Voltage Control Strategy Research Articles

Optimal Secondary Control to Suppress Voltage Fluctuations in an HVDC-Linked Wind Farm Grid

High-voltage direct-current (HVDC) systems using voltage source converters (VSCs) have been developed to suppress voltage fluctuations in offshore grids due to the intermittent power generation of offshore wind farms (OWFs). This paper proposes a new strategy for the optimal secondary voltage control of HVDC-linked OWFs. In the proposed strategy, optimal coordinated control of an HVDC system and OWFs is achieved to minimize real-time voltage fluctuations throughout an offshore grid. A full-order dynamic model of HVDC-linked OWFs is implemented, and its unobservable and uncontrollable state variables are then eliminated using a model reduction algorithm. Given the dynamic model, a linear quadratic Gaussian regulator is designed for optimal coordinated control, and an eigenvalue analysis is conducted focusing on the effects of communication time delays on the stability of the proposed strategy. Simulation case studies are also carried out to verify that the proposed strategy more effectively improves real-time regulation of offshore grid voltages, compared to conventional strategies, while ensuring voltage stability even for relatively large delays in communications systems.

Distributed Optimal Voltage Control and Berth Allocation of All-Electric Ships in Seaport Microgrids

The uncoordinated berth allocation of all-electric ships (AES) will impose great challenges on voltage profiles in seaport microgrids. This paper proposes a distributed optimal voltage control and berth allocation strategy to mitigate voltage violations while balancing the benefits between microgrids and AES. The voltage control problem is formulated as a second-order cone program (SOCP) to minimize network power loss, while the berth allocation problem is formulated as a mixed-integer linear program (MILP) to minimize total service time. To build the bridge between microgrids and berthed-in AES, the power interaction model at the seaport is proposed. The proposed model introduces a time-cost factor to evaluate the economic cost of service time and also unifies the timescales between voltage control and berth allocation. Considering different system operators and objectives of microgrids and AES, this paper develops an alternating direction of multipliers (ADMM)-based distributed optimization algorithm to solve the problem. The method can help reach the convergence of ADMM in solving models with discrete variables. The proposed approach is tested on the EU 16-bus microgrid embedded with a seaport and simulation results verify the effectiveness both in mitigating voltage violations and maximizing the total benefits of microgrids and AES.

A Nine-Level Split-Capacitor Active-Neutral-Point-Clamped Inverter and Its Optimal Modulation Technique

This article proposes a novel hybrid nine-level inverter for high-efficiency applications based on the popular five-level active-neutral-point-clamped (5L-ANPC) topology. Compared with conventional ANPC-based topologies, this inverter reduces the required number of active and passive components while avoiding the use of higher voltage switches. Only nine active switches with eight gate drivers and two discrete diodes are used in each phase leg. Moreover, the proposed inverter not only reduces the number of flying capacitors (FCs) but also their rated voltages, which has a positive impact on the reliability, volume, and cost of the converter. Based on switching state redundancies, a voltage control strategy for FCs and neutral points is developed and incorporated into the phase-disposition pulsewidth modulation technique. The developed control method is able to balance the voltages of the FCs with only one sensor under active and reactive power operation. Based on the designed modulation strategy, a mathematical analysis is carried out to determine the capacitance of the FCs. A comprehensive comparison with other nine-level ANPC-based topologies is included to highlight the advantages of the proposed inverter. The performance of the proposed topology with its control strategy is validated through simulation studies and experimental implementation under different conditions.

Hierarchical Event-Triggered MPC-Based Coordinated Control for HVRT and Voltage Restoration of Large-Scale Wind Farm

This paper proposes a hierarchical event-triggered model predictive control (HEMPC)-based voltage control strategy to coordinate the reactive power (Var) outputs among all WTs for improving the high-voltage ride-through (HVRT) and post-fault voltage restoration capability of the large-scale wind farms (WFs), while reducing the heavy computational task of the WF controller. The event-triggering condition and optimal controller are designed to activate the proposed HEMPC strategy based on the system state deviation and input-to-state stability (ISS) mechanism, which can substantially decrease data communication in a non-periodic sampling fashion. A hierarchical solution method based on the alternating direction method of multipliers (ADMM) is developed, aiming to further improve the computing efficiency of the voltage optimization problem of the large-scale WF. Two time-domain WF simulation cases in Matlab/Simulink corroborate that the proposed HEMPC strategy is more efficient in minimizing the WT terminal voltage deviations and reducing the computational burdens of the WF controller compared with traditional control strategies.

An improved fuzzy logic based DC-link voltage control strategy for smoothing output power of the PMSG-WECS

Direct power smoothing control methods relying on the wind turbine itself have been widely studied to improve wind turbines’ output power quality, and the DC-link voltage control based on the capacitor of the wind power converter is one of them. In this paper, the principle of DC-link capacitor voltage control is studied at first, then an improved time interval DC-link voltage control strategy on the basis of a low-pass filter (LPF) is proposed and compared with existing DC-link voltage power smoothing controls. To further improve the power smoothing effect, the fuzzy logic control is used to adjust the time constant of LPF, and the maximum time constant in the fuzzy logic control is determined through convolution calculation. For the sake of ensuring the system stability, the transfer function of proposed DC-link voltage control is derived to determine proper parameters of voltage loop. The proposed fuzzy logic-based DC-link power smoothing strategy is verified through effective simulation results of the permanent magnet synchronous generator wind energy conversion system (PMSG-WECS).

Control Strategy for Offshore Wind Farms with DC Collection System Based on Series-Connected Diode Rectifier

The DR-HVDC (Diode rectifier-based HVDC) transmission topology was recently proposed for integration on large offshore wind farms due to its low investment cost and high reliability. To further reduce the investment, a DC collection topology based on the series-connected diode rectifiers (DR) is proposed, where no offshore platform is needed. However, units of series-connected topology (SCU) show coupling issues, such as overvoltage, energy curtailment, and fault isolation. First, the coupling mechanism is analyzed, and a suitable operation mode for SCUs is selected to ensure the safe operation of the DC system. Then, the linear relationship of active power and output DC current and DC voltage of SCUs is analyzed, and a novel coordinate control strategy for DC wind farms is proposed, where an onshore converter adapts a DC current controller and wind turbines adapt a mediate output voltage control strategy. The mediate output voltage control strategy includes a triple loop with power loop, mediate output voltage loop, and current loop. Also, the DC open line fault, DC grounding fault, and AC grounding fault of the onshore grid are investigated, and a protection strategy is proposed. A 160 MW wind farm with a DR-SCU DC collection system is built in PSCAD/EMTDC to verify the validity of the proposed control strategy under unequal wind speeds, DC fault, and onshore AC fault, and the results validate the performance of the proposed strategy.

Fast Supplementary Voltage Control Strategy to Mitigate Catastrophic Trip-off Risk of Wind Turbines

Fast Supplementary Voltage Control Strategy to Mitigate Catastrophic Trip-off Risk of Wind Turbines

Impacts of integration of very large‐scale photovoltaic power plants on rotor angle and frequency stability of power system

The increase in penetration of large-scale renewable generation significantly affects power system characteristics. Thus, it is necessary to assess the steady state and dynamic performance of a power system for different installing scenarios of renewable power plants such as photovoltaic (PV) ones. The paper specifically investigates the impacts of very large-scale photovoltaic (VLS-PV) generation on the power system dynamic performance including rotor-angle and frequency stabilities. For this purpose, a detailed equivalent dynamic model for a multi-unit VLS-PV system and corresponding controllers are developed. The paper also proposes a comprehensive set of replacement scenarios comprising different PV penetration, location, and voltage control strategies as well as simulating various large disturbances. To investigate the impacts of VLS-PV generation on dynamic system performance, a study framework for frequency and rotor-angle stability involving the modal and transient analysis is presented as well. By using the VLS-PV detailed model along with other dynamic components of a power system such as synchronous generators, automatic voltage regulators (AVRs), and governors, the stabilities studying instructions are employed for the IEEE 39-bus and 118-bus test systems.

Hybrid voltage and current control strategy for virtual synchronous generator under unbalanced voltage conditions

AbstractThe traditional VSG strategy does not consider the negative sequence component, which will lead to current imbalance and power oscillation under unbalanced voltage. Several improved control strategies of VSG are usable for operating under unbalanced voltage nowadays. These control strategies usually require a complex PLL (phase‐locked loop) to detect sequence components, and a current reference generator to convert voltage references to current references. This paper proposes an improved control strategy for VSG. The main contributions are in threefold: (1) A hybrid voltage and current control strategy by simultaneously controlling the positive sequence output voltage and negative sequence output current is proposed, which can achieve different operation demands under unbalanced voltage conditions without changing the original characteristics of VSG. (2) A positive and negative sequence decoupling method is designed to acquire the synchronization phase information directly from the positive sequence voltage control branch, thereby avoiding the use of complex PLL (phase locked loop). (3) A general formulation for the negative sequence current references is presented to control the power oscillations or the current balance flexibly. The correctness and effectiveness of the control strategy are verified by simulation and experimental results.

Increasing Green Energy Penetration and Efficient Utilization Through a Finite Control Set Model Predictive-based Virtual Inductance Droop Control

As global energy demand hikes up, the search is to provide adequate penetration of green energy to the power grid to solve this threat of energy crisis and preserve the globe from pollution. Thus, this paper proposes an inverter control algorithm in the form of a finite control set model predictive-based virtual inductance droop voltage control strategy for a hybrid microgrid structure, which will provide a more effective platform for high penetration and efficient utilization of green energy. The new control algorithm as tabled in this paper considers the following aspects: DC voltage control, enhancement of reactive power issues with conventional control, fast dynamic response to AC load voltage regulation under balanced, unbalanced, and unbalanced non-linear loads, and power quality delivery in an autonomous hybrid microgrid system. To verify the performance of the proposed control method, simulation models are built under various operating modes of load variations and source unavailability. When analyzed, the findings show the control method's great efficacy to achieve desirable efficiency of green energy utilization and its greater accommodation into the microgrid. Additionally, it alleviates pressure on conventional generation and the utility system, avoiding the need for expensive grid improvements.

Reactive Voltage Control Strategy for PMSG-Based Wind Farm Considering Reactive Power Adequacy and Terminal Voltage Balance

To improve the ability of the power system to accommodate high penetration wind power, wind turbines (WTs) need to realize the mode transformation from grid-following to grid-forming, thus actively participating in the voltage regulation of the power grid with a high proportion of wind power. In this work, a reactive voltage control strategy for wind farms considering reactive power adequacy and terminal voltage balance is proposed. Firstly, the expression of the maximum reactive power regulation capacity of WT, namely reactive power adequacy, is derived under the complete wind condition based on the mathematical model and operating characteristics of WT, to study the influence of wake effect on reactive power adequacy of a wind farm. Then, the point of common coupling (PCC) voltage and terminal voltage are expressed analytically based on the radiative topology equivalent model of a wind farm, to analyze the influence of electrical distance on active power loss of wind farm. Finally, the calculation method of the adaptive gain coefficient of WT is put forward, which comprehensively considers the input wind speed and the electrical distance, to regulate the PCC voltage and terminal voltage simultaneously. The comprehensive effectiveness of the proposed strategy is demonstrated on a permanent magnet synchronous generator (PMSG)-based wind farm integration simulation model. While supporting the PCC voltage, the proposed strategy maintains the balance of the terminal voltage in the wind farm, thereby improving the friendliness of wind power grid connection.

Hierarchically Coordinated Voltage Control in Seaport Microgrids Considering Optimal Voyage Navigation of All-Electric Ships

The uncoordinated scheduling and control of all-electric ships (AESs) will impose great challenges on voltage profiles in seaport microgrids. This article proposes a three-stage hierarchically coordinated voltage control strategy in seaport microgrids considering the optimal voyage navigation and scheduling of AES. In stage one, we first propose an arrival time estimation method for AES to reduce the potential voltage violation risk in different seaport microgrids. The optimal power flow (OPF) is then performed in each microgrid to determine the day-ahead dispatch of on-load tap changer (OLTC) and inverter-based renewable energy resources (RESs). In stage two, the optimal route planning is carried out based on the sea states and the Dijkstra algorithm. The voyage and generation scheduling of AES are formulated as optimization problems and solved by CPLEX solvers. In stage three, a real-time local voltage control strategy is performed to mitigate fast voltage violations. To fully exploit the voltage support capability of energy storage systems (ESSs) on berthed-in AES, both constant charging mode and voltage control mode are designed to regulate voltages while ensuring sufficient SOC levels of ESS when leaving the seaport. The simulation results verify the effectiveness of the proposed method both in saving the generation cost of AES and improving voltage profiles in seaport microgrids.

Transient Stability Analysis and Improved Control Strategy for DC-Link Voltage of DFIG-Based WT During LVRT

In this paper, the transient stability for dc-link voltage of doubly-fed induction generator (DFIG)-based wind turbine (WT) is studied in detail during low voltage ride-through (LVRT). Firstly, referring to the rotor swing equation of synchronous generator (SG), the nonlinear large-signal model of dc-link voltage is established. In addition, the damping power, static slip power, and dynamic slip power are derived. Consequently, the transient performance of dc-link voltage under the different operation conditions of DFIG is revealed during LVRT. Furthermore, the instability form of dc-link voltage with the moving process of equivalent power angle is analyzed by the energy function of dc-link voltage. In addition, the impacts of dc voltage control loop's parameter, the active current of the stator, and the slip on the stabilization process of dc-link voltage are studied. Analysis result indicates that the insufficient damping and large unbalanced power would deteriorate the transient behavior and steady-state level of dc-link voltage. Therefore, an additional damping and slip power feedforward control strategy is proposed, which can make the transition process of dc-link voltage smoother and significantly improve the voltage steady-state level. Finally, simulation and experimental results validate the effectiveness of theoretical analysis.

Enhancement of the LVRT Capability for DFIG-Based Wind Farms Based on Short-Circuit Capacity

In this article, an advanced transient voltage control (A-TVC) strategy is proposed to enhance the low-voltage ride-through (LVRT) capability of a doubly fed induction generator (DFIG)-based wind farm (WF). In severe voltage dip (VD) conditions, the DFIG-based WF should stay online to support the grid voltage. However, it may lead to transient voltage instability in the grid. This article proposes a novel A-TVC, which follows the VD conditions and, consequently, improves the LVRT capability of the DFIG-based WF. In this approach, a lookup table is presented to control the transient voltage based on the short-circuit capacity at the point of common coupling (PCC), VD index on the grid, and generated power limitations of the DFIG. By applying the proposed control strategy, the DFIG could not only smooth the dc-link voltage fluctuations and reduce the stator and rotor overcurrents in a faulty grid but also increase the PCC and stator voltages and, thus, enhance the transient voltage stability. Simulations and control-hardware-in-the-loop experimental results validate the effectiveness of the proposed control strategy for improving the LVRT capability of the DFIG in severe balanced and unbalanced VD conditions.

A Modified Neutral-point Voltage Control Strategy for Three-level Inverters Based on Decomposition of Space Vector Diagram

Capacitor voltage imbalance is a significant problem for three-level inverters. Due to the mid-point modulation of these inverter topologies, the neutral point potential moves up or down depending on the neutral point current direction creating imbalanced voltages among the two capacitors. This imbalanced capacitor voltage causes imbalanced voltage stress among the semiconductor devices and causes increase output voltage and current harmonics. This paper introduces a modified voltage balancing strategy using two-level space vector modulation. By decomposing the three-level space vector diagram into two-level space vector diagram and redistributing the dwell times of the two-level zero space vectors, the modified voltage balancing method ensures minimal NP voltage ripple. Compared to the commonly used NP voltage control method (using 3L SVM [9]), the proposed modified NP voltage control method offers a slightly higher neutral-point voltage ripple and output voltage harmonics but, it has much lower switching loss, code size and execution time.

Voltage control to maximize the transmission efficiency of a multi‐input and multi‐output wireless power transfer system

AbstractIn multi‐input and multi‐output (MIMO) wireless power transfer (WPT) systems, the destructive interference between multiple transmitters and receivers brings a serious impact on power transmission efficiency. This paper proposes a voltage control strategy to maximize the transmission efficiency of a MIMO WPT system by controlling the amplitude and phase of the transmitter voltages. However, the complexity of the realistic WPT circuit topology and the uncertainty in the related parameters obstruct the establishment of an accurate closed‐form description of the WPT model to optimize the transmission efficiency of the system. In this paper, a simple multi‐port network model is introduced to simulate a MIMO system to avoid the complex circuit topology and a large number of circuit elements. To determine the parameters of the proposed model, a simple yet efficient methodology, by sequentially changing the excitations of the input ports and sampling the responses at the out ports, is proposed. A voltage control methodology to optimize the transmission efficiency of a MIMO wireless power transfer system is finally developed. To validate the proposed model and voltage control methodology, a three‐input and three‐output WPT prototype is designed and fabricated and used as the experimental platform. The performances of the proposed voltage control methodology are compared to both the experimental ones and those without the proposed voltage control methodology.

Study on abnormal power failure shutdown control of magnetic suspension centrifugal compressor

The rotor of a magnetic suspension centrifugal compressor is supported by magnetic bearing, and a power failure will cause the high-speed rotor to fall directly onto the protective bearings, affecting the reliability of the compressor. To solve this problem, this paper proposes an abnormal power failure shutdown control method with semi-switching modulation of the inverter. Firstly, the overall scheme of abnormal power failure shutdown control is designed, and its feasibility is analysed in detail through mathematical modelling. Secondly, aiming at the non-linear, time-varying and gain uncertainty characteristics of the control system, a voltage control strategy based on parametric self-regulating sliding-mode variable structure is proposed, and the stability is analysed. Finally, the method proposed in this paper is experimentally verified, the results show that this method does not depend on the electric angle of the motor compared to existing methods, and the rotor can achieve a zero-drop speed shutdown, which greatly improves the reliability of the magnetic suspension centrifugal compressor.

An Adaptive Voltage Control Using Local Voltage Profile Mode and Similarity Ranking

Emergency voltage control provides a real-time online response to maintain the long-term voltage stability of a power system. Searching for an optimal emergency control solution is a hard combinatorial optimization problem because of the highly dynamic and nonlinear characteristics of power systems with discrete control variables. Additionally, real-time response is required. A new mode-matching–based adaptive voltage control is proposed in this article to achieve a fast response by exploring local voltage profile modes and the technique of similarity ranking. First, some typical emergencies are studied in advance, and their control knowledge is collected for preparation. The set of local voltage profile modes and their corresponding optimal control solutions are stored as knowledge for each emergency. Second, fast online control is realized and the control solution is configured according to a similarity ranking among local voltage profile modes. Furthermore, a learning process is applied to accumulate knowledge in case of unprepared situations. Thus, improved control performances in the future are ensured. This article depends on a new concept of local voltage profile mode with which knowledge preparation, emergency identification, configuration of control, and knowledge accumulation work together to perform an effective response. Simulations of the IEEE 39-bus system verify the outcomes of this voltage control scheme.

Modified Voltage Control Strategy for DC Network with Distributed Energy Storage using Fuzzy Logic Controller

This research provides an extraordinary distributed energy storage (DES) technique for DC distribution networks. Under exhibited virtual inertia and capacitances, the parameters of the AC and DC networks are briefly assessed. The proposed control approach for DES is based on many interactive features, which allows the DES to respond to both DC network voltage variations and make use of AC grid frequency changes. To reduce the burden of DC network voltage degradation, a cascade droop management approach with fuzzy is proposed for DES in DC microgrid. The simulation outcomes give a response that is fruitful for improving the voltage stability in DC distributed network as compared to other existing techniques. INDEX TERMS: Distributed energy storage, flexible voltage controlstrategy, AC and DC networks, fuzzy,voltage stability.

Real Time Power Control in a High Voltage Power Supply for Dielectric Barrier Discharge Reactors: Implementation Strategy and Load Thermal Analysis

Atmospheric-pressure plasma treatments for industrial and biomedical applications are often performed using Dielectric Barrier Discharge reactors. Dedicated power supplies are needed to provide the high voltage frequency waveforms to operate these nonlinear and time-dependent loads. Moreover, there is a growing technical need for reliable and reproducible treatments, which require the discharge parameters to be actively controlled. In this work, we illustrate a low-cost power supply topology based on a push–pull converter. We perform experimental measurements on two different reactor topologies (surface and volumetric), showing that open loop operation of the power supply leads to a temperature and average power increase over time. The temperature increases by ΔTvol~120 °C and ΔTsup~70 °C, while the power increases by ΔPvol~78% and ΔPsup~60% for the volumetric (40 s) and superficial reactors (120 s), respectively. We discuss how these changes are often unwanted in practical applications. A simplified circuital model of the power supply–reactor system is used to infer the physical relation between the observed reactor thermal behavior and its electrical characteristics. We then show a control strategy for the power supply voltage to ensure constant average power operation of the device based on real-time power measurements on the high voltage side of the power supply and an empirical expression relating the delivered power to the power supply output voltage. These are performed with an Arduino Due microcontroller unit, also used to control the power supply. In a controlled operation the measured power stays within 5% of the reference value for both configurations, reducing the temperature increments to ΔTvol~80 °C and ΔTsup~44 °C, respectively. The obtained results show that the proposed novel control strategy is capable of following the transient temperature behavior, achieving a constant average power operation and subsequently limiting the reactor thermal stress.