Presentation on theme: "CH.4 Full-wave and Three- phase rectifiers (Converting AC to DC)"— Presentation transcript:
1CH.4 Full-wave and Three- phase rectifiers (Converting AC to DC) 4-1 IntroductionThe average current in AC source is zero in the full-wave rectifier, thus avoiding problems associated with nonzero average source currents, particularly in transformers.The output of the full-wave rectifier has inherently less ripple than the half-wave rectifier.Uncontrolled and controlled single-phase and three-phase full-wave converters used as rectifiers are analyzed.
24-2 Single-phase full-wave rectifiers Fig. 4-1 Bridge rectifier：The lower peak diode voltage make it more suitable for high-voltage applications.
9 Discontinuous current is analyzed like section 3-5. For continuous current operation, the only modification to the analysis that was done for R-L load is in the dc term of the Fourier series .The dc component of current in this circuit is. The sinusoidal terms in the Fourier analysis are unchanged by the dc source, provided that the current is continuous. Discontinuous current is analyzed like section 3-5.
11=？ solved numerically for Assuming ideal diodes：the angle where the diodes become reverse biased, which is the same as for the half-wave rectifier and is=？ solved numerically for Peak-to-peak variation(ripple)：
12minimal output voltage occurs at In practical circuits where ωRC,minimal output voltage occurs at is half that of the half-wave rectifier.
13 voltage doubler(sw. closed) Fig. 4-7 (a) Voltage doublerFig. 4-7 (b) Dual voltage rectifier =full-wave rectifier(sw. open)+ voltage doubler(sw. closed)
14L-C filtered output： Fig.4-8 C holds the output voltage at a constant level, and the L smoothes the current from rectifier and reduces the peak current in diodes.
15=0 , full-wave rectified Continuous Current： The variation in can be estimate from the first Ac term (n=2) in the Fourier series. The amplitude of the inductor current for n=2 is where For Continuous current,
16Discontinuous current： When is positive ( at ) ,
17 Procedure for determining Vo： Estimate a Value for Vo slightly below Vm, and solve (2) Solve numerically,(3) Solve (4) Slove Vo=(5) Repeat step (1)~(4) until the computed Vo in step(4) equals the estimated Vo in step(1)Output Voltage for discontinuous current is larger than for continuous current.(see Fig4-8(d))
21 For discontinuous current Analysis of the controlled full-wave rectifier operating in the discontinuous current mode is identical to that of the controlled half-wave rectifier, except that the period for the output current is .
25R-L Source load ： Fig.4-14 The SCRS may be turned on at any time that they are forward biased, which is at an angle
26average load current is For continuous current case, the average bridge output voltage isaverage load current is The ac voltage terms are unchanged from the controlled rectifier with an R-L load. The ac current terms are determined from circuit. Power absorbed by the dc voltage isPower absorbed by resistor in the load is
27Controlled Single-phase converter operating as an inverter： seeing Fig 4-14. 4-15.
28 Vdc and Vo must be negative For inverter operation, power is supplied by the dc source, and power is absorbed by the bridge and is transferred to the ac system. Vdc and Vo must be negative rectifier operation inverter operation
31 A transition of the highest line-to-line voltage must take place every 上、下半部Diode，每次僅一個ON；同相上、下Diode不可同時ON；Diode ON由瞬間最大線電壓決定。 A transition of the highest line-to-line voltage must take place every . Because of the six transitions that occur for each period of the source voltage, the circuit is called a six-pulse rectifier. vo(t)之基頻為 電源頻率之6倍 Diode turn on in the sequence 1,2,3,4,5,6,1,..
32 Each diode conducts one-third of the time, resulting in Apparent power from the three-phase source is
33Since the output voltage is periodic with period 1/6 of the ac supply voltage, the harmonics in the output are of order 6kω, k=1,2,3,…Adevantage：output is inherently like a dc voltage, and the high-frequency low-amplitude harmonics enable filters to be effective.
34 For a dc load current (constant I0) --- Fig4.17
35which consists of terms at fundamental frequency of the ac system and harmonics of order 6k Filters(Fig.4-18) are frequently necessary to prevent harmonic currents to enter the ac system. Resonant filters for 5th and 7th harmonics. High-pass filters for higher order harmonics.
39 Twelve-pulse rectifiers：using two six-pulse bridges
40This results in inputs to two bridges which are The purpose of the transformer connection is to introduce phase shift between the source and bridge.This results in inputs to two bridges which are apart. The two bridge outputs are similar, but also shifted by. The delay angles for the bridge are typically the same. The peak output of the twelve-pulse converter occurs midway between alternate peaks of the six-pulse converters. Adding the voltages at that point for gives
41 Since a transition between conducting SCRs every , there are a total of 12 such transitions for each period of the ac source. The output has harmonic frequencies which are multiple of 12 times the source fre. (12k k=1,2,…)Cancellation of harmonics 6(2n-1) , n=1, 2, … has resultedfrom this transformer and converter configuration.
42 This principle can be expanded to arrangements of higher pulse number by incorporating increased number of six-pulse converters with transformers which have the appropriate phase shifts. The characteristic ac harmonics of a p-pulse converter will be pk , k=1,2,3… More expense for producing high-voltage transformers with the appropriate phase shifts.
43Three-phase converter operating as a inverter： seeing 4-22.
454-6 DC power transmission ․ By using controlled twelve-pulse converter (generally).․ Used for very long distances of transmission lines.Advantages：(1) , voltage drop↓ in lines(2) , line loss ()(3) Two conductors required rather than three(4) Transmission towers are smaller.(5 ) Power flow in a dc transmission line is controllable by adjustment of delay angles at the terminals.(6) Power flow can be modulated during disturbances on one of the ac system. System stability increased.(7) The two ac systems that are connected by the dc line do not need to be in synchronization.Disadvantages：costly ac-dc converter, filter, and control system required at each end of the line to interface with the ac system.
47For current being ripple free Power supplied by the converter at terminal 1 is Power supplied by the converter at terminal 2 is
48Fig.4-24 using twelve-pulse converter (a bipolar scheme)
49One of the lines is energized at and the other is energized at - One of the lines is energized at and the other is energized at In emergency situations, one pole of the line can operate without the other pole, with current returning through the ground path.
54 During Commutation from , The voltage across La is Current in starts at I0 and decreases zero in the commutation interval
55 During the commutation interval from , the converter output voltage is Average output Voltage： 類似 Single-phase rectifier Source inductance lowers the average output voltage of three-phase rectifiers.