Power Electronics Chapter 5 DC to AC Converters ( Inverters ) Power Electronics 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 2 Power Electronics Outline 5.1 Commutation 5.2 Voltage source inverters 5.3 Current source inverters 5.4 Multiple-inverter connections and multi-level inverters 3 Power Electronics 5.1 Commutation types Basic operation principle of inverters uo S1 Ud S2 io Load uo S3 S 4 io t1 t2 t 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 Power Electronics 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 5 Power Electronics Load commutation Condition: Load current is leading load voltage Application: capacitive load, synchronous motor 6 Power Electronics Forced commutation (capacitance commutation) Direct-Coupled With Coupling-Inductor 7 Power Electronics Another classification of commutations 4 types of Commutations Device commutation Self-commutation For fully-controlled devices Forced commutation Line commutation External commutation For thyristors Load commutation 8 Power Electronics 2 classes of inverters Voltage Source Inverter (VSI) Current Source Inverter (CSI) 9 Power Electronics 5.2 Voltage source inverter (VSI) + V3 VD 1 C V1 R io Ud VD3 L uo V2 VD 2 VD4 V4 - 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) 10 Power Electronics Single-phase half bridge VSI V1 Ud 2 VD io R Ud Ud 2 1 L uo VD V2 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 Ud/2. 11 Power Electronics Single-phase full bridge VSI Operation principle + V3 VD 3 VD1 C V1 R io Ud L uo V2 VD2 VD 4 V4 - 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. 12 Power Electronics Single-phase full bridge VSI Quantitative analysis Fourier series extension of output voltage 4U d ⎛ 1 1 ⎞ t + t + t + " ω ω sin 5 ω sin 3 sin (5-1) ⎜ ⎟ π ⎝ 5 3 ⎠ Magnitude of output voltage fundamental component uo = U o1m = 4U d π = 1.27U d (5-2) Effective value of output voltage fundamental component U o1 = 2 2U d π = 0.9U d (5-3) 13 Power Electronics Single-phase full bridge VSI Output voltage control by phase-shift uG1 + O V3 VD1 C V1 R io U d L uo V2 VD2 - VD 3 VD 4 V4 t uG2 O uG3 O uG4 t θ t O uo io O t io t1 t2 uo t3 t 14 Power Electronics Inverter with center-tapped transformer —push-pull inverter Load io uo + Ud - V1 V2 VD1 VD2 15 Power Electronics Three-phase VSI 180o conduction Dead time (blanking time) to avoid “shoot through” 16 Power Electronics Three-phase VSI Basic equations to obtain voltage waveforms For line voltage For phase voltage of the load U UN + U VN + U WN = 0 17 Power Electronics Three-phase VSI Quantitative analysis Fourier series extension of output line-to-line voltage u UV = = 2 3U d ⎛ 1 1 1 1 ⎞ sin 11ω t + sin 13 ω t − " ⎟ ⎜ sin ω t − sin 5ω t − sin 7ω t + π 13 5 7 11 ⎝ ⎠ 2 3U d ⎡ ⎢ sin ω t + π ⎣ ∑ n ⎤ 1 ( − 1) k sin n ω t ⎥ n ⎦ (5-8) Magnitude of output voltage (line-to-line) fundamental component 2 3U d U UV1m = = 1.1U d (5-10) π Effective value of output voltage (line-to-line) fundamental component U UV1 = U UV1m 2 = 6 π U d = 0.78U d (5-11) 18 Power Electronics 5.3 Current source inverter (CSI) Features DC side is constant current, high impedance (current source, or large inductor) AC side current is quasisquare 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. 19 Power Electronics Single-phase bridge CSI Parallel Resonant Inverter Ld Id A VT1 VT3 C LT1 io LT2 R VT2 LT3 uo LT4 L VT4 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. 20 Power Electronics Three-phase self-commutated CSI 120o conduction 21 Power Electronics Three-phase force-commutated CSI 22 Power Electronics Three-phase load-commutated CSI 23 Power Electronics 5.4 Multiple-inverter connections and multi-level inverters Series connection of 2 single-phase VSIs 24 Power Electronics Series connection of 2 3-phase VSIs 25 Power Electronics Multi-level Inverters 3-level inverter 26