Voltage Side Research Articles

Analysis and Control of DC Side Voltage Fluctuation Characteristics under Unbalanced Fault of AC Side in AC/DC Distribution Network

Analysis and Control of DC Side Voltage Fluctuation Characteristics under Unbalanced Fault of AC Side in AC/DC Distribution Network

Research on dynamic voltage restorer based on self-coupled proportional–integral controller

Aiming at dynamic voltage restorer (DVR) needs real-time detection of voltage sag and fast compensation. However, the traditional proportional–integral (PI) control of DVR is difficult to take into account the fast following, immunity, and robustness of the system. This paper presents a DVR based on a self-coupled PI (SC-PI) controller. Because it contains a differential to a given voltage signal, it improves the speediness of the system. This control method defines unknown, uncertain dynamic, and external disturbance as a total disturbance d. The disturbance rejection and robustness of the system are enhanced by the cooperative control of output and given deviation through the controller speed factor. Through the cooperative control of the output and the given deviation by the controller speed factor, the immunity and robustness of the system are enhanced, and an adaptive speed factor model is introduced to avoid overshoot and oscillation due to integral saturation in the initial stage of dynamic response. The system’s stability is proved, and the effect of voltage sag compensation is analyzed by simulation. The response time of DVR based on SC-PI is 4 times faster than that of traditional PI, the voltage variation is 2.6923 times lower than that of traditional PI, and the harmonic distortion rate of load side voltage is 8.8333 times lower than that of traditional PI control. The results show that the DVR controlled by SC-PI can simultaneously take into account the fast following, immunity, and robustness of the system, proving the scheme’s feasibility and effectiveness.

Grid Integration of Renewable Energy Generating System Using Nonlinear Harmonic Observer Under Nonideal Distribution System

The renewable energy sources are intermittent in nature with high availability-based uncertainties. These resources when integrated to the utility grid significantly hinder the grid power quality (PQ). This article deals with PQ issues arising in the topology including the grid connected renewable energy generating system, which is a combination of wind turbine (WT) driving the synchronous generator (SG) and solar photovoltaic (PV) array and control strategies to improve the same. The solar PV array is associated at the dc link eliminating the dc–dc converter stage via a common dc link. The variable revolutions per minute (RPM) operation is achieved by the back to back connected grid side VSC (GVSC) and the machine side voltage source converter (MVSC). The GVSC is controlled by using the nonlinear harmonic observer (NHO) control. It provides robust implementation at various perturbations faced practically, i.e., discontinuous jump of frequency, phase, and amplitude. The fundamental load current component is effectively extracted. The NHO control is deployed within the framework to allow the system to dynamically adapt to the changes in the operating environment while maintaining the PQ of the utility grid. A special attention is paid to the condition when the power generated by the solar PV array is zero (during night operation) and the GVSC acts in the distribution static compensator (DSTATCOM) mode fulfilling the reactive power requirement. The regulation of MVSC is achieved by the vector control. Independent optimal power extraction techniques are featured for both wind and solar PV array. Test conditions are realized on the laboratory developed prototype and test results are acquired for PQ, loss of supply from solar PV array, effects of randomness of the wind speed, and solar insolation along with load alterations.

An efficient hybrid tunicate swarm algorithm and radial basis function searching technique for maximum power point tracking in wind energy conversion system

Purpose The purpose of this paper is to track the maximal power of wind energy conversion system (WECS) and enhance the search capability for WECS maximum power point tracking (MPPT). Design/methodology/approach The hybrid technique is the combination of tunicate swarm algorithm (TSA) and radial basis function neural network. Findings TSA gets input parameters from the rectifier outputs such as rectifier direct current (DC) voltage, DC current and time. From the input parameters, it enhances the reduced fault power of rectifier and generates training data set based on the MPPT conditions. The training data set is used in radial basis function. During the execution time, it produces the rectifier reference DC side voltage that is converted to control pulses of inverter switches. Originality/value Finally, the proposed method is executed in MATLAB/Simulink site, and the performance is compared with different existing methods like particle swarm optimization algorithm and hill climb searching technique. Then the output illustrates the performance of the proposed method and confirms its capability to solve issues.

Power Sharing Control in a Grid-Tied DC Microgrid: Controller Hardware in the Loop Validation

This article presents the development of a low-cost control hardware in the loop platform for the validation and analysis of controllers used for the management of power sharing between the main grid and a DC microgrid. The platform is made up of two parts: a main grid interconnection system emulator (MGISE) and a controller under test (CUT). The MGISE operates on a 260 V DC bus and includes a 1000 W photovoltaic array, a DC variable load and a single H full bridge converter (HFBC). The CUT includes a phase locked loop and a main cascade control structure composed of two PI controllers. Both the MGISE and the CUT were embedded on an NI myRIO-1900 development board and programmed using LabVIEW virtual instrumentation software. These devices communicate with each other using analog signals representing the AC side current, the DC side voltage, and the HFBC control signal. The MGISE operates with an integration time of 6 µs and its performance is validated by comparing it with a simulation in PSIM. The integration time of the MGISE, the development boards used, as well as its programming environment, and the results obtained from the comparison with PSIM simulation, show that the proposed platform is useful for the validation of controllers for power sharing, with a simple implementation process compared to other hardware description methods and with a low-cost platform.

Cascaded Multilevel Rectifier with Common High Voltage Direct Current (HVDC) Bus and Its Control Method

In order to realize the voltage equalization and minimize the total harmonic distortion rate of current of cascaded multilevel rectifier with common high voltage Direct Current (HVDC) bus, the cascaded multilevel rectifier with common high voltage DC bus and its control method are studied. The output side DC voltage of the cascade multilevel rectifier with common high voltage DC bus is sampled, and the output DC current signal amplitude is obtained according to the calculated output side DC voltage signal. On this basis, the AC side voltage corresponding to the applied switching state and the switching state corresponding to the rectifier in the sampling period are solved. The driving signal of the switch device is generated and sent to the corresponding switch device to realize the active power factor correction and sinusoidal input current, so as to realize the balanced control of output DC capacitor voltage of cascaded multilevel rectifier with common High-Voltage Direct Current (HVDC) bus. The test results show that this method can effectively control the three-phase output current of the rectifier and ensure the maximum output of the current. The lowest total harmonic distortion rate of the output phase voltage is about 40%, which can effectively ensure the stability of the output phase voltage after control.

Control Characteristic Analysis and Coordinated Strategy Design for Hybrid HVDC With Multi-Infeed MMC Inverters

The hybrid HVDC with cascaded multi-infeed MMC inverters is proposed in recent years and it will be put into reality in near future. But the control characteristic of this hybrid HVDC have not been discussed in details. This paper investigates the control strategy characteristic of the hybrid HVDC using cascaded multi-infeed MMC inverters. By combining the UI curves (the DC voltage and DC current curves) of LCC rectifiers, LCC inverter and MMC inverters, the complete UI curves of the hybrid HVDC are obtained in different control modes. Through the complete UI curve, the characteristic of the control strategy is investigated in different operation situations. It is found that the MMC inverter in DC voltage control could shift to the rectifying mode if the fault occurs or active power orders are intensively changed. To solve such problem, the coordinated control strategy based on the dynamic limiter, diodes and LCC-MMC active orders is proposed in this paper, which can not only prevent the MMC inverter from becoming a rectifier at DC side but also can improve the AC side voltage stability. Nevertheless, the proposed strategy can further endow the MMC with fault ride through abilities. The simulations in PSCAD software verify the correctness and effectiveness of the proposed control strategy.

Analysis and design of four-plate capacitive wireless power transfer system for undersea applications

This paper presents a four-plate undersea capacitive wireless power transfer (CPT) system for underwater applications such as autonomous underwater vehicles (AUVs). Generally, a CPT system transfers the power based on electric fields. The complex resonant compensation networks are used to make the CPT system work in the resonant condition. The resonant voltage is always very high. It will be a big challenge to the human safety. In this paper, a virtual electrons periodic reciprocating flow theory is proposed for the CPT system. In one switching cycle, the electrons firstly flow in the forward direction through the forward path and then flow in the inverse direction through the inverse path. The CPT system has been deeply studied with the vacuum dielectric or the air dielectric. However, for the CPT system, there are few papers to show the underwater application. In this paper, an undersea four-plate CPT system is designed and studied in the underwater condition. The two coupling capacitors and other elements of the CPT system could build a closed-loop path. A small value inductor is adapted as a resonant compensation network for the four-plate CPT system. The DC voltage is inverted to the AC voltage in the primary side with the single-phase full-bridge inverter. The resonant voltage is rectified to the DC voltage in the secondary side with the single-phase full-bridge diode rectifier. A 100 W power level CPT system is constructed to verify the theory analysis and the calculation. The theory analysis is verified by the simulated and experimental results. The stable output voltage and load power are achieved in this paper.

Generating lithium fluoride-abundant interphase on layered lithium-rich oxide cathode with lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide

Layered Li-rich oxides (LLROs) attract much attention due to their high capacities. However, the electrolyte decomposition and interfacial reactions especially driven by the catalysis effect of high-valance transition metal ions under high voltage cause severe performance recession, hindering their practical applications. Lithium fluoride (LiF) is an effective component in both cathode electrolyte interphase (CEI) and solid electrolyte interphase (SEI) films. In this work, lithium 1,1,2,2,3,3-hexafluoropropane-1,3-disulfonimide (LiHFDF) is employed as a novel electrolyte additive for generating LiF-abundant interphase on LLRO. With the addition of LiHFDF, the capacity retention and Coulombic efficiency of the LLRO are improved noticeably. In addition, the interfacial side reactions and voltage decay issues are mitigated. Transmission electron microscopy and X-ray photoelectron spectroscopy measurements confirm that a compact and thin CEI film with more LiF is formed on the cathode surface for the cell using LiHFDF, which is beneficial for the enhancement of the electrochemical performance. Moreover, differential scanning calorimetry (DSC) investigations reveal that the LLRO electrode with LiHFDF has improved safety compared to that with the blank electrolyte. This work provides an effective LiF-generating electrolyte additive for high-voltage LLROs, which is believed to be positive for other high-voltage cathode materials.

Impedance Modeling and Stability Analysis of MMC Flexible DC System at the DC Side

Aiming at the stability of modular multi-level converter (MMC) on the dc side, this paper adopts harmonic linearization method to establish impedance model of MMC dc side. This method ignores the higher-order components of voltage and current, and considers the influence of the control loops to guarantee high accuracy at the same time. The high-frequency band of impedance presents inductive characteristics, and the mid- and the low-frequency band is related to the control loop. The parameters of the current loop have a greater influence on the impedance characteristics of the mid-frequency band while the voltage loop parameters have a greater influence on the low-frequency band. For the back-to-back transmission system based on MMC, the voltage at the dc side has obvious oscillation when the impedance mismatch or the stability margin is small. The risk of dc side voltage instability can be eliminated by optimizing the controller parameters and phase angle margin can be improved.

Comprehensive Modeling and Control of Grid-Connected Hybrid Energy Sources Using MPPT Controller

Nuclear and renewables energies are the two variants for low-carbon energy and the evolving grid suggests possible synergies between them. Nuclear energy introduces supple operations based on power demand, while renewables such as PV and wind hybrid systems depend on the presence and strength of sunlight or wind. For grid stability, there is need to improve their performance in order to overcome the impact of this disadvantage. The paper is a step in this direction as it addresses a detailed comprehensive dynamic modeling and an efficient control of grid-connected energy sources such as PV or wind system to increase system reliability and to ensure the power quality and safe operation of critical demands. The behavior of the suggested hybrid system is tested at different climate circumstances such as variation of sun radiation and wind speed. The PV is equipped with a boost converter and a three-phase pulse width modulation (PWM) inverter. The wind energy comprises a doubly fed generator (DFIG) based on a variable-speed wind turbine. The two controllers’ rotor-side and grid-side converters of DFIG have the ability to generate and observe reactive power, to keep constant speed of the rotor and control the DC-link voltage. The proposed scheme was investigated using MATLAB software. The maximum power point tracking (MPPT) was used for two systems, PV and wind, in varying weather conditions. The simulation results prove that the voltage at the point of common coupling was constant. Furthermore, the injected current of the grid side was in sinusoidal form and was synchronized with grid side voltage. In addition, the injected power-to-utility grid was around power delivered by the hybrid PV and wind system.

Control of linear generator based on hysteresis‐SVPWM current rectification and bidirectional buck/boost converter used for energy storage

The free-piston Stirling linear generation system has tremendous applications in space power supply, biomass generation, solar thermal generation, factory waste heat generation, and other industrial fields. However, the free-piston Stirling linear generation system suffered from the disadvantages of fluctuation of output electric energy and self-starting. To solve the aforesaid problems, a two-level electric energy conversion control mode which combines three-phase AC/DC and bidirectional buck/boost DC to DC converter is proposed in this paper to make the generation system with stable output DC power. Then, both the linear and non-linear mathematical model of the free-piston Stirling engine are established. Additionally, to improve the performance of frequency and displacement tracking, the traditional proportional-integral (PI) in the position loop is replaced by Proportional Resonance (PR) controller. Moreover, the Hysteresis-SVPWM current control strategy with high robustness and dynamic response is applied to the rectifier to realize the unit power factor control of the AC side of the generator and the stability and controllability of the DC side voltage. Finally, the three-phase voltage Source Rectifier (VSR) and Bi-directional DC/DC with an appropriate inductance and capacitance for energy storage are modelled. The effectiveness of the proposed system is validated using theoretical analysis and simulations in Matlab.

Modular DC‐DC auto‐transformer: Topology, operation, and system design

Here, a modular DC-DC auto-transformer is proposed with a developed scheme of two-port submodules. The proposed two-port submodule is composed of six switches and single capacitor, and it is featured with three general working states. In the proposed DC-DC auto-transformer, all two-port submodules are connected in the input-series-output-series structure. Then the general primary and secondary ports of the submodules of the proposed DC-DC auto-transformer are connected in series to achieve the auto-transformer operation capability. All submodule capacitors are actively connected in the transformer to form the primary and secondary side voltages. The capacitor utilization is increased therefore. Different voltage ratios can be achieved by arranging different submodule numbers in each working state simultaneously. The volume is reduced due to the developed two-port submodule which eliminates of AC transformers. Detailed topology, working principles, modulation method, and control strategy are provided in the paper. Voltage and current fluctuations are analysed, and the system parameter design method is proposed in detail. Both a simulation system with 40 submodules and a down-scaled experimental system with three submodules were set up and verified the proposed approach.

Stability Analysis of AC/DC Microgrids in Island Mode

This study aims to introduce a new structure based on a nonlinear controller for controlling and analyzing the stability of the microgrids. In the proposed model, AC and DC resources and loads are located on two different sides. In addition, an AC/DC bidirectional interface converter is applied to supply loads by AC/DC sources. There are AC/DC products on both sides of the converter and each side can supply the load of the other side via a bidirectional interface converter and its load. Alternatively, an energy storage system is used for the system stability on the DC side. The nonlinear microgrid controller is designed to adjust the AC bus side frequency and the DC bus side voltage properly. In this structure, the coordinated optimal power exchange and precise regulation of control signals lead to constant improvement. Thus, system performance is improved. The results show that the proposed model is efficient for both reduction of the fluctuations and improvement of the system stability

Research on DC side power decoupling control of photovoltaic inverters

Inverter is a vital component in photovoltaic power generation system, and it is related to the performance and efficiency of photovoltaic power generation. When the inverter is connected to the grid, the instantaneous power on the DC side and the AC side is unbalanced, and the instantaneous power pulsation of double frequency will be generated on the DC side, which directly affects the current stability and power utilization rate of the inverter on the DC side. This paper proposes a Power Decoupling Circuit (PDC) based on a single-phase photovoltaic inverter. This circuit uses a closed-loop feedforward power decoupling control strategy to compensate for the unbalanced pulsating power on both sides of the inverter. Eliminate low-frequency harmonics on the DC side, achieve the purpose of power decoupling, stabilize the DC side voltage of the photovoltaic inverter, and improve the performance and efficiency of photovoltaic power generation.

Active power improvement of three-phase grid-connected inverter under unbalanced grid faults

Unbalanced grid voltage causes fault-phase over-current, affects the life span of devices and even leads to control system instability. The conventional current limiting strategy reduces the output power of grid-connected inverter. In order to increase the active power injected by the grid-connected inverter into the grid, this paper proposes a new current limiting strategy. On the premise of ensuring that the DC side voltage fluctuation and grid-connected current harmonics meet the grid standards, the fault-phase current and the normal phase current are respectively limited. The simulation results verify the proposed strategy can effectively improves the active power output of the grid-connected inverter.

Power balance modulation strategy for hybrid cascaded H-bridge multi-level inverter

Under the traditional hybrid modulation strategy, the output powers of the hybrid cascaded H-bridge inverter are not balanced between the cascaded units. With a hybrid cascaded H-bridge 13-level inverter with a DC side voltage ratio of 1:1:1:3 (referred to as III-inverter in the paper) as the research object, a hybrid modulation power equalization strategy based on carrier rotation is proposed. Under the control of which, the output voltage of inverters is formed by superposition of multiple basic voltage waveforms. Firstly, the hybrid modulation strategy is adopted to modulate the inverter so that the high- and low-voltage units, respectively, operate in the low-frequency and high-frequency states. Then, the pulse sequence within the 3/2 output voltage cycle is made to circulate among all the cascaded units by carrier rotation. Finally, the output voltage of each cascaded units is made to contain all the basic voltage waveforms, thus implementing the output power balance between all the cascaded units. Based on simulation and experiment, the effectiveness of the strategy is verified, and it is shown that under the control of this power balance modulation strategy, the output voltage waveform quality of the inverter is high, there is no current backflow, and the output power can be balanced within 3/2 cycles.

Electrostatic levitation: an elegant method to control MEMS switching operation

This paper investigates the characteristics of a micro-switch that uses two side electrodes to open a normally closed switch. The side electrodes surround the fixed electrode in the well-known gap-closing electrode configuration. The side electrodes can open a closed switch and be tuned to respond appropriately to outside forces. The combined electrode system dramatically improves the control of a standard gap-closing electrode configuration. In conventional switches, a DC voltage above a certain value closes the switch. To reopen the switch, the voltage difference is reduced to peel off the moving electrode. Currently, the contact area is carefully designed to avoid stiction, but the degradation over time and stiction forces can cause a permanent failure. In this work, opening occurs by feeding the side electrodes a voltage beyond a certain value that results in a levitation force. Even if the degradation in the surfaces happens, the switch can open by increasing the side voltages. The characteristics of the combined actuation system are thoroughly analyzed and include the static pull-in, static displacement, release voltage, dynamic pull-in, frequency response, and basins of attraction. The results are validated by the experimental tests. The levitation-based micro-switch improves the system tunability as the sensitivity and switching thresholds can be adjusted.

Analysis on unbalanced compensation of star - connected Cascade H - bridge inverter

In unbalanced compensation current situation, it is necessary to take appropriate control strategies to ensure that the star connection cascade static reactive power generator of the dc side voltage balance, the commonly used methods for injecting zero sequence voltage, this paper introduce current unbalance degree to analyze the compensation ability of plant, gives the scope of compensation is derived, by means of simulation demonstrate the validity of the derived, for the star connection series H bridge the application and selection of multilevel SVG has the significance.

Control parameters optimization of three-level neutral-point clamp rectifier for EAST low-frequency resonance suppressor

The low-frequency resonance suppressor effectively reduce the low-order harmonics and resonance amplification generated by the EAST poloidal field power system during the operation. This article takes the control parameters of the three-level neutral-point clamp rectifier in the low-frequency resonance suppression device as the research object to reduce the steady-state delay of the system and to ensure the stability of the DC side voltage balance. The system model of the three-level midpoint clamp rectifier in dq rotating coordinate system is constructed to study the effect of various control parameters on the stability of the rectifier system. Furthermore, combined with variation regularity of system eigenvalue, the control parameters of the rectifier are optimized, and the anti-interference ability, dynamic performance and robustness of rectifier system are improved. Finally, the correctness and effectiveness of the control parameters optimization are verified through RT-LAB simulation.