Power Electroni cs Chapter 5 DC to AC Converters ( Inverters ) Power E l e ct r o n i cs 2 Applications of Inverters Conversion of electric power from DC type energy sources to AC type load – Battery – Photovoltaic cell (Solar cell) – Fuel cell As a part of composite converter – AC-DC-AC frequency converter (for AC motor drive) – AC-DC-AC constant-voltage constant-frequency converter (for uninterruptable power supplies) – AC-DC-AC Converters for induction heating – AC-DC-AC-DC switching power supplies Power E l e ct r o n i cs 3 Outline 5.1 Commutation 5.2 Voltage source inverters 5.3 Current source inverters 5.4 Multiple-inverter connections and multi-level inverters Power E l e ct r o n i cs 4 5.1 Commutation types Basic operation principle of inverters t u o i o t 1 t 2 -PBE S 1 S 2 S 3 S 4 i o u o U d A classification of inverters – Square-wave inverters (are discussed in this chapter) – PWM inverters ( will be discussed in Chapter 6) The concept of commutation Power E l e ct r o n i cs 5 4 types of commutation Device commutation: Fully-controlled devices: GTO, IGBT, MOSFET Line commutation Phase-controlled rectifier Phase-controlled AC controller Thyristor cycloconverter Load commutation Forced commutation Power E l e ct r o n i cs 6 Load commutation Condition: Load current is leading load voltage Application: capacitive load, synchronous motor Power E l e ct r o n i cs 7 Forced commutation (capacitance commutation) Direct-Coupled With Coupling-Inductor Power E l e ct r o n i cs 8 Another classification of commutations Self-commutation Device commutation Forced commutation Line commutation Load commutation 4 types of Commutations For fully-controlled devices For thyristors External commutation Power E l e ct r o n i cs 9 2 classes of inverters Voltage Source Inverter (VSI) Current Source Inverter (CSI) Power E l e ct r o n i cs 10 5.2 Voltage source inverter (VSI) +  C R L U d V 1 V 2 V 3 V 4 VD 1 VD 2 VD 3 VD 4 u o i o Features DC side is constant voltage, low impedance (voltage source, or bulk cap) AC side voltage is square wave or quasi-square wave. AC side current is determined by the load. Anti-parallel diodes are necessary to provide energy feedback path. (freewheeling diodes , feedback diodes) Power E l e ct r o n i cs 11 Single-phase half bridge VSI t t O O ON u o U m U m i o t 1 t 2 t 3 t 4 t 5 t 6 V 1 V 2 V 1 V 2 VD 1 VD 2 VD 1 VD 2  R L U d i o u o V 1 V 2 VD 1 VD 2 U d 2 U d 2 The current conducting path is determined by the polarity of load voltage and load current. (This is true for analysis of many power electronics circuits.) The magnitude of output square-wave voltage is U d /2. Power E l e ct r o n i cs 12 Single-phase full bridge VSI Operation principle +  C R L U d V 1 V 2 V 3 V 4 VD 1 VD 2 VD 3 VD 4 u o i o The magnitude of output square-wave voltage is Ud. The effective value of output voltage (or fundamental output voltage) can be changed by changing Ud. Power E l e ct r o n i cs 13 Single-phase full bridge VSI Quantitative analysis Fourier series extension of output voltage Magnitude of output voltage fundamental component Effective value of output voltage fundamental component ? ? ? ? ? ? +++="ttt U u ωωω π 5sin 5 1 3sin 3 1 sin 4 d o d d o1m 27.1 4 U U U == π d d 1o 9.0 22 U U U == π (5-1) (5-2) (5-3) Power E l e ct r o n i cs 14 Single-phase full bridge VSI Output voltage control by phase-shift t O t O t O t O t O θ u G1 u G2 u G3 u G4 u o i o t 1 t 2 t 3 i o u o +  C R L U d V 1 V 2 V 3 V 4 VD 1 VD 2 VD 3 VD 4 u o i o Power E l e ct r o n i cs 15 Inverter with center-tapped transformer —push-pull inverter -PBE +  i o u o U d V 1 V 2 VD 1 VD 2 Power E l e ct r o n i cs 16 Three-phase VSI 180 o conduction Dead time (blanking time) to avoid “short through” Power E l e ct r o n i cs 17 Three-phase VSI Basic equations to obtain voltage waveforms For phase voltage of the load 0=++ WNVNUN UUU For line voltage Power E l e ct r o n i cs 18 Three-phase VSI Fourier series extension of output line-to-line voltage Magnitude of output voltage (line-to-line) fundamental component Effective value of output voltage (line-to-line) fundamental component Quantitative analysis ? ? ? ? ? ? ?+= ? ? ? ? ? ? ?++??= ∑ n k tn n t U ttttt U u ωω π ωωωωω π sin)1( 1 sin 32 13sin 13 1 11sin 11 1 7sin 7 1 5sin 5 1 sin 32 d d UV " d d UV1m 1.1 32 U U U == π dd UV1m UV1 78.0 6 2 UU U U === π (5-8) (5-10) (5-11) Power E l e ct r o n i cs 19 5.3 Current source inverter (CSI) Features DC side is constant current, high impedance (current source, or large inductor) AC side current is quasi- square wave. AC side voltage is determined by the load. No anti-parallel diodes are needed. sometimes series diodes are needed to block reverse voltage for other power semiconductor devices. Power E l e ct r o n i cs 20 Single-phase bridge CSI A C RL L d I d VT 1 VT 2 VT 3 VT 4 L T1 L T2 L T3 L T4 u o i o Switching frequency is a little higher than the resonant frequency so that the load becomes capacitive and load current is leading voltage to realize load commutation. Parallel Resonant Inverter Power E l e ct r o n i cs 21 Three-phase self-commutated CSI Power E l e ct r o n i cs 22 Three-phase force-commutated CSI Power E l e ct r o n i cs 23 Three-phase load-commutated CSI Power E l e ct r o n i cs 24 5.4 Multiple-inverter connections and multi-level inverters Series connection of 2 single-phase VSIs Power E l e ct r o n i cs 25 Series connection of 2 3-phase VSIs Power E l e ct r o n i cs 26 Multi-level Inverters 3-level inverter