Unified Control based Adaptable Converters with a Fractional Harmonic Inter Converter Signalling channel for Dynamic tracking of PCC and Managing Multiple Converters Connected in Parallel in a Microgid
The increasing demand of energy over the past few years as well as the growing environmental concerns have forced the mankind to look out for non conventional sources of energy such as solar, wind etc. Consequently a large number of these energy resources are integrated to the existing power grid in a distributive manner. These resources known as distributed energy resources (DERs) are interfaced to the grid through power electronic converters. More and more DERs are being integrated to the grid. The increased penetration of DERs interfaced through power electronic converters have led to the concept of microgrids. The installation of microgrids have become a common scenario across the globe. However, the microgrid is also not devoid of technical issues like any other system. In this thesis, the author looks at some of the persistent issues in the microgrid and proposes some relevant practical solutions. The thesis is broadly divided into four parts. The first part deals with the concept of a Utility Interactive Uninterruptible Power Converter (UIUPC) and its role in a microgrid. In the first part, a new UIUPC, making use of a concept known as unified control is proposed. The proposed unified control is based on controlling the perturbations in the magnitude and speed of the point of common coupling (PCC) space vector with respect to a reference space vector applied continuously along the direction of the PCC space vector. The proposed unified control based UIUPC inherently transfers from a grid follower to a grid former and vice -versa according to the modes without the help of any islanding detection algorithms or external synchronising mechanisms. The second part of the thesis looks at the issue of communication between power converters connected in a microgrid. The thesis proposes a novel method of inter converter communication through the existing power line. The proposed method is based on the concept of using a fractional harmonic space vector to carry the data. The data is modulated and demodulated in the corresponding fractional harmonic d-q domains. The use of fractional harmonic d-q domains make the data appear as bits like a conventional communication system. The proposed method thus puts forth an inter converter communication technique which is economical and at the same time carries forward the advantages of a traditional communication system. The third part of thesis proposes a control architecture for managing a number of power converters connected in parallel in a microgrid. The proposal divides the microgrids into smaller units called cells. Each cell is made up of unified control based UIUPCs called as adaptable converters and current controlled converters. The adaptable converters ensure a smooth transition of the cell from a grid connected mode to an autonomous mode and vice versa without any interruption or breakage. The issue of energy imbalance persistent in a microgrid is also taken care of by using the inter inverter communication technique proposed. The proposed control architecture also takes care of the feature of redundancy of adaptable converters. This assures that the system remains functional in the event of a failure of the determining adaptable converters. The final part of the thesis is about the hardware implementation of the whole thesis. As a contribution towards the thesis, in this part the author introduces a new generation power converter which can be used in future. This part of the thesis also puts forward a novel integrated gate driver card. The proposed gate driver is based on interleaving of two forward converters. The proposed gate driver achieves the objectives of magnetic isolation and a wide duty cycle band along with the other necessary requirements of a gate driver card.