Weak Grid Conditions Research Articles

Optimization and Configuration of Control Parameters to Enhance Small-signal Stability of Hybrid LCC-MMC HVDC System

This paper investigates the small-signal stability of the hybrid high-voltage direct current (HVDC) transmission system. The system is composed of line commutated converter (LCC) as rectifier and modular multi-level converter (MMC) as inverter under weak AC grid condition. Firstly, the impact of short-circuit ratio (SCR) at inverter side on the system stability is investigated by eigen-analysis, and the key control parameters which have major impact on the dominant mode are identified by the participation factor and sensitivity analysis. Then, considering the quadratic index and damping ratio characteristic, an objective function for evaluating the system stability is developed, and an optimization and configuration method for control parameters is presented by the utilization of Monte Carlo method. The eigenvalue results and the electromagnetic transient (EMT) simulation results show that, with the optimized control parameters, the small-signal stability and the dynamic responses of the hybrid system are greatly improved, and the hybrid system can even operate under weak AC grid condition.

Analysis and stabilization control of a voltage source controlled wind farm under weak grid conditions

Analysis and stabilization control of a voltage source controlled wind farm under weak grid conditions

Stability Assessment and Enhanced Control of DFIG-Based WTs During Weak AC Grid

According to the latest grid codes, wind turbines (WTs) are required to inject certain amount of reactive and active current into the faulty grid during voltage sags. However, studies have shown that the doubly fed induction generator (DFIG)-based WTs may lose stability under weak grid condition. So the impact of grid code requirement on DFIG system stability is worthy of investigation. To address such issue, a comprehensive small-signal admittance model of DFIG system is established in this paper. Then the influence of reactive/active current settings during grid fault is analyzed in detail. Furthermore, the stable operation region is derived, considering three key factors, i.e., the current demand of grid codes, capacity of WTs and stability constraint. Besides, in order to extend the stable operation region of the DFIG system, an enhanced control strategy based on voltage disturbance compensation is put forward. Finally, simulations and experiments are performed to validate the correctness of the theoretical analysis and effectiveness of the proposed control.

Research on Control Strategy for Improving Stability of Multiple Inverter Parallel System Under Weak Grid Condition

Research on Control Strategy for Improving Stability of Multiple Inverter Parallel System Under Weak Grid Condition

Robust Control of GTIs under wide grid impedance ranges: An approach combining metaheuristics and LMIs

This paper provides a new design procedure for robust state feedback current controllers suitable for grid-tied inverters under wide grid impedance ranges. The proposed procedure is based on the combination of particle swarm optimization and linear matrix inequalities. The control gains, designed offline, are obtained through the minimization of a novel objective function that takes into account the grid current tracking error in the stationary reference frame, limits for the control signal, and robust stability. Linear matrix inequalities are used in the design stage to theoretically certify the closed-loop robust stability systematically and efficiently, based on a polytopic model of the grid-tied inverter, which avoids the need for exhaustive and time-consuming a posteriori tests. Real-time simulations and experimental results are presented, attesting that the closed-loop system with control gains provided by the proposed procedure is robustly stable for wide ranges in the grid inductances. Moreover, the grid currents exhibit an optimized transient performance and steady-state responses in compliance with harmonic distortion limits from IEEE 1547 Std., even encompassing operation from stiff to weak grid conditions and in the presence of distorted grid voltages. To provide a comparison, results based on other well-established state feedback design techniques are also presented, highlighting the superior performance obtained with the proposed procedure.

AES-FLL control of RES powered microgrid for power quality improvement with synchronization control

In this paper, AES-FLL (Advanced Extremum-Seeking Frequency Locked Loop) control with DC-offset-rejection capability for power quality improvement is proposed. The attributes of the AES-FLL are a precise estimation of fundamental load current with low steady-state oscillations, fast dynamic response, DC-offset rejection capability and low THD. Further, the AES-FLL is capable of extracting the fundamental component of grid voltages and generates harmonic free reference grid current under weak grid conditions. Moreover, reactive power compensation, harmonics mitigation, unity power factor and coordinated power flow operation are accomplished using the AES-FLL control approach. A seamless transition is achieved from the grid-connected operating mode to the standalone operating mode during the grid outage to ensure power continuity. The grid synchronization is achieved through the LPN-PLL (Low-Pass Notch Filter Phase Locked Loop) control approach. Comparative analysis of the VSC (Voltage Source Converter) control approach AES-FLL and grid synchronization approach LPN-PLL with the conventional methods is presented. The performance of the microgrid is demonstrated by simulation and then presented a prototype experimental validation under nonlinear loads.

Optimal Controller Design for Transient Stability Enhancement of Grid-Following Converters Under Weak-Grid Conditions

Modeling and design-oriented control of transient stability of grid-following converters have attained an increasing interest in recent years. Despite novel nonlinear models enabling a design-oriented enhanced transient stability controller, the focus has so far been limited to study only the synchronization dynamics of the phase-locked loop. To expand upon the knowledge of the large-signal performance and stability, this article proposes a systematic analysis procedure of a grid-following converter under weak-grid conditions and large-signal disturbances including the outer dc-link and ac-side voltage control loops. A reduced-order large-signal model is used to analyze the large-signal nonlinear behavior of the system using the area of the basin of attraction as a measure for large-signal robustness. Here, stabilizing and destabilizing trends for outer-loop controller parameters are given. Through a surrogate-model expensive black-box optimization algorithm, a computational-efficient optimal design of the outer-loop controller parameters is proposed to maximize the large-signal robustness. Finally, a recommendation and a design guideline for converter constraints and outer-loop controller parameters are given. This can be used to identify the influencing parameters for grid-following converters under large-signal disturbances, and as a tool for fast controller optimization toward large-signal robustness.

A Robust Vector Current Controller with Negative-Sequence Current Capability for Grid-Connected Inverters

This paper presents a vector current controller (in the synchronous reference, or the dq, frame) with negative-sequence current injection capability for three-phase grid-connected converters. This capability is desired for the operation of the converter during unbalanced conditions and also for a certain type of islanding detection. The proposed controller first determines the double-frequency current references and then uses a sixth-order two-input two-output proportional-integral-resonance (PIR) structure, which is optimally designed. Compared with the existing similar approaches, the proposed controller has a simpler structure and more robust performance, e.g., against system parameter uncertainties and weak grid conditions. The proposed controller is developed for converters with both the L-type and LCL-type filters. For the LCL-type converter, a suboptimal partial state feedback control is also proposed to achieve robust stability and active damping of resonance poles without requiring additional sensors. Detailed experimental results are presented to illustrate the properties and performances of the proposed controller.

Control strategies for power quality enrichment in Distribution network using UPQC

Unified Power Quality Conditioner (UPQC) is a cutting-edge Custom Power Device (CPD) that is being used to improve power quality in the distribution network. UPQC contributes to the distribution network by compensating for both voltage and current-related power quality disturbances. Its control strategy in unbalanced and distorted weak grid conditions is of particular interest for research. In addition, the performance of UPQC in the presence of unbalanced and non-linear harmonic loads is critical for maintaining desirable power quality. This paper investigates the implementation of UPQC control strategies and algorithms for improving power quality (PQ) and proposes a versatile control strategy to improve UPQC performance. When tested on a critical distribution network, the proposed dynamic control strategy provides better steady-state and dynamic response. Models developed in MATLAB/SIMULINK are used to validate the proposed control scheme.

Observer-based neural network dynamic surface control of single-phase LCL-filtered grid-connected inverters under parametric uncertainties and weak grid condition

This paper focuses on the current control of single-phase LCL-filtered grid-connected inverters in the presence of parametric uncertainties and weak grid condition. Therefore, a novel neural network dynamic surface control (NNDSC) method is proposed by overcoming the problem of ‘explosion of complexity’. In addition, radial basis function neural networks (RBFNNs) are employed to approximate the system parametric uncertainties. Furthermore, by considering practical considerations, a novel state observer (SO) is designed to estimate the inverter-side current and capacitor voltage. As a result, additional current and voltage sensors are not required, which makes the implementation of the proposed approach straightforward and reliable. The origin neighbourhood convergence of estimated and tracking errors is assured through Lyapunov stability theorem and Young’s inequality. The effectiveness and performance of the proposed NNDSC+SO approach is demonstrated through MATLAB/Simpower simulations in view of the reference current changes, LCL filter parametric uncertainties and weak grid condition.

An RTDS-Based Testbed for Investigating the Impacts of Transmission-Level Disturbances on Solar PV Operation

The increasing penetration of renewable energy resources such as solar and wind via power electronic inverters is challenging grid dynamics, as well as grid planning, operation, and protection. Recently, the North American Electric Reliability Corporation (NERC) has reported a series of similar events of the unintended loss of solar generation in Southern California over a large geographic area following the transmission-level disturbances. These events highlight the importance of understanding the characteristics of the transmission-side disturbances propagating into the distribution systems and their impacts on the operation of inverter-based resources. In this paper, a real-time electromagnetic simulation testbed is constructed for real-time electromagnetic simulations to generate realistic transmission-level disturbances and investigate their impacts on the solar PV operation under different fault types and locations, solar penetration levels, and loading levels. Through the simulation analysis and grid strength assessment, it is found that the grid strength at points of integration (POIs) of solar PVs significantly affects the transient stability of solar generators. Particularly, undesirable transient stability events are more likely to occur at the weak POIs following the transmission-level disturbances. Moreover, undesirable transient stability events become severer when the transmission-level disturbance is closer to the weak POIs or the disturbances become more serious. Additionally, the impact of the transmission-level disturbances on the solar PVs at the weak POIs exacerbate with the increasing solar penetration levels and loading levels. Thus, it is important to study and develop new technologies for grid planning, operation, and protection in weak grid conditions to address the emerging issues of integrating the high penetration of solar PVs and other IBRs.

A study of a single phase grid connected pv inverter performance under a weak grid conditions and distorted grid voltage for Cambodia

<span lang="EN-US">A single-phase grid-connected PV inverter performance under a weak grid is a model designed to penetrate PV energy with a weak grid. Usually, this model gets complex and unstable in power system control such as THD growth, harmonic effects, voltage surges, inverter performance. Experimental results would present the impacts on the system which would lead to instability in the grid system. This study was based on inverter performance control, a weak grid control, the grid distortion examination, and harmonic effects. To optimize this grid system, hence, the proposed methods of oscilloscope and power meter were proportionally used to control the grid impacts and stabilities. The results showed that current THD in distorted grid voltage system surged to 8.88%. V<sub>PCC</sub> growth was 238.11V, equivalent to 8.23% in a weak grid (X<sub>L</sub> and R). Such a huge increase could prevent the grid system from transferring power for later operation. During the experiment, the inverter, S power and Q power performances stood for good operating processes without impacts on the grid performance. This method should be applied with a weak grid system because it provided information about grid stability control.</span>

Current Control of a Three-Phase, Grid-Connected Inverter in the Presence of Unknown Grid Parameters Without a Phase-Locked Loop

Three-phase inverters for grid-connected applications typically require some form of grid voltage phase detection in order to properly synchronize to the grid and control real and reactive power. This phase detection is usually based upon some type of grid voltage sensing. However, in this work, a method is proposed, whereby the phase angle of the grid can be accurately identified solely via the grid current feedback. In the proposed current-control scheme, the only measurement is the output inverter current, and no phase-locked loop (PLL) exists within the scheme. This phase observer is incorporated into a current controller, which can manage the injected power to the grid. The design of this combined observer/controller system is motivated and validated via a Lyapunov stability analysis. The performance of the proposed control scheme has been compared with the conventional PI PLL-based control scheme that needs both a grid voltage and a current sensors. Experimental results utilizing a Controller+ Hardware-in-the-Loop test bed have been presented in order to validate the proposed observer/controller scheme under normal operation, weak grid, distorted grid, and faulty grid conditions. To summarize, this scheme archives current control for a grid-connected inverter using only current feedback. In comparison, traditional methods require voltage and current feedback along with a PLL to synchronize with the grid creating a cascaded closed-loop control system.

Internal Model Based Grid Voltage Estimation and Control of a Three-Phase Grid Connected Inverter for $PV$ Application

Grid connected inverter controllers require the magnitude and phase of the point of common coupling voltage for the generation of unit vectors from a phase locked loop. However, weak grid conditions or grid voltage sensor failures might substantially increase control difficulty in the inverter due to unknown grid impedance which causes instability in the phase locked loop. In this paper, an internal model based grid voltage estimation architecture has been presented. The proposed control architecture relies on the current measurements along with nominal values of filter parameters for the point of common coupling voltage estimation and unit vector generation. Detailed stiffness and robustness analyses are presented to show that the proposed control and estimation architecture is nearly insensitive to parameter variations. In comparison to conventional current control with both voltage and current sensors, the proposed current control scheme provides better transient response during both ideal and non-ideal grid voltage conditions. Experimental results on a reduced scale laboratory prototype validate the efficacy of the proposed architecture.

Voltage disturbance compensation based on impedance modeling of DFIG under weak grid

This paper proposes a grid voltage disturbance compensation control for the stability of the DFIG system under weak grid conditions. A detailed impedance model of the DFIG system, including the generator, the inner and outer control loops of the rotor side converter (RSC) and grid side converter (GSC), the DC-link and phase locked loop (PLL) dynamic, is developed in the d-q synchronous reference frame. Based on the proposed model, the relationship between the outputs of the controllers and the grid voltage disturbance at the point of common coupling (PCC) can be obtained. Then, the compensation control of the grid voltage disturbance is integrated to the current controllers of RSC and GSC to improve the stability of the DFIG system connected to the weak grid. In this paper, the effectiveness of the proposed compensation control strategy is proved using the generalized Nyquist criterion (GNC), which illustrates that the compensation control strategy implemented in RSC and GSC simultaneously has a better ability to maintain the stability of the DFIG system under weak grid conditions than only one side applied. The simulation results verify the analysis.

Finite Control Set Model Predictive Control With Floating Virtual Voltage Vectors for Grid-Connected Voltage Source Converter

This article presents a new algorithm of finite control set model predictive control (FCS-MPC) with floating virtual voltage vectors (VVs) applied to grid-connected voltage source converter. The new solution improves the conventional FCS-MPC and FCS-MPC with virtual VVs, assuring a fixed switching frequency. In comparison with FCS-MPC using virtual VVs, the proposed algorithm ensures a shorter distance between floating virtual VVs and actual voltage vector achieves lower THDi and ripples of the grid current for the same sampling time. The proposed control method has been experimentally validated using a 5-kW prototype. The algorithm was examined and compared in steady and transient states as well as under grid voltage disturbances, such as higher harmonics, dips, and unbalance. The simulation results and experimental measurements confirm that the developed control technique shows a high quality of the grid current (low THDi value), high dynamic performance, and immunity in case of weak grid conditions.

MOSAIK and FMI-Based Co-Simulation Applied to Transient Stability Analysis of Grid-Forming Converter Modulated Wind Power Plants

The grid integration of renewable energy sources interfaced through power electronic converters is undergoing a significant acceleration to meet environmental and political targets. The rapid deployment of converters brings new challenges in ensuring robustness, transient stability, among others. In order to enhance transient stability, transmission system operators established network grid code requirements for converter-based generators to support the primary control task during faults. A critical factor in terms of implementing grid codes is the control strategy of the grid-side converters. Grid-forming converters are a promising solution which could perform properly in a weak-grid condition as well as in an islanded operation. In order to ensure grid code compliance, a wide range of transient stability studies is required. Time-domain simulations are common practice for that purpose. However, performing traditional monolithic time domain simulations (single solver, single domain) on a converter-dominated power system is a very complex and computationally intensive task. In this paper, a co-simulation approach using the mosaik framework is applied on a power system with grid-forming converters. A validation workflow is proposed to verify the co-simulation framework. The results of comprehensive simulation studies show a proof of concept for the applicability of this co-simulation approach to evaluate the transient stability of a dominant grid-forming converter-based power system.

Energization of windfarm infrastructure as part of microgrid based restoration

Studies on windfarms have focussed on their integration under normal and short duration weak grid conditions. With their increasing penetration at the same time that there is increasing disaster related blackouts, there is need to investigate their role during restorative conditions assuming a bottom-up approach in which local generation is used to re-energize the network. This is commonly referred to as microgrid based restoration. Wind Energy Conversion Systems (WECSs) are not blackstart units thus they require cranking from designated blackstart units in order to play a role in re-energizing the network. Prior to the WECSs being cranked up, the windfarm infrastructure, which includes underground cables, reactors (applicable to long HV cables), WECS transformers and yaw-pitch motor system, needs to be re-energized. This paper investigates the energization of different components within the windfarm considering availability of a single medium sized (5 MVA) blackstart synchronous source. Firstly, diesel based generation (DBG) will be used to study the different challenges associated with the energization of the different components considering different re-energization sequences after which thermal (TBG) and hydro (HBG) based generation will be used for comparison. High frequency modelling has been considered for static network components such as transformers, cables and reactors while dynamic models have been considered for rotating network components such as generators and motors. Excitation limiters have been included in the generation systems in order to obtain a near accurate response on energization of the different components. Discussion is provided on the different factors that influence the success of energization. Analysis is also provided on the effect of size of the excitation source and influence of the synchronous condenser which has been considered as part of the windfarm. Resonance, using a simple empirical formula, is briefly discussed as a factor in influencing and determining the energization sequence. A typical windfarm layout has been considered in this study with MATLAB/Simulink being used as the simulation platform due its flexibility in modelling.

Simple current control without grid voltage sensor for traction solid-state transformer

This paper proposes a simple current control method without a grid voltage sensor for a rectifier of traction solid-state transformer. The proposed control scheme estimates the phase angle of grid voltage from the fundamental frequency current of the grid. The extracted phase signal is normalized to generate a sinusoidal current reference by adopting a scaling factor to achieve a unity gain. Since the proposed control method operates without the grid voltage sensor for power factor correction (PFC), it offers a cost-effective solution with a simple implementation structure. However, this method cannot guarantee its control performance under weak grid conditions, where the frequency of grid voltage often varies. Thus, the control parameters of the proposed method are discussed and analyzed to minimize performance degradation. To verify the feasibility of the proposed control method, simulations and experiments are carried out on a 2 kW single-phase PFC converter. The proposed method provides as much control performance as the conventional method, which uses the grid voltage sensor.

Tuning and experimental assessment of second-order generalized integrator –frequency locked loop grid synchronization for single-phase grid assisted system

The phase-locked loop (PLL) is an essential part of the grid-tied system to synchronize control of converter with grid voltage, particularly affects the converter stability as well as performance under weak grid conditions. During abnormality in a grid, its bandwidth ought to be a11dequately brought down to achieve appropriate disturbance rejection capability with a compromise to slower detection speed. The researchers have done intended work in advanced PLLs to improve phase-angle detection speed by modifying the pre/in-loop filtering stage. A most concerning challenge with the PLLs is the means by which to additionally provide superior dynamic performance and reduced settling time without bargaining stability of system along with the capability of disturbance elimination To overcome this challenge, this paper describes the second-order generalized integrator (SOGI-Frequency Look loop (FLL) which offers filtering capability like band-pass filter, low-pass filter, and notch filter for adaptive frequency tuning as well as an orthogonal signal generation for the grid-tied photovoltaic inverter. The impact of control parameters on their dynamic performance in SOGI-FLL is tabulated from a step response of frequency estimation by taking a frequncy shift. Furthermore, Low-cost DSP based STM32F407VGT the microcontroller is employed to implement a SOGI-FLL to test under adverse grid conditions using Waijung Block-set of SIMULINK/MATLAB. The experimental results of SOGI-FLL have proven superior dynamic performance over type-2 PLLs by choosing the appropriate value of the control parameter of SOGI-FLL.