Investigations Of Novel Power Conversion And Control Method In High Power Microwave Systems

Power supply is the basic element of high power microwave systems, and power conversion and control method is what lies at the heart of power supply technology. Investigations of novel power conversion and control method are necessary to meet the developments of high power microwave systems towards high power, compact volume and light weight direction.Power supplies have DC-Link in general use, which consists of bulky electrolytic capacitors with heavy weight and great volume. Power density of supplies reduces due to these capacitors. In addition, input power quality of power condition systems is low with high harmonic current. Power factor correction circuit must be inserted to overcome the problem. This reduces the power density furthermore. Therefore, the significance of research on the supplies basing novel power conversion and control method to improve the power density become obvious. AC-Link based converter has high efficiency, high power density, and high reliability without bulky DC-Link component. It can provide high input power quality with sinusoidal input current, adjustable input power factor, and regeneration capability, and it’s very attractive in areas where volume, efficiency and reliability are of importance.This paper summarizes the present status and trend in development of power conversion and control method analysis the power conversion and control method process history in detail. Drawbacks of DC-Link technology are generalized, and the focal point is the technology related to AC-LinkTM and matrix converter.The study work is undertaken around high voltage capacitor charging power supply (CCPS). First, Topological structure and control method related to AC-LinkTM series resonant AC-DC converter and AC-DC matrix converter are discussed in detail. AC-LinkTM series resonant AC-DC converter made up of AC input filter, switch matrix, LC resonant charge and discharge circuit, free-wheel switch and output filter. Fault-immune operation and natural fault current limiting (zero-current turn off, no PWM shoot-through) can be achieved. In the AC-LinkTM converter, current is moved from the input to the output by rapidly transferring small packets of charge, one at a time. This transfer is accomplished by charging up a central capacitor with current from the input phases, then fully discharging the central capacitor into the output phases. By controlling the amounts of charge taken from each phase, the current on each of the three inputs can be precisely controlled. The power throughput can be controlled in two ways, through the adjustment of the inverter frequency and through the residual voltage control. The shortage is exposed when charging capacitive load, so development of structure is processed to meet the capacitive load. Using new structure, constant current mode will be chosen when load voltage is low, and constant power mode is inserted when load voltage exceeds a level. Power capacitance delivered to CCPS can be reduced with this mode, but high voltage stress of output and free-wheel switch can not neglected.Bidirectional switch and parallel resonant structure are adopted in the studied AC-DC matrix converter, and zero-current switch is achieved. Two control approaches are applied, the first control approach considered was the use of a predictive input controller, such as that to control, directly, the real and reactive components of the input filter variables (voltage/current). This is achieved by predicting the effect that the application of each switching state will have on the input filter variables for each half cycle of resonant operation. However, it was found that this control approach could not maintain sufficient control of the resonant tank to produce the required specification of output voltage unless the Q factor of the resonant circuit was high. Consequently, a new predictive control approach was required. The principle of the predictive tank controller is to switch the converter to maintain a closely regulated operation of the resonant tank, rather than maintain the input power quality, as was the case with the predictive input controller. Unfortunately, this basic algorithm leads to a poor input current waveform with low frequency distortion. The proposed solution to improve the input line current is to prevent the controller from applying the same line voltage for more than one cycle at a time. This can be achieved by storing the previous switching state of the converter in the controller, and assigning its opposing state a high error in the cost function. The predictive input controller is implemented by sampling the sign of voltage across input filter capacitor, input line current and resonant inductor current, and the predictive tank controller is based on the sign of the voltage across the input filter capacitor, resonant inductor current and voltage across the resonant capacitor, moreover, to keep output constant load voltage must be detected. More variables lead to complex data-processing. An FPGA together with DSP platform is used to execute real time data processing. This leads to complex design of controller.Taking capacitor load into account, a novel AC-Link topological structure is proposed considering the shortages and merits of these two structures as before stated. By controlling the amounts of charge taken from each phase, the current on each of the three inputs can be precisely controlled. Four process mode and three process mode are presented according to the characteristic of the power supply. These expressions of inductor current, the voltage across resonant capacitor and charge from input phases in every process are solved by analytic model building, and controller equation is presented also. The research results indicate that effective control can not be implemented when load voltage goes high, even though low ripple arcoss the input filter capacitor is get in four process mode. The controlling complexity will be dropped in three process mode, but higher ripple arcoss the input filter capacitor happens. We choose three process modes in response to qualitative comparison result. A simplified controlling equation is put forward, and it is effective after compared with mentioned modes before.State-plane analysis is first introduced into the analysis of series resonant AC-Link CCPS. State-plane diagram shows the geometrical relationship between different variables depicted in the paper. Controlled variable can be solved making use of the corresponding control strategy in the state-plane diagram, and the relationships between switch phase, charging period, free-wheel period, voltage across the resonant capacitor and output voltage, input AC voltage show. Limit terms of the output voltage is analysed at the end of the chapter, and corresponding expressions are deduced. State-plane analysis was found to be a simple, yet powerful method which can clearly portray the steady-state and transient operation of resonant converters. The anomalous sequences of conduction observed often in practical systems can be easily explained with the aid of the state plane. Also, the properties of control of resonant converters can be better understood and their relative merits and demerits assessed. Significantly the state plane directly indicates the resonant tank energy level at which the system is operating. Hence, the ability of a control method to keep tank energy levels within bounds under transient conditions can be evaluated with state-plane analysis.Simulation model of series resonant AC-Link CCPS with simulink is established. Firstly, waveforms of major devices and ports show with three process mode, and analysis project are done. Secondly, this chapter is focus on the "Y" model LC filter. Voltage ripple across the filter capacitors and ripple of input line current are got. It have an effect on voltage ripple, current ripple, current harmonic and power factor when parameters L and C change, and the influence is studied. In the end, design principle and influence of "C" model snubber circuit on the performance of the series resonant AC-Link CCPS are analyzed.Design and implementation of a60kJ/s capacitor charging power supply with output voltage50kV is described, basing on the theoretical, simulating and experimental research of AC-Link technology. Designing details of the power supply were reported, which consists of EMI filter, matrix switch, LC resonant bank and high frequency transformer. To take FPGA control chip as the core, hardware and software part of the controller has developed. In the hardware design part, electro-magnetic compatible problems happen because of high output power and compact packaging, so interference-free design of different parts is represented in detail. In the software design, a new phase voltage detecting method using phase measuring is put forward, and phase measuring technique is discussed in detail, which reduces the storage memory and accelerates the process speed of controller.Experiments are implemented, digital signal of grid voltages, IGBT driver signal, input line current, switch current, resonant current and output voltage were tested. The experiment results show:reliability and stabilization of control systems are performed in the working electro-magnetic compatible environment.The average charging rate is60kJ/s, and a higher power density of above0.60W/cm3is achieved with the CCPS, by utilizing various technologies on AC-Link topology and proper packaging. Power factor is0.99with input line current waveforms following the input phase voltage. Total voltage harmonic is below2%, and total current harmonic is below10%. High efficiency can be achieved with soft switch and natural commutation utilized, which is0.90with resistive load. By analyzing the reason that input line current waveforms distort, a linear compensation algorithm is proposed. It improves the distortion of the input line current waveform obviously, but output power drops at the same time.