AC Supply System Research Articles

Transient stability analysis of the standalone solar‐storage AC supply system based on grid‐forming and grid‐following converters during sudden load variation

AbstractThe standalone solar‐storage AC supply system is an efficient form of utilizing renewable energy. The solar‐storage supply system normally includes two types of converters, grid‐forming (GFM) and grid‐following (GFL) converters. Nevertheless, the incompatibility of the dynamic performance between the GFM and GFL converters due to different control schemes may introduce the transient instability risk to the solar‐storage supply system. To address this issue, it is extremely necessary to perform transient stability analysis and stability‐enhanced control for such a system. In this paper, a sixth‐order model of the dc‐link timescale for the solar‐storage supply system is developed, whose accuracy and applicability are verified by comparing with the detailed electromagnetic model. Based on the derived model, the analysis demonstrates that the solar‐storage supply system faces the potential risk of transient instability under large disturbances of sudden load variation, due to the mismatch of the transient response speeds between virtual excitation control of the GFM converter and dc voltage control of the GFL converter. The transient stability boundary of the system is quantitatively investigated, by depicting the basin of attraction of the post‐disturbance equilibrium point. Furthermore, from the perspective of improving the coordination of the transient behaviours between the GFM converter and the GFL converter, the parameters related to virtual excitation control and dc voltage control are designed to enhance the transient stability of the system. Time‐domain simulations and real‐time simulations are conducted to validate the theoretical analysis.DC–AC power convertors, microgrids, power system control, power system transient stability, renewable energy power conversion

Analysis of rectification techniques and autonomous hybrid power plants potential for railway power supply systems

The paper considers optimization approaches for AC and DC traction supply systems. The recent developments in the area of traction supply systems operation optimization mean the application of modern energy storage systems, renewable energy sources, and AI-based control systems considering traction system operation features. The authors provide a traction substation layout with an autonomous hybrid power plant and its components. The main requirements for the control system of an autonomous hybrid power plant are formulated. The operational areas and modes of the proposed system are investigated: voltage stabilization, external power supply volume decreasing, voltage quality improving. The modernization methods for AC and DC side of the power system is analyzed for improvement of the power supply. Also, the paper deals with developing a multi-pulse rectifier for DC traction systems. The key features of the rectifier and its performance vs pulse number are analyzed. The influence of different rectifier structures on voltage quality and the harmonic spectrum was examined. The optimal rectifier layout for DC traction systems is proposed and experimentally verified with a physical model. The theoretical data on the proposed solution is verified with the physical model.

Impact Assessment of Energy Storage Systems Supporting DC Railways on AC Power Grids

Energy storage systems (ESSs) represent an established solution for energy saving and voltage regulation in DC urban railway systems. In particular, ESSs can store the braking energy of light rail vehicles (LRVs) and support the DC feeder system during traction operations. Moreover, ESSs can significantly improve the operating conditions of the AC supply system by reducing voltage drops and current spikes. This paper investigates the impact on the bus voltages and branch currents of the AC grid of wayside and on-board ESSs supporting the DC railway infrastructure. An iterative algorithm solves the decoupled AC/DC power flow considering the 3-phase bridge rectifier model of traction substations. The work presents a novel mathematical formulation of the optimization problem to solve the positioning and sizing of supercapacitor-based wayside ESSs, considering both DC and AC network constraints. The effectiveness of the proposed method is proved through numerical simulations on a real Italian DC railway system. Obtained results are presented and discussed comparing the proposed methodology to several existing literature solutions.

Complex Design Method of Filtration Station Considering Harmonic Components

The paper deals with a new methodology for calculating the filter parameters. The basis is respect for the fact that the real filter current consists of other harmonic components, which filter is tuned. The proposed methodology was used to design filters for traction substation 25 kV/50 Hz. The operation of the locomotives in the AC supply systems of 25 kV/50 Hz leads to a rising of higher order harmonic currents. Due to the 1-phase supply system, these are mainly the 3rd and 5th harmonics. By simulation and subsequent measurement of the proposed traction power station filters the proposed methodology was verified. Thus, the filter design can also be used for filter compensating stations of the standard 3-phase distribution network. The described method presents an optimal filter design without unnecessary oversizing. This fact reduces the size and cost of the filter. It is shown that it is possible to design a filter that meets the requirements for power quality under extreme load and to minimise distortion of line voltage.

Reconfigurable Line-Side Converter for DC Voltage Matching and Ripple Suppression in Multisystem Locomotives

The multisystem locomotive traction drive system is fed by different kinds of railway traction power supply systems (TPSSs). The direct connection of the dc TPSSs and the traction inverter are broadly adopted. However, the dc-link voltage is influenced by the variable and fluctuant catenary voltage, therefore wasting the capacity of the devices, complicating the wiring of motor as well as reducing the reliability of system. In this article, the line-side converter, which is used as an ac/dc four-quadrant converter in ac supply system, is reconfigured as a dc/dc chopper for dc voltage matching in dc supply systems. The quadruple interleaved structure is adopted to eliminate the current ripple caused by the chopper. First, structures and corresponding switching method of the reconfigurable line-side converter are introduced. With the help of modal analysis, the principle of chopper operation, the mechanism, and the influencing factors of current ripple suppression are revealed. The system control method and configuration switching algorithm for voltage matching and stabilizing are then presented. Finally, the proposed conceives are validated by hardware-in-the-loop-based system.

Design, Modeling and Fabrication of a Portable Solar Powered Suction Pump for Use in Industries and Autoclinics

Solar powered systems have been built for various applications ranging from public lighting to water supply units but robust efforts have not been made in the development of pumping systems that can effectively replace the manually operated pumps used in field applications. Designing a machine that can pump the fluid without the need of so much energy on the part of the operator as well as boycott the use of direct electricity will hence be a reasonable contribution to field of maintenance and other relevant fields where such pumps will be required. The methodology adopted for this study involves detailed system design and analysis of a prototype of the pump system for actual implementation. In respect to the energy source, a Solar Photovoltaic system has been developed which is backed up by a battery bank such that the system can function effectively even when disconnected from the PV array. A 65 W PV panel was used alongside a 36 Ah battery bank. The system is designed to be a 12 V DC Charging system and 220-230V AC supply system. A 500 W DC to AC power inverter was used to achieve the transition for implementation. A review of pump technologies and alternative energy applications (Solar Photovoltaic energy technologies) in the study location was undertaken. Finally, the characteristics relationship between the flow rate and the head of the solar powered pump was determined and represented on a graph. It was observed from the graph that the rate of change of the flow rate is inversely proportional to the increasing head. And the highest flow rate is obtained when the head is 0.25m with a corresponding flow rate of 0.040 m3/s and highest efficiency of 91.153%. The outcome of this study will find useful applications as well as give reasonable contribution to the field of maintenance, and other relevant fields where such pumps will be required such as vegetable and palm oil production companies.

Demand Side Management for Stand-Alone Microgrid Using Coordinated Control of Battery Energy Storage System and Hybrid Renewable Energy Sources

Distributed generation with battery energy storage (BES) system is an alternative solution for stand-alone AC supply systems. This article addresses the issue of voltage variability of stand-alone microgrid with the adoption of intelligent micro-source controllers. The high energy density of the BES system is used to compensate the voltage fluctuation. A control mechanism is designed to regulate the flow of electrical energy from BES system. The control structure of DC–AC microgrid integrated with BES system is proposed, and the study is extended to explore the mechanism of demand side management (DSM) with the incorporation of voltage-droop characteristics. The BES system is interlinked to maintain the voltage to a desirable range and the proposed work is intended to show the introductory concept on DSM accomplished through voltage-reduction at the consumption-side. The proposed control strategy not only achieves frequency-regulation in adherence to IEEE Standard-1547, but also maintains customers’ quality of service while customizing voltage for regulating DSM. The basis of regulating the nickel-metal-hydride battery is state-of-charge of BES system and voltage generated by the hybrid sources consist of photovoltaic and wind energy systems. The adequacy of the proposed control scheme with BES-module is validated using time-domain simulation studies considering voltage-dependent-loads.

Simple Controller Configuration for Decentralized Parallel Operation of Inverters

In this paper, a simple controller configuration for the decentralized parallel operation of single-phase inverters in stand-alone ac supply systems is proposed. The configuration is designed considering the unavailability of the filter capacitor voltage and grid current information to the controller. Due to the unavailability of voltage information, internally estimated voltage references and the measured value of the converter-side inductor current are used for the calculation of the active and reactive power. Frequency and voltage droop characteristics are used, and hence, communication is not required. The operation of the inverter at the reference voltage magnitude and frequency is achieved by the direct control of the modulating wave. Therefore, inability to implement closed loops caused by the absence of certain voltage and current information is mitigated. The proposed controller has the provision for protection of switching devices during overcurrent situation. The accuracy and feasibility of the proposed controller are validated using the simulation and experimental results.

Evaluation and Reduction of Total Harmonic Distortion in AC Supply System

Evaluation and Reduction of Total Harmonic Distortion in AC Supply System

State-of-charge-based droop control for stand-alone AC supply systems with distributed energy storage

The droop method is an advantageous technique for stand-alone AC supply systems, allowing for power sharing among various inverters with no need for communication cables. However, in stand-alone systems with multiple distributed energy storage units, the conventional droop methods are unable to control the storage unit state-of-charge (SOC) in order to change simultaneously. Existing techniques endeavor to solve this problem by changing the slope of the P–f curve however this solution compromises the power response performance. As an alternative, this paper proposes a new SOC-based droop control, whereby the P–f curve is shifted either upwards or downwards according to the battery SOC. The proposed technique makes it possible to select the time constant for the battery SOC convergence and, at the same time, to optimize the power response performance. The paper also shows how the SOC changes when the ratios between the battery capacity and the inverter rated power are different and how the proposed technique can limit the SOC imbalance. Simulation and experimental results corroborate the theoretical analysis.

Frequency-Based Energy-Management Strategy for Stand-Alone Systems With Distributed Battery Storage

Distributed generation is an attractive solution for stand-alone ac supply systems. In such systems, the installation of two or more energy-storage units is recommended for system redundancy and may also be required when there is a consumption increase following installation. However, energy management with multiple energy-storage units has been, but vaguely analyzed in the literature and the few studies made are based on communication cables with a central supervisor. This paper proposes an energy-management strategy for a multiple-battery system which makes it possible to avoid the use of communication cables, rendering the system more cost effective and reliable. The strategy modifies the conventional droop method so that the power becomes unbalanced, allowing for the regulation of one or more battery voltages or currents, as required. Furthermore, whenever the frequency is high, the PV inverters reduce their power in order to prevent the battery from overcharge or high charging currents. On the other hand, whenever the frequency is low, then either the noncritical loads are regulated or the system stops in order to prevent the battery from overdischarge or high discharging currents. Simulation and experimental validation are performed for a system with two-battery inverters, two-PV inverters, and a number of loads.

Performance Comparison of VSC-Based Shunt and Series Compensators Used for Load Voltage Control in Distribution Systems

In this paper, the performance of voltage-source converter-based shunt and series compensators used for load voltage control in electrical power distribution systems has been analyzed and compared, when a nonlinear load is connected across the load bus. The comparison has been made based on the closed-loop frequency response characteristics of the compensated distribution system. A distribution static compensator (DSTATCOM) as a shunt device and a dynamic voltage restorer (DVR) as a series device are considered in the voltage-control mode for the comparison. The power-quality problems which these compensator address include voltage sags/swells, load voltage harmonic distortions, and unbalancing. The effect of various system parameters on the control performance of the compensator can be studied using the proposed analysis. In particular, the performance of the two compensators are compared with the strong ac supply (stiff source) and weak ac-supply (non-stiff source) distribution system. The experimental verification of the analytical results derived has been obtained using a laboratory model of the single-phase DSTATCOM and DVR. A generalized converter topology using a cascaded multilevel inverter has been proposed for the medium-voltage distribution system. Simulation studies have been performed in the PSCAD/EMTDC software to verify the results in the three-phase system.

Implementation of a three-phase high-power-factor rectifier with NPC topology

A three-phase four-wire power factor corrector based on neutral-point-clamped (NPC) topology is adopted to reduce the current harmonics and increase the input power factor. Using the NPC topology, the voltage stress of power switches can be reduced to the half of the dc-link voltage. With the dc-link voltage controller and the phase locked loop circuit, the balanced and sinusoidal line currents can be drawn from the ac supply system under the balance and unbalance mains voltages. The hysteresis current comparators are adopted in the current control loop to track the line current commands. Three voltage levels are generated on the ac terminal to the neutral point. Simulations and experiments are provided to verify the validity and the effectiveness of the proposed control scheme.

AC power line conditioner with switch-clamped topology

A new AC line conditioner with high input power factor and clean AC output voltages for isolating the linear or nonlinear loads is introduced. An AC/DC rectifier with switch-clamped topology is adopted to draw the balanced sinusoidal line current from the AC supply system. A three-leg inverter with switch-clamped topology is adopted in the system to provide clean AC output voltages to the critical or sensitive loads. Experimental results show the validity and effectiveness of the proposed control strategy.

Análise para pequenos sinais de um sistema ca composto de inversores conectados em paralelo

Este trabalho mostra a análise de estabilidade para pequenos sinais de um sistema CA composto de vários inversores de tensão conectados em paralelo sem a presença de uma barra infinita. O controle de cada unidade inversora é baseado nas curvas características frequência versus potência ativa e tensão versus potência reativa, o qual não depende da comunicação entre as respectivas unidades de controle. Os resultados de simulação e experimentais comprovam a eficácia do modelo para pequenos sinais, facilitando os estudos de estabilidade do sistema.

Small-signal stability for parallel-connected inverters in stand-alone AC supply systems

This paper presents a small-signal analysis for parallel-connected inverters in stand-alone AC supply systems. The control technique of the inverters is based on frequency and voltage droops, which depends on the local variable measurements and does not need control interconnections. Simulation and experimental results show that the system is well represented by the small-signal model. Some root locus plots for the system are provided, which make the stability studies and design easier.

Harmonic Reduction in Adjustable-Speed Synchronous Motors

Adjustable-speed synchronous motors (ASSM) are very popular in high-power high-speed applications. The Chilean mining industry uses ASSM in milling operations in ranges of several thousands hp. Although a high pulse number for the inverter would be beneficial, it never goes beyond 12, due to complicated connections. Thus, a distorted harmonic-rich current feeds the synchronous motor, increasing the inverter commutation angle and causing additional rotor-heating and pulsating electric torque. Harmonic suppression by filters is not viable due to the variable frequency nature of the motor. In this paper a new design for the inverter, without complicated circuitry, is proposed, so that it works with 36 pulses and then the motor is fed with a highly sinusoidal current. Also, the same technique is applied to the rectifier end so that an effective harmonic reduction is carried out at both ends of the system, i.e., the motor as well as the ac supply system. A 20-kVA laboratory development drive system has been built and experimental waveforms for the conventional (12-pulse) and proposed (36-pulse) configurations are included.

Harmonic reduction in adjustable-speed synchronous motors

Summary form only given, as follows. Adjustable-speed synchronous motors (ASSM) are very popular in high-power high-speed applications. The Chilean mining industry uses them in milling operations in ranges of several thousands hp. Although a high pulse number for the inverter would be beneficial, it never goes beyond 12, due to complicated connections. Thus, a distorted harmonic-rich current feeds the synchronous motor, increasing the inverter commutation angle and causing additional rotor-heating and pulsating electric torque. Harmonic suppression by filters is not viable due to the variable frequency nature of the motor. In this paper a new design for the inverter, without complicated circuitry, is proposed, so that it works with 36 pulses and then the motor is fed with a highly sinusoidal current. Also, the same technique is applied to the rectifier end, so that an effective harmonic reduction is carried out at both ends of the system, i.e., the motor as well as the AC supply system. A 20-kVA laboratory development drive system has been built and experimental waveforms for the conventional (12-pulse) and proposed (36-pulse) configurations are included.

Determination of Worst-Case Capacitor-Switching Overvoltage of MRT System Using Genetic Algorithms

This paper analyzes the worst-case DC transient voltages of mass rapid transit (MRT) systems due to utility-capacitor switching on AC supply systems. The various factors affecting this phenomenon are analyzed in detail using extensive EMTP (Electromagnetic Transients Program) simulations. These factors include the size and location of utility capacitor, point-on-wave switching angles on the three phases, and DC link parameters. In this paper, genetic algorithms (GAs) are applied to determine the worst-case DC overvoltage caused by nonsimultaneous energization of utility capacitor. The effectiveness of the proposed GA approach is verified by comparing results with traditional statistical approaches.

Adaptive SVM to compensate DC-link voltage ripple for four-switch three-phase voltage-source inverters

An adaptive space vector modulation (SVM) approach to compensate the DC-link voltage ripple in a B4 inverter is proposed and examined in detail. The theory, design, and performance of this pulsewidth modulation (PWM) method are presented, and the method effectiveness is demonstrated by extensive simulations and experiments. High-quality output currents are guaranteed by this approach even with substantial DC-voltage variations that might be caused by an unbalanced AC supply system, the diode rectification of the line voltages, and circulation of one output phase current through the split capacitor bank. The application of this approach to induction machine drives is also discussed. It is concluded that the DC-voltage ripple effect on the B4 inverter output can be minimized by an adaptive SVM algorithm with the advantage of improving the response of the DC-link filter and the output quality of the inverter becoming high.