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PowerElectronics Chapter 4 DC to AC Converters( Inverters )
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
Outline • 4.1 Commutation • 4.2 Voltage source inverters • 4.3 Current source inverters • 4.4 Multiple-inverter connections and multi-level inverters
S S 3 1 i Load o U d u S S o 4 2 4.1 Commutation types Basic operation principle of inverters • A classification of inverters • Square-wave inverters (are discussed in this chapter) • PWM inverters ( will be discussed in Chapter 6) • The concept of commutation
4 types of commutation • Device commutation: • Fully-controlled devices: GTO, IGBT, MOSFET • Line commutationPhase-controlled rectifier • Phase-controlled AC controller • Thyristor cycloconverter • Load commutation • Forced commutation
Load commutation • Condition: Load current is leading load voltage • Application: capacitive load, synchronous motor
Forced commutation (capacitance commutation) Direct-Coupled With Coupling-Inductor
Another classification of commutations 4 types of Commutations For fully-controlled devices Device commutation Self-commutation Forced commutation Line commutation For thyristors External commutation Load commutation
2 classes of inverters Voltage Source Inverter (VSI) Current Source Inverter (CSI)
+ V VD 3 3 VD 1 C V 1 R i L o U d u V o 2 VD 4 VD 2 V 4 - 4.2 Voltage source inverter (VSI) 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)
U G1 V 1 U d VD 2 1 U G2 L i R o U u d o u U o m U d VD 2 2 V 2 - i o t t 3 4 t t t t 1 2 5 6 V V V V 1 2 1 2 VD VD VD VD 1 2 1 2 Single-phase half bridge VSI • 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 Ud/2.
U G1,4 U G2,3 u o U m i o t t 3 4 t t t t 1 2 5 6 V V V V 1 2 1 2 V V V V 4 3 4 3 VD VD VD VD 1 2 1 2 VD VD VD VD 4 3 4 3 Single-phase full bridge VSI Operation principle + V VD 3 3 VD 1 C V 1 R i L o U d u V o 2 VD VD 4 2 V 4 - • 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.
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 (4-1) (4-2) (4-3)
+ V VD 3 3 VD 1 C V 1 R i L o U d u V o 2 VD 4 VD 2 V 4 - Single-phase full bridge VSI Output voltage control by phase-shift
Three-phase VSI • 180o conduction • Dead time (blanking time) to avoid “shoot through”
Three-phase VSI Basic equations to obtain voltage waveforms For line voltage For phase voltage of the load
Three-phase VSI Quantitative analysis • 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 (4-8) (4-10) (4-11)
4.3 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.
L A d I d VT VT 1 3 C i L L o T 1 T3 u o L L T 2 T4 R L VT VT 2 4 Single-phase bridge CSI Parallel Resonant Inverter • 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.
Three-phase self-commutated CSI • 120o conduction
4.4 Multiple-inverter connections and multi-level inverters Series connection of 2 single-phase VSIs
Multi-level Inverters • Ways to deal with higher voltage and achieve better waveform • Series connection of multiple converters • Series connection of multiple switch devices • Major type of multi-level inverters • Neutral point clamped multi-level inverter • Flying-capacitor multi-level inverter • Cascade H-bridge( series connected H-bridges) • In broad sense, previously discussed series connection of multiple inverters is also called multi-level inverter. • In narrow sense, only NPC and FC structures are called multi-level inverters.
