Power Electronic Drives - DC to AC converter / Inverter EET421 Power Electronic Drives - DC to AC converter / Inverter Abdul Rahim Abdul Razak Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical Summary dc-to-ac converters are known as inverters The function of an inverter is to change the dc input voltage to an ac output voltage of desired magnitude and frequency The output voltage waveforms of ideal inverters should be sinusoidal However, the output of practical inverters contains harmonics For high power applications, low distorted sinusoidal waveforms are required Harmonic contents could be minimized by the use of high-speed semiconductor switching techniques Inverters are widely used in industrial applications - motor drives, UPS, induction heating, standby power supplies, etc. - input may be a battery, fuel cell, solar cell, or there dc source dc-to-ac inverters can make smooth transition into the rectification mode, where the flow of power reverses from the ac side to the dc side Two types of inverters: single-phase inverters and three-phase inverters Systems Engineering School of Electrical ABD RAHIM 2008

Switch-Mode DC-AC Inverters Applications: ac motor drives Uninterruptible ac power supplies Where a sinusoidal ac output is required whose magnitude and frequency both have to be controlled Systems Engineering School of Electrical Terminal voltage is adjustable in its magnitude and frequency ABD RAHIM 2008

Switch-Mode DC-AC Inverter: Bi-directional power flow

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical Voltage control of 1-phase inverter The needs to have a controllable output voltage : To cope with variations of DC input voltage For inverter voltage regulation For constant volts/frequency control requirement Most efficient techniques is by incorporating PWM control within the inverter : Single pulse width modulation - PWM Multiple pulse width modulation - MPWM Sinusoidal pulse width modulation - SPWM Modified sinusiodal pulse width modulation - MSPWM Phase displacement control Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical Single pulse width modulation - PWM Modulation index: M=Ar/Ac Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical Single pulse width modulation - PWM Modulation index: M=Ar/Ac Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs DF increased badly significantly at low output voltage 3rd harmonic more dominant on single PWM Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical b) Multiple pulse width modulation - MPWM fo will set the output frequency while fc will determines the pulses per half cycle,p. fc / fo = mf, frequency modulation ratio Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical b) Multiple pulse width modulation - MPWM By using several pulses, (p=5) The harmonic content are reduced compared to single PWM the DF is also reduced significantly. but switching loss would increased. if p increased, the amplitude of lower order harmonics would be lowered but it will increase the high-orders harmonics fo will set the output frequency while fc will determines the pulses per half cycle,p. fc / fo = mf, frequency modulation ratio Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical c) Sinusoidal pulse width modulation - SPWM Pulse width varied proportional to the amplitude of sinewave ref signal. the gating signal generated by comparing the triangular carrier wave and sinusoidal ref signal. Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical Sinusoidal pulse width modulation - SPWM Modulation index: M=Ar/Ac Varying Ar from 0 to Ac, will increase the δ thus the output will vary from 0 to Vs DF is less compared to MPWM eliminates the lower order harmonics of 2p-1, for this p=5, lowest order harmonics is 9th. the harmonics were pushed to the high frequency range of fc. So it would be much easier for low-pass filtering process. Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical Overmodulation leads to squarewave operation and add harmonics to the system Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical d) Modified sinusiodal pulse width modulation - MSPWM instead of 100% pulses, carrier wave is applied on 1st and last 60 degree of half cycle. advantages : a) Fundamental component increased b) Harmonics characteristics are improved. c) Reduce switching loss. Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical d) Modified sinusiodal pulse width modulation - MSPWM advantages : a) Fundamental component increased b) Harmonics characteristics are improved. c) Reduce switching loss. Systems Engineering School of Electrical ABD RAHIM 2008

Systems Engineering School of Electrical e) Phase displacement control multiple inverter are used, output is taken from summation of output voltage of the individual inverter. 2 half bridge inverter output Output with 180 degree displacement. Output with β degree displacement Systems Engineering School of Electrical ABD RAHIM 2008

Three-Phase Inverter Used to supply three-phase loads Three single-phase inverters could be used with 120 degree displacement or shifting between phases, however, 12 switches are necessary Consists of three legs, one for each phase One of the two switches in a leg is always ON at any instant Output of each leg depends on Vd and the switching status

Current source inverters a large inductor component inserted at the input input behaves like a current source The output current will maintained constant at any loads but output voltage forced to change Diodes in series are required to block the reverse voltage on the transistors. when two device in different leg conduct – IL flows through load when two device in same leg conduct – IL is bypassed from load (buck-bost regulator circuit) Systems Engineering School of Electrical ABD RAHIM 2008

Current source inverters Systems Engineering School of Electrical when two device in different leg conduct – IL flows through load when two device in same leg conduct – IL is bypassed from load (buck-bost regulator circuit) ABD RAHIM 2008

Current source inverters CSI are simpler than VSI  only required capasitor for comutation a large inductor component inserted at the input input behaves like a current source The output current will maintained constant at any loads but output voltage forced to change Diodes in series are required to block the reverse voltage on the transistors. Systems Engineering School of Electrical ABD RAHIM 2008

Current source inverters - operations assume T1 and T2 conducting, cap C1 and C2 are charged per picture once T3 and T4 are fired, T1 and T2 will be reverse biased and turned OFFF (impulse commutation). the circuit now flows thru T3C1D1-load-D2C2T4. the caps will be discharged and recharged at constant rate. Systems Engineering School of Electrical once fully charged, caps will act as open circuit. Current falls to zero. Load current will now be transferred thru D3 –load-D4. caps C1 and C2 is now ready to turn OFF T3 and T4 once T1 and T2 fired in next half cycle. the commutation time will depend on magnitude of load current and voltage. ABD RAHIM 2008

Current source inverters – 3-phase Systems Engineering School of Electrical only 2 thyristor conducts at same time each device conducts for 120 degrees phases current can be stated as: ABD RAHIM 2008

Current source inverters – 3-phase Other advantages of CSI: 1) the input DC current is controlled and limited, so misfiring of switching devices or short circuit would not be serious problem. 2) The peak current of power device is limited. 3) The commutation circuit for thyristor are much simpler. 4) each device conducts for 120 degrees 5) Requires no freewheeling diodes. Systems Engineering School of Electrical Disadvantages : Requires large reactor. Need extra front converter stage. Dynamic respond is slow. Due to current transfer between switches, output filter is required to suppress the output voltage spikes. ABD RAHIM 2008