Three-Phse Synchronous Mchines The synchronous mchine cn be used to operte s: 1. Synchronous motors 2. Synchronous genertors (Alterntor) Synchronous genertor is lso referred to s lterntor since it genertes lternting voltge. Synchronous mchine is designed to be operting t synchronous speed, n S. Sttor Two min prts: Rotor Construction detils of field nd rmture: Types of synchronous mchines clssified s ccording to the rrngement of the field nd rmture windings. 1. Rotting-Armture type: The rmture winding is on the rotor nd the field system is on the sttor. The generted current is brought out to the lod vi three (or four) slip-rings. Insultion problems, nd the difficulty involved in trnsmitting lrge currents vi the brushes, limit the mximum power output nd the generted electromgnetic field (e.m.f) this type is only used in smll units, nd its min ppliction is s the min exciter in lrge lterntors with brushless excittion system. (Limited pplictions). الصفحة ١
2. Rotting-field type The rmture winding is on the sttor nd the field system is on the rotor. Field current is supplied from the exciter vi two slip-rings, while the rmture current is directly supplied to the lod. This type is employed universlly since very high power cn be delivered. (Used in commercil ppliction). The prt of the mchine in which voltge is induced is clled rmture. Advntges of rotting field nd sttionry rmture system 1. Ese of construction for lrge three-phse SYG. Since the rmture winding is more complex thn the field winding. 2. Number of slip-rings required only two slip-rings. 3. It is esier to insulte the rmture coils from the core if the windings re plced on the sttor insted of on the rotor. 4. The weight of the field system plced on the rotor is comprtively much lower thn the rmture windings plced on sttor. 5. Improved ventiltion rrngement ir-cooling or/nd hydrogen cooling for lrge gen. cn esily be mde on sttionry rmture Construction of synchronous genertors: The rmture winding, usully on the sttor nd the field windings usully on the rotor is d.c excited. 1- Sttor (rmture): Three-phse windings connected on str or delt. 2- Rotor (field) The types of synchronous mchines clssified s ccording to the shpe of the field. الصفحة ٢
) Cylindricl rotor, round rotor, non-slient-pole rotor. b) Slient-pole rotor Types of rotor constructions Slient-poles rotor mchine الصفحة ٣
Round rotor mchine الصفحة ٤
Construction of synchronous genertor depends upon the type of prime mover used to rotte the rotor. Three types of prime mover re generlly used: 1- Therml or nucler power sttion (stem turbine). Stem turbines re designed to rotte t high speed (3000 r.p.m.) s t high speed. The efficiency of the stem turbine is high. The rotor of SYG is non slient-pole (cylindricl rotor). Stem turbine genertor set is mounted horizontlly. الصفحة ٥
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2-Hydroelectric power sttions: The wter turbine nd the lterntor re coupled together nd mounted verticlly. The speed of such prime mover vry from 50 to 500 r.p.m, the rotor (field) hve lrge number of poles nd lrge dimeters. 3-Diesel engines: Diesel engines re used s prime movers for low rting synchronous genertors. Diesel engines re t low speeds s compred to stem turbines. The genertors hve more thn two poles nd lwys mounted horizontlly. الصفحة ٧
Excittion for rotting field system: ) In older mchines, the excittion current ws typiclly supplied through slip-rings from D.C mchine, referred to s the exciter, which ws often mounted on the sme shft. b) In more modern system, the excittion is supplied from A.C exciter. The output of exciter is rectified through rectifier nd then fed to the field windings. c) On lrger genertors, brushless exciters re used to supply the D.C field current to the mchine. Mny SYG which include brushless exciters lso hve slip-rings nd brushes, so tht n uxiliry source of D.C field current is vilble in emergencies. الصفحة ٨
According to the shpe of the field, the synchronous mchines my be clssified s: 1. Non-slient pole (operte t high speeds usully two poles). Why non-slient pole mchine usully hs smll dimeter-to-length rtio? In order to void excessive windge loss nd excessive mechnicl stress on the rotor due to the lrge centrifugl forces nd lso void noise. 2. Slient pole (operte t low speeds). Why slient pole mchine usully hs lrge dimeter-tolength rtio? Since frequency of 50 Hz is required, we must use lrge number of poles on rotor of slow-speed lterntors. Mgnitude of induced e.m.f in ech phse: The mgnitude of the voltge induced in ech phse depends upon the rotor flux, the number nd position of conductors in the phse nd the speed of rottion the rotor. E g K.. n Where: K: Is constnt representing the construction of the mchine. n: Mechnicl speed (r.p.m). φ: Flux it depends on the current flowing in the rotor field circuit. Frequency: The frequency of induced e.m.f in the rmture conductors depends upon speed nd the number of poles. f n S P. P 60 Where: f: Frequency of e.m.f in Hz. n s : Rotor speed in (r.p.m), synchronous speed. P P : Number of rotor pir pole. الصفحة ٩
Equivlent circuit model: We will explore the effects of the first three fctors nd derive mchine model from them, the effects of slient-pole shpe on the opertion of synchronous mchine will be ignored, in other words, ll the mchines ssumed to hve non slientpole rotors. E O : Is the internl generted voltge produced in one phse. E: It is the induced e.m.f fter llowing for rmture rection. V: Terminl voltge. E O E I ( jx ) AR E V ph I ( R jx ) L E O V ph I R ji ( X X ) AR L الصفحة ١٠
Synchronous Rectnce (X S ): The sum of rmture lekge rectnce (X L ) nd rectnce of rmture rection (X AR ) is clled synchronous rectnce (X S ). Note tht ll quntities re per phse: X X X S L AR Synchronous impednce Z E S O R V ph jx I S Z S الصفحة ١١
Phsor Digrm of Synchronous Genertor I R V F E O When the synchronous genertor non-slient pole is supplying lod; t unity power fctor (purely resistive lod) E O jx S I I V I R Lod t lgging P.F E O I R j X S I S I الصفحة ١٢
Lod t leding P.F E O I j X S I S I R Power nd Torque in Synchronous Genertors A genertor converts mechnicl energy into electricl energy; hence the input power will be mechnicl prime mover, e.g. diesel engine, stem turbine, wter turbine or nything similr. Regrdless of the type of prime mover, the rotor velocity must remin constnt to mintin stble system frequency.. The power-flow digrm for synchronous genertor is shown الصفحة ١٣
Since X R S The rmture resistnce R is ignored, Simplifying the phsor digrm, n ssumption my be mde whereby the rmture resistnce R A is considered to be negligible nd ssuming tht lod connected to it is lgging in nture. This gives phsor digrm s shown: E O j X S I E O X sin S I cos I X I cos E sin S O الصفحة ١٤
I cos E O sin X S P 3V ph E X O S sin Notice lso tht mximum power tht the genertor cn be supply occurs when 90, t 90,sin 1 P Mx. 3V phe X S O The mximum power indicted by this eqution is clled the sttic stbility limit of the genertor. The Effect of Lod Chnges on Synchronous Genertor Operting Alone SYG 3-Phse Lod الصفحة ١٥
The behvior of synchronous genertor under lod vries gretly depending on the power fctor of lod. Assume genertor is connected to lod. Wht hppens when we increse the lod on the genertor operting lone? Assumptions 1- The field current is constnt, therefore the flux is constnt. 2- The prime mover keeps constnt speed the mgnitude of the E O K.. n internl generted voltge Eo constnt 3- The lod t the sme power fctor At lgging power fctor Increse the lod t the sme power fctor, then rmture current increses. Therefore, the rmture rection voltge is lrger thn before. E V 0 jx I O ph. S الصفحة ١٦
Note tht E o hs to remin constnt (refer to the ssumption stted erlier) Hence the only element which would chnge to compenste would be V. This chnge my be seen in the phsor digrm. At unity-power fctor The lod increses due to the voltge decreses At leding power fctor In this cse n increse in the lod in the genertor produced n increse in the terminl voltge. الصفحة ١٧
Voltge Regultion The voltge regultion of n lterntor is defined s the chnge in terminl voltge from no-lod to full-lod (the speed nd field excittion being constnt) divided by full-lod voltge VR% E O V V F. L F. L 100 An lterntive wy to explin this is vi the voltge regultion formule. For lgging lods, VR would be positive. For leding lods, VR would be negtive. For unity lods, VR would positive الصفحة ١٨
Conditions for Prlleling Alterntor with Infinite Bus-brs The proper method of connecting n lterntor to the infinite bus-brs is clled synchronizing. A sttionry lterntor must not be connected to live bus-brs. It is becuse the induced e.m.f. is zero t stndstill nd. In order to connect n lterntor sfely to the infinite bus-brs, the following conditions re met: 1- The r.m.s line voltges of the incoming lterntor nd the infinite bus-br must be equl. 2- Frequency of the generted voltge of the incoming lterntor must be slightly higher thn the frequency of the running system. 3- The incoming lterntor nd the infinite bus-br must hve the sme phse sequence. Generted voltge of the incoming lterntor cn be djusted by djusting the field excittion by {utomtic voltge regultion (AVR) or mnully by resistnce Frequency of the incoming lterntor cn be controlled nd mde equl to bus-br frequency by controlling the speed of the prime mover driving the lterntor ( governor mechnism) Phse sequence of the lterntor nd the bus-br cn be checked by phse sequence indictor (lmps) or specil instruments clled synchroscopes الصفحة ١٩
Three lmp method If ll the lmps glow together nd become drk together then the phse sequence of the incoming lterntor is the sme s tht of the bus-br. If the lmps brighten in succession, then the systems hve the opposite phse sequence, nd one of the sequences must be reversed Infinite Bus-br Synchroscope: A synchroscope is n instrument tht indictes by mens of revolving pointer the phse difference nd frequency difference between the voltges of the incoming lterntor nd the bus-brs. الصفحة ٢٠
In lrge genertors belonging to power system, this whole process of prlleling new genertor to the line is utomted, nd computer does this job. Opertion of genertors in prllel with lrge power systems: An infinite bus is power system so lrge tht its voltge nd frequency do not vry regrdless of how much rel nd rective power is drwn from or supplied to it. When genertor is connected in prllel with lrge system, the genertor will be essentilly floting on the line, supplying smll mount of rel power nd little or no rective power. الصفحة ٢١
The behvior of lterntors connected to n infinite bus-br: Cse1: Wht hppens when it governor mechnism set point (prime-mover) re increses, but tht the field excittion is kept constnt? Any chnge mde in the operting conditions of one lterntor will not chnge the terminl voltge or frequency of system. V T, E O (constnt) since E O =Kφ n P in T pp S Conclusion: incresing the mechnicl input power to the prime mover will not chnge the speed ultimtely but will increse the power ngle (). As result, the chnge of driving torque controls the ctive output power incresed الصفحة ٢٢
After the rel power of the genertor hs been djusted to the desired vlue, the phsor digrm of genertor look like t this time the genertor is ctully operting slightly leding power fctor, supplying negtive rective power. How cns the genertor be djusted so tht it will supply some rective power to the system? Cse 2: How cn the genertor be djusted so tht will supply some rective power to system? Cn be done by djusting the field current of the lterntors, when the field current is incresed, the flux increses, nd therefore E O =Kn, nd V=constnt. Incresing the field current due to increses the rective power output of the genertor الصفحة ٢٣
Summry: When genertor is operting by itself supplying the system lods (lone): 1. The rel nd rective power supplied by the genertor will be the mount demnded by the ttched lod. 2. The governor set points of the genertor will control the operting frequency of the power system. 3. The field current set points control the terminl voltge of the power system. When genertor is operting in prllel with n infinite bus-br: 1. The frequency nd terminl voltge of the genertor re controlled by the system to which it is connected. 2. The governor set points of the genertor control the rel power supplied by the genertor to system. 3. The field current in the genertor controls the rective power supplied by the genertor to the system. Advntges of Prllel Opertion of Alterntors The following re the dvntges of operting lterntors in prllel: 1. Continuity of service. The continuity of service is one of the importnt requirements of ny electricl pprtus. If one lterntor fils, the continuity of supply cn be mintined. الصفحة ٢٤
through the other helthy units. This will ensure uninterrupted supply to the consumers. 2. Incresed Efficiency: The lod on the power system vries during the whole dy; being minimum during die lte night hours. Since lterntors operte most efficiently when delivering full-lod, units cn be dded or put off depending upon the lod requirement. This permits the efficient opertion of the power system. 3. Mintennce nd repir cn be done without power disruption. It is often desirble to crry out routine mintennce nd repir of one or more units. For this purpose, the desired Unit/units cn be shut down nd the continuity of supply is mintined through the other units 4. Lod growth. The lod demnd is incresing due to the incresing use of electricl energy. The lod growth cn be met by dding more units without disturbing the originl instlltion. الصفحة ٢٥