Control of Advanced Power Converter Topologies for Transmission Grid Management

Currently, there is common agreement that the power grid needs to be upgraded and modernized. The transmission grid assets are getting aged, especially in the US, and growing number of variable renewable energy sources has made the situation even worse. This requires a huge upfront investment to renew the current transmission infrastructure and further increase its capacity. The most valuable assets in the transmission grid are high power transformers. Transformers have the single highest cost value of the equipment installed in high voltage substations; therefore, utilities and system operators are most interested to extend the utilization time of this expensive component of the transmission grid. This dissertation seeks to integrate flexible active solutions to existing transmission assets to control power flow and increase power transmission capacity.;First, the concept of Convertible Static Transmission Controller (CSTC) using Modular Transformer Converter (MTC) as the building block is investigated. The MTC is a bidirectional back-to-back AC/AC power conversion unit and the CSTC is a versatile transmission controller asset in case of transmission system contingencies and normal conditions for dynamic power flow control and contingency management of the transmission grid. The proposed CSTC with new functions has several advantages compared to existing FACTS controllers. System modularity for manufacturers and utilities/system operators using standard high power electronic systems is one of the advantages of this structure. In this thesis, algebraic models of the CSTC are derived in three main operation modes (series shunt, series-series, and shunt-shunt connecting configurations). The proposed algebraic models are used to define the reference values for active and reactive power flow of the CSTC converters based on the desired operating points for the meshed power system, the power transformers in particular. The dynamic performance of the CSTC with the proposed control structures and algebraic models will be investigated based on the PSCAD simulation.;The vulnerability of electric power grid to man-made and natural disasters, such as hurricanes, tsunami, earthquake, geomagnetic storms etc. has become crucially important. In this dissertation, the concept of transmission-level Active Mobile Substations (AMS) is introduced for disaster management. The AMS is a mobile substation enabled by integrated power electronics. In this thesis, a detailed comparative analysis of different power converter topologies for AMS is presented. Also, a modular topology with high frequency isolation is proposed for AMS application. In this configuration, several single-phase AC/DC power conversion building blocks are connected in series. The isolation in each building block is provided through a high frequency transformer within a DC/DC converter. In the high frequency isolated modular converter configuration, several floating DC capacitors in all three phases are connected in series, and voltage balancing control of these floating dc capacitors is required. In this dissertation, an appropriate control structure with the capacitor voltage balancing controller is proposed.;Also, the concept of embedded multi-terminal dc grid in meshed ac power system is explored. The proposed technology can provide back-up in case of transmission line failure, and it can enhance power transmission capacity and flexibility in existing ac grids. In this thesis, the algebraic model of multi-terminal DC grid is derived and validated in a meshed ac power system. Algebraic model will present the behavior of ac grid with respect to various operating points of dc grid. The control structure of dc grid based on master/slave and droop control scheme is explored. Also, a droop control structure with dead-band controller is proposed for multi-terminal DC grid control. To verify the model and the control structure, dynamic performance of the integrated multi-terminal DC grid in a reduced order three-bus AC equivalent NYPA power system is investigated through PSCAD and real time digital simulation.;Furthermore, the dynamic performance of the proposed solutions under fault operating conditions is explored. The AMS must be designed to operate satisfactorily under fault operating conditions. In this dissertation, component design considerations in development of the AMS under AC fault operating condition will be provided, and a new control strategy is proposed to control AMS under AC fault when component design is not sufficient to prevent overcurrent and trips. The appropriate control algorithm of AMS and multi-terminal DC grid for DC fault operation and recovery after DC fault is also proposed. The dynamic performance of multi-terminal DC grid with master/slave control scheme and droop control with dead-band controller is also explored and compared under loss of terminal station.;Finally, to prove the MTC and high frequency isolated modular converter based AMS as transmission assets and to verify the control structure and algorithm, ultra-high fidelity Controller Hardware-in-the-Loop (CHIL) testing has been conducted and comparative results will be presented. Lab-scale experimental results for both configurations are also reported to verify the proposed control schemes.