Generalized Design, Analysis and Control of Grid side converters with integrated UPS or Islanding functionality

Today there is a growing need to use similar power and control hardware and even similar control methods for various applications in utility connected converters operating in grid and island modes. It can be shown that even an AC motor drive control has many similarities to the grid side converter control. Only the application layer of the control is different in each case.

The main focus of this thesis is to find the various needs of these applications and how to meet them, address the design and analysis methods, study of topologies, control methods etc. The theory is supported with analysis, simulation and practical results wherever possible.

Though there has been a substantial literature available on the topic of concern, there is no generalized systematic approach made available for the converter system design for a given set of basic specifications. The utilization of the converter is completely dependent on the application. For example, consider an IGBT based full bridge line side converter. When the converter is used for unity power factor applications, most of the time diodes conduct when the dc link draws power as compared to the IGBTs. Therefore, heating of diodes limit the fundamental power delivered by the converter. However, when the same converter is used as an active filter, the inverter IGBTs and diodes share current almost equally. This leads to more uniform losses in IGBT’s and diodes as compared to a unity power factor application. This in turn means that the kVA rating of the same converter when used for active filtering can be significantly higher than that of unity power factor applications. In medium and high power converters (1-5MW), the magnitudes of ripple currents approach the magnitudes of the fundamental because of low switching frequencies. Therefore, to maximize the utility of converters it is important to calculate various design parameters as accurately as possible. Accurate calculations means that the safety margins on all components used in the converter are minimized thereby increasing the ratings of the converter. Considering these facts, a generalized strategy to design and analyze the PWM converters is necessary.

Because of the goal to have common hardware in a variety of applications it is necessary to comprehensively review various requirements of the converters connected to the utility in both grid and island operation. A study of various topologies used in these applications is necessary to find the most optimum topology from a cost and reuse point of view.

Apart from hardware there is also a necessity to find synergies in the control system. For this a study of various existing control systems is made to find the most appropriate control, which can be implemented in a modular way. The control shall consider LCL filters and their damping. Other requirements of the control scheme include the active harmonic rejection of the supply voltage harmonics, load harmonic control in case of island operation, short circuit handling in both modes, active detection of loss of mains in case of distributed generation equipment. In some applications such as STATCOMs it is necessary to operate the system under various supply fault conditions such as line to ground and line-to-line. Some applications, which have high performance transformers with magnetizing currents of the order of 0.5%, need the DC current injection of the converter to be lower than 0.05%.

In islanding there is a need of parallel operation without communication between inverters. A scheme, which seamlessly works with single and multiple converters, is needed. Often in islanding it is necessary to supply the loads with a neutral wire. Effective schemes need to be used and analyzed. Also in some cases it is needed to continue supplying the loads on healthy phases when there is a short circuit fault on other phases. It is even necessary to be able to operate with a momentary three-phase short circuit on load side due to a motor start or three phase fault. The probability of a three-phase fault as shown is much lower than the line to line or line to neutral faults. These strategies increase the equipment availability.

As more and more such converters are using LCL filters to meet the standard requirements, active damping is an important topic to be investigated. With active damping it is possible to reduce the losses in the filter significantly. The damping is even more necessary for large converters, which have low inductances and higher capacitances with both components having lesser resistances.

Also with more powerful controllers there is now a need to build more and more intelligence in the system with respect to the operation of the equipment. Various online system modeling and analysis possibilities are to be studied and implemented which can be useful in increasing the availability of the equipment.