Overview of Power Electronic Converter Topologies Enabling Large-Scale Hydrogen Production via Water Electrolysis

Renewable power-to-hydrogen (P2H) technology is one of the most promising solutions for fulfilling the increasing global demand for hydrogen and to buffer large-scale, fluctuating renewable energies. The high-power, high-current ac/dc converter plays a crucial role in P2H facilities, transforming medium-voltage (MV) ac power to a large dc current to supply hydrogen electrolyzers. This work introduces the general requirements, and overviews several power converter topologies for P2H systems. The performances of different topologies are evaluated and compared from multiple perspectives. Moreover, the future trend of eliminating the line frequency transformer (LFT) is discussed. This work can provide guidance for future designing and implementing of power-electronics-based P2H systems

A Review on the Need of HVDC Transmission System

With the growing demand of electricity on a daily basis, we cannot rely on conventional electric authority systems like long-haul distributed power stations as well as complicated and heavy load / distribution networks. High voltage direct current (HVDC) transmission systems include an extremely imperative role in authority systems. Without the appropriate study of the HVDC system, it is unfeasible to obtain an accurate mathematical model of the system and in the absence of proper modeling the influence transmitted in the HVDC system cannot be considered. The power transmitted through the HVDC system depends upon the competence of the controller and the converter station.Conservatively, the PID controller was used for the polar current control of the rectifier and the excitation control on the inverter side. This paper is an indication of the HVDC system and covers the essential part of the foundation of the HVDC system. Due to enlarged demand for power at the load center and concentration to distributed power generation, a lot of high capacity long distance HVDC systems are requisite and are intended to achieve various advantages. As growth in the power electronics field advances, HVDC systems are more consistent

Modeling and control design of a Vienna rectifier based electrolyzer

Hydrogen production is an interesting alternative of storing energy. Electrolyzers produce hydrogen through water electrolysis; the resulting hydrogen is later used to generate electricity by using fuel cells, that reverse the process. Electrolyzers use rectifiers to convert the grid ac voltage into dc voltage for supplying the electrolyzer cells. Previous research used a rectification process based on conventional rectifiers (diode-or thyristor-based) which draw non-sinusoidal current from the main grid. This requires increased filtering to prevent power quality problems and equipment malfunctioning/failure. In addition, previous literature assumed simplified models for the power electronics converters and lacked a detailed control system. The Vienna rectifier is a non-regenerative converter that produces sinusoidal currents with low losses due to the reduced number of active switches. This manuscript proposes using the Vienna rectifier as an interface to connect electrolyzers to the ac grid. The dc voltage applied to the electrolyzer is regulated by using another DC-DC converter, which is selected to be a synchronous buck converter for simplicity and maximum efficiency. In this paper, the models of the Vienna rectifier, synchronous buck converter, and the electrolyzer are developed along with their respective controls. The control system has the ability to function in two operation modes for the overall reference: hydrogen production and power demand. The first one is adequate for grid-connected operation and the later for off-grid operation. Simulation results are given to show the validity of the proposed procedures

Medium Voltage DC Network Modeling and Analysis with Preliminary Studies for Optimized Converter Configuration Through PSCAD Simulation Environment

With the advancement of high capacity power electronics technologies, most notably in high voltage direct current (HVDC) applications, the concept of developing and implementing future transmission networks through a DC backbone presents a realistic and advantageous option over traditional AC approaches. Currently, most consumer electrical equipment requires DC power to function, thus requiring an AC/DC conversion. New forms of distributed generation, such as solar photovoltaic power, produce a direct DC output. Establishing an accessible and direct supply of DC power to serve such resources and loads creates the potential to mitigate losses experienced in the AC/DC conversion process, reduce overall electrical system infrastructure, and lessen the amount of power generated from power plants, as well as other advantages. For the reasons listed, medium voltage DC (MVDC) networks represent a promising, initial platform for interconnecting relatively low voltage generation resources such as photovoltaic panels, serving loads, and supplying other equipment on a common DC bus bar. Future industrial parks, ship power systems, hybrid plug-in vehicles, and energy storage systems are all avenues for future implementation of the concept. This thesis introduces an initial design and simulation model of the MVDC network concept containing renewable generation, power electronic converters, and induction machine loads. Each of the equipment models are developed and modeled in PSCAD and validated analytically. The models of the represented system equipment and components are individually presented and accompanied with their simulated results to demonstrate the validity of the overall model. Finally, the equipment models are assembled together into a meshed system to perform traditional preliminary studies on the overall power system including wind speed adjustments, load energizing, and fault-clearing analysis in order to evaluate aspects of various operational phenomena such as potential overvoltages, system stability issues, and other unexpected occurrences

Advanced wind energy convertors using electronic power conversion.

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN013000 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Advanced and Innovative Optimization Techniques in Controllers: A Comprehensive Review

New commercial power electronic controllers come to the market almost every day to help improve electronic circuit and system performance and efficiency. In DC–DC switching-mode converters, a simple and elegant hysteretic controller is used to regulate the basic buck, boost and buck–boost converters under slightly different configurations. In AC–DC converters, the input current shaping for power factor correction posts a constraint. But, several brilliant commercial controllers are demonstrated for boost and fly back converters to achieve almost perfect power factor correction. In this paper a comprehensive review of the various advanced optimization techniques used in power electronic controllers is presented

Power control, fault analysis and protection of series connected diode rectifier and VSC based MTDC topology for offshore application.

A multiterminal high-voltage dc (MTDC) system is a promising method for transmitting energy generated from an offshore windfarm (OWF). The creation of MTDC systems became easier by the introduction of voltage source converter (VSC) due to the flexibility and controllability it provides. This technology is newer than the line-commutated converter technology (LCC). Power systems can include any number of windfarms together with converters for both offshore and onshore power conversion. Therefore, this thesis suggests a three-terminal MTDC model of two offshore windfarms and one onshore inverter. The electric energy generated by the two windfarms is rectified into dc and transmitted to the shore using dc cable. Although a VSC or a diode rectifier (DR) can convert ac to dc, a series connection of a VSC and two DRs was proposed at the windfarm side to convert the generated power to achieve controllability of the uncontrollable diode rectifiers and reduces the high cost of badditional VSCs. The proposed topology converts the ac power by dividing the windfarm power so that one-third is the share of the VSC and two-thirds is the share of the DRs. The same topology is used to convert the power produced from the other windfarm. Then, the dc power is transmitted via an undersea dc cable to the onshore location, and is then inverted into ac before it is supplied to the neighbouring ac grid using a grid-side VSC. The proposed topology has many advantages, including a significant save in windfarm VSC (WFVSC) capital cost and a significant reduction in the loss of power of the converter without losing the overall controllability. However, although this topology is suitable for windfarm applications, it might not be suitable for high-voltage direct current (HVDC) that requires bidirectional power flow unless making changes to the topology such as disconnecting the diode rectifiers. Furthermore, fault analyses were investigated, including dc faults and ac faults. Ac faults are categorised as symmetrical or unsymmetrical faults. For comparison purposes, a Simulink model was designed, implemented, and simulated as a reference model. The reference model can operate as VSC-, DR-based MTDC, or a mix of both in a way that any component can be added to or removed from the model at any time during the simulation run. The contribution to the dc fault current from various parts such as dc capacitor and the adjacent feeder was investigated thoroughly, and detailed mathematical formulae were developed to compute fault current from these contributors. In addition, the results of the system response due to both fault types are illustrated and discussed. Both symmetrical and unsymmetrical ac faults were initiated on the onshore grid side, and the system response results are presented for those faults. A generalised control scheme (GCS) was proposed in this thesis, which add the ability the model to control the reactive power and is suitable for both balanced and unbalanced ac faults conditions. A protection against faults was investigated and implemented using dc circuit breakers. The protection system was built to ensure safe operation and to fulfil the grid code requirements. Many grid codes are available and presented in the literature, such as Spanish, British, and Danish; however, a grid code by E.ON was chosen. The protection scheme in VSC-based MTDC networks plays a vital role during dc faults. It is vital that this protection be sensitive, selective, fast, and reliable. Specifically, it must isolate the fault reliably from the system within a short time after the fault occurrence, while maintaining the remaining components of the system in a secure operational condition. For optimal performance, the protection scheme discussed in this thesis employs solid-state circuit breakers. A literature survey relevant to the tasks mentioned above was conducted.PhD in Energy and Powe

Harmonic Impact of Rectifiers Served by Unbalanced Three-Phase Sources

A converter is a rectifier or inverter which is intended to transfer electrical energy between AC and DC busses. A common industrial converter usually employs the familiar Graetz bridge configuration and usually has a rating in the kilowatt through lower megawatt range. The operation of such a device is nominally in the balanced three-phase mode in which the phase currents are nonsinusoidal. The Fourier series components of these currents, or harmonics, have been studied extensively, but relatively little has been done in the unbalanced operating mode. The principal goal of this research is to examine how unbalance in magnitude and phase of the AC supply alters the frequency spectrum of a line-commutated power converter. The topics considered in this thesis are for cases of small unbalance, infinite inductance in the DC circuit (Ldc = OO), and pure resistance in the DC circuit (Ldc= 0). Also, symmetrical component analysis is made, and reasons for the presence of uncharacteristic harmonics are studie

Improvement of resonant harmonic filter effectiveness in the presence of distribution voltage distortion

Resonant harmonic filters (RHFs), are the most common devices installed in distribution systems for reducing distortion caused by harmonic generating loads. When such filters are applied in systems with a distorted distribution voltage their effectiveness may decline drastically. This dissertation explores the causes of degradation of RHFs effectiveness and suggests methods of their improvement both by optimization algorithms and by modification of the filter structure. An optimization based design method is developed for the conventional RHF. It takes into consideration the interaction of the filter with the distribution system and provides a filter which gives the maximum effectiveness with respect to harmonic suppression. The results for the optimized filters, applied in some typical cases, are given, and the limits of effectiveness for a common application are explored. For cases where the conventional RHF cannot be applied due to low effectiveness, a resonant harmonic suppressor, referred to as a RHF with line inductor, is investigated. It is formed by the addition of a line inductor to a conventional RHF, and it has a higher effectiveness in the presence distribution voltage distortion. A similar method of optimization based design is developed and evaluated for the RHF with line inductor as for the conventional RHF. Also, the limits of its effectiveness are explored. One major disadvantage of the RHF with line inductor is the load voltage reduction due to the additional impedance between the distribution system and load. For loads with variable reactive power, the voltage drop across the line inductor may reach an unacceptable level. Also, the fluctuation of the load voltage could increase. In order to reduce these effects, an adaptive capability with respect to load reactive power compensation is added to the filter. Such a filter, referred to as a semi-adaptive RHF, is obtained when a RHF is combined with a thyristor switched inductor (TSI). The addition of the TSI also increases flexibility in the design of the filter with respect to the line inductor’s value. Design aspects of the semi-adaptive RHF are explored and simulation results are presented