Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
2001-08-10
2003-04-08
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C363S044000, C363S084000, C363S047000
Reexamination Certificate
active
06545887
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of 3-phase power factor corrected rectifiers, active power filters, and grid-connected inverters and in particular to control methods based on one-cycle control.
2. Description of the Prior Art
The invention in this document covers vast applications spanning from power factor corrected rectifiers in front of and active power filters in parallel to electronic equipement such as computers, communication, motion control, aviation, space electronics, etc. to the grid-connected inverters for distributed power generation.
Power factor corrected rectifiers
In recent years, the usage of modern electronics equipment has been widely proliferating. The electronics equipment usually have a rectifier of single-phase or three-phases in the front end. Three-phases are more desirable for high power applications. A three-phase rectifier is a device that converts three-phase sinusoidal ac power into dc power. Traditional rectifiers draw pulsed currrent from the ac main as shown in
FIG. 1
, which causes significant harmonic pollution, low power factor, reduced transmition efficiency, harmful electromagnetic interference to neighborhood appliances, as well as overheating of transformers.
In order to solve these problems, many international agencies have proposed harmonic restrictions to electronic equipment. As a result, a vast number of power factor corrected (PFC) rectifers have been proposed to comply with these regulations.
A three-phase power factor corrected rectifeir is a device that converts three-phase sinusoidal ac power into dc power while the input currents are sinusoidal and unity power factor, as shown in FIG.
2
. Many three-phase topologies are suitable for implementing PFC function for rectification. Usually, high frequency active switches are used in the rectifiers to realize the PFC function.
The control methods that modulate the pulse width of the switches are an important issue in the power electronics research. A third harmonic injection method was reported for a dual-boost converter with center-tapped dc-link and split output capacitors. This method achieves low current distortion. However, it is not convenient to generate the third harmonic signal tuned to the right frequency and right amplitude.
Hysteresis control and d-q transformation control were frequently used control approaches. Hysteresis control results in variable switching frequency that is difficult for EMI filter design. The d-q approach is based on digital implementation that leads to complicated systems. An analog control method with constant switching frequency modulation was reported for a particular rectifier, where several multipliers are necessary to implement the three phase current references. Due to the disadvantages of variable frequency or complexity of implementation, three-phase PFC rectifiers are not commercially practical.
Active power filters:
One alternative for dealing with the current harmonics generated by treaditional rectifiers is to use active power filters (APF). Considering electronic equipment with traditional rectifier as nonlinear loads to the ac main, a three-phase APF is a device that is connected in parallel to and cancels the reactive and harmonic currents from one or a group of nonlinear loads
110
so that the resulting total current drawn from the ac main is sinusoidal as shown in FIG.
3
. In contrast to PFC, where a PFC unit is usually inserted in the energy pass, which processes all the power and corrects the current to unity power factor, APF provides only the harmonic and reactive power to cancel the one generated by the nonlinear loads. In this case, only a small portion of the energy is processed, which may result in overall higher energy efficiency and higher power processing capability.
Most APF control methods proposed previously need to sense the three-phase line voltages and the three-phase nonlinear load currents, and then manipulate the information from these sensors to generate three-phase current references for the APF. Since the reference currents have to reflect the load power of the nonlinear load, several multipliers are needed to scale the magnitude of the current references. A control loop is necessary to control the inverter to generate the reactive and harmonic current required by the nonlinear load. These functions are generally realized by a digital signal processing (DSP) chip with fast analog-to-digital (A/D) converters and high-speed calculations. The complex circuitry results in high cost and unreliable systems, preventing this technique from being used in practical applications.
Some approaches that sense the main line current were reported for single-phase APF and for three-phase APF. The overall circuitry is reduced. However, multipliers, input voltage sensors are still necessary. High speed DSPs are still used in three-phase systems due to the complexity of the systems.
Grid-connected inverters:
Distributed power generation is the trend in the future in order to promote new power generation technologies and reduce transmition costs. An effective use of natural resources and renewable resources as alternatives to fossil and nuclear energy for generation of electricity has the effect of protecting the environment. In order for the alternative energy sources to impact the energy supply in the future, they need to be connected to the utility grid. Therefore, grid-connected inverters are the key elements for the distributed power generation systems. A grid-connected inverter is a device that converts dc power to ac power of single phase or three-phase power that is injected to the utility grid. In order for an alternative energy source to be qualified as a supplier, sinusoidal current injection is required as shown in FIG.
4
.
Again, control methods are crucial. In the past, d-q transformer modulation based on digital implementation was often employed for a standard six-switched bridge inverter topology. The complexity results in low reliability and high cost. In addition, short-through hazard exists in this inverter.
What is needed is a design for 3-phase power factor corrected rectifiers, active power filters, and grid-connected inverters which overcomes each of the foregoing limitations of the prior art.
BRIEF SUMMARY OF THE INVENTION
The method of the invention is an unified constant-frequency integration (UCI) control method based on one-cycle control. It employs an integrator with reset as its core component along with logic and linear components to control the pulse width of a three-phase recitifier, active power filter, or grid-connected inverter so that the all three phase current draw from or the current output to the utility line is sinusoidal. No multipliers are required, as used in many control approaches to scale the current references according to the load level. Furthermore, no reference calculation circuitry is needed for controlling active power filters.
The UCI control employs constant switching frequency and operates in continuous conduction mode (CCM) that is desirable for industry applications. This control approach is simple, general, and flexible and is applicable to many topologies with slight modification of the logic circuits, while the control core remains unchanged. Although, a DSP is not required to implement the UCI control; if in some cases a DSP is desired for other purposes, the unified constant-frequency control function may be realized by a low cost DSP with a high reliability, because no high speed calcutation, high speed A/D converter, or mutipliers are required.
The implementation of UCI control can be roughly classified into two categories: (1) vector control mode and (2) bipolar control mode. A three-phase system in the vector control mode has only two switches operating at a switching frequency at a given time, while a three-phase system in bipolar control mode has three switches operating at a switching frequency.
Power factor corrected rectifiers:
The power train of a three-phase rectifier is usually a boost-deri
Qiao Chongming
Smedley Keyue M.
Dawes Daniel L.
Myers Dawes & Andras LLP
The Regents of the University of California
Vu Bao Q.
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