Alessando Costabeber

Comparative stability analysis of droop control approaches in voltage-source-converter-based DC microgrids

Droop control has been widely applied in dc microgrids (MGs) due to its inherent modularity and ease of implementation. Among the different droop control methods that can be adopted in dc MGs, two options have been considered in this paper: I-V and V-I droop. I-V droop controls the dc current depending on the dc voltage while V-I droop regulates the dc voltage based on the output current. The paper proposes a comparative study of V-I/I-V droop control approaches in dc MGs focusing on steady-state power-sharing performance and stability. The paper presents the control scheme for current-mode (I-V droop) and voltage-mode ( V-I droop) systems, derives the corresponding output impedance of the source subsystem, including converters dynamics, and analyzes the stability of the power system when supplying constant power loads. The paper first investigates the impact on stability of the key parameters including droop gains, local control loop dynamics, and number of sources and then performs a comparison between current-mode and voltage-mode systems in terms of stability. In addition, a generalized analytical impedance model of a multisource, multiload power system is presented to investigate stability in a more realistic scenario. For this purpose, the paper proposes the concept of “global droop gain” as an important factor to determine the stability behaviour of a parallel sources based dc system. The theoretical analysis has been validated with experimental results from a laboratory-scale dc MG.

Distributed current control for multi-three phase synchronous machines in fault conditions

Among challenges and requirements of on-going electrification process and future transportation systems there is demand for arrangements with both increased fault tolerance and reliability. Next aerospace, power-train and automotive systems exploiting new technologies are delving for new features and functionalities. Multi-three phase arrangements are one of these novel approaches where future implementation of aforementioned applications will benefit from. This paper presents and analyses distributed current control design for asymmetrical split-phase schemes composed by symmetrical three phase sections with even number of phases. The proposed design within the dq0 reference frame in nominal, open and short circuit condition of one three-phase system is compared with the vector space decomposition technique and further validated by mean of Matlab/Simulink® simulations.

The series bridge converter (SBC): a hybrid modular multilevel converter for HVDC applications

This paper presents a novel hybrid modular multilevel voltage source converter suitable for HVDC applications. It has the advantages of other modular multilevel topologies and can be made more compact making it attractive for offshore stations and city infeed applications. The Operating principle of the converter and internal energy management are discussed with simulation results from a scaled medium voltage demonstrator presented to validate the concepts.

Stability analysis of single-phase grid-feeding inverters with PLL using Harmonic Linearisation and Linear Time Periodic (LTP) theory

A general method for the stability analysis of power converters is presented in this paper. The method is based on Harmonic Linearisation and Linear Time Periodic (LTP) analysis techniques and a single-phase grid-feeding inverter with PLL is considered as case study. Although stability analysis has been developed using the average model of the converter, the obtained results can be extended to the switching model and it is possible to evaluate precisely the boundary between stability and instability.

Control of a modular multilevel converter with pulsed DC load

This paper focuses on a Modular Multilevel Converter grid interface for a klystron modulator system that behaves as a pulsed DC load. With such a load, and without mitigating control, the MMC suffers from cell capacitor voltage imbalance between the converter arms which leads to distortion (fluctuation) of the absorbed AC power. This paper proposes an augmented modulation strategy, with a tailored distribution of the modulation signals between the arms within a phase, to ensure low AC power fluctuation. The effectiveness of the method has been verified through simulation and has been experimentally proven on a 7 kW MMC prototype operating with a 3 kA pulsed DC load.

Modular Multilevel Converter Grid Interface for Klystron Modulators: An Augmented Modulation Scheme for Arm Balancing

This paper discusses the control of a modular multilevel converter (MMC) used as a grid interface for the klystron modulators in the compact linear collider (CLIC). The converter has a DC side load which takes short-duration power pulses, causing high DC side power fluctuations that are not tolerable if seen by the AC grid. The DC–AC power decoupling capability of the MMC enables mitigation of the power ripple on the AC side, guaranteeing compliance with power quality requirements. However, the pulse repetition rate of the CLIC modulators is synchronized the 50-Hz AC grid and this induces permanent power imbalance in the arms of the MMC, causing voltage deviation and overmodulation unless appropriate balancing strategies are implemented. Unlike existing arm balancing methods that control 50-Hz circulating currents to balance the arm powers, the method proposed in this paper introduces an augmented modulation strategy where modulation signals are redistributed among arms based on the demand from a balancing controller. The resulting controller has lower complexity and its simple structure enables an easier design of the balancing loop, which guarantees predictable dynamics in operation. The effectiveness of the method has been demonstrated in simulation for the full-scale CLIC converter ratings and experimentally on a 7-kW MMC prototype operating with a 3.3-kA pulsed DC load.

Distributed speed control for multi-three phase electrical motors with improved power sharing capability

This paper proposes a distributed speed control with improved power sharing capability for multi-three phase synchronous machines. This control technique allows the speed to be precisely regulated during power sharing transients among different drives. The proposed regulator is able to control the time constant of the current within the dq0 reference frame to a step input variation. If compared to current set-point step variations, the proposed droop controller minimises devices stress, torque ripple, and thus mechanical vibrations. Furthermore, since distributed, it shows improved fault tolerance and reliability. The design procedure and the power sharing dynamic have been presented and analysed by means of Matlab/Simulink and validated in a 22kW experimental rig, showing good agreement with the expected performances.

ESBC: An enhanced modular multilevel converter with H-bridge front end

This paper presents the Enhanced Series Bridge Converter (ESBC), a hybrid modular multilevel converter with H-bridge front end suitable for high power grid applications. It retains the advantages of other modular multilevel topologies while offering compact structure, making it attractive for offshore stations, back-back HVDC stations, and city centre infeeds. The structure, operating principles and energy management of the converter are discussed. Simulation results from a scaled down medium voltage demonstrator are presented to validate the concept.

Arm-Balancing Control and Experimental Validation of a Grid-Connected MMC With Pulsed DC Load

This paper focuses on the operation of a grid-connected modular multilevel converter (MMC) supplying a pulsed dc load. The goal is to achieve minimum ac power fluctuation despite the high power fluctuation present on the dc side. The MMC has been selected for its inherent ability to decouple ac and dc current controllers. However, if no additional provisions are taken, the pulsed load causes imbalance of cell capacitor voltages between upper and lower arms in each phase. This paper presents the theoretical analysis of the imbalance problem and proposes a simple arm-balancing controller to enable the operation of the converter under pulsed dc load. The effectiveness of the controller has been successfully verified on a 7 kW MMC experimental prototype with a 3 kA pulsed dc load.

The series bridge converter (SBC): Design of a compact modular multilevel converter for grid applications

This paper presents a novel hybrid modular multilevel voltage source converter suitable for grid applications. The proposed converter retains the advantages of other modular multilevel topologies and can be made more compact making it attractive for offshore stations and other footprint critical applications like city in feeds. In this paper, the basic operating principle and design criteria for the converter implementation are presented. The submodule capacitor requirements which have significant influence on the size of a converter station are also evaluated and compared to the MMC. The performance of the converter is supported by simulation results from a representative medium voltage scaled demonstrator.