Power Conversion System Research Articles

Global Maximum Power Point Tracking in Dynamic Partial Shading Conditions Using Ripple Correlation Control

Under partial shading conditions (PSC), a photovoltaic (PV) system may produce multiple local maximum power points (LMPPs). Traditional maximum power point tracking (MPPT) techniques are not able to distinguish between LMPPs and the global maximum power point (GMPP), leading to sub-optimal PV array power outputs. This article proposes a two-level algorithm for tracking the GMPP. The first level is a discretized global search, allowing the system to hone in on the neighborhood containing the GMPP. In the second level, the well-known ripple correlation control (RCC) technique is used to swiftly converge to the GMPP. Using the proposed two-level algorithm, it can be guaranteed that the GMPP is successfully found and tracked. A benchmark analysis involving other state of the art algorithm reveals that the proposed method is the most superior algorithm in terms of accurately and swiftly tracking the global maximum power point in dynamic partial shading conditions. The algorithm is implemented with a simple inexpensive microcontroller, and therefore can be readily adopted in a myriad of dynamic partial shading applications.Therefore, this work allows the penetration of future photovoltaic power conversion systems to be more efficient.

Power Conversion Systems Enabled by SiC BiDFET Device

The BiDirectional Field-Effect Transistor (BiDFET) can enable circuit topologies requiring four-quadrant switches, that were earlier designed using discrete combinations of MOSFETs, IGBTs, GaN HEMTs, and PiN diodes. The monolithic nature of the BiDFET allows lower device count, smaller switch volume, lower inductance, and simpler packaging, and hence more reliable and commercially viable implementation in power electronics converters. The matrix converter topologies, now feasible using BiDFETs, can eliminate the bulky and unreliable dc link capacitors or inductors required for conventional voltage-source or current-source converters in ac–ac and ac–dc applications. The 1.2 kV BiDFET has the potential to disrupt all the applications utilizing 1.2 kV switches, including electric vehicle (EV) drivetrain, bidirectional EV chargers, industrial motor drives, solid-state transformers, datacenter power supplies, elevator drives, dc microgrids, energy storage grid integration, solid-state breakers, etc.

Operational characterization of tokamak and stellarator type fusion power plants from an energy system perspective

Fusion power plants are not yet considered specifically in European long-term energy system studies. In order to include them in such studies a corresponding and valid parametrization of their operating performance has to be established despite the fact that fusion reactor design is still an ongoing effort.The goal of the present paper is to specify and energetically represent the prospect of feasible operation and dynamics of tokamak and stellarator type fusion power plants from an energy system perspective. Special focus is given on time and operation mode dependent self-consumption. The basis of the parametrization is a one GWel power output plant. As a result, we propose the representation of fusion power plants as a system with three main components (fusion reactor, thermal energy storage (TES) and power conversion system), followed by a set of parameters for both tokamak and stellarator type devices. Five different operating states are defined for a fusion plant, depending on the required and active auxiliary subsystems. The comparison between operational dynamics of conventional and fusion power plants showed no tremendous differences due to the TES utilization. However, fusion plants had a lower full-load operation efficiency due to higher self-consumption as well as extensive pre-production losses.

Wind turbine emulator control improvement using nonlinear PI controller for wind energy conversion system: Design and real‐time implementation

SummaryWind turbine emulators (WTE) have become a necessity for testing, developing, and improving design and control strategies in the renewable energy domain. The aim of this paper is to realize an experimental standalone wind energy conversion system emulator (WECSE) with improved torque and current control strategy using a nonlinear PI controller. The prototype was developed with a separately excited DC motor to simulate the wind turbine by providing the required speed and torque for power generation using a directly driven wound‐rotor synchronous generator and a power conversion system controlled by a modified drift‐free Perturb and Observe (P&O) Maximum Power Point Tracking algorithm (MPPT). The DC motor torque is controlled by an nonlinear PI controller regulator for an estimated current reference through a chopper driven by a dSPACE control board where the MATLAB/Simulink platform is used for wind turbine simulation. The proposed method was validated by experimental tests for different wind speeds and was compared with the conventional method. The experimental results have demonstrated that the control, emulation, and MPPT performances of the proposed method are significantly better.

Bifurcation Phenomena in Open-Loop DCM-Operated DC–DC Switching Converters Feeding Constant Power Loads

Constant power loading is an effect that appears in multiple-stage energy conversion systems with individually regulated switching power converters. In a two-stage system, an upstream or source converter drives one or more downstream or load converters. The downstream converters in a two-stage power conversion system are designed to provide the fastest transient response in stand-alone operation. Consequently, they behave as a constant power load (CPL) to the upstream converter within their control bandwidth. In the past, open-loop power converters feeding CPLs and operating in discontinuous conduction mode (DCM) were considered to be stable. In this paper, it is shown that these systems can undergo instabilities, which have been so far overlooked. First, numerical simulations from the switched model of an open-loop boost converter under DCM operation and loaded with a tightly regulated buck converter and the same converter loaded by an ideal CPL are presented to show that they exhibit similar nonlinear behavior and bifurcation phenomena. Then, the three elementary open-loop DC–DC converters operating in the DCM were considered and their bifurcation phenomena were revealed. It is shown that the period-doubling route to chaos in the DC–DC boost converter is interrupted by a sudden appearance of dangerous destructive dynamics due to the excessively unlimited load current in the CPL. For the buck converter, only the first period-doubling bifurcation is observed before the destructive behavior appears. The open-loop buck–boost converter under DCM and feeding a CPL is always unstable and exhibits no periodic orbit. Based on the observed phenomena, approximate discrete-time models were derived, which despite their simplicity, were seen to display the most-important and -essential features of the corresponding switching converters before destructive dynamics occurs.

Design of the secondary circuit for the WCLL BB option of the EU DEMO power plant based on the new Energy Map

EU-DEMO will be a DEMOnstration Fusion power plant that is being designed to demonstrate the production of electricity from nuclear fusion at the level of a few hundred MW. The Primary Heat Transfer System (PHTS) transfers heat from the reactor to the Power Conversion System (PCS, the secondary circuit), responsible for converting the thermal power into electricity. According to the recent DEMO Energy Map data, In-Vessel components such as Breeding Blanket (BB), Divertor (DIV), Limiters, Vacuum Vessel and Auxiliaries feature uncertainties in generated thermal power. Some of them serve PCS as heat sources, namely: BB and DIV. Two reference concepts of the BB have been selected as a result of the DEMO pre-conceptual studies: the Helium Cooled Pebble Bed and the Water Cooled Lithium-Lead (WCLL). Because of the mentioned uncertainties regarding the distribution of energy released from fusion among the different reactor heat sources, several possible operational scenarios are considered. One of the challenges faced by the DEMO plant designers is the pulsed operation of the tokamak: plasma burn pulses (2 h) will be separated by dwell phases (10 min). To compensate drastic power reduction of the reactor heat sources during dwell, an Energy Storage System (ESS) of different sizes is included in considered DEMO plant configurations.Our work is focused on the design and analysis of the DEMO PCS circuit for the option WCLL BB with the small ESS. We create the GateCycle model of the “maximum of maximum” PCS cycle, in which the plant components are sized to incorporate the maximum power of all the reactor heat sources. Operation of the PCS circuit during the pulse (in the “Design” mode) and during dwell (in the “Off-design” mode with the fixed design of all the circuit components) is simulated, to verify the system performance and the potential feasibility of the considered PCS concept.

Transient stability enhancement using optimized PI tuning of static synchronous series compensator in wind power conversion system

Power systems are expanding comprehensively with the increase in load demand from both residential and industrial usage. Renewable energy is penetrating the power system to satisfy the power needs of the load demand. With its potential to generate power and compensate for a large portion of the load demand, wind generators make a major renewable power contribution. Power oscillations inherent in wind generator integration with the grid are a power quality issue to be addressed. Oscillation damping using flexible AC transmission system (FACTS) devices is a relevant solution for the power quality issue. There are multiple reasons for power oscillation. Mainly, power systems encounter fault conditions. The faults can be cleared, and the power system tries to retain stability. Sometimes, the system fails due to a longer settling time. A series-connected FACTS device utilized as a series compensator is referred to as a static synchronous series compensator (SSSC). Controlling the flow of electricity over a transmission line using this method is incredibly efficient. The capacity to switch between a capacitive and an inductive reactance characteristic is necessary. The SSSC regulates the flow of power in transmission lines to which it is linked by adjusting both the magnitude of the injected voltage and the phase angle of the injected voltage in series with the transmission line. This allows the SSSC to manage the power flow in the transmission lines. It does it by inserting a voltage that can be controlled into a transmission line in series with the fundamental frequency. This paper develops the optimally tuned SSSC in the wind-integrated grid system to dampen the oscillation. Teacher–learner-based optimization (TLBO) and gray wolf optimization (GWO) algorithms are used to tune the PI controller to improve the damping response. The obtained results show that the damping performance of the proposed controller is better than that of the other traditional controllers.

Analyzing the engineering feasibility of the direct fusion drive

The Direct Fusion Drive (DFD) and its terrestrial counterpart, the Princeton Field Reversed Configuration (PFRC) reactor, have seen significant developments in the past decade. Various groups conducted detailed research on the required specifications of the engine and associated technology for power delivery to onboard avionics and payloads. Multiple studies have also addressed the thrust generation mechanism using empirical specific power scaling relations and plasma flow simulations. Recent studies have designed spacecraft for missions to Earth’s second Lagrange point, Mars, transneptunian bodies like Pluto, and the neighboring star systems Alpha Centauri A and B. However, significant work is needed to design the engine components in detail using scientific scaling relations and ab inito calculations to develop the physical systems for prototyping and testing. After critically analyzing the reference design of the DFD and the underlying fusion reactor, this paper addresses the technological gaps and suggests avenues to improve specifications toward targets outlined in previous studies while considering costs. Further, the authors present a prototype engine and magnetohydrodynamic power conversion system design to study the engineering hurdles relevant to the practical implementation of the DFD.

Techno-economic comparison of DEMO power conversion systems

The transition to low-carbon technologies lies in the support of modern energy sources, which are also represented by fusion power plants. One of the first fusion power plants will be the European Union DEMOnstration fusion power plant. Among the key attributes that influence the entire design of this power plant, belongs the power conversion system. In addition to the classic Rankine steam cycle, it is possible to use different cooling media, such as supercritical CO2 in the Brayton cycle. The advantages of supercritical CO2 are mainly in the compactness of the power conversion system, that is, lower investment costs for the entire power plant while maintaining the high efficiency of the entire power plant. The key question that can influence the choice of the power conversion system is the levelized cost of electricity. The direct cost of the supercritical CO2 cycles is lower compared to the Rankine steam cycles, in general. On the other hand, supercritical CO2 cycles for the DEMOnstration fusion power plant offer slightly lower efficiency (by approximately 1.5 percentage points), which negatively influences supercritical CO2 cycles levelized cost of electricity by 10%–20% against Rankine steam cycles.

Application of a Model-Based Method to the Online Detection of Rotating Rectifier Faults in Brushless Synchronous Machines

Converters are one of the most sensible components of any power conversion system when it comes to electrical faults. Moreover, if these converters are used in a rotating system, as is the case with rotating rectifiers used in brushless synchronous machines, apart from also being exposed to mechanical effects and thus having a greater likelihood of failure, no access is available directly, causing a lack of available measurements for condition monitoring. This paper applies a model-based method to the online detection of open-diode faults, shorted-diode faults and exciter open-phase faults in the rotating rectifiers of brushless synchronous machines. The applied method relies on the comparison between the measured and the theoretical exciter field currents, the latter computed through a healthy machine model from the machine actual output values. The proposed protection strategy stands out for its computational simplicity and its non-invasiveness, which makes its industrial application straightforward without the need of any further equipment or adaptation. Its applicability has been verified through a double approach, on the one hand, through computer simulations, and, on the other hand, through experimental tests, achieving satisfactory results. The research conducted proves that with the proposed method, given reasonable measurement and model estimation typical errors of less than 5%, positive differences between the measured and the theoretical exciter field currents of more than 13%, 200% and 30% for open-diode faults, shorted-diode faults and exciter open-phase faults, respectively, are detectable with at least a 95% confidence interval.

Modelling and Simulation of Two-Float Oscillating Flapping-Wing Wave Energy Converter

In order to achieve an efficient collection of wave energy and stable power output, a two-float oscillating flapping-wing wave energy converter is proposed. Based on the hydraulic conversion system of the multi-chamber cylinder, a hydraulic conversion and merging system of the multi-chamber cylinder is created. The composition and working principle of the system, as well as the parameters of the acquisition mechanism, hydraulic conversion and merging system, are determined. AQWA is used to simulate the flapping-wing angular displacement response, and AMESim is used to build the simulation model of the acquisition mechanism, the hydraulic conversion and merging of multi-chamber cylinders, and the power conversion and energy storage system. The research results show that the system can work stably under level 3 and 4 sea conditions, and the input power of the battery is stable at about 46.5kW and 56.5kW, respectively. The rationality and feasibility of the two-float oscillating flapping-wing wave energy converter are verified by simulation.

Research on Control Strategy of High Voltage Cascaded Energy Storage Converters

High voltage cascaded energy storage power conversion system, as the fusion of the traditional cascade converter topology and the energy storage application, is an excellent technical route for large capacity high voltage energy storage system, but it also faces many new problems. How to use the control strategy to play better the advantages of high voltage cascaded energy storage has gotten more and more attention. This paper summarizes the research on power control, balance control, and fault-tolerant control of high voltage cascaded energy storage to provide a reference for related research and engineering application.

Event-Triggered ESO-Based Robust MPC for Power Converters

An event-triggered control technique has been developed recently. This technique explicitly reduced the signal transmission by introducing a flexible design of threshold inequalities. It was later extended to event-triggered model-predictive control for power converter systems. In this letter, by incorporating this control technique into an extended state-observer-based finite-control-set model-predictive control framework, we have developed a new model-predictive control architecture for power converter systems with parametric uncertainties. Meanwhile, a novel cost function with respect to the angle minimization term is embedded into this proposal. The novelty of our development lies not only in integrating the event-triggered mechanism with the suggested finite-control-set model-predictive control architecture for facilitating the alleviation of performance deterioration caused by parameter variations and model uncertainties, but also in a multiobjective optimization design that allows the switching frequency in a low value. Finally, extensive simulative and experimental investigations for a modular multilevel converter confirm the interest and the viability of the proposed design methodology.

An improved robust coupled sliding mode control strategy for solar photovoltaic‐based single‐phase inverters

SummarySolar photovoltaic (SPV)‐based single‐phase inverters are universally accepted and commercially utilized renewable power converter system. Generally, the magnitude of the inverter output is perturbed on load variations, and the intermediate DC‐DC boost converter output is perturbed because of the solar availability. The main intermediary component is the DC‐Link capacitor. The regulation of DC‐Link voltage is the key factor in maintaining the stability of the system with bilateral power converters. In the existing methods, individual Sliding Mode Control (SMC) are used to maintain the stability of the converters independently. However, the individual SMC's do not have any corelated control to address the common problem of DC‐Link voltage regulation. This paper proposes an alternative solution with Coupled SMC (CSMC) to address the problem of regulating the DC‐Link voltage. Both the Boost‐SMC and Inverter‐SMC are individually developed and controlled in a unified approach by having mutual adjustments in the converter and inverter switching pulses. The CSMC strategy is developed in Matlab and experimented using dSPACE‐MicrolabBox. The validation tests are compared with conventional SMC, which shows the betterment of CSMC in robustness and waveshape tracking.

Hybrid-Model-Based Digital Twin of the Drivetrain of a Wind Turbine and Its Application for Failure Synthetic Data Generation

Computer modelling and digitalization are integral to the wind energy sector since they provide tools with which to improve the design and performance of wind turbines, and thus reduce both capital and operational costs. The massive sensor rollout and increase in big data processing capacity over the last decade has made data collection and analysis more efficient, allowing for the development and use of digital twins. This paper presents a methodology for developing a hybrid-model-based digital twin (DT) of a power conversion system of wind turbines. This DT allows knowledge to be acquired from real operation data while preserving physical design relationships, can generate synthetic data from events that never happened, and helps in the detection and classification of different failure conditions. Starting from an initial physics-based model of a wind turbine drivetrain, which is trained with real data, the proposed methodology has two major innovative outcomes. The first innovation aspect is the application of generative stochastic models coupled with a hybrid-model-based digital twin (DT) for the creation of synthetic failure data based on real anomalies observed in SCADA data. The second innovation aspect is the classification of failures based on machine learning techniques, that allows anomaly conditions to be identified in the operation of the wind turbine. Firstly, technique and methodology were contrasted and validated with operation data of a real wind farm owned by Engie, including labelled failure conditions. Although the selected use case technology is based on a double-fed induction generator (DFIG) and its corresponding partial-scale power converter, the methodology could be applied to other wind conversion technologies.

Consensus-based multi-converter power allocation strategy in battery energy storage system

Battery energy storage system (BESS) commonly consists of multiple power conversion systems (PCSs) under parallel operation, which are controlled by a centralized controller to realize power allocation. As the number of PCSs increases, the topology and communication structure of the BESS become more complex, reducing the ability of centralized controller to respond quickly and flexibly. An effective approach to address this problem is to develop a distributed power allocation strategy. Consensus theory, as an advanced distributed algorithm, is based on partial information interaction to achieve the state consistency of multiple agents. This paper presents a consensus-based strategy to realize efficient power allocation under distributed framework. The system convergence is analyzed under communication failure to demonstrate the robustness of the consensus-based control strategy. Considering state-of-charge (SOC) equalization, operation efficiency and battery life loss, the consensus factor of the consensus-based power allocation model is switched according to specific battery states and application scenarios. Finally, the effectiveness of the proposed strategy is validated by simulation and real-world experiment in peak shaving and mitigating wind farm fluctuation scenarios.

Recent Progress on High Temperature and High Pressure Heat Exchangers for Supercritical CO2 Power Generation and Conversion Systems

Heat exchangers for supercritical CO2 power generation and waste heat to power conversion systems have a significant impact on the overall cycle efficiency and system footprint. Key challenges for supercritical CO2 heat exchangers include ability to withstand high temperature and high pressure (typical temperature range of heat source 350 to 800 °C and typical required operating pressure range 150 to 300 bars), and large pressure differential between fluid streams. Other requirements are low pressure drop, high effectiveness and high reliability under thermal cycling. This paper presents recent developments in supercritical CO2 heat exchangers in terms of material selection, design, manufacture, and operation. Since heat exchangers represent a significant portion of the total system cost, another key challenge is to find a compromise between the heat exchanger type, cost, durability, and performance. This paper explores heat exchanger technologies, manufacturing techniques and materials for high temperature and high pressure heat exchangers for supercritical CO2 applications. It also identifies technology gaps and research needs to accelerate the development of effective designs to facilitate the commercialization of both supercritical CO2 heat exchanger technologies and power cycles.

Medium-Voltage Seven-Level Multiplexed Converter for AC Applications

After several decades of technological evolution, power electronics applications have become extremely important for electric/hybrid propulsion systems, smart grids, renewable energy systems, energy saving, and bulk energy storage, besides the typical applications in industrial automation and high-efficiency energy systems. In all these applications, a power conversion system with high efficiency and power density is required without sacrificing the system's power quality. The paper describes the specific characteristics and the scalar modulation strategy of a new multi-level multiplexed inverter topology for medium-voltage AC applications. The proposed power converter configuration is intended for high-power applications aiming to reduce the active device's voltage rating and losses exhibiting a resulting low switching frequency of the power switches. The proposed modulation strategy aims to reduce the computational burden and simplify the implementation process, which are at the basis of any possible industrial adoption. Simulation results and full Hardware-In-the-Loop verification support the analysis. The Hardware-In-the-Loop assessment has enabled the development of a suitable modulation strategy, and the testing of the power converter is a safe environment.

SSR Stable Wind Speed Range Quantification for DFIG-Based Wind Power Conversion System Considering Frequency Coupling

The subsynchronous resonance (SSR) characteristics of doubly-fed induction generation-based wind power conversion system (DFIG-WPCS) varies with the wind speed. Obtaining the small signal SSR stable wind speed range is urgently important for wind farm dispatch planning. However, there are few methods to quantify the small signal SSR stable wind speed range of DFIG-WPCS. Moreover, the inherent frequency coupling when DFIG-WPCS is integrated to modular multilevel converter-high voltage direct current (MMC-HVDC) may lead to inaccurate stability assessment results. In this article, the frequency coupling mechanism between WPCS and MMC is revealed by analyzing the law of harmonic interaction in the interconnected system. The wind speed-frequency two-variable equivalent admittance model of DFIG-WPCS is established by taking the frequency coupling account. Relying on the two-variable open-loop transfer function, the multiple phase margin contours plot approach is proposed to quantify the small signal SSR stable wind speed range and damping properties. The effectiveness of two-variable admittance and analysis method are validated against electromagnetic transient (EMT) testbed with MATLAB /SimPowerSystems. The analysis results can provide a reference for the early planning and stable operation of DFIG-WPCS.

AC-DC Fuzzy Linear Active Disturbance Rejection Control Strategy of Front Stage of Bidirectional Converter Based on V2G

Aiming at the problems of output voltage fluctuation and current total harmonic distortion (THD) in the front stage totem-pole bridgeless PFC of two-stage V2G (Vehicle to Grid) vehicle-mounted bi-directional converter, a fuzzy linear active disturbance rejection control strategy for V2G front-stage AC-DC power conversion system is proposed. Firstly, the topological working mode of the totem-pole bridgeless PFC is analyzed, and the mathematical model is established. Combined with the system model and the linear active disturbance rejection theory, a double closed-loop controller is designed with the second-order linear active disturbance rejection control as the voltage outer loop and PI control as the current inner loop. The controller can realize self-adaptive tuning of the proportional gain coefficient of the active disturbance rejection controller through fuzzy reasoning and realize self-adaptive control. Simulation and experimental results show that this method can better solve the problems of slow system response and high total harmonic distortion rate of input current and effectively improve the system’s robustness.