Unit 10 Converters PDF

Unit 10 Converters 1. Type of convertors 2. IGBT transistors 3. Discuss the diagram of an inverter 4. Describe applications and harmonics filtering for inverters Industrial power supplies are used in applications where a variety of voltages is required, such as power for PLC processo...

Unit 10 Converters 1. Type of convertors 2. IGBT transistors 3. Discuss the diagram of an inverter 4. Describe applications and harmonics filtering for inverters Industrial power supplies are used in applications where a variety of voltages is required, such as power for PLC processor and their analog modules and other specialty modules. You’ll find a power supply in every electrical device on factory floor. Any equipment that has electronic circuits in it must have a DC supply voltage available. Since all power in the factory originates as AC voltage, converters must be used in the power supply to convert AC power to DC power. These circuits are called converters. Types of power electronic converters 1. Rectifiers (AC to DC converters): These converters convert constant AC voltage to variable DC output voltage. 2. Choppers – or switching regulators (DC to DC converters): DC chopper converts fixed DC voltage to a controllable (variable) DC output voltage (used to control the speed of DC motors). 3. Inverters (DC to AC converters): An inverter converts fixed DC voltage to an AC output voltage (used for variable frequency AC motor drives, power plant/stations, uninterruptible power supplies (UPS)/commercial/industrial, induction heating, AC power supply from photovoltaic (PV) array and welding applications). 4. AC voltage controllers: These converters convert fixed AC voltage to a variable AC output voltage at same frequency. 5. Cycloconverters: These circuits convert input power at one frequency to output power at a different frequency through one stage conversion (without first converting the voltage to DC). 350 Unit 10 IC 1 Power semiconductor devices - Power Diodes (6000 V, 4500 A) - Power Transistors BJT's (1200 V, 400 A) - Power MOSFET’s (1000 V, 100 A) - Thyristors (6000 V, 4500 A) - IGBT's (2500 V, 2400 A) Power Diodes The diodes have the following advantages: - High mechanical and thermal reliability - High peak inverse voltage - Low reverse current - Low forward - Voltage drop - High efficiency and compactness. Power Transistors Power transistors are devices that have controlled turn-on and turn-off characteristics. These devices are used a switching devices and are operated in the saturation region resulting in low on-state voltage drop. They are turned on when a current signal is given to base or control terminal. The transistor remains on so long as the control signal is present. The switching speed of modern transistors is much higher than that of thyristors and is used extensively in dc-dc and dc-ac converters. However, their voltage and current ratings are lower than those of thyristors and are therefore used in low to medium power applications. Power transistors are classified as follows: - Bipolar junction transistors(BJTs) - Metal-oxide semiconductor filed-effect transistors (MOSFETs) - Insulated-gate bipolar transistors (IGBTs) 350 Unit 10 IC 2 Advantages of BJT’s - BJT’s have high switching frequencies since their turn-on and turn-off time are low - The turn-on losses of a BJT are small - BJT has controlled turn-on and turn-off characteristics since base drive control is possible - BJT does not require commutation circuits Demerits of BJT - Drive circuit of BJT is complex. - It has the problem of charge storage which sets a limit on switching frequencies. - It cannot be used in parallel operation due to problems of negative temperature coefficient. Thyristors – Silicon Controlled Rectifiers (SCR’s) A silicon controlled rectifier or semiconductor-controlled rectifier is a four- layer solid-state current controlling device. SCRs are mainly used in electronic devices that require control of high voltage and power. This makes them applicable in medium and high AC power operations such as motor control function. Resistance Firing circuit for SCR and corresponding waveforms In this, the triggering angle is limited to 90 degrees only. Because the applied voltage is maximum at 90 degrees so the gate current has to reach minimum gate current value somewhere between zero to 90 degrees. 350 Unit 10 IC 3 Insulated Gate Bipolar Transistor (IGBT) An IGBT combines the advantages of BJT’s and MOSFET’s. Similar to the MOSFET is controlled by the gate voltage, the IGBT has a high input impedance, which requires only a small amount of energy to switch the device. Like the BJT, the IGBT has a small/low on-state voltage (conduction losses) even in devices with large blocking voltage ratings (for example, Von is 2-3 V in a 1000 V device). Insulated gate bipolar transistors have turn-on and turn-off times on the order of 1 μs and are available in module ratings as large as 2500 V, and 2400 A. IGBT – Symbol: G = Gate C = Colector E = Emitter IGBT Characteristics: As IGBT is a voltage controlled device, when VGE is less than VGET (gate emitter threshold voltage), IGBT is in off state. The controlling parameter is gate - emitter voltage (VGE). a) b) c) 350 Unit 10 IC 4 a) Equivalent diagram of IGBT b) Forward (static) characteristics (V-I) of IGBT c) Transfer characteristics of IGBT DC to AC Converter (Inverter) Converts DC to AC power by switching the DC input voltage (or current) in a pre-determined sequence so as to generate AC voltage (or current) output. General block diagram of an inverter: Typical applications: – Uninterruptible power supply (UPS), industrial (induction motor) AC motors drives, HVDC (high-voltage direct current, also called a power superhighway or an electrical superhighway) power plant/stations, induction heating, AC power supply from photovoltaic (PV) array. Typical application for an inverter (UPS) 350 Unit 10 IC 5 How it works? When a DC voltage is applied to the inverter, one of the transistors will saturate (closed circuit) and the other one is in cut-off (open). Suppose the transistor T1 has closed the circuit first. The collector current of T1 produces a magnetic field in the half of the primary of the transformer (closed circuit) and T2 is opened circuit (OFF state/cut-off). When the T1 is OFF (open circuit/cut-off) and T2 has closed, the current from battery will flow through the second half of the primary of the transformer in a reverse direction (it causes the voltage induction to be the opposite polarity in the secondary coil of a transformer). The T1 and T2 will still conduct current until the transformer core reaches saturation. After that, this process of the closed-opened circuit between T1 and T2 will be the same again. L1 and C2- harmonic filter (Low – pass filter) - to make the output AC voltage to be smoothed or low noise. Harmonics Filtering Voltage and Harmonic Control A periodic waveform that has frequency, which is a multiple integral of the fundamental power with frequency of 60 Hz is known as a harmonic. Total harmonic distortion (THD) on the other hand refers to the total contribution of all the harmonic current frequencies. Output of the inverter is “chopped AC voltage with zero DC component”. It contains harmonics. 350 Unit 10 IC 6 An LC section low-pass filter is normally fitted at the inverter output to reduce the high frequency harmonics. In some applications such as UPS, “high purity” sine wave output is required. Good filtering is a must. In some applications (AC motor drive), filtering is not required. Output voltage harmonics/distortion - Harmonics cause distortion on the output voltage. - Lower order harmonics (3rd, 5th etc) are very difficult to filter, due to the filter size and high filter order. They can cause serious voltage distortion. - Study of harmonics requires understanding of wave shapes. Fourier Series is a tool to analyze wave shapes. Spectra of square wave: Spectra (harmonics) characteristics: – Harmonic decreases with a factor of (1/n). – Even harmonics are absent – Nearest harmonics is the 3rd. If fundamental frequency is 50 Hz, then nearest harmonic is 150 Hz. (60 Hz  180 Hz) – Due to the small separation between the fundamental and harmonics, output low-pass filter design can be very difficult. 350 Unit 10 IC 7