Conventional Voltage Control Research Articles

Clustered Voltage Control and Modulation Scheme With Switching Loss Reduction for High Power Hybrid Cascaded Converter

The hybrid cascade converters (HCC) is a potential solution to achieve harmonic current compensation in medium and high voltage applications due to the characteristics of having high frequency module. However, the high switching frequency inevitably leads to an increase of undesirable switching loss. In addition, since the reduction of power loss and the balance of clustered voltage are mutually interrelated, the existing modulation and control schemes cannot cope with them decently. To address this problem, the HCC is divided into a high-frequency module (HFM) and a low-frequency module (LFM), and a corresponding hybrid modulation scheme is proposed to reduce the power loss while maintaining a high harmonic compensation performance. Then, based on the analysis of the system voltage constraints, the dc-link voltage and threshold voltage under different parameters are redesigned. Furthermore, since the power flow between modules has been changed with the hybrid modulation scheme, the conventional clustered voltage control strategy is not capable of clustered control. Therefore, based on the re-modeling of the power flow, a negative-sequence voltage injection control strategy is proposed to achieve voltage balance. Finally, simulation and experimental results are demonstrated to verify the effectiveness of the proposed scheme.

A Second-Order Sliding Mode Voltage Controller with Fast Convergence for a Permanent Magnet Synchronous Generator System

This paper studies an improved super-twisting sliding mode controller (IST-SMC) for the permanent magnet synchronous generator (PMSG) voltage loop to improve the anti-disturbance capability of the system. Compared to conventional voltage controllers, the control algorithm provides advantages in terms of system resistance to load disturbances. Conventional voltage controllers have significant voltage fluctuations and long recovery times during sudden load changes. To solve this problem, a voltage loop controller based on a super-twisting sliding mode (ST-SMC) is designed to enhance the immunity of the system. Also, the ST-SMC was improved to further increase the convergence rate of the system and enhance the dynamic performance. The convergence of the system away from the balance point is accelerated by introducing an exponential term, which in turn provides an improvement in the dynamic performance of the system. The effectiveness of the proposed control scheme was verified on a PMSG.

Coupled-driven high-speed and high-torque switchable transmission with a large transmission ratio

Electric motors are used globally, especially in industrial applications, and achieving high energy efficiency is a major problem. Variable transmissions are effective in reducing the energy consumption of motors, but practical variable transmissions are bulky and heavy, making them unsuitable for robots. To overcome this problem, two motor-driven mechanisms have been proposed. The two motors are operated independently and assigned to the high-speed drive and high-torque drive, and one motor always becomes dead weight. Therefore, we propose a coupled-driven switchable transmission system that can switch the high-speed and high-torque drives by combining the rotation directions and utilizing the output of both motors. The developed device uses two 22-W motors and can switch the reduction ratio from 1/15 to 1/375. The maximum torque, maximum rotation speed, and weight are 10 Nm, 500 rpm, and 905 g, respectively. The experimental results show that the relative speeds of two motors are significant for the coupled drive; nevertheless, this device can be controlled by conventional voltage control without precise speed control.

Accurate consensus-based distributed secondary control with tolerance of communication delays for DC microgrids

Distributed secondary control is widely used in DC microgrids (MGs), which can achieve average voltage recovery and accurate power/current sharing among the distributed generators (DGs). In the distributed control architecture, a consensus-based average observer is usually desired to estimate the global average of the output voltage. However, the inevitable communication delay between DGs will cause the output average voltage to deviate from the rated. To tackle this problem, a novel dynamic average consensus observer based on bias compensation has been proposed in this paper, which can accurately converge to the average of the variables being tracked with tolerance of the bounded communication delay while getting rid of the harsh requirement for identical initial value of conventional distributed observer. Based on it, the proposed distributed secondary controller can ensure that the average output voltage converges to the rated without steady-state deviation and that the output currents are proportionally shared accurately. Instead of the conventional voltage and current dual secondary controller, the proposed secondary controller estimates the average of the voltage-current coupling variables and achieves the average voltage recovery and accurate current sharing simultaneously. Moreover, the communication delay margin is determined quantitatively. Finally, the performance of the proposed controller and the efficacy of the calculated communication delay margin is verified by real-time hardware-in-loop (HIL) tests.

Performance evaluation of isolated three-phase voltage source inverter with LC filter adopting different MPC methods under various types of load

As the variety of loads in microgrids continues to grow rapidly, the drawbacks of using conventional voltage and current double closed-loop PI control are becoming increasingly intolerable. These disadvantages include poor static tracking of the output and the need for complicated tuning of optimal PI control parameters. Model predictive control (MPC) is widely studied in recent years and gradually applied to the inverter as a modern control strategy. Currently, the research on MPC mainly focuses on reducing the computational burden of algorithms and improving computing skills. In this paper, the control theory of continuous-control-set model predictive control (CCS-MPC), optimal switching vector model predictive control (OSV-MPC), quasi-sliding mode model predictive control (QS-MPC) and optimal switching sequence model predictive control (OSS-MPC) are elaborated. Also, the disadvantages of adopting conventional voltage and current double closed-loop PI control on microgrids are clarified. Moreover, aiming at evaluating the influence of connecting and/or removing the common loads, such as electric vehicles to the microgrid on the output performance of microgrids in practical engineering applications, single three-phase voltage source inverter with LC filter system with two-phase pure resistance load, three-phase diode rectifier bridge nonlinear load, constant power load and constant current source load applying four different MPC methods, are simulated. The simulation results show that under different types of load, system with nonlinear load applying OSV-MPC method has the biggest value of total harmonic distortion (THD) of the output voltage and the biggest steady state root mean square (RMS) tracking error of the output voltage, which are 1.33% and 2.1 V, respectively. The performances of the output voltage of the system adopting MPC are generally improved compared to the system using conventional voltage and current double close-loop PI control. Regarding the output voltage performance of a system under load step change, a system using the OSS-MPC method with a nonlinear load exhibits the smallest overshoot of the output voltage and the smallest root mean square value of the tracking error (RMSE) between the output voltage and the reference value within 20ms after the load step change. Specifically, the overshoot and RMSE are 2.804 V and 0.810 V, respectively. The simulation results provide valuable guidance for selecting the appropriate model predictive control strategy for microgrid integrating complex loads, such as electric vehicles in practical applications.

Performance analysis of voltage sensorless based controller for two-stage grid-connected solar PV system

<a name="_Hlk98601430"></a><span lang="EN-US">In this research, a sensor free DC-link voltage controller based multifunctional inverter is proposed for a two-stage three-phase grid-connected solar photovoltaic (PV) system. First, the proposed scheme consists of an outer voltage controller requiring DC-link voltage knowledge, has the ability to manage power-sharing between the PV system and the utility grid, and second, has the ability of the VSI to act as a compensator for harmonics in the grid current polluted by non-linear loads. The suggested active power controlling grid reference current signal scheme replaces the standard controller, requiring the high voltage sensor to sense DC-link voltage, which is pricey, increasing the hardware complexity and reducing reliability. The performance of the proposed active power management approach is comparable to that of a conventional voltage sensor-based voltage controller. The effectiveness of the proposed method for two stage three-phase grid-connected solar PV system under fixed and variable irradiance feeding nonlinear loads is rigorously verified using MATLAB/Simulink software</span><span lang="EN-US">.</span>

Tube-based MPC of hierarchical distribution network for voltage regulation considering uncertainties of renewable energy

• A novel model for low-voltage distribution areas based on genetic algorithm is developed, which simplifies the complexity of online computation and suppresses the inner voltage deviations in a safe section. • The coordination of PVs and ESs at different voltage levels is promoted to improve the voltage regulation while short of the reactive power capacity of PVs. • A tube-based MPC controller is proposed for voltage fluctuation under uncertainties introduced by power disturbance and a genetic-algorithm-based equivalent model of low-voltage areas, with which a distribution network can work in common conditions under variable operations of PVs. With the ever-growing penetration of distributed generators (DGs), voltage deviation caused by randomness and fluctuation has become increasingly prominent in distribution networks. Conventional voltage control cannot sufficiently utilize the rapid and flexible characteristics of DGs. In this paper, a tube-based model predictive control (TMPC) method is proposed to address voltage violation and fluctuation difficulties considering the uncertainties of renewable energy. Firstly, a simplified model approach based on a genetic algorithm (GA) is proposed to unify numerous DGs in low-voltage distribution areas. Then, based on this simplified model combined with the voltage sensitivity model, the TMPC controller is designed for the whole distribution network, which consists of two subsidiary controllers to suppress voltage deviation caused by uncertainties. The former is used to generate a central path for different operating points of the grid, and the latter is designed to track the path in a bound tube. The proposed method can cope with the uncertainties caused by external disturbances such as drastic power fluctuations. Finally, the numerical simulation in the case studies demonstrates the effectiveness of the proposed method in accuracy and efficiency.

Phase-Duty Modulated Loop Decoupling and Design Optimization for a Triple Active Bridge Converter for Light Electric Vehicle Charging

This article presents a comprehensive generalized harmonics approximation based modeling and design of a triple active bridge (TAB) dc–dc converter topology aimed to facilitate simultaneous—main and auxiliary—battery charging solutions for light electric vehicle charging applications. A three-loop control scheme, incorporating the duty and phase control parameters, is proposed in this article to address the issue of inherent coupling of two output bridges and their control loops (as observed in conventional phase modulation-based voltage control techniques), along with an objective to enhance the operational efficiency of the system, in affiliation with a real-time power flow optimization algorithm. Adhering to the objective of achieving minimized losses, the optimization algorithm provides insightful design limitations in terms of selecting the required leakage inductances to ensure desired power flow at the output bridges. Furthermore, to validate the effectiveness of the proposed three-loop control, a detailed set of simulation and experimental results are presented at various steady state and dynamic load change conditions. A detailed analytical loss model of the TAB topology is also presented along with a comparative study elucidating the effectiveness of the proposed three-loop control over the conventional control schemes. The analyses portray a peak efficiency of 96.24% at the rated load of 300 W, which is 2.94% and 4.97% higher than the decoupled and the conventional phase shift control schemes, respectively.

Fast Bus Voltage Control of Single-Phase Grid-Connected Converter With Unified Harmonic Mitigation

This paper presents a comprehensive analysis of the harmonic sources of the single-phase grid-connected voltage source converter (VSC), which leads to an alternative approach for DC bus voltage control of the VSC under grid voltage distortion with a significant switching dead time. A selective current harmonic controller with the zero-reference structure plays a vital role in rejecting the harmonic components in the grid reference current created by the bus voltage control loop and the harmonic components in the grid voltage and in the VSC caused by the switching dead time. Therefore, the bus voltage control can adopt a conventional proportional-integral regulator tuned at a fast bandwidth. The proposed control scheme implemented in the multiple unbalanced synchronous reference frames was validated with a bidirectional 2-kVA VSC under grid voltage distortion and a significant dead time. Furthermore, the proposed control scheme exhibited the transient response and grid current quality superior to the conventional bus voltage control methods with a notch filter and a low-pass filter. The proposed control scheme has inherent frequency adaption.

Multi-timescale voltage control for distribution system based on multi-agent deep reinforcement learning

Nowadays a large number of distributed generators and controllable elements connecting to the distribution network have resulted in higher requirements for voltage control. Existing voltage control methods require a specific physical model and are time-consuming with the complexity increasing. To overcome these challenges, this paper proposes a multi-timescale voltage control scheme using multi-agent deep reinforcement learning (MADRL). Firstly, considering the control requirements of different timescale regulators, a multi-timescale voltage control model is formulated. Then the control variables are assigned to multiple agents, and a MADRL-based voltage control scheme is proposed, which is based on the multi-agent deep deterministic policy gradient (MADDPG) algorithm and applies Gumbel-Softmax distribution to solve discrete actions. Through centralized learning and decentralized execution, this scheme can obtain the optimal coordinated control strategy of multiple regulators adaptively and achieve the effect of distributed control. Finally, case studies are conducted on the modified IEEE-123 bus system, and the performance of the proposed method is compared with the conventional model-based optimal voltage control scheme and other DRL-based methods. The results show that the MADRL-based method possesses better performance than other methods.

Active Disturbance Rejection Control of DC-Bus Voltages Within a High-Speed Aircraft Electric Starter/Generator System

Wide-speed range operation capabilities in both motoring and generation modes are essential for an aircraft electrical starter/generator (ESG) drive system for more-electric aircraft applications. When running as a generator, the ESG is supplying power to the main HVdc bus within the onboard electrical system. A fast and robust dc bus voltage would be beneficial to electrical loads connected to it. In this article, an active disturbance rejection control (ADRC) is proposed to realize a fast and robust regulation of onboard dc bus voltages. The ADRC is achieved with a second-order extended state observer (ESO) to generate torque reference for the ESG. All the key characteristics, such as the stability of ESO, dc voltage tracking performance, and disturbance rejection capability of the ADRC for aircraft ESG applications, are thoroughly analyzed. To enhance system stabilities, an innovative and simple design method for the ADRC and ESO gains is introduced based on the gain-margin and phase-margin criteria. Experimental results have shown that, compared with the conventional PI-based dc voltage control, the proposed method can realize a faster and more robust dc voltage control.

Investigation Into the Adjustable Dynamic Characteristic of the High-Speed / Valve With an Advanced Pulsewidth Modulation Control Algorithm

High-speed <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> / <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> valve (HSV) has been widely concerned in the field of hydraulic engineering, mainly focusing on improving its dynamic characteristic. However, relatively little attention is paid to the adjustment of HSV switching characteristic, which limits the applications of HSVs in some specific systems. Therefore, an advanced pulsewidth modulation (PWM) control algorithm is proposed in this article to change the dynamic behaviors of the experimental HSV and makes it adjustable in a wider scope to adapt to more applications. Applying the voltages of opening preduty ratio and holding duty ratio, the opening initial current and closing initial current can be generated, respectively. Combining with the voltages of opening duty ratio and closing duty ratio, the opening and closing dynamic behaviors can be fully controlled and adjusted in a wide range. The effectiveness of the proposed algorithm is proved by numerical analysis and experimental studies. Results show that the opening delay time can be controlled in a range of 0.2–18.3 ms and the closing delay time can be adjusted in a range of 0.1–6.8 ms, while the opening moving time can be adjusted from 1.9 to 14.1 ms and the closing moving time can be adjusted from 1.8 to 6.1 ms. Consequently, the total opening time and total closing time can be controlled in the ranges of 2.1–32.1 ms and 1.9–12.9 ms, respectively. Compared with the conventional single voltage control strategy, the advance PWM control algorithm is verified to be a promising method to improve or even adjust the dynamic behaviors of HSVs and adapt to more complex working conditions.

Cost-Effective Coordinated Voltage Control in Active Distribution Networks With Photovoltaics and Mobile Energy Storage Systems

High penetration of intermittent renewable energy resources (RES) will impose great challenges on the conventional voltage control in distribution networks. This paper proposes a two-stage cost-effective coordinated voltage control strategy to mitigate fast voltage violations while minimizing the total voltage regulation cost. In the first stage, the optimal power flow (OPF) is performed to determine the day-ahead dispatch results of on-load tap changers (OLTC) and capacitor banks (CBs). The hourly charging schedule of mobile energy storage systems (MESS) is also obtained through a spatial-temporal decision model embedded in the optimization problem. In the second stage, the real-time voltage control strategy is implemented to regulate the power output of photovoltaics (PVs) and MESS with one minute resolution. A novel fine-tuning control method is proposed to adjust the power output, which can further reduce voltage regulation cost within a certain voltage dead band. To reserve more fast-response power resources for PVs and MESS, the voltage regulation cost curves are designed as a piecewise linear function with different per unit cost in each range. The proposed approach is tested on the IEEE 33-bus and 123-bus distribution systems and comparative simulation results show the enhanced control effect in mitigating voltage violations and saving costs.

Deep Reinforcement Learning Enabled Physical-Model-Free Two-Timescale Voltage Control Method for Active Distribution Systems

Active distribution networks are being challenged by frequent and rapid voltage violations due to renewable energy integration. Conventional model-based voltage control methods rely on accurate parameters of the distribution networks, which are difficult to achieve in practice. This paper proposes a novel physical-model-free two-timescale voltage control framework for active distribution systems. To achieve fast control of PV inverters, the whole network is first partitioned into several sub-networks using voltage-reactive power sensitivity. Then, the scheduling of PV inverters in the multiple sub-networks is formulated as Markov games and solved by a multi-agent soft actor-critic (MASAC) algorithm, where each sub-network is modeled as an intelligent agent. All agents are trained in a centralized manner to learn a coordinated strategy while being executed based on only local information for fast response. For the slower time-scale control, OLTCs and switched capacitors are coordinated by a single agent-based SAC algorithm using the global information with considering control behaviors of the inverters. Particularly, the two-level agents are trained concurrently with information exchange according to the reward signal calculated from the data-driven surrogate model. Comparative tests with different benchmark methods on IEEE 33- and 123-bus systems and 342-node low voltage distribution system demonstrate that the proposed method can effectively mitigate the fast voltage violations and achieve systematical coordination of different voltage regulation assets without the knowledge of accurate system model.

Implementation of Power Balance Control Scheme for a Cascaded Matrix-Based Dual-Active-Bridge (CMB-DAB) MVAC-LVDC Converter

Though the merits of matrix-based dual-active-bridge (MB-DAB) ac–dc converter as a submodule for grid-connected multilevel medium-voltage ac (MVac)–low-voltage dc (LVdc) conversion have been presented in recent literature, its operational challenges in the cascaded configuration have not been explored. The objective of this article is to present the inevitable challenge of unequal power routing through the submodules of cascaded matrix-based dual-active-bridge (CMB-DAB) and its mitigation. The detailed and simplified small-signal model of the CMB-DAB converter along with the proposed power balance control (PBC) strategy required to eliminate the power imbalance issue are explained comprehensively. The simulation results for a 3-cell 52.65-kVA CMB-DAB converter along with scaled-down 5.85 kVA experimental results are presented, to demonstrate the unavoidable power imbalance issue and the proposed PBC strategy as a mitigation technique. The proposed scheme is observed to perform seamlessly with the conventional LVdc bus voltage control and remediate power-routing imbalance issue during the steady-state, load-change and hot plug-in/hot plug-out operations.

Advanced voltage control method for improving the voltage quality of low-voltage distribution networks with photovoltaic penetrations

As the number of photovoltaic (PV) power generators connected to the distribution grid increases, applications of on-load tap changers (OLTCs), power conditioning systems, and static reactive power compensators are being considered to mitigate the problem of voltage violation in low voltage distribution systems. The reactive power control by power conditioning systems and static reactive power compensators can mitigate steep voltage fluctuations. However, it creates losses in generation opportunities. On the other hand, OLTCs are installed at the bases of distribution lines and can collectively manage the entire system. However, the conventional voltage control method, i.e., the line drop compensation (LDC) method, is not designed for the case in which a large number of PV systems are installed in the distribution network, which results in voltage violations above the limit of the acceptable range. This study proposes a method to determine the optimal LDC control parameters of the voltage regulator, considering the power factor of PV systems to minimize the magnitude of voltage violations based on the voltage profile analysis of low-voltage (LV) distribution networks. Specifically, during a measurement period of several days, the voltages at some LV consumers and pole transformers were measured, and the optimal parameters were determined by analyzing the collected data. The effectiveness of the proposed method was verified through a numerical simulation study using the actual distribution system model under several scenarios of PV penetration rates. Additionally, the difference in the effectiveness of voltage violation reduction was verified in the case where all the LV consumer’s consumer voltage data measured per minute were used as well as in the case where only the maximum and minimum values of the data within the measurement period were used. The results reveal that the proposed method, which operates within the parameters determined by the voltage analysis of the LV distribution network, is superior to the conventional method. Furthermore, it was found that even if only the maximum and minimum values of the measurement data were used, an effective voltage violation reduction could be expected.

PQ Theory Based Power Quality Improvement in Distribution Level with D-STATCOM

The Transient response of the distribution static compensator (DSTATCOM) is very important while compensating rapidly varying unbalanced and nonlinear loads. Any change in the load affects the dc link voltage directly. The sudden removal of load would result in an increase in the dc link voltage above the reference value where as a sudden increase in the load would reduce the dc link voltage below its reference value. The proper operation of DSTATCOM requires variation of the DC link voltage within the prescribed limits. Conventionally, a proportional-integral (PI) controller is used to maintain the dc-link voltage to the reference value. It uses the deviation of the capacitor voltage from its reference value as its input. However, the transient response of the conventional PI dc-link voltage controller is slow. In this paper, a fast acting dc-link voltage controller based on the energy of the dc-link capacitor is proposed and proposed shunt controller method by using PQ-Theory. Mathematical equations are given to compute the gains of the conventional controller based on the fast-acting dc-link controllers to achieve similar fast transient response. The detailed simulation studies are carried out to validate the proposed controller..

Multilevel converter integration for low voltage ride through controlling renewable wind energy conversion systems

In this study, a new control of grid-connected doubly-fed induction generators (DFIGs) is introduced to attain lowvoltage ride-through capability. The grid-side converter is proposed to be a multilevel converter (MLC) controlled by conventional DC-link voltage controller. The MLC is designed and integrated into the grid-connected DFIG. The rotor-side converter is considered a two-level conventional converter controlled by the proposed virtual voltage strategy. The virtual strategy is to use the designed input control signal rather than the actual measured one during system disturbance. The system disturbance in this study is the voltage dip. The proposed virtual controller is designed to use busbar voltage during normal operation, whereas it utilizes the designed virtual voltage during wide range conditions of low grid voltages. The energy conversion system currents are increased due to voltage dip disturbances. The use of designed virtual voltage values is proposed as inputs to the controller to limit these currents at the rating values. These virtual voltages are extracted using direct-quadrature machine representation and fixed machine currents at the rated values or lower. On the basis of the proposed control concerning the virtual voltage concept, the generator currents are limited to the rated values, thereby protecting the energy conversion system during low grid voltages. Simulation results prove the validity of the proposed control scheme during extreme low voltages.

A Novel Control Strategy to Achieve SOC Balancing for Batteries in a DC Microgrid Without Droop Control

In a dc microgrid, several batteries with their converters are controlled to share the bus deficient/excessive power among themselves. A battery management system is required to improve the reliability and appropriate power-sharing. A novel control strategy is proposed in this article that ensures power sharing among the various batteries in accordance with their power rating and state of charge (SOC). In the proposed scheme, a common time reference (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ref</sub> ) is generated using the conventional dc-bus voltage control loop and SOC information of the batteries with the help of communication. An attractive feature of the proposed scheme is that it does not depend on output current of the converters. Voltage regulation is also maintained within limits in case of wide deviation in the converter output voltage at interfacing point by monitoring and using corrective measures. A low-speed communication is sufficient to convey the reference time to the controllers of the batteries. The batteries are charged or discharged in the same time interval irrespective of the initial SOC, which significantly increases their life and performance. The proposed system is more reliable as it is tolerant to various fault scenarios. All the analytical, simulation and experimental results are presented.

A Customized Voltage Control Strategy for Electric Vehicles in Distribution Networks With Reinforcement Learning Method

The increasing electric vehicles (EVs) at charging stations will impose great challenges on the conventional voltage control in distribution networks. In this article, a two-stage voltage control strategy based on deep reinforcement learning is proposed to mitigate voltage violations caused by the uncertainty of EVs and load. In the first stage, the charging demand of EVs is predicted based on trip chain theory and simulated by Monte Carlo simulation. The optimal power flow is then performed to determine the day-ahead dispatch of on-load tap changer and capacitor banks. In the second stage, the real-time voltage control problem is formulated as a Markov Game considering both reactive power control and vehicle to grid modes of EVs. The problem is solved by the deep deterministic policy gradient algorithm to develop a well-trained control strategy that can be implemented online. Moreover, a novel customized charging criterion is proposed to conduct the charging behavior of EVs and guarantee full charging at the departure time. The proposed approach is tested on the IEEE 33-bus and 123-bus distribution systems and comparative simulation results show the effectiveness in addressing voltage problems.