Abstract I. INTRODUCTION. Keywords: Synchronous Generator, Short pitch

Transcription

1 A Novel Approach to Design Stator Winding of Synchronous Generator Sachin N. Gaikwad*1,Poonam D. Daunde*2, Rakesh Kumar Jha*3 *1 Research Scholar, M Tech(Power system), IET, Alwar, *2 PG Student, M E (Power System), SNDCOE&RC,Yeola, *3 Asst. Prof., SNDCOE&RC,Yeola, sachingaikwad86@live.com*1,poonamdaunde91@gmail.com*2,rakeshshanti@gmail.com*3 Abstract Stator winding of synchronous generator is design with various types winding for manufacturing of Synchronous alternator in an industry and the steel sheet laminations design is always kept constant. Lamination cannot be changed frequently because it needs tool (die) development, for designing of new laminations the time required and cost increases enormously which is not acceptable to the manufacturers. At an instant very less information/ data is available about the fact that a synchronous generator can also causes harmonics at the time of generation of EMF, So it becomes paramount to deal with this important equipment as far as harmonics and related losses. Considering the harmonics present in the machine which, is mainly due to unequal distribution of field fluxes in the air gap. Many of researchers studies have been addressed that Synchronous machines are also source of harmonic currents on two counts: the frequency conversion effect and the non-linear characteristic due to magnetic saturation. Therefore we concentrated our investigation work on the distribution type of winding only. Considering the harmonic present in the machine, our concentration was on pitch factor. Keywords: Synchronous Generator, Short pitch winding, Harmonics, 2/3 rd, 4/5 th, 5/6 th Pitch factor I. INTRODUCTION The Synchronous generator consisting stator and rotor. The stator of the AC generator is manufactured with placing winding with some pitch factor. The distance between the two sides of an individual coil of an AC armature winding is termed the coil pitch. A coil whose sides are separated by one pole pitch (i.e., coil span is 180º electrical) is called a full-pitch coil. With a full-pitch coil, the emfs induced in the two coil sides are in phase with each other and the resultant emf is the arithmetic sum of individual emfs. However the waveform of the resultant emf can be improved by making the coil pitch less than a pole pitch. Such a coil is called short-pitch (or fractional or partial pitched) coil. The emf induced in a short-pitch coil is less than that of a full pitch coil. Winding given by, Kp= cosα/2 which can be made by keeping a 2/3 rd, 4/5 th or a 5/6 th winding pitch. Winding pitch of AC generators has influence on the shape of waveform (harmonic contents) and on the level of the fundamental voltage. The winding pitch will also have an influence on neutral circulating current in case of parallel operation and also will impact on the type of neutral grounding method. Paralleling generators with different winding pitch will require careful consideration if interconnecting star points of all running generators should be necessary. 1

2 The stator winding pitch of a generator is a design parameter that can be used to optimize the generator waveform shape and minimize the generator cost, because shorter pitch (lower pitch ratios) use the generator stator less effectively and require the use of more electrical steel and copper for the same kw output than higher pitch machines. Quality of generated emf in the AC generator is how close to a true sine wave. The actual voltage waveform from rotating machinery is never being sinusoidal. Internal generator and external load characteristics cause distortions in the sinusoidal waveform called as harmonic. In brief, harmonics are energy levels existing in the system at multiples of the fundamental wave s frequency. The main source of harmonic is non-linear load connected to AC generator. The MMF distribution (flux per pole) is sinusoidal which determines the EMF generated in an armature. As load is applied on the alternator the generated sinusoidal waveform gets distorted For balanced three-phase loads, distortion is caused by voltage drop due to the harmonic currents in the sub transient reactance of the generator. The sub transient reactance of a generator is not a function of coil pitch. Therefore, the coil pitch does not affect waveform distortion. The impact of generator pitch on load generated harmonic currents is highly dependent on the system configuration. Controlling circulating currents when paralleling generators in a power system that shares a common neutral can be difficult. In any paralleling operation, it is extremely important that voltages produced by the generating equipment are as closely matched as possible. To properly match voltages, not only do the RMS values need to be similar but the instantaneous values, which are determined by the voltage wave shapes, should be similar as well. When this is not possible, as in paralleling of generators with different winding pitch configurations, circulating currents may appear in the common neutral which bonds the wye connections of the generating sources. These circulating currents can cause overheating in the generator windings II. LITERATURE REVIEW Lyon and Waldo V was described that, the flux distribution in the air gap of a synchronous machine consists of a series of component distributions that are simple harmonic wave trains, either stationary or moving at constant velocities. Methods are suggested for determining the effects of slots and the saturation of the magnetic circuit on the magnitudes of these component distributions. Expressions for the voltage generated and the power developed thereby are given. The theory is applied to the operation of a three-phase, synchronous machine under different conditions of load, both qualitatively and quantitatively. Chalmers, B.J, explained in his paper describes a type of double-layer winding known as an interspersed winding, which may be used in A.C. machines to reduce harmonic components of induced E.M.F. or load MMF. The principles underlying the design of interspersed windings are described and compared with the standard analysis of conventional windings in terms of harmonic winding factors. Eleschova Zaneta, Belan Anton and Martin Mucha, Department of Power Engineering, Slovak University of Technology, Ilkovicova, Bratislava, Harmonic Distortion Produced by Synchronous Generator in Thermal Power Plant, Slovak Republic Published Paper Sept.2006.This paper discussed about voltage harmonics produced by synchronous machine. If the magnetic flux of the field system is distributed perfectly 2

3 sinusoidal around the air gap, the flux is never exactly distributed, particularly in salient pole machines non/sinusoidal field distribution can be expressed in harmonic series. The machine can be consider to have 2p fundamental poles together with 6p, 10p, 2np harmonic poles, all individually sinusoidal and all generating electromotive forces in an associated winding. The winding e.m.f. can be expressed in harmonic series. The magnitude of the harmonic e.m.f. is determined by the harmonic fluxes the effective electrical phase spread of the winding, the coil span, and the method of inter phase connection. For an integral slot winding with m slots per pole per phase and an electrical angle α between slots, the distribution factor for the nth harmonic can be calculated. By suitable choice of Kd and Ks many troublesome e.m.f. harmonics can be minimized or even eliminated. The triplen harmonics in a three-phase machine are generally eliminated by phase connection, and it is usual to select the coil span to reduce 5th and 7th harmonic. Slotting (the slots being on the stator) produce variation of permeance. The fundamental rotor m.m.f. (magneto motive force) can be represented as a travelling wave. The slot ripple component of flux density. Tsujikawa, T. Tokumasu, M. Kakiuchi, D. Hiramatsu, M, Fujita, T. Ueda, K. Ikeda, M.Ichimonji and T.Otaka, Special winding to reduce space flux harmonics caused by fractional-slot, IEEE Transactions on Electrical Machines and Systems, October The authors said that space flux harmonics caused by fractional-slot winding and proposed a new winding method that reduces the low order space flux harmonic components (special winding method based on intersperse one coil winding), The advantage of this proposed winding method was confirmed using a model winding. Here several numbers of research papers from IEEE and other reputed journals have been studied and came to know that due to unequal distribution of field flux harmonics are introduced in output voltage waveform and also temperature rises. Thus, the proposed problem is unique related to synchronous machine which the main equipment of our power system. So, it the challenge to minimize above problem by using two different short pitches stator winding of synchronous alternator. Paolo mattavelli published paper relate to an implementation of synchronous-frame control for selected frequencies in the output voltage. The regulation of the fundamental output voltage, as well as that of some loworder harmonics, is achieved using a synchronous-frame controller for each selected frequency in addition to a conventional control III. CONSTRUCTION OF SYNCHRONOUS GENERATOR Except for small ratings, the synchronous generators have a stationary armature and a rotating field. The field is housed in the rotor for technical and economic reasons. It makes the rotor light, reduces the size and losses in the bearing, makes the cooling simpler, and the response fast. Also, it is not possible to extract large power produced in a rotating armature through slip-rings and brushes. Therefore rotating fields are used from about a size of 500 Kw. The generators coupled to steam turbines in a thermal power station run at a high speed 3000 rpm in India. The armature is housed in the stator of the alternators and the field in the rotor. The stator is made up of sector-shaped laminations of high-grade Cold Rolled Grain Oriented Steel. Normally full pitched winding has been employed as stator winding. Which results in more use of copper hence copper loss, more harmonic introduced in machine. The Stator frame is made of sheet metal which reduces the overall weight of the machine and is aesthetically 3

4 better. The sheet metal enclosures are fixed on the steel bars welded on the stator core. The armature coils are made from dual coated, class 200 copper wire. The stator core is made of high quality low content silicon steel stampings with C-4 coating for better welding core packs. These are oriented 90 deg. after every one forth length for better magnetic properties. The slots are skewed to reduce the tooth ripples in voltage waveform. IV. DESIGN APPROACH TO SYNCHRONOUS GENERATOR Quality of electrical performance is a measure of how close the electrical output of the generator is to a true sine wave. The actual voltage waveform from rotating machinery is never perfect. Internal generator and external load characteristics cause distortions in the wave. These factors impair the consistency of the generator output, and can result in voltage regulator sensing errors and incorrect instrument reading. In brief; harmonics are energy levels existing at multiples of the fundamental wave s frequency. Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50 or 60Hz (50Hz for Asian and European power and 60Hz for American power). For example, if the fundamental power frequency is 60 Hz, then the 2nd harmonic is 120 Hz, the 3rd is 180 Hz, etc. In modern test equipment today harmonics can be measured up to the 63rd harmonic. When harmonic frequencies are prevalent, electrical power panels and transformers become mechanically resonant to the magnetic fields generated by higher frequency harmonics. When this happens, the power panel or transformer vibrates and emits a buzzing sound for the different harmonic frequencies. Harmonic frequencies from the 3rd to the 25th are the most common range of frequencies measured in electrical power systems. The main source of harmonic is non-sinusoidal field form, which can be made sinusoidal and the total harmonics can be eliminated. But it is not an easy job because air gap offers maximum reluctance to the flux path due to which air gap cannot be made to vary sinusoidally, if air gap is made to vary sinusoidally around the machine, then field form would be sinusoidal and total harmonics would be eliminated. But field form cannot be sinusoidal due to saturation in iron parts and hence we cannot totally eliminate all harmonics but can decrease them. Field form is nothing but distribution of flux in machine. This flux distribution determines the wave shape of generated voltage in armature winding. In case of alternators, the voltage and currents induced are having sinusoidal waveforms. But practically we cannot get sinusoidal waveforms when such alternators are loaded. Due to the loading conditions, the generated waveform deviates from ideal waveforms. Such a non-sinusoidal waveform is called complex wave. Additionally, harmonics are caused by and are the byproduct of modern electronic equipment such as personal or notebook computers, laser printers, fax machines, telephone systems, stereos, radios, TVs, adjustable speed drives and variable frequency drives, battery chargers, UPS, and any other equipment powered by switchedmode power supply (SMPS) equipment. The abovementioned electronic SMPS equipment is also referred to as non-linear loads. This type of non-linear loads or SMPS equipment generates the very harmonics they re sensitive to and that originate right within your building or facility. SMPS equipment typically forms a large portion of the electrical non-linear load in most electrical distribution systems. There are basically two types of non-linear loads: single-phase and three-phase. Singlephase non-linear loads are prevalent in modern office buildings while three-phase non-linear loads are widespread in factories and industrial plants. 4

5 In today s environment, all computer systems use SMPS that convert utility AC voltage to regulated low voltage DC for internal electronics. These non-linear power supplies draw current in high amplitude short pulses. These current pulses create significant distortion in the electrical current and voltage wave shape. This is referred to as a harmonic distortion and is measured in Total Harmonic Distortion (THD). The distortion travels back into the power source and can affect other equipment connected to the same source. To give an understanding of this, consider a water piping system. Have you ever taken a shower when someone turns on the cold water at the sink? You experience the effect of a pressure drop to the cold water, reducing the flow of cold water. The end result is you get burned! Now imagine that someone at a sink alternately turns on and off the cold and hot water. You would effectively be hit with alternating cold and hot water! Therefore, the performance and function of the shower is reduced by other systems. This illustration is similar to an electrical distribution system with non-linear loads generating harmonics. Any SMPS equipment will create continuous distortion of the power source that stresses the facility s electrical distribution system and power equipment. Harmonic cancellation is performed with harmonic canceling transformers also known as phase-shifting transformers. A harmonic canceling transformer is a relatively new power quality product for mitigating harmonic problems in electrical distribution systems. This type of transformer has patented built-in electromagnetics technology designed to remove high neutral current and the most harmful harmonics from the 3rd through 21st. The technique used in these transformers is call "low zero phase sequencing and phase shifting". These transformers can be used to treat existing harmonics in buildings or facilities. This same application can be designed into new construction to prevent future harmonics problems. In case of alternators the voltage generated is mostly symmetrical as the field system and coils are all symmetrical. So the generated voltage or current will not have any even harmonics in most of the cases. The waveform distortion can cause problems in voltage regulation, generator and load overheating, and inaccurate instrument readings. Both voltage and current may have harmonic components. The current components produce heat and are therefore derating factors for the generator as well as system motors. TABLE NO.1 EFFECT OF HARMONICS ON VARIOUS MACHINES Harmonic Order Frequency (Hz) Phase sequence Effect on # # Harmonic Order Frequency (Hz) Phase sequence Effect on Equipm # Heating, # heating plus motor problem, $ heating plus noise problem V. TECHNIQUES FOR REDUCTION OF HARMONICS A. WAVE FORM SHAPING: Field flux waveform can be made sinusoidal by decreasing the air gap at the pole centre and large air gap towards the pole end in salient pole synchronous machine. Skewing the pole faces if possible. In turbo- alternators the air gap is uniform. So field winding is distributed in 5

6 the slots in such a manner as to make the field flux waveform almost a sine wave. B. SKEWING By skewing the armature slots, only tooth harmonics or slot harmonics can be eliminated. C. FRACTIONAL SLOT WINDING The slot harmonic emfs can be drastically reduced and even completely eliminated from output voltage waveform by using fractional slot windings. In fact, present day synchronous generators invariably use fractional slot windings on account of the fact that these windings give a much smaller distribution factor for harmonics as compared with that of fundamental. D. LARGER LENGTH OF AIR GAP If we use a large air gap length, the reluctance is increased and therefore the magnitude of slot harmonics is reduced. The flux pulsations can be reduced by having the number of slots per pole arc as an integer plus 0.5. Alternator connections: Star or delta connections of alternators suppress triplen harmonics from appearing across the lines. E. CHORDING Coil span less than the pole pitch the emf generated is proportional to Cos (n*α/2) Where, α is angle of chording and n is order of harmonic. The harmonic emf can therefore, be considerably reduced or entirely eliminated by choosing a proper value of α. To understand chording we should first go through the term coil span, It is the distance on the periphery of the armature between two coil sides of a coil. It is usually expressed in terms of number of slots or degree electrical. So if coil span is 180 degree electrical then the coil is called full pitched coil. As against if coils are used in such a way that coil span is slightly less than a pole pitch i.e. less than 180 degree electrical, the coils are called short pitched coils. ADVANTAGES OF SHORT PITCH COILS The length required for the end connections of coils is less i.e less copper is required, hence economical. Short pitching eliminates high frequency harmonics which distort the sinusoidal nature of emf. Hence waveform of an induced emf is more sinusoidal due to short pitching. As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimized. This results in increase in efficiency. Not only higher order harmonics but with specific degree of chording the corresponding order of harmonic can be eliminated. Chording also helps to reduce the vibration level of the machine to a certain magnitude. STATOR WINDING PITCH Stator winding pitch Reduces or eliminates certain harmonics, but not all harmonics. In general, when coil is pitched short or long by (or), no harmonic of order n survives in the coil emf.the triplen harmonics that may be generated in a three phase machine are normally eliminated by star connection of the phases.it is therefore usual to select the coil span to reduce as much as possible the 5th and 7th harmonics a pitch of 83.3% is most useful in this aspect, as it gives the following factors. A. Distribution Factor Corresponding to Order of Harmonics 6

7 TABLE NO 2: DISTRIBUTION FACTOR CORRESPONDING TO ORDER OF HARMONIC Order Fundamental harmonic Distribut factor The 5th& 7th harmonic factors are both small while 3rd and 9th harmonic emf will not appear in the line in case with star and delta connection.hence at time of winding design the attention is mainly directed for the attenuation of 5th & 7th order harmonics by adopting a suitable chording angle. Chording angle of 300 i.e. coil- pitch = 1500 electrical is most useful,since it gives the above values. B. Wave Distortion Factor The distortion factor of a voltage wave is the ratio of effective value of the residue after the elimination of the fundamental to the effective value of the original wave. The distortion factor, Fdi of a wave is obtained by dividing the rms harmonic content, that is the square root of sum of the square of the rms value of all frequency component except the fundamental, by the rms value of the wave including the fundamental. Fdi= ( En2) / Erms, Where, E = sum of the square of the rms value of all the components of the voltage except the fundamental,erms= rms value of the voltage A coil whose sides are separated by one pole pitch (i.e. coil span is 180deg electrical) is called a full- pitch coil, the emf s induced in the two coil sides are in phase with each other and the resultat emf is the arithmetic sum of individual emfs. However the waveform of resultant emf can be improved by making coil pitch less than a pole pitch such a coil is called as short pitch or fractional pitched coil. 1. Full pitch winding: Full pitch = pole centre (S) to pole centre (N). For 48 slots stator Slot/pole= 48/4=12 slots Coil span = 1-13 Fig. 1: Full Pitch Winding Machine Design 2. 5/6th Short pitch winding: This winding pitch equals to 5/6th of full pitch winding. For 48 slots stator= 5/6th of 12 slots=10 slots Coil span = 1-11 VI. OPTIMIZATION TECHNIQUES OF HARMONICS WITH VARIOUS SHORT PITCH WINDING A. WINDING PITCH 7

8 Fig. 2: 5/6th Short Pitch Winding Machine Design 3. 2/3rd Short pitch winding: This winding pitch equals to 2/3rd of full pitch winding. For 48 slots stator= 2/3rd of 12 slots= 08 slots Coil span = 1-09 Fig. 3: 2/3rd Short Pitch Winding Machine Design VII. CASE-STUDIES OF VARIOUS MANUFACTURER ON DESIGN OF ALTERNATOR Mirus International Company; 2/3-pitch generators produce little third harmonics current, they do produce much higher fifth and seventh harmonics when compared with 4/5- and 5/6-pitch generators. This increases heating in motors, which can shorten life. If inductive loads make up the majority of the load, 4/5- or 5/6-pitch generators can be used with correct sizing. These generators also result in phase-to-neutral faults much lower than 2/3-pitch unit. There has been much written and even more speculated about the pros and cons of 2/3-pitch generators vs. 4/5- (and 5/6-) pitch machines. Because the effects of third harmonics on electrical systems are installation-specific, few hard and fast rules apply. However, in general, the following points are consistent across all generator and electrical systems: 1. Third harmonics current is generated almost totally by connected load computer systems, UPS, variable-speed and fluorescent lighting. Only a negligible amount is produced by the generator, no matter what its winding pitch. 2. Third harmonic currents in identical paralleled gensets are no problem if gensets are carrying equal load. However, it may be a problem if two generators of different pitches are paralleled. 3. While 2/3-pitch generators have very little third harmonic current compared to other pitches, the fifth and seventh harmonics are nearly maximum at 2/3 pitch. Further, if a phase-to-neutral fault (the cause of 65% of all faults) occurs on a 2/3-pitch machine, there will be higher fault currents, with the potential of more system damage and the need for higher interrupting capability circuit breakers - adding cost to the installation pitch factors for synchronous generators of various pitch types. These pitch factors are multiplied by the respective harmonic fluxes to predict the harmonic voltages [2]. Since differently pitched machines have different pitch factors for each harmonic number, their harmonic voltages and voltage waveshapes will be different as well. Cummins Generator Technologies; 8

9 Alternators are compatible if they have compatible voltage waveform shapes. To assure the optimal compatibility between current and potential future machines, always specify the use of 2/3 pitch alternators for line voltage applications. When faced with paralleling dissimilar machines, some of which are not 2/3 pitch, the most desirable practice is to replace the dissimilar alternator(s) with a compatible alternator, so that all the machines are 2/3 pitch. TABLE 3: PITCH FACTOR IMPACT ON HARMONIC VOLTAGE MAGNITUDES IN SYNCHRONOUS GENERATORS Pitch Fundamental 3 rd 5 th 7 th 9 th 2/ / / / D. 6/7 Pitch: A 6/7 pitch will eliminate the 7th harmonic. E. 5/6 Pitch: A 5/6 pitch will: 1. Minimize the 5th harmonic, but not eliminate it as will a 4/5 pitch. 2. Minimize the 7th harmonic, but not eliminate it as will a 6/7 pitch. It is important to note that it is not the generator s specific pitch value that causes the circulating current but rather the difference in voltage wave shape of the two differently pitched generators. Therefore, the fact that a 2/3 pitch generator has a very low pitch factor for the 3 rd harmonic does not mean that it will perform any better in paralleling operations. In fact, a 2/3 rd pitch generator has very low zero sequence reactance and therefore, has less impedance to reduce the flow of circulating neutral current. Circulating currents can result with any generator pitch type when it is not matched with a similarly pitched unit or it is paralleled with the Utility. VIII. CONCLUSION One of the design considerations in selecting an appropriate pitch factor is the harmonic content of the generated voltage wave form. Pitch factor can be used to reduce or eliminate specific harmonic frequencies in the generated voltage waveform as follows: A. Full Pitch: A full pitch will have no damping effect on any harmonic frequency. B. 2/3 Pitch: A 2/3 pitch will eliminate the third harmonic and subsequent triplens i.e.: 9th, 15th, 21st, 27th, etc. C. 4/5 Pitch: A 4/5 pitch will eliminate the 5th harmonic. IX. REFERENCES [1] Dr Jawad Al-Tayie, Chris Whitworth, Dr Andreas Biebighaeuser AC Generators with 2/3rd and 5/6th winding pitch, Issue WP105: Technical Information from Cummins Generator Technologies. [2] Pradipta K. Das And Amulya K. Das Gupta, Inductance Coefficients of Three-Phase Inductor Alternators: Part- I Analytical Study, IEEE Transactions on Power Apparatus and Systems, vol. PAS- 88, no.-11, pp , November [3] Chalmers, B.J, A.C. machine windings with reduced harmonic content, IEEE Transactions on Electric Machines & Drives, vol.-111, no.-11, pp , November [4] Xola B. Bomela and Maarten J. Kamper, IEEE Members, Effect of Stator Chording and Rotor Skewing on Performance of Reluctance Synchronous Machine IEEE Transactions On Industry Applications, vol.-38, no.-1, pp , January/February [5] Paolo Mattavelli, IEEE Member, Synchronous Frame Harmonic Control For High Performance Ac Power Supplies, IEEE Transactions On Industry Applications, vol.-37, no.-3, pp , May-June

10 [6] Seok Myeong Jang, You, Dae Joon, Kyoung Jin Ko and Sang Kyu Choi, Design And Experimental Evaluation Of Synchronous machine Without Iron Loss Using Double Sided Halbach Magnetized PM Rotor In High Power FESS, IEEE Transactions On Magnetics, vol.- 44, no.-11, pp , November [7] Zhu,Xi Xiao And Yongdong Li, China, Permanent Magnet Synchronous Motor Current Ripple Reduction With Harmonic Back- Emf Compensations, IEEE Transactions on Electrical Machines and System, pp , October [8]Rakesh Kumar Jha, Arvind S. Pande, Prof.Harpreet Singh Harmonics Reduction in Synchronous Alternator by using Short Pitch Winding, International Journal of Emerging Technology and Advanced Engineering ISSN , ISO 9001:2008 Certified Journal, Volume 3, Issue 9, September