Grid Synchronization Research Articles

Improvement of Power System Frequency Stability With Universal Grid-Forming Battery Energy Storages

Large-scale integration of inverter-based renewable generation leads to the reduction of power systems’ natural inertia. Therefore, the dynamics of the future power systems will be more sensitive than in the traditional systems with high-inertia rotating synchronous generators. This development is a potential risk for frequency stability and requires utilization of rapidly controllable resources for dynamic frequency stability support. Simultaneously, development of new synchronization and control methods for inverter-based resources is needed in order to ensure the frequency and synchronization stability of future power systems. In this paper, a grid-forming and supporting universal frequency-locked-loop -based control and grid synchronization for inverter-based resources is utilized to improve the frequency stability of a small high-voltage network. The simulations are done with PSCAD software and the main focus is on the battery energy storages to evaluate the effect of their location, enhanced control schemes as well as operation mode on frequency stability. In the studies, for example, the effect of battery storages location, active power response related control parameters, communication time delay and input frequency determination on frequency support are studied during charging and discharging of the batteries. Based on the simulations, also new solutions to improve the frequency stability of future variable inertia power systems with universal grid-forming battery storages are proposed.

Initial analysis of the impact of the Ukrainian power grid synchronization with Continental Europe

Following the Russian invasion of Ukraine in February 2022, the Ukrainian power grid was synchronized to the Continential Euopean grid. We show how power flows changed, while frequency statistics remained almost unchanged.

Resilient Control Algorithm for Wind-Hydro Based Distributed Generation System with Grid Synchronization Capability

A microgrid is key to an efficient and resilient power grid operation. Thus, in this paper, a robust control scheme is implemented namely a modified frequency locked loop (MFLL), proficient in eliminating the voltage and current fluctuations because of phase/frequency variation. Therefore, enhancing the consistency of the supply across local nonlinear load through a smooth transition from utility-interactive mode to islanded mode. It also achieves accelerated convergence and reduced steady-state error. MFLL is an easy control technique, with only one interfacing voltage source converter, which delivers a stable supply to critical loads during different operating modes. The implemented system has the main advantage of the capability of power-sharing between distributed generation (DG) systems while regulating the voltage at the DC link. It also provides power quality improvement through harmonics suppression; voltage regulation during contingencies like unbalance in load and compensation in reactive power at the common coupling point following the prerequisite of the system. Performance through the implemented control algorithm is brought within MATLAB 15b/Simulink platform and validated through experimental results. Performances are compared with already reported control strategies. Thus, simulated results comparison with existing work presents the competence of the implemented control strategy under dynamic conditions.

Self-reliant solar PV based microgrid with seamless transition capabilities at weak grid conditions

• The extended proportional complex filter (EPCF) control allows a fast estimation of the quadrature and in-phase fundamental load current components with reduced harmonics within the prescribed limit. • The EPCF-based control has extracted the fundamental component of load current with high rate of convergence and better system response. The variations on load, utility, and solar array radiation are well established in simulation and test results. The PQ is enhanced at unbalancing of load, change in solar insolation and grid adverse conditions of voltages unbalances. The performance of the system is found satisfactory and its response is enhanced at adverse grid conditions. Experimental results have demonstrated that the THD are as recommended by the IEEE-519 standard even during weak grid conditions. • A comparative analysis is included, which validates the superior filtering capability and improved power quality during weak grid conditions. In addition a tabular comparative analysis is included to support its effectiveness while addressing abnormalities of the grid. • The EPCF-PLL grid synchronization and phase angle estimator scheme has provides fast mode transition, frequency tracking and uninterrupted supply to the load during non availability of the grid. This has given precise phase angle estimation. Conventional PLL controllers are non-adaptive towards large frequency. EPCF-PLL has provided effective frequency adaptability. This makes the synchronization process immune from frequency variation and voltage unbalance. • The proportional resonant controller with harmonic compensation (PRHC) controller provides satisfactory performance under all the intermittent conditions overcoming the tradeoff between improved steady-state and transient performance. This paper presents a solar photovoltaic (PV) based energy conversion system with seamless transfer to the grid. To provide satisfactory performance in grid connected mode, a control based on extended proportional complex filter (EPCF) is used. The EPCF is used to estimate the fundamental current component (LCFC) of local loads At the linkage point (LP), the power quality (maintaining IEEE-519 standard) is improved using this control. The EPCF maintains balanced grid currents whether PV array generation is available or not. It provides effective mitigation of harmonics and monitors the flow of active power, which improves the grid current quality.This control provides versatile actions such as extraction of LCFC, harmonics elimination, DC offset removal, robust performance during voltages unbalance etc. The same control technique (EPCF) along with phase locked loop is used for smooth synchronization; it reduces the use of another algorithm, which improves the dynamic response of the system. This control estimates exact phase and frequency variation for the seamless mode switching. It provides frequency adaptation during mode shifting. The comparative results validate that the control based on EPCF has a faster detection of grid in comparison with existing controls. During the grid-outage, the proportional resonant (PR) controller with harmonics compensation (HC) based control algorithm enhances performance of the microgrid. This provides successful solution for harmonics compensation and minimizing steady state errors. Simulated and experimental results, in various scenarios, have verified the effectiveness of the controllers and demonstrate the favourable solution in controlling the unfavorable grid conditions.

Internal Energy Based Grid-Forming Control for MMC-HVDC Systems With Wind Farm Integration

The virtual synchronous generator (VSG) control is regarded as an effective solution for operating converters in weak grid conditions due to its excellent grid support functions. However, successful implementation of the VSG control of an AC/DC converter relies on the existence of a steady DC voltage at the DC side, while this is not easy to achieve for the cascaded converter system of this work, i.e., the Modular Multilevel Converter-based High-Voltage Direct-Current (MMC-HVDC) transmission systems with offshore wind farm integration. To address this issue, a novel grid-forming control strategy with real-time inertia support and direct DC-link voltage regulation is proposed for the Receiving End Converter (REC) of the MMC-HVDC. The intrinsic power balancing regime of the internal energy stored in MMC's submodule (SM) capacitors is utilized for grid synchronization rather than emulating the swing equation as requested by the VSG control. For being more robust to sudden power variations, proper insertion strategies of the REC are developed to decouple the DC-link voltage of MMC-HVDC from the voltage of SM capacitors. Furthermore, in terms of the grid fault-ride-through issue of the REC, an associated FRT control strategy is proposed aiming for power angle stability and the stability of SM capacitor voltage. Finally, simulation results in PSCAD/EMTDC show that the proposed control can provide fast inertia support with satisfactory control of DC-link voltage and FRT capability, etc.

PLL-Less Active/Reactive Power Control of Photovoltaic Energy Source with Applying pq-Theory in Single-Phase Grid System

Converter systems are applied to manage active/reactive power control of photovoltaic (PV) source during integration with the electric grid system. The control algorithm of the conventional converter system consists of a reference generation unit, a dc-link voltage control loop, two power control loops and a phase lock loop (PLL) system. PLL unit is used to lock in phase angle of the electric grid, and to perform the coordinate transformation for calculations of the active/reactive powers. However, the control algorithm has a slow dynamic response because of utilisation of a PLL structure. In addition, additional complex mathematical computations are required with the use of a PLL algorithm. Furthermore, the interaction of the PLL and the power control loops may lead power oscillation problems under weak grid, and also result in instability of the PV system. In this study, to avoid the aforementioned issues and to enhance the power flow capability of the grid-connected PV panels, a PLL-less control algorithm in pq-theory is studied for the active/reactive power management and the grid synchronization. In addition, the mathematical formulations of the current control algorithm are presented in detail. To show the effectiveness of the PLL-less controller, a PV system model with using real PV panel groups is designed and constructed in a simulation environment. The proposed control method is tested under various operation cases such as dynamic environmental conditions, reactive power support and voltage variations. The proposed method shows efficient performance under applications of the different operation situations.

Design and Development of a B-Type Inverter for Harmonic Mitigation in a Grid Integrated System Using Whale Optimization Algorithm

This paper introduces a B-type inverter and an improved Y source DC-DC converter that use a selective harmonic elimination (SHE) method based on whale optimization to inject power into the grid with reduced harmonic content. The suggested topology contains four unidirectional switches, two bidirectional switches and three DC sources to produce the required output voltage level. The proposed topology can be functioned with equal and unequal DC sources. The developed structure can generate the negative voltages without the use of an H-bridge. It has more redundant states, and it is partially fault tolerant in nature. The proposed structure provides a high value of level to switch ratio. A single input multiple output high gain modular Y-source DC-DC converter is implemented for effective utilization of the PV modules in the suggested system. The triggering angles for the suggested system is generated using the SHE based whale optimization algorithm (WOA). Using the SHE-WOA method a total harmonic distortion (THD) of is obtained. The developed system is integrated with the grid and for grid synchronization PI based WOA method is used. Simulation study has identified the presence of only 2.47% of harmonics in the grid current and 2.03% in experimental analysis. Both these harmonic distortions are less than the IEEE-519 standards.

Design and control of Takagi-Sugeno-Kang fuzzy based inverter for power quality improvement in grid-tied PV systems

The addition of solar photovoltaic energy penetration to the electric grid is widely increasing across the world due to the many advantages such as clean energy source, easy to install and improved performance of power converters for the grid synchronization, and so on. Instead of many stages taking place for the grid-tied, a single stage is preferable to avoid the greater number of power electronic converters connected in the system which in turn reduce the cost of the overall system. In general, the DC-to-DC converters are used in the PV systems to track the maximum power point. However, an inverter circuit is needed to synchronize the source to grid and it can be utilized to extract maximum power from PV systems through proper controlling techniques. The proper control of inverter avoids the DC-to-DC power converter usage for the maximum power point tracking purpose. On the other hand, this technology can be used for small scale solar power plants. An irregular irradiance of solar is a bottleneck for the solar systems and to overcome it, a Takagi-Sugeno-Kang Fuzzy (TSKF) controller is designed for controlling the inverter operation in this paper. The TSKF controller responds very quickly to the input rapid changes of solar irradiance than the conventional proportional integral controllers. In this paper, a single stage 1 MW rating solar PV grid system is designed and verified the performance of the proposed controller under different loading conditions. Further, the proposed system is loaded with local loads and their reactive compensation is evaluated through hardware-in-loop on the platform of OPAL-RT software.

Small-Signal Stability Boundary of Heterogeneous Multi-Converter Power Systems Dominated By the Phase-Locked Loops’ Dynamics

While renewable resources are increasingly integrated into the electric power system, the small-signal instability risk may be induced by grid-following converters using phase-locked loops (PLLs) for grid synchronization, especially in low short-circuit strength systems. The stability analysis complexity is further increased in such a heterogeneous multi-converter system (HMCS), where multiple converters may have different PLLs’ control parameters provided by various vendors. To understand how the different PLLs’ dynamics collectively affect the small-signal stability of the HMCS, we transform the HMCS into an equivalent homogeneous multi-converter system for the small-signal stability analysis. Then, we analytically derives the small-signal stability boundary condition of the HMCS dominated by the PLLs’ dynamics (HMCS-DPLL). The derived stability boundary allows us to obtain analytical results about how the small-signal stability of the HMCS-DPLL is collectively affected by network structure, converter operating conditions, and different PLLs’ control parameters. Based on the stability boundary condition, a computationally efficient method is also proposed to identify the design rationality of PLL control parameters as well as the small-signal stability and stability margin of the HMCS-DPLL. The analytical results and proposed method are validated by modal analysis and electromagnetic transient simulation with the full-order models on a 9-converter heterogeneous system.

Direct and Sensorless Grid Synchronization of DFIG—Control the Availability of WECS/HEPS for Disturbed Grid Requirements

Grid integration of DFIG based-WECS/HPES has gained special attention in recent codes, especially under voltage disturbances. The main object of this requirement is to ameliorate the grid stability from the frequency point of view and increase the integration ratio of renewable energy conversion systems like WECS and HPES. The availability of such grid requirements depends heavily on the viability of the grid synchronization procedure of DFIG, which has not been thoroughly investigated under weak-grid conditions. It also depends on withstanding capacity of DFIG against over-voltages or so-called HVRT capability. In this regard, the present paper introduces a smooth and sensor-less grid synchronization strategy of DFIG under disturbed grid voltage to ensure HVRT capability. The feasibility of the proposed method is proved analytically and assessed via experimental and simulation results.

Revisiting Grid-Forming and Grid-Following Inverters: A Duality Theory

Power electronic converters for integrating renewable energy resources into power systems can be divided into grid-forming and grid-following inverters. They possess certain similarities, but several important differences, which means that the relationship between them is quite subtle and sometimes obscure. In this article, a new perspective based on <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">duality</i> is proposed to create new insights. It successfully unifies the grid interfacing and synchronization characteristics of the two inverter types in a symmetric, elegant, and technology-neutral form. Analysis shows that the grid-forming and grid-following inverters are <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">duals</i> of each other in several ways including a) synchronization controllers: frequency droop control and phase-locked loop (PLL); b) grid-interfacing characteristics: current-following voltage-forming and voltage-following current-forming; c) swing characteristics: current-angle swing and voltage-angle swing; d) inner-loop controllers: output impedance shaping and output admittance shaping; and e) grid strength compatibility: strong-grid instability and weak-grid instability. The swing equations are also derived in dual form, which reveal the dynamic interaction between the grid strength, the synchronization controllers, and the inner-loop controllers. Insights are generated into cases of poor stability in both small-signal and transient/large-signal. The theoretical analysis and simulation results are used to illustrate cases for single-inverter systems, two-inverter systems, and multi-inverter networks.

Improved Cascaded SOGI Control for Islanding-Synchronization in Photovoltaic System

This work presents an improved cascaded second-order generalized integrator (I-CSOGI) controlled photovoltaic system (PVS), which functions as an islanded system during grid interruption without energy storage. Here, a single-stage PVS structure is used, simplifying the control by automatically regulating the PV array operating point and dc-link voltage during the grid outage. The I-CDSOGI algorithm is exercised to determine the sensed voltages angle, frequency, and harmonics free positive sequence components of the sensed voltages and load currents. The advantage of the presented I-CDSOGI over the existing CDSOGI algorithm is that the gain of its frequency-locked loop is changed adaptively. It reduces the spurious change in the frequency during the sudden phase jump, which is experienced by the load voltages during synchronization. Hence, the repeated tripping of the power electronics switches, which isolates the PVS from the local grid, is reduced, and smooth synchronization is achieved. Furthermore, the cascading of two second order generalized integrator (SOGI) blocks eliminates the dc offset from the sensed signals and boosts the overall harmonics rejection capability, which enhances the angle and frequency estimation performance. A comparison is made to showcase the benefits of I-CSOGI compared with the existing SOGI-based methods in the PVS.

Robust Control and Optimization Method for Single-Phase Grid-Connected Inverters Based on All-Pass-Filter Phase-Locked Loop in Weak Grid

In a distributed generation system, the all-pass-filter phase-locked loop (APF-PLL) is a commonly used method for grid synchronization. However, the coupling effect between APF-PLL and current control loop increases the risk of oscillation instability for the inverter in the weak grid. At present, there are few effective methods to solve the adverse effect of APF-PLL on the inverter-grid interconnection system in the weak grid. Therefore, a small-signal impedance model of the inverter considering the dual d-q frame brought by APF-PLL is first established. Then the reason for the inverter instability caused by APF-PLL in the weak grid is analyzed. Subsequently, an impedance reshaping method based on a modified first-order filter PLL with a complex coefficient filter (CCF-MFOF-PLL) and its parameter optimization design method are proposed. Finally, the experimental results verify that the proposed method widens the stable range of the inverter and ensures the stable operation of the inverter even with the large grid impedance.

Modified LMS synchronization technique for distributed energy resources with DC-offset and harmonic elimination capabilities

Grid synchronization techniques are needed to improve the distribution system’s power quality with the Shunt Active Power Filter (SAPF). The traditional synchronization technique or the Phase Locked Loop (PLL) works well under ideal grid conditions; however, its performance deteriorates under non-ideal grid conditions. In this paper, a new scheme has been proposed to function as PLL. The Modified Least Mean Square (MLMS)-PLL has been proposed for tracking the phase angle under non-ideal grid conditions such as phase shift, frequency deviation, harmonics in the signal, DC offset and combinations of these grid voltage issues. The proposed method has added the advantage of DC-offset estimation and perfect synchronization template generation over conventional PLL. The MLMS-PLL algorithm precisely estimates phase, amplitude and frequency information and generates a synchronizing signal under abnormal grid conditions. The designed algorithm is extensively tested and shows advantages such as least steady-state error, faster dynamics and DC-offset rejection capability. Experimental findings of the proposed synchronizing technique show the effectiveness of the proposed technique and experimental results are also compared with other modern synchronization techniques. The application of MLMS-PLL for synchronization, reactive power compensation and harmonic elimination in a grid-tied PV system has been validated in the experimental prototype.

Discrete Multiple Second-Order Generalized Integrator With Low-Pass Filters and Frequency-Locked Loop for DC Rejection

The multiple second-order generalized integrator (MSOGI), which is composed of parallel second-order generalized integrators (SOGI), is widely used in grid synchronization and signal extraction, and it usually suffers from dc offset issue. In continuous domain, low-pass filter (LPF) can be added to SOGI to eliminate the dc offset in its quadrature channel. However, in digital implementation, directly discretized LPF-based methods may not be able to eliminate dc offset and ac discretization error simultaneously, where the steady-state error will also be introduced by the conventional frequency-locked loop (FLL). In this article, a modified impulse invariant method is adopted to discretize SOGIs, which presents zero ac errors and good dynamics, but shows dc offset in both output channels. Then, an LPF is introduced to eliminate the dc offset of the discrete SOGI, forming a discrete SOGI-LPF. And, a discrete MSOGI-LPF (DMSOGI-LPF) is formed by paralleling multiple DSOGI-LPFs, which can extract different frequency components. Furthermore, a discrete FLL is proposed to estimate the frequency of the input signal, which suppresses the influence of the dc offset and achieves good dynamics. Finally, the stability analysis of the proposed method based on discrete linear-time periodic (LTP) model is provided. The effectiveness of the proposed method and its discrete LTP model are validated through experimental results.

Reduced-Order Generalized Integrator-Based Phase-Locked Loop: Performance Improvement for Grid Synchronization of Single-Phase Inverters

Generalized integrator-based orthogonal signal generator phase-locked loops (GI-OSG PLLs) are very popular in single-phase grid synchronization. Generally speaking, the OSG of this PLL is realized from a second-order integrator perspective. As a result, the OSG implementation impacts the PLL dynamics. Improving the PLL performance by employing a reduced-order OSG implementation is the main motivation for this paper. Therefore, a reduced order-OSG PLL is proposed. In the proposed PLL, a single state and two output equations are employed to obtain a pair of orthogonal signals. This results in the loss of orthogonality between the OSG outputs and the single-phase supply voltage. Also, harmonic filtering capability provided by SOGI-OSG becomes non-existent. These issues are resolved by incorporating steady-state phase angle-offset error compensation at the PLL output and a moving average filter (MAF) within the PLL loop. To counteract the influence of MAF on the PLL’s stability and dynamics performance, a phase-lead compensator is designed. Through experimental studies, it is shown that the proposed reduced order-OSG PLL offers excellent dynamic performance. Also, the proposed PLL achieves minimal fluctuation of PV power when employed for grid synchronization of a PV system.

Assessing Maximal Capacity of Grid-Following Converters With Grid Strength Constraints

The increasing penetration of renewable resources via power-electronic converters is turning the modern power grid into a multi-converter system (MCS). In an MCS, most renewable resources currently use grid-following converters (GFLCs) for grid synchronization. The increasing integration of renewable resources via GFLCs can cause small-signal stability problems, especially under weak grid conditions. One way to prevent the stability issue is limiting the capacity of GFLCs in an MCS. Thus, it is important to assess the maximal capacity of GFLCs in the MCS while considering small signal stability constraints (SSSCs). This assessment is challenging due to 1) the complexity of assessing the small signal stability resulting from the complicated interaction between the power network and a large number of GFLCs, especially for a heterogeneous GFLCs with unknown inner parameters; 2) the difficulty of finding the optimal solution to the relevant nonlinear optimization problem for maximal capacity assessment. To address these challenges, this paper proposes a semi-definite programming (SDP)-based method to assess the maximal capacity of GFLCs with SSSCs. In the proposed method, we first formulate the SSSCs based on the generalized short-circuit ratio (gSCR), which is a grid strength metric that can significantly reduce the complexity of quantifying small signal stability in an MCS. Then, we convert the formulated gSCR-based nonlinear optimization problem into an SDP that can conveniently find the optimal solution to the maximal capacity of GFLCs and their optimal allocations in the MCS. The efficacy of the proposed method is demonstrated on a 39-bus test system and a practical wind power system.

Design and performance of self adaptive SRF-PLL technique for grid synchronisation

ABSTRACT This paper proposes an improved version of Synchronous Reference Frame (SRF) based phase locked loop (PLL). The conventional SRF-PLL structure is modified and made adaptive using a Least Means Square (LMS) algorithm. The benefit of the proposed scheme is that the tuning of proportional controller (PI) gain parameters can be achieved automatically without having to optimise these parameters manually. The PLL is utilised to track important electrical grid parameters such as phase angle, unit synchronisation templates and voltage magnitude. This performance study investigates several cases such as voltage sag/swell, wide frequency deviation, phase jump, presence of noise and harmonics in the supply voltage. The impact of constant and adaptive PI parameters influencing PLL performance is investigated. Finally, a fair comparison is presented which discusses the proposed adaptive SRF-PLL(ASRF-PLL), conventional fixed gain SRF-PLL, unit template-based synchronisation technique and Delayed Signal Cancellation based PLL (DSC-PLL).

Seamless Operation of Master–Slave Organized AC Microgrid With Robust Control, Islanding Detection, and Grid Synchronization

In this article, islanding detection, control, grid synchronization, and power share techniques have been considered for the seamless operation of ac microgrid in grid-connected mode, islanded mode, and during the transition between both modes. A master–slave organization coordinates the power distribution among microgrid sources. The control scheme of each source is designed with a nested loop robust linear quadratic regulator and mixed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H_{2}/H_{\infty }$</tex-math></inline-formula> optimal controller. The islanding detection algorithm is developed with synchro-extraction transform and optimal support vector machine (OSVM). An optimal linear Kalman filter-based synchronous reference frame phase locked loop (OLKF-SRFPLL) method is considered for the reconnecting of the microgrid to the utility grid. For optimal parameter tuning in the OSVM and LKF-SRFPLL methods, a particle swarm optimization technique is used. The seamless operation of the proposed ac microgrid system is verified in MATLAB/SIMULINK, and the practical feasibility is verified with an experimental photo-type setup and dSPACE 1202 control kit. Simulation and experimental results are presented to illustrate the seamless operation of the proposed ac microgrid system.

Interactive Grid Synchronization-Based Virtual Synchronous Generator Control Scheme on Weak Grid Integration

While high volume of power electronic converters with renewable energy are being integrated into the grid, low system strength of grid causes reduction in system inertia and loss of synchronization with power converters. This paper proposes a novel interactive control scheme of voltage source converter (VSC) connected to weak grid. Firstly, a grid synchronization approach is performed using VSC side voltage transformed on rotating reference frame with respect to grid voltage, which is carried out based on lowest short circuit ratio (SCR) as approved by utilities such as Australian Energy Market Operator’s system strength report. The transformed voltage is then fed into the active power and reactive power control through PLL. Secondly, the grid synchronization approach through PLL is made interactive by adding virtual synchronous generator (VSG) control into the outer loop, which generates virtual inertia to mimic the synchronous generators of a power system. The design of the VSG control parameters is realized based on small signal stability analysis. Furthermore, the control method is evaluated as per the National Electricity Rules clauses S5.2.5.1-S5.2.14 under AEMO and National Service Providers of Australian strict Industry guidelines. The time-domain simulation results are conducted on PSCAD platform.