Investigation of Control Strategies For Multimode Operation of Series Interfacing Converter

Abstract

The upcoming use of sensitive and critical equipments in the distribution system has resulted in the awareness of the power quality (PQ) issues. The PQ problems are not only concerned by end users of the electric power but also became major concern for electric utilities. Among all the PQ problems voltage related issues viz., voltage sag, unbalance and har- monic distortion are more prominent. The devices which are employed for power quality improvement are technically termed as Custom Power Devices (CPDs). The CPD includes shunt connected , series connected and a combination of series-shunt active devices. Among these the main focus of thesis is series connected converter as are considered to be more economical to compensate voltage disturbances. This thesis investigates control schemes for multifunctional operation of series converter in dis- tribution and hybrid microgrid systems. When installed in distribution system the series converter acts as Dynamic Voltage Restorer (DVR) for compensation of symmetric and asymmetric voltage sags and harmonics, and also compensates faults at load side (downstream faults). Later the control schemes for operating the series converter as interlinking converter in hybrid microgrid system are investigated. For the effective utilization of DVR control strategy plays significant role. This thesis proposes a dual role Cascaded Delay Signal Cancella- tion (CDSC) based Dual Vector Control (DVC) for DVR to compensate the symmetrical and asymmetrical voltage sags and harmonics. Based on the numerical analysis it is found that CDSC prefilter is promising solu- tion when grid voltage is distorted by symmetric, asymmetric harmonics and voltage sag. The compensation voltage injected by DVR includes both fundamental and nonfundamental component. The proposed dual role CDSC has a feature of generating both components simultaneously. First, the prefilter extracts ISC of grid voltage and is given to the controller to generate fundamental component. Apart from that, to achieve the har- monic mitigation of load voltages, an extractor based on the modified CDSC strategy is designed which generates nonfundamental component of compensation voltage and added in 1800 phase opposition to the DVC ii algorithm. Intially simulation studies are carried out for different cases in PSCAD/EMTDC platform to validate the system. Later experimental studies are conducted on scaled down (100 V, 0.5 kVA) laboratory pro- totype DVR to verify the effectiveness of the proposed control algorithm under unbalanced and distorted grid conditions. Further in this thesis a Pseudo Derivative Feedback (PDF) based voltage control of DVR is pro- posed. The issues in the conventional P and PI based voltage controllers with and without feedforward path is studied. The efficacy of the PDF based DVR voltage control is demonstrated by comparative study with aforementioned conventional controllers using time response and relative stability analysis. The comparative study is validated through simulation (PSCAD/EMTDC) and experimental studies (on aforementioned scale down laboratory prototype system) To protect the PCC voltages of the system during faults on load side, a Vir- tual Impedance (VI) based Downstream Fault Current Limiter (DFCL) for DVR is proposed. By employing the DFCL the fault current is suppressed as the DVR act as virtual impedance in series to the system during the downstream fault conditions. The main advantages of proposed DFCL is no extra passive devices are required to be installed such that the DVR is bypassed during the fault condition and moreover these devices introduce additional losses to the system. In this thesis apart from VI-based DFCL the limitations of other DFCL methods are discussed in detail to show the robustness of proposed DFCL. The proposed DFCL model is simulated using PSCAD/EMTDC and the respective results prove its efficacy. Apart from the conventional DVR topology (with injection transformer) this thesis work also proposes a control algorithm for Transformerless DVR (TDVR) topology. Due to its reduced cost weight, size, and losses the TDVR topology is more efficient than conventional topology but the main challenge with this topology is design of control structure. Differ- ent control algorithms are investigated for TDVR topology and finally Uncertainty and Disturbance Estimator (UDE) based control algorithm is proposed. The UDE based control algorithm overcomes the parame- ter variation issue in addition to the voltage sag compensation. It posses simple structure for implementation compared to other control algorithms iii mentioned in literature. Simulation (MATLAB/SIMULINK) and experi- mental studies are carried out for evaluation of system under symmetric and asymmetric voltage sags. Finally the operation of series converter as Interlinking Converter (IC) is discussed in the present thesis. The hybrid AC/DC systems with DC and AC sources/loads are considered to be the most likely future distribution or even transmission structures. This is achieved by an IC. The IC can act as a rectifier or an inverter depending on the direction of power flow needed at each instant. IC topology selection depends primarily on the control objectives needed to be fulfilled by it. The IC typically controls the DC bus voltage when HMG is in grid conneccted mode. There is an increasing demand for executing additional functions other than the primary function of power management using a single IC rather than employing multiple ICs or additional active/passive components. Addi- tional control objectives include, storage coordination, stability improve- ment, managing power and voltage unbalances among phases, grid current control, harmonics mitigation, islanding detection, synchronization, fault limiting, etc. This demand for multi-functional ICs has led to an inves- tigation of various modes of operation in hybrid microgrid which mainly focus on AC side and DC side voltage control, bidirectional power flow. In this approach DC subgrid is integrated with main utility grid via single interlinking converter. The control strategy adopted ensures the multi- mode operation of proposed single IC which includes the bi-directional operation also. Furthermore, the control strategy allows the IC to pro- vide stable DC bus voltage for dc subgrid during transients and exchange power between the ac and dc buses.A hybrid ac/dc MG is simulated using PSCAD/EMTDC software and simulation studies of various modes are presented to validate the effectiveness of the proposed single multifunc- tional IC.