Voltage Fluctuation Suppression Research Articles

DC-Link Voltage Fluctuation Suppression Method for Modular Multilevel Converter Based on Common-Mode Voltage and Circulating Current Coupling Injection under Unbalanced Grid Voltage

Grid voltage imbalance conditions often occur. Modular multilevel rectifiers (MMCs) have high DC-link voltage fluctuation under an unbalanced grid, which affects the normal operation of DC-side equipment. To suppress voltage fluctuation under an unbalanced grid, a coupling injection strategy composed of third zero-sequence common-mode voltage (TZCV) and secondary circulating current (SCC) was designed in this paper. In this paper, we calculated the coupling time-domain expression of the TZCV and SCC under an unbalanced grid voltage. Then, the influence of an SCC and TZCV coupling injection on DC-link voltage fluctuation was analyzed. The converter power flow of different system control objectives under an unbalanced grid was calculated, and the overall control method of the converter based on the arm current was proposed. The advantage of the method proposed in this paper is that it can realize online control under different grid voltages and input power conditions in real time and effectively suppress DC-link voltage fluctuation. The simulation was carried out on the MATLAB/Simulink platform, and a hardware-in-the-loop experimental platform was built; the results verify the effectiveness of the proposed strategy.

An adaptive power control method for soft open points based on virtual impedance

The fluctuations in power output from distributed power sources are rapid and dramatic, causing voltage fluctuations in the distribution network that threaten the safety of electricity consumption. Soft open points (SOPs) can replace traditional contact switches and are expected to suppress voltage fluctuations. However, traditional power regulation methods based on the reference values are unable to address rapid and large voltage fluctuations. Therefore, this paper proposes an adaptive power control method for SOPs based on virtual impedance. The SOP is modeled as a series link of adjustable impedance and a voltage source. Then, the voltage difference component is used to calculate the reference for the DQ domain to regulate the power flow in real time automatically. By doing so, the proposed method can smooth the voltage fluctuations in the distribution network. Additionally, the virtual impedance is also optimized to minimize the power loss. Finally, the method is validated through simulation and experiments, demonstrating that this method can automatically regulate power and significantly reduce voltage fluctuations.

Low-voltage and high-penetration distributed photovoltaic access control technology based on intelligent measurement

Abstract This project takes high-penetration distributed photovoltaic access as the background and studies the control method for low voltage and high penetration distributed photovoltaic access. Firstly, by analyzing the measured data of the low-voltage distribution network, the daily load and measured values of the low-voltage distribution network were obtained. On this basis, the topology structure of a high-penetration distributed photovoltaic grid-connected system is studied, and the voltage, current, power, and other parameters of the grid-connected system are obtained. A low-voltage and high-penetration distributed photovoltaic grid-connected control method based on a genetic algorithm is adopted. Experiments have shown that the maximum allowable capacity of this scheme is smaller than the standard value and can effectively suppress voltage fluctuations. The experimental results show that the system has good grid connection performance, which is of great significance for improving the operational stability and efficiency of low-voltage distribution systems.

Grid control strategy of wind turbines based on LADRC voltage control

Traditional grid-controlled wind turbines are required to rely on AC power grid operation, but when the power grid fluctuates, it will cause a series of oscillation instability problems in wind turbines, such as low-frequency oscillation, subsynchronous oscillation, etc. The safe and stable operation of the crisis power grid needs to be ensured. In this study, the virtual synchronous machine is supposed to be adopted as the main control strategy for grid-type control, transforming the traditional system into a more robust and reliable one. Additionally, a converter model of the grid-type control network is set up. By implementing grid-type control, voltage support and frequency support can be offered without relying solely on AC power grid operation. This approach effectively addresses issues caused by preventive such as reliance on wind turbines. The stability of the networked control system is dependent on maintaining stable system power. When there are fluctuations in system power, it impacts the stability of the networked control system. To mitigate these effects and enhance overall stability, this paper proposes using linear active disturbance rejection control (LADRC) based voltage and grid controls to suppress voltage fluctuations through linear active disturbance rejection techniques. The overall system performance is demonstrated through simulations conducted using Matlab/Simulink platform.

Microgrid Pre-Synchronization Scheme for Suppressing Voltage Fluctuations and Avoiding Frequency Out-of-Limits

Pre-synchronization control is needed when the microgrid changes from an off-grid state to a grid-connected state. Aiming to resolve the problems of frequency overstep and voltage fluctuation in traditional pre-synchronous grid-connection schemes, a micro-grid pre-synchronous grid-connection scheme is proposed that can solve both frequency overstep and voltage fluctuation at the same time. Firstly, the feedforward disturbance compensation V/f control strategy of the load voltage and current is introduced to the energy storage system to suppress voltage fluctuations caused by power changes. Then, the V/f control strategy of the energy storage system is improved to suppress voltage fluctuations caused by frequency fluctuations when the starting phase of pre-synchronization is enacted. Finally, PI parameters are optimized based on the golden jackal optimization algorithm to meet the requirements of frequency fluctuation and grid-connection speed in the pre-synchronization process. A large number of simulation experiments demonstrate the effectiveness of the proposed method.

A novel capacitor voltage fluctuation suppression method for the low-frequency operating stage of MMC driven pumped storage system

The pumped storage power plant, which has the energy conversion and storage capacity, is an important solution for improving the power grid absorption ability to the unstable and fluctuating new energy from wind farms, as well as photovoltaic power stations. The modular multilevel converter (MMC) is a competitive topology for driving the pumped storage motor due to the advantages of high output power level and low output current harmonics. However, the voltages of the sub-module capacitors fluctuate largely when the MMC is operating at low-frequency stage, which would lead to an abnormal start-up process or an unstable low-speed running state of the variable speed pumped storage system. In order to suppress the low-frequency voltage fluctuation of the MMC sub-module capacitors, this paper proposes a charge channel topology including in-phase and inter-phase channel between the sub-modules for transferring the capacitor charge. Then, the reverse fluctuating fundamental component and third harmonic circulating component, as well as the synchronizing fluctuating second harmonic component, of the sub-module capacitor voltages could be counteracted, respectively. Eventually, the start-up operation performance, as well as the low-speed operation performance of the variable speed pumped storage system are enhanced greatly. The effectiveness of the proposed low-frequency capacitor voltage fluctuation suppression method is verified through simulation results.

Iterative Learning-Based Distributed Secondary Control for AC Microgrid

Existing distributed secondary control strategies relying on the microgrid system model have good theoretical performance but are difficult to implement in practice due to the need for real-time observation of numerous system states. This article addresses this issue by proposing a distributed secondary control strategy for ac microgrids based on ILC. During operation, this strategy neither relies on the microgrid system model nor imposes any computational burden. First, this article introduces the microgrid hierarchical control structure and establishes a system model for primary control. Then, the proposed ILC-based secondary control strategy is elaborated, and the corresponding stability criterion is derived using the small-signal analysis method. Finally, the simulation results in MATLAB/Simulink demonstrate the effectiveness of the proposed ILC-based strategy. Specifically, the proposed strategy can limit the transient time within 0.4 s when handling various disturbances, which is comparable to the existing strategies based on MPC and SMC. However, the proposed strategy is easier to implement compared with the MPC-based and SMC-based strategies, which rely on the system model. Furthermore, in plug-and-play and load change scenarios, the proposed control strategy can suppress voltage fluctuation within 1 V, which is better than existing strategies.

DC‐link voltage control of small capacitor PMSM drive system based on MPC algorithm

SummaryThe permanent magnet synchronous motor (PMSM) drive system with small dc‐link capacitor will have the problem of unstable drive system due to the small damping of the LC filter and the negative impedance characteristics of the constant power load. The input LC filter can suppress interference and is essential for the normal operation of the drive, but it may not match the impedance of the post‐stage inverter, causing the drive system to become unstable. In this paper, by constructing the dc‐link voltage prediction model, combined with the PMSM current predictive control, the weighted cost function of the model predictive control (MPC) strategy is designed to realize the motor optimal control and the dc‐link voltage fluctuation suppression control. To avoid introducing additional sensors when constructing the dc‐link voltage model, a source state observer that can estimate the dc‐link voltage and input current is constructed. Simulation and experimental results are provided to verify the effectiveness of the proposed method.

Predictive Control of Modular Multilevel Converters: Adaptive Hybrid Framework for Circulating Current and Capacitor Voltage Fluctuation Suppression

Modular multilevel converters (MMCs) are widely used in voltage-sourced, converter-based high-voltage DC systems due to their modular design, scalability, and fault tolerance capabilities. In MMCs, multi-variable control objectives can be employed by using model predictive control (MPC) due to its fast dynamic response and ease of implementation. Nonetheless, conventional MPC techniques for MMCs have shortcomings, including high computational requirements, poor circulating current, and capacitor voltage fluctuation suppression. First, this study proposes an adaptive MPC technique that adapts the number of candidate combinations to the steady and transient states, significantly reducing the computational burden. Second, an improved hybrid combination of an MPC with a proportional-resonance (PR) controller enhances the circulating current and capacitor voltage fluctuation suppression performance. According to the phase difference between the circulating current and the capacitor voltage, the circulating current and capacitor voltage can be suppressed at different times by changing the circulating current reference of the PR controller. The switching frequency can be reduced by using the PR controller’s output to adjust the input submodule number instead of changing the duty cycle. The proposed techniques were validated by simulations and experimental case studies with a three-phase grid-connected MMC.

Nonlinear transient voltage and frequency-coordinated control strategy for the renewable energy sending system

In order to ensure that the voltage and frequency of the sending system with a high proportion of renewable energy access under transient port energy impact can be controlled within the allowed fluctuation range, a nonlinear multi-objective transient voltage and frequency-coordinated control strategy for the renewable energy sending system is proposed in this paper. First, the dynamic balance characteristics of the renewable energy sending system under transient energy impact are analyzed by establishing an equivalent power source model, and the frequency and voltage state equation of the sending system considering transient energy impact is studied and established. Then, considering the supporting effect of energy storage on the voltage and frequency of the renewable energy sending system, a nonlinear transient frequency and voltage-coordinated control model of the renewable energy sending system is proposed. Finally, the aforementioned conclusions are verified by simulation analysis. The simulation results show that the proposed control method can realize voltage and frequency stability control under transient energy. The effective energy tracking control algorithm can better utilize the fast energy regulation characteristics of battery energy storage, which can effectively reduce the fluctuation amplitude of the transmission system and restore synchronization in a short time. The voltage frequency control algorithm can significantly reduce the amplitude of voltage and frequency fluctuations, and can realize the rapid suppression of voltage and frequency fluctuations.

Research on a hybrid optical storage control strategy based on a DC virtual synchronous generator and MPC

There are many problems in optical storage systems, including fault ride-through and disturbance, inadequate DC bus voltage support, and high state variable overshoot. The main problem is caused by the slow dynamic reaction speed of the energy storage system based on traditional PI control to the DC voltage of the optical storage system. A hybrid optical storage system control optimization strategy based on a DC virtual synchronous machines and improved model predictive control (MPC) was proposed in the paper as a solution to the aforementioned issues. A strategy based on DC virtual synchronous generator can improve the damping braces and overshoot suppression of DC voltage fluctuation during the disruption process of the energy storage system in the optical storage system. The dynamic response speed of energy storage in optical storage systems may be further increased based on the improved MPC. The effectiveness and validity of the control strategy proposed in the paper were verified by the simulation examples.

Dynamic control strategy for bi-directional DC/DC converters to suppress the dc-link voltage fluctuation in motor driver systems

This paper presents an improved dc-link voltage control strategy for bi-directional DC/DC converter to reduce the fluctuation of dc-link voltage. The bi-directional DC/DC converter is the top widely-adopted topology for machine drives. However, a wild dc-link voltage fluctuation may cause the overvoltage accident if the load is suddenly altered. Therefore, a novel energy-balanced dc-link voltage control scheme is presented. Through the analysis of the instantaneous energy balance of the whole system, this paper draws a conclusion that the energy stored in the dc-link capacitor will fluctuate in the dynamic process. Then, when the output power changes, the influence of the proposed control strategy on the dc-link voltage fluctuation is analyzed. Moreover, some deadbeat predictive control method has been applied within the current inner loop to improve systematically dynamic response ability. As the energy consumption can be compensated, the fluctuation of dc-link voltage is constrained in sudden load change. Small-signal model has been presented to analyze dynamic performances. As a result, experimental outcomes verify that above method is equipped with superior dc-link voltage fluctuation suppression capability, and the current overshoot is reduced and dynamic performance is improved. What is more, the controller design is simple to realize and has a bright application prospect.

A Local Control Strategy for Voltage Fluctuation Suppression in a Flexible Interconnected Distribution Station Area Based on Soft Open Point

Fluctuations in the output power of a high penetration distributed generation (DG) will result in severe voltage fluctuations/violations in the power distribution station area (DSA). This paper proposes the use of Soft Open Point (SOP) for DSA interconnection to alleviate voltage violations and frequent fluctuations. Firstly, the typical topology of a flexible interconnected DSA is introduced, and then the correlation between active/reactive power fluctuations and voltage fluctuations in DSA is analyzed, according to which a local control strategy is proposed to achieve voltage control by relying only on the local electrical information of the nodes connected to the SOP. Finally, the effectiveness of the proposed control strategy is verified by simulation case. The simulation results show that the voltage fluctuation caused by the DG output power fluctuation can be suppressed within tens of milliseconds through the control strategy. This strategy greatly reduces the communication burden and has good real-time performance. It can be used as an SOP control strategy under conditions of communication failure or to realize a plug-and-play SOP, which has significant engineering importance for solving the voltage violations and frequent fluctuations in DSA and improving the consumption capacity of DGs.

High-precision bus voltage control based on NLESO and TSMC predicted load current and grid voltage differential feedforward

Power battery test equipment is widely used to produce and test new energy vehicles and storage containers. To address the problem of DC bus voltage fluctuations that affect the accuracy of output voltage and current, a high-accuracy bus voltage control strategy is based on nonlinear extended state observation (NLESO) and terminal silding mode controller (TSMC) load current estimation. Simulation and experimental results show that the control strategy has a good suppression effect on the DC bus voltage fluctuation and can effectively improve the accuracy of the DC side output voltage and current. Grid voltage differential feedforward is adopted, which can achieve fast suppression of DC bus voltage fluctuations in power battery test equipment and meet the high precision output requirements of the system DC bus.

Novel Voltage-Balance Control of Four-Level Hybrid-Clamped Converter With Open-Loop Optimized Common-Mode Voltage Injection

The four-level hybrid-clamped converter (HCC) is a new high-efficiency dc–ac converter. However, its dc-link capacitors face challenging voltage fluctuation issues at low frequencies, resulting in the distorted ac current and oversize converter volume. This letter presents a simple and effective control method of the HCC using open-loop common-mode voltage (CMV) injection. The mathematical model of the CMV and the capacitor voltage fluctuations is derived. The optimal CMV algorithm with the maximum suppression effect of capacitor voltage fluctuations is proposed as well as the implementation method. The microlevel mechanism of the proposed method in suppressing voltage fluctuation is clarified. The capability of the proposed method in improving dc voltage utilization is explored. Compared with existing methods, the proposed one reduces dc-link voltage sensors and computation complexity and mitigates CMVs at the load side. The optimal CMV is approximately 25% of the output voltage magnitude, and enhances the dc-bus voltage utilization by 12%. Results demonstrate that voltage fluctuations of HCC capacitors are reduced by over 60%. The proposed method has a solid dynamic balancing capability and works well for the wide-range frequency and modulation index.

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.

Voltage fluctuation mitigation with coordinated OLTC and energy storage control in high PV penetrating distribution network

The surge in the growth of renewable energy resources integration such as solar photovoltaic (PV), wind energy, and tidal energy, etc., is introducing fast fluctuating voltage challenges to the power grid. A battery energy storage system (BESS) can suppress voltage fluctuations up to certain limits that are introduced by intermittency in solar photovoltaic. Although battery energy storage system (BESS) corresponds to faster responses, the lack of coordination between conventional voltage fluctuation mitigation strategies such as On-load tap changer (OLTC) and BESS might lead to an overuse of BESS and underuse of OLTC. This research presents an intelligent and real-time coordinated-control of BESS and OLTC that mitigates the voltage fluctuations, enhances the life of BESS and optimizes the use of existing OLTCs. In the proposed control technique the BESS controls the bus voltage at local level and OLTC controls the voltage at global level. The devised scheme monitors the real-time bus voltages using phasor measurement units (PMU). The scheme assigns respective weights to individual buses that decide the optimal operation of OLTC and BESS. In control of OLTC all the buses are assigned some weights which decide the optimal operation of OLTC and BESS. The proposed scheme is demonstrated on IEEE-13 bus system and results are being compared with the conventional scheme. The proposed control scheme surpasses the voltage regulation by reducing the average voltage violation time to one-fifth compared to conventional control scheme and significant reduction in BESS energy cycle.

Dynamical Reconfigurable Master–Slave Control Architecture (DRMSCA) for Voltage Regulation in Islanded Microgrids

Voltage fluctuation suppression and reactive power proportional sharing in islanded microgrids (MGs) are often difficult to maintain simultaneously. Hence, a dynamical reconfigurable master–slave control architecture is proposed herein. When a load fluctuates, the leader will be dynamically selected through specific features (such as the output power, output voltage, and power capacity); subsequently, the leader sends his information to all other distributed generators (DGs) as a unified signal in the MG. Meanwhile, the other DGs adjust their local output based on the unified reference to achieve accurate power sharing in each DG. Moreover, system voltage recovery can be realized simultaneously since each DG contains closed-loop voltage control. In addition, for the automatic selection of the leader, a dynamically selected method based on a weight-voting-based differential delay method is proposed herein; the method realizes the unicity and automatic selection of the leader during communication failure. Additionally, the stability of the proposed method is proven mathematically. The results show that the proposed method is stable for any communication period and DG quantity. Furthermore, the communication delay and communication failure are derived mathematically. The results show that the proposed method demonstrates robustness against communication delay. Finally, experimental results verify the effectiveness of the proposed method under communication delays, failures, and power failures.

Analysis and Suppression of Voltage Violation and Fluctuation with Distributed Photovoltaic Integration

In recent years, the violation and fluctuation of system voltage has occurred with greater frequency with the integration of high-penetration distributed photovoltaic generation. In this paper, the voltage violation and fluctuation in a high-penetration distributed photovoltaic integrated system is analyzed, and then a corresponding suppression strategy is proposed. Firstly, based on solar cell and photovoltaic control system models, the influence factors of photovoltaic output are analyzed. Secondly, the voltage violation and fluctuation caused by photovoltaic integration is analyzed, and the quadratic parabola relationship between bus voltage fluctuation and photovoltaic power variation is constructed. Next, according to the virtual synchronous generator characteristic of distributed photovoltaics, a double-hierarchical suppression strategy is proposed to make full use of reactive power regulation capability, which can maintain the symmetry of power supply while meeting standard requirements. The proposed strategy can conveniently realize quick response and support the photovoltaic extensive access. Moreover, with the employment of the proposal, the system voltage violation and fluctuation can be suppressed effectively. Finally, considering the photovoltaic access location, capacity, and partial shading, the effectiveness of the proposed strategy is verified in IEEE 33-bus distribution system with field measured data. After distributed photovoltaic accesses the system, more than 60% of buses appear to have undergone bus voltage violation. With the proposed method, more than 20% of the voltage deviation and more than 6% of the voltage fluctuation are effectively suppressed so that the system voltage can be kept below 1.07 p.u. and the voltage fluctuation can be kept within 4%, meeting the requirements of power quality standards.

Suppression of DC-Link Voltage Fluctuation in the Motor Drive System with Reduced Capacitance

Suppression of DC-Link Voltage Fluctuation in the Motor Drive System with Reduced Capacitance