Converter Capacity Research Articles

Rapid Power Compensation-Based Frequency Response Strategy for Low-Inertia Power Systems

To reduce the frequency deviation and the rate of change of frequency (RoCoF) in a low-inertia power system, some converters are required to provide the frequency response (FR) power normally associated with the frequency deviation and/or the RoCoF, by droop/inertia/PD control. In this article, a rapid power compensation (RPC)-based FR strategy is developed to optimize the ability to compensate grid imbalance power, by fully exploiting the converter idle capacity. To this end, first, mathematical proof demonstrated the improved performance of the RPC strategy in terms of frequency deviation suppression versus droop control, and in terms of RoCoF suppression versus inertia control, with identical converter capacity limit. Moreover, it is proven that the RPC strategy can achieve consistent FR performance with respect to the optimal PD control, i.e., it can maximize the suppression of frequency deviation and RoCoF simultaneously, yet avoiding the limitations due to unknown grid parameters. Finally, by analyzing the operation modes and identifying the pertinent switching logic, the detailed implementation of the proposed RPC strategy is developed. Its superb FR performance is verified by the experiment results in a two-converter low-inertia system, and simulation results in an IEEE four-machine two-area system.

Real-Time LCC-HVDC Maximum Emergency Power Capacity Estimation Based on Local PMUs

The maximum active power that a Line-Commutated-Converter High Voltage Direct Current (LCC-HVDC) can provide instantly, called LCC-HVDC-MC Maximum Emergency Power Capacity, is an important information for determining the emergency control strategy of a power system subject to a large disturbance. However, it is a challenging task to calculate this capacity accurately due to system model uncertainties and contingencies. To address this problem, we propose to estimate this capacity by real-time tracking the Thevenin equivalent (TE) parameters of the AC system based on local PMUs. This new TE estimation method can handle the large disturbances and yield an accurate HVDC-MC while considering various constraints. Specifically, we first investigate the impacts of the TE potential changes on the TE impedance estimation under a large disturbance. To reduce the estimation error caused by the TE potential change, we develop an adaptive current screening process of the current measurements. Moreover, we use the total least square estimation method to further filter out the PMU errors. Finally, we calculate LCC-HVDC maximum emergency power capacity while accounting for the limitations of the control performance, voltage dependent current order limit, the converter capacity, and the AC voltage. The simulations show that the proposed method can accurately track the TE parameters and the real-time LCC-HVDC maximum emergency power capacity following a large disturbance.

A Selective Power Droop Control for Hybrid Interlinking Converter in AC/LVDC Microgrid

This article proposed a selective power droop control for a hybrid interlinking converter (HIC) in connecting ac grid with low-voltage DC (LVDC) grid. The direct connection to the LVDC means relatively lower converter capacity, lower switching loss, higher efficiency, and lower cost in HIC. However, the small capacity converter and large capacitive coupling impedance of HIC can lead to a relatively lower reactive power compensation range. Therefore, the practical overcapacity situation may be more likely to occur in HIC, which deteriorates the system performance. To address this issue, the unique P-δ and Q-P-V droops are proposed for HIC with automatic selective factors adjustment control. With the proposed control method, the HIC can effectively regulate active/reactive powers and improve power quality. To demonstrate the validity of the proposed structure and its control method, a 5-kW HIC is built, and results are presented for poor power quality situations. The results convincingly indicate that the proposed HIC is efficient and capable of low capacity advantage, multifunction control strategy, and overcapacity operation.

Дизель-генераторная электростанция с вентильным генератором по схеме машины двойного питания

The article considers the problem of improving power efficiency of the diesel generator power plants as one of the priority objectives for development of low power generation in Russia. Improving energy efficiency for autonomous facilities is directly related to the optimization of hydrocarbon fuel consumption by marine diesel engines. It is possible to optimize the fuel consumption of internal combustion engines by creating variable-speed diesel generators (VSDG), which are diesel-generator units with semiconductor converters, i.e. systems with valved generators. Optimization of specific fuel consumption by VSDG is provided by the forced regulation of the rotational speed of the shaft of the internal combustion engine in accordance with its multi-parameter characteristic in the shared load conditions of the power plant. The synchronous electric machine is used as a generator as part of a ship power station. Its use in the classic constant-speed diesel generators is more preferable than an asynchronous electric machine. In the development of VSDG - valved power plant - the use of an electric machine with a phase rotor as a generator is technically justified, because the installed capacity of the frequency converter in the rotor circuit of an asynchronous generator with a phase rotor is determined by the sliding power and, with a limited range of speed control, SDGV significantly reduces the installed capacity of the electrical equipment. Such power topology of a ship’s power plant is called VSDG based on a dual-power machine. There has been proposed to consider a variant of a ship power plant based on VSDG with a dual-power machine. Its functional diagram and mathematical model are developed. A structural diagram is presented and dynamic modes of the amplitude and frequency of the generated voltage are modeled during electrical load switching.

A novel model of the lower active power limit (LAPL) of DFIG under overspeed mode

The downward dispatch to the doubly-fed induction generators (DFIGs) may prevent the synchronous generators (SGs) from frequent shut-down during light loading. There are papers using power reserve of the DFIG under the overspeed mode for power dispatch or frequency control, and other papers quantify the upper active power limit of the DFIG, but none quantifies the lower active power limit (LAPL) of the DFIG.In this paper, it is newly found that the capacity of the rotor-side converter (RSC) limits the LAPL of the DFIG under the overspeed mode. By including this constraint, the analytical solution to the LAPL of the DFIG is newly proposed. A sensitivity model is newly proposed to quantify the dependency of the LAPL on the DFIG’s parameters. The ratios of the reserved power and the reserved kinetic energy with respect to the maximum output of the DFIG, i.e. RRP, RRE, and the ratios of the reserved power and the reserved kinetic energy with respect to the rated capacity of the RSC, i.e. RRPC and RREC, are newly defined to evaluate the benefit of the LAPL of the DFIG. The LAPL of the DFIG is newly included in the optimal power flow (OPF) to quantify its benefit to the SGs and the power systems.

A new hybrid decision-making framework to rank power supply systems for government organizations: A real case study

This paper aims to select the best hybrid renewable energy system (HRES) using a new multi-criteria decision-making model for supplying the power consumption of a government institution. The model considers social, environmental, technical, and economic criteria for providing a clear overview of the most suitable HRES design configurations. The proposed model has been implemented in two scenarios related to economic conditions. In this model, Analytic Hierarchy Process, Best-Worst, and Full Consistency methods are used for the weighting of criteria and Evaluation Based on Distance from Average Solution methods and Technique for Order of Preference by Similarity to Ideal Solution are used for selecting the best alternative. The results of the first scenario show that the crucial criteria were technical and social issues. Finally, alternative 4079 was the best alternative that included 450 kW converter capacity, 300 kW diesel generator, 550 kW photovoltaic systems, 800 kW battery storage, and 500 kW wind turbines and indolence of the main grid. In the second scenario, economic criteria were the fundamental criteria and the 4865th alternative was the best alternative that included the main grid, 130 kW converter capacity, 40 kW PV systems, 40 kW battery storage, and 40 kW wind turbines.

Improving the Reliability of Semiconductor Converters

This paper considers the opportunities for improving the reliability of power semiconductor converters by increasing the installed capacity margin and modifying the configuration of power circuit diagrams. As shown by regression analysis, the unit cost of an autonomous voltage inverter with an increase in the installed capacity of the frequency converter rises by about 25%. It is shown that the machine and the semiconductor converter are used most efficiently in six phases, whereas the number of valves in the semiconductor converter is 4m, where m is the number of phases. A further increase in the number of phases does not significantly reduce the phase current, but significantly increases the number of keys in the valve converter. It is shown that an increase in the phase of power circuit layouts allows using the electric machine more fully in terms of capacity and reduce the phase current in semiconductor converters by more than twofold. It is established that the new solutions allow reducing by almost twofold the number of failures in electric drives of more than 500 kW in capacity, while the costs of power electric equipment increase by no more than 25%.

Engineering practices for the integration of large-scale renewable energy VSC-HVDC systems

With the continuous development of power electronic devices, intelligent control systems, and other technologies, the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current (HVDC) technology will continue to increase, while the system losses and costs will gradually decrease. Therefore, it can be foreseen that VSC-HVDC transmission technology will be more widely applied in future large-scale renewable energy development projects. Adopting VSC-HVDC transmission technology can be used to overcome issues encountered by large-scale renewable energy transmission and integration projects, such as a weak local power grid, lack of support for synchronous power supply, and insufficient accommodation capacity. However, this solution also faces many technical challenges because of the differences between renewable energy and traditional synchronous power generation systems. Based on actual engineering practices that are used worldwide, this article analyzes the technical challenges encountered by integrating large-scale renewable energy systems that adopt the use of VSC-HVDC technology, while aiming to provide support for future research and engineering projects related to VSC-HVDC-based large-scale renewable energy integration projects.

The corrosion protection afforded by a commercial rust converter doped with graphene oxide

The present work is devoted to study the effect of the Graphene Oxide (GO) addition in the corrosion protection capacity of a commercial Rust Converter (RC). These substances are designed to be applied over corroded surfaces. It is supposed that they react with rust generating a stable layer, which protects to the metallic substrate. The RC used here is a commercial product that contains gallic acid as reactive compound, which reduces the GO added to reduced-GO (r-GO).Electrochemical Impedance Spectroscopy (EIS) was used to characterize the barrier properties of RC and RC + r-GO treatments, both as in free-standing films and in supported coatings on rusted steel. The results suggest that the RC + r-GO treatment generates more stable and protective films. For the interpretation of the impedance diagrams, the semiconducting character of the film obtained from the RC + r-GO treatment was considered. The initial lower impedance values observed in these samples are related to the presence of electronic paths in parallel with the ionic paths. The impedance values remain stable during long-term immersion. On the contrary, samples coated with the RC treatment undergo a continuous impedance decrease indicating a loss of the barrier properties of the coating.

Evaluating the likely temporal variation in electric vehicle charging demand at popular amenities using smartphone locational data

‘Destination charging’ – in which drivers charge their battery electric vehicles (EVs) while parked at amenities such as supermarkets, shopping centres, gyms and cinemas – has the potential to accelerate EV uptake. This study presents a Monte Carlo-based method for the characterisation of EV destination charging at these locations based on smartphone users' anonymised positional data captured in the Google Maps Popular Times feature. Unlike the use of household and travel surveys, from which most academic works on the subject are based, these data represent individuals' actual movements rather than how they might recall or divulge them. Through a fleet EV charging approach proposed in this study, likely electrical demand profiles for EV destination charging at different amenities are presented. Use of the method is presented first for a generic characterisation of EV charging in the car parks of gyms, based on a sample of over 2000 gyms in around major UK cities, and second for a specific characterisation of hypothetical EV charging infrastructure installed at a large UK shopping centre to investigate the impact of varying the grid and converter capacity on the expected charging demand and level of service provision to the vehicles charging there.

Accurate active power flow diagrams of Brushless doubly-fed induction machine in motoring and generating modes

The brushless doubly-fed induction machine (BDFIM) shows interesting features for incorporating as wind generator or variable speed drive. The BDFIM operation is based on magnetic flow modulation of two stator winding fields through a special squirrel cage rotor. Hence, the power flow is very different from that of a regular induction machine. Taking into account, accurate analysis of the power flow is very valuable for the comprehension and design of BDFIM as well as the selection of converter capacity. In this paper, the power balance for BDFIM is discussed in the motoring and the generating modes and for both super- and sub-synchronous speed intervals. For this purpose, the power flow equations are extracted and power flow diagrams taking into account all the power loss components are presented. The accuracy of the proposed power flow diagrams is verified by comparing the obtained results with the experimental and finite element ones.

Design of Renewable and System-Beneficial District Heating Systems Using a Dynamic Emission Factor for Grid-Sourced Electricity

In future energy scenarios with a high share of renewable energies within the electricity system, power-to-heat technologies could play a crucial role for achieving the climate goals in the heating sector. District heating systems can integrate volatile wind and photovoltaic energy sources and resolve congestions within the electricity grid, leading to curtailment of renewable electricity generation. This paper presents a design approach for setting up system-beneficial power-to-heat-based district energy systems. Within the scope of the project QUARREE100 an existing district in the provincial town Heide in Northern Germany is examined. A linear investment and unit commitment optimization model is applied. By considering local dynamic emission factors for grid-sourced electricity, which contain information on local wind energy curtailment as well as the emission intensity of the overall electricity generation, a renewable and system-beneficial design can be derived. With this method, the minimal rated power and capacity of energy converter and storage units can be determined to achieve emission reductions with respect to minimum costs. The approach of using different methods for the consideration of the emissions of grid-sourced electricity is analyzed based on different scenarios. By using a dynamic emission factor for grid-sourced electricity, lower emissions with fewer costs can be achieved. It is shown that a dynamic assessment leads to different design decisions and far-reaching deviations in the unit commitment. The results clearly show that a constant emission factor is no longer an option for grid-sourced electricity in urban energy system models.

Increasing the information capacity of a fiber-optic multi-sensor converter of binary mechanical signals into electrical signals

The design and operation principle of a multi-sensor Converter of binary mechanical signals into electrical signals based on a partitioned fiber-optic digital-to-analog Converter with a parallel structure is considered. The digital-to-analog Converter is made from a set of simple and technological (three to five digit) fiber-optic digital-to-analog sections. The advantages of the optical scheme of the proposed. Converter in terms of metrological and energy characteristics in comparison with single multi-bit converters are justified. It is shown that by increasing the number of digital-analog sections, it is possible to repeatedly increase the information capacity of a multi-sensor Converter without tightening the requirements for its manufacturing technology and element base. A mathematical model of the proposed Converter is developed that reflects the features of its operation in the mode of sequential time conversion of the input code vectors of individual fiber-optic sections into electrical analogues and the formation of the resulting output code vector.

Optimal Energy Management and Techno-economic Analysis in Microgrid with Hybrid Renewable Energy Sources

Microgrids with hybrid renewable energy sources are increasing and it is a promising solution to electrify remote areas where distribution network expansion is not feasible or not economical. Standalone microgrids with environment-friendly hybrid energy sources is a cost-effective solution that ensures system reliability and energy security. This paper determines the optimal capacity, energy dispatching and techno-economic benefits of standalone microgrid in remote area in Tamilnadu, India. Microgrids with hybrid energy sources comprising photovoltaic (PV), wind turbine (WT), battery energy storage system (BESS) and diesel generator (DG) are considered in this paper. Various case studies are implemented with hybrid energy sources and for each case study a comparative analysis of techno-economic benefits is demonstrated. Eight different configurations of hybrid energy sources are modeled with renewable fractions of 50%, 60%, 65%, and 100%, respectively. The optimization analysis is carried out using Hybrid Optimization Model for Electric Renewable (HOMER) software. Impact of demand response is also demonstrated on energy dispatching and techno-economic benefits. Simulation results are obtained for the optimal capacity of PV, WT, DG, converter, and BESS, charging/discharging pattern, state of charge (SOC), net present cost (NPC), cost of energy (COE), initial cost, operation cost, fuel cost, greenhouse gas emission penalty and payback period considering seasonal load variation. It is observed that PV+BESS is the most economical configuration. COE in standalone microgrid is higher than the conventional grid price. The results show that CO 2 emissions in hybrid PV+WT+DG+BESS are reduced by about 68% compared with the traditional isolated distribution system with DG.

Stationary Energy Storage System for Fast EV Charging Stations: Simultaneous Sizing of Battery and Converter

Optimal sizing of stationary energy storage systems (ESS) is required to reduce the peak load and increase the profit of fast charging stations. Sequential sizing of battery and converter or fixed-size converters are considered in most of the existing studies. However, sequential sizing or fixed-converter sizes may result in under or oversizing of ESS and thus fail to achieve the set targets, such as peak shaving and cost reduction. In order to address these issues, simultaneous sizing of battery and converter is proposed in this study. The proposed method has the ability to avoid the under or oversizing of ESS by considering the converter capacity and battery size as two independence decision variables. A mathematical problem is formulated by considering the stochastic return time of electrical vehicles (EVs), worst-case state of charge at return time, number of registered EVs, charging level of EVs, and other related parameters. The annualized cost of ESS is computed by considering the lifetime of ESS equipment and annual interest rates. The performance of the proposed method is compared with the existing sizing methods for ESS in fast-charging stations. In addition, sensitivity analysis is carried out to analyze the impact of different parameters on the size of the battery and the converter. Simulation results have proved that the proposed method is outperforming the existing sizing methods in terms of the total annual cost of the charging station and the amount of power buying during peak load intervals.

The Role of Supercapacitors in Regenerative Braking Systems

A supercapacitor module was used as the energy storage system in a regenerative braking test rig to explore the opportunities and challenges of implementing supercapacitors for regenerative braking in an electric drivetrain. Supercapacitors are considered due to their excellent power density and cycling characteristics; however, the performance under regenerative braking conditions has not been well explored. Initially the characteristics of the supercapacitor module were tested, it is well known that the capacitance of a supercapacitor is highly dependent on the charge/discharge rate with a drop of up to 9% found here between the rated capacitance and the calculated value at a 100 A charge rate. It was found that the drop in capacitance was significantly reduced when a variable charge rate, representative of a regenerative braking test, was applied. It was also found that although supercapacitors have high power absorbing characteristics, the state-of-charge significantly impacts on the charging current and the power absorbing capacity of a supercapacitor-based regenerative braking system. This owed primarily to the current carrying capacity of the power electronic converters required to control the charge and discharge of the supercapacitor module and was found to be a fundamental limitation to the utilisation of supercapacitors in a regenerative braking system. In the worst cases this was found to impact upon the ability of the motor to apply the desired braking torque. Over the course of the tests carried out the overall efficiency was found to be up to 68%; however, the main source of loss was the motor. It was found that measurement of the state-of-charge using the rated capacitance significantly over-estimates the efficiency of the system.

Novel Equivalent Circuit Model and Theoretical Analysis of Doubly Fed Machine

Doubly fed induction machine (DFIM) has been widely used in variable speed constant frequency electricity generation systems due to its small power converter capacity. Then another machine—brushless doubly fed machine (BDFM)—is widely concerned because of its similar characteristics but better reliability than DFIM. In fact, according to common features of DFIM and BDFM, these two machines can be collectively called doubly fed machine (DFM). Theoretical study on characteristics of these two machines has not formed concise, practical, and all-sided theoretical analysis system. Proceeding from basic theory of DFIM, this paper derived the physical model and established a novel synchronous–asynchronous series equivalent circuit model of DFIM at first. Based on the analysis method of DFIM, a novel equivalent circuit model of BDFM was established. These two models illustrated that the characteristic of DFM is the superposition of synchronous characteristic and asynchronous characteristic. Based on these circuit models, the characteristic under different operation mode was analyzed and corresponding analytical curves were drawn, which revealed the operation mechanism of DFM clearly. To verify the theoretical analysis of two novel equivalent circuit models, simulation models of DFIM and BDFM were established in Simulink and finite element analysis (FEA) software. Corresponding characteristic curves were drawn to compare with that obtained by circuit models. At last, combined with these characteristic curves, practical significance that circuit models and theoretical analysis bring to machine design and control theory was analyzed.

Research on capacity optimization configuration of AC/DC hybrid microgrid interconnect converter

Interconnected converters are an important part of AC/DC hybrid microgrids. This paper fully considers various factors affecting the capacity configuration of interconnected converters, and proposes the principle of optimal capacity allocation to meet the power exchange requirements between AC and DC hybrid microgrids and meet the requirements of reliable power supply for important loads. According to the operation requirements of AC/DC hybrid microgrid under different scenarios, two optimal capacity allocation methods for power supply continuity and economy are given. Finally, the AC/DC hybrid microgrid project with island operation is taken as an example to verify the effectiveness of the proposed method, and the influence of important parameters is discussed in depth. The optimal configuration of interconnected converter capacity is given.

Reliability evaluation of integrated energy system considering the dynamic behaviour of loads and operation strategy

Abstract The dynamic behaviour of loads and operation strategy both have a great impact on the reliability evaluation of the integrated energy system (IES). This paper proposes a novel reliability evaluation method with the comprehensive consideration of the dynamic behavior and operation strategy. Firstly, the IES model based on the energy hub is presented. The dynamic behaviour is modeled in the form of differential equations, and the operation strategy is set by the difference between exergy. Then, the reliability evaluation method based on the performance of loads is proposed. The matrix of converter capacity and support capacity are defined to reflect the assistance process of multiple operation. The Markov chain Monte Carlo method is employed to calculate the reliability indexes in different scenarios. Finally, case studies are carried out on a practical user-level IES, and the results indicate that the multiple operation of subsystems in the IES can effectively increase the reliability.

Multi-Timescale Active and Reactive Power-Coordinated Control of Large-Scale Wind Integrated Power System for Severe Wind Speed Fluctuation

Wind speed fluctuations are extremely easy to cause the voltage change frequently. The existing voltage control methods are lacking in adjustment range and response speed. When wind speed fluctuates substantially or rapidly, voltage exceeding limit and the resulting grid safety problems cannot be avoided. Therefore, reasonably distributing and utilizing the reactive power of variable speed wind turbines in the whole network are an inevitable choice. However, the controllable reactive capacity of wind turbines is limited by converter capacity and active power, and thus, the grid demand may not be met. A new idea of active and reactive power-coordinated control for preventing voltage exceeding limit due to wind speed fluctuations was proposed. The effects of wind speed fluctuations on voltage were analyzed, and the controllable reactive capacity of variable speed wind turbines under wind speed fluctuations was studied. The relationship between voltage and the active and reactive power of wind farms was deduced. The theory and method of active and reactive power coordinated control before wind speed fluctuations were proposed based on the model predictive control theory. The control model based on multi-timescale was established. Finally, it was verified that the method can solve the voltage problems and maximize the system economy.