Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2000-02-29
2001-07-31
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S138000
Reexamination Certificate
active
06269010
ABSTRACT:
FIELD OF INVENTION
The invention generally relates to the field of power electronics. Conjointly, the invention also relates to a drive for an alternating current (a.c.) induction motor which employs a current source inverter and features improved voltage regulation and suppressed resonance between the drive and motor. The improved drive is suitable for high power multi-motor applications.
BACKGROUND OF INVENTION
Pulse width modulated current source inverter (CSI) based a.c. motor drives are increasingly used in high power (e.g., 1,000~10,000 hp) applications. See, for instance, P. M. Espelage and J. M. Nowak, “Symmetrical GTO Current Source Inverter for Wide Speed Range Control of 2300 to 4160 volt, 350 to 7000 hp, Induction Motors”, IEEE IAS Annual Meeting, pp 302~307, 1988. Compared with voltage source inverter fed drives, the CSI drive features simple structure, reliable short circuit protection, four quadrant operation capability and nearly sinusoidal output voltage and current waveforms. In addition, the symmetrical gate turn-off thyristor (GTO) switching devices typically used in CSI drives can be easily connected in series, which makes the CSI drive particularly suitable for implementation at medium/high voltage levels such as 4160 Volts and up. Further details concerning the benefits of the CSI drive can be found in F. DeWinter and B. Wu, “Medium Voltage Motor Harmonic Heating, Torques and Voltage Stress When Applied on VFDs”, IEEE 43rd PCIC Conference, pp 131-139, 1996.
In many industrial applications, it is often necessary to control multiple motors in some manner. In these cases, it will be more economical to drive all motors by a single drive system rather than implementing individual drive/motor systems. To date, however, the CSI drive has typically been applied to singlerive/single-motor applications.
The CSI drive is not problem-free. In the CSI drive with a single a.c. induction motor, there exists a resonance mode due to the parallel connection of the output filter capacitor and the motor. This makes it difficult to stabilize the system if the drive operates at a frequency which is close to the resonant frequency. Further details concerning this problem can be found in the following two references, both of which are incorporated herein in their entirety: B. Wu, F. DeWinter, “Elimination of Harmonic Resonance in High Power GTO-CSI Induction Motor Drives”, IEEE PESC Conf. pp 1011-1015, 1994; and R. Itoh, “Stability of Induction Motor Drive Controlled by Current-source Inverter”, IEE Proc. Vol. 136, Pt. B, No. 2, pp 83-88, 1989. The situation becomes even worse when the motor is unloaded since the inverter output current in this case is minimal whereas the resonant current flowing between the capacitor and the motor magnetizing inductance is substantial.
A similar resonance problem also exists when a PWM rectifier is employed in the drive to provide direct current to the CSI from a power source. In this case, a resonance mode exists between an input a.c. filter capacitor of the rectifier and the system impedance of the line voltage source. If the resonance frequency is close to a characteristic harmonic of the rectifier an oscillation will occur, which makes the stability of the PWM rectifier sensitive to the system impedance. Unfortunately it is difficult to measure the system impedance accurately, which complicates the design of a compensating filter. In addition, even when the resonance frequency is not close to any characteristic harmonic of the rectifier, undesired oscillations will also occur during transient states. See additionally, N. R. Zargari, G. Joos, and P. D. Ziogas, “Input Filter Design for PWM Current-Source Rectifiers”, IEEE Trans. on Ind. Appl., vol. 30, No. 6, pp 1573-1579, 1994.
In a CSI drive with multiple motors, there are two major technical challenges which must be overcome to make such a drive practical. First, the motors connected to the inverter may have different sizes, which may produce multiple resonant modes. The effect of these and other resonant modes on drive stability should be minimized, and the drive should be able to operate steadily over a fill speed range. Second, the inverter output voltage should be kept constant both in steady and transient states for a given output frequency. In other words, the inverter output voltage should be stiff, not affected by changes in multiple motor loads. Otherwise an interaction between the motors and inverter will occur when one or more motors are loaded or unloaded, making the system unstable. The invention seeks to overcome various of these problems.
SUMMARY OF INVENTION
The general utility of the invention(s) described herein relate to improved CSI-based motor drives. However, those skilled in the art will understand that the various aspects of the invention may be employed more generally in the field of power electronics.
One aspect of the invention reduces the resonance existing between one or more output filter capacitors of a current source inverter and an a.c. induction motor. This is achieved through the use of active damping control, wherein the invention essentially simulates the use of a physical damping resistor connected in parallel with each output capacitor. This is accomplished by determining how much current would flow through the resistor had it been there, and this current, which is equal to the voltage across the output filter capacitor divided by the value of the resistor, is deducted from a command or reference current used to control the inverter. From a control point of view, the invention provides damping similar to the use of a real damping resistor, but without the corresponding energy loss.
Under this aspect of the invention, an inverter is provided which includes a switching bridge for converting direct current into alternating current. The direct current is coupled to a line side of the switching bridge and the alternating current is provided at a load side of the switching bridge. A switching pattern generator controls switches in the switching bridge based on a reference current. At least one capacitor is connected at a terminal thereof to the load side of the switching bridge. The terminal is connectable to a load having an inductance, such as an a.c. induction motor. Control circuitry connected to the switching pattern generator
(i) measures the load current or voltage and generates a nominal reference current based on an error therein;
(ii) determines a damping current based on the voltage at the terminal; and
(iii) determines the reference current supplied to the switching pattern generator by subtracting the damping current from the nominal reference current.
The active damping control may also be applied to a pulse width modulated (PWM) rectifier to reduce resonance caused between one or more input a.c. shunt capacitors of the PWM rectifier and the system impedance of a power source. In this variant of the invention, a PWM rectifier is provided which includes a switching bridge for converting alternating current into direct current. The alternating current is coupled to a line side of the switching bridge and the direct current is provided at a load side of the switching bridge. A switching pattern generator controls switches in the switching bridge based on a reference current. At least one shunt capacitor is connected at a terminal thereof to the line side of the switching bridge. An alternating current power source having a system inductance may be connected to the terminal. Control circuitry connected to the switching pattern generator:
(i) measures a load current or voltage and generates a nominal reference current based on an error therein;
(ii) determines a damping current based on the voltage at the terminal, and
(iii) determines the reference current supplied to the switching pattern generator by subtracting the damping current from the nominal reference current.
According to another variant of the invention, a drive is provided which comprises a PWM rectifier coupled to a current source inverter via a d.c. link choke. The PWM rectifier an
Ma Daming
Rizzo Steven C.
Wu Bin
Zargari Navid R.
Amin Himanshu S.
Gerasimow A. M.
Laxton Gary L.
Rockwell Technologies LLC
Walbrun William R.
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