Inverter Control Parameters Research Articles

Stability and Fault Detection Algorithm of Grid-Connected Inverter in Complex Power Grid Environment

In order to improve the stability, this paper designs a control strategy based on adaptive grid impedance. By monitoring the grid impedance in real time and dynamically adjusting the inverter control parameters, the influence of grid impedance change on inverter stability is effectively reduced. At the same time, LCL filter is introduced, which significantly filters out the higher harmonics in the inverter output current and improves the output waveform quality. In addition, a voltage compensation device is applied to quickly generate a compensation voltage signal when the grid voltage fluctuates to ensure the stability of the inverter output voltage. In the aspect of fault detection, a data-driven fault detection algorithm is proposed, which combines machine learning and real-time monitoring technology to realize early detection and accurate diagnosis of inverter faults by collecting and analyzing inverter operation data. The experimental results show that the algorithm performs well in the detection of power switch device faults, sensor faults and other types, and has high classification accuracy. The research shows that the control strategy based on adaptive grid impedance, the addition of LCL filter and the application of voltage compensation device have significantly improved the stability of grid-connected inverter in complex grid environment. The proposed data-driven fault detection algorithm provides strong support for rapid fault location and accurate treatment of inverter.

A Generic Multicell Network Control for Three-Phase Grid-Connected Inverters

The inherent resonance of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> filters threatens the inverter stability seriously. Moreover, the strong <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dq$</tex-math></inline-formula> -axis cross-coupling of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LCL</i> -filtered grid-connected inverters in a synchronous frame goes against the independent regulation of active ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$P$</tex-math></inline-formula> ) and reactive ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula> ) power that is required in various applications. Besides, the imbalance and harmonics of the grid voltage will deteriorate the injected grid current, so the disturbance suppression performance of grid-connected inverters is important. Furthermore, excellent steady-state and dynamic performance is essential for grid-connected inverters. To solve the above four issues simultaneously, a complex-domain model of grid-connected inverters is established, and a generic multicell network control (GMNC) structure is proposed in this article. Based on the proposed GMNC structure, a number of control schemes are obtained, and the quantitative relationship between inverter performance and control parameters can be given directly and precisely for each scheme. Therefore, it is convenient and simple for engineers to select proper control schemes depending on the practical conditions and to design all the control parameters for an expected steady-state, dynamic, and antidisturbance performance without the resonance and the coupling issues. Finally, the effectiveness of the proposed control is verified by the experiments.

Real-Time Frequency Stabilization of Standalone PV System with SMC

This paper aims to obtain suitable control for the Standalone Photo Voltaic (SAPV) system to meet consumer demands in weather conditions. A SAPV system is developed with a solar array with Maximum Power Point Tracking (MPPT), a Boost converter, and an inverter. This system uses controllers for maximum power output tracking, constant DC output of the Boost converter, and constant output voltage and frequency with Pulse Width Modulation (PWM) control mode. MPPT, boost converter, and inverter controller parameters are designed and tuned, respectively. Proportional gain (Kp) and Integral gain (Ki) values and the Sliding Mode Method (SMC) tune the controller parameters. The innovative aspect of this work is to propose a standalone PV system with controllers based only on the sliding mode control approach. Moreover, the current controller provides an output current of high quality with a THD of 1.6 %. Effectiveness and robustness of the proposed scheme modeled and simulated under OPAL-RT real-time Software in Loop (SIL) platform with MATLAB Simulink under fast variations of irradiance and temperature. Various analyses have been carried out for examining the proposed Sliding Mode Controller-based standalone photovoltaic system.

Data-driven adjustable robust solution to voltage-regulation problem in PV-rich distribution systems

Adjustable robust counterpart (ARC) methodology has been recently utilised in distribution system operation to allow fast-acting devices, such as PV inverters, to take live recourse actions in response to operating point deviations. Available ARC models are used to obtain the controller parameters based on the worst-case realisations. However, we show that such a strategy might lead to over-conservative results when uncertainty realises away from the worst-case. To close this gap, this paper extends ARC to allow exploiting probabilistic information of uncertainty within the optimisation problem. This results in controller parameters that not only secure the worst-case but also work better in expectation. We formulate our approach as a linear program that minimises the real power curtailment (RPC) and reactive power usage of PV inverters while maintaining network voltages in an acceptable range. In contrast to the conventional ARC, our approach considers a segmented uncertainty set, and assigns a probability to each segment based on historical data. It then optimises to find inverter controller parameters in the form of piecewise affine functions, with each piece associated with one segment in the segmented uncertainty set. In live operation, the controllers use the real-time local measurements and assigned piecewise affine functions to generate real and reactive power set-points for the inverters. The performance of the proposed approach is validated using Monte-Carlo simulations on IEEE 37-bus and 906-bus networks. The simulations show that the proposed approach improves the performance of ARC by 30% to 60% over a variety of experiments.

Optimal Controllers to Improve Transient Recovery of Grid-Following Inverters Connected to Weak Power Grids

Leading to the enormous growth in renewable and power electronics technologies and the global drive towards environmental friendliness and sustainability, a significant number of renewable energy sources are being connected to the power system via inverter-based systems. The inverter-based generations (IBG) have no stored energy and less fault current injection capability compared to the conventional synchronous machines. Consequently, a large penetration of IBG creates challenges to maintaining the stability of the power system, especially the transient stability. The weaker the power system, the higher the significance of instability. Few solutions exist in the literature to improve the fault recovery of IBG connected to weak power systems. This paper considers the method of storing energy in sub-module capacitors of the Modular-Multi-level Converter (MMC) along with temporarily boosting the inverter’s current limit. Conversely, increasing the ratings of the inverter will result in high manufacturing costs. Hence an optimization strategy is proposed in this paper, for obtaining a robust set of inverter control parameters that enhances fault recovery without excessively increasing the manufacturing cost of MMC. A frequency scanning technique supplemented with Generalized Nyquist criteria is incorporated into the optimization methodology to constrain the search space for the optimization algorithm. This enables the optimization algorithm to converge to an acceptable solution with a reasonable computing time. Furthermore, validation of the resultant set of parameters for different system conditions is presented. Finally, IBG with optimized fault recovery controllers is integrated into a simplified real-world power system, and the applicability of the proposed optimized controllers is illustrated.

Stability analysis of microgrid inverter off-grid mode considering nonlinear load

With the large-scale utilization of distributed generation in microgrid, inverter as the connection hub of new energy grid connection, directly affects the operation performance of microgrid. In order to improve the output voltage quality and load capacity of the inverter in the off-grid mode of distributed energy, the stability region of the inverter with load is analyzed by using the impedance analysis method of cascade converter and control theory. The quasi proportional resonant (QPR) double loop control is adopted to realize no static error tracking voltage while increasing bandwidth, and the influence of control parameters on performance is analyzed. At the same time, in order to improve the capacity of inverter with nonlinear load, odd harmonics are introduced into the controller to suppress the influence of low harmonics of load current on output voltage. Finally, the influence of inverter output impedance change, load level, controller parameters and filter parameters on system stability is analyzed through impedance ratio Nyquist curve, which provides corresponding theoretical support and parameter optimization reference for the design of actual system.

Minimal Output Impedance Required for Stability of Grid-Supporting Inverters

In the field of power system dynamics, a main challenge is to properly tune the inverter control parameters, where one important parameter is the inverter's output impedance, physical or virtual. In this work, we provide a proof showing that a minimal output impedance must be used in order to keep the system stable. Although this result is well-known among practicing engineers, our proof reveals a fundamental property which causes instabililty, namely the pole excess of the inverter dynamic model. To prove this claim we model the system as a signal flow diagram, where the output impedances of the inverters are described using static gain feedback. Based on this model, we show that the typical dynamic model of grid supporting inverters has a pole excess larger than 2, which causes instability when high feedback gain is used. This result may be used to design a numerical method for estimating the minimal impedance. The theorem is validated based on an array of photovoltaic plants in the Ramat Negev region, which is part of the Israeli power grid.

Influence of Inverter Controller Parameters on the Small-signal Stability of LCC-HVDC System Based on a Practical Project

Unreasonable control parameters of the Line Commutated Converter based High Voltage Direct Current (LCC-HVDC) system may induce small-signal instability. This paper studies the impact of inverter controller parameters on steady response of the LCC-HVDC system under weak AC grid condition. Firstly, according to a practical project, the small-signal model of LCC-HVDC system is established based on the switching function. Then, the eigen-analysis method is adopted to study the impact of the controller parameters on the inverter side on the oscillation mode and damping characteristics of the LCC system, and the correctness of results is verified by PSCAD/EMTDC simulation. The conclusion shows that reasonable controller parameters can improve the stability margin of the system.

Investigations into series resonant inverter power control parameters for an effective metal induction surface hardening

Metal hardening depends on the material properties and the way the heat is being applied; therefore, a careful induction heating system parameter should be carefully selected, and in particular to meet the hardening requirements. In this study, a focus on a cost-effective design of the induction coil and the converter is presented. For that, a sample bar metal made of XC48 is considered. The methodology exploits an indirect procedure. It starts from the coil design according to the metal sample size, followed by a 2D finite element method for parameters and losses determination for the specified hardening depth. The power supply using resonant series converters is evaluated through electrical modelling simulation and which integrates a phase-locked loop scheme to account for load parameter variation. A comparison between the electrical and heat power is conducted to highlight the importance of the inverter control parameters leading to high efficiency and an appropriate and satisfactory hardening profile. A prototype system is built to demonstrate the effectiveness of the approach described.

Lyapunov model predictive control to optimise computational burden, reference tracking and THD of three‐phase four‐leg inverter

Due to the evolution of high processing microprocessors, the model predictive control (MPC) has been widely used in power electronic applications. In spite of simplicity, flexibility and fast dynamic response, the conventional MPC (C-MPC) has a drawback of computational burden. This study focuses on Lyapunov MPC (L-MPC) method, in which Lyapunov control law is employed in the cost function to minimise the error between the desired control variables and the actual control variables of three-phase four-leg inverter to optimise the closed-loop system performance. The proposed control algorithm takes advantage of a predefined Lyapunov control law which minimises the required calculation time by the Lyapunov model equations just once in each control loop to predict the future variables. L-MPC technique improves the digital speed by 23.8% as compared to C-MPC. It reduces current tracking error and total harmonic distortion (THD) in the variation of inverter control parameters. The stability of the system is established through Lyapunov function with the help of backsteping control method. The LabVIEW field programmable gate array (FPGA) rapid prototyping controller is used to validate the proposed control method. The experimental results showed that the proposed system has better performance as compared to C-MPC.

Design and implementation of a virtual capacitor based DC current suppression method for grid-connected inverters

Integration of renewable energy (RE) sources, such as wind energy and photo voltaic (PV) energy, to a power network (grid) is usually achieved through an intermediate power electronic inverter. Ideally, the inverter is not expected to inject any form of DC component into the grid. However, this is not the case in practice as DC component are invariably generated and, subsequently, injected into the grid by the power converter, impacting on the overall power quality. To mitigate this problem, the International Grid Certification stipulates the maximum extent of DC component that can be injected into the grid current. Thus, various techniques have been proposed to maintain the DC component within the stipulated limit, however these techniques have the drawbacks of complicated control algorithm, extra power losses, and increased high costs. To solve these problems, this paper proposes a virtual-capacitor based DC current suppression control technique for grid-connected inverters, which has the advantages of fast implementation and good DC component suppression performance in utility. An LCL filter interfaced 3-phase inverter is used and virtual capacitors are incorporated into the inverter which connects the inverter with the grid. The allowable region and design methods of the inverter controller parameters which are combined with the active damping and grid voltage feedforward strategies are described in detail. Furthermore, both simulation and experimental results show that the DC component can be successfully mitigated and could satisfy the International Grid Certification standards.

Evaluation of Dead-Time Effect of Grid-Connected Inverters Using Broadband Methods

Power electronic inverters are devices that are used to interface renewables, such as photovoltaic panels or wind turbines, with the electrical power system. The inverter should control its output current to follow a sinusoidal reference to avoid distorting the voltage waveform of the power system. However, in real inverters dead-time is used in the control signals which causes unwanted harmonics, leading to lower power quality. The amount of harmonics is difficult to predict due to nonlinear nature of the dead-time. Moreover, the harmonic distortion depends on many variables, such as inverter control parameters and power system impedance. This paper shows that frequency response from inverter control signal (duty ratio) to grid current is linked to the detrimental dead-time effect and poor power quality. The frequency response is identified by perturbing the inverter control signal with a maximum length binary sequence (MLBS). The magnitude of the identified frequency response is shown to follow the same trend as the amount of current distortion. Therefore, the effect of dead-time on power quality could potentially be modeled using a linearized model allowing optimal control design in the presence of dead time.

Two‐step method for identifying photovoltaic grid‐connected inverter controller parameters based on the adaptive differential evolution algorithm

Photovoltaic (PV) grid-connected inverter is the core component of PV generation system; quickly and accurately obtaining the parameters of inverter controller has great significance in analysis of transient characteristics of PV generation system. Parameter identification relies on disturbance signals and measurements selection, and the trajectory sensitivities of inverter controller parameters are calculated with different system outputs as the measurements, the disturbance signals include AC three-phase short-circuit fault and DC voltage reference jump, accordingly, a two-step identification method is proposed, the first step use a three-phase fault to identify all voltage loop parameters and the proportional coefficient of current loop, and the second step consider a DC voltage reference jump disturbance to identify the integral coefficient of current loop and the setting inductance value. The adaptive differential evolution is taken as the identification algorithm, which adopts adaptive strategies for algorithm parameters in order to speed up the convergence and enhances the balance between the global search and local mining. The simulation analysis validates the effectiveness of this method. In addition, the impact of the inductance error on the identification is discussed.

Sequence-Impedance-Based Harmonic Stability Analysis and Controller Parameter Design of Three-Phase Inverter-Based Multibus AC Power Systems

Three-phase inverter-based multibus ac power systems could suffer from the harmonic instability issue. The existing impedance-based stability analysis method using the Nyquist stability criterion once requires the calculation of right-half-plane (RHP) poles of impedance ratios, which would result in a heavy computation burden for complicated systems. In order to analyze the harmonic stability of multibus ac systems consisting of both voltage-controlled and current-controlled inverters without the need for RHP pole calculation, this paper proposes two sequence-impedance-based harmonic stability analysis methods. Based on the summary of all major connection types including mesh, the proposed Method 1 can analyze the harmonic stability of multibus ac systems by adding the components one by one from nodes in the lowest level to areas in the highest system level, and accordingly, applying the stability criteria multiple times in succession. The proposed Method 2 is a generalized extension of the impedance-sum-type criterion to be used for the harmonic stability analysis of any multibus ac systems based on Cauchy's theorem. The inverter controller parameters can be designed in the forms of stability regions in the parameter space, by repetitively applying the proposed harmonic stability analysis methods. Experimental results of inverter-based multibus ac systems validate the effectiveness of the proposed harmonic stability analysis methods and parameter design approach.

D–Q Impedance Based Stability Analysis and Parameter Design of Three-Phase Inverter-Based AC Power Systems

Small-signal stability is an important concern in three-phase inverter-based ac power systems. The impedance-based approach based on the generalized Nyquist stability criterion (GNC) can analyze the stability related with the medium and high-frequency modes of the systems. However,. the GNC involves the right-half-plane (RHP) pole calculation of return-ratio transfer function matrices, which cannot be avoided for stability analysis of complicated ac power systems. Therefore, it necessitates the detailed internal control information of the inverters, which is not normally available for commercial inverters. To address this issue, this paper introduces the component connection method (CCM) in the frequency domain for stability analysis in the synchronous d–q frame, by proposing a method of deriving the impedance matrix of the connection networks of inverter-based ac power systems. Demonstration on a two-area system and a microgrid shows that: The CCM-enabled approach can avoid the RHP pole calculation of return-ratio matrices and enables the stability analysis by using only the impedances of system components, which could be measured without the need for the internal information. A stability analysis method based on d–q impedances, the CCM, and the determinant-based GNC is also proposed to further simplify the analysis process. Inverter controller parameters can be designed as stability regions in parameter spaces, by repetitively applying the proposed stability analysis method. Simulation and experimental results verify the validity of the proposed stability analysis method and the parameter design approach.

Investigating SSO risks of large‐scale photovoltaic generation with the series compensated transmission lines

With the increasing of photovoltaic (PV) installed capacity, large-scaled development of PV and centralized grid connection has become an important form of solar energy resources utilization. The series compensated transmission lines could enhance the long-distance transmission capacity of large-scale PV plants. Some PV plants in China are planned to adopt this technology. However, series capacitor compensation transmission lines are one of important sources of sub-synchronous oscillation (SSO). This SSO risk is a potential threat for the operation of PV plants and the stability of the utility grid. It is urgent to investigate the SSO risk of large-scale PV generation with the series compensated transmission lines. This study first developed a dynamic model of large-scale PV generation connected to the utility grid with series compensated lines on PSCAD platform. The PV dynamic model is built with detailed circuits of the power converter. Using the proposed model, the interaction of PV inverter control parameters, reactive power compensation device and the series compensation degree is investigated in detail. Then, the dominant parameters that affect the SSO are identified. From the results above, the SSO damping expression is deduced to reveal the SSO performance of large-scale PV generation with series compensated transmission lines.

Analytical Determination of the Control Parameters for a Large Photovoltaic Generator Embedded in a Grid System

It appears that, until now, no work has been reported on evaluating the operational impacts of large photovoltaic generators (PVGs) embedded in a high-voltage grid system. To meet this goal, the authors have proposed in this paper that the pulsewidth-modulated inverter that interfaces a PVG with a grid Bus be modelled in terms of its control parameters in the load flow analysis. This will then provide a direct answer as how to control a PVG and what will be the real and reactive power, and voltage at the interfacing and other grid Buses. For this it is necessary to predetermine the PVG inverter control parameters for a given steady state, i.e., given insolation (incoming solar radiation) and system load. So an analytical method for this predetermination has been proposed considering the available insolation and the boundary conditions of the interfacing grid Bus. Also the way the impact evaluation using the control parameters incorporated load flow analysis is made has been shown for two test systems having different characteristics. The proposed method being very fast can be applied for setting in real-time the control parameters for inverters of large PVGs embedded in a conventional power system.