Weak Grid Research Articles

Simulation of Influence of Load Model and Parameters on Frequency and Voltage Stability of Yunnan Power Grid

After fully asynchronous networking with Yunnan Power Grid and China Southern Power Grid, a multi-DC sending-end grid with a relatively weak grid structure and low load level, has prominent frequency and voltage stability, which makes Yunnan Power Grid have complex security and stability problems. Therefore, beginning with the load model and its variable, in this study, the effects of four static load models and four dynamic load models are compared with regard to the frequency and voltage stability of the Yunnan power grid’s asynchronous interconnection system. It simulates and analyzes how load model parameters affect the frequency and voltage stability of the transmitting end power grid of the asynchronous interconnection system while Puqiao DC is experiencing unipolar and bipolar blocking faults.

Distributed Energy Resources Based EV Charging Station With Seamless Connection to Grid

The charging of electric vehicles (EVs) via common DC bus charging infrastructure based on hybrid renewable energy sources such as solar photovoltaic (PV) and fuel cell is presented here. The requisite to incorporate renewable energy based distributed energy resources (DERs) is attributed to the escalating concern for decarbonisation with improved power quality requirements. Furthermore, the bidirectional flow of power enables the charging/discharging of EVs during the grid presence/absence modes of operation. In addition, the utilization of common DC bus charging mechanism for EVs, facilitates fast charging capability at higher voltage levels. The satisfactory operation during the grid availability/unavailability is attained through the current and voltage based control mechanisms, along with the seamless transition capability via switching (STS-1/0) of the static transfer switches. Furthermore, in compliance with the IEEE standards, the power quality (PQ) improvement is obtained with the utilization of an adaptive comb-filter based current control during the grid-tied mode of operation. The need for improving PQ, stems from the fact that an uninterrupted supply is essential to the critical loads along with an improved power quality. Thus, for validation and corroboration of the system behavior, its performance is authenticated during weak grid conditions, in conjunction with grid connected/islanded modes of operation.

Voltage and Frequency Regulation With WT-PV-BESS in Remote Weak Grids via Fixed-Time Containment Control

In this paper, a novel voltage and frequency regulation framework for the WT-PV-BESS in a remote weak grid is proposed. The BESSs are utilized to flexibly absorb or output power to address the issue of bus overvoltage caused by PV generation and system level frequency deviation caused by power output randomness. In this scheme, the BESSs are controlled by a kind of novel distributed control method called containment control. Its key feature is that it treats frequency and voltage as two independent containment dimensions, such that the BESSs will be controlled in a cooperative way to simultaneously restrict the system frequency and bus voltage within the allowable range. Meanwhile, the containment control can be achieved within fixed time to ensure the responsiveness of the BESSs. Simulation case studies are conducted to validate the performance and superiority of the method.

The parameters optimization and stable boundary expansion method of current controller for VSC to avoid low frequency oscillations

The low frequency oscillations that are raised by the interactions between the voltage source converter (VSC) and the weak grid, are becoming a critical threat to power system stability. Identifying the proper parameters for VSC is essential for preventing the appearance of oscillation in advance. To achieve this target, this paper proposes a quantitative evaluation method for the system’s stability to clarify the stable boundary of current control parameters. Then, an active damping controller is proposed and supplemented in the control of VSC, to expand the stable boundary of current control parameters. Thus, the VSC can stably operate within a wider range of parameters selection, the oscillation can be damped without weakening the dynamic control capability of the VSC. The simulation tests are performed to verify the correctness of the quantitative evaluation method and the effectiveness of the proposed stability boundary expansion control.

Grid-Connected Converters: A Brief Survey of Topologies, Output Filters, Current Control, and Weak Grids Operation

Grid-connected converters (GCCs) are used extensively for the integration of DC power sources with AC power sources. However, since it is a complex topic, there are many possibilities for regulating grid-injected currents, as well as different modulation techniques for generating full-bridge PWM voltages. The control techniques are directly related to the type of output filter, as well as to the topology of the converter, since a complex plant can require more sophisticated controllers to keep the system stable, and with good regulation performance. Furthermore, a discussion of the applicability of these converters in weak and very weak grids with high inductance content has recently been growing, which adds a greater degree of complexity to the control structure of the converter. In this brief overview are outlined some topics about topologies, output filters, and control, focusing on the current regulation of grid-connected converters. In addition, a discussion of the main challenges and critical areas in operating on weak and very weak grids is also presented.

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.

A phase compensation method combined with power correction for suppressing sub-synchronous resonance of wind generation under weak grid

Since three-phase grid-connected inverters (GCI) play a crucial role as the connection between renewable energy systems and the grid. They are extensively used in wind farms. However, issues related to sub-synchronous resonance (SSR) have been observed in wind farms, which pose a significant threat to the safe and stable operation of large-capacity wind farms and the grid. In this paper, the mechanism and characteristics of the SSR occurring in the GCI system under weak grid are analyzed based on the impedance model. And derives that the origin affecting the stability of the system is the asynchrony of system frames and control frames. Then, a phase compensation method employed in phase-locked loop (PLL) is proposed to suppress SSR, which can introduce a compensation frame to reduce the angle difference with the system frame. The grid strength can be enhanced equivalently. Meanwhile, virtual resistance and virtual reactance will also be introduced through the introduction of reactive power. It will further provide damping to the system, but will limit power transfer capability. Thus, the power needs to be corrected to achieve precise power control by change the value of current reference. By employing the proposed method, the SSR issues of the GCI under weak grid can be effectively solved and achieve precise power control. Finally, the experimental results verify the correctness of the theoretical analysis and the effectiveness of the proposed method

Research on Fuzzy Active Disturbance Rejection Control of LCL Grid-Connected Inverter Based on Passivity-Based Control in Weak Grid

In response to the problems of impedance variation and rich background harmonics in the weak grid environment of traditional LCL grid-connected inverters, which cause distortion of the grid-connected current and reduce the stability of the entire grid-connected system, this paper proposes a passive fuzzy-active disturbance rejection control strategy. Firstly, passivity-based control (PBC) is introduced into the grid-connected inverter system to ensure the global stability of the entire system. Secondly, in order to solve the problem of difficult selection of inverter-side current, active disturbance rejection control (ADRC) is used to directly track the grid-connected current, avoiding the tracking error of AC signal and suppressing the interference of background harmonics of grid voltage. The combination of fuzzy control and ADRC simplifies the system parameter tuning process, and the FUZZY-ADRC grid-connected current outer loop and PBC inductance current inner loop are combined to form a multi-loop control system. Finally, simulation comparison verifies that the proposed control strategy can effectively suppress the harmonics of grid-connected current, improve the quality of grid-connected current, and enhance the stability of the entire grid-connected system in a weak grid environment.

PLL phase margin design and analysis for mitigating sub/super-synchronous oscillation of grid-connected inverter under weak grid

Under weak grid, the grid-connected inverter can easily cause sub/super-synchronous oscillations, which are determined by the oscillation modes of system. Firstly, based on the eigenvalue analysis, the sub/super-synchronous oscillation modes of the grid-connected inverter are revealed with considering the phase-locked loop (PLL) and control delay. With the decline of grid stiffness, the typical roots of sub-synchronous oscillation (SSO) mode gradually shift from the left-half plane to the right-half plane, which means that the grid-connected inverter will oscillate under weak grid. By analyzing the participation factor of SSO mode, it finds that PLL is the dominant factor. Based on the root locus analysis, the stable regions of PLL parameter are obtained. Then, according to the stable regions of PLL parameter, the phase margin of PLL can be calculated, which can make the grid-connected inverter operate stably. It finds that PLL phase margin should be increased for suppressing this SSO. However, when the PLL phase margin is greater than a certain value, the system will enter a supersynchronous oscillation mode, which can cause a supersynchronous oscillation. Thus, to suppress the sub/super-synchronous oscillations of the grid-connected inverter under weak grid, the phase margin of PLL should neither be too small nor too large. Finally, the proposed suppression strategy of the sub/super-synchronous oscillations for the inverter integrated into a weak grid is validated with the simulation and experimental studies.

Sustainable development and applications of variable frequency transformer (VFT)

Power system interconnection is achieved by incorporating phase shifting transformers (PST) and back-to-back high voltage direct current (B2B HVDC) systems. Incorporating PSTs in an electrical power system is highly undesirable owing to its step wise variation of power transition. On the other hand, B2B HVDC systems are rarely employed in weak grids, due to their massive consumption of reactive power for device commutation. The variable frequency transformer (VFT) is capable of resolving the concerns described above by delivering improved performance on account of lower reactive power consumption besides providing smooth controllable power. This paper presents a critical review of VFT depicting its sustainable development for power system interconnection. To start with, an overview of VFT is presented followed by a brief description of other power flow controllers. These power flow controllers are compared with VFT to highlight its advantages over other conventional power flow controllers. This paper further identifies role of VFT in improving system stability and suppressing low frequency oscillations. This study also identifies the possible gaps for incorporation of VFT in power systems that have not been reported in literature and intends to serve as a useful reference for researchers working in the field of power system interconnection.

Robust Loop Correction Scheme Based on Reference Model for Inverters in Weak Grids

Grid-connected inverters are widely used to interface renewable energy and energy storage resources into power grids. However, the impedance of the power transmission line is relatively large in weak grids, which can affect the stability of grid-connected inverters. In order to improve the stability of inverters in weak grids, a robust loop correction method is proposed in this article. First, a reference model with good stability is designed. Then, a high-pass filter is used to obtain the high-frequency components of the reference model response, and a low-pass filter is used to obtain the low-frequency components of the actual system response. The outputs of the two filters are added together to form the actual feedback current. Consequently, this method can correct the frequency characteristics of an inverter in high-frequency range, so that the inverter still has a good stability margin in a weak grid. The effectiveness of the proposed method is verified by both simulation and experimental results.

Grid Current Control for Active-Front-End Electric Propulsion Systems in AC Ship Microgrids

This article proposes a finite control set (FCS) current controller for grid-tied converters, specifically tailored to meet the characteristics of ac ship microgrids in all-electric ships (AESs). Contrary to a pulsewidth modulator (PWM)-based design, which often employs an LCL filter, the proposal is able to meet applicable harmonic regulations using an L filter with the same total inductance as the LCL filter and the same switching frequency required by PWM-based solutions. Moreover, the proposed design provides a low sensitivity in a wide frequency range from dc to the switching frequency of the power converter. This low sensitivity permits to quickly attenuate low-order disturbances and it is particularly convenient in marine applications with a weak grid, where the frequent connection and disconnection of high power nonlinear loads can significantly affect the voltage waveform quality.

A Multiobjective Control Strategy for Harmonic Current Mitigation With Enhanced LVRT Operation of a Grid-Tied PV System Without PLL Under Abnormal Grid Conditions

This article proposes a novel multiobjective control strategy (MOCS) for improving the performance of grid-tied photovoltaic (GTPV) system without phase-locked loop (PLL) operating under abnormal grid voltage conditions such as harmonically distorted voltages, grid faults, and weak grid. The proposed control strategy effectively mitigates the harmonics in grid injected current, eliminates the oscillations in grid injected power, and enhances the low-voltage ride-through (LVRT) capability simultaneously during abnormal grid conditions. The LVRT capability of the GTPV system is enhanced by incorporating an active power regulator (APR) with dynamic reactive power support (DRPS) controller in the proposed control strategy. The proposed MOCS is designed in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> stationary reference frame. Furthermore, the proposed MOCS consists of an advanced phase compensated multiresonant (APCMR) current controller and a phase-shifted reference current generator (PSRCG) integrated with the APR with DRPS controller. The APCMR suppresses the grid current harmonics under weak and distorted grid voltages and the PSRCG eliminates active and reactive power oscillations at the grid side caused by grid faults. The synchronization mechanism of the GTPV system is included in the PSRCG, and therefore, the requirement of PLL is avoided. Consequently, the associated computational burden on the digital controller is reduced. The effectiveness of the proposed control strategy is validated using the OPAL-RT digital simulator and offers superior performance compared to the conventional control strategy (COCS).

The Stability of LCL-Type Grid-Tied Inverter Based on Repetitive Control and Grid Voltage Feed-Forward

The grid voltage feed-forward can improve the dynamic performance of the repetitive controller (RC), but it will seriously weaken the stability under non-ideal grid conditions. This article focuses on the stability and dynamic characteristics of a grid-tied inverter with RC and grid voltage feed-forward. The internal model filter of RC uses an infinite impulse response (IIR) filter to improve the steady-state performance. First, an equivalent digital control model with RC and feed-forward is established, the parameters of RC are designed, and the system’s stability is analyzed based on the small gain theorem. Then, it is verified that the stability of RC in the presence of grid impedance can be weakened by the feed-forward term, thus a better feed-forward scheme is required. In addition, the solution for weak grid condition is proposed, and different feed-forward algorithms are compared from various perspectives, which is concluded that the improved proportional and first-order differential feed-forward scheme has a better performance. Finally, the validity of the controller is verified by the experimental prototype of a single-phase <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LCL$ </tex-math></inline-formula> -type grid-tied inverter.

Robust PLL Synchronization Unit for Grid-Feeding Converters in Micro/Weak Grids

A grid-feeding voltage source converter (GFD-VSC) requires a phase-locked loop (PLL) synchronization unit to be connected to the grid. The PLL critically affects the dynamic performance and stability of the GFD-VSC. In particular, a PLL with pre/in-loop filtering, for working under distorted/polluted conditions, possesses a narrow stability margin and deficient performance in weak grid connections and fault ride-through (FRT) transients, also poor performance in frequency estimation. To address these problems, a robust PLL with several enhanced characteristics is proposed in this article. The robust PLL with a dynamic state feedback controller is designed using an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\bm{H}}}_\infty $</tex-math></inline-formula> robust control. The feedback controller is designed to improve the dynamic stability/response of the PLL, exposed to control uncertainties and exogenous disturbances, weak-grid connection, FRT transients and to improve its performance in frequency estimation. Numerical simulations validate the effectiveness of the proposed PLL.

Artificial Neural Network-Based Constrained Predictive Real-Time Parameter Adaptation Controller for Grid-Tied VSCs

This article proposes a real-time algorithm for identifying the grid parameters, which is concurrently used for online tuning of the predictive controller in each iteration, in a grid-tied active front end (AFE) voltage source converter (VSC) applications. The algorithm is designed by inspiring from the concepts of the extended Kalman filter (EKF) and the model predictive control (MPC). The performance of the algorithm highly depends on the weighting factors of the algorithm. The artificial neural network (ANN)-based algorithm is used to find the optimal set of weighting factors among the ones in a parameter search block. An offline particle swarm optimization (PSO) is run to provide the data source for the parameter search block. The algorithm identifies not only the inductance but also the resistance of the grid. In addition, the hard constraints on the amplitude of the input and output variables are guaranteed. The validation of the proposed approach is performed experimentally and compared with the state-of-the-art conventional methods. The experimental results show that the proposed method could effectively stabilize the system in weak grid conditions and under wide impedance variations. In addition, the accuracy of the proposed impedance identification method is 96%.

On Transient Instability Mechanism of PLL-Based VSC Connected to a Weak Grid

This paper analyzes the transient process of Phase Locked Loop (PLL) based grid-connected Voltage Source Converters (VSCs) under large disturbances by modeling the PLL dynamic model and active power loop model. With a focus on the effect of transient grid voltage changes on the stability of the PLL-based VSCs, it reveals that the mechanism of transient instability of PLL-based VSCs is different under various disturbance levels. It can be concluded that prior to the PLL losing synchronization, the power loop has already become unstable. In addition, the influence of the PLL prefilter used for asymmetrical faults on the stability criterion is analyzed. Moreover, based on the transient instability mechanism, two solutions are proposed to deal with different instability occasions, and the adaptability of these solutions is analyzed. Finally, hardware-in-the-loop experimental results validate the proposed transient instability mechanism and the proposed control strategies.

A phase feed-forward method to enhance inverter-grid system stability

The interaction between the inverter and the grid in weak grids may produce resonances that lead to reduced power quality and instability concern. The recently proposed control methods based on eliminating disturbances in the control loop avoid the challenging control parameter design faced by the existing stability-enhancing control methods. They are effective to overcome the instability concern, but may not be able to maintain stable operation of the system in certain situations. Moreover, they affect the steady-state output current of the system under the fluctuation of the grid. This paper proposes a phase feed-forward method in the dq-frame to overcome the problems encountered by these control methods, such as duty cycle perturbation and grid phase jump. The proposed method constructs an integrator and a high pass filter path outside the PLL. It is simple and readily realized in the traditional control structure. Simulation and experimental results are presented to demonstrate the effectiveness of the proposed control method.

Comparison of the Grid and Off-Grid Hybrid Power Systems for Application in University Buildings in Nigeria

The Nigerian Universities rely on weak and unreliable fossil-based electric grids with diesel engine generators (DEG) as a backup. However, there is a potential to light up the campuses using power systems derived from primary renewable power systems (RPS) like wind turbine (WT) and solar photovoltaic (PV), that can be on or off-grid to improve the energy mix and duration reliably. This study presents the comparative analysis of the optimal hybrid grid and off-grid systems (OGS &amp; OOGS) for serving the demand load of university buildings in four climatic regions of Nigeria. HOMER Pro is used to design and select the systems based on minimal net present cost (NPC) and cost of electricity (COE). The impact of a minimal renewable fraction of 95% on the optimal system architecture (OSA) and COE is studied for both grid and off-grid modes. Also, sensitivity analysis of the impact of key variables on performance for the sites is carried out. It is found that the OGS in the four regions is PV/Converter (Conv), while for the OOGS, it is PV/WT/DEG/battery (BB)/Conv except in Port Harcourt (PH), where it is PV/DEG/BB/Conv. The COE for the OGS in the Savana and monsoon climes of Enugu and PH are 10 and 19% more than that in the warm-semi arid climate zones of Maiduguri and Kano, which is approximately 0.09 $/kWh. The COE ($/kWh) for the OOGS is 0.21 in Maiduguri, 0.245 in Kano, 0.275 in Enugu and 0.338 in PH. An obligatory 95% RF changes the architecture and increases COE in all the locations except Maiduguri, with a slightly improved COE but higher NPC like other locations. It is established that the suggested hybrid system is beneficial and feasible for supplying more reliable and clean energy to educational buildings in different Nigerian locations.

Comparative Study of the Transmission Capacity of Grid-Forming Converters and Grid-Following Converters

The development trend of high shares of renewables and power electronics has increased the demand for new energy converters in the power system, but there is a lack of systematic research on the stability of different types of converters when transmitting power, which is worth exploring in depth. In this study, the power transfer capabilities of grid-forming and grid-following converters are investigated separately through an equivalent circuit diagram and phasor diagram when connected to the grid, and a quantitative relationship between converters’ power transmission limit and short circuit ratio under static stability conditions is obtained, leading to the conclusion that, in terms of power transmission, grid-forming converters are more suitable for weak grids with high damping and low inertia, whereas grid-following converters are more suitable for strong grids with high inertia. The conclusions are further verified by constructing the converter grid-connected models for different grid strengths through the PLECS simulation platform and the real-time simulation RTBOX1 and F28379D launchpad platform.