Plug and Play DC-DC Converters for Smart DC Nanogrids with Advanced Control Ancillary Services

This paper gives a general view of the control possibilities for dc-dc converters in dc nanogrids. A widely adopted control method is the droop control, which is able to achieve proportional load sharing among multiple sources and to stabilize the voltage of the dc distribution bus. Based on the droop control, several advanced control functions can be implemented. For example, power-based droop controllers allow dc-dc converters to operate with power flow control or droop control, whether the hosting nanogrid is operating connected to a strong upstream grid or it is operating autonomously (i.e., islanded). Converters can also be equipped with various supporting functions. Functions that are expected to play a crucial role in nanogrids that fully embrace the plug-and-play paradigm are those aiming at the monitoring and tuning of the key performance indices of the control loops. On-line stability monitoring tools respond to this need, by continuously providing estimates of the stability margins of the loops of interest; self- tuning can be eventually achieved on the basis of the obtained estimates. These control solutions can significantly enhance the operation and the plug-and-play feature of dc nanogrids, even with a variable number of hosted converters. Experimental results are reported to show the performance of the control approaches

Analysis of an On-Line Stability Monitoring Approach for DC Microgrid Power Converters

An online approach to evaluate and monitor the stability margins of dc microgrid power converters is presented in this paper. The discussed online stability monitoring technique is based on the Middlebrook's loop-gain measurement technique, adapted to the digitally controlled power converters. In this approach, a perturbation is injected into a specific digital control loop of the converter and after measuring the loop gain, its crossover frequency and phase margin are continuously evaluated and monitored. The complete analytical derivation of the model, as well as detailed design aspects, are reported. In addition, the presence of multiple power converters connected to the same dc bus, all having the stability monitoring unit, is also investigated. An experimental microgrid prototype is implemented and considered to validate the theoretical analysis and simulation results, and to evaluate the effectiveness of the digital implementation of the technique for different control loops. The obtained results confirm the expected performance of the stability monitoring tool in steady-state and transient operating conditions. The proposed method can be extended to generic control loops in power converters operating in dc microgrids

Simultaneous Measurement of Bus Impedance and Control Loop Gains in Multi-Converter Systems

acceptedVersionPeer reviewe

Dynamical Characterization of Multi-Converter System : Simultaneous Measurement of Bus Impedance and Control Loop Gains

Interconnected power-electronics converters connected to a common bus have become increasingly important in various power-distribution systems. Due to interactions between converter subsystems the multi-converter system can often have stability issues even though each converter is standalone stable. Recent studies have presented a passivity-based stability criterion with which the stability of a multi-converter system can be analyzed by measuring the system bus impedance. The technique provides the stability of the complete system but does not reveal the dynamics of a single converter. The dynamics of single converters can be studied by other methods such as loop-gain measurements. The loop gains provide direct information on the operation of single converters and their stability margins but not the information about the global stability. This paper combines these techniques to simultaneously analyze the single converters and the complete multi-converter system. In the method, several orthogonal perturbations are injected into the converter control loops. The current and voltage responses are measured in the loop and from the converter outputs. After this, Fourier techniques are applied to obtain the spectral information of the loop gains and the bus impedance. The applied perturbations can be designed to have a very small amplitude, and thus, the process does not cause the system to deviate too much from its normal operation. Therefore, the method is well suited, for example, in online analysis and adaptive control. Experimental measurements are presented from a complex multi-converter system. The work is a revised and extended version of a presentation at ECCE2022.acceptedVersionPeer reviewe

Stability Monitoring and Controller Autotuning of Power Converters in DC Microgrid

Recently, there has been an increasing interest towards grouping several power resources together with some loads as well as some energy storage systems in a microgrid environment. This is mainly because a high number of distributed energy resources (DERs), such as renewable energies and energy storage systems can be integrated in a microgrid environment, that, in turn, will lead to a reduction in the transmission and distribution losses, the overall system costs, as well as the CO2 emissions. In addition, as the generation is going to be mostly near to the consumption point, the power quality, eciency and reliability will be signicantly increased. Microgrids are also a smart choice for the remote locations that are beyond reach of the current grid. Dc microgrids bring with some advantages over their ac counter part. For instance, they are more compatible with the dc nature of many DERs such as photovoltaics and energy storage systems. Also, the inductive voltage drop is removed in a dc system. Thus, a large number of DERs can be integrated into a dc microgrid by taking advantage of power electronic converters, that introduce several control and operation benets. Power converters used in dc microgrids are usually equipped with several control loops. When many converters are connected to a common dc bus, the dynamic performance of some control loops may be dierent from the behavior designed for the stand-alone converter, due to possible eects of the interconnected converters. This issuewhich is typically referred to as the `interaction effect' of multiple parallel converters can lead to stability and performance concerns in a dc microgrid. Thus, interaction eect on a generic control loop depends on the interconnected power converters, for instance, on their topology, control structure, parameters, etc. In order to know the real-time control performance and stability of the control loops within dc microgrid power converters, it is important to equip the converters with online stability monitoring tools. The monitored data will not only include the internal stability conditions of each loop, but also take the interaction eect into account. Subsequently, some corrective actions can be introduced in the system to maintain a desired dynamic performance and avoid instability. In addition, in the context of smart microgrids, the advanced monitoring tools, as well as adaptive control and management actions are of a wide interest. This work rstly, investigates an on-line stability monitoring technique that is inspired by the Middlebrook's injection method. This method allows to estimate and monitor the stability margins of a generic control loop (e.g., current loop, voltage loop, droop loops, etc.) within dc microgrid power converters. Since we target a multi-converter environment, the presence of multiple perturbations coming from the monitoring units of several converters is also taken into account. Secondly, two dierent on-line tuning techniques are proposed, that both aim to achieve the desired phase margin for a generic control loop at the reference bandwidth. These methods are based on injecting a small-signal perturbation at the desired reference crossover frequency into the loop under study. In other caseswhere a full picture about the performance of dierent loops over the entire bandwidth is desiredmultiple orthogonal pseudo-random binary sequences (PRBSs) are proposed to be simultaneously injected in several control loops. This will provide the frequency responses of all the loops in a single measurement cycle. Finally, in order to further assess the microgrid-level stability and dynamic performance, some of the monitored data are eectively used to estimate the dc bus impedance, which has been shown to provide a measure of the stability and performance of the entire microgrid. All the stability monitoring and adaptive tuning functions are experimentally validated in a laboratory setup that emulates a dc microgrid

Stability Monitoring and Controller Autotuning of Power Converters in DC Microgrid

Recently, there has been an increasing interest towards grouping several power resources together with some loads as well as some energy storage systems in a microgrid environment. This is mainly because a high number of distributed energy resources (DERs), such as renewable energies and energy storage systems can be integrated in a microgrid environment, that, in turn, will lead to a reduction in the transmission and distribution losses, the overall system costs, as well as the CO2 emissions. In addition, as the generation is going to be mostly near to the consumption point, the power quality, eciency and reliability will be signicantly increased. Microgrids are also a smart choice for the remote locations that are beyond reach of the current grid. Dc microgrids bring with some advantages over their ac counter part. For instance, they are more compatible with the dc nature of many DERs such as photovoltaics and energy storage systems. Also, the inductive voltage drop is removed in a dc system. Thus, a large number of DERs can be integrated into a dc microgrid by taking advantage of power electronic converters, that introduce several control and operation benets. Power converters used in dc microgrids are usually equipped with several control loops. When many converters are connected to a common dc bus, the dynamic performance of some control loops may be dierent from the behavior designed for the stand-alone converter, due to possible eects of the interconnected converters. This issuewhich is typically referred to as the `interaction effect' of multiple parallel converters can lead to stability and performance concerns in a dc microgrid. Thus, interaction eect on a generic control loop depends on the interconnected power converters, for instance, on their topology, control structure, parameters, etc. In order to know the real-time control performance and stability of the control loops within dc microgrid power converters, it is important to equip the converters with online stability monitoring tools. The monitored data will not only include the internal stability conditions of each loop, but also take the interaction eect into account. Subsequently, some corrective actions can be introduced in the system to maintain a desired dynamic performance and avoid instability. In addition, in the context of smart microgrids, the advanced monitoring tools, as well as adaptive control and management actions are of a wide interest. This work rstly, investigates an on-line stability monitoring technique that is inspired by the Middlebrook's injection method. This method allows to estimate and monitor the stability margins of a generic control loop (e.g., current loop, voltage loop, droop loops, etc.) within dc microgrid power converters. Since we target a multi-converter environment, the presence of multiple perturbations coming from the monitoring units of several converters is also taken into account. Secondly, two dierent on-line tuning techniques are proposed, that both aim to achieve the desired phase margin for a generic control loop at the reference bandwidth. These methods are based on injecting a small-signal perturbation at the desired reference crossover frequency into the loop under study. In other caseswhere a full picture about the performance of dierent loops over the entire bandwidth is desiredmultiple orthogonal pseudo-random binary sequences (PRBSs) are proposed to be simultaneously injected in several control loops. This will provide the frequency responses of all the loops in a single measurement cycle. Finally, in order to further assess the microgrid-level stability and dynamic performance, some of the monitored data are eectively used to estimate the dc bus impedance, which has been shown to provide a measure of the stability and performance of the entire microgrid. All the stability monitoring and adaptive tuning functions are experimentally validated in a laboratory setup that emulates a dc microgrid

Effect of Torsional Interactions on the Output Impedance of PMSG-Based Wind Turbines

This paper investigates the output impedance of a permanent magnet synchronous generator (PMSG) in dq reference frame, taking the torsional interactions into account. The dynamic behavior of the PMSG drive-train is modelled through both one-mass and two-mass mechanical models. The obtained model is included in the overall output impedance of a grid-connected wind energy conversion system (WECS). According to the analytical model found in this paper, which is also verified by simulation results in Matlab/Simulink, low frequency resonances may appear in the output impedance of the WECS, due to torsional interactions