He has done extensive research in power electronics and motor drive areas,.. Written for upper-level undergraduates, graduate students, and professionals, this book presents the basics of. Then, the recent advances of power semiconductor devices, converters, variable- frequency AC drives, and advanced control and estimation techniques will be.
Modern Power Electronics and Ac Drives treadanophthi. Formerly, the mercury arc valve , the high-vacuum and gas-filled diode thermionic rectifiers, and triggered devices such as the thyratron and ignitron were widely used in power electronics. As the ratings of solid-state devices improved in both voltage and current-handling capacity, vacuum devices have been nearly entirely replaced by solid-state devices.
Power electronic devices may be used as switches, or as amplifiers.
Semiconductor devices used as switches can approximate this ideal property and so most power electronic applications rely on switching devices on and off, which makes systems very efficient as very little power is wasted in the switch. By contrast, in the case of the amplifier, the current through the device varies continuously according to a controlled input. The voltage and current at the device terminals follow a load line , and the power dissipation inside the device is large compared with the power delivered to the load.
Several attributes dictate how devices are used. Devices such as diodes conduct when a forward voltage is applied and have no external control of the start of conduction. Power devices such as silicon controlled rectifiers and thyristors as well as the mercury valve and thyratron allow control of the start of conduction, but rely on periodic reversal of current flow to turn them off. Devices such as gate turn-off thyristors, BJT and MOSFET transistors provide full switching control and can be turned on or off without regard to the current flow through them.
Transistor devices also allow proportional amplification, but this is rarely used for systems rated more than a few hundred watts.
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The control input characteristics of a device also greatly affect design; sometimes the control input is at a very high voltage with respect to ground and must be driven by an isolated source. As efficiency is at a premium in a power electronic converter, the losses that a power electronic device generates should be as low as possible. Devices vary in switching speed. Some diodes and thyristors are suited for relatively slow speed and are useful for power frequency switching and control; certain thyristors are useful at a few kilohertz.
Vacuum tube devices dominate high power hundreds of kilowatts at very high frequency hundreds or thousands of megahertz applications.
Faster switching devices minimize energy lost in the transitions from on to off and back, but may create problems with radiated electromagnetic interference. Gate drive or equivalent circuits must be designed to supply sufficient drive current to achieve the full switching speed possible with a device. A device without sufficient drive to switch rapidly may be destroyed by excess heating. Practical devices have non-zero voltage drop and dissipate power when on, and take some time to pass through an active region until they reach the "on" or "off" state.
These losses are a significant part of the total lost power in a converter. Power handling and dissipation of devices is also a critical factor in design. Power electronic devices may have to dissipate tens or hundreds of watts of waste heat, even switching as efficiently as possible between conducting and non-conducting states. In the switching mode, the power controlled is much larger than the power dissipated in the switch.
The forward voltage drop in the conducting state translates into heat that must be dissipated. High power semiconductors require specialized heat sinks or active cooling systems to manage their junction temperature; exotic semiconductors such as silicon carbide have an advantage over straight silicon in this respect, and germanium, once the main-stay of solid-state electronics is now little used due to its unfavorable high temperature properties.
Semiconductor devices exist with ratings up to a few kilovolts in a single device.
Where very high voltage must be controlled, multiple devices must be used in series, with networks to equalize voltage across all devices. Again, switching speed is a critical factor since the slowest-switching device will have to withstand a disproportionate share of the overall voltage. Mercury valves were once available with ratings to kV in a single unit, simplifying their application in HVDC systems. The current rating of a semiconductor device is limited by the heat generated within the dies and the heat developed in the resistance of the interconnecting leads.
Semiconductor devices must be designed so that current is evenly distributed within the device across its internal junctions or channels ; once a "hot spot" develops, breakdown effects can rapidly destroy the device. Certain SCRs are available with current ratings to amperes in a single unit. Topologies for these converters can be separated into two distinct categories: voltage source inverters and current source inverters.
Voltage source inverters VSIs are named so because the independently controlled output is a voltage waveform. Similarly, current source inverters CSIs are distinct in that the controlled AC output is a current waveform. DC to AC power conversion is the result of power switching devices, which are commonly fully controllable semiconductor power switches.
The output waveforms are therefore made up of discrete values, producing fast transitions rather than smooth ones. For some applications, even a rough approximation of the sinusoidal waveform of AC power is adequate.
Where a near sinusoidal waveform is required, the switching devices are operated much faster than the desired output frequency, and the time they spend in either state is controlled so the averaged output is nearly sinusoidal. Common modulation techniques include the carrier-based technique, or Pulse-width modulation , space-vector technique , and the selective-harmonic technique.
Voltage source inverters have practical uses in both single-phase and three-phase applications. Single-phase VSIs utilize half-bridge and full-bridge configurations, and are widely used for power supplies, single-phase UPSs, and elaborate high-power topologies when used in multicell configurations. They are also used in applications where arbitrary voltages are required as in the case of active power filters and voltage compensators.
Current source inverters are used to produce an AC output current from a DC current supply. This type of inverter is practical for three-phase applications in which high-quality voltage waveforms are required. A relatively new class of inverters, called multilevel inverters, has gained widespread interest. Normal operation of CSIs and VSIs can be classified as two-level inverters, due to the fact that power switches connect to either the positive or to the negative DC bus. If more than two voltage levels were available to the inverter output terminals, the AC output could better approximate a sine wave.
It is for this reason that multilevel inverters, although more complex and costly, offer higher performance. Each inverter type differs in the DC links used, and in whether or not they require freewheeling diodes. Either can be made to operate in square-wave or pulse-width modulation PWM mode, depending on its intended usage.
MODERN POWER ELECTRONICS AND AC DRIVES
Square-wave mode offers simplicity, while PWM can be implemented several different ways and produces higher quality waveforms. Voltage Source Inverters VSI feed the output inverter section from an approximately constant-voltage source. The desired quality of the current output waveform determines which modulation technique needs to be selected for a given application.
The output of a VSI is composed of discrete values. In order to obtain a smooth current waveform, the loads need to be inductive at the select harmonic frequencies. Without some sort of inductive filtering between the source and load, a capacitive load will cause the load to receive a choppy current waveform, with large and frequent current spikes. The single-phase voltage source half-bridge inverters, are meant for lower voltage applications and are commonly used in power supplies.
Low-order current harmonics get injected back to the source voltage by the operation of the inverter. This means that two large capacitors are needed for filtering purposes in this design. If both switches in a leg were on at the same time, the DC source will be shorted out.
Inverters can use several modulation techniques to control their switching schemes. If the over-modulation region, ma, exceeds one, a higher fundamental AC output voltage will be observed, but at the cost of saturation. For SPWM, the harmonics of the output waveform are at well-defined frequencies and amplitudes. This simplifies the design of the filtering components needed for the low-order current harmonic injection from the operation of the inverter.
The maximum output amplitude in this mode of operation is half of the source voltage. If the maximum output amplitude, m a , exceeds 3. As was true for PWM, both switches in a leg for square wave modulation cannot be turned on at the same time, as this would cause a short across the voltage source. Therefore, the AC output voltage is not controlled by the inverter, but rather by the magnitude of the DC input voltage of the inverter.
Using selective harmonic elimination SHE as a modulation technique allows the switching of the inverter to selectively eliminate intrinsic harmonics. The fundamental component of the AC output voltage can also be adjusted within a desirable range. Since the AC output voltage obtained from this modulation technique has odd half and odd quarter wave symmetry, even harmonics do not exist.
The full-bridge inverter is similar to the half bridge-inverter, but it has an additional leg to connect the neutral point to the load. Any modulating technique used for the full-bridge configuration should have either the top or the bottom switch of each leg on at any given time. Due to the extra leg, the maximum amplitude of the output waveform is Vi, and is twice as large as the maximum achievable output amplitude for the half-bridge configuration. The AC output voltage can take on only two values, either Vi or —Vi. To generate these same states using a half-bridge configuration, a carrier based technique can be used.
Unlike the bipolar PWM technique, the unipolar approach uses states 1, 2, 3 and 4 from Table 2 to generate its AC output voltage. Therefore, the AC output voltage can take on the values Vi, 0 or —V i.
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To generate these states, two sinusoidal modulating signals, Vc and —Vc, are needed, as seen in Figure 4. The phase voltages VaN and VbN are identical, but degrees out of phase with each other. The output voltage is equal to the difference of the two phase voltages, and do not contain any even harmonics. Therefore, if mf is taken, even the AC output voltage harmonics will appear at normalized odd frequencies, fh. These frequencies are centered on double the value of the normalized carrier frequency.