16 DESEMBER AC to AC VOLTAGE CONVERTERS

Transcription

1 DSMBR AC to AC VOLTAG CONVRTRS

2 THR PHAS AC RGULATORS

3 Instructional Objectives Stuy of the following: The circuits use for the threephase ac regulators (ac to ac voltage converters) The operation of the above circuits with threephase balance resistive (R) loa, along with the waveforms The important points of comparison of the performance with ifferent types of circuits Introuction In the last lesson first one in the first half of this moule, various circuits of the glephase ac regulators, also terme as ac to ac voltage converters, are escribe. In the basic circuit, one Triac, or two thyristors, connecte back to back, are use. The operation of the above circuits with ifferent types of loas resistive (R) an inuctive (RL), along with the waveforms, is then iscusse. Lastly, the output voltage waveform is analyse. In this lesson the secon one in the first half, firstly, the circuits of the threephase ac regulators, also terme as ac to ac voltage converters, are escribe. The operation of the above circuits with threephase balance resistive (R) loa, along with the waveforms, is then iscusse. The two basic circuits are threephase threewire type with loa connecte in star an threephase eltaconnecte one. Lastly, the important points of comparison of the performance with ifferent types of circuits, incluing the above two, are presente. Keywors: Threephase ac regulator circuits, AC to AC voltage converter, balance threephase star an eltaconnecte loas Threephase AC Regulators There are many types of circuits use for the threephase ac regulators (ac to ac voltage converters), unlike glephase ones. The threephase loas (balance) are connecte in star or elta. Two thyristors connecte back to back, or a triac, is use for each phase in most of the circuits as escribe. Two circuits are first taken up, both with balance resistive (R) loa Threephase, Threewire AC Regulator with Balance Resistive Loa The circuit of a threephase, threewire ac regulator (terme as ac to ac voltage converter) with balance resistive (starconnecte) loa is shown in Fig. 7.. It may be note that the resistance connecte in all three phases are equal. Two thyristors connecte back to back are use per phase, thus neeing a total of six thyristors. Please note the numbering scheme, which is same as that use in a threephase fullwave brige converter or inverter, escribe in moule or 5. The thyristors are fire in sequence (Fig. 7.), starting from in ascening orer, with the angle between the triggering of thyristors & being (onesixth of the time perio ( T ) of a complete cycle). The line frequency is 5 Hz, with T f = ms. The thyristors are fire or triggere after a elay of from the natural commutation point. The natural commutation point is the starting of a cycle with perio, ( T ) of output voltage waveform, if six thyristors are replace by ioes. Note that the output voltage is similar to phasecontrolle waveform for a converter, with the ifference that it is an ac waveform in this case. The current flow is biirectional, with the current in one irection in the positive half, an then, in other (opposite)

4 irection in the negative half. So, two thyristors connecte back to back are neee in each phase. The turning off of a thyristor occurs, if its current falls to zero. To turn the thyristor on, the anoe voltage must be higher that the cathoe voltage, an also, a triggering signal must be applie at its gate. A C AN CN I L BN L B T T T T T T 5 i b b i a an R bn i c a R n R cn c Fig. 7. Threephase, threewire ac regulator The proceure for obtaining the expression of the rms value of the output voltage per phase for balance starconnecte resistive loa, which epens on range of firing angle, as shown later, is escribe. If is the rms value of the input voltage per phase, an assuming the AN s voltage, as the reference, the instantaneous input voltages per phase are, ean s t, ebn s ( t ) an ecn s ( t ) Then, the instantaneous input line voltages are, eab s ( t ), ebc s ( t 9) an eca s ( t 5)

5 AB BC CA AB BC AB BC CA AB t t AN BN CN AN AN BN CN t t I g I g I g5 I g I g I g an t t I g t I g5 t t I g t t I g t t I g t 5 5 t t AB BC CA AB BC 5 5 an t t.5 AB.5 AC.5 AB.5 AB.5 AC (a) For = (b) For = Fig. 7. Waveforms for threephase threewire ac regulator I g The waveforms of the input voltages, the conuction angles of thyristors an the output voltage of one phase, for firing elay angles ( ) of (a) an (b) are shown in Fig. 7.. For ( ), immeiately before triggering of thyristor, two thyristors (5 & ) conuct. Once thyristor is triggere, three thyristors (, 5 & ) conuct. As state earlier, a thyristor turns off, when the current through it goes to zero. The conitions alternate between two an three conucting thyristors. 5

6 At any time only two thyristors conuct for 9.Although two thyristors conuct at any time for 5 9, there are perios, when no thyristors are on. For 5, there is no perio for which two thyristors are on, an the output voltage becomes zero at ) (5 5. The range of elay angle is 5. The expressions of the rms value of the output voltage per phase for balance starconnecte resistive loa are as follows. Please note that t. For : ) ( e AN s 8 s For 9 : 5 5 s cos s 8 ( s For 5 9 : s cos 5 s 8 ) ( 5 s Threephase Deltaconnecte AC Regulator with Balance Resistive Loa The circuit of a threephase, eltaconnecte ac regulator (terme as ac to ac voltage converter) with balance resistive loa is shown in Fig. 7.. It may be note that the resistance connecte in all three phases are equal. Two thyristors connecte back to back are use per phase, thus neeing a total of six thyristors. As state earlier, the numbering scheme may be note. It may be observe that one phase of the balance circuit is similar to that use for glephase ac regulator escribe in the previous lesson () of the moule. Since the phase current in a balance threephase system is only ( ) of the line current, the current rating of the thyristors woul be lower than that if the thyristors are place in the line.

7 A I L i a i ab a R T T 5 AB L B CA i b T b R T i bc T R i ca c BC C i c T Assuming the line voltage Fig. 7. Delta connecte threephase ac regulator AB as the reference, the instantaneous input line voltages are, eab s t, ebc s ( t ) an eca s ( t ) It may be note that s is the rms value of the line voltage in this case. The waveforms of the input line voltages, phase an line currents, an the thyristor gating signals, for are shown in Fig

8 AB BC CA AB BC m t I g I g I g I g I g5 I g i ab i bc i ca t t t t t t t t t i a t i b t i c t For = Fig. 7. Waveforms for threephase eltaconnecte ac regulator 8

9 The rms value of the output phase voltage is obtaine as ( ) eab ( ) s s When, the maximum value of the output voltage is obtaine, an the control range of elay angle is 8( ). The line currents, which can be etermine from the phase current, are, i i i, i i i an i i i. a ab ca b bc ab c ca bc From Fig. 7., it can be observe that the line currents epen on the elay angle, an may be iscontinuous. The rms value of line an phase currents in this case can be etermine by numerical solution or Fourier analysis. If I n is the rms value of the n th harmonic component of a phase current, the rms value of the phase current is obtaine from I I I I I I I I ab n For elta connection, the triplen harmonic components (i.e., those of orer, n m, where m is an o integer) of the phase currents flow aroun the elta, an woul not appear in the line. This is ue to the fact that these harmonic currents are like the zero sequence component, being in phase in all three phases of the loa. So, the rms value of the line current is, I I I I I I a 5 7 n As a result, the rms value of the line current woul not follow the normal relationship of a threephase system such that I I. a ab Comparison of the Different Circuits use for Threephase AC Regulators Besies the two circuits shown in figures 7. & 7., other circuits use for threephase ac regulators (ac to ac voltage converters), are shown in Fig. 7.5ac. As given in point #, the balance loa for the circuits (Fig. 7. & 7.) is taken as inuctive (RL) one, not resistive as shown. The important points of comparison between the circuits are state. A T T B T T C T T 5 Fig. 7.5(a) wire elta loa 9

10 N A T T B T T C T T 5 Fig. 7.5(b) wire star loa A T T B T T C T T 5 Fig. 7.5(c) Control in elta. In two circuits (Fig. 7. & 7.5b), the iniviual phase controllers control their own loas inepenently of the other. As state earlier, they can, therefore be stuie as three glephase controllers.. In other circuits, the iniviual phase controllers affect the other phase loas also, an they have to be stuie as complete threephase circuits, as state earlier in one case (Fig. 7.).. The peak voltages occur across thyristors at or near the fully off state. In case of two circuits (Fig. 7. & 7.5c), the maximum thyristor voltage is the peak of the line voltage, whereas in the circuit (Fig. 7.5b), it is the peak of the phase voltage; in two circuits (Fig. 7. & 7.5a), the maximum thyristor voltage will be somewhere between the peak of the phase an line voltages epening on the leakage currents of the thyristors, the metho of firing an the presence of voltagesharing resistors across the thyristors.. All the five circuits can be use uner phase control.

11 5. The range of phase angle require to achieve full output range from zero to maximum varies between the circuits, an are given in Table 7... The maximum current in the thyristors is ecie from the fully on conition, an the size of the thyristors to be use shoul be chosen from this conition. The peak, mean an rms values of the thyristor currents (Table 7.) are relate to the rms value of the input (ac) current, which shoul be foun by applying the full supply voltage to the loa circuit. The loa impeance per phase is equal in magnitue (Z) an angle ( ), which is taken as positive, as it is mostly inuctive (RL). 7. The ifference between the two circuits (Fig. 7. & 7.5b) is that in the secon one, the neutral point is available, making it a wire one. 8. The ifference between the two circuits (Fig. 7. & 7.5a) is that in the secon one, the threephase balance loas are connecte in elta, which can be converte into its equivalent star, making it ientical to the first circuit (Fig. 7.). Also, the current in the thyristors in the secon case (Fig. 7.5a) are the line currents, which are higher than the phase currents, which are flowing in the thyristors in the first one. 9. The ifference between the two circuits (Fig. 7. & 7.5c) is that, in the secon one, the thyristors connecte back to back, are in elta, with the loa connecte in the three lines. Also, the current in the thyristors in the secon case (Fig. 7.c) are the phase currents, which are lower than the line currents, which are flowing in the thyristors in the first one. Table 7. summarises the current an voltage rating parameters associate with all these circuits use as threephase ac controllers. ac an I ac are the rms values of the line voltage an line current respectively. It may be mentione that other types of circuits for threephase ac regulator can be use, but either the circuits are not biirectional, i.e. uniirectional, or if they are biirectional, in one half, only ioe connecte back to back per phase, instea of thyristor, is use. In the secon case, only in one half with the thyristor per phase, controlle output voltage as shown earlier is obtaine, but in the other one, uncontrolle output voltage, same as input one, is obtaine. In the first case, where only one thyristor per phase is use, in one half, controlle output voltage is obtaine, but in the secon half, output voltage is zero, as only one evice, but not two evices, is use. The reaers are requeste to refer to text books. In this lesson the secon one in the first half, firstly the stuy of two basic circuits one with star connection an the other with elta connection, for threephase ac regulator (ac to ac voltage converter) are taken up. The operation with threephase balance resistive loa, along with waveforms, is then escribe. Lastly, the important points of comparison of the performance with ifferent types of circuits, incluing the above two, are presente. In the next, i.e. thir an final lesson in the first half, the control circuit for ac regulators will be escribe in etail.

12 Table 7. Rating of the Parameters use in Threephase AC Regulators Circuits (Fig. No.) Delay angle, for full control (egrees) a 5 7.5b 8 7.5c 5 Maximum input line current ( rms) ac Z Maximum loa power issipation Thyristor Thyristor current V Peak Mean RMS RWm V I ac ac R ac R Z R ac Z ac Z ac Z R R Perhatian Tolong ipelajari juga topik ini i buku LKTRONIKA DAYA JILID kalau tiak salah MAINILDA punya bukunya