Constant Power Applications Research Articles

Analytical modelling and sizing of supercapacitors for spacecraft hybrid energy storage systems

The vast majority of Earth-orbiting satellites carry an electrical power subsystem (EPS) which main components are solar panels and secondary batteries. During eclipse periods, satellites are powered only by rechargeable batteries which have a large energy density but a limited power density. This fact limits the power capabilities of small satellites during eclipse periods. Due to the large power density of Supercapacitors (SCs), ground and on-orbit tests have been conducted to verify their applicability on satellite EPSs. At the moment, no studies are underway on the issue of sizing SCs for eclipse operations. Hybrid configuration could reduce the mass and volume of EPS or maintain those reference values but increasing peak power capabilities. This paper deals with this issue. On the one hand, novel analytical expressions of a variable capacitance SCs are derived, including time–voltage dependence in constant power applications. Also an equivalent formulation for a SCs bank (SCsB) is derived. On the other hand, a SCsB sizing procedure is presented, considering the energy and power requirements of a particular space mission as an input. Finally, a simple mission is analysed, the results showing an improvement on the hybrid EPS design with respect to the traditional, being the mass reduction of 36%.

A study on the selection of electrode materials for hepatic radiofrequency ablation: A numerical approach

Radiofrequency Ablation (RFA) is one of the most extensively practiced and studied tumor ablation therapy to date. This treatment therapy is most commonly used to treat chronic pain, liver tumors, arthritis of the spine, etc. Previous studies suggest that material properties of the electrode used during RFA tend to gain comparatively larger temperature zones leading to better thermal lesions. Material properties of electrodes, especially thermal conductivity and electrical conductivity, play an essential role in achieving a larger ablation zone. In this regard, the present study has analyzed the effect of thermal and electrical conductivity of different electrode materials such as Ni-Ti, Pt-Ir, Au, and Cu used in RFA. A FEM-based numerical approach has been accounted for understanding the behavior of electrode materials and the temperature profile attained post ablation of 10 mins. The results show that with a constant power application for 10 mins of RFA, there is no significant difference in ablation volume and temperature zones for different electrode materials. Moreover, this study suggests that higher thermal and electrical conductivity of electrode materials doesn’t have a significant effect on the thermal lesions. Though it was observed that using Au and Cu electrodes resulted in a larger thermal lesion as compared to Ni-Ti and Pt-Ir, the current study suggests minor benefits in terms of improving the efficacy of the treatment process. The observed results would be highly beneficial for medical practitioners to decide when choosing the electrode for the RFA treatment process.

Thermal model of supercapacitors operating in constant power applications: New mathematical expressions for precise calculation of temperature change

Supercapacitors (SCs) are key enablers that can bridge the gap between batteries and conventional capacitors. Certainly, their different merits in high power density, rapid rates of charging/discharging, long life cycles, their ability to handle high levels of charge and discharge currents, and the small values of their internal parameters have drawn attention in modern power systems and energy applications. Unfortunately, although SCs have simple equivalent circuits, the analyses, testing, and mathematical description of these circuits have not been completely resolved. This paper deals with the presentation of new accurate mathematical formulations that allow the calculation of the temperature change of SCs engaged in constant power-based applications precisely in charging and discharging. To achieve this goal, firstly, a comparison is made between well-known mathematical expressions in literature to describe all electrical variables (current, internal voltage, external voltage, power loss, etc.). Secondly, since the discharge process can be accurately represented by a transcendent equation with two solutions, a fast iterative method for solving this equation is proposed and tested. Thirdly, a new mathematical expression is derived for the temperature change of SCs during both the charge and discharge processes without assuming or neglecting SC parameters. Further, a comparison of the proposed expressions with literature-known solutions, as well as numerical results, is presented and discussed. Fourthly, the results of SC temperature change during both charging and discharging are also compared with changing parameters and operating conditions. The findings and analyses confirm that the derived mathematical formulations, proposed in this work, are precise and very close to the numerical results that can facilitate the integrity of the investigation and control of SCs contributing to the work in constant power-based applications.

Experimental study of water freezing process improvement using ultrasound

The phase change process of freezing water is an important application in several fields such as ice making, food freezing technologies, pharmaceuticals, etc. Due to the widespread usage of ice-related products, process improvements in this technology can potentially lead to substantial energy savings. It is well known that supercooling has a negative effect on the overall time and energy consumption of the freezing process. Therefore, ultrasound is proposed as a technique to improve the freezing process by eliminating the supercooling effect and the resulting energy savings is investigated. An experimental study was conducted to analyze the energy expenditures in the freezing process with and without the application of ultrasound. After a set of preliminary experiments, an intermittent application of ultrasound at 3.52 W & 8.25 W power levels was found to be more effective than constant-power application. The supercooling phenomenon was thoroughly studied through iterative experiments. It was also found that the application of ultrasound during the freezing process led to the formation of shard-like ice crystals. From the intermittent ultrasound experiments performed at 3.52 W & 8.25 W power levels, energy savings relative to no-ultrasound processes of 12.4% and 10.8% were observed, respectively.

A multipurpose power supply for constant current, constant voltage, and constant power applications.

Constant current (CC), constant voltage (CV), and constant power (CP) find numerous applications in different stages of the power supply chain and are often a part of various industrial and commercial needs. Although several conventional power supplies are available in the market to provide CC, CV, and CP, their outputs are specific and limited to one or two of these power modes. In light of this, this paper introduces a multipurpose power supply that can deliver power in all three modes (CC, CV, and CP) irrespective of load dynamics. A novel branch type resonant converter comprising three inductors and a capacitor is investigated for the designand development of the power supply. Consumers can choose their preferred mode of operation by switching the operating frequency. This multipurpose power supply subsidizes the space, technology, time, and economy required for three individual supplies and bags magnificent features such as simple control with efficiency greater than 95% in the CC mode and above 80% in CV and CP modes, subjected to loading constraints. The versatility of the power supply is validated by a hardware prototype tested at different load conditions. Regardless of the changes in load resistance, the proposed power supply remained rigid and efficient in each of the mentioned modes.

Supercapacitors in Constant-Power Applications: Mathematical Analysis for the Calculation of Temperature

A set of analytical equations for the calculation of the temperature in supercapacitors operating in constant-power applications is presented in this paper. Although the main operation modes of supercapacitors are constant-current and constant-power charge and discharge, this study was focused on the latter, since both sources and loads act as constant-power systems in a wide range of power conversion facilities. The starting point of this study is the classical supercapacitor model based on electrical and thermal parameters provided by manufacturers or also obtained by experimental means. The proposed mathematical analysis is based on the so-called incomplete gamma function that presents two major advantages over previously existing methods. Firstly, it is not necessary to solve any differential equations system by means of numerical methods, which reduces the required computational effort. Secondly, no simplifications to relief the calculations are made in the computation of any variable. The new formulation renders valid solutions even for high-power demand situations. Moreover, the temperature of the supercapacitor can be expressed as a function of time or any other electrical variable in the charging and discharging processes. Therefore, the proposed formulas are especially remarkable for the electrical and thermal dimensioning of supercapacitors.

Towards accurate calculation of supercapacitor electrical variables in constant power applications using new analytical closed-form expressions

Supercapacitor (SC) is one of the most trending energy storage solutions. The SCs equivalent circuit models have been extensively applied to energy management because of their accuracy and simplicity. However, there is a difficulty in solving the conventional differential equation-based model used to characterize the electrical behavior of SCs operating in constant power applications. Hence, numerical or metaheuristic techniques have been used in the literature to derive SC's internal voltage at any time. In this work, a thorough mathematical analysis that enables a precise calculation of the electrical variables implied in the charge/discharge processes of SCs operated at constant power as a function of time is presented. First, the transcendental discharge voltage expression of SCs operating at constant power is formulated, and then it was solved using the Special Trans Function theory (STFT). The precision of calculation of the method used for solving the transcendental expression is presented and discussed. Second, the transcendental voltage of charging expression of SCs operating at constant power as a function of time is also formulated, and then it was solved using Lambert W equation. Third, a comparison of different methods for solving mentioned equations is presented. Fourth, the electrical variables involved in the charge/discharge processes of SCs – voltage, current, power, energy, state of charge, and power loss are investigated. Furthermore, the results obtained for the variation of parameters and their operating conditions demonstrate the proposed equations’ applicability and accuracy. Finally, the results obtained validate that the closed-form expressions suggested in this paper are accurate and straightforward, which can contribute to proper modeling, investigation, sizing, regulation, and control of constant power SCs in modern energy systems.

Lambert W function based closed-form expressions of supercapacitor electrical variables in constant power applications

The traditional model used to represent the electrical behavior of supercapacitors (SCs) operating at constant power leads to a well-known differential equation which allows to obtain the charge/discharge time of the device as a function of its internal voltage. However, the opposite is not true, i.e. it is necessary to resort to numerical methods to derive the internal voltage of the SC at any specific time. In this paper, new explicit expressions for the evolution of the electrical variables involved in the charge/discharge process of a SC bank operated at constant power are derived. The proposed formulation, which is based on the use of the Lambert W function, does not only allow a straightforward calculation of all the electrical variables as a function of time, but also sheds light on the direct relations between those variables. In the assumption of validity of the classic model, the results derived in this work can be considered exact, as no further approximations are made. The accuracy of the proposal is demonstrated by comparing the results derived from the new formulation with those yielded from the classical iterative resolution of the differential equations using numerical methods. The new closed-form expressions presented in this paper have the potential to simplify the sizing, regulation and control of power applications with embedded SC banks operated at constant power.

A Three-Phase Asymmetrical Dual-Active-Bridge Converter With Series/Parallel-Reconfigurable Output for Wide Voltage Range Applications

A three-phase asymmetrical dual active bridge (TP-ADAB) dc-dc converter with series/parallel-reconfigurable output (SPRO) is proposed for wide output range and constant peak power applications. The three outputs of the TP-ADAB converter are able to be connected in series or parallel to achieve wide output voltage range. The output voltage range, maximum output voltage/current of each power module, as well as the power rating of devices, are reduced significantly. Moreover, automatic voltage sharing in output series mode and current sharing in output parallel mode is achieved by adopting the three-phase structure. Furthermore, soft-switching of all power switches is achieved in a wide operation range. Operation principles, output characteristics, soft-switching performances, and control strategy of the proposed TP-ADAB converter are analyzed in detail. Experimental results of a 3 kW prototype are provided to verify the effectiveness and feasibility of the proposed converter.

Design and implementation of electric spring for constant power applications

Electric spring has recently been proposed as a special type of reactive power compensator to improve various attributes of the smart grid. Its extended functionality and applications are yet to be explored. This paper presents a new application of electric spring to ensure constant power across the loading elements under impedance variations. Two control methodologies are proposed to carry out this task: a linear approximation configuration and a closed-loop configuration. Design and mathematical analysis are provided, and the results of both control configurations are analyzed for a simulation model as well as a hardware setup. It has been found that experimental results are congruent with those obtained from the simulation model, hence ensuring the efficacy of theoretical framework.

Fuzzy Controlled ZVS Asymmetrical PWM Full-bridge DC-DC Converter for Constant load High Power Applications

This paper proposes a fuzzy logic controlled new topology of high voltage gain zero voltage switching (ZVS) asymmetrical PWM full-bridge DC-DC boost converter for constant load and high power applications. The APWM full-bridge stage provides high voltage gain and soft-switching characteristics increase the efficiency and reduce the switching losses. Fuzzy logic controller (FLC) improves the performance and dynamic characteristics of the proposed converter. A comparison with a classical proportional-integral (PI) controller demonstrates the high performances of the proposed technique in terms of effective output voltage regulation under different operating conditions. Simulation is done by integrating two different simulation platforms <TEX>$PSIM^{(R)}$</TEX> and <TEX>$Matlab^{(R)}/Simulink^{(R)}$</TEX> by using SimCoupler tool of <TEX>$PSIM^{(R)}$</TEX>. Experimental results using 120W load have been provided to validate the results.

A Novel Phase-Shift Dual Full-Bridge Converter With Full Soft-Switching Range and Wide Conversion Range

This paper proposes a novel phase-shift dual full-bridge converter with shared leading leg and dual outputs in series. Full soft-switching range of the active switches can be achieved based on the parallel full-bridge configuration of the converter, and the dual outputs of the proposed converter are connected in series. The output voltage can be regulated by primary-side phase-shift and secondary-side phase–shift dual-mode control scheme, and a wide conversion range can be achieved. Furthermore, the circulating current in the primary-side full-bridge circuits can be extinguished. Therefore, the proposed converter would be useful for constant peak power and wide output voltage range applications. Steady-state operation and relevant analysis results of the proposed converter are presented and verified on a 1.5-kW hardware prototype. The experimental results show that the proposed converter can achieve a peak efficiency of 95.3%.

Open-Delta VSC Based Voltage Controller in Isolated Power Systems

&lt;p&gt;This paper proposes a reduced switch Open-Delta (OD-VSC) voltage controller for an standalone asynchronous generator (SAG), also known as the self-excited induction generator (SEIG),used in constant power applications such as pico hydro uncontrolled turbine driven isolated induction generator (IAG) for feeding three-phase loads. The proposed reduced switch voltage controller is used to regulate and control the generator terminal voltage as it is subjected to voltage drops, dips or flickers when the isolated power system is subjected to various critical loads. Generally this purpose is carried out by a STATCOM comprising of a three-leg six-switch inverter structure. Here, in this work the DSTATCOM is realized using a three-leg four-switch insulated gate bipolar transistor (IGBT)-based current controlled voltage-sourced converter (CC-VSC) and a self-supporting dc bus containing two split capacitors. The proposed generating system along with the controller is modeled and simulated in MATLAB along with Simulink and power system blockset (PSB) toolboxes. The system is simulated and the capability of the isolated generating system along with the reduced switch based voltage controller is presented here where the generator feeds linear and non-linear loads are investigated.&lt;/p&gt;

A Reconfigurable Structure DC–DC Converter With Wide Output Range and Constant Peak Power

This paper presents a reconfigurable structure dc-dc converter with wide output range and constant peak power (RS-WOCP). This converter is an extension of the traditional isolated dc-dc converter with two output channels by adding an active switch and two diodes. Theoretical analysis indicates that the proposed RS-WOCP converter is able to smoothly reconfigure the output channels in parallel or series to realize wide output voltage range. With this approach, the output voltage is shared equally in the series operating mode and so is the output current in the parallel operating mode. Therefore, the power rating of devices is significantly reduced and the output rectifiers and filters can distribute power losses and thermal stresses. Operating principle, control strategy, modeling, and stability analysis are illustrated in this paper. An experimental prototype of a 200-W DSP-based RS-WOCP converter was built. The experimental results have verified the steady-state performance and fast dynamic response in the whole output voltage range. Comparisons with two close relatives of wide output converters show that the proposed RS-WOCP converter is advantageous for wide output range constant peak power applications.

Design of a Microcontroller Based Electronic Load Controller for a Self Excited Induction Generator Supplying Single-Phase Loads

The generation of electric power using self excited induction generation (SEIG) is a viable option in remote and rural areas where grid electricity is not available. The generated voltage and frequency of these machines, however, varies with varying loads. This characteristic can be resolved either by adjusting the values of the excitation capacitance or by controlling the prime mover speed. Further, in a single-point constant power application, where the machines deliver a fixed amount of power, the electronic load controller (ELC) can be used to switch-in or switch-out a dump load whenever the consumer load decreases or increases respectively. This paper presents a detailed analysis and the design of a microcontroller based SEIG -ELC system intended for stand-alone pico hydro power generation. The simulated performance of the controller is supplemented by experimental results.

The control of a free-piston engine generator. Part 2: Engine dynamics and piston motion control

Free-piston engines are under investigation by a number of research groups due to potential fuel efficiency and exhaust emissions advantages over conventional technology. The main challenge with such engines is the control of the piston motion, and this has not yet been fully resolved for all types of free-piston engines. This paper builds on the fundamental investigations presented in the accompanying paper and investigates the dynamics of the engine and the feasibility of classical control approaches. The response of the engine to rapid load changes are investigated using decentralised PID, PDF and disturbance feedforward. It is found that the engine is sensitive to rapid load changes but that in constant power applications standard control techniques provide satisfactory performance. The influence of cycle-to-cycle variations in the combustion process are investigated, but not found to be critical for engine operation.

Five-leg inverter PWM technique for reduced switch count two-motor constant power applications

Industrial applications often require a number of variable speed electric drives. In the majority of cases, these multi-motor drive systems require independent control of individual motors. Over the past decade, research efforts have been made to reduce the number of power electronic devices required in multi-motor drive systems in order to reduce the overall complexity and hence cost of the drive. It has been shown recently that it is possible to independently control two three-phase induction machines using a five-leg voltage source inverter (VSI) as the supply, with one inverter leg being common to both machines. The existing pulse-width modulation (PWM) methods for this supply topology either limit the amount of DC bus voltage available to each machine or lead to uneven and increased switching frequency across five legs of the VSI. A new PWM method that effectively utilises the standard three-phase modulators in conjunction with appropriate modifications to generate modulation signals for all five legs of the VSI is presented. It enables an arbitrary distribution of the available DC bus voltage between the two machines. The considered supply topology, when utilised in conjunction with the developed PWM technique, is well suited to constant power applications such as centre-driven winders. Verification of the developed modulation method and its applicability to winder systems are demonstrated by experimental testing that includes both steady state and transient operation.

An Improved Electronic Load Controller for an Isolated Asynchronous Generator Feeding 3-Phase 4-Wire Loads

This paper deals with a voltage and frequency control of an isolated asynchronous generator feeding 3-phase 4-wire loads in constant power applications where terminal voltage, excitation capacitor, speed and generated power of the generator remain constant under all operating conditions. The proposed controller is functioning as an improved electronic load controller (IELC) to regulate the frequency and as a source of reactive power to regulate the terminal voltage. Three single-phase voltage source converters (VSCs) are connected to each phase of the generator through three single-phase transformers and there is an auxiliary load with a chopper at the self-regulated DC bus of the IELC. These VSCs along with transformers provide their function as a voltage regulator, a harmonic eliminator, a load balancer and a neutral current compensator whereas the chopper and the auxiliary load at the DC bus are used to regulate the frequency. The complete system is modelled and simulated in MATLAB along with Simulink and Power system Block set toolboxes. The simulated results are presented to demonstrate the capability of an isolated generating system driven by uncontrolled pico hydro turbine and feeding 3-phase 4-wire linear/non-linear balanced/unbalanced loads.

Decoupled Voltage and Frequency Controller for Isolated Asynchronous Generators Feeding Three-Phase Four-Wire Loads

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper deals with a decoupled voltage and frequency controller (DVFC) for an isolated asynchronous generator (IAG), also known as the self-excited induction generator (SEIG), used in constant power applications such as pico hydro uncontrolled turbine driven IAG for feeding three-phase four-wire loads. The proposed controller is used to control the voltage and frequency at the generator terminal independently. The proposed decoupled controller is a combination of a STATic synchronous COMpensator (STATCOM) for regulating the voltage and an electronic load controller (ELC) for controlling the power which in turn maintains the system frequency constant. The STATCOM is realized using a 4-leg insulated gate bipolar transistor (IGBT)-based current controlled voltage-sourced converter (CC-VSC) and a self-supporting dc bus, while the ELC consists of a three-phase diode bridge rectifier, a chopper switch and an auxiliary load. The proposed generating system is modeled and simulated in MATLAB along with Simulink and power system blockset (PSB) toolboxes. The simulated results are presented to demonstrate the capability of an isolated generating system for feeding three-phase four-wire loads with the neutral current compensation. </para>

A Novel Solid State Controller for Parallel Operated Isolated Asynchronous Generators in Pico Hydro Systems

This paper deals with a novel solid state controller (NSSC) for parallel operated isolated asynchronous generators (IAG) feeding 3-pbase 4-wire loads in constant power applications, such as uncontrolled pico hydro turbines. AC capacitor banks are used to meet the reactive power requirement of asynchronous generators. The proposed NSSC is realized using a set of IGBTs (Insulated gate bipolar junction transistors) based current controlled 4-leg voltage source converter (CC-VSC) and a DC chopper at its DC bus, which keeps the generated voltage and frequency constant in spite of changes in consumer loads. The complete system is modeled in MATLAB along with simulink and PSB (power system block set) toolboxes. The simulated results are presented to demonstrate the capability of isolated generating system consisting of NSSC and parallel operated asynchronous generators driven by uncontrolled pico hydro turbines and feeding 3-phase 4-wire loads.