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**Chapter 4 DC to AC Converters ( Inverters )**

Power Electronics Chapter DC to AC Converters ( Inverters )

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**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

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**Outline 4.1 Commutation 4.2 Voltage source inverters**

4.3 Current source inverters 4.4 Multiple-inverter connections and multi-level inverters

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**4.1 Commutation types Basic operation principle of inverters**

Load S 1 2 3 4 i o u 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

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**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

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**Load commutation Condition: Load current is leading load voltage**

Application: capacitive load, synchronous motor

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**Forced commutation (capacitance commutation)**

Direct-Coupled With Coupling-Inductor

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**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

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**2 classes of inverters Voltage Source Inverter (VSI)**

Current Source Inverter (CSI)

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**4.2 Voltage source inverter (VSI)**

+ - C R L U d V 1 2 3 4 VD u o i 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)

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**Single-phase half bridge VSI**

U d i o u V 1 2 VD u o U m i t 1 2 3 4 5 6 V VD G1 G2 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.

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**Single-phase full bridge VSI**

o U m i t 1 2 3 4 5 6 V VD G1,4 G2,3 Operation principle + V 3 VD 3 VD 1 C V 1 R i L U o d u V 2 o 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.

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**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)

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**Single-phase full bridge VSI**

Output voltage control by phase-shift + - C R L U d V 1 2 3 4 VD u o i

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**Inverter with center-tapped transformer —push-pull inverter**

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**Dead time (blanking time) to avoid “shoot through”**

Three-phase VSI 180o conduction Dead time (blanking time) to avoid “shoot through”

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**Basic equations to obtain voltage waveforms**

Three-phase VSI Basic equations to obtain voltage waveforms For line voltage For phase voltage of the load

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**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)

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**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.

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**Single-phase bridge CSI**

Parallel Resonant Inverter A C R L d I VT 1 2 3 4 T T3 T4 u o i 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.

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**Three-phase self-commutated CSI**

120o conduction

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**Three-phase force-commutated CSI**

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**Three-phase load-commutated CSI**

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**4.4 Multiple-inverter connections and multi-level inverters**

Series connection of 2 single-phase VSIs

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**Series connection of 2 3-phase VSIs**

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**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.

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**Neutral-Point-Clamped 3-level inverter**

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**Neutral-Point-Clamped 5-level inverter**

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**Flying-Capacitor 3-level inverter**

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**Series connection of 3 H-bridges**

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