Candidate: Aram Kirakosyan
Title: Control of Voltage Source Converter Based Multi Terminal DC and Hybrid AC/DC Systems
Date: April 20, 2020
Time: 11:00 AM
Place: REMOTE PARTICIPATION
Supervisor(s): El-Saadany, Ehab - Salama, Magdy
The journey of power systems started with the development of the dc technology pioneered by Thomas Edison in the late 19th century. Meanwhile, Nicola Tesla led the investigation of the ac technology, which soon surmounted the dc paradigm in the "War of Currents" in the ongoing urge to favor higher efficiency systems. For about a century ac became the preferred choice in all stages of power systems including generation, transmission, sub-transmission, and distribution. However, with recent advances in power electronic technology, the conversion between ac and dc become more practical, leading to the potential for the development of dc and hybrid ac/dc grids.
Specifically, on the transmission side, the High Voltage DC (HVDC) systems become a preferable alternative over their ac counterpart for certain application: for example, transmitting bulk power over long distances. What is more, the utilization of Voltage Source Converter (VSC) based HVDC systems have already allowed formation of Multi Terminal HVDC (MT-HVDC) grids. The latter systems are dc grids on large geographical areas enabling dc interconnection of several ac systems and facilitating integration of bulk renewable energy, especially offshore wind resources.
On the distribution side, on the other hand, the portion of the dc generation has increased due to the advancement of renewable energy sources such as solar power and dc storage systems. What is more, modern appliances such as elevators, computers, mobiles, LED lighting and electric cars increase the portion of dc technology in the customer loads. The advancement of VSC based dc microgrids, therefore, facilitates the integration of dc energy sources with the mentioned DC load technologies. Thus, the formation of dc grids in both transmission and distribution systems and their integration with the existing ac grids has gained large attention in recent years.
The control of the converters interfacing the dc grids is one of the challenges for the future expansion of such grids and is the subject matter of this thesis. Droop control is a widely accepted strategy for the control of parallel converters in both MT-HVDC and dc microgrid applications. The main advantage of the droop control is that it allows several converters to simultaneously regulate the dc grid voltage and share power imbalance in the system. In this thesis, the issues related to the dc voltage regulation, the mutual frequency support, and power-sharing between the droop-controlled converters are investigated and solutions are sought.
First, a new control strategy is proposed for MT-HVDC systems that allows accurate dc voltage regulation and maintains adequate power-sharing among the converter stations. The incorporation of the AVS loop allows avoiding dc system average voltage deviation from the nominal value without affecting the current flow between the converters. Furthermore, the controller is extended to incorporate a new power-sharing loop. The latter strategy ensures accurate sharing of the active power between the droop controlled converter stations based on the pre-determined portions even in the presence of large latencies in the communication system.
Furthermore, a new algorithm is developed to manage the power distribution between droop-controlled converters of MT-HVDC system considering the frequency profiles of the ac systems interfaced with those converters. This strategy allows achieving enhanced primary frequency regulation in the individual ac grids interconnected through the MT-HVDC system by enabling mutual frequency support between those ac grids. The strategy is to be extended to consider an operational state of each ac grid before determining the extent of the mutual frequency support. That is, an algorithm is to be developed to distinguish between the affected and supporting grids and enable frequency deviation sharing based on the pre-specified ratios.
Finally, a communication-less power-sharing strategy is developed between droop-controlled converters in the dc microgrid. Identical steady-state voltage feedback is used for all droop controlled converters, which allows overcoming the inaccurate current sharing resulted from the difference between converter terminal voltages. The information about the common voltage is acquired in a communication-free manner by embedding it in the frequency of the small ac signal superimposed on the dc voltage. Therefore, the algorithm does not rely on the information of the grid structure and line parameters and is applicable for the multibus microgrids.