Converter Capacity Research Articles

Rotating speed pulling-back control and adaptive strategy of doubly-fed variable speed pumped storage unit

When the doubly-fed variable speed pumped storage unit (DFVSPSU) adopts the power priority control, the rotating speed may change excessively during the transient process, potentially causing the rotor side power to exceed the converter capacity, which may damage the converter. Therefore, this study aims to analyze the mechanism underlying rotor side power overlimit and conventional rotating speed pulling-back control (RSPBC), proposing an adaptive rotating speed pulling-back control (ARSPBC). First, the unit's speed operating range and the converter capacity were determined based on the characteristic curve of the pump-turbine. Second, the optimal speed and speed limit were compared, indicating that the unit's operation is closer to the lower speed limit, which increases the likelihood of power overlimit occurrences. Third, the mechanism of rotor side power overlimit and conventional RSPBC were analyzed. It was noted that although conventional RSPBC can prevent rotor side power overlimit, it can result in significant generator power fluctuations. Finally, an ARSPBC incorporating speed change rate feedback was proposed. The numerical simulation showed that the ARSPBC can not only prevent the rotor side power overlimit, but also reduce the generator power fluctuations. Compared to the conventional RSPBC, the power fluctuation was reduced from 70 % to 62.5 %–4 %.

A novel high-efficient lithium-ion battery serial formation system scheme based on partial power conversion

The formation of the battery is a crucial step in its manufacturing process. The existing battery formation system suffers from low efficiency and high energy consumption costs due to long energy flow paths, high DC bus line losses, and additional balancing circuit applications. Therefore, a novel high-efficient battery series formation system (BSFS) that combines partial power processing architecture (PPPA) with the modular converter is proposed to address this problem in this article. The PPPA-BSFS includes two-stage isolated sub-modules consisting of Buck-Boost converter and LLC converter with input-serial and output-parallel connection. The isolated converter only processes the serial compensation power, about 1/3 of the full power. Consequently, the converter capacity can be shrunk in the PPPA-BSFS, which can reduce the system's cost and power loss. Furthermore, the paper establishes the steady-state mathematical model and energy efficiency model of PPPA-BSFS based on analyzing the power compensation principle, and the key factors affecting the operational efficiency of PPPA-BSFS are studied. Finally, we construct the simulation models with 10 cells and 120 cells to compare and analyze energy efficiency between PPPA-BSFS and existing battery formation topology under various operating conditions. Our simulation results show that the highest operating efficiency of the PPPA-BSFS proposed is 99.37 % when 120 cells are formed normally, where there is an increment of 5.95 %, 2.36 %, and 0.85 % compared with others, respectively. The high efficiency of the PPPA-BSFS is verified, and the PPPA-BSFS shows great potential for application in battery formation scenarios.

Evaluation of the efficiency of energy complexes with the production of hydrogen, oxygen, heat and electricity

RELEVANCE of the research. The global trend of decarbonization of the national economies of the leading countries of the world implies an increase in energy production due to renewable energy sources and hydrogen. The most environmentally friendly way to produce hydrogen is the electrolysis of water using wind and solar energy. Combined production of thermal, electric energy, hydrogen and oxygen carried out by energy complexes ensures reduction of harmful emissions and increase of their economic efficiency.METHODS. In solving this problem, the method of computational experiment was used, taking into account the geographical and climatic data of the location of the energy complex, as well as the nature of consumption of thermal, electric energy and hydrogen. The calculation was implemented in Visual Basic.RESULTS. The article describes the relevance of the topic, examines the influence of the installed capacity of photovoltaic converters, geographical, climatic and cost characteristics on the efficiency of the energy complex.CONCLUSION. The structure is determined and a schematic diagram of a multi-purpose energy complex is proposed. A methodology for calculating quantitative and economic indicators of the installation has been developed. In the course of the study, it was noted that there is an optimal value of the installed capacity of a photovoltaic installation, further increase of which is inexpedient from an economic point of view. It was also determined that the combination of hydrogen gas stations based on solar installations with traditional sources of energy supply can reduce the cost of hydrogen produced, which in the future may be a solution to the problem of creating a hydrogen infrastructure.

IGBT Gate Boost Drive Technology for Promoting the Overload Capacity of Traction Converter.

Under certain circumstances, a high-speed railway may require constant acceleration or emergency braking, in which case the inverter may experience short-term overload conditions and the current passing through the IGBT will go beyond the rated design tolerance. Under overload conditions, the IGBT loss will increase instantly, raising the power semiconductor device's junction temperature in the process. This research examines the boosting-gate-voltage-driven IGBT control technology. It increases the gate drive voltage and the IGBT current capacity and decreases the conduction voltage drop of IGBT under short-term overload conditions, reducing the instantaneous loss and temperature rise undulation of IGBT. The working characteristics of IGBT devices are studied, and the influence of gate drive voltage on device loss and temperature rise fluctuations is analyzed. Based on the emergency acceleration and brake conditions of the actual train operation, the short-term overload characteristics of the inverter are analyzed. The optimization analysis of the boosting gate voltage under emergency conditions is carried out, and the IGBT drive circuit with gate voltage pumping function is designed. The effectiveness of the driving circuit is verified through PSpice simulation and actual switching characteristic test. According to the analysis of experimental data, it can be verified that increasing the gate voltage technology can reduce IGBT losses.

Partial-Power Conversion for Increased Energy Storage Capability of Li-Ion Battery Energy Storage System

Full-power converters are used in battery energy storage systems (BESS) because of their simple structure, high efficiency and relatively low cost. However, cell-to-cell variation, including capacity, state of charge (SOC), and internal resistance, will decrease the available capacity of serially connected battery packs, thereby negatively affecting the energy utilization rate (EUTR) of BESS. This article proposes a novel BESS scheme that combines a modular converter with partial power conversion architecture to make a modular partial power converter (MPPC) that addresses the issue. The MPPC consists of input-serial and output-paralleled (ISOP) isolated phase-shift full-bridge (PSFB) sub-modules. It processes only the serial compensation power, about 1/3 of the full power. Consequently, the MPPC shrinks the converter capacity, which can reduce the cost and power loss. Furthermore, this article develops a BESS model considering cell-to-cell variations to analyze the energy storage capability of the MPPC-BESS compared with the existing full-power BESS. To test the model, we run a simulation using parameter values from 100 real retired batteries. Our simulation results show that the MPPC can significantly alleviate the reduction of EUTR as the voltage level increase. Finally, we construct a 36V/720W MPPC-BESS prototype with two battery packs and PSFB sub-modules to verify the bidirectional operating stability and energy storage capability.

Thermal Safety Assessment Method for Power Devices in Natural Air-Cooled Converters

The junction temperature of a power device in a natural air-cooled power converter fluctuates randomly due to the variation in airflow rate in ambient environments. Most of the existing thermal analysis methods do not pay attention to the uncertain influence of airflow on the heat-dissipation capacity of such converters, making it difficult to accurately evaluate the thermal safety of these devices. To address this issue, a thermal safety assessment method for power devices in natural air-cooled converters is proposed in this paper. In the proposed method, convective heat resistance samples of converter housing are extracted with an equivalent thermal network model and the historical operation temperature of the converter. Wavelet packet transform is used to analyze the time–frequency domain characteristics of the convective heat resistance, and Monte Carlo simulation is employed to simulate the random influence of the airflow rate on the device junction temperature. The thermal safety of power devices is assessed in the form of over-temperature probability, which is expressed by a two-variable growth function. An experimental platform is designed to validate the effectiveness of the proposed method. The results show that the proposed method can accurately estimate the over-temperature risk of a power device in a natural air-cooled converter under different ambient temperature and current levels, thus effectively improving the thermal reliability of converters.

AC Networks With Microgrids: Passivity-Based Controllers in Single Phase Electric Network

—This paper is an investigation of the possible integration of distributed energy resources (DERs) in single-phase alternating current networks, using voltage-controlled converters, based on passivity-based control theory. In closed-loop operation, Hamiltonian representations facilitate the development of passive controllers that guarantee stability. The trajectory tracking problem can be solved as a regulation problem by transforming the non-autonomous dynamical model into an incremental model. In this work, the primary contribution is the ability to control active and reactive power flows between DERs and the grid according to the availability of primary energy resources and converter capacity. Simulated results indicate that all passive controllers achieve the control objective, maintaining asymptotic stability and achieving the same dynamic performance as integral proportional controllers. SimPowerSystems library is used to develop all simulations under the MATLAB/Simulink environment. The paper presents a novel approach to integrating distributed energy resources into single-phase alternating current networks, utilizing voltage-controlled converters and leveraging passivity-based control theory.

Coordinated control strategy of reactive power compensation based on a flexible distribution network transformer

In order to solve the problem of the power quality caused by distributed power access to the distribution network, this paper proposes a coordinated control strategy of reactive power compensation based on a flexible distribution transformer. First, the working principle of the flexible distribution transformer is analyzed, and the mathematical model of energy acquisition and the regulation converter of the flexible distribution transformer are studied as well. The device-level control strategies of energy acquisition and regulation converters are proposed, respectively. Then, in order to maintain the stability of the bus voltage and quickly respond to the reactive power changes of the system, a coordinated control strategy for the reactive power compensation of flexible distribution transformers is proposed. The priority herein is to maximize the reactive power compensation capacity of the energy harvesting converter. When the energy harvesting converter reaches the compensation upper limit, the control converter is used for reactive power compensation to further suppress the grid voltage fluctuation. Finally, it is verified through simulations that the flexible distribution transformer can realize the reactive power compensation of the distribution network and effectively improve the power quality of the distribution network.

A multi‐objective low‐voltage ride‐through control strategy for three‐phase grid‐interfaced solar power plant during symmetrical and asymmetrical faults†

SummaryThis paper presents a low‐voltage ride‐through capability (LVRT) solution for the photovoltaic (PV) grid‐connected inverters compliant with the Indian grid code. The LVRT strategy ensures the uninterrupted connection of solar energy to the grid, safeguarding against system shutdown in the event of various grid faults. During this time, the inverter supplies reactive power to the utility grid, effectively utilizing converter capacity and aiding the restoration of the voltage profile at the point of common coupling (PCC). The system used in this study involves a two‐stage grid‐interfaced solar PV setup with a boost converter that tracks the maximum power point (MPP) and an inverter that maintains the DC‐link voltage. A modified dynamic braking chopper control (MDBCC) has also been implemented to minimize the DC‐link voltage oscillations during asymmetrical faults while simultaneously injecting the sinusoidal current into the grid. To improve grid synchronization in the presence of unbalanced abnormalities, a modified dual second‐order generalized integrator phase‐locked loop (DSOGI‐PLL) has been employed to track the grid voltage angle precisely. Furthermore, a new control system has been developed in conjunction with the LVRT approach. The suggested system is simulated in MATLAB/Simulink to validate the efficacy of the LVRT control method during symmetrical and asymmetrical faults using a 100‐kW two‐stage grid‐interfaced solar PV facility. Finally, for experimental validation, a real‐time test bench (RTS) OPAL‐RT 4510 is used to implement the setup with its control, and the practical results are compared with the simulation outcomes.

Adaptive power capability control scheme for voltage source converter to improve transient stability

Applications of Power Electronic Devices (PED) are rapidly increasing because of their controllability. A Direct Current (DC) transmission system is used for the interconnection of Renewable Energy Sources (RES), and a DC grid research reached the stage of demonstration. Since power electronic devices are controllable, as opposed to conventional ac power facilities, various studies have been carried out on control schemes. Based on fast response speed, the device can affect the stability of the power system by improving the transient stability according to the control method. However, the operation range of converters is limited based on the normal state of the AC system. Therefore, the entire converter capacity cannot be used, and the same standard is applied even in the transient state. In this paper, a novel control scheme is proposed in order to improve the voltage and frequency stability by using the maximum capacity of the converter. The control scheme calculates an Adaptive Power Capability Index (APCI) based on the range of variation of frequency and voltage in the transient state, then applies this index to the controller. In order to verify the new control scheme, a dynamic model is developed and verified in a Transient Stability Analysis (TSA) program. Following the verification of the model, the model is applied to the actual power system data and the power system stability is analyzed.

Optimization of Algorithm for Solving Railroad Power Conditioner Compensation Power Reference Value and System Power Quality Analysis Based on Optimal Compensation Model

At present, electrified railroads with complex road conditions are facing the problems of the existence of a single power supply method, the deterioration in power quality, and the difficulty in recycling regenerative braking energy. In order to improve the above problems, this paper establishes a traction power supply system containing photovoltaic units, proposes an optimized compensation model for the RPC (railroad power conditioner), and improves the particle swarm algorithm for solving the reference value of RPC compensation power. First, the structure of the RPC-based traction photovoltaic power generation system and the establishment of the integrated energy management strategy of the system are constructed. Then, the back-to-back converter compensation model with power quality index parameter constraints is established, which establishes the optimization function with the objectives of minimizing the negative sequence current, maximizing the power factor, and minimizing the RPC compensation power, as well as establishes constraints on the active converter capacity and three-phase voltage imbalance. Then, the particle swarm algorithm for solving the RPC compensation power reference value is improved, specifically in the original PSO by introducing dynamically changing inertia weights and learning factors. This not only solves the problem of the single power supply and realizes the nearby consumption of photovoltaic units in the traction system, but also realizes the recycling of regenerative braking energy and the coordinated control of the traction photovoltaic power generation system, improves the power quality of the system, and meets the demand of the RPC for real-time control. In order to verify the effectiveness of the optimized compensation model established in this paper and improve the convergence of the particle swarm algorithm for solving the RPC compensation power reference value, a simulation model of the traction PV power generation system is established in MATLAB/Simulink, and a real-time simulation is carried out to verify it based on the preset working condition data. The results show that the RPC optimization compensation model developed in this paper can coordinate the control system energy flow and improve the system power quality (the power factor increases and the negative sequence current decreases). The improved particle swarm algorithm is more convergent.

Over-current low voltage ride-through operation of grid-connected converters based on thermal analysis

The grid-connected converters are becoming an important role in modern power grid. The grid codes have been proposed for the operation of grid-connected converters. The low voltage ride-through, namely one of the grid codes, is the operation of grid-connected converters under grid fault. The reactive power is generated by gird-connected converters to support grid under low voltage ride-through operation. However, the maximum reactive power is restricted by the capacity of converter, which is designed for long-term operation. Since the low voltage ride-through is a short-term operation, the maximum output reactive power can be enhanced. An over-current low voltage ride-through operation is proposed, and it is designed based on the maximum short-term capacity. The maximum short-term capacity is designed to ensure the junction temperature of semiconductor devices in the converter under the reliable operation, which is calculated based on thermal analysis. Furthermore, the different approaches to enhancing the maximum short-term capacity is analyzed.

Hardware-In-the-Loop test for the optimal damping identification of oscillating buoy wave energy converters

The Power-Take-Off (PTO) damping is a significant factor on energy harvesting capacity of Wave Energy Converters (WEC), which cannot be determined through the traditional laboratory test. In order to consider the complexity of the PTO damping mechanism, the Hardware-In-the-Loop (HIL) methodology is employed to focus on the PTO performance and the energy harvesting capacity in this paper. The dynamic numerical model, servo drives, and data transmission system are integrated in the HIL system, and the established system is then applied to the optimal damping identification of the WEC device which is assembled with the PTO containing a Mechanical Motion Rectifier (MMR). The experimental results indicate that the maximum power appears at the different damping values since the multi-stage energy capture characteristics are affected by the damping mechanism within the mechanical PTO, and the wave height has little effect on the damping values corresponding to the maximum power. This phenomenon is worth noting during the process of the PTO test, and could provide intuitive guidance for the reasonable design and control strategy optimization of the PTO.

Three-level ANPC rectifier loss calculation and balance control method

Traditional three-level neutral-point clamped converter has the problem of the uneven loss distribution, which restricts the increase of converter capacity. Three-level active neutral-point clamped (3L-ANPC) converter can reduce the imbalance of power device loss distribution. At present, three traditional SPWM modulation strategies are used in single-phase three-level ANPC modulation. Considering the loss distribution, it is generally required to input the current and voltage sampling values in real-time and calculate the temperature of the switching device online for control or collect the temperature of the switching device in real-time for feedback control, which is more complicated to realize. This paper proposes a modulation scheme based on two carrier cycles, combined with traditional SPWM1 and 2, to analyze the switching conditions and loss characteristics of each switching device in this scheme. The simulation of a single-phase three-level ANPC rectifier was built, and the experiment was carried out on the Yuankuan HIL semi-physical platform. The results show that this scheme is simple, easy to implement, and can effectively reduce the loss distribution imbalance.

Optimal dynamic power allocation for electric vehicles in an extreme fast charging station

With the ever-increasing penetration of electric vehicles (EVs), extreme fast charging stations (XFCSs) are being widely deployed, wherein battery energy storages (BESs) are also installed for reducing the peak charging power. However, integrating the XFCS with a high-capacity power converter into the power distribution network (PDN) is difficult and uneconomical due to the restrictions regarding urban planning and high investment in PDN expansion. Considering the fluctuation in the EV charging demand and the limited capacity of the power converter, a collaborative policy for real-time EV charging power allocation and BES discharging power control is proposed based on Markov Decision Process (MDP), which is solved by the constraint deep deterministic policy gradient (CDDPG). The proposed model makes it possible to integrate the XFCS with reduced capacity power converter into the PDN with a minimal negative impact on the quality of service (QoS) of EV owners. Finally, the experimental evaluation with real-word data sets demonstrates that the proposed approach is more effective than benchmark methods in dynamically allocating charging power for XFCS.

Low-voltage ride-through strategy for offshore wind turbines based on current relaxation region

This paper proposes a current relation region (CRR)-based low-voltage ride-through (LVRT) strategy for offshore wind turbines (OWTs). The proposed strategy seeks to minimize the detrimental shortfall of OWT's active current on the premise of maintaining the reactive support ability by considering short-time overcurrent capacity of grid-side converter (GSC) during low-voltage fault. Firstly, the extended transient current region (ETCR) is introduced to improve the transient current ability of OWTs with comparison to conventional transient current region (CTCR) based on rated current capacity of GSC. Combined with the introduced ETCR, grid code constraint and wind speed constraint are investigated for detailed description of OWT's transient current boundary of CRR. Furthermore, we design the CRR-based LVRT strategy with look-up tables to facilitate dispatching the transient current instructions to OWTs’ controllers. Finally, extensive case studies based on PSCAD are performed and the results show that the proposed strategy can improve active current and reactive current simultaneously over the conventional strategy. In the scenario of the rated wind speed (11.4 m/s) and critical voltage (0.2 p.u.), this achievement can be clearly seen by the value improvement in the active current and reactive current by 0.328 p.u. and 0.05 p.u., respectively.

Contemporary materials for semiconductor switches in an isolated bidirectional DC-DC converter using a closed loop system with linear quadratic regulator control

This research seeks to create a novel bidirectional DC-DC converter capable of high step-up/down power transfer for diverse applications, such as grid systems, energy backup systems, fuel cell vehicles and hybrid electric vehicles. The converter features an inductor linked to an electric power grid to facilitate multi-layer energy conversion with galvanic isolation for improved voltage ratio and galvanic isolation. Notable among its features is this converter's capacity to recycle energy from a leaky inductor without needing additional snubber mechanisms or clamped circuits. To facilitate efficient operation, the converter employs power switches capable of soft switching both step-up and step-down modes. This option permits using MOSFETs with low on-state resistance, which is advantageous due to reduced voltage endurance at the primary side. This research explores the application of a linear quadratic regulator (LQR) for use with a constant frequency bidirectional DC-DC converter. Leveraging the high-order nature of converters, LQR controllers make full use of their potential by offering rapid dynamic response time, reliability and minimal changes in settling time for an array of operating conditions. Thorough analyses were carried out into the availability and stability conditions of an LQR controller in step-up mode using MATLAB/Simulink simulation results as evidence. This study also emphasizes the significance of performance and effectiveness in electronic power-switching devices, which rely heavily on material properties for operation. While silicon has traditionally been employed for these switches, advances in material science have seen wider band-gap semiconductors adopted to improve switching device performance. Therefore, this research encompasses selecting and implementing appropriate semiconductor materials into power electronic switches used by DC-DC converters.

Boost power factor correction converter with adaptive harmonic compensation control

AbstractIn low‐voltage distribution systems, the increased penetration of power electronic devices leads to the decentralization of harmonic sources, which puts the sensitive loads or communication systems in the risk status. The conventional methods of point‐to‐point harmonic control are not applicable. In order to solve this problem without increasing additional investments, this paper focuses on mitigating the harmonic interference through the power factor correction (PFC) converter. Hence, a critical conduction mode (CRM) single‐phase boost PFC converter with adaptive harmonic compensation (AHC) control is proposed to reduce the harmonic interference at user side. Firstly, the feed‐forward control loop of a PFC converter is decoupled into the fundamental branch and harmonic branch to trace the harmonic interference and generate the compensation current. Then the compensation gain is designed by the compensation loop gain and compensation capacity of PFC converter. Meanwhile, the characteristics of the PFC converter are analyzed to study the effect of the AHC control on the active power transmission and output load, which can provide important support for the proposed method in application. Finally, a 160‐W experimental prototype is built to verify the effectiveness of the proposed AHC control.

Optimization of a Virtual Synchronous Control Parameter for a Wind Turbine Generator Considering the Physical Constraint Boundary of Primary Frequency Regulation

The wind turbine generator participates in the primary frequency regulation of the power system by releasing kinetic energy from the rotor. It is necessary to ensure that the rotor speed and converter capacity are within the safe range during the frequency regulation process; otherwise, it will have serious negative effects on the frequency stability of the power system. As an important primary frequency regulation parameter, the dead zone affects the evaluation of the frequency regulation ability of WTG. Therefore, the influence of the dead zone should also be further considered. In order to evaluate the frequency regulation capability of wind turbine generators more comprehensively and accurately, this paper proposes an optimized method for the parameter of virtual synchronous control for wind turbine generators by considering the dead zone and physical constraint boundary of primary frequency regulation. After establishing the time domain expression by considering the frequency regulation dead zone, the real-time frequency regulation capacity of the wind turbine generator is quantified by considering the speed limit of the rotor and the capacity limit of the converter. Furthermore, the optimal value of the frequency regulation coefficient can be derived. Simulation results show that the proposed method can effectively reduce the frequency deviation and frequency change rate of the power system, which can also keep the response within the physical constraint boundary. Consequently, the proposed method can fully utilize the ability for frequency regulation of the wind power generation system and effectively improve the frequency stability of the power system.

Study on the characteristic of the grounding fault on the cascaded midpoint side of the hybrid cascaded HVDC system

The hybrid cascaded high-voltage direct current (HVDC) system combines the system support capabilities of the modular multilevel converter (MMC) with the capacity of the line-mutated converter’s (LCC’s) advantage of high-power transmission. The HVDC system is among the key elements of a smart grid where artificial intelligence is applied extensively. However, the characteristics of a grounding fault on the cascaded midpoint side of a hybrid cascaded HVDC system remain unclear. This study analyzes fault characteristics and the impact of faults using analytical methods. First, the topology and basic control strategy are presented. The fault response process is then analyzed by dividing systems into the MMC and LCC parts at the inverter side. A separate theoretical analysis is also conducted. In addition, the impacts of faults on HVDC and alternating current (AC) networks are analyzed. Therefore, even after the HVDC system is disabled, the AC network can supply fault currents using an antiparallel diode. The simulation results show that the proposed analysis method is feasible, and the theoretical analysis is correct. The proposed method can provide a theoretical basis for the selection of equipment for HVDC systems and smart grid construction.