Control system and method for four-quadrant switches in...

Electric power conversion systems – Frequency conversion without intermediate conversion to d.c. – By semiconductor converter

Reexamination Certificate

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Reexamination Certificate

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06459606

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention generally relates to control systems and methods for three-phase pulse-width-modulated (PWM) alternating-current (AC) voltage regulators (also referred to as AC choppers). More particularly, the present invention relates to a system and method for implementing a set of four-quadrant switches in a pulse-width-modulated alternating-current voltage regulator based on commutation of the load currents among different converter switches.
A number of applications, such as air-conditioning or refrigeration applications, utilize multi-phase induction motors. The starting, or inrush, current for multi-phase motors tends to be several times the rated full-load current. This high inrush current may have many detrimental effects on the equipment and the power system in general, as well as the economics of power usage. By means of example only, drawing high inrush current over a long power line may cause the voltage to essentially collapse, leaving insufficient voltage for equipment to operate. Furthermore, with high motor inrush current, other customers along the same power line may experience undesirable voltage fluctuations during the start of the motor. To discourage this situation, power companies sometimes impose penalties if a customer's starting or inrush currents are excessive. This is particularly true in regions with “weak” power grids, such as Europe. Thus, it is desirable to minimize the current drawn by a multi-phase induction motor during starting.
Several known methods exist which allow for the reduction of induction motors' inrush current. Use of an autotransformer is one known method for achieving lower motor starting currents. Autotransformers, however, are relatively inflexible in that the turns ratio of an autotransformer is established up front and remains fixed by the design of the components. Another approach employs the use of series elements such as inductors, resistors, and the like, to limit starting current. The latter approach, however, requires significantly higher line currents than autotransformer starters to provide the same amount of torque. Yet another approach consists in employing the so-called wye-delta motor starters. This type of equipment configures the connection of induction motor windings in a different manner during the motor start-up than during the regular motor operation. This allows the motor to start with a reduced inrush current.
The above methods for achieving reduced motor inrush currents can all be characterized as electro-mechanical methods. They require a set of electro-mechanical contactors in order to alter the connection of an induction motor to the power line. This altering of connection further results in a reduced voltage being applied to each of the motor's windings, which in turn results in reduced inrush currents. Electro-mechanical contactors have the disadvantage of being expensive and prone to reliability problems due to wear and tear. In addition, their transitions can cause voltage or current spikes with potentially damaging effects to the system.
The problems associated with electro-mechanical starting methods for induction motors can be avoided by employing electronic (or solid-state) starting methods. Electronic motor starters reduce the voltage supplied to an induction motor during its startup by means of a power electronics converter. One such converter technology employs thyristors, also called silicon-controlled rectifiers (SCRs). SCRs are semiconductor switches that can be turned on by means of an electronic signal. However, they cannot be forcefully turned off, i.e. they can be turned off only if the current through them naturally extinguishes itself. In a typical SCR-based motor starter, two SCRs are back-to-back connected between each of the motor's phases and the power line. During the motor start-up, the SCRs are turned on only once in every line cycle, and this is done in a delayed fashion, so that the motor is actually connected to the power line for only a portion of each line period. This results in a reduced voltage being applied to the motor, and therefore a reduced inrush current being drawn from the power line. The amplitude of the fundamental voltage being supplied to the motor is controlled by the time instant when an SCR is turned on within a line cycle. This type of control is usually referred to as phase control.
With no electro-mechanical contactors needed, SCR-based solid-state motor starters represent an improvement over electro-mechanical starters in terms of reliability and cost. However, SCR-based electronic starters have the disadvantage of distorting motor's current and voltage waveforms during the start-up. In addition, they offer no possibility for improving the power factor (power factor is intended as the phase displacement between the fundamental component of voltage and current at the line terminals feeding the motor starter). A good power factor is generally a desired feature in any electrical system.
Alternatively, PWM AC voltage regulators can be used for starting large induction motors, as they allow for a significant reduction in inrush current and provide better quality motor current and voltage waveforms during start-up than SCR-based motor starters. Similar to SCR-based technology, a PWM AC voltage regulator includes power electronics converter capable of supplying an output voltage of a fixed frequency, but at a variable magnitude, to AC loads. PWM AC voltage regulators differ from phase-controlled SCR-based AC voltage regulators, in that, with PWM AC voltage regulators, the switching of power semiconductors occurs at a frequency (called the switching frequency) many times higher than the input line frequency (usually equal to 50 or 60 Hz). Such high rate of semiconductor switching can be achieved with modern power semiconductors with full turn-on and turn-off capability, such as, for example, insulated gate bipolar transistors (IGBTs). The control of the fundamental amplitude of the output voltage of a PWM AC voltage regulator is achieved through the control of the width of the pulses of which the output waveform in such a regulator consists. A single-phase PWM AC voltage regulator circuit is described in U.S. Pat. No. 5,923,143 to Cosan et al. entitled “Solid State Motor Starter with Energy Recovery.”
PWM AC voltage regulators, when used for starting of induction motors, have several advantages compared to SCR-based motor starters. First, they are able to start a motor with a smaller fundamental component of line inrush current. Typically, if an SCR-based motor starter requires an inrush current equal to 45% of motor's locked-rotor current (LRA), a PWM AC voltage regulator used with the same motor shall require around 20% of LRA. Second, PWM AC voltage regulators generate better-quality motor current and voltage waveforms during the start-up. This results in lower pulsating torque produced by the motor, which, in turn, benefits the motor's mechanical driveline. Finally, PWM AC voltage regulators offer the possibility for power factor correction.
Practical implementation of three-phase PWM AC voltage regulators requires semiconductor switches that can conduct, or block, electric current flow in either direction in a fully controllable manner. Such switches are also referred to as four-quadrant switches. No such single semiconductor device is available commercially nowadays. Therefore, four-quadrant switches are implemented as a combination of two two-quadrant switches (i.e. switches that can fully control the current flow in one direction only). Examples of two-quadrant semiconductor switches are bipolar junction transistors (BJTs), gate turn-off thyristors (GTOs), field-effect transistors (FETs) and previously mentioned IGBTs. When both two-quadrant switches in a four-quadrant switch are turned on, the four-quadrant switch conducts electric current in either direction. When both two-quadrant switches are turned off, the four-quadrant switch blocks electric current from either direction. When o

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