Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2000-07-24
2002-04-02
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S037000, C363S041000
Reexamination Certificate
active
06366483
ABSTRACT:
FIELD OF INVENTION
The invention generally relates to the field of power electronics. More particularly, the invention relates to a pulse-width modulated (PWM) rectifier wherein the reactive and active powers are controlled independent of one another for improved performance. The PWM rectifier has particular utility in a current source inverter (CSI) based drive for controlling one or more high power alternating current (a.c.) induction motors.
BACKGROUND OF INVENTION
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 7000hp, 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 single-drive/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.
When the PWM rectifier is used to provide direct current to the CSI it can sometimes be difficult to tune the control compensators of the PWM rectifier. This will be better understood by reference to
FIG. 11
which shows a block diagram of a typical control scheme for a PWM rectifier
100
with unity power factor control which, in conjunction with a smoothing d.c. link inductor L
dc
, provides direct current for CSI
110
. The rectifier comprises two control loops: a power factor control loop
112
and a d.c. link current control loop
114
. In the power factor control loop
112
, the phase angle between the source voltage {right arrow over (v)}
s
and the source current {right arrow over (i)}
s
is detected by a phase detector
116
and sent to a p.i. proportional, integral) compensator
118
which controls the phase angle &agr; of the rectifier output current in order to ensure a unity power factor on the voltage source
22
. The d.c. link current i
dc
is controlled by adjusting the modulation index M of the rectifier
100
by another p.i. compensator
120
so as to minimize the error between a requested d.c. link current i*
dc
(this signal is provided by CSI controller) and the actual or measured value of the d.c. link current i′
dc
. The source voltage {right arrow over (v)}
s
is detected to provide the synchronizing signal for the rectifier
100
. This control scheme is not entirely satisfactory because the phase angle control loop
112
effects not only the power factor but also the d.c. link current. Similarly, the modulation index control loop
114
effects the power factor. Consequently, the coupling between these control loops
112
and
114
can make it difficult to tune the p.i. compensators
118
and
120
. Another drawback of this control scheme is that the rectifier
100
maybe saturated under some operating conditions due to the power factor compensation.
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 full 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. A solution to these problems is described herein.
Furthermore, when the PWM rectifier is used to provide d.c. current to the CSI, the problem of tuning the control compensators is present. The invention seeks to overcome this problem.
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.
Generally speaking, the invention provides a rectifier which has independent power factor and d.c. link current control loops. This is accomplished by separately controlling the active power and the reactive power of the rectifier.
According to one aspect of the invention a rectifier is provided which comprises a switching bridge for converting alternating current into direct current. The bridge features a line side and a load side. The bridge has m input filter capacitors, each connected at a terminal thereof, to the line side. These terminals are used to connect an m-phase a.c. power supply (m>=1) having a per phase system inductance to the bridge. A current smoothing inductor is connected to the load side of the bridge and enables a load to be connected thereto. A switching pattern generator controls the switches in the switching bridge based on a reference output current. A first control loop is provided for determining an active portion of the reference output current based on a desired power level of the load and an m-phase voltage at said terminals. A second control loop is provided for det
Ma Daming
Rizzo Steven C.
Wu Bin
Zargari Navid R.
Amin Himanshu S.
Gerasimow Alexander M.
Patel Rajnikant B.
Rockwell Automation Technologies Inc.
Walbrun William R.
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