Link DC-AC Research Articles

Frequency Stability of Multi-source Power System using Whale Optimization Algorithm

The Whale Optimization Algorithm (WOA), an evolutionary computing approach, is presented in this study and is used to auto regulated frequency of many composed power systems including thermoelectric power station, hydroelectric, and gas power plants. The purpose of this process follows the concept of a hunting mechanism of fish through water bubbles. The WOA is first applied to a single region with a multi-source power system for optimal gain adjustment of proportional integral controllers (PID). This approach is then applied to two areas, each having six generating sources with AC and AC-DC links. The credible achievements of the WOA-based PID controller are compared with previously constructed optimization approaches like Teaching Learning Based Optimization (TLBO), Differential evolution (DE) and Optimal Controller (OC), which is demonstrated in terms of frequency error, settling time and damping ratio. The performance indices of the purposed controller are analyzed through different objective functions like integral square error (ISE), integral time absolute error (ITAE), integral absolute error (IAE) and integral time squared error (ITSE). By using step load agitation, the simulation results indicate that the suggested technique is computationally stable.

Offline Reconfigurability Based Substation Converter Sizing for Hybrid AC–DC Distribution Links

Hybrid ac–dc distribution systems with multiple parallel operating ac and dc links are being developed to maximize system capacity during (n-1) contingencies. This paper shows that offline reconfigurability between such ac and dc links post-fault clearance allows an additional design flexibility of reducing the required substation converter rating to maintain the same required capacity by maximizing the utilization of healthy infrastructure. The associated variation of system capacity with power rating and number of substation converters is determined for different types of component faults. It is discussed that while converter downsizing is possible, as a consequence the parallel ac–dc link system is forced to operate at a sub-optimal efficiency during normal conditions. Using case studies, the economics of this trade-off is quantified for determining the optimal de-rating factor for varying grid voltage, link length, and conductor area.

A Three-Phase Three-Level Isolated DC–AC Converter With Line Frequency Unfolding

A three-level three-phase single-stage high-frequency link dc–ac converter is reported in this article for grid integration of photovoltaic sources. The proposed topology employs two three-level neutral point clamp (NPC) half-bridge legs on the dc side. The advantages of using three-level legs over conventional two level are: 1) the three-level legs can be implemented with the devices with lower blocking voltage which are economical; and 2) the low voltage blocking devices have lower on -state drop and lower turn on – off energy losses compared to high voltage blocking devices used in a two-level leg. These help to improve the converter efficiency. The sinusoidal pulsewidth modulation is implemented with the three-level NPC legs. The modulation strategy ensures reduced neutral current drawn by the NPC legs. The inner switches of the NPC legs are zero voltage switched. The turn- on transitions of the outer switches are with zero current. The proposed solution employs two high-frequency transformers to provide galvanic isolation which results in compact, low cost isolated converter solution. The intermediate dc link is pulsating and does not require any filtering. A low-frequency unfolder is employed to generate line frequency ac from pulsating dc. The switching loss of the unfolder is negligible. The proposed topology can support stand-alone load up to $\pm 0.866$ PF. The converter operation is verified on a 2-kW hardware prototype.

Optimal Power Flow Control in Parallel Operating AC and DC Distribution Links

DC links with back to back voltage source converters are usually built alongside the existing medium voltage ac distribution grids for infrastructure reinforcement. The distribution network operators need to run multiple of such parallel ac and dc links between two substations at optimal efficiency. This article shows that the active power steering capability of the dc link can be used to dynamically vary the share of power flow in the ac link such that the system operating efficiency is maximized for varying power demand, grid voltage, dc-link voltage, converter efficiency, link length, and conductor area. The algorithm developed based on the derived exact and estimated solution for this parallel ac-dc link power sharing ratio is proved through simulations on a 10 kV, 30 MVA system. The concept is validated using experiments on a scale-down lab model. Using case-study with adapted measured substation data of hourly average power demand profile for one year, it is shown that annual energy saving potential in the range of 8-92 MWh can be achieved with varying link length between 10-20 km for 5-20 kV grid voltage and 185-630 mm$^2$ conductor area if the proposed optimal power flow control is used.

Optimal Power-Frequency Control in Deregulated Thermal, Hydro and Hydrothermal Power Systems with AC-DC Links

Background: A comparative analysis of Automatic Generation Control (AGC) of two-area electric power systems interconnected by AC and AC-DC links under deregulated environment is conducted. Each area has Thermal-Thermal (TT), Thermal-Hydro (TH) and/or Hydro-Hydro (HH) multiple power sources. A maiden attempt is made to study the demeanour of HH power system under restructured mode. Methods: The state space models of the power systems have been developed to simulate all market transactions probable in a deregulated power environment and optimal proportional integral structured controller is applied to improve the dynamic performance. The concept of DISCO participation matrix is harnessed to simulate the transactions. Results: Eigenvalue analysis is carried out to assess the comparative stability analysis of the power systems with/without AC-DC links. Further, the dynamic responses of TT, TH and HH power systems are contrasted in the presence of AC link and AC-DC links. The inclusion of AC-DC links improves the dynamic performance of all the systems remarkably, however, the responses of HH system are sluggish/poor with large undershoots in comparison to TT and TH systems. Also, TH system exhibits degraded dynamic performance compared to TT system. Conclusion: Moreover, optimal controller is found competent to demonstrate the matching of generation with power demand under different market transactions.

Adaptive Neuro-Fuzzy Scheduled Load Frequency Controller for Multi Source Multi Area System Interconnected Via Parallel AC-DC Links

Adaptive Neuro-Fuzzy Scheduled Load Frequency Controller for Multi Source Multi Area System Interconnected Via Parallel AC-DC Links

A Novel Method for Fault Detection in Future Renewable Electric Energy Delivery and Management Microgrids, Considering Uncertainties in Network Topology

The use of solid state transformers (SSTs) in microgrids has created a new kind of network called Future Renewable Electric Energy Delivery and Management (FREEDM) microgrid. The FREEDM microgrid provides an appropriate means for enhanced energy management, loss reduction, and network flexibility by reducing the number of converters used for a variety of AC-DC links. In this work, we propose a novel method for fault detection in FREEDM microgrids when considering uncertainties in network topology. The proposed method makes use of the Clarke and S-transforms to characterize the transients in three-phase current and voltage waveforms in the event of a fault. The extracted features of the waveforms will be used to form appropriate indices for detection, location, and characterization of the fault. The main feature of the proposed method is its capability to operate in a dynamic microgrid with varying topology. The performance of the proposed method is investigated by applying it to a sample FREEDM microgrid with ring and radial structures. It is shown that the proposed method is well capable of fault detection and diagnosis while being able to differentiate between short-circuit faults and switching transients due to variations in the network topology.

An Isolated Bidirectional Single-Stage DC–AC Converter Using Wide-Band-Gap Devices With a Novel Carrier-Based Unipolar Modulation Technique Under Synchronous Rectification

A novel carrier-based unipolar-sinusoidal pulse width modulation (SPWM)-oriented modulation technique with synchronous rectification for isolated bidirectional single-stage high-frequency-ac link dc–ac converters using SiC MOSFET is presented in this paper. The dc–ac converter is composed of a full-bridge (FB) inverter cascaded with a cycloconverter through a high-frequency transformer. A carrier-based unipolar-SPWM-oriented modulation technique with synchronous rectification is proposed to realize zero-voltage-switching (ZVS) for the FB inverter and zero-current or zero-voltage-switching (ZVS/ZCS) for the cycloconverter in all load ranges, and to suppress the voltage spikes introduced by the transformer leakage inductance as well. In order to increase the switching frequency, efficiency, and power density, this paper proposes to utilize SiC MOSFETs for the converter. Synchronous rectification is implemented to further increase the converter efficiency. With the novel modulation technique, there are two switches in the cycloconverter that are continuously on at each interval, which eliminates on-fourth of the switching loss. A simulation model and a 400 VDC–240 VAC, 1.2=kW prototype have been developed to validate the effectiveness and performance of the proposed unipolar soft-switching modulation technique and SiC converter.

Application of Modified Biogeography Based Optimization in AGC of an Interconnected Multi-Unit Multi-Source AC-DC Linked Power System

An attempt has been made for the effective application of biogeography based optimization and its modified version to solve load frequency control (LFC) problem. Two-area interconnected multi-unit multi-source power system having thermal, hydro and gas power plant without and with AC-DC link is considered for study. Proportional-integral-derivative controller is used as secondary controller in LFC system and its gains are tuned by proposed algorithms through minimization of integral time absolute error based objective function. The results confirm the effectiveness of proposed algorithms after comparing results with other evolutionary algorithms like differential evolution (DE), teaching learning based optimization (TLBO) for the similar test system. The robustness of proposed algorithm is checked with different objective functions like integral square error, integral absolute error, integral time square error criterions and under different loading conditions. Critical analysis of results reveals that proposed method gives better performance than that obtained with DE, TLBO.

Bidirectional three‐phase high‐frequency ac link dc–ac converter used for energy storage

In this study, a bidirectional three-phase high-frequency ac link dc–ac converter is proposed for energy storage with low dc voltage. The operation of the converter changes with the direction of power flow. The proposed topology has the following advantages: high voltage conversion gain, high power factor, high conversion efficiency and high power density. The operation principle of the general control strategy, mode analysis and gate signals generation are discussed. The proposed architecture and the general control strategy have been validated by experimental results of a 300 W laboratory prototype using supercapacitors as the storage device.

風力発電装置の系統連系におけるACリンクとDCリンクの一考察

In this paper, grid-connection of wind power generators was evaluated from the viewpoint of frequency fluctuation. Wind power generation is a power generation method that depends on natural energy, and there is some concern that it may exert a negative influence on electric power quality. As a result, it is necessary to maintain high electric power quality when wind power generation is connected to the grid. The AC link method, the AC-DC link method, and the DC link method are alternatives for grid connection of wind power generators. The model system was constructed with the use of a synchronous generator and an induction generator as wind power generator, and verification experiments were performed. Verification experiments using the various grid-connection methods for each generator were conducted. The steadiest frequency characteristic was obtained in the DC link method. In particular, the stability level was highest in the wind power generation system using the DC link method with a wound-rotor induction generator. Generally, induction generators are grid-connected by the AC link method. Therefore, grid-connection of induction generators by the DC link method appears effective from the viewpoint of frequency fluctuation. © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 178(3): 21–30, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21196

SOFT SWITCHING TECHNIQUES TO IMPROVE EFFICIENCY OF A NONRESONANT HF LINK DC–AC CONVERTER

This paper presents some new ideas on high frequency (HF) link DC-DC or DC-AC converters. Simulation and experimental results will be presented in the case of the design of a DC-AC converter for driving wheel motors for an electric vehicle. The purpose of such an application is to boost energy stored in low-voltage batteries (or other energy storage system like fuel cells) to high-AC voltage output to feed permanent magnet brushless DC motors. In this direct conversion structure, transistors (IGBTs and MOSFETs) operate under soft switching conditions that is to say that this converter will present very low losses. Another advantage will be discussed with the HF link bus that permits the adaptation of voltage levels thanks to new planar technology for transformers. For the experimental test bench an 18kW DC-AC HF (20kHz) link converter has been designed to feed 400V brushless wheel motor with a low voltage simulated battery source of about 67V.

Operation analysis of quasi-resonant high-frequency transformer link DC-AC converter applied for phase-shift PWM control in secondary side

In recent years, the soft-switching techniques have attracted attention for its peculiar advantages such as low swtching loss, high-power density, EMI/RFI noise reduction and so on. Authors have already reported before about quasiresonant DC-DC converter using new phase-shift PWM control scheme. By using proposed control scheme, circulating current is elimineted and ZVS (Zero Voltage Switchig) is achieved with small commutating current. As a result, the conduction losses caused by their currents are substantially reduced.In this paper, authors apply a proposed control scheme to a quasi-resonant high-frequency transformer link DC-AC converter. As a result, all switching devices in this DC-AC converter can achieve soft-switching with small commutating current irrespectively of inverter mode and rectifier mode. Its operating principle and unique features are described as compared with the symmetrical control scheme of DC-ACconverter. Operating performance of this DC-AC converter in steady-state are illutrated by means of simulating results.

Steady-state analysis of a self-excited induction generator with an AC-DC conversion scheme for small-scale generation

In the present paper, the steady-state analysis of a three-phase self-excited induction generator (SEIG) having discrete switching capacitors and an AC-DC link is investigated. A model of the whole system is developed to accommodate the various combinations of converters in the AC-DC link. A computer algorithm for predicting the system performance is developed. The number of switching capacitors required for loading the machine from no load to full load is computed. A discrete capacitor switching scheme for the generator to feed the isolated load directly while maintaining its terminal voltage within certain limits from no load to full load is investigated. The effect of speed on the excitation requirement of the machine is also studied.

Stabilization A High Voltage ACDC Power Systems I. Evaluation Of Control Strategies

Various optimal and suboptimal stabilizing signals for a single machine infinite bus system connected through an ACDC link have been investigated. The controls in order of effectiveness of transient control are the linear state regulator control, a quasioptimal proportional control, output regulator controls from terminal voltage and frequency, and frequency and acceleration. From implementation viewpoint the proportional control is preferred. This scheme can also be easily extended to Include the nonlinearities in the system dynamics.

Equivalent Circuit of AC-DC Link During Unbalanced Fault and Its Analytical Method

Equivalent Circuit of AC-DC Link During Unbalanced Fault and Its Analytical Method