Inverter Terminals Research Articles

Lyapunov-Krasovskii passivity based stability analysis of grid-tied inverters

The non-linear dynamics of a grid-interfaced inverter (GII) in a distributed generation system results in dynamic instability and power quality issues. The conventional linear methods depend on parametric variations in controller parameters hence, tuning should be proper. Also, linearization of dynamical states around steady-state operating point works only if the controllers operate around a fixed operating point thus, only ensuring local stability. The feedback and feedforward damping methods can address some of the above challenges but requires additional sensors and also incapable of maintaining robustness against grid impedance variations and system parametric changes. To address aforementioned issues, this journal presents a non-linear Lyapunov Krasovskii Passivity (LK-PBC) based dynamic controller for Grid-interfaced Voltage Source Converters (VSCs). A generalized admittance model is derived which is used to calculate Non-passive regions of the VSC where the output admittance will have a negative real part. The damping provided by the proposed controller also eliminates the resonance instability issues in the concerned frequency scale regardless of the admittance seen at the inverter terminals. It also takes care of time delay and norm-bounded state uncertainties. The evaluation of the theoretical studies and efficacy of the proposed LK-PBC controller is confirmed by hardware in loop analysis on a laboratory experimental setup.

Active power decoupling for differential boost inverter with linear and nonlinear loads using inverse model approach

In this paper, a novel control system structure is introduced for generating a sinusoidal wave on the differential boost inverter terminals using inverse model approach. To address the high-frequency resonance issues, an active damping technique is proposed. Proportional-resonant controllers are designed for tracking and harmonic rejection. Pulling direct and constant current from photovoltaic panels as the DC source is essential in maximum power point tracking. Therefore, a decoupling controller is utilised to suppress current oscillations. An innovative combination of tracking controller, harmonic rejector, and decoupling controller is proposed in order to provide a sinusoidal voltage output for linear and nonlinear loads with low total harmonic distortion. The proposed method reduces the number of controllers by half compare to conventional methods. In addition to simulation, experimental results show the effectiveness of the proposed method.

A Hybrid Adaptive Control Architecture for Grid-Connected Inverter With Optimal Policy Generation

A nonlinear optimization-based hybrid control architecture for grid-connected inverters is presented in this article. The proposed control approach augments the existing inverter control with an identification-based adaptive control architecture. Grid dynamics are first represented as energy signals at the inverter terminals using the concept of an energy function. The nonlinear optimization function will act on this dynamic energy signal and derive an optimal control gain for the augmented hybrid control architecture. The optimization function derived in this work considers both the state energy and controller effect. The proposed architecture developed considering the nonlinearities in the system ensures excellent performance during high-frequency grid dynamics. Based on validation on a real power grid, it was observed that the proposed architecture provides better control accuracy (more than 20%) when compared to static PI controller acting alone.

DC‐side current compensation control in the rectifier terminal for power variations in back‐to‐back converters

Back-to-back (BTB) converters, which consist of an AC/DC rectifier cascaded with a DC/AC inverter, have been widely adopted in the interconnection of power systems. The DC link, being the interface between the rectifier and inverter terminals, is susceptible to disturbances, particularly from power fluctuations. Regardless of the magnitude of power or changes in the power flow direction, stability degradation will occur. To lessen the influence of power, a DC-link current compensation control scheme is proposed and applied to the rectifier terminal. The proposed scheme reduces the DC link impedance gain of the rectifier terminal to balance the impedance at both ends, and thus enhance stability and controllability. The scheme is applied to a BTB converter to improve the system's resistance to disturbance, regardless of variations in power magnitude or power flow direction. The accuracy of the theoretical analysis and the effectiveness of the proposed scheme are verified using simulations and a scaled-down experimental setup.

Active Slip Frequency Based Islanding Detection Technique for Grid-Tied Inverters

In this article, an islanding detection technique (IDT) is proposed based on applying positive feedback of wide area measurement based slip frequency at the inverter reactive power control loop. The classical Q--f droop characteristic based IDT is affected due to variations in load and active power mismatch (APM) conditions that may exist during islanding condition. To overcome these issues, an adaptive reactive power injection based IDT is proposed. Adaptiveness in the proposed method is achieved by considering the gain of the reactive power control loop as a product of rate of change of susceptance with respect to frequency seen at the inverter terminals, square of per unit point of common coupling voltage and slip frequency, which is the difference between distribution generator frequency and a reference substation bus frequency. The application of slip frequency as a positive feedback parameter injects negligible reactive power during grid side frequency disturbances and also associates with under frequency and over frequency ride through requirements. The method is least effected by load quality factor variations and APM conditions. The IDT is able to detect islanding conditions with a negligible non detection zone. Real time simulation studies are performed using dSPACE DS1104 real time controller and RTDS/RSCAD.

Power Control of Virtual Oscillator Controlled Inverters in Grid-Connected Mode

This paper proposes a novel method to determine and control active and reactive power output of the virtual ocillator control-based inverter in grid-connected mode. An analytical expression is introduced to describe the quasi-steady-state relationship between the parameters of the inverter including its controllers, and the power injection and voltage at the inverter terminals. We introduce a new, complex valued parameter ( K ) to the conventional virtual oscillator control (VOC) scheme in order to control the inverter output power. We compare analytic and numeric solutions of the above analytical expression with time-domain simulation results, using different K values and line impedances to validate the analytical expression. In the second part of the paper, we design a controller: we use the above expression to derive K for given reference powers and use adaptive integrators to compensate for the remaining errors. We compare this control scheme to a method that uses PI controllers, and also to a simple decoupling method for independent control of active and reactive power. The controller based on the analytical method exhibits very good dynamic and decoupling performance. A single VOC-controlled inverter connected to a nonstiff grid is simulated in MATLAB/Simulink for demonstration, and is also validated using real-time simulations.

State‐space analysis based pole‐to‐ground line fault isolation strategy for LCC–HVDC systems

The common way to isolate the line fault in line-commutated converter-based high-voltage direct current (LCC-HVDC) transmission grids is regulating the trigger angle of converters on both terminals of the faulted line, which usually depends entirely on the conventional controller and goes with large current peak and lengthy overcurrent duration. In this study, a trigger-angle setting approach with adaptability is, therefore, proposed to provide a theoretical basis for the generation of fault isolation strategies under different scenarios. First, equivalent circuits of solid pole-to-ground line faults at rectifier and inverter terminals are established, and time-domain expressions of the fault and converter currents are obtained. Secondly, based on the analysis of current characteristics, the non-linear programming method is utilised to obtain the optimal trigger-angle sequence after the fault, which can be applied in the following fault isolation. Finally, the proposed approach is verified via power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) simulation. The results show that it can effectively reduce the current peak and the overcurrent duration of the converter, which is of positive significance for maintaining the power system security and stability.

Influence of motor cable on common-mode currents in an inverter-fed motor drive system

Induction motor drive systems fed by cables are widely used in industrial applications. However, high-frequency switching of power devices will cause common-mode (CM) voltages during operation, leading to serious CM currents in the motor drive systems. CM currents through the cables and motors in the drive systems can cause electromagnetic interference (EMI) with the surrounding electronic equipment and shorten the life of induction motors. Therefore, it is necessary to analyze the CM currents in motor drive systems. In this paper, high-frequency models of unshielded and shielded power cables are formulated. The frequency-dependent effects and mutual inductances of the cables are taken into account. The power cable parameters are extracted by the finite element method and validated by measurements. High-frequency models of induction motors and inverters are introduced from existing works. The CM currents at the motor and inverter terminals are obtained, and the influence of the cable length and cable type on the CM currents is analyzed. There is a good agreement between the experimental results and the CM currents predicted by the proposed models.

On Stability of Voltage Source Inverters in Weak Grids

As the number of inverters increases in the power grid, the stability of grid-tied inverters becomes an important concern for the power industry. In particular, a weak grid can lead to voltage fluctuations at the inverter terminals and consequently cause inverter instability. In this paper, impacts of circuit and control parameters on the stability of voltage source inverters are studied using a small-signal state-space model in the synchronously rotating $dq$ -frame of reference. The full-order state-space model developed in this paper is directly extracted from the pulsewidth modulation switching pattern and enables the stability analysis of concurrent variations in the three-phase circuit and control parameters. This paper demonstrates that the full-order model of a grid-tied active (P) and reactive (Q) power (PQ)-controlled voltage source inverter (VSI) can be significantly reduced to a second-order model, preserving the overall system stability in the case of grid impedance variations. This paper also shows that a decrease in the grid inductance does not necessarily improve the stability of grid-tied VSIs. The system stability is a function of both the grid R/X ratio and grid inductance. Despite the grid-side inductor of the LCL filter is in series with the grid impedance, they have different impacts on the stability of a grid-tied PQ-controlled VSI, i.e., an increase in the filter inductance may improve the system stability in a weak grid. These findings are verified through simulated and experimentally obtained data.

A low cost single phase grid connected reduced switch PV inverter based on Time Frame Switching Scheme

Abstract The domain of Grid Connected Inverter (GCI) and reduced switch topology in multi-level inverter (MLI) are gaining its attention in terms of reducing cost and optimization. For such a motivation, many researchers are addressing their efforts in proposing new MLI topologies or in modifying the existing ones, aiming at improving the quality of the energy available at the inverter terminals. The proposed split capacitor based topology uses only a single dc supply to generate any required output levels. The split capacitors are considered to obtain stepped output by adopting a Time Frame based Switching Scheme (TFSS). The numbers of semiconductor switches are reduced with an improvement in the harmonic factor. The power output is then filtered using passive inductor component and then synchronized to feed the grid power system. The result of the proposed topology is verified by Matlab/Simulink simulation and hardware implementation using a dsPIC30F4011 controller. The simulation and experimental results are compared to previous studies. The proposed scheme offers an excellent performance with significant result in lowest THD obtained and also enables the inverter to operate under unhinged PV conditions comprehensively.

SHE-PWM을 적용한 STATCOM에 의한 저차고조파 제거 방법

In HVDC converters that employ a line-commutated control, reactive power is absorbed by the rectifier and inverter terminals during AC/DC conversion. An AC filter usually consists of filters and large shunt capacitors to supply reactive power to the HVDC station. When STATCOM is used to supply reactive power to the HVDC system with AC filter, the low-order harmonics generated from STATCOM can result in a resonance between the shunt capacitor and AC network. Therefore, a control strategy based on selective harmonic elimination is adopted to minimize the low-order harmonics from STATCOM. The cancellation of harmonic instabilities is verified through simulations in PSCAD/EMTDC.

Neuro-evolutionary approaches to power system harmonics estimation

This paper focuses on exploiting two computational intelligence techniques such as artificial neural network and evolutionary computation techniques in estimation of harmonics in power system. Accurate estimation of harmonics in distorted power system current/voltage signal is essential to effectively design filters for eliminating harmonics. No standard design is available for handling of local minima and training of NN but Evolutionary Computation (EC) techniques are capable of resolving local minima. Neural Network and Evolutionary Computing (Bacterial Foraging Optimization (BFO)) are combined to achieve accurate estimation of different harmonics components of a distorted power system signal. First estimation of unknown parameters are carried out using BFO, then optimized output of BFO are taken as initial values of the unknown parameters for Adaline. Amplitude and phase of fundamental and harmonics components are determined from final updated values of unknown parameters using Adaline. This Adaline based Bacterial Foraging Optimization (Adaline-BFO) hybrid estimation algorithm addresses the problems of slow convergence and reduction of time generation of off-springs happening in Genetic Algorithm (GA), and to avoid local minima in Particle Swarm Optimization (PSO). The proposed Adaline-BFO algorithm has been applied for estimation of harmonics of the voltage obtained across the inverter terminals of a prototype Photovoltaic (PV) system. From the obtained results, it is confirmed that the proposed Adaline-BFO algorithm provides superior estimation performance in terms of improvement in % error in estimation, processing time in computation and performance in presence of inter and sub-harmonic components when compared with the Discrete Fourier Transform (DFT), Kalman Filter (KF) and BFO algorithms.

Performance evaluation of a transformerless multiphase electric submersible pump system

Using of low-voltage variable-frequency drive followed by a step-up transformer is the most preferable way to feed an electrical submersible pump motor. The existence of long feeder between the motor and drive systems usually causes over-voltage problems because of the travelling wave phenomenon, which makes the employment of filter networks on the motor or inverter terminals mandatory. The so-called boost-inverter inherently can solve this problem with filter-less operation as it offers a direct sinusoidal output voltage. As boost inverters have voltage boosting capability, it can provide a transformer-less operation as well. This study investigates the performance of a five-phase modular winding induction machine fed from a boost-inverter through a long feeder. A simulation study using a 1000 Hp system and experimental investigation on a 1 Hp prototype machine are used to support the presented approach.

Real-Time Implementation of Solar Inverter with Novel MPPT Control Algorithm for Residential Applications

Solar energy is a fast growing energy resource among the renewable energy resources in the market and potential for solar power is huge to contribute towards the power demand almost in all the countries. To capture the maximum power from the sun light in order to generate maximum power from the inverter, control system must be an equally efficient with the well designed power electronic circuits. Maximum power point tracking (MPPT) control system in general is taking care of extraction of maximum power from the sun light whereas current controller is mainly designed to optimize the inverter power to feed to power grid. In this paper, a novel MPPT algorithm using neuro fuzzy system is presented to ensure the maximum MPPT efficiency in order to ensure the maximum power across the inverter terminals. Simulation and experimental results for residential solar system with power electronic converters and analysis have been presented in this paper in order to prove the proposed algorithm.

Overvoltage Mitigation of Inverter-Driven Motors With Long Cables of Different Lengths

This paper presents an intensive discussion on an overvoltage appearing at the motor terminals of an adjustable-speed drive that consists of a voltage-source pulsewidth-modulated inverter, an induction motor, and a three-phase symmetric long cable. It describes a design procedure for a simple passive electromagnetic-interference filter based on parallel connection of an inductor and a resistor, which is intended for mitigating the overvoltage at the motor terminals. The modeling of the cable and the motor is characterized by focusing on the natural frequency or ringing frequency inherent in the cable used, which is inversely proportional to cable length. Impedance mismatch at the inverter and motor terminals causes the overvoltage that may reach double the inverter dc-link voltage. The effectiveness and validity of the simple design procedure of the filter are confirmed on the 400-V 15-kW experimental system with either a 100- or 200-m-long cable, as well as on computer simulation based on the modeling of the cable and the motor.

Diode-Based HVdc Link for the Connection of Large Offshore Wind Farms

This paper includes a technical feasibility study on the use of diode-based HVdc links for the connection of large offshore wind farms based on synchronous generators. A technique for the voltage and frequency control of the offshore ac grid is presented. The proposed control technique allows the operation of the rectifier end of the HVdc link in current or voltage control mode. Fault response to onshore voltage sags of up to 80% has been shown to be comparable to that of thyristor-based rectifiers. Moreover, the complete system shows an adequate fault-ride-through operation to solid short circuits at onshore inverter terminals. PSCAD® simulations are used to prove the technical feasibility of the proposed control techniques both in steady state and during transients.

Effects of Cabling and Grounding Configuration on Surge Voltage in Inverter-Fed Motors

In inverter-fed motor systems, output voltage of the inverter has a rectangular waveform with rise time of a few tens of nanoseconds, and consequently a motor suffers from repetitive surge pulses which may have adverse effect on motor insulation. Therefore the behavior of surge pulses should be clarified to devise the optimum insulation design of a motor. We have investigated how cable structure and grounding wire connecting inverter and motor affect the surge behavior with particular regard to peak voltage between motor input terminal and ground and peak voltage across the first coil in serially connected multi coils. As a result, we discovered phenomena whereby, when cross sectional structure of the cable is asymmetrical, the surge voltage of a specific phase can become higher than that of the other phases resulting from asymmetry among capacitances between feeder line and ground line. And we have confirmed these phenomena can be prevented by the following three methods: using cables that have symmetrical capacitance to ground among 3 phases (e.g., a 3-phase shielded cable), providing the grounding wire as a separate cable to reduce the capacitance, inserting capacitors at motor terminals or inverter terminals to cancel out capacitive asymmetry.

Design and implementation of an inverter output LC filter used for dv/dt reduction

This paper presents the design and analysis of a LC output filter for use with IGBT-based motor drive inverters. The LC filter is used to limit the rate of rise of the inverter output voltage and reduce common mode noise to the motor. In typical applications where dv/dt is limited to 100-500 V//spl mu/s, the resonant frequency of the filter is above the switching frequency. A diode bridge must therefore be used to clamp the resonant voltage. Resistors are also used to help dissipate the energy stored in the resonant circuit. Crucial to the design of the filter is the handling of the additional losses in the filter. The paper describes the design considerations for such a filter for use in a 460 V induction motor drive. The losses in the circuit are estimated to show the limitations of the filter. Experimental results illustrate the control of dv/dt at the inverter terminals, and the reduced peak voltage at the motor end of a long cable.

Surge propagation and overvoltages for PWM-inverter-driven motors

The use of PWM inverters with high switching characteristics to provide variable frequency supply to motors causes overvoltage with steep-fronted waveform on the motor terminals. Insulation failure in motor winding can be caused by this steep-fronted transient voltage in inverter drive systems. From the point of view of wave propagation with the skin effect of cable conductors, these phenomena have not been reported previously. The main purpose of this paper to describe the exact surge propagation considering the skin effect of cable conductors and the wave deflections at inverter and motor terminals. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 65–72, 2000

Surge propagation and overvoltages for PWM‐inverter‐driven motors

The use of PWM inverters with high switching characteristics to provide variable frequency supply to motors causes overvoltage with steep-fronted waveform on the motor terminals. Insulation failure in motor winding can be caused by this steep-fronted transient voltage in inverter drive systems. From the point of view of wave propagation with the skin effect of cable conductors, these phenomena have not been reported previously. The main purpose of this paper to describe the exact surge propagation considering the skin effect of cable conductors and the wave deflections at inverter and motor terminals. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 65–72, 2000