Overview of AC microgrid controls with inverter-interfaced generations

Distributed generation (DG) is one of the key components of the emerging microgrid concept that enables renewable energy integration in a distribution network. In DG unit operation, inverters play a vital role in interfacing energy sources with the grid utility. An effective interfacing can successfully be accomplished by operating inverters with effective control techniques. This paper reviews and categorises different control methods (voltage and primary) for improving microgrid power quality, stability and power sharing approaches. In addition, the specific characteristics of microgrids are summarised to distinguish from distribution network control. Moreover, various control approaches including inner-loop controls and primary controls are compared according to their relative advantages and disadvantages. Finally, future research trends for microgrid control are discussed pointing out the research opportunities. This review paper will be a good basis for researchers working in microgrids and for industry to implement the ongoing research improvement in real systems

STEROWANIE ORAZ SYNCHRONIZACJA DWUPOZIOMOWEGO FALOWNIKA NAPIĘCIA W WARUNKACH PRZEJŚCIOWEJ ASYMETRII NAPIĘĆ SIECI

This paper presents the operation of grid tied, two level voltage source inverter (VSI) during network voltage unbalance. The control system was implemented in synchronous rotating reference frame dq0 (SRF). Two types of control structures were investigated herein. First utilizes the Double Decoupled SRF Phase-locked loop (DDSRF-PLL) synchronisation with positive and negative sequence currents control. Second one is simplified system that does not provide symmetrical components decomposition and decoupling for synchronisation. Simulation results exhibited a superior performance of the DDSRF-PLL control system under grid voltage unbalance.Niniejszy artykuł przedstawia pracę dwupoziomowego falownika napięcia współpracującego z siecią, podczas przejściowej asymetrii napięć. System sterowania został zaimplementowany w wirującym układzie synchronicznym dq0. Przeanalizowano dwa typy sterowania. W pierwszym zastosowano metodę synchronizacji z odprzęganiem DDSRF-PLL wraz z możliwością kontroli prądów składowej zgodnej i przeciwnej. Drugi natomiast w swoje uproszczeni formie nie pozwalała na sterowanie obu składowych symetrycznych, zabrakło również odprzęgania podczas synchronizacji z siecią. Wyniki symulacji pokazały o wiele lepsze działanie pierwszej metody sterowania

Improvement of Stability of a Grid-Connected Inverter with an LCL filter by Robust Strong Active Damping and Model Predictive Control

This study addresses development and implementation of robust control methods for a three-phase grid-connected voltage source inverter (VSI) accompanied by an inductive-capacitive-inductive (LCL) filter. A challenge of current control for the VSI is LCL filter resonance near to the control stability boundary, which interacts with the inverter control switching actions and creates the possibility of instability. In general, active damping is needed to stabilize the system and ensure robust performance in steady-state and dynamic responses. While many active damping methods have been proposed to resolve this issue, capacitor-current-feedback active damping has been most widely used for its simple implementation. There has been no clear consensus regarding design of a control system including capacitor-current-feedback active damping. This is due to the fact that simulation/experiment results are not congruent with the design analyses on which the control is designed. This study explains the incoherence between theory and practice when it comes to a capacitor-currents-feedback active damping system. Proposed capacitor-current-estimate active damping utilizing a developed posteriori Kalman estimator gives coherent simulation results as expected from the design analyses. This reveals that the highly oscillatory capacitor currents containing the inverter switching effects bring about uncertainty in the system performance. The switching effects are not incorporated in the analyses and control system design. Therefore, it is required to remove the switching noise from the capacitor currents in order to yield consistent results. It has been confirmed that the proportional-negative feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor. In a digitally controlled system, the computation delay causes the equivalent resistance of the virtual impedance to become negative in the frequency range of fs/6 to fs/2, which produces a pair of open-loop unstable poles in RHP. This happens when the displaced resonance peak by active damping is in that region. Thus, an a priori Kalman estimator has been developed to generate one-sample-ahead state variable estimates to reconstruct the capacitor currents for active damping, which can compensate for the delay. The one-sample-ahead capacitor-current estimates are computed from the inverter-side and grid-side current estimates. The proposed method provides extended limits of the active damping gain that improve robustness against system parameter variation. It also allows strong active damping which can sufficiently attenuate the resonance. Grid condition is another significant factor affecting the stability of the system. In particular, a weak grid tends to provide high impedance. The system employing the proposed active damping method stably operates in a weak grid, ensuring robustness under grid impedance variation. The developed Kalman estimators offer an effective and easy way of determining the stability status of a system in addition to the functions of filtering and estimation. Stability analysis can be easily made since state variable estimates go to infinity when a system is unstable. As a promising approach, model predictive control (MPC) has been designed for the system. This study suggests that MPC including active damping can be employed for a grid-connected VSI with an LCL filter with good dynamic performance

A Novel Control Method For Grid Side Inverters Under Generalized Unbalanced Operating Conditions

This thesis provides a summary on renewable energy sources integration into the grid, using an inverter, along with a comprehensive literature research on variety of available control methods. A new generalized method for grid side inverter control under unbalanced operating conditions is also proposed. The presented control method provides complete harmonic elimination in line currents and DC link voltage with adjustable power factor. The method is general, and can be used for all levels of imbalance in grid voltages and line impedances. The control algorithm proposed in this work has been implemented by using MATLAB Simulink and dSPACE RT1104 control system. Simulation and experimental results presented in this thesis are in excellent agreement

Dynamics estimation and generalized tuning of stationary frame current controller for grid-tied power converters

The integration of AC-DC power converters to manage the connection of generation to the grid has increased exponentially over the last years. PV or wind generation plants are one of the main applications showing this trend. High power converters are increasingly installed for integrating the renewables in a larger scale. The control design for these converters becomes more challenging due to the reduced control bandwidth and increased complexity in the grid connection filter. A generalized and optimized control tuning approach for converters becomes more favored. This paper proposes an algorithm for estimating the dynamic performance of the stationary frame current controllers, and based on it a generalized and optimized tuning approach is developed. The experience-based specifications of the tuning inputs are not necessary through the tuning approach. Simulation and experimental results in different scenarios are shown to evaluate the proposal.Peer ReviewedPostprint (published version