Fixed DC Bus Research Articles

Smooth control strategy for emergency switching of multi-port flexible interconnected distribution system modes

IntroductionWith the rise of distributed energy resources, the interconnection of distribution networks and Flexible Multi-State Switch (FMSS) has become a key technology in the construction of new distribution networks. FMSS plays a significant role in enhancing the reliability and flexibility of the system.MethodsThis paper investigates the impact of mode-switching in FMSSs on voltage shocks, current shocks, and power fluctuations in the event of a feeder fault in a multi-port flexible interconnected distribution system. Firstly, an improved state-tracking control method is proposed to effectively mitigate these impacts. Secondly, for feeder faults connected to the fixed DC bus voltage (Udc-Q) control port, a reselection method for the Udc-Q control port at the main station is introduced, aiming to select the optimal port to maintain the stability of the DC bus voltage.ResultsSimulation experiments have validated the effectiveness of the proposed control method in reducing voltage and current shocks as well as power fluctuations. Additionally, the proposed control strategy has demonstrated an enhancement in the safe and stable operation of the system under emergency fault conditions.DiscussionThe control strategy presented in this paper is capable of addressing the challenges posed by feeder faults and ensuring the stability of the DC bus voltage and reliable power supply to feeder loads, thereby enhancing the overall performance and reliability of the flexible interconnected distribution system. The research findings have been verified on the MATLAB/Simulink simulation platform.

Asymmetrical Pulse-Width Modulation Strategy for Current-Fed Dual Active Bridge Bidirectional Isolated Converter Applied to Energy Storage Systems

This paper proposes an asymmetrical pulse-width modulation (PWM) strategy for current-fed dual-active bridge (CFDAB) converters applied to energy storage systems (ESS). The ESS application considers low-voltage and high-capacity batteries, for low-power applications, such as data centers, residential photovoltaic systems (PV), and uninterruptable power supplies (UPS). The proposed modulation permits the use of an isolation transformer with negligible leakage inductance and, therefore, avoids the use of auxiliary circuits such as snubbers, active-clamp, or resonant cells. Hence, the converter implementation is simplified. The modulation also benefits the design of the control system because the converter can be modeled and controlled using simple strategies. A straightforward, large-signal model for the battery charge mode, which is valid over all the operation range of the converter, is obtained. Also, the converter operates with a fixed dc bus voltage on both charge and discharge modes. These characteristics represent a significant advantage when the CFDAB with PWM modulation is compared with phase-shifted or frequency modulations, commonly applied in these converters.

Constant power start-up control strategy for modular multilevel converter

HVDC system based on modular multilevel converter(MMC) usually adopts fixed DC bus voltage to realize the start-up control of converter, which will have a great impact on AC network during charging process. This paper presents a constant power start-up control strategy based on MMC arm current control, which can effectively control active power, reactive power, output current and submodule(SM) capacitor voltage. Based on the existing two-stage start-up control mode, according to the output characteristics of arm voltage in controlled charging process, the circuit topology of DC side open circuit of converter is proposed, and the mathematical model is established. Based on the derived instantaneous arm power calculation method and the average sliding filter of SM capacitor voltage, the double closed-loop fast response controller of fixed AC power in controlled precharging stage is given. The safety of the power electronic equipment in the converter is effectively guaranteed and the impact of converter on ac power grid is greatly reduced. Finally, simulation models of conventional start-up method and proposed start-up method are built on MATLAB/Simulink platform to verify the validity and correctness of the proposed start-up control strategy.

Maximum Efficiency Tracking of an Integrated Two-Staged AC–DC Converter Using Variable DC-Link Voltage

This paper presents an approach to achieve maximum conversion efficiency of a two-staged-cascaded ac–dc converter. In this study, the integrated ac–dc converter consists of a three-phase boost-type power factor correction rectifier and a phase-shifted full-bridge dc–dc converter. The detailed loss modeling of the integrated stage implies that the total power loss varies in a nonmonotonic fashion with intermediate dc-link voltage. In order to ensure maximum conversion efficiency, an optimum dc-link voltage level can be set depending on the load power level. A comparison between the efficiency variation under two different cases is carried out with extensive mathematical analyses: 1) fixed dc bus and 2) variable dc-bus voltage control. In addition, the experimental results obtained from a 6-kW-integrated ac–dc hardware prototype show a unity power factor along with a peak efficiency of 97.2%, which is 2.5% more than the fixed dc-link control approach.

Integrated Control of an IPM Motor Drive and a Novel Hybrid Energy Storage System for Electric Vehicles

A dc bus voltage control for a motor drive that uses a hybrid energy storage system (HESS) for electric vehicle applications is proposed in this paper. The HESS is composed of a high energy density battery pack along with a high power density ultracapacitor. The motor drive of an interior permanent magnet (IPM) motor is conventionally designed based on a fixed dc bus voltage. However, with the voltage controllability of the HESS, it is possible to enhance the system efficiency by selecting the optimum dc bus voltage. Unlike conventional designs, the proposed HESS processes only a portion of the vehicle power through an interfacing dc/dc converter. This new concept reduces converter losses, enables full usage of the energy stored in the storage devices, and provides a boosting capability of the dc bus voltage. Consequently, the motor drive operates with maximum torque per ampere control for an extended operating range. An electric vehicle, equipped with a HESS and an IPM drive is simulated under different driving conditions. Experimental results of a prototype system are provided to verify the system effectiveness.

Application of cascaded H-bridge distribution-static synchronous series compensator in electrical distribution system power flow control

In this study, the power flow control of the electrical distribution network using multilevel converter-based distribution-static synchronous series compensator is presented. The conventional radial distribution network can be changed into a meshed grid because of this device. Using multilevel converter topology, the device can be installed directly in series with the line, connecting two separate feeders from different substations together. Two control methods are proposed to control the power flow in the line. The first control strategy uses a fixed DC bus voltage. Enhancing the performance of the multilevel converter, a novel control strategy using variable DC bus voltage is introduced while maintaining the appropriate dynamic response. The new strategy can reduce the total harmonic distortion (THD) of the injected voltage and losses of the power converter compared with the fixed DC bus method. A new soft start method is also proposed to connect the separate feeders together safely. The feasibility of the control strategies is investigated through simulation and experimental results.

Control of grid inverter based on opposite vectors hodographs

Purpose – This paper presents control strategy of VSC connected to the unbalanced grid in stationary coordinates system. The algorithm shown in the paper can be applied to typical inverter to reduce or even eliminate oscillation of DC‐link voltage under unbalanced operation. That has a direct influence on constant flow of p component of power which is essential for drives system for reducing torque ripples.Design/methodology/approach – The presented strategy of control assumes that orientation of grid voltage and line current vectors hodographs have to be opposite. Mathematical analysis and simulation has been done. Laboratory tests on low power has also been carried out.Findings – This method leads to fixed p component of power flow and fixed DC bus voltage. The presented idea of control may cause reduction of voltages asymmetry in three‐phase network with significant impedance (local grid, microgrid, isolated grid).Research limitations/implications – For 50 Hz power system, the presented control strateg...

Harmonic Performance Analysis of an OPWM-Controlled STATCOM in Network Applications

The method of selective harmonic elimination is often used as an efficient means for obtaining harmonic-reduced waveforms from voltage sourced converters (VSC). However, the switching angles required for its implementation are usually pre-calculated on the basis of a fixed dc bus voltage. In practice, there are harmonics present in the dc source because of the finite dc bus capacitance and other realistic operating conditions, which can potentially introduce additional noncharacteristic harmonics. This paper examines the harmonic performance of a VSC-based static synchronous compensator as a function of design and operating parameters. These include the value of dc capacitor, the strength of the connected ac network and ac system unbalance. The results are generated using theoretical analysis for special cases and numerical simulation using electromagnetic transients simulation for the more general case.