Synchronous Power Controller Research Articles

Modulation-Compensated Power Synchronization Control for Modular Multilevel Converters With Enhanced Performance Under Varying SCR

Modulation-Compensated Power Synchronization Control for Modular Multilevel Converters With Enhanced Performance Under Varying SCR

Analysis and Suppression Strategy of Synchronous Frequency Resonance for Grid-Connected Converters With Power-Synchronous Control Method

The power-synchronization control (PSC) method of grid-connected converters, which is based on the active power orientation, is a promising solution for high-penetration renewable power generation systems. Unfortunately, synchronous frequency resonance (SFR) is inevitably found in PSC-controlled converters, especially when connected to inductive networks, which seriously damages the stability of the system. The relatively small equivalent resistance of the inductive network is regarded as the main cause of SFR, and a virtual resistor (VR) is usually employed to dampen this kind of oscillation. However, another important reason for SFR is the non-minimum phase effect of the loop gains, which is discussed in this paper. With considering the dynamics of the grid equivalent inductance, the small-signal model of PSC-controlled converter is established. By this, the mechanism of SFR and the non-minimum phase effect are revealed. Furthermore, a conjugate poles elimination control method based on auxiliary branches introduced into the power control loops is proposed to suppress SFR, which can also broaden the control bandwidth compared with VR method. Finally, the theoretical analysis is verified by simulations and hardware-in-loop experimental results.

A Power-Angle-Based Adaptive Overcurrent Protection Scheme for Grid-Forming Inverter Under Large Grid Disturbances

As the capacity of renewable energy generation increases, grid-forming (GFM) inverters are deemed as promising solutions for low inertia power grids. However, power-electronic-based inverters have limited overcurrent capability, so additional overcurrent protection schemes are necessary. More importantly, the stability should not be jeopardized by using additional overcurrent protection schemes. However, GFM inverters with the conventional current reference limiting method tend to be unstable under large grid disturbances such as grid frequency drop. To address this problem, this paper proposes a virtual power angle limiting method. The main idea of this method is to limit the output power by limiting the power angle instead of directly limiting the current reference. Thus, the power synchronization control law is still satisfied when the overcurrent protection is triggered, so that the stability can be maintained. Besides, after redesigning the power angle limiting value, the proposed method is also suitable for grid voltage sag cases. A significant advantage of the proposed method is that the output current can be limited automatically during grid voltage sag or frequency drop events without the need of fault detection or tuning control structures and parameters. Simulation and experimental results have verified the effectiveness of the proposed method.

Stability analysis of DFIG controlled by hybrid synchronization mode and its improved control strategy under weak grid

AbstractIn order to improve the stability of doubly‐fed induction generator (DFIG)‐based wind turbines under weak grid, previous works have proposed the power synchronization control strategy in the rotor side converter (RSC). However, the potential instability risk of dc‐link voltage control in the grid side converter (GSC) has been neglected. Here, DFIG with power synchronization and phase‐locked synchronization for RSC and GSC respectively, is called as hybrid synchronization mode (HSM)‐controlled DFIG and, the full‐order state space model of HSM‐controlled DFIG are established. Eigenvalue and participation factor analysis shows that the HSM‐controlled DFIG is unstable under extremely weak grid due to the interaction of phase‐locked loop (PLL) and current‐loop in GSC. Furthermore, dominant pole analysis indicates that the HSM‐controlled DFIG exhibits weak damping at the AC side in strong grid. To enhance the stability of HSM‐controlled DFIG under extremely weak grid, an improved hybrid synchronization control strategy is proposed. The improved control strategy utilizes active power and dc‐link voltage to achieve PLL‐free operation of DFIG. The effects of the parameters in dc‐link voltage synchronous loop on control bandwidth, system stability and dynamic coupling are analysed. Finally, the validity of the theoretical analysis is investigated with experiments based on 2MW control‐hardware‐in‐loop platform.

GRID INTEGRATION OF VOLTAGE SOURCE CONVERTER USING POWER SYNCHRONIZATION CONTROL

Renewable Energy Sources (RES) are integrated with electric power systems around the world at an accelerated pace. Due to their intermittent nature, integrating RES into the power grid is a challenging task. Because of their good controllability and superior performance in weak AC grids, the three phase Voltage Source Converters (VSCs) are generally used for this purpose. The grid connected converter has to correctly detect the grid voltage signal during voltage distortion and unbalance condition. In this research work, a control method using power synchronization for grid integration of VSC is developed and simulated. In this control technique, the electricity fed to AC grid from VSC is controlled using the internal synchronization mechanism as like in conventional synchronous machine. Utilizing this type of control, the instability of VSC affected by a standard Phase Locked Loop during weak grid condition can be avoided. Moreover, a VSC terminal could support a weak AC system with strong voltage, just similar to an ordinary synchronous machine organizer. Simulations are done in Matlab/Simulink to validate the system performance of grid attached VSC with the Power Synchronization Control.

Robust Grid-Forming Control With Active Susceptance

This letter proposes a grid-forming control scheme that is robust against a wide range of short-circuit ratio (SCR) of the ac grid. The approach modifies the traditional power synchronization control by adding an active susceptance loop, which enhances system damping for high stability robustness and enables to use a standard voltage-oriented vector current control. The latter can avoid major software changes on a classical grid-following control structure. Both small-signal dynamics analysis and experimental results confirm the effectiveness of the approach from ultraweak (SCR = 1, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ratio = 7) to stiff (SCR = 38.5) grids.

Phaser-Based Transfer Function Analysis of Power Synchronization Control Instability for a Grid Forming Inverter in a Stiff Grid

The grid forming inverter (GFM) is considered a key technology to solve the issues of inverter-based resource (IBR)-rich grids. Many studies have investigated how GFM affects and improves power system stability. However, few studies have considered the instability of GFM as it relates to various grid conditions. This paper discusses the mechanism of power synchronization control instability of GFM by conducting transfer function analysis with focus on various grid conditions, the control logic of GFM, and relevant parameter setting. Based on the above analysis, a procedure for power synchronization controller design of GFM is proposed. Experimental tests in the control hardware in the loop (CHIL) environment are performed to verify the validity of the transfer function analysis and reveal that the instability of GFM is likely to occur when it is connected to stiff grids or experiences a larger phase delay due to a longer power measurement delay and lower damping of the controller.

Impedance modelling and stability analysis of modular multilevel converter with different types of grid‐forming control schemes

With the increasing penetration of power electronics, grid-forming modular multilevel converters (MMCs) have attracted great attention in the upcoming MMC-based high-voltage direct current transmission (HVDC) projects. Power synchronization control (PSC) and direct power control (DPC) are two typical grid-forming control schemes for MMCs. This paper sets out to investigate the impedance-based stability characteristics of PSC-MMC and DPC-MMC. Utilizing the harmonic state space (HSS), equivalent impedance models of PSC-MMC and DPC-MMC are developed with the consideration of complete controllers and MMC internal dynamic characteristics. Impedance shaping effects of the major controllers are further analyzed to identify the frequency bands where the MMCs have negative resistive impedance characteristics. Finally, potential instability phenomena of the interconnected system are shown through the case studies, and the virtual impedance method is introduced as the stabilization control scheme. The electromagnetic transient simulation results based on PSCAD/EMTDC verify the accuracy of the impedance models and the effectivity of the stabilization control scheme.

Generalized power-angle control for grid-forming converters: A structural analysis

Several control schemes have been recently proposed and studied as grid-forming controls for power converters. In all these schemes, the power-angle control loop is the part which defines the fundamental capabilities of the grid-forming control: that control loop governs in fact the inherent synchronization mechanism of the power converter, the power sharing with the other generation sources in the system and the oscillatory characteristics of the converter-based resource. This article introduces a general formulation for the power-angle control characterizing the grid-forming concept for power converters. The generalized power-angle control is based on a polynomial fraction formulation, and it is arranged according to physical requirements and constraints. The structural analysis provides an insight into the capabilities and the design of the power-angle control loop of a grid-forming scheme. The generalized formulation is eventually applied to some common grid-forming controls like the power-synchronization control and the virtual synchronous machine, showing how the proposed generalized power-angle control can effectively realize different control structures.

Power Dynamic Decoupling Control of Grid-Forming Converter in Stiff Grid

This article proposes a grid-forming based decoupling approach, which effectively mitigates the dynamic interactions between active and reactive power, especially suitable for system stabilization in ultrastrong grids with short-circuit ratio over 30. The physical coupling behaviors are first revealed by analytical studies, where two potential resonances, respectively, located in 40–50 Hz and 0–10 Hz are discovered with mathematical proofs. Based on the results, the decoupling controller is then designed with transfer function approximation for practical usage. The control method not only suppresses the couplings but also cancels two pairs of conjugate poles through decoupling paths. Thanks to the cancellation, the stability margin is remarkably increased in stiff grids, and therefore, a significant bandwidth boost on the power synchronization control can be realized if desired. The experimental results are finally performed to verify the proposed decoupling method.

HVDC Interconnections with SMES Device for PV and Wind Hybrid Power Transmission System

Abstract: The consumption of power shows a steeply increasing behavior in the present scenario which demands the integration of various novel techniques to the existing grid. With the penetration of PV and wind energy units into the grid, the evolution from a generator dominated grid to an inverter dominated grid arises in the near future. The paper discusses the characterization of HVDC convertors with the behavior of SG emulating virtual inertia for frequency stabilization and proper power oscillation damping during transmission of renewable energy based power. The conventional control of HVDC converters is integrated with synchronous power controller with second order characteristics inside power control loop for proper damping essentialities. The functioning of HVDC link is enhanced by means of SMES coil based energy storage operation. The proposed formulation is applied to a four area model in IEEE 14 bus system incorporated with PV, wind and HVDC links. The dynamic behavioral improvements are tested by means of MATLAB simulation.

Generalized Multivariable Grid-Forming Control Design for Power Converters

The grid-forming converter is an important unit in the future power system with more inverter-interfaced generators. However, improving its performance is still a key challenge. This paper proposes a generalized architecture of the grid-forming converter from the view of multivariable feedback control. As a result, many of the existing popular control strategies, i.e., droop control, power synchronization control, virtual synchronous generator control, matching control, dispatchable virtual oscillator control, and their improved forms are unified into a multivariable feedback control transfer matrix working on several linear and nonlinear error signals. Meanwhile, unlike the traditional assumptions of decoupling between AC and DC control, active power and reactive power control, the proposed configuration simultaneously takes all of them into consideration, which therefore can provide better performance. As an example, a new multi-input-multi-output-based grid-forming (MIMO-GFM) control is proposed based on the generalized configuration. To cope with the multivariable feedback, an optimal and structured $H_{\infty}$ synthesis is used to design the control parameters. At last, simulation and experimental results show superior performance and robustness of the proposed configuration and control.

Review on grid-forming converter control methods in high-proportion renewable energy power systems

Grid-forming (GFM) converters can provide inertia support for power grids through control technology, stabilize voltage and frequency, and improve system stability, unlike traditional grid-following (GFL) converters. Therefore, in future “double high” power systems, research on the control technology of GFM converters will become an urgent demand. In this paper, we first introduce the basic principle of GFM control and then present five currently used control strategies for GFM converters: droop control, power synchronization control (PSC), virtual synchronous machine control (VSM), direct power control (DPC), and virtual oscillator control (VOC). These five strategies can independently establish voltage phasors to provide inertia to the system. Among these, droop control is the most widely used strategy. PSC and VSM are strategies that simulate the mechanical characteristics of synchronous generators; thus, they are more accurate than droop control. DPC regulates the active power and reactive power directly, with no inner current controller, and VOC is a novel method under study using an oscillator circuit to realize synchronization. Finally, we highlight key technologies and research directions to be addressed in the future.

Employing Machine Learning for Enhancing Transient Stability of Power Synchronization Control During Fault Conditions in Weak Grids

Grid-connected converters are exposed to the loss of synchronisation with the grid during severe voltage sags particularly when operating under weak grid condition which introduces voltage and frequency volatility. This paper presents employing machine learning methods besides modifying the converter control scheme to enhance the transient stability of power synchronization control (PSC). For early detection of synchronization instability of PSC to provide adequate time for taking correcting control actions, an encoder stacked classifier is proposed which is trained to be robust against data corruption and added noise. Then, by integrating the proposed instability detection scheme to the synchronization loop of PSC, a phase freezing mode is introduced to avoid losing synchronism during grid faults. It is disclosed that the frozen synchronization loop, which is activated by the proposed instability detection scheme, can ensure synchronization stability of PSC. Time-domain simulations are conducted to confirm the presented findings.

Frequency stability improvements based on automatic adjustment of synchronous power controller parameters

Frequency stability improvements based on automatic adjustment of synchronous power controller parameters

Architecture design of battery energy storage coordinated control system based on Multi-Agent mechanism

Abstract Energy storage can realize rapid voltage, frequency support and energy adjustment, and is an important guarantee for promoting the efficient consumption of renewable energy and the safe and stable operation of the power system. The flexible adjustment of energy storage is not only affected by its own characteristics, but also closely related to the operation control mode of energy storage. Based on the PQ constant power and virtual synchronization control strategy of the battery energy storage system, this paper constructs the operation architecture of the battery energy storage system based on the Multi-Agent cooperative mechanism, and further gives the energy storage system Multi-Agent cooperative control system’s application scenarios in active frequency modulation and reactive voltage regulation. Research shows that this architecture helps to fully tap the potential of aggregation and regulation of multi-point decentralized energy storage facilities and meet the flexible regulation needs of the power system under the access of high-permeability intermittent renewable energy.

Modelling and Analysis of Electromagnetic Time Scale Voltage Variation Affected by Power Electronic Interfaced Voltage Regulatory Devices

This paper establishes an instantaneous voltage model to illustrate the dynamic characteristics of grid voltage in electromagnetic time scale affected by power-electronic interfaced voltage regulatory devices with two typical categories of control systems, i.e., power-current cascading control and power synchronization control, exemplified by the STATCOM and virtual synchronous condenser (VSCON) respectively. In this paper, a linearized instantaneous voltage model is firstly established to analyze the impact paths of control loops in the STATCOM and VSCON. Based on the model, the dynamic process of grid voltage is divided into three stages. Various factors on the dynamic process of grid voltage are analyzed, especially cascading control loops prior to the functioning of AC voltage control. Moreover, stiffness characteristic is proposed to evaluate the effects of power electronic interfaced voltage regulatory devices with different control structures. In addition, the effectiveness of the stiffness compensation based on the VSCON is validated by the study cases.

Electromechanical Design of Synchronous Power Controller in Grid Integration of Renewable Power Converters to Support Dynamic Stability

Nowadays, modern power converters installed in renewable power plants can provide flexible electromechanical characteristics that rely on the developed control technologies such as Synchronous Power Controller (SPC). Since high renewable penetrated power grids result in a low-inertia system, this electromechanical characteristic provides support to the dynamic stability of active power and frequency in the power generation area. This goal can be achieved through the proper tuning of virtual electromechanical parameters that are embedded in the control layers of power converters. In this paper, a novel mathematical pattern and strategy have been proposed to adjust dynamic parameters in Renewable Static Synchronous Generators controlled by SPC (RSSG-SPC). A detailed dynamic modeling was obtained for a feasible design of virtual damping coefficient and virtual moment of inertia in the electrometrical control layer of RSSG-SPC’s power converters. Mathematical solutions, modal analysis outcomes, time-domain simulation results, and real-time validations of the test in IEEE-14B benchmark confirm that the proposed method is an effective procedure for the dynamic design of RSSG-SPC to provide these dynamic stability supports in grid connection.

Transient Synchronization Stability Control for LVRT With Power Angle Estimation

Power synchronization control (PSC) is an effective control method for a voltage-source converter (VSC) connected with a weak grid, where the short-circuit ratio is close to 1. The grid voltage sag will induce the synchronization instability of PSC-VSC and then trigger the catastrophic cascading failure in the power system. This letter presents the transient power angle nature of PSC under the grid disturbance by phase portrait, and the critical condition of transient synchronization instability for PSC is derived. The virtual orthogonal power (VOP) is elaborated and the power angle estimation method is presented by VOP. Then, a mode adaptive transient synchronization stability control scheme is proposed for the low-voltage ride through with the power angle estimation scheme. By using the proposed transient synchronization stability control scheme, the PSC-VSC can maintain synchronization with the grid during the fault of grid voltage sag. The control method theoretical analysis is verified by the experimental tests.

Centralized Synchronous Controller Based on Load Angle Regulation for Photovoltaic Power Plants

The integration of renewable energies based on photovoltaic power plants (PVPPs) is increasing exponentially fast. In consequence, stability problems associated with the lack of inertia are becoming a serious concern. Transmission and distribution system operators are demanding more requirements in PV plants to provide grid functionalities in terms of frequency and voltage support. Therefore, power converters based on virtual synchronous controllers have been proposed to emulate the dynamic behavior of synchronous generators. Control strategies such as the synchronous power controller have been proposed for PV applications to fulfill this requirement. However, most of these control strategies are integrated locally as an add-on firmware embedded in each converter, providing the demanded support in their connection point. For PVPPs, the inherent synchronous characteristic is expected at the connection point with the electrical grid, which is not always the connection point of each power converter; thus, unexpected power support levels can be obtained. The aggregation of several power converters operating in a coordinated synchronous action would solve this issue, due to the fact that they can provide voltage and frequency support in a specific point. In this article, a centralized control strategy based on a synchronous central architecture is proposed. The control strategy emulates an aggregated synchronous control to regulate a PVPP as a coordinated system. The simulation and experimental results validate the effectiveness of the control proposed.