Performance investigation of linear and nonlinear controls for a fuel cell/supercapacitor hybrid power plant

In this paper, linear proportional–integral (PI) and nonlinear flatness-based controllers for dc link stabilization for fuel cell/supercapacitor hybrid power plants are compared. For high power applications, 4-phase parallel boost converters are implemented with a switching interleaving technique for a fuel cell (FC) converter, and 4-phase parallel bidirectional converters are implemented with a switching interleaving technique for a supercapacitor converter in the laboratory. As controls, mathematical models (reduced-order models) of the FC converter and the supercapacitor converter are given. The prototype small-scale power plant studied is composed of a PEMFC system (the Nexa Ballard FC power generator: 1.2 kW, 46 A) and a supercapacitor module (100 F, 32 V, based on Maxwell Technologies Company). Simulation (by Matlab/Simulink) and experimental results demonstrate that the nonlinear differential flatness-based control provides improved dc bus stabilization relative to a classical linear PI control method.     

Power and energy management of grid/PEMFC/battery/supercapacitor hybrid power sources for UPS applications

This paper presents a hybrid power and energy source supplied by a proton exchange membrane fuel cell (PEMFC) as the main power source in an uninterruptible power supply (UPS) system. To prevent the PEMFC from fuel starvation and degradation and realize their seamless linking in the hybrid UPS system, the power and energy are balanced by the battery and/or supercapacitor (SC) as two alternative auxiliary power sources. Based on the modeling and sizing of hybrid power and energy components, the power and energy management strategies and efficiency measurements of four operating modes in UPS system are proposed. To evaluate the proposed strategies, an experimental setup is implemented by a data acquisition system, a PEMFC generating system, and a UPS system including AC/DC rectifier, DC/AC inverter, DC/DC converter, AC/DC recharger and its intelligent control unit. Experimental results with the characteristics of a 300 W self-humidified air-breathing of PEMFC, 3-cell 12 V/5 Ah of batteries, and two 16-cell 120 F/2.7 V of SCs in parallel corroborate the excellent management strategies in the four operating modes of UPS system, which provides the basis for the optimal design of the UPS system with hybrid PEMFC/battery/SC power sources.     

Performance of printable supercapacitors in an RF energy harvesting circuit

We report the fabrication of a supercapacitor on a plastic substrate with mass-production-compatible methods and its characterisation using galvanostatic and voltammetric methods. The supercapacitor is prepared in ambient conditions using activated carbon and an aqueous, non-acidic electrolyte. The obtained capacitances are 0.45 F and 0.21 F for device sizes of 4 cm² and 2 cm², respectively. Additionally, we demonstrate the utilisation of the supercapacitor in an autonomous energy harvesting and storage system. The RF energy harvester comprises a printed loop antenna and a half-wave organic diode rectifier operating at 13.56 MHz frequency. The harvested energy is stored in two supercapacitors connected in series to increase the maximum operating voltage. In order to power a device such as a sensor or a small indicator display, voltage regulation is needed. A voltage regulator, implemented as an application specific integrated circuit (ASIC), was designed for this purpose, and fabricated commercially. We demonstrate the ability of the harvester storage unit to power the regulator for hours with a constant regulator output voltage and power. The effect of supercapacitor charging time on the actual supercapacitor charging state is also discussed, as a slower charging rate is found to have a significant effect on the output of the supercapacitor.     

Design and control of LCL filter interfaced grid connected solar photovoltaic (SPV) system using power balance theory

In this paper solar photovoltaic (SPV) system connected to the utility grid is designed and simulated. The utility grid and SPV system are coupled with current controlled voltage source converter (VSC) and LCL filter. The design of LCL filter, MPPT algorithm and power quality improvements are discussed and simulation results are shown for the performance analysis of grid-coupled PV system under different load condition. The system is controlled through power balance theory method. The principle behind the control implementation is to evacuate the solar power generated during the daytime and the reactive power demand for the load should be supplied by the PV. The grid coupled system consists of SPV system, dc–dc boost converter, maximum power point tracking (MPPT), voltage source converter (VSC), LCL filter, different loads and three phase utility grid. This system is capable of eliminating harmonic and load balancing by supplying unbalanced current from the PV as a compensator. The system is simulated with 10 kW SPV array using indirect current control scheme.     

The effect of converter configurations of HVDC links on sub- and super-synchronous disturbances to turbine units

In this paper, the rotor torque disturbances to turbine–generator units arising from harmonic interactions between converters of a HVDC system are studied. It is shown that a distinct-pulse converter configuration will be a better choice for avoiding sub- or super-synchronous resonance. For an asynchronous HVDC link, the risk of electromechanical resonance could be completely got rid of on either side of the link, no matter what directions the powers flow, if an 18-pulse converter were used on the 60 Hz side and a 12-pulse converter on the 50 Hz side. Such a configuration could still perform well even under the large frequency deviations. Furthermore, it shows that power system type plays a significant role on the possible excitation of resonance and on the probably damaging sections of a turbine–generator unit. Good match of the converter configuration and power system type is significant for a HVDC link to exempt from sub- and super-synchronous resonance.     

Improving power quality of wind energy conversion system with unconventional power electronic interface

The increasing interest to utilize wind energy as a power source prompted more researches to be dedicated to the unconventional integration of this power source into the current grid. In this paper, one avenue to achieve this efficient utilization, through the use of integrated wind energy conversion system (WECS) using doubly fed induction generator (DFIG) is presented. Wind grid integration brings the problems of voltage fluctuation and harmonic distortion. This paper presents an Unconventional Power Electronic Interface (UPEI) to reduce the total harmonic distortion (THD) and enhance power quality during disturbances. The models used in the paper includes a pitch-angled controlled wind turbine model, a DFIG model, power system model and an UPEI having controlled converters. A phase to phase fault is simulated on 132 kV bus and the measured results obtained from grid connection of the wind generation system are presented. The results have demonstrated the ability of UPEI to regulate pitch angle, VAR and to reduce THD. The proposed system increases the effectiveness of the utilization of wind energy.     

Switching Power Supplies with Ferrite Inductors in Sustainable Saturation Operation

This paper investigates the Sustainable Saturation Operation (SSO) of Ferrite Core Power Inductors (FCPIs) in Switch Mode Power Supplies (SMPSs). A ferrite inductor is considered in SSO if its current ripple, power losses and temperature rise are acceptable and reliable for both the device and the SMPS, despite the inductance drop determined by the core saturation. An algorithm is discussed, which identifies SSO-compliant FCPIs with minimum size and volume, given the SMPS specifications about the allowed power losses, temperature rise and peak-to-peak current ripple of the inductor. The experimental results relevant to a 465 kHz/3.3 V/1.5 A buck converter show that SSO-compliant inductors allow to increase the SMPS power density, while preserving the overall converter efficiency. Despite the proposed low power application, the findings relevant to the utilization of power inductors in partial saturation have general conceptual valence and similar investigations can be prospectively re-assessed for few kW output power DC/DC converters.     

Grid capacity and efficiency enhancement by operating medium voltage AC cables as DC links with modular multilevel converters

It is anticipated that with the thrust towards use of clean energy resources such as electric vehicles, future distribution grids will face a steep increase in power demand, forcing the utility operators to invest in enhancing the power delivering capacity of the grid infrastructure. It is identified that the critical 5–20 km medium voltage (MV) underground ac distribution cable link, responsible for bulk power delivery to the inner urban city substation, can benefit the most with capacity and efficiency enhancement, if the existing infrastructure is reused and operated under dc. Quantification of the same is offered in this paper by incorporating all influencing factors like voltage regulation, dc voltage rating enhancement, capacitive leakage currents, skin and magnetic proximity effect, thermal proximity effect and load power factor. Results are presented for three different ac and dc system topologies for varying cable lengths and conductor cross-sections. The computed system efficiency is enhanced with use of modular multilevel converters that have lower losses due to lower switching frequency. A justified expectation of 50–60% capacity gains is proved along with a generalized insight on its variations that can be extrapolated for different network parameters and configurations. Conditions for achieving payback time of 5 years or lower due to energy savings are identified, while the socio-economic benefits of avoiding digging and installing new cable infrastructure are highlighted. The technical implications of refurbishing cables designed for ac to operate under dc conditions is discussed in terms of imposed electric fields, thermal profile and lifetime. A novel opportunity of temperature dependent dynamic dc voltage rating to achieve additional capacity and efficiency gains is presented.     

Energy management strategy for fuel cell/battery/ultracapacitor hybrid vehicle based on fuzzy logic

In order to enhance the fuel economy of hybrid vehicle and increase the mileage of continuation of journey, a fuzzy logic control is utilized to design energy management strategies for fuel cell/battery (FC + B) hybrid vehicle and fuel cell/battery/ultra-capacitor (FC + B + UC) hybrid vehicle. The models of hybrid vehicle for FC + B and FC + B + UC structure are developed by electric vehicle simulation software ADVISOR which uses a hybrid backward/forward approach. The results demonstrate that the proposed control strategy can satisfy the power requirement for four standard driving cycles and achieve the power distribution among various power sources. The comprehensive comparisons with the power tracking control strategy which is wide adopted in ADVISOR verify that the proposed control strategy has better rationality and validity in terms of fuel economy and dynamic property in four standard driving cycles. Therefore, the proposed strategy will provide a novel approach for the advanced energy management system of hybrid vehicle.     

H桥级联多电平变流器的直流母线电压平衡控制策略

H桥级联多电平变流器是一个非线性、多变量、强耦合系统,其中单个功率模块的直流母线电压平衡问题严重影响该类型装置的输出性能,并限制其应用范围。对变流器与系统之间的功率交换模型进行详细的数学推导,得出可以通过控制变流器负序电流、零序电压或负序电压的方式达到控制单相直流电压平衡的控制规律。结合全局平均直流电压控制策略和单模块直流电压控制策略,提出基于正负序电流分离解耦控制的通用三级直流母线电压控制方法,解决了H桥级联多电平变流器的直流母线电压平衡问题,并提高了装置的输出性能。仿真和实验验证了所提直流电压控制策略的有效性和可行性。     

A high efficiency input/output magnetically coupled interleaved buck–boost converter with low internal oscillation for fuel-cell applications: Small signal modeling and dynamic analysis

Dynamic behavior of DC–DC converters plays a crucial role in stability of renewable energy exploitation systems. This paper presents small signal modeling of an input/output magnetically coupled interleaved buck–boost converter for fuel-cell applications to help the designers with the better understanding of converter dynamics. Aiming to have a continuous converter transfer function for a smooth transition between the operation modes and an improved inner dynamics, a damping network and an input/output coupling have been added to the interleaved structure of well-known cascaded buck–boost converter. Having the same step-up/step-down voltage transfer ratio, smooth transition and improved inner dynamics make this converter quite suitable for renewable energy applications. The paper presents a small signal ac equivalent circuit model of the proposed converter based on state space averaging (SSA) method. Simulation results show remarkable improvements in converter dynamic behavior in both time and frequency domains. Prototype setup of 360 W and 36 V output voltage for a fuel cell with a brand of “FCgen 1020ACS” Ballard Power Systems, Inc. was implemented. Experimental results are presented to verify the theoretical model and its expected merits.     

Effect of varying concentration and temperature on steady and dynamic parameters of low concentration photovoltaic energy system

Low-concentration photovoltaic (LCPV) system has huge potential for further cost reduction of solar photovoltaic (PV) power as compared to flat panel PV. The dependence of steady state and dynamic parameters on concentration and temperature is crucial to extract maximum power from solar photovoltaic system. This article aims to present the effect of varying concentration and temperature on steady state and dynamic parameters of LCPV system under actual test conditions (ATC). The rate of change in I_SC with solar irradiation i.e., dI_SC/dG is found as 0.25 A/W assuming ≈±1 °C change in module temperature. The effect of temperature on inherent material properties responsible for photo-conversion efficiency is studied using impedance spectroscopy technique. A linear response of series resistance of LCPV module is observed with respect to change in module temperature, i.e. dR_S/dT from 297 to 333 K is in the range of 1.15–1.20 Ω with a rate of 1 mΩ/K. From real-time analysis of LCPV system open-circuit voltage found decreasing from 21 to 20.6 V with temperature coefficient of voltage ≈−0.061 V/K. The dynamic resistance has a positive coefficient of module temperature i.e., dr_d/dT given by 0.49 Ω/K.     

锂电池/超级电容混合动力系统及控制策略

混合动力轻轨车(Hybrid Light Rail Vehicle,HLRV)以锂电池作为主动力源,超级电容作为辅助动力源,具有良好的技术性与经济性。提出锂电池与超级电容分别通过Buck/Boost双向变换器并联于直流母线侧的储能主电路结构,以实现功率的双向调节并提高超级电容器利用率。为了提高系统输出功率以及减小电流纹波,采用三相交错并联结构。针对锂电池响应速度慢的问题,提出了超级电容响应负载变化,锂电池响应超级电容低频分量的间接功率控制策略。仿真结果验证了该系统以及控制策略的正确性。     

Experimental analysis and optimization of key parameters of ZVS mode and its application in the proposed LLC converter designed for distributed power system application

This paper deals with experimental analysis of zero-voltage switching mode targeting high-frequency operation of chosen MOSFET type. After selection of specific type of transistor (IPW60R165CP) the experimental investigation has been made by changing parameters (e.g. dead-time, auxiliary capacitance of MOSFET, transistor current), that are influencing the ZVS commutation process. For these purposes we constructed the universal testing device, which is capable to secure realistic conditions of various types of commutation modes (hard switching, zero-voltage switching, zero-current switching). Afterwards the best settings of commutation mode have been utilized in proposed LLC converter suited for distributed power system application. Prototype is operating in ZVS region with optimized parameters. Switching frequency is from 130 kHz (input voltage 325 Vdc) to 210 kHz (input voltage 415 Vdc) with the output power of 1500 W. It is clear from the results that experimental analysis of the ZVS commutation mode brings expectation of transistor behavior which was totally confirmed also in the case of experimental analysis of LLC resonant converter.     

Control of cyclic fluctuations in an independent microgrid by an SOFC triple combined cycle inertia system

Technology was investigated to control cyclic fluctuations in an independent microgrid powered with unstable renewable energy by use of a solid oxide fuel cell (SOFC, 1 MW) in a triple combined cycle (SOFC-TCC) that included a gas turbine (G/T, 0.8 MW) and a steam turbine (S/T, 0.2 MW). A large-scale solar power system (0.8 MW) and a wind farm (0.8 MW) were interconnected with the electrical power network through an inverter. The cyclic fluctuations ingredient of the network was controlled by a suitably designed inertia system and by governor-free control of the G/T and S/T. The SOFC-TCC’s control block diagram was submitted to MATLAB/Simulink R 2013a, and the deviation of electrical power and frequency in the independent microgrid caused by the SOFC-TCC and renewable energy interconnection was clarified. As a result, a range of suitable inertial constants for G/T and S/T and the electrical output characteristics were determined. Selecting a small inertial constant for the simulation resulted in a large frequency deviation of G/T and S/T, with frequency stabilized for a short time. On the other hand, selecting a large inertial constant resulted in a controlled frequency deviation, although the unstable frequency of the power grid continued for a long time.     

A new optimal unified power flow controller placement and load shedding coordination approach using the Hybrid Imperialist Competitive Algorithm-Pattern Search method for voltage collapse prevention in power system

As power systems become more complex and heavily loaded, voltage collapse has become one of the most destructive events in modern power systems leading to blackouts in electric utilities worldwide. Voltage collapse is mainly caused by operating power systems at lower stability margins due to a surge in electric power demand. This paper presents an optimal unified power flow controller (UPFC) placement and load shedding coordination approach for voltage collapse prevention in N − K (K = 1, 2 and 3) contingency condition using Hybrid Imperialist Competitive Algorithm-Pattern Search (HICA-PS). ICA is the main optimizer of the proposed algorithm while pattern search is applied to further fine tune the results of the ICA. To show the effectiveness of the proposed approach in preventing voltage collapse in complex power systems, we implemented it on the New-England 39 bus power system. Its performance was also compared to that of some classical optimization techniques. Decrease in load shedding amounts, continuity of energy supply and voltage collapse prevention is the main positive features of the proposed approach.     

A performance comparison of a nonlinear and a linear control for grid connected PMSG wind energy conversion system

With the increased penetration of wind energy on modern power systems all over the world, the Wind Farm Systems (WFS) are today required to participate actively in electric network operation by an appropriate generation control strategy. This paper presents a comparative study of two control strategies for wind farm based on Permanent Magnet Synchronous Generator (PMSG) and interconnected to the distribution network. The 4 MW wind farm consists of 2 PMSGs based on 2 MW generators connected to a common DC-bus system. Each PMSG of the WFS is connected to the DC-bus through a rectifier, but the DC-bus is connected to the grid through only one inverter system. The proposed control laws are based on a sliding mode algorithm and classical Proportional Integral (PI) controllers to regulate both generator and grid-side converters. The control strategy combines a pitch control scheme and Maximum Power Point Tracking (MPPT) to maximize the total generated power of WFS. Furthermore, the aim of the control strategy is to maximize the extracted power with the lowest possible impact in the power network voltage and frequency for fault conditions as well as for normal working conditions. Finally, simulation results with Matlab/Simulink environment confirm that the proposed strategy has excellent performance.     

Analysis and comparison of current control methods on bridgeless converter to improve power quality

In this paper, the performance analysis of power factor correction (PFC) current control methods is presented for a bridgeless converter operating under continuous conduction mode (CCM). The bridgeless converter has been proposed using proportional-integral control (PIC), average sliding control (ASC) and predictive current control (PCC) methods to obtain unity power factor (PF) and lower total harmonic distortion (THD) of input current. Several PFC methods have been developed to satisfy the international standards such as IEC 61000-3-2 and IEEE 519-1992. The detailed steady-state theoretical analysis of the bridgeless converter is presented, which is verified by simulations and experiments carried out on 600 W and 50 kHz. The performance of the current control methods for the bridgeless converter is investigated by a Matlab/Simulink program. The experiments performed in the laboratory under input voltage and load variation ranges verify the theoretical and simulation studies. The control methods are programmed by the TMS320F2812 DSP microprocessor.     

A lossless switching technique for smart grid applications

Smart grid is an upgrade of the existing electricity infrastructure in which integration of non conventional energy sources are an integral part. This leads to the introduction of harmonics and increased switching losses in the system. Thus there is a need of loss less switching techniques for smart grid applications. Switched mode power supplies (SMPSs) are being extensively used in most power processes [1]. Developments were carried out centered on hard switched converters, where switching frequency is limited to 10 s of kHz [2]. The uses of soft switching techniques, [3], [4], [5], [6] zero voltage switching (ZVS) or zero current switching (ZCS), is an attempt to substantially reduce the switching losses and hence attain high efficiency at increased switching frequency. The soft-switching topologies belong to families namely resonant load converters [3], resonant switch converters [2], [4], resonant transition converters [5], [6], and most recently active clamped PWM converters [7], [8], [9]. The active clamp topology adds an active clamp network, consisting of a small auxiliary switch in series with a capacitance plus the associated drive circuitry to the traditional hard switch converters. The proposed paper basically deals with the design, modeling and simulation of a ZVS–PWM active clamp/reset forward converter having features like zero switching power losses, constant frequency and PWM operation, Soft-switching for all devices and Low voltage stresses on active devices due to clamping action.     

A new control algorithm of active power line conditioner for improving power quality

In this paper, a new control algorithm for active power line conditioner (APLC) is proposed. The proposed APLC contains two power converters, a series power converter and a shunt power converter. The series power converter is operated as a current source, and it has the function of a harmonic isolation to block the harmonic current from the nonlinear load to mains and the harmonic voltage from the mains to load. The shunt power converter is operated as a voltage source to supply a clean and regulated output voltage to the load. Both power converters use the same dc bus. To demonstrate its performance, a prototype is developed and tested. The tested results show that the proposed APLC has the expected performance.