Rafael Pena-Alzola

Analysis of the Passive Damping Losses in LCL-Filter-Based Grid Converters

Passive damping is the most adopted method to guarantee the stability of LCL-filter-based grid converters. The method is simple and, if the switching and sampling frequencies are sufficiently high, the damping losses are negligible. This letter proposes the tuning of different passive damping methods and an analytical estimation of the damping losses allowing the choice of the minimum resistor value resulting in a stable current control and not compromising the LCL-filter effectiveness. Stability, including variations in the grid inductance, is studied through root locus analysis in the z-plane. The analysis is validated both with simulation and with experiments.

Systematic Design of the Lead-Lag Network Method for Active Damping in LCL-Filter Based Three Phase Converters

Three-phase active rectifiers guarantee sinusoidal input currents and unity power factor at the price of a high switching frequency ripple. To adopt an LCL-filter, instead of an L-filter, allows using reduced values for the inductances and so preserving dynamics. However, stability problems can arise in the current control loop if the present resonance is not properly damped. Passive damping simply adds resistors in series with the LCL-filter capacitors. This simplicity is at the expense of increased losses and encumbrances. Active damping modifies the control algorithm to attain stability without using dissipative elements but, sometimes, needing additional sensors. This solution has been addressed in many publications. The lead-lag network method is one of the first reported procedures and continues being in use. However, neither there is a direct tuning procedure (without trial and error) nor its rationale has been explained. Thus, in this paper a straightforward procedure is developed to tune the lead-lag network with the help of software tools. The rationale of this procedure, based on the capacitor current feedback, is elucidated. Stability is studied by means of the root locus analysis in z-plane. Selecting the lead-lag network for the maximum damping in the closed-loop poles uses a simple optimization algorithm. The robustness against the grid inductance variation is also analyzed. Simulations and experiments confirm the validity of the proposed design flow.

LCL -Filter Design for Robust Active Damping in Grid-Connected Converters

Grid-connected converters employ LCL-filters, instead of simple inductors, because they allow lower inductances while reducing cost and size. Active damping, without dissipative elements, is preferred to passive damping for solving the associated stability problems. However, large variations in the grid inductance may compromise system stability, and this problem is more severe for parallel converters. This situation, typical of rural areas with solar and wind resources, calls for robust LCL-filter design. This paper proposes a design procedure with remarkable results under severe grid inductance variation. The procedure considers active damping using lead-lag network and capacitor current feedback. Passive damping is also discussed. The design flow, with little iteration and no complex algorithms, selects the proper ratios between the switching and resonance frequency, the grid and converter inductance, and the filter capacitance and total inductance. An estimation for the grid current total harmonic distortion (THD) is also proposed. Simulation and experiments validate the proposals.

A Self-commissioning Notch Filter for Active Damping in a Three-Phase LCL -Filter-Based Grid-Tie Converter

LCL-filters are a cost-effective solution to mitigate harmonic current content in grid-tie converters. In order to avoid stability problems, the resonance frequency of LCL-filters can be damped with active techniques that remove dissipative elements but increase control complexity. A notch filter provides an effective solution, however tuning the filter requires considerable design effort and the variations in the grid impedance limit the LCL-filter robustness. This paper proposes a straightforward tuning procedure for a notch filter self-commissioning. In order to account for the grid inductance variations, the resonance frequency is estimated and later used for tuning the notch filter. An estimation for the maximum value of the proportional gain to excite the resonance is provided. The resonance frequency is calculated using the Goertzel algorithm, which requires little extra computational resources in the existing control processor. The discrete Fourier transform coefficients are therefore obtained, with less calculations than the running sum implementation and less memory requirements than with the fast Fourier transform (FFT). Thus, the self-commissioning technique is robust to grid impedance variations due to its ability to tune the grid-tie inverter on-site. Finally, the analysis is validated with both simulation and experiments.

Review of modular power converters solutions for smart transformer in distribution system

While the use of power electronics based Smart Transformer (ST) is becoming a reality in traction applications, and it has been considered as an interesting option for interfacing different transmission systems, the possibility to use it in distribution systems is still considered futuristic. Replacing primary distribution transformers with ST can lead to more flexible handling of the distribution feeders, while replacing secondary distribution transformers can allow decoupling of distribution network. This paper reviews different power converter solutions for the ST focusing on modularity, control and communication needs, to meet high reliability requirements. Five topologies for the ST are considered and compared.

Control Design of a PFC With Harmonic Mitigation Function for Small Hybrid AC/DC Buildings

Unprecedented expansion of native dc powered equipment (LEDs, computers, and consumer electronics) has increased commercial and residential dc electricity usage over the past decade. Thus, it is foreseeable that hybrid ac/dc buildings featuring both ac and dc infrastructures will coexist. A hybrid ac/dc building will involve an efficient centralized rectifier that supplies all the dc loads, while legacy ac loads will remain connected to the existing ac infrastructure. This paper explores the opportunity of harmonic mitigation at distribution level in small hybrid ac/dc building by using a centralized power factor corrector (PFC) with large bandwidth. The current reference generator for the harmonic mitigation function (HMF) is explained along with power considerations. The PFC uses a proportional resonant controller, instead of a PI controller, without requiring additional sensors in the rectifier. A computationally inexpensive implementation of the phase-locked loop is also proposed along with considerations on parameter selection. The proposals provide all the steps for the straightforward control design of the PFC+HMF with fast calculations. The HMF requires only software modifications in the PFC and one sensor to measure the nonlinear load. Simulation and experiments validate the proposed procedures.

Robust design of LCL-filters for active damping in grid converters

Grid converters require a simple inductor or an LCL-filter to limit the current ripples. The LCL-filter is nowadays the preferred solution as it allows lower inductance values. In order to solve the stability concerns, active damping is preferred to passive damping since it does not use dissipative elements. However, large variations in the grid inductance and resonances arising from parallel converters may still compromise the system stability. This calls for a robust design of LCL-filters with active damping. This paper proposes a design flow with little iteration for two well-known methods, namely lead-lag network and current capacitor feedback. The proposed formulas for the resonance frequency, grid and converter inductance ratio, and capacitance of the LCL-filter allow calculating all the LCL-filter parameters. An estimation for the achieved Total Harmonic Distortion (THD) of the grid current is also provided. Experimental results show very robust designs to the parameter variations.

Self-commissioning notch filter for active damping in three phase LCL-filter based grid converters

LCL-filters are used to mitigate the harmonic current content in grid converters. The LCL-filter resonance must be damped in order to avoid stability problems in the current control. Active damping avoids resistors at the expense of increased control complexity. Large grid impedance variations can challenge the LCL-filter stability. Active damping by using a notch filter on the reference voltage for the modulator is simple to implement and does not require additional sensors. With the notch frequency tuned for the resonant frequency the voltage reference does not contain any component susceptible of exciting the LCL-filter. However, the notch filter tuning requires considerable design effort and the variations in the resonance frequency limit the LCL-filter robustness. This paper proposes a simple tuning procedure for the notch filter that results in proper robustness. In order to cope with the grid inductance variations it is proposed to estimate the resonance frequency by means of Fourier analysis. The Goertzel algorithm, instead of the FFT, is used to reduce the calculation and memory requirements. Thus, the proposed self-commissioning notch filter results robust and consumes little computational resources. Finally, the analysis is validated with both simulation and experiments.

Control of flywheel energy storage systems as virtual synchronous machines for microgrids

Microgrids are an attractive option in remote areas with elevated renewable resources. However, with or without grid connection, microgrids often results in weak grids. Hence, microgrids are much affected by the power variations and require energy storage systems to smooth them out. Flywheel based energy storage systems (FESSs) are gaining momentum in microgrids as, despite the limited amount of stored energy, they allow to interchange high power and have long useful lifetime. In addition, the state-of-charge is very simple to estimate as it only consists on measuring the spinning speed. In FESSs, the flywheel is attached to an electrical machine, which is connected to the grid through a back-to-back converter. Controlling the three phase converter as a virtual synchronous machine allows to overcome instability issues of the PLL-based control in a weak grids. For his reason, his paper proposes to control the grid-side converter of the FESS as a virtual synchronous machine. Small signal analysis is applied to the equations of the virual synchronous machine. The DC-link voltage is regulated by the machine-side converter. Simulation results are provided to illustrate the proposed concepts.

DC-Link Control Filtering Options for Torque Ripple Reduction in Low-Power Wind Turbines

Small wind energy conversion systems (WECSs) are becoming an attractive option for distributed energy generation. WECSs use permanent-magnet synchronous generators (PMSGs) directly coupled to the wind turbine and connected to the grid through a single-phase grid-tie converter. The loading produced on the dc link is characterized by large ripple currents at twice the grid frequency. These ripple currents are reflected through the dc bus into the PMSG, causing increased heating and ripple torque. In this paper, the PMSG inverter is used to control the dc-link voltage. In order to avoid reflecting the ripple currents into the PMSG, the feedback dc-link voltage is passed through a filter. The Butterworth filters, notch filters, antiresonant filter (ARF) and moving average filter (MAF) are considered. For a fair comparison, formulas are provided to tune the filter parameters so that dc-link voltage control will achieve the selected bandwidth. The different filtering options produce different levels of torque ripple reduction. The notch filter, ARF, and MAF obtain the best results and there is a tradeoff between the filter implementation complexity, bandwidth, overshoot, and the torque ripple reduction. Simulations and experiments using a 2.5-kW PMSG turbine generator validate the proposals.