AC Power Distribution Systems Research Articles

Comparative Analysis of Various Topologies for AC and DC Power Distribution Systems on Efficiency Grounds

Efficiency is the factor that once wiped DC out of the power system at the birth of electricity, in nineteenth century. The battle of currents between AC and DC has reignited with the aggravating employment of DC in the sectors of generation, transmission, and utilization. This calls for cutting edge research in the field of comparative efficiency analysis of AC and DC, on distribution scale. The current research effort compares AC and DC distribution systems on efficiency grounds, for various topologies. The two systems are compared employing normal distribution technique for residential load profile. The varying efficiency of power electronic converters (PECs) with stochastic load profiles is modeled using MATLAB/SIMULINK, for each topology. DC takes the lead against AC with efficiency advantage for energy efficient load-based architectures. The architectures with variable speed drive-based loads, the efficiency advantage in favor of DC is between 1% and 2%, however, the efficiency advantage reaches to 12% with inherent DC loads. In contrast, AC takes the lead with conventional distribution, with an efficiency advantage between 4% and 5%. The findings of current research work can prove to be beneficial in making policy decisions for the adoption of DC at distribution scale.

Switched-Capacitor-Based Multi-Source Multilevel Inverter With Reduced Part Count

This paper proposes a switched-capacitor (SC) based multi-source multilevel inverter (MSMLI) for high-frequency AC power distribution systems. The proposed topology is a potential alternative in applications involving asymmetric dc voltage sources (in the case of renewable source-based AC microgrids). Using the available dc sources as an aggregated input eliminates the need for paralleling of inverters. In addition, the proposed topology offers a common ground among the multiple input sources and, thus, has a more straight forward design and avoids stacking voltage sources. By employing nine switches, two diodes, and two SCs, an 11-level voltage waveform is synthesized. The SC voltages are self-balanced owing to their series-parallel charging technique. A simple selective harmonic elimination technique is used to obtain the desired output voltage level and simultaneously eliminate the lower-order harmonics. A detailed comparison of the proposed MSMLI is carried out, and the test results obtained from a laboratory prototype are presented to validate the feasibility and highlight the benefits of the proposed topology.

A Low-Voltage AC, Low-Voltage DC, and High-Voltage DC Power Distribution System with Grid: Design and Analysis

Low-voltage (LV) and high-voltage (HV) DC distribution systems are being investigated as alternatives due to the growth of DC distribution energy resources (DER), DC loads such as solar and wind power systems, and energy storage sources (ESSs). Furthermore, an HV/LV DC distribution system offers various advantages, including lower conversion losses, an easier connecting strategy for DC DERs, and less complex power management techniques. As renewable energy sources are increasingly incorporated into the electrical grid, it is important to create novel, effective approaches for connecting such sources and loads. It would hence be effective to merge DC distribution with AC distribution to fulfill the energy demands of both DC and AC consumers. To this end, this study proposes a multizone design with four buses: low-voltage direct current (LVDC), high-voltage direct current (HVDC), low-voltage alternating current (LVAC), and an electrical grid. A model of this system that covers crucial elements, including power systems, DER systems, and power electronic devices, to serve as a foundation for the analysis and design of this architecture is proposed. MATLAB/Simulink is used to conduct a simulation study to verify the performance of the proposed design. In this study, a hybrid electrical grid with an LVDC, HVDC, and LVAC distribution network test is used and implemented. Additionally, a transient and steady-state characteristic analysis of the test system is performed.

A brief review on hardware structure of AC electric spring

<span lang="EN-US">Decarbonizing power generation and reducing greenhouse gas emissions require renewable energy sources. Intermittent nature of renewable energy sources can cause blackouts, fluctuation in voltage, grid-connected inverter usage, inability to predict power generation, fluctuations in voltage and frequency. It is possible to balance between the supply and demand of power to overcome these problems by using an electric spring (ES). It is necessary to study its work, types, and controlling actions for realizing the benefits of the electric spring. This paper reviews the hardware structure of an ES based on voltage-source inverter (VSI) and current-source inverter (CSI) topologies, in single-phase and three-phase AC power distribution systems with renewable energy sources. The structure, control strategies, operating modes, advantages and disadvantages of each ES topology are elaborated to make it a suitable alternative for resolving different power quality issues caused due to the high penetration of renewable energy sources.</span>

Control Strategy for Resonant Inverter in High Frequency AC Power Distribution System with Harmonic Suppression and Transient Performance Improvement

In high frequency AC (HFAC) distribution system, the resonant inverter is used to improve power quality and keep the stability of the output AC voltage. Aiming at the problems of poor output power quality and slow transient performance caused by unreasonable filter parameter design and load change during inverter operation, a single-phase H-bridge LCLC resonant inverter based on analog circuit controller implement is introduced in this paper for HFAC power distribution system (PDS). In this study, to design harmonic compensator and analyze the responsiveness of the inverter, it is necessary to analyze the output voltage total harmonic distortion (THD) of LCLC resonant inverter and the performance of the open loop system in detail. On the one hand, a proportional-integral-resonant (PIR) controller is designed to maintain the zero static error of the voltage output and suppress the output voltage THD of LCLC resonant inverter. On the other hand, an integral controller combines with phase-shift modulation (PSM) method is presented to effectively improve the transient performance of resonant inverter and provide the fixed frequency of the output voltage. On the basis of the above, the experimental prototype is implemented with the output AC voltage root mean square of 28 V, and the output voltage frequency for resonant inverter is equal to switching frequency. A rated output power of 130 W experimental platform is built to verify the effectiveness of the theoretical analysis, control strategy, and modulation method.

Analysis of DC distribution efficiency based on metered data in a typical Hong Kong office building

This paper aims to analyze the distribution efficiency of DC power distribution power system in commercial building based on measured data from a typical office building in Hong Kong. In this research, a real-scene comparative testbed was built in a Hong Kong office to compare the energy efficiencies of AC and DC power distribution system. Similar load characteristics and use schedules are set and used in the experiment. The distribution efficiency has been found to be affected by load type, voltage level and the loading conditions. The efficiency of this DC power system is derived, considering the PFC model was working in a good condition. For LED loads, the average distribution efficiencies after converter are 77.7% and 51.8 % under the condition of full lightning and normal lightning mode. The main loss in this system is transmission loss and switching loss. The average AC/DC converter efficiencies of LEDs are 92.21% and 88.85% respectively with the load working under full power and normal power, while the average AC power factors of LEDs are 85.31% and 56.22% respectively with the load working under the same as above. The average AC/DC converter efficiencies of the fan coil are 97.41%, 96.03%, 92.74%, 89.10%, 85.71%, 78.72%, 67.61%, 63.57% respectively with the load working under level 8 to level 1 while the average AC power factors of the fan coil are 98.0%, 99.0% and 99.4% respectively with the load working under low middle and high condition. As the PFC circuit does not have ideal power factor correction when the load is small, it is not recommended to add PFC circuit when the load power is low. Using uncontrollable rectifier circuit is going to lead to a better performance.

Energy Efficiency Evaluation and Revenue Distribution of DC Power Distribution Systems in Nearly Zero Energy Buildings

With the advantage of integrating distributed energy, storage and DC load with high efficiency, the DC distribution network recently attracted wide attention in the field of nearly zero energy buildings. Considering the large number of buildings and the enormous energy-saving potential, the distribution form and the revenue distribution are key factors affecting energy efficiency and operational economy. Aiming at the characteristics of AC and DC distribution topologies in nearly zero energy buildings, an energy efficiency evaluation method is proposed based on time-sequential power flow simulation, where the piecewise linear efficiency function of converters and dynamic line power loss model are established. Based on a DC distribution demonstration project, an AC power distribution system with the same boundary conditions is designed. The results show that the efficiency of DC distribution topology is about 6% higher than the AC system, while the efficiency advantage in office, hotel, commercial, educational and residential buildings in Beijing, Shenzhen and Shanghai varies from 0.6–4.96%, which shows a high correlation with the proportion of a high-power load, with Pearson correlation coefficient of 0.794. With the increase of DC load ratio and distributed power supply access capacity, the efficiency of the system will be improved accordingly. On the basis of this, a method to equalize DC distribution income in the nearly zero energy buildings power market is proposed. In addition, 15 typical scenarios of different cities were selected to evaluate efficiency and influencing factors. Finally, assessing the multi-stakeholder impact of DC retrofitting to balance the revenue generated by the DC distribution model suggests that transport costs should be raised by 8.2–13.1% to balance revenue with the equipment prices decreased by about 0.1 yuan/W.

Dynamic Model and Converter-Based Emulator of a Data Center Power Distribution System

Data centers have become a widespread power electronics (PE) load, which has significant impact on the power grid. In order to investigate the data center load characteristics, this article proposes a complete dynamic model for a typical data center ac power distribution system. A generalized model with mode transition is proposed to coordinate different power stages in the data center power system. Meanwhile, to help evaluate the grid dynamic performance and transient stability, an all-in-one load data center power emulator is developed on a reconfigurable PE converter-based hardware testbed (HTB). The dynamic power model is digitized and simplified to be implemented in two local voltage source inverters on the HTB. This proposed data center power emulator has been verified experimentally in a regional network. Dynamic performances during voltage sag events and server load variations are emulated and discussed. The article details the design, development, and verification of the data center model and power emulator. The proposed model and emulator provide an effective, easy-to-use tool to better design data centers and study the interaction with the power system.

Comparison of voltage drop in AC and DC shipboard electrical power distribution systems: A case study of 17,500 DWT tanker vessel

The recent emergence of power electronics equipment raises the possibility of implementation of DC shipboard electrical distribution systems. Several investigations on the effectiveness of DC shipboard electrical power distribution systems have proven that it is more profitable than AC distribution systems both in technical and economical aspect. However, known investigations are still limited to the system in ship with electric propulsion, which the consumption of electric power is enormous. The current study investigates the technical performance of both distribution systems in a non-electric propulsion ship, especially in the aspect of power losses. Both of the original AC distribution systems in a 17,500 DWT tanker ship and its equal DC distribution systems are modelled in a computer simulation program. The electric power flow simulation for both models are performed with the data of electric load during operating conditions, to compare the power losses in the distribution systems. The results show that the DC distribution systems has fewer drop voltage than its AC counterpart.

Virtual synchronous generator based pole control in high-voltage DC transmission systems

The use of renewable energy to realize decarbonization is rapidly increasing, and considerable research is being performed to achieve highly efficient high-voltage direct current (HVDC) transmission for offshore wind power generation over long distances. However, in the case of onshore AC power transmission and distribution systems, with the mass introduction of distributed energy resources (DERs), voltage and frequency fluctuations often occur. It is thus necessary to implement the control function of smart inverters, which can facilitate the stabilization of the AC system, in the grid-connected inverters of DERs. Virtual synchronous generator (VSG) control is a smart inverter control strategy that can contribute to the stability of the AC system. In this study, conventional power control and VSG control were applied to the control strategy of modular multi-level converter-based poles of the HVDC transmission system, and the AC system fault tolerances of the entire system were examined.

Soft-Switching Full-Bridge Converter with Multiple-Input Sources for DC Distribution Applications

Due to the advantages of power supply systems using the DC distribution method, such as a conversion efficiency increase of about 5–10%, a cost reduction of about 15–20%, etc., AC power distribution systems will be replaced by DC power distribution systems in the future. This paper adopts different converters to generate DC distribution system: DC/DC converter with PV arrays, power factor correction with utility line and full-bridge converter with multiple input sources. With this approach, the proposed full-bridge converter with soft-switching features for generating a desired voltage level in order to transfer energy to the proposed DC distribution system. In addition, the proposed soft-switching full-bridge converter is used to generate the DC voltage and is applied to balance power between the PV arrays and the utility line. Due to soft-switching features, the proposed full-bridge converter can be operated with zero-voltage switching (ZVS) at the turn-on transition to increase conversion efficiency. Finally, a prototype of the proposed full-bridge converter under an input voltage of DC 48 V, an output voltage of 24 V, a maximum output current of 21 A and a maximum output power of 500 W was implemented to prove its feasibility. From experimental results, it can be found that its maximum conversion efficiency is 92% under 50% of full-load conditions. It was shown to be suitable for DC distribution applications.

Two-Port Characterization and Transfer Immittances of AC-DC Converters—Part I: Modeling

This two-part paper presents a general method to model three-phase ac-dc converters in the frequency domain for power system stability studies. In the developed approach, an ac-dc converter is treated as a two-port network connecting an ac and a dc power system, and is characterized by a set of self and transfer immittances. The transfer immittances describe two types of small-signal coupling that are unique to three-phase ac-dc converters: The first type describes the coupling between the ac and dc port and involves a frequency shift by the fundamental; the second type models the coupling over frequency in the ac port current response and involves a frequency shift by twice the fundamental. Together with the self-immittances of each port, the developed models can be used as building blocks to support impedance-based stability analysis of various power systems that involve such converters, including ac, dc, and hybrid ac-dc power generation, transmission and distribution systems. Part I of the paper explains the two-port modeling approach and develops the associated models for a two-level voltage-source converter. Part II presents applications of the developed models for stability analysis of different systems.

Comparative Evaluation of AC and DC Power Distribution Systems for Underwater Vehicles Based on Multiobjective Optimization Techniques

This paper presents a multiobjective optimization technique for the submarine cables of high-power remotely operated vehicles (ROVs). A detailed design flowchart with integrated electrical and mechanical designs of the ROV submarine cables is provided to quantitatively compare AC and DC-based power distribution systems and identify the optimal design from multiple standpoints, prioritizing electrical loss and cable weight. The electromechanical cable design optimization plays a vital role in the ROV industry and in all those applications where the power distribution system is employed vertically from the support platform/vessel. The entire procedure is exemplified by carrying out a particular case study; for an ROV power hub with rated power of 5 MW and operational depth of 6000 m, the DC submarine cable design with distribution voltage of 33 kV results in an electrical loss reduction of 54.3% compared to the AC submarine cable design with distribution voltage of 22 kV at similar cable weights of 63 ton. Additionally, the proposed method can be widely applied to any type of AC-DC comparison to achieve the optimal ROV submarine cable design with a considerably short process time in comparison to the approaches with finite element analysis.

Switched Capacitor-Based 13L Inverter Topology for High-Frequency AC Power Distribution System

The power converters are taking responsibility to produce high-quality output voltage and current waveforms from the various PV sources. Recently, PV applications focus on high-frequency ac power distribution (HF-AC-PD). In particular, dc/ac switched capacitor multilevel inverters (SCMLIs) significantly reduce the power conversion stages and components, which result in high conversion efficiency. This article proposes a new SCMLI topology with a reduced number of switches and a self-balanced floating capacitor (FC) voltage. The proposed topology is configured as a single dc source and three dc sources. The proposed topology generates thirteen level output voltage waveforms, and the output voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{o}$ </tex-math></inline-formula> ) is two times higher than the input voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{in}$ </tex-math></inline-formula> ). The discharging time is less than the recently suggested topologies. Furthermore, the voltage stress on switches is reduced to 50% concerning the load voltage. Thus, a remarkable reduction in the bulky size of the FC is achieved. The simulation results are discussed for different fundamental frequencies of 360 Hz, 400 Hz, and 1 kHz and considering various loads. The proposed converter prototype is designed in the laboratory for 2 kW, and the results are presented. The results show a good agreement in terms of total harmonic distortions, power loss, and efficiency.

Analysis and damping of high-frequency oscillations at the presence of distributed constant power loads

Distributed power system can be subjected to a high penetration level of constant power loads (CPLs). In this circumstance, the system stability is a matter of concern. In this paper, the distribution effect of grid-connected CPLs is analyzed in an AC distributed power system. In fact, the interaction dynamics between different arrangements of distributed CPLs and the grid are completely compared. To this aim, the total power of all arrangements of distributed CPLs is considered constant that is called fixed-power arrangements in this work. Accordingly, first, the linearized model of tightly regulated voltage source rectifier (VSR), treating as a CPL, is developed. Then, the obtained model is generalized for multiple VSRs to provide distributed CPLs model. Further, by applying the bode diagram method and eigenvalues-based analysis, the distribution effect of CPLs on their stability is evaluated. The conducted analysis reveals that the distribution of CPLs is accompanied by different responses in their interactions with the grid, which is mainly highlighted at their high-frequency (HF) behavior. Moreover, it is obtained that more distribution of CPLs can deteriorate their HF interaction and lead to a reduction in system stability. According to these achievements, it can be deduced that ideal models of CPLs by negative impedances are not valid anymore, especially at higher frequencies. Afterward, a proper active damper is implemented to reduce the mentioned HF oscillations. The simulation studies of several distributed CPLs cases, as well as a centralized CPL, confirm the validity of the performed analysis.

Analysis of a series resonant AC/DC converter with integral cycle mode control for high frequency AC distribution systems

The study of a resonant AC/DC converter, thought for high frequency sinusoidal AC power distribution systems, is performed in this article. The control switch is commuted at the resonant current zero crossings, staying closed (or open) for a certain number of complete cycles. This commutation strategy, known as “integral cycle mode control”, gives soft-switching which improves efficiency, minimizes EMI and increases reliability and converter life span. Also, the input current has a low THD and a good power factor. A suitable scheme to control the converter is proposed. It consists of an internal loop based on sliding mode techniques, for regulating the amplitude of the resonant current, and an external PI loop with an added feed-forward action, that sets the DC output voltage. The main objective is minimizing the disturbing effect that variations in the AC bus voltage and load consumption have over the DC output voltage. For the design of the control laws, a model expressed in terms of rectified and averaged sinusoidal variables is employed. Computer simulations have been performed, considering typical surrounding conditions for this kind of application. The obtained results show that the converter DC output voltage stay always close to its reference value, presenting good rejection against the aforementioned disturbances

Early AC Power: The First Long-Distance Lines [History

In 1883, A 2,000-V, SINGLE-PHASE, 133 1/3-Hz power distribution system, designed and built by French inventor Lucien Gaulard and his British backer John Dixon Gibbs for the Metropolitan Railway of London, stretched for 15 mi and provided electric power for arc and incandescent lights at five railway stations. In 1884, a single-phase, 3,000-V, 133 1/3-Hz Gaulard?Gibbs power distribution system in Italy (Figure 1) extended about 25 mi from Turin (elevation 240 m, population 250,000 in 1881) to Lanzo Torinese (elevation 530 m, population 2,700 in 1881). It was the first multiple-user ac power-distribution system in the world. Having a small town at the foothills of the Italian Alps as the destination demonstrated the practicality of bringing electric power to remote, sparsely populated locations by using high-voltage ac power transmission. This would not have been possible profitably with Thomas Edison's low-voltage dc power transmission.

Generalized Topology of a Hybrid Switched- Capacitor Multilevel Inverter for High- Frequency AC Power Distribution

The generalized topology of a hybrid switched-capacitor multilevel inverter (SCMLI) for high-frequency ac (HFAC) power distribution system is presented. By employing the series–parallel conversion technique, the proposed SCMLI can achieve self-balancing of capacitor voltages and voltage step-up with arbitrary numbers of SC cells. The output power quality is improved by the intermediate voltage levels provided by the bidirectional switched converter connected to the dc source string. Besides, several active switches can be replaced with diodes if the large phase difference between output current and voltage is not required. Since the voltage ripple magnitude of the SC units is inversely proportional to the fundamental output frequency, the proposed SCMLI is more preferable for HFac applications. This paper elucidates the working principle of the proposed inverter with the detailed circuit and switching states’ description under staircase modulation. The mathematical analyses associated with the capacitor voltage ripples, sizing, and power loss are also provided. A 19-level MLI with a voltage gain of three was realized by cooperating two SC units with a voltage source string consisting of three symmetric dc sources. The operation and performance of the proposed inverter have been validated by the simulation and experiment on the inverter. The results showed that the inverter performed better at HFac condition and the power quality was satisfactory with the 19-level staircase output modulated at the fundamental frequency.

A New Voltage Balancing Technique for a Three-Stage Modular Smart Transformer Interfacing a DC Multibus

DC smart grids represent an alternative to traditional ac power distribution systems. The power conversion stage, interfacing the ac and the dc distribution systems, is the enabling technology to manage multiple dc loads and sources. The three-stage smart transformer (ST) is a promising solution, because it includes a dc–dc power conversion stage providing a dc multibus in output. The modular ST architecture supplies several dc outputs, which allow interfacing a dc smart grid with the ac power system. In this paper, the three-stage ST is based on a cascaded H-bridge (CHB) converter for the ac–dc power conversion, whereas dual active bridge (DAB) converters are adopted for the dc–dc power conversion stage. The peculiarity is that the dc voltage balancing is performed by the dc–dc power conversion stage instead of the CHB converter. It is advantageous for dc smart grids applications where the power sharing among the dc sources and loads is often not balanced. The design of the entire control system is based on the detailed small signal model of the ac–dc and dc–dc power conversion stages of the ST. High dynamic performance of the voltage balancing is fulfilled due to the commonly high switching frequency of the DAB converters.

Nine‐level high‐frequency inverter

In the high-frequency AC (HFAC) power distribution system, problems such as high switching frequency, a complicated circuit configuration and difficult parameter design still exist in the traditional inverters. A multi-level high-frequency inverter topology based on a forward converter is proposed in this study, which implements the electrical isolation of input and output. With the introduction of variable turns ratio technology, the nine-level output is achieved, which is conductive to the decrease of output harmonics. The selected harmonic elimination method is also adopted to further reduce the output harmonic distortion through the Fourier analysis of output waveforms. Besides, the HFAC output can be achieved at a lower switching frequency for the maximum switching frequency is only double the output frequency. The detailed analysis on operating principles and parameter design of the proposed inverter are provided in this study. Finally, the feasibility of the inverter is verified by the simulations and experiments.