of the electrical machine insulation along with the electromagnetic tensions. Therefore with respect to insulation system improvement of stator, the HV generator can be optimized. Dielectric parameters such as insulation thickness, spacing, material types, geometry of winding and slot are major design consideration. A very powerful method available to analyze electromagnetic performance is Finite Element Method (FEM) which is used in this paper. The analysis of various stator coil and slot configurations are used to design the better dielectric system to reduce electrical stresses in order to increase the power of generator in the same volume of core. These processes of optimization have been done according the proposed algorithm. In this algorithm the technical constraints have been considered. This paper describes the process used to perform classical design and improvement analysis of stator slot’s insulation with respect to objective function and constraints

A. Gholami

D. Azizi

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ETASR   Engineering, Technology & Applied Science Research  Vol. 1,  o. 3, 2011, 49 53  49    www.etasr.com  Azizi and Gholami: Optimum Scheme for Insulation System in HV Generator…    Optimum Scheme for Insulation System in HV Generator based on Electromagnetic Analysis  D. Azizi Dept of EE IUST Tehran, Iran azizi@iust.ac.ir A. Gholami Dept of EE IUST Tehran, Iran   Abstract—  Electrical  insulations  are  one  of  the  basic  parts  of electrical machinery in any sizes and characteristics. Focusing on insulating,  studies  on  the  operation  of  industrial electrical machinery came to the fact  that the most important part of a machine  is  the  Stator.  This  fact  reveals  the  requirement  for inspection  of  the  electrical  machine  insulation  along  with  the electromagnetic  tensions.  Therefore  with  respect  to  insulation system  improvement  of  stator,  the  HV  generator  can  be optimized.  Dielectric  parameters  such  as  insulation  thickness, spacing, material types, geometry of winding and slot are major design  consideration.  A  very  powerful  method  available  to analyze electromagnetic performance is Finite Element Method (FEM) which is used in this paper. The analysis of various stator coil and slot configurations are used to design the better dielectric system to reduce electrical stresses in order to increase the power of  generator  in  the  same  volume  of  core.  These  processes  of optimization have been done according the proposed algorithm. In this algorithm the technical constraints have been considered. This paper describes the process used to perform classical design and improvement analysis of stator slot’s insulation with respect to objective function and constraints.  Keywords  insulation; FEM; machine; magnetic; thermal  I.   INTRODUCTION  Safe  operation  of  electrical  machine,  particularly  high power  generators strongly  depends  upon the health of stator coil insulation. Industrial researches show that troubles initiated in the stator  winding insulation  are one of the primary  root causes of electric machine failures [1, 2 and 3].  It is shown in [4, 5] that 30 40 % of ac machine failures are stator related and also shown in [6] that 60 70 %  high voltage machine failures result from stator insulation troubles. The winding and core of stator  integrity  plays  essential  role  in  the  reliability  of  the alternator [7 11]. Thus, performing the optimal design consist of electromagnetic study with respect to thermal limits in core, winding and insulation of stator slots is necessary. To attain an improved design electromagnetic analysis with observe to real condition is performed which completes pervious researches.   The  reason  of  applying  the  method  is  investigating  the effects  of  electrical,  magnetically  and  thermal  stresses  on insulation parts. The main idea involves finding the optimum scheme  of  stator  slot’s  insulation  with  respect  to  possible stresses and constraints. FEM analysis is used to simulate the electric, magnetic and thermal field simultaneously. The main supremacies  of  the  simulations  in  comparison  with  other researchers are: •  Simulating ordinary rotation of rotor. •  Considering magnetic saturation of core. •  Considering of technical constraints. According to this method, various configurations of stator (winding,  core,  slot,  insulation)  for  operation  conditions  are proposed  to  investigate  the  possibility  of  improvement  high voltage  generator  characteristics  with  respect  to  proposed optimization algorithm.   II.  CLASSICAL DESIGN   M phase machine classical design has definite steps which  are  considered  in  this  section.  The  induced  KVA  by armature can be obtained from:  3ph 10 ) T φ f 2 (− ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅ = ph w I k m Q π                   (1) Where: φ: total flux, Iph: phase current, kw: winding factor, f: frequency, Tph: winding turn in phase The  equation  (1)  can  be  rewritten  as  (regarding  to 2s pnf = , ph z I I =  and ph mT z 2 =  )   3z 10 ) I )( φ (2 2− ⋅ ⋅ ⋅ ⋅ = w s k n Z p Qπ            (2)   Total  magnetic  flux  in  air  gape  is  called  magnetic loading which is used to calculate especial magnetic loading (Bav) as bellow: Total magnetic loading = φ p⋅              (3)               πDLφ p Ba = ν                             (4) Where:  D: diameter,  L: length. ETASR   Engineering, Technology & Applied Science Research  Vol. 1,  o. 3, 2011, 49 53  50    www.etasr.com  Azizi and Gholami: Optimum Scheme for Insulation System in HV Generator…      Total  current  loading  and  especial  current  loading introduced by the following equations: Total current loading =  Z Iz ⋅               (5) D πZ Iacz =                   (6)   By  replacement   and   from  equations  (4)  and (6) in equation (2), it can be shown that: = ⋅ =−3s αν 10 Dαc)n )( (πDLB2 2w k Q ππ  s Ln D2 3w2 ) 10 ck 11 . 1 (− ⋅ Β α π αν   (7) Where:  3w20 10 ck 11 . 1− ⋅ Β = α π αν C                          (8) kw is winding factor which is calculated by multiplying kc and kd. kc is assumed one and kd is obtained from equation (9).                                      2sin2sinaqqakd =                      (9) III.  CASE STUDY According to section 2, a generator with different insulation is  designed  as  the  case  study.  The  selected  generator  is synchronous,  three  phase  and  two  poles.  Rated  frequency, voltage and power are respectively 50Hz,   20 KV and 1 MVA. Wiring is form wound multi turn type and has many insulation layers  with  different  specifications.  In  this  study  the  turn insulation  and  the  strand  insulation  are  the  same,  i.e.  nylon type. Ground wall insulation type is PTEF and semi conductive coating is Si(c) with characters identified in table 1.  TABLE I.   ELECTRICAL AND THERMAL SPECIFICATIONS OF USED INSULATIONS  Quantities  Symbols & Dimensions    ylon insulation  PTEF insulation  Si(c) insulation heat capacity  C [J/(kg*K)] 1700  1420  700  young’s modulus E  [Pa] 2e9  3e9  170e9 Thermal expansion coeff. Α  [1/K] 280e 6  70e 6  2.6e 6 relative permittivity ε  4  2  11.7 thermal conductivity K [W/(m*K)] 0.26  0.19  130 density  Rho [kg/m^3] 1150  1190  2329 IV.  ELECTROMAGNETIC MODELS [12] Ampere’s law is the main part to  derive electromagnetic system equation.     tDJ B v EtDJ He∂∂+ + × + =∂∂+ = × ∇ σ σ        (10) Where:  •  E is the electric field intensity •  D is the electric displacement or electric flux density  •  H is the magnetic field intensity  •  B is the magnetic flux density  •  J  is the current density  •  Je is the externally generated current  •  σ is the electrical conductivity •  v  is the velocity Time variant harmonic field’s effect can be introduced by equations (11) and (12):                                      A B × ∇ =                   (11)                                 tAV E∂∂− −∇ =            (12) Ampere’s  law  is  rewritten  by  equations  (11)  and  (12) Combining  with  constitutive  relationships  B= 0(H+M)  and D=ε0E+P , as:             ( ) ( )( ) ( ) P j J V j A vM A A je ω ωε σ σ  ε ω ωσ+ = ∇ + + × ∇ × −− × ∇ × ∇ + −−010 02    (13) In which ω, ε0,  0, M and P respectively refer to Angular frequency,  Relative  permittivity,  Relative  permeability, magnetization vector and electric polarization vector. In the case of 2 dimensional plane, there are no variations in  z direction,  so  the  electric  field  is  parallel  to  z axis.  , therefore  V ∇ is written as − V/L, where  V is the potential difference  over  the  distance  L.  Now  these  equations  are simplified to:     ( )zezz zxyzP j JLVA j A vMMAω σε ω ωσ σ  + + =  − + ∇ + −− ∇ ∇ −−02 10 . .                                              (14) In  the  ax symmetric  case,  another  form  of  the  electric potential  gradient  has  been  used  (r 2πloop VV−= ∇ )  as  the electric field is only present in the azimuthally direction. The above equation, in cylindrical coordinates, becomes: ETASR   Engineering, Technology & Applied Science Research  Vol. 1,  o. 3, 2011, 49 53  51    www.etasr.com  Azizi and Gholami: Optimum Scheme for Insulation System in HV Generator…           [ ]( )P j JrVu V u j rzuruv rMMuzzurur z re looprrzωπσσ ε ω ω σ σ   ϕ + + =+ − +  ∂∂∂∂+    −− +∂∂∂∂∂∂∂∂ −− −22 .0.021010     (15)                  The dependent variable u is the nonzero component of the magnetic potential divided by the radial coordinate r, so that:                                     rAuϕ =                 (16)         The application mode performs this transformation to avoid singularities on the symmetry axis. V.  OPTIMIZATION ALGORITHM The optimization procedure is presented in the optimization algorithm (Figure 1).    Fig. 3.   Flow chart of the optimization procedure  VI.  SIMULATION AND RESULTS Following  the  development  of  the  study,  magnetic  field distribution  for  cylindrical  windings  with  two  layers  is simulated  (Figure  2).  Coupling  the  equations  makes  the capability  of  simulating  electrical  field  and  potential distribution as shown in (Figures 3, 4). Varying configuration can include winding’s geometry, slot depth, slot width, winding layers and material types of insulation. To study the effects of each varying on operation conditions some configurations are proposed.  It  should  be  mentioned  that  these  changes  of configuration  are  done  according  to  optimization  algorithm (Figure  1).  In  first  step,  cubic  windings  in  the  same  cross section are used as Figure 5. The results of simulations show that electric field distribution has changed but the maximum electric field value is constant. The electric field distribution between winding layers has reduced intensively which allows adding a more layer (Figure 6). This means that the operation power can be enhanced 50 %. The length and depth of slots are two other options which impress design parameters. For this purpose, in the final step, the width of slot is increased. This is required the reduction of core volume which causes relatively core saturation (Figure 7).   Fig. 4.   Magnetic field distribution and flux lines   Fig. 5.   Potential distribution in a sample slot ETASR   Engineering, Technology & Applied Science Research  Vol. 1,  o. 3, 2011, 49 53  52    www.etasr.com  Azizi and Gholami: Optimum Scheme for Insulation System in HV Generator…     Fig. 6.    Electrical field in a sample slot (mode 1)  Fig. 7.   Electrical field distribution for cubic winding (mode 2)  Fig. 8.   Electrical field distribution for cubic windings with three layers (mode 3)   Fig. 9.   Flux lines and magnetic distribution in generator with larger slot (mode 4) TABLE II.   MAXIMUM VALUES OF ELECTRICAL, MAGNETICAL AND THERMAL FIELDS IN ALL MODES     Electrical field (kV/mm) Magnetic flux density (T)  Temperature (˚ ˚ ˚ ˚C) Mode 1  2.77  1.5  78 Mode 2  2.77  1.5  79 Mode 3  2.97  1.5  87 Mode 4  1.86  1.73  112  Comparing  between  varied  configurations  indicates  that changing in the shape of winding geometry and insulation class are more effective. Thus, it can be claimed that the model of Figure 6 (mode 3) is the optimum configuration.   VII.  CONCLUSION The majority of high voltage machine failures result from stator  insulation  problems.  On  the  other  hand  electrical  and magnetic  stresses  are  the  important  factors  of  exhaustion  in generators. Therefore selection of proper  wiring scheme can decrease  these  stresses.  Different  schemes  with  regard  to different  construction  have  different  characters  and specification. The finite element electromagnetic and thermal analyses are useful to improve the configuration of stator which conquests  the  previous  problems  in  classical  design.  In  this paper, some proposed configurations presented and compared. It can be extracted from results that the configuration of Figure 6 is the optimum case with respect to mentioned algorithm. REFERENCES [1]  F.  T.  Emery,  D.  Pavlik,  “Electrostatic  field  analysis  of  high  voltage generator stator insulation systems”, IEEE Annual Report Conference on Electrical Insulation and Dielectric Phenomena, Vol. 2, pp. 510 – 513, 2000 [2]  M. Arshad, A. Khaliq, S. M. Islam, “Turbo generator stator winding condition  assessment”,  POWERCON  International  Conference  on Power System Technology, Vol. 2, pp. 1399   1403, 2004. ETASR   Engineering, Technology & Applied Science Research  Vol. 1,  o. 3, 2011, 49 53  53    www.etasr.com  Azizi and Gholami: Optimum Scheme for Insulation System in HV Generator…    [3]  S.  B.  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Cacciari, “A probabilistic insulation life model for combined thermal electrical stresses”, IEEE Transactions on Electrical Insulation Vol. EI 20 No. 3, 1985. [8]  G.  J.  Anders, J. Endrenyi,  G.  L.  Ford, G.  C. Stone, “A probabilistic model for evaluating the remaining life of electrical insulation in rotating machines”, lEEE Transactions on Energy Conversion, Vol. 5, No. 4, pp. 761 – 767, 1990. [9]  S.  B.  Pandey,  “Estimation  for  a  life  model  of  transformer  insulation under combined electrical and  thermal  stress”,  IEEE Transactions on Reliability, Vol. 41, No. 3, pp. 466 – 468, 1992. [10]  H. S. Endicott, B. D. Hatch, R. G. Sohmer, “Application of the eyring model to  capacitor aging  data”,  IEEE Transactions Component parts, Vol. 12, No. 1, pp. 34 41, 1965 [11]  T. S. Ramu, ‘‘On the estimation of life power apparatus insulation under combined  electrical  and  thermal  stress,’’  IEEE  Transactions  on Electrical Insulation, Vol. EI 20, pp. 70 78, 1985. [12]  D. Azizi, A. Gholami, A. Vahedi, “Analysis of the deterioration effects of  stator  insulation  on  the  its  electro thermal  property”,  International Journal of Electrical and Electronics Engineering, Vol. 3, No. 11, pp. 697  701, 2009.   AUTHORS PROFILE  Diako  Azizi was born in 1985. He has received B.Sc. degree in Electrical Engineering from Tabriz University, Tabriz, Iran in 2007. And he received the Master degree in Electrical Power Engineering from  the  University  of  Science  and  Technology, Tehran, Iran in 2009. He is presently pursuing the Ph.D. degree in Electrical Power Engineering, Iran University  of  Science  and  Technology.  His research  interests  are  aging  of  insulations  in electrical machines.    Ahmad Gholami has received his B.Sc. Degree in electrical engineering from IUST, Tehran, Iran, in 1975, the M.Sc. and PhD Degrees in electrical  engineering from UMIST, Manchester, England, in 1986 and 1989 respectively. He is currently an associate professor in the Electrical Engineering  Department  of  Iran  University  of  Science  and Technology. His main research activities are high voltage  engineering,  electrical  insulation, insulation  coordination,  transmission  lines  and substations planning.          

Optimum Scheme for Insulation System in HV Generator Based on Electromagnetic Analysis

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