Wide Range Of Load Variations Research Articles

Analysis of the possibility of reducing the heating time of thick-walled cylindrical components with holes

Thermal power plants, both fossil-fuel-fired and nuclear power plants operating in a modern energy system with a high share of renewable energy sources, should be characterised by high flexibility, i.e. operate safely over a wide range of load variations as having short start-up and shutdown times. Many years of experience in power plants and stress calculations in power plant components have shown that thick-walled pressure parts with complex shapes or holes in the walls determine the maximum allowable raising or lowering of the working fluid temperature. A new method for determining the optimum fluid temperature variations in a cylindrical component with openings was developed. The optimum fluid temperature variation was determined from the condition that the sum of the circumferential stresses from thermal and pressure load is equal to the allowable stress at the edge of the opening. The optimum fluid temperature variation over time was determined at constant pressure and with a linear change in pressure over time using the Heat Balance Integral Method (HBIM). HBIM is preferred over other numerical and analytical methods because it can accurately determine temperature distribution at the beginning of optimum heating for very short times.

Soft-Switching Bridgeless Buck PFC With Low THD

In this paper, a modified soft-switching bridgeless buck converter is introduced. In the proposed converter input current dead angle is eliminated by applying a simple auxiliary circuit. Also, by operating under DCM and DCVM condition, high efficiency and low THD is achieved for a wide range of load variation. The proposed converter is analyzed and design considerations are discussed. A prototype converter is implemented. The efficiency of 93.7% and THD of 4.3% are achieved using low-cost IGBT switches and diodes for 24 V output voltage. The proposed converter is controlled by a simple PWM controller.

Contactless-Sensor-Based Output Feedback Control With Maximum Efficiency Tracking Technique in Inductive Power Transfer Systems

To maximize the overall efficiency of a series-series inductive power transfer (SSIPT) system, the maximum efficiency tracking (MET) control should be employed. Based on a contactless current sensor which is designed for the mutual inductance estimation and the output voltage feedback, this paper proposes a novel MET control method for the SSIPT system with front-side and load-side converters. In this method, the output voltage of the rectifier on the load side is estimated and used as a control reference on the front side. Moreover, this paper analyzes the small-signal model of the SSIPT system and presents an impedance-based stability criterion. The factors which influence output feedback control are investigated, and a comprehensive control strategy based on the impedance stability criterion is developed. Finally, a 600 W prototype is built to validate the proposed method. MET control is implemented in a wide range of load variations, with a maximum efficiency drop of approximately 2.3% over a 100 mm misalignment. The results show that the proposed scheme significantly improves the robustness and response performance of the SSIPT system.

Research on fuzzy weighted gain scheduling water level control system of U-tube steam generator

U-tube steam generator (UTSG) is one of the key components of the pressurized water reactor (PWR) in nuclear power plant (NPP). The Traditional UTSG water level control system (WLCS) copes with a wide range of load variations by changing the gain of the water level controller. The water level controller is designed based on a specific power level. At other power levels simply changing the gain of the controller will not perform well. Therefore, it is necessary to study a full power range UTSG WLCS. In this paper, based on the transfer function, the traditional controller is designed, including water level controllers and flow rate controllers under five power levels. However, the traditional controller is designed based on fixed working points, and the performance is poor when applied to other power levels. In order to solve this problem, the fuzzy weighted (FW) controller is designed by using the triangle fuzzy membership function and trapezoidal fuzzy membership function respectively. The simulation results show that the FW controller using the triangle fuzzy membership function is recommended.

A ZVS High Step-Up DC–DC Converter for Renewable Energy Systems With Simple Gate Drive Requirements

In this article, we propose a high step-up interleaved dc–dc converter with zero-voltage switching, which is based on the coupled-inductor technique and voltage multiplier cell. A capacitor is placed in series with the secondary windings of the coupled inductors to increase voltage gain so low-voltage-rated and <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -resistance <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s can be employed to reduce conduction losses. By using auxiliary switches, soft switching is provided for a wide range of load variations. Due to the presence of the leakage inductor, all diodes are turned <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> with zero-current switching condition, so the reverse recovery problem is alleviated. Besides, the thermal stress of the phases and the input current ripple are low due to the interleaved structure applied. The operation principle and steady-state analysis of the presented converter are discussed. Finally, in order to verify the operation of the proposed converter, a 28 V input voltage to 460 V output voltage prototype with a rated power of 250 W is implemented and tested in the laboratory.

The flexible boiler operation in a wide range of load changes with considering the strength and environmental restrictions

Thermal steam units cooperating in one power system with wind and photovoltaic farms must have high flexibility, i.e., operate over a wide range of load variations, typically between 40 and 100%. In addition, the start-up and shutdown time of the unit should be short. Thick-walled pressure components that limit the fast start-up of the units are turbines, steam boilers, and pipeline branches due to the occurrence of high thermal stresses in them. This paper determines the allowable heating rates of thick-walled components of a 97.22 kg/s natural circulation steam boiler. The boiler drum is the critical element, with a heating time of about 6000 s from the cold state. Computer simulations of the boiler combustion chamber were also performed at three different loads, i.e., 100, 60 and 40%. The calculations show that the maximum emissions of harmful substances do not exceed the permissible values given in European regulations: CO≤140mg/mn3 and NOx≤100mg/mn3. The maximum content of CO = 136.8 mg/mn3 occurs at 60% boiler load. The maximum content of NOx = 217.3mg/mn3 in the exhaust gas upstream of the SCR (Selective Catalytic Reduction) system occurs at 40% load, but downstream of the SCR system is reduced below the limit value 100mg/mn3.

Efficient interleaved high step‐up converter with wide load range soft switching operation

The output voltage of the renewable energy resources such as photovoltaic and fuel cell systems is relatively low. Therefore, in the renewable energy-based power generation systems, high step-up (HSU) DC-DC converters are demanded between the renewable energy resource and the grid. The required characteristics of such HSU converters are low size, cost, number of elements, current ripple, and high efficiency. In response, in this paper, a new non-isolated HSU converter with interleaved structure and resonant operation is presented. The proposed converter achieves high voltage gain and full soft switching performance over a wide range of load variations while utilizing low number of components. In this converter, the voltage gain is adjusted by tuning the coupled inductors turns ratio and the resonant frequency. The semiconductors voltage stress is low in comparison to the other counterpart converters with resonant operation. Also, the converter uses interleaving technique to reduce the input current ripple. Comprehensive theoretical analysis of the converter is presented and its performance is compared with the recent counterparts. In order to validate the converter effectiveness, a laboratory prototype is implemented and the experimental results are provided.

Off‐design performance of supercritical carbon dioxide cycle combined with organic Rankine cycle for maritime nuclear propulsion system under different control strategies

A combined power conversion system, which consists of a supercritical carbon dioxide recompression cycle with an organic Rankine cycle as the bottoming cycle, has broad potential for maritime propulsion applications. However, the performance of the combined system under the off-design condition and operating states of compressors with a wide range of load variations and different control strategies have not been comprehensively investigated. Therefore, a simulation model for the combined system under off-design conditions is established in the present study to investigate the compressors operating states and the performance of the system under different control strategies. The result shows that a peak efficiency of 42.82% and total production unit cost of 11.6 $/GJ are achieved at the design point for the combined system, where the pressure ratio, flow split ratio, and evaporating pressure are 2.7, 0.27, and 770 kPa, respectively. When the relative load varies from 100% to 10%, the efficiency of the inventory-controlled system decreases by 19.07%, and the efficiency of the bypass-controlled system decreases by 34.3%. For the 10% load scenario, the compressors tend to surge under the inventory-controlled strategy and approach the choke area under the bypass-controlled strategy. The adoption of the inventory-bypass hybrid control strategy keeps the compressors of the low-load system away from the surge and choke areas, and the efficiency is between those of the systems controlled by the two separate strategies.

Optimal Design of Solid State Transformer-Based Interlink Converter for Hybrid AC/DC Micro-Grid Applications

The conventional low-frequency transformer (LFT) in conjunction with power electronic converters is used as the conscientious device to provide galvanic isolation and voltage regulation for the hybrid micro-grid (HMG) applications. However, it may not be suitable for HMG applications where the high power density and active power flow control over a wide operating range are major concerns. This article presents the solid state transformer (SST)-based interlink converter (IC) used to provide high frequency (HF) galvanic isolation between the ac/dc micro-grid (MG) networks. The proposed structure of SST intends to meet out most of the aspects of HMG networks such as active control over the bidirectional power flow, high efficiency, high power density, and better voltage regulation. In SST, nonoptimal parasitic components of HF transformer (HFT) are used as a part of power transfer elements and are unable to assist soft-switching operation over the wide range of load variations that degrade its overall performance. In view of these concerns, the comprehensive design and analysis are carried out at different stages to decide the design constraints for the optimization of nonoptimal magnetizing components and their significant effect on the system performance. Furthermore, the influence of the system parameters is analyzed with analytical expression and graphical analysis. The proposed SST configuration is designed for minimum kVA rating and better voltage regulation at different buses by using the proposed dual degree control (DDC) scheme over the wide range of load variations. The 4-kW hardware prototype is built up in the laboratory which provides isolated dc dual-port output terminals. The design and testing are carried out under different loading conditions to validate the significant performance of the proposed SST structure.

A Load-Insensitive Hybrid LSK Back Telemetry System With Slope-Based Demodulation for Inductively Powered Biomedical Devices.

This paper presents a hybrid load-shift keying (LSK) modulation for a load-insensitive back telemetry system to realize near-constant voltage changes in a primary coil (L1) against a wide range of load variations. The hybrid-LSK-enabled full-wave rectifier enables the sequential combination of open- and short-coil functions for hybrid-LSK modulation in addition to wireless power conversion operation. Load-insensitive L1 voltage changes can be demodulated using the proposed slope- based demodulator, which utilizes the threshold slope of L1 voltage changes over the back data pulse width, enabling robust data recovery regardless of the load conditions. The 0.56-mm2 0.18-μm standard CMOS hybrid-LSK prototype demonstrated that the variation of L1 voltage changes could be minimized to 60 mV under load changes between 50 Ω and 50 kΩ at coil separation distance of 10 mm, achieving 88.2% reduction compared to the conventional short-coil LSK with 510 mV variation. The proposed back telemetry system also achieved a bit error rate (BER) of < 9.1 × 10-10 under load ranges from 50 Ω to 50 kΩ and data rate of 1 Mbps, ensuring reliable back data recovery against load variations.

Influence of frequency and power control modes on the technical condition of hydraulic units of hydroelectric power plants

The features of the operation of hydroelectric power plants, which are used to regulate the frequency and power in the power system with a wide range of load variations and with varying degrees of intensity of operation mode variations, are considered. An approach is proposed to assess the impact on the reliability of hydraulic turbine equipment used in these modes. When examining the equipment of existing hydroelectric power plants, it has been revealed that the mode of automatic secondary regulation of frequency and power flows in the power system has the greatest impact on the technical condition of hydraulic turbines. Intense participation in regulation leads to an increase in dynamic loads on the hydraulic unit, loads on the friction pairs of the regulating elements of the turbine compared to the basic, semipeak and peak modes and leads to additional wear, reduced reliability and deterioration of the technical condition of the equipment. The operation in the automatic secondary regulation mode of hydraulic units at different HPPs differs depending on the function performed in the power system. To determine the loading of a hydraulic unit with transient load variation modes during operation in the mode of automatic secondary regulation of frequency and power flows, it is proposed to introduce a parameter characterizing the intensity of operation of hydraulic units of HPP in the aforesaid mode and to take it into account in order to accumulate data on the actual operating modes of hydraulic units and use it in assessing and predicting the technical condition of equipment.

Design of a Dual-Loop Controller with Two Voltage-Dependent Current Compensators for an LLC-Based Charger.

A two-stage charger (PFC+LLC) is developed for Power Mobility Device (PMD) charging applications (i.e., power wheelchairs). The LLC topology is selected due to its ability to maintain ZVS operation over a wide range of load variations. This paper discusses the challenges of designing a dual-loop controller for an LLC-based charger with a wide operation region. It proposes modifying the inner current loop to maintain stable operation over a wide input and output range-the proposed internal loop control switches intelligently between two current controllers based on the output voltage level. A 300-W prototype is designed and tested with a resistive load. Moreover, simulation and experimental results are compared to validate the robustness and stability of the proposed controller.

Secondary Side Cascaded Winding-Coupled Bidirectional Converter With Wide ZVS Range and High Conversion Gain

In this article, a secondary side cascaded bidirectional dc-dc converter (SSC-BDC) is proposed with the winding-coupled structure for the bidirectional operations of power flow, which features high voltage gain, full range of soft switching, low current ripple in low voltage side, and extended duty cycle range. Derived from a current-fed dual active bridge converter, the interleaved buck/boost circuit in the primary side is cascaded with the secondary side of full-bridge circuit through the winding coupled inductor to achieve high voltage gain. The differential voltage is designed to generate the bias current to realize soft switching of the eight power switches within a wide range of load variation. Meanwhile, pulse width modulation (PWM) plus phase-shift modulation is used to eliminate backflow power and increase conversion efficiency. The operation principle, steady-state analysis, and comparison result are presented in detail. Finally, a 1-kW experimental prototype is constructed to verify the feasibility of the SSC-BDC, and the correctness of the theoretical analyses is validated by experimental results.

Soft Switching Interleaved High Step-Up Converter With Multifunction Coupled Inductors

This article presents an efficient interleaved soft switching dc-dc converter for high step-up applications. In the proposed converter, two multifunction coupled inductors are employed to increase the voltage gain, accomplish soft switching operation for all semiconductors in a wide range of load variations, and smooth the input current. Besides, the leakage inductance energy is recovered to enhance the voltage gain and efficiency while assisting soft switching condition for the switches and diodes. In addition, no auxiliary switch or extra magnetic element is employed in the proposed converter while the voltage stress of switches and diodes is rather low. These have led to simple structure, high density, low cost, and improved efficiency. Theoretical analysis and design considerations for the proposed converter are presented. Moreover, the experimental results of a 48 to 500 V laboratory prototype approve the effectiveness of the proposed topology.

Load Effect Analysis and Maximum Power Transfer Tracking of CPT System

Owing to the flexibility of coupling structure, low standing losses and low Electromagnetic Interference (EMI), capacitive power transfer (CPT) has drawn much attention as one of the new wireless power transfer (WPT) technologies. However, to date, the load effect on the system performance has not been analyzed comprehensively, and the CPT system is usually fully tuned (tank network resonant frequency fully tuned to the fundamental components of the nominal switching frequency), which is challenging to maintain practically. Based on these considerations, this paper first provides a detailed analysis of how the load can affect the performance of a CPT system. An optimal load resistance is shown to exist for a CPT system with a non-fully/partially tuned LCC tank (tank network resonant frequency is designed to be lower than the fundamental components of the switching frequency). By dynamically transforming the load to its optimal value, the system can transfer maximum power to the load while maintaining zero voltage switching (ZVS) operation. Then, a controlled buck-boost converter based on perturbation and observation algorithm is introduced to the system to obtain the optimal equivalent resistance against the actual load variations via tracking the maximum power transfer point. The final prototype has demonstrated that a maximum power of 10 W is obtained, with an end-to-end power efficiency of about 70% over a wide range of load variations from $5~\Omega $ to 1 $\text{k}\Omega $ .

Obtaining Maximum Efficiency of Inductive Power-Transfer System by Impedance Matching Based on Boost Converter

This article presents a maximum efficiency tracking method for an inductive power-transfer (IPT) system based on a Boost dc–dc converter. Impedance matching (IM) is achieved by maintaining the equivalent load impedance to be constant against load variations. The expression of the optimal equivalent load impedance corresponding to the maximum system-transfer efficiency is derived, and the impedance-transfer characteristics of the Boost converter in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM) are analyzed. The duty cycle of the Boost converter is controlled to maintain the optimal equivalent load with extended range in DCM mode, and the switching frequency of the Boost converter is further adjusted to improve the accuracy of the IM. Both simulation and experimental results have demonstrated that the proposed IM method can help achieve the maximum power-transfer efficiency under a wide range of load variations.

Investigation on Operating Characteristics in Three Level Full Bridge AC–DC Converter using Different Control Strategies for Telecom

This paper presents the improvement of operating characteristics of three level full bridge ac–dc converter by proposing four different closed loop control techniques for telecom applications. Comparisons between different controllers have been considered to evaluate the power quality parameters with fast dynamic response under various operating conditions. Simulations have been done with Matlab/Simulink with four different voltage and current controllers, namely; PID with hysteresis current controller, Fuzzy Logic with hysteresis current controller (FL-HCC), Hysteresis Modulation based Sliding Mode Controller (HM-SMC) and Genetic Algorithm based Average Current Control (GA-ACC). Comparative analysis has been carried out with power factor, total harmonic distortion (THD) for various loading conditions upto 300 W. Based on the comparative analysis, GA-ACC technique provides better power quality parameters with improved dynamic response than the other three controllers. GA-ACC technique improved the power factor to 0.9911 and source current THD of 5.11% for converter. Also, voltage ripple and efficiency of the converter were calculated and tabulated under a wide range of load variations. Simulated results of the converter with GA-ACC have been validated with the experimental results.

Grid Integration of Three-Phase Single-Stage PV System Using Adaptive Laguerre Filter Based Control Algorithm Under Nonideal Distribution System

The increasing integration of utility with renewable sources of energy has resulted into serious issues of power quality (PQ) deterioration primarily in a scenario of weak distribution grid. In order to improve the PQ, efficient implementation of a control algorithm for the solar energy conversion system interfaced to the grid is paramount. This article illustrates the usage of an adaptive Laguerre filter based control technique for optimal operation providing distribution static compensator capabilities along with load balancing, power factor correction and harmonics mitigation. Moreover, it provides the active power transfer to the grid together with feeding the nonlinear and linear loads. In addition, the system performance according to an IEEE-519 standard has been verified, hence it is proficient in maintaining the PQ. The Laguerre filter offers the benefits of reduced computational time in addition with the alleviation in model complexity predominant during abnormal grid conditions. By utilizing maximum power point tracking, the efficient utilization of solar PV array is accomplished with a perturb and observe method. For validating performance of the proposed system, it is examined through simulation results. Furthermore, a prototype is developed for validation and test results corroborate reliable operation under nonideal grid conditions comprising of wide range of load variations, voltage sag, swell, distortion, and unbalance conditions.

Structural-and-Parametric Optimization of Automatic Control System for Power Units of 300 MW in Wide Range of Load Variations

The structural-parametric optimization of the automatic control system for power units (ACSPU) of 300 MW of Lukoml’skaya GRES (Lukoml Local Condensing Power Plant) in the mode of both the permanent and the variable superheated steam pressure upstream of the turbine is under consideration. During 1974–1979, eight units of the Lukoml’skaya GRES implemented the ACSPU with a leading boiler power control. At the moment, these systems no longer meet all the frequency control quality requirements. In 2016, the daily schedule of electric loads of the Belarusian power system was as follows: the basic part of the schedule of electric loads was covered by combined heat and power plants (CHP) and by mini-CHP (which are the least maneuverable of the power plants), the semi-peak part of it–by local condensing power plants (Lukoml’skaya GRES and Berezovskaya GRES), the peak part–by import electric energy from neighboring power systems. However, this year the first unit of the Belorussian NPP will be put into operation, while the second one–in 2020. After the launch of the Belorussian NPP, it will cover basic part of load curve; CPPs will cover the semi-peak part, while the peak part of load curve will be covered by local condensing power plants. Correspondingly, due to the alteration of the structure of daily schedule of electric loads of the Belarusian power system, it is necessary to improve the efficiency of power units of Lukoml’skaya GRES as well as of the entire Lukoml’skaya GRES in general. This can be achieved with the help of the method of parametric optimization of the typical ACSPU proposed in the present paper. As a result, the quality of control of power and pressure upstream of the turbine will be improved; the flow of fuel will be reduced, as well as the turbine regulation valve displacement; environmental performance of entire power plant will be improved, too. The proposed technique has been confirmed by the results of computer simulation of transient processes in the automatic control system under external and internal disturbances.

Characterization of variable inductors using finite element analysis

The variable inductor (VI) uses a small DC current to regulate the permeability of magnetic cores. Nonetheless, this magnetic device is composed of magnetic cores and multiple windings, which leads to a more complex geometry. Its magnetic properties variations and non-linear behavior complicate its modeling, analysis and its characterization. This paper uses Finite Element Analysis tool MagNet Infolytica software to characterize the VI in order to verify its feasibility for DC–DC converters with wide range of load variations. The inductance characterization, the losses and improvements in the saturation level are used to quantify the capability of the VI. The characterization and analysis presented in this paper will be a good base for researchers and designers to integrate the VI in converters and help them in choosing suitable VI configurations for their application. Moreover, the theoretical aspects presented in this paper will be essential for development of analytical ways for the characterization of such kind of devices. The inductance characterization obtained from FEM simulations agree with the experimental values with a mean squared error of 1.6%. Furthermore, for a current of more than twice the rated current, the differential inductance is increased by 56 times with control current in the auxiliary winding. This reveals the potential significance of VI in improving the current handling capability and core utilization leading to core size reduction in power electronic converters.