Hamed Nademi

A method for charging and discharging capacitors in Modular Multilevel Converter

In this paper, a method for charging and discharging capacitors in Modular Multilevel Converter (MMC) is explained. The proposed method helps to start the converter from a de-energized condition and does not require any auxiliary voltage source. An additional resistance is inserted in the MMC arm and by appropriately switching this resistance, both charging and discharging of the MMC cells can be accomplished. A theoretical background is presented for drives applications and design examples are included. Simulation and experimental results are included at the end to validate the proposed concept.

Modular Multilevel Converter With an Adaptive Observer of Capacitor Voltages

A modular multilevel converter (MMC) is an attractive solution for power conversion without transformers. The MMC consists of cascade connections of floating dc capacitors. In this paper, an adaptive observer design has been proposed to estimate the capacitor voltages from the measurement of arm currents. This work introduces the capacitance value of the cell capacitors as a parameter uncertainty for making the system performance robust with unknown constant parameters. It may be used for predictive control, condition monitoring for capacitors, and diagnosis check for capacitor health. In addition, a pulsewidth modulation (PWM) technique for MMC has been explored. The PWM technique is performed using a carrier-based level-shifted PWM strategy. It does not necessitate the calculation of duty cycles, and can be easily implemented in a DSP. By using the PWM technique, harmonics in the phase voltage is shifted to twice the switching frequency. Theoretical analysis is included in this paper for showing stability and convergence of the proposed observer. Analytical expressions are verified by simulation and experimental results.

A Pulse Width Modulation technique for reducing switching frequency for modular multilevel converter

In this paper, a Pulse Width Modulation (PWM) technique for modular multilevel converter has been explored. The PWM technique is implemented using carrier based level shifted PWM strategy. It does not necessitate the calculation of duty cycles, and can be easily implemented in a DSP. With the proposed technique, harmonics in the phase voltage is shifted to twice the switching frequency. Thus, the devices of the converter can be switched at a reduced switching frequency to maintain the same output voltage quality. A detailed explanation of the carrier based level shifted PWM for implementing the proposed technique is included in the paper. This is verified by computer simulation on a 6.6kV, 6MW drive system.

A New Circuit Performance of Modular Multilevel Inverter Suitable for Photovoltaic Conversion Plants

This paper investigates a new circuit topology of the modular multilevel converter (MMC) for deploying in photovoltaic (PV) distributed generation systems. In the conventional MMC, two arm inductors are placed in each phase to limit the circulating current. In the proposed topology, the inductors are replaced by a transformer. The proposed circuit gives a 50% reduction in the voltage rating of the power devices and the capacitor size in comparison with the basic MMC topology. The required dc-link voltage that is directly fed by PV panels is also reduced by half. In addition, the transformer helps to limit the fault current in the dc bus. This paper presents a pulsewidth modulation method to control the solar inverter output voltage, in which there is no need to calculate the duty cycle of individual cells. A brief description of the charging process for the capacitors is also included. The converter behavior is evaluated by numerical simulation to validate the concept. Experimental verification is achieved through a downscaled real test bench.

An adaptive backstepping observer for modular multilevel converter

This paper deals with the estimation problem in three-phase, three-level Modular Multilevel Converter (MMC) using nonlinear observer to realize an accurate estimation of unmeasured variables. It is shown that the desired states to be exponentially convergent if all system signals are bounded. An observer based on adaptive backstepping approach is used for capacitor voltages estimation from the arm currents measurement. The observer can track the capacitance value of module capacitor which is unknown as a parameter uncertainty with acceptable precision. In practical, it results attracting to evaluate the possibility of reducing the number of sensors, or even using these soft sensors as redundant for fault detection purposes. The nonlinear observer stability study by means of a Lyapunov theory guarantees the stability and convergence of the estimated quantities. The feasibility of the proposed solution is demonstrated by simulation results.

An analytical frequency-domain modeling of a Modular Multilevel Converter

The Modular Multilevel Converter (MMC) is a new topology for multilevel converters with potential for medium voltage and high voltage applications. This paper focuses on the modeling technique in the frequency domain of an MMC. Using this frequency domain modeling it is possible to analyze and predict the harmonic content in the arm current and capacitor voltages. In MMC, a strong harmonic content exists in the arm current, although it is not reflected in the load current. This essentially controls the dynamic behavior of the converter. Moreover, explicit derivation of the equations that describe the harmonic evaluation on the floating capacitor voltage has been developed based on the switching functions concept. It can be employed for reducing the ripple voltage using harmonic components in the arm currents. The most dominant harmonics (2nd and 4th) in the arm current are derived analytically and compared in the paper.

Implicit Finite Control Set Model Predictive current Control for Modular Multilevel Converter based on IPA-SQP algorithm

This paper investigates a Finite-Control-Set Model Predictive Control (FCS-MPC) for the precise control of (un)balanced load currents in Modular Multilevel Converter (MMC). The control objectives are circulating currents minimization inside the converter arms, achieve a capacitors voltage balance and load current control. To achieve the converter constrained optimization and facilitate the implementation on embedded systems, an integrated perturbation analysis and sequential quadratic programming (IPA-SQP) solver is also utilized. As a case study the proposed approach is applied to a grid-connected five-level MMC. The introduced FCS-MPC formulation reduces sensitivity of the converter output voltage to disturbances in grid side and measurement noise with reducing the computational burden. Simulation results reveal the effectiveness of the developed control scheme in cases when operational objectives, e.g., load current reference tracking and disturbance rejection are considered under system model uncertainties.

Analysis of a Modular Multilevel inverter under the predicted current control based on Finite-Control-Set strategy

This paper introduces a method for the precise control of (un)balanced load currents in Modular Multilevel Converter (MMC). The algorithm is based on the Finite-Control-Set Model Predictive Control (FCS-MPC) which is a promising predictive control approach for power converters to minimize the circulating currents inside the converter legs, achieve the voltage balance in the capacitors and load current control. The grid-connected MMC topology, its analytical modeling and the controller design procedure are described and design example is studied. Developed predictive control is also employed to reduce sensitivity of the converter output voltage to disturbances in load side and measurement noise. In addition, system stability has been analytically verified in terms of model uncertainties. Simulation results are included to validate the proposed controller especially for good load current reference tracking and disturbance rejection properties despite the variations in the system parameters.

Robust controller design for governing steam turbine power generators

The turbine control system is one of the key control loops in the dynamic performance of steam power generation units. In this paper a multivariable PID controller is designed for the governing system of steam turbine power generators. The necessary and sufficient conditions for existence of a strong robust H∞ dynamic compensator are established in terms of linear matrix inequality (LMI) approach. This controller is designed in succeeding to the existing proportional controller in a power plant. To reach to this goal, the complete dynamic model of an actual turbine-generator including the governor, turbine and generator, are derived. Simulation results show that proposed controller has good tracking and disturbance rejection properties despite the variations in the system parameters.

Comparative analysis of different MPPT schemes for photovoltaic integration of modular multilevel converter

This paper demonstrates a comparative study of two well-known Maximum Power Point Tracking (MPPT) algorithms in photovoltaic (PV) plants based on Modular Multilevel Converter (MMC). The Perturb and Observe (P&O) technique which is widely deployed throughout industry is considered for the comparison evaluation together with the Ripple Correlation Control (RCC) strategy. The efficiency of entire PV generation system heavily depends on the power conversion unit in addition to the employed MPPT performance, in particular during fast varying environmental conditions. This paper presents an outcome of a detailed evaluation of the simulated PV system utilizing both MPPT methods for the MMC with respect to dynamic performance, solar panels mismatch, solar irradiance variation and practical implementation. The proposed PV generation system with a 5-level MMC inverter is evaluated through simulation to verify the main findings.