This thesis deals with the problem of control system design for power electronic controllers when high performance is desired despite unaccounted for internal and external conditions. Factors such as parameter variations, operating condition changes, and filtering and measurements delays, may adversely impact the performance of a circuit whose controller design is not immune to external and internal disturbances. The thesis explores the method of internal model design as a viable approach for designing controllers with superior performance despite system variations. Following a presentation of the theoretical background of the internal model design, the thesis considers two examples of state variable models, improving the stability of a voltage source converter and speed control of an induction motor. Conclusions show the new control system is more stable and offers better controllability despite unexpected system variations, compared to classical control system.

Voltage Source Converter (VSC) applications include but are not limited to HVDC, Flexible AC Transmission System (FACTS) devices such as STATCOM, SSSC, UPFC and Wind generators and active filters. The VSC based HVDC system is a feasible option for bulk power transmission over long or short distances and the grid integration of renewable energy sources in existing transmission and distribution systems. The main requirement in a power transmission system is the precise control of active and reactive power flow to maintain the system voltage stability. The VSC operating with the specified vector control strategy can perform independent control of active/reactive power at both ends. This ability of VSC makes it suitable for connection to weak AC networks or even dead networks i.e. without local voltage sources. For power reversal, the DC voltage polarity remains the same for VSC based transmission system and the power transfer depends only on the direction of the DC current. This is advantageous when compared to the conventional Current Source Converter (CSC) based HVDC system. Furthermore, in case of VSC, the reactive power flow can be bi-directional depending on the AC network operating conditions. In this thesis, a 3-phase, 2-level, 6-switch VSC connected to an active but weak AC system at both ends of the HVDC link is developed using EMTP-RV. The VSC-HVDC transmission system model is developed using both direct control and vector control techniques. The direct control method is an approximate method in which the active power, AC voltages at both ends of HVDC link and DC link voltage are controlled directly by using PI-controllers. In vector control method, closed loop feed-forward control system is used to control the active power, reactive power at both ends and DC voltage. By comparing the simulation results, it is concluded that the vector control method is superior to the direct control because of the removal of the coupling between control variables to achieve the independent control of active and reactive powers at both ends of the HVDC link. / UOIT

The thesis describes the development of a model of the three-phase Voltage Source Converter (VSC) in the Harmonic State Space (HSS) domain, a Linear Time Periodic (LTP) extension to the Linear Time Invariant (LTI) state space. The HSS model of the VSC directly captures harmonic coupling effects using harmonic domain modelling concepts, generalised to dynami- cally varying signals. Constructing the model using a reduced-order three-phase harmonic signal representation achieves conceptual simplification, reduced computational load, and direct inte- gration with a synchronous frame vector control scheme. The numerical switching model of the VSC is linearised to gain a small-signal controlled model, which is validated against time-domain PSCAD/EMTDC simulations. The controlled model is evaluated as a STATCOM-type system, exercising closed-loop control over the reactive power flow and dc-side capacitor voltage using a simple linear control scheme. The resulting state- space model is analysed using conventional LTI techniques, giving pole-zero and root-locus analyses which predict the dynamic behaviour of the converter system. Through the ability to independently vary the highest simulated harmonic order, the dependence on the closed-loop response to dynamic harmonic coupling is demonstrated, distinguishing the HSS model from fundamental-only Dynamic Phasor models by its ability to accurately model these dynamics.

Modular multilevel converter type voltage source converter (MMC-VSC) is a potential candidate for present and future HVdc projects. The d-q decoupled control system is widely used to control MMC-VSC systems. Selection of PI-controller parameters for MMC-VSC systems is a challenging task as control loops are not completely decoupled. Since there is no widely accepted method to tune these control loops, the industry practice is to use the trial and error approach that requires a great amount of time. Therefore, it is required to develop a systematic procedure to tune PI-controllers considering necessary system dynamics and also to propose guidelines for control system design. This thesis introduces a systematic procedure to determine PI-controller parameters for the d-q decoupled control system. A linearized state-space model of an MMC-VSC system is developed to calculate the frequency-domain attributes. The control tuning problem is formulated as an optimization problem which is general and any meta-heuristic method can be used to solve the problem. In this thesis, the simulated annealing is applied to solve the problem. The efficacy of the tuned parameters is tested on the electromagnetic transient model of the test system on the real-time digital simulators (RTDS). In addition, it is shown that the proposed method is suitable to tune PI-controller parameters for MMC-VSC systems connected to strong as well as weak ac networks. Further, this thesis investigates the effects of d-q decoupled controller parameters, phase-locked loop (PLL) gains, and measuring delays on the stability and performance of the MMC-VSC test system. It is shown that the converter controllers have greater influence on the system stability and the impact of PLL gains is negligible unless very high PLL gains are used. In addition, the negative impact of measuring delays in instantaneous currents and voltages is also analysed by performing eigenvalue and sensitivity analysis. Finally, a set of guidelines for control design of MMC-VSC systems is summarized. In general, the proposed control tuning procedure would be useful for the industry to tune PI-controllers of MMC-VSC systems. Furthermore, the proposed methodology is generic and can be adapted to tune of any dynamic device in power systems. / February 2017

In this master thesis control systems for a voltage-source converter HVDC connected to weak ac networks are investigated. HVDC stands for high voltage direct current and is a way to transfer power in the electrical power system. A HVDC uses direct current (dc) instead of alternate current (ac) to transfer power, which requires transformation between ac and dc since most power grids are ac networks. The HVDC uses converters to transform ac to dc and dc to ac and the converter requires a control system. A complete control system of a voltage source converter HVDC contains many different parts. The part investigated in this thesis is the active power control. Different structures containing PID controllers have been tested and evaluated with respect to stability and performance using control theory. The impact of weak ac networks has been evaluated in regards to the different control structures. The investigations have been conducted using mainly steady-state simulations. Based on the simulation and analyzes of the simulation results a promising control structure has been obtained and suggested for further investigation.

The deterioration in power quality due to the increase in non linear loads has sparked a new interest in the filtering techniques used in transmission and distribution systems. Unlike passive filters, active filters are adaptable to rapidly changing source impedance and provide the necessary harmonic compensation for varying non-linear loads. This thesis models a Voltage Source Converter (VSC) based shunt active filter (SAF) to filter harmonics due to large non linear loads. SAF compensates the harmonics by injecting a compensating current which is equal in magnitude but opposite in phase to the disturbance in the system. The power circuit of this SAF consists of a three-phase VSC and the switching signals for this converter is generated by hysteresis based current modulation method. The controller uses the sinusoidal current control strategy of the generalized instantaneous p-q control theory to calculate the reference compensating current. Proposed SAF is simulated using EMTP-RV simulation package under steady state and dynamic conditions and its effectiveness in mitigating harmonics is tested. The stability and response of the SAF is also tested satisfactorily under transient load and severe AC / DC fault conditions. / UOIT

The interest in the utilization of offshore wind power is increasing significantly. Due to the shortage of in-land locations for wind farm and the wind speed offshore is potentially higher than that of onshore, which leads to a much higher power production. In this thesis a large offshore wind farm is modeled using Matlab simulation package. In the simulations active stall regulated wind turbines driving fixed speed asynchronous generators are used. Two different types of interconnections are modeled and compared, one is the Voltage Source Converter (VSC) based HVDC link and the other one uses high voltage AC (HVAC) cable interconnection. Transmission faults are simulated in each system and the transient response are examined. Three phase fault and single line to ground fault are used to compare the performance of the VSC based HVDC interconnection system and HVAC interconnection. It is found that compared to the traditional HVAC transmission, the VSC based HVDC transmission would have better performance under various system disturbances.

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Due to its unique features such as controllable power factor, controllable bi-directional power flow, and rapid dynamic response, Voltage Source Converters (VSCs) have been widely used in various industrial applications such as distributed generation systems, power distribution systems, uninterruptible power supplies (UPS), AC motor drives, etc. To optimize the performance of the VSC, many control algorithms have been proposed. This thesis investigates development of the nonlinear control for the VSC in two applications: power factor control and active power filtering. A detailed description of the dynamic model of the VSC system is presented in different reference frames. A linearization-based control scheme is introduced for power factor regulation and verified by switched simulation and real-time experiment on a test stand which has been constructed at the Applied Nonlinear Control Lab (ANCL), University of Alberta. In addition, an internal model-based control scheme is introduced to perform active power filtering. This algorithm is verified by simulation. / Controls

The Medium Voltage DC (MVDC) architecture of shipboard power system (SPS) with higher power density and enhanced power control is seen as a future prospect in warships by US Navy. Optimal operation of SPS is essential to enable efficient power and energy usage in the tightly coupled and power limited systems. It helps in obtaining adequate and reliable power supply by rescheduling generator output and energy storage devices for different operating scenarios and can also ensure power supply to critical loads during fault conditions. The self-commutated Voltage Source Converters (VSCs) with high dynamic performance and independent control over the real and reactive powers are best suited in the MVDC architecture. Therefore, an optimal operation tool is developed for the multi-terminal VSC based MVDC SPS which minimizes the system operating costs by optimally coordinating generators and energy storage, and will also implement preventive and corrective actions for managing credible contingencies.