Pre-synchronization Control Research Articles

Active Support Pre-Synchronization Control and Stability Analysis Based on the Third-Order Model of Synchronous Machine

When traditional grid-forming converters directly participate in the grid-connected operation of the power grid, due to the lack of a pre-synchronization control system, the voltage amplitude and initial phase on both sides of the grid-connected point will deviate, resulting in voltage and current distortion during grid-connected mode. An active support phase-locked loop free pre-synchronization control strategy based on the third-order model of a synchronous generator is proposed to address the grid-connected problem of the grid-forming converter mentioned above. First, a model of active support control with frequency integral feedback at small signal levels was constructed. The root locus method was employed to examine how system parameters affect the stability of the active support control system. Second, by adding phase pre-synchronization controllers and amplitude pre-synchronization controllers to the active frequency loop and excitation voltage loop of the third-order model, it was ensured that the frequency, phase, and voltage amplitude of the unit are consistent with the power grid, achieving a fast and smooth grid-connected mode of the unit. Finally, by using a DC source to simulate all types of new energy power generation equipment, the active support pre-synchronization control system based on the three-order model of synchronous generator is built in the MATLAB/Simulink simulation environment, and the accuracy and effectiveness of the control strategy in this paper is verified.

A Virtual Synchronous Generator-Based Control Strategy and Pre-Synchronization Method for a Four-Leg Inverter under Unbalanced Loads

Virtual synchronous generator (VSG) control has positive effects on the stability of microgrids. In practical power systems, both single-phase loads and three-phase unbalanced loads are present. The four-leg inverter is an alternative solution for the power supply of unbalanced loads and grid connections. The traditional VSG control strategy still faces challenges when using a four-leg inverter to provide a symmetrical voltage and stable frequency in the load power supply and pre-synchronization. This paper proposes a VSG-based control strategy along with a pre-synchronization method for four-leg inverters. An improved VSG control strategy is put forward for four-leg inverters. The improved virtual impedance control and power calculation methods are integrated into the control loop to suppress the voltage asymmetry and frequency ripples. Building on the improved VSG control strategy, a pre-synchronization control approach is proposed to minimize the amplitude and phase angle discrepancies between the inverter output voltage and the power grid voltage. In addition, an optimized design method for control parameters is presented to improve VSG dynamic performance. A hardware prototype of the four-leg inverter is built; the results show that the voltage unbalance ratio can be reduced by 89%, and the response time can be shortened by 50%.

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.

Presynchronization control for ship microgrid of merchant marine inverters based on VSG algorithm with MFAC

The paper studies a presynchronization control of grid connection for large merchant marine microgrid inverters. We present a virtual synchronous generator (VSG) algorithm with model-free adaptive control (MFAC) to optimize the stable grid connection of ship microgrid and shore-to-ship power. To solve poor precision of presynchronization control under nonideal ship microgrid condition, an MFAC controller and its presynchronization method are developed for grid connection of ship-distributed generation inverters. The proposed presynchronization control method effectively avoids a high transient overcurrent and achieves a seamless grid connection to different types of shore power. The simulation results verify the effectiveness of the proposed control method.

Control strategies for active distribution networks based on soft open point

The application of SOP instead of traditional tie line switches in distribution networks can im-prove the controllability, flexibility, and renewable energy accommodation of the distribution grids. Compared with traditional tie line switches, SOP have high costs and large land occupation. When power transfer is not considered, the location and capacity of SOP can be optimized through planning algorithms to reduce the overall costs. However, the main function of tie line switches is power transfer. If SOP as alternative devices do not meet this requirement, their ex-tensive application will be restricted. Therefore, this paper designs the direct current volt-age-power smooth switching control strategy, phase angle pre-synchronization control strategy, and voltage support control strategy based on the parallel structure of SOP and tie line switches to ensure stable operation during the power transfer process, through this complete control strategy, smooth switching can be achieved to keep the DC voltage stable during the switching process, effectively suppress the impulse current and maximize the utilization of SOP capacity Simulations on the SIMULINK plat-form verify the feasibility of the control strategies.

Research on the Hybrid Wind–Solar–Energy Storage AC/DC Microgrid System and Its Stability during Smooth State Transitions

The hybrid AC/DC microgrid is an independent and controllable energy system that connects various types of distributed power sources, energy storage, and loads. It offers advantages such as a high power quality, flexibility, and cost effectiveness. The operation states of the microgrid primarily include grid-connected and islanded modes. The smooth switching between these two states is a key technology for ensuring the flexible and efficient operation of the microgrid. In this paper, the typical structure of an AC–DC hybrid microgrid and its coordination control strategy are introduced, and an improved microgrid model is proposed. Secondly, an adaptive current–voltage–frequency integrated control method based on signal compensation is proposed to solve the impulse current and voltage generated during the switching between a grid-connected state and an off-grid state. Finally, in response to unplanned grid-connected scenarios, an improved pre-synchronization control strategy based on BP neural networks is introduced to rapidly restore stable operation. The proposed control strategies enhanced the steady-state and transient stability of the hybrid wind–solar–energy storage AC/DC microgrid, achieving seamless grid-connected and islanded transitions without disturbances. The simulation and experimental results validated the correctness and effectiveness of the proposed theories.

Presynchronization Control for Grid-Connected Inverters Without Grid Voltage Sensors

The grid voltage sensorless control for grid-connected inverters samples the line current to estimate the voltage at the point-of-common-coupling and achieve grid synchronization. The sensorless control tends to enlarge in-rush currents and fails to connect to the grid. For addressing this issue, this letter proposes a presynchronization control strategy to achieve a fast and reliable connection to the grid. By applying periodic pulses with a designed duty cycle, the converter operates in the rectifier mode with unit power factor before being connected to the ac grid as an inverter. The initial phase of the ac grid can thereby be estimated by the sampled current, where a synchronous reference frame phase-locked loop is applied to obtain the current phase. The proposed strategy is validated by experimental tests.

A Novel Pre-Synchronization Control Method Based on Open-Loop Phase Detection for the Maritime VSC-HVDC System

Maritime island grids based on renewable energy are being rapidly developed, which poses a new challenge to the traditional pre-synchronization process of grid connection. Aiming at the problems of impulse current out of limit and long steady-state regulation time, a novel pre-synchronization control method is provided based on open-loop phase detection, which is realized by replacing the frequency detection method of the difference judgment part. The open-loop detection model of voltage phase and amplitude based on the synchronous rotating coordinate system is established first. Then the voltage phase is linearized to control the output frequency and finally reduces the voltage deviation through the compensation of virtual power to achieve the effect of a smooth grid connection. The novel pre-synchronization method has a fast response speed and no regulator effect, which can synchronize the actual values of output voltage amplitude, phase, and frequency. Simulation results verify the excellent performance of the proposed method in island stable operation and grid connection under disturbance.

A novel pre-synchronization control strategy for microgrid connections based on improved droop control

Microgrid control strategies based on traditional droop control often exhibit output voltage and frequency return errors. As such, this study proposes a novel pre-synchronization control strategy to improve both the accuracy and stability of voltage and frequency, suppress harmonics generated by an inverter, and reduce the control errors. First, an improved droop control strategy was proposed to automatically adjust the microgrid output voltage and frequency in order to achieve power sharing and suppress voltage harmonics. A pre-synchronization controller was then used to compensate for the angular frequency during droop control, based on deviations in output voltage and frequency between the microgrid and power grid. This ensured stability and smooth switching between the two grids. Finally, a series of numerical simulations were conducted with the Matlab/Simulink software. The simulated results exhibited a frequency fluctuation range of 49.95–50.02 Hz, and 49.5–50.2 Hz for traditional droop control. In addition, a voltage deviation of only ±2.89% was observed from the nominal voltage (in both connected and islanded operation). The total harmonic distortion (THD) rate was 1.50% when the microgrid was connected to the power grid and 2.57% for traditional droop control. These results suggest the proposed control strategy to be highly effective and reliable.

Research on the control strategy of grid connection between shore power supply and ship power grid

In order to meet the requirements of smooth connection and gradual load transfer between power supply and ship’s power grid. A control strategy based on an AC/DC/AC converter topology is studied in this paper. Rectification of the grid side by a double closed-loop control model using SVPWM (Space Vector Pulse Width Modulation). The V/f control strategy combined with pre-synchronization control is adopted for grid connection, in order to reduce the shock. A droop control strategy is used after grid connection. Generator gradually exits operation. The output power of the power supply increases gradually. The load is transferred in this way.

Control strategy for seamless switching of virtual synchronous generators based on secondary frequency and voltage regulation

In microgrid, virtual synchronous generators can enhance the system stability by simulating the operation mechanism of synchronous generators. However, a large impact current could be triggered during the grid-access of VSG inverters, resulting in switching failure. Aiming at this problem, based on real-time digital simulator (RTDS), this paper proposes a complete and simple pre-synchronization control strategy based on secondary frequency modulation and voltage regulation. Firstly, the grid-access structure of VSG was defined, and a mathematical model of VSG was constructed, followed by the design of power-frequency and excitation regulators. Next, secondary frequency and voltage regulation was performed on the microgrid in islanded mode to eliminate the frequency and voltage bias induced by load changes. On this basis, the grid phase was collected by phase-locked loop, the phases were synchronized through phase difference detection, and the grid-access synchronization was completed, realizing the seamless switching between islanded and grid-connected modes. Compared with the universality of applying matlab simulation software, real-time digital simulator (RTDS) has great advantages compared with physical simulation and general simulation software. Finally, the proposed strategy was proved effective with a self-designed RTDS simulation model.

Research on Pre-Synchronization Control Strategy of Optical Storage VSG Off-Grid Switching

In the optical storage DC microgrid in island mode, in view of the large inrush current problem of the inverter controlled by the optical storage VSG when switching from off-grid to grid-connected operation mode, a pre-synchronization control strategy based on secondary frequency regulation and voltage regulation was proposed. This strategy can adjust the large frequency and voltage fluctuations caused by active and reactive load changes under the independent DC microgrid of the optical storage. At the same time, it can complete the effective tracking of the grid phase before connecting to the grid, and reduce the impulse current when the optical storage microgrid is isolated and connected to the grid. The real-time digital simulation platform (RTDS) is used to simulate and analyze the proposed control scheme, and the simulation results verify the feasibility of the proposed control strategy.

Improved Seamless Switching Control Strategy for AC/DC Hybrid Microgrid

Aiming at the problems of transient over-current and over-voltage in the switching process of AC/DC hybrid microgrid in grid-connected mode and island mode, which leads to the sudden change of transmission power and seriously affects the power transmission quality. In this paper, an improved seamless switching control strategy of droop control with disturbance observer is designed. The main work is as follow: according to the diffident power transmission calculation methods of droop control in grid-connected mode and island mode, the adaptive switching is realized by detecting the current and frequency at Point of Common Coupling(PCC) of public power grid. Secondly, the switching from island mode to grid-connected mode will cause impact due to the difference of frequency, phase and amplitude between microgrid and public grid, and even cause system collapse. So the pre-synchronization control of frequency, phase and amplitude is designed. Based on the droop control, the disturbance observation(DOB) is added, which can quickly track the sudden change of system current, and suppress the sudden change by the difference between the tracking value and the actual value, so as to realize the smooth switching from island to grid. Finally, the effectiveness and feasibility of the control strategy are verify in Matlab/Simulink.

A Novel Pre-Synchronization Control for Grid Connection of Virtual Synchronous Generator

The virtual synchronous generator (VSG) introduces rotational inertia and damping through the droop control to take part in frequency regulation of the grid. By building the VSG model, the VSG controller, including both the frequency controller and the excitation controller, are analysed and designed. The analysis shows that the large phase difference Δθ between the VSG voltage and the grid voltage at the grid-tied moment may produce large power and damage the VSG. The proposed pre-synchronization control minimizes the phase difference, which is implemented by replacing Δθ with sinΔθ to avoid any step change from Δθ. The proposed method is able to simplify control, shorten pre-synchronization tracking time, and achieve high accuracy without a phase-lock loop. The built 12 kW VSG simulation and prototype verify the theoretical analysis, the proposed control and its associated design.

Research on multi-VSG fast pre-synchronization grid connection with dual power loop

This paper researched on the pre-synchronization grid connection method of a virtual synchronous generator (VSG), and proposed a fast pre-synchronization method based on the virtual power loop. According to the self-synchronization characteristics of the VSG power loop, the double power loop design was adopted in this paper. The two power loops had the same structure, which the power loop 1 was used to implement the basic VSG function, and the power loop 2 was used to achieve rapid pre-synchronous control. Therefore, the mathematical model of the VSG was similar to the synchronous generator in the pre-synchronization control. This method did not require a phase-locked loop and VSG has a good inertia and Damping performance. Moreover, this method was suitable for the networking of multiple VSGs.

Transient Analysis of Microgrids With Parallel Synchronous Generators and Virtual Synchronous Generators

In recent years, the increasing penetration of distributed generation in microgrids challenges the control and coordination of energy resources. Especially in microgrids with virtual synchronous generator (VSG)-controlled converters and conventional synchronous generators (SG), the inherent inertia difference (i.e., the VSG and SG) results in a poor transient performance when the VSG and/or loads are cut in/out. Thus, this paper explores the transient performance of microgrids with parallel VSG and SG systems. More importantly, a novel pre-synchronization control method is proposed to eliminate the phase jump while meeting the requirements in case of closures or re-closures of generation units. A small-signal dynamic model is presented, and accordingly, the VSG inertia and its damping can be designed considering the capacity ratio of VSG and SG units. In addition, with the power angle stability analysis, an active power provision strategy is introduced to suppress the transient power oscillation due to the inertia difference. Finally, the feasibility of the proposed methods is verified by simulations on a microgrid consisting of parallel VSG and SG units.

Research on virtual synchronous generator grid-connection based on phase-locked loop

When a single Virtual Synchronous Generator (VSG) is connected to the grid, the output voltage amplitude and frequency are significantly different from the grid voltage, which will impact the power grid. In order to solve this problem, the paper proposes a grid-connected pre-synchronization control method. By controlling the virtual synchronous generator based on the Phase Locked Loop (PLL) pre-synchronization unit, the voltage amplitude, frequency and phase of the VSG output before the grid connection can be consistent with the grid side, reducing the instantaneous impact of the grid. At the same time of the current, the output voltage of the virtual synchronous generator is stabilized. The single-machine VSG grid-connected model was built on the MATLAB/Simulink simulation platform and the effectiveness of the method was verified.

Research on VSG Control Strategy of Microgrid

In order to solve the low inertia problem and damping effect brought by the large-scale integration of Distributed Generators (DGs) into the grid, the Virtual Synchronous Generator (VSG) control strategy is introduced, The VSG control strategy is applied to the distributed generators grid-connected inverter to make it have the characteristics of synchronous generator, which can provide voltage and frequency support for the grid.Based on the established model, a detailed and complete VSG parameter calculation process is given,and aiming at the problem of seamless switching operation of microgrid based on VSG control, a simplified presynchronization control method is presented, which can simplify the operation and improve the safety and stability of the microgrid. The simulation results based on Real-Time Digital Simulator prove that the proposed method can effectively improve the safety and stability and has good practicability.

Doubly-Fed Induction Generator Coordination Control Strategy Compatible with Feeder Automation

To solve the problem of incompatibility and mutual repulsion between a distribution network (DN) and distributed generation (DG), this paper first analyzes the conflicts between current feeder automation (FA) and doubly-fed induction generation (DFIG). Then, a DFIG coordination control strategy compatible with FA is proposed to cause the DFIG operating continuously during fault isolation and power supply recovery. The strategy consists of three aspects: (1) a DFIG islanding active synchronization control technology to support islanding and distant presynchronization control; (2) DFIG senses the status of circuit breakers/tie-switch through generic object oriented substation event (GOOSE) messages to achieve rapid switch of control modes and distant synchronization; (3) tie-switch senses the status of DFIG through GOOSE messages for closing. Finally, the effectiveness of the strategy is validated by multiple sound and detailed time-domain simulation cases. This study indicates future possible development trends and provides a simple and effective strategy for researches in the field of smart grid DN–DG compatibility.

An Islanding Signal-Based Smooth Transition Control in AC/DC Hybrid Micro-Grids

Smooth transition is one of the most important issues of micro-grids. The resulting transition is much impacted by the state step of the regulator. To suppress this mutation, this paper proposes a smooth transition control based on an islanding signal, which updates the state of the regulators by detecting the change of islanding signal. Pre-synchronization control is applied during the transition from islanding mode to grid-connected mode. In comparison, the proposed approach is superior over direct transition control and state follower-based transition control, with both easier regulator parameter configuration and better performance during transition time.