Voltage control strategies for loss minimzation in autonomous microgrids

This dissertation investigates the novel idea of flexible-voltage autonomous microgrids (MG), employing several interconnectable dc buses operating in a minimum-voltage mode. In comparison with the traditional fixed-voltage MGs, the proposed MGs reduce losses to gain significant enhancement in efficiency. It is widely believed that energy systems of the future will heavily depend on MGs rich in power electronics converters (PECs). This dissertation is focused on MGs with a high degree of self-sufficiency, without precluding sporadic links with the power grid. Potential applications of those MGs include: (a) distributed generation power systems, (b) ships, land vehicles, aircraft, and spacecraft, (c) users in need of power supply impervious to vulnerabilities of the grid, and (d) localities lacking an access to a grid.Modern pulse-width modulated PECs allow rapid and wide-range changes of voltages and currents. High switching frequencies result in high power quality and fast dynamic response, but each switching event causes energy loss related to the magnitudes of input voltage and output current. In the existing MGs, the bus voltages are maintained at a fixed level. However, many heavy loads, such as electric drives, operate most of the time with a reduced voltage, which is adjusted by decreasing the voltage gain of the feeding converter. This makes the voltage pulses high and narrow. If instead the pulses were made wide and low, then with the current unchanged the conduction losses would remain unchanged, but the switching losses would greatly decrease. This observation leads to the main idea of the dissertation, namely MGs whose dc-bus voltages are allowed to fluctuate and which are maintained at the lowest possible level. Loss minimization, apart from energy savings, may be critical for autonomous MGs with a tight balance of power.In this dissertation, two methods are proposed for calculating the minimum (optimum) required dc voltage level. In the first method, a central control unit allocates the minimum required dc voltages to individual buses by employing the information obtained from control systems of the adjustable voltage loads. For example, most of the variable-speed ac motors employ the so-called constant volts per hertz strategy, in which the relation between frequency and voltage is clearly specified. In the more sophisticated high-performance drives, the instantaneous values of the desired speed, torque, and current are available, allowing the required voltage estimation from the equation of power balance.In the second method, the problem of determining the optimal dc voltage and power settings is formulated as an optimization problem with the objective function of minimizing the converter losses. Genetic algorithm is utilized in solving the optimization problem. Due to limited available power from renewables, reducing the converter losses will enhance the survivability of the microgrid and ease the cooling requirements, resulting in a more compact system. A model of a 20-bus microgrid with the dc distribution network is employed to verify the effectiveness of the proposed methods

Matrix converters

Voltage and/or current back-to-back converters are traditionally used to interface an ac source with an ac load. An energy storage element is used to couple the dc-link of the front-end ac-dc rectifier to the back-end dc-ac inverter. A matrix converter (MC), however enables ac-ac conversion without any intermediate energy storage element. Conventional MCs, known as direct matric converters (DMCs), are single-stage converters that connect an m-phase voltage source to an n-phase output load through an m x n array of bidirectional switches. On the other hand, an indirect matrix converter (IMC) requires separate stages for the voltage and current conversion. In this chapter, the most popular MC topologies along with their control and modulation strategies are presented. A brief discussion on the technological and practical issues facing MCs, and a comparative assessment of their performance with the voltage back-to-back converters is given

Modeling and Sizing of an Undersea Energy Storage System

This paper presents modeling and sizing of an undersea energy storage system (USS). The USS, which is placed at the seabed, consists of a concrete sphere, a reversible pump-turbine unit, a permanent magnet synchronous machine, and a steel pipe through which water flows into/out of the sphere from/to the deep ocean. A novel mathematical model is derived to describe the governing equations of the USS operation. It is assumed that the USS operates in parallel with a direct-drive wave energy converter (DDWEC) and a novel sizing algorithm is developed to obtain the required USS capacity/size for regulating the DDWEC output power fluctuations during a designated period of time. In order to determine the DDWEC output power profile, wave spectrums are required. Real significant wave height and dominant wave period data, which are measured and collected by a buoy station in the Gulf of Maine, are used to generate wave profiles based on the modified two-parameter Pierson-Moskowitz spectrum. Eventually, Monte Carlo simulations with a thousand generated wave profiles are carried out to determine the optimal size of the USS for all possible scenarios. Furthermore, through promising results obtained from hardware-in-the-loop studies, the effective role of the USS in regulating the wave energy converter output power fluctuations is demonstrated

Voltage and power control for minimising converter and distribution losses in autonomous microgrids

Voltage source converters (VSCs) are widely used in microgrids to interface the renewable resources with the electrical network. In autonomous microgrids with the dc distribution network, the optimal dc voltage reference for the one VSC that operates in the voltage regulator mode and the optimal reference power settings of the remaining VSCs working in the power dispatcher mode have to be pre-determined. Emulating the utility grid, these settings and control modes are commonly selected such that the dc voltage is maintained within desired margins, typically ±10% around the rated value. In this study, the objective function is minimisation of the converter and distribution line losses. All the operational modes and limits of VSCs have been taken into account. Genetic algorithm has been utilised in solving the optimisation problem. Owing to limited available power from renewables, reducing the converter and distribution system losses will enhance the survivability of the microgrid and ease the cooling requirements, resulting in a more compact system. A model of a 20-bus microgrid with the dc distribution network is employed to verify the effectiveness of the presented optimisation algorithm

Wireless power transfer coil design for transmitter and receiver LCC compensation based on time-weighted average efficiency

The impact of changing inner diameter of wireless power transfer (WPT) coils on coupling coefficient is studied and it is demonstrated that at a certain outer and inner coil diameter, turn space variation has minor effect on the coupling coefficient. Next, two compensations networks that offer constant output voltage and zero voltage switching for WPT are presented. Eventually, by defining a time-weighted average transfer efficiency (TWATE), and based on measured values of resistance and inductance of a WPT prototype and experimental charging curve of a Li-ion battery, a design procedure for both compensation networks is proposed. The proposed design leads to high TWATE as well as low material usage

Energy flow control and sizing of a hybrid battery/supercapacitor storage in MVDC shipboard power systems

The propulsion system of a medium-voltage dc (MVDC) ship is subject to large thrust/torque variations due to interactions of the ship and the propeller with sea waves. These variations induce steep power fluctuations on the MVDC bus, adversely impacting the stability, efficiency and power quality. Hybrid energy storage system (HESS) is a promising solution for mitigating these power fluctuations. Dictating the energy that the HESS components must deliver/absorb, the energy management strategy (EMS) impacts the size/capacity of the energy storage system (ESS). Based on this consideration, sizing and EMS of a battery/supercapacitor (SC) HESS are jointly optimised by using a deep reinforcement learning-based method. The proposed method splits the power between the HESS components such that while the operational constraints are satisfied, energy storage size and losses are minimised. Its features are adaptability to varying sea states, real-time implementation feasibility, while obviating the requirement for knowledge of the ship propulsion power profile. The efficacy of the joint sizing/EMS on reducing the ESS size is validated by comparing the sizes when battery-only, SC-only and HESS are employed in the MVDC shipboard power system. Real-time implementation feasibility and adaptability to various ship propulsion power profiles is confirmed through real-time simulations

A comparative study of battery, supercapacitor and undersea energy storage systems in wave energy applications

A technical comparison between two standard energy storage technologies, i.e. battery and supercapacitor (SC), and a novel alternative, i.e. undersea energy storage system (UESS), in wave energy applications is presented. Various sea states with different significant wave heights are considered for investigating the efficiency and lifetime of the storage devices. Comparisons are carried out assuming that the storage device operates in parallel with a wave energy converter to deliver fixed power, i.e., average harvested wave power, to the grid during a designated period of time. While battery and SC outperform the UESS in terms of efficiency, extreme oversizing is required for achieving lifetime periods that satisfy maintenance and reliability requirements, however

Improved Design and Space Vector Modulation of a Z-Source Ultrasparse Matrix Converter: Analysis, Implementation, and Performance Evaluation

Space vector modulation (SVM) of a Z-source ultrasparse matrix converter (ZSUSMC) can be realized by either using or not using a zero state in the space vector rectification. In this paper, these two modulation schemes are investigated for a cascaded ZSUSMC. It is demonstrated that the input current under the modulation scheme without a zero state is nonsinusoidal, and its harmonics content depends on the output and impedance network inductor currents. In addition, restrictions on operation of the ZSUSMC with low voltage gains are unveiled. Next, aiming at improving the input current quality and suppressing the harmonics content to an acceptable level, an approach for design of the cascaded Z-source network is proposed. Furthermore, an optimal switching pattern, which results in minimum possible number of changes in the switching states of the ZSUSMC, is developed. Using the developed switching pattern, an approach for digital implementation of the SVM schemes on a single conventional digital signal processor (DSP) without employing an additional field programmable gate array is proposed. Hardware-in-the-loop studies of the ZSUSMC modulated under the proposed SVM schemes are carried out to verify their digital implementation feasibility on a conventional single DSP, as well as effectiveness in producing high-quality input/output currents

An In-Depth Investigation of a Z-Source Ultrasparse Matrix Converter in Buck and Boost Modes of Operation

Z-source networks are recently being employed to enhance the boosting capability of matrix converters. This paper concentrates on cascaded Z-source ultrasparse matrix converter (ZSUSMC), where the Z-source network is placed between the three-switch input rectifier stage and the six-switch output inverter stage. Two space vector modulation schemes, with and without a zero state in the rectifier stage modulation, are presented and their advantages and disadvantages are discussed. In addition, restrictions imposed on the converter operation in buck mode, which arise from unidirectional nature of the ultrasparse matrix converter, are discussed and a solution is proposed for offsetting those limitations. Furthermore, an optimal switching pattern, which results in minimum possible number of changes in the switching states as well as even distribution of the shoot-through state over the entire control period, is proposed. Also, common-mode voltages of the converter in all possible switching configurations are calculated. Hardware-in-the-loop studies of a ZSUSMC-based permanent magnet synchronous motor drive are carried out to evaluate the performance of the drive under the proposed modulation techniques. The obtained results are compared with a recent study and the superiority of the proposed method in terms of converter input/output current quality is demonstrated

Improved Design and Space Vector Modulation of a Z-Source Ultrasparse Matrix Converter: Analysis, Implementation, and Performance Evaluation

Space vector modulation (SVM) of a Z-source ultrasparse matrix converter (ZSUSMC) can be realized by either using or not using a zero state in the space vector rectification. In this paper, these two modulation schemes are investigated for a cascaded ZSUSMC. It is demonstrated that the input current under the modulation scheme without a zero state is nonsinusoidal, and its harmonics content depends on the output and impedance network inductor currents. In addition, restrictions on operation of the ZSUSMC with low voltage gains are unveiled. Next, aiming at improving the input current quality and suppressing the harmonics content to an acceptable level, an approach for design of the cascaded Z-source network is proposed. Furthermore, an optimal switching pattern, which results in minimum possible number of changes in the switching states of the ZSUSMC, is developed. Using the developed switching pattern, an approach for digital implementation of the SVM schemes on a single conventional digital signal processor (DSP) without employing an additional field programmable gate array is proposed. Hardware-in-the-loop studies of the ZSUSMC modulated under the proposed SVM schemes are carried out to verify their digital implementation feasibility on a conventional single DSP, as well as effectiveness in producing high-quality input/output currents