Even-harmonic repetitive control for circulating current suppression in Modular Multilevel Converters

Grid emulators provide efficient manners to test grid-connected distributed power generators. They test the power generator such as photovoltaics, wind turbines without hardware damages. Recent studies investigated conventional voltage source inverters such as a two-level voltage source inverter and a three-level voltage source inverter as grid emulators. However, they have limited voltage capabilities and weak transient responses. This paper presents a technique for the control of a modular multilevel converter as a grid emulator in fault scenarios, where voltage sags, harmonics distortions, and frequency deviations are emulated. The steady-state performance in these scenarios is analyzed and compared with that of a two-level voltage source inverter. Energy balancing between the sub-modules in a modular multilevel converter is always a crucial topic in terms of stability, particularly under fault conditions. In this paper, the transient response of the point of common coupling voltage and the sub-module energy in the modular multilevel converter with a delta-wye configuration is analyzed and compared with a two-level voltage source inverter. Simulations of a 24 sub-modules modular multilevel converter verify the control method with S70 V at the dc-link and a 400 V line-to-line ac voltage at the point of common coupling. The modular multilevel converter grid emulator with a delta-wye transformer configuration shows better steady-state performances than a two-level voltage source inverter. The energy stored in each sub-module is well balanced during the fault scenarios, which indicates strong stability.

Two optimized control schemes of shunt active power filter (SAPF) in balanced three-phase three-wire system are proposed in this paper, including a fast fractional-order repetitive control (RC) strategy and a computation reduced harmonic detecting method. The improved RC is implemented in the synchronous reference frame (SRF) and based on the fractional-delayed-filter. Compared with the conventional RC (CRC), the proposed strategy can reduce the repetitive period from one fundamental cycle to one-sixth of the fundamental cycle, which results in the faster dynamic response. The fractional-delayed-filter, as the delay part of the proposed improved RC strategy, is used to achieve the high tracking accuracy of reference signals for control systems with different sampling rates. This paper also proposes a harmonic detecting method in the SRF based on the sliding Discrete Fourier transform (SDFT) with reduced computational burden, catering for selective harmonic elimination. Simulations and experiments show the validity of the proposed solutions.

This paper presents a general parallel structure repetitive control (PSRC) scheme for multiphase dc-ac pulse-width modulation (PWM) converters to cancel output total harmonic distortion more efficiently. With parallel structure, categorized harmonic frequency internal models are connected in parallel. Each categorized internal model has its own independent control gain and PSRC can optimize the total convergence rate by tuning the control gains independently according to the harmonic distribution. Compared with a multiresonant controller, PSRC has more compact structure and yields much less computation burden. Compared with conventional repetitive control (CRC), PSRC can achieve a much faster dynamic response without any loss of tracking accuracy or any added data memory. Moreover, PSRC is a general parallel structure RC for housing various existing RC, such as CRC, odd-harmonic RC, and dual-mode-structure RC. Experimental results of the PSRC-controlled three-phase dc-ac PWM converters show the validity and advantages of the proposed PSRC scheme.

The modular multilevel converter (MMC) is one of the most promising topologies for high- and medium-voltage applications. However, the conventional MMC relies on large arm inductance and complicated feedback control to maintain submodule (SM) capacitor voltages balanced, which requires a large number of voltage sensors and complicated arm current control loops. In contrast to the conventional perspective, the MMC can be regarded as a derivative from the general multilevel topology based on switched-capacitor (SC) structure. The SC structure endows excellent properties to the MMC, such as minimum or no arm inductor and automatic SM capacitor voltage balancing capability. This MMC with minimum or no arm inductor is named as SC-MMC. Since the arm inductances are very small, the SM capacitor voltages can be balanced automatically. Small stray/parasitic inductance is enough to limit inrush current during SM's switching instants to the rated current level. To unleash the automatic SM capacitor voltage balancing capability of the SC-MMC, a Y-matrix modulation (YMM) has been proposed, which does not require closed loop control or any sensor, has no limitations on the number of SMs, and works for both half-bridge and full-bridge SMs. An unresolved issue of the emerging YMM is that, in the published literature, the existence of the Y matrix with full rank has not been proved theoretically beyond the observation for the SC-MMC when the number of SMs is large. In this paper, an analytical proof of the existence of the full rank Y matrix is provided, which implies the automatic voltage balance of SMs for the MMC with arbitrary number of SMs in each arm. Simulation results using the YMM on a 21-level SC-MMC with minimum arm inductance are provided to verify the voltage balance theory/observation of the SC-MMC using the YMM.

Model-predictive current control (MPCC) is a promising candidate for modular multilevel converter (MMC) control due to its advantages of direct modeling and fast dynamic response. The conventional MPCC, which obtains the optimal control input by evaluating a cost function for all the possible switching states, may make the MPCC impractical due to the exponentially increasing computation burden with the increasing number of submodules (SMs). On the other hand, the MPCC experiences high load current and circulating current tracking errors, since only one switching state is selected and applied during one control period. To address these issues, this paper proposes an MPCC with phase-shifted pulsewidth modulation (PS-PWM) for improving the steady-state control performance. The arm voltages are considered as a whole to implement the proposed MPCC. The optimal duty cycle is obtained based on the load and circulating current tracking error minimization and applied using the PS-PWM. As a result, the computation burden is unrelated to the number of SMs by avoiding the exhaustive evaluation process for all the possible switching states. A better steady-state performance with smaller tracking errors is achieved with the similar switching frequency, and the tedious tuning process of the weighting factor is eliminated. Experimental results are presented to demonstrate the effectiveness of the proposed MPCC.

The paper presents a discrete-time controller for high voltage direct current (HVDC) based on modular multilevel converter. Voltage source converter HVDC (VSC-HVDC) systems are mainly built with pulse-width modulation converter technology. As it operates at high frequency, its high switching losses is an obstacle to its progress. Modular multilevel converter is another kind of converter built with serial-connected half-bridges which only need to switch at line frequency. Therefore it does not need to exert high voltage on power switch and can be easy to enlarge, and modular multilevel converter has been applied to HVDC recently. The control system of modular multilevel converter HVDC can be cascade control like other VSC-HVDC. For its low switching frequency, its sampling period has to be long which would result in slow dynamic response. This paper proposes a high dynamic controller based on a discrete-time model. In the control the reference for the inner loop has taken account of the transition ...

Modular Multilevel Converters (MMC) are known for their high modularity and good quality output power. The problems of circulating current and capacitor voltage balancing for the MMC have been widely studied in available literature but little work is done towards grid current control with fast dynamic response. Conventional current controllers use PI or PR controllers which are slow and thus degrade transient performance of the MMC. Recently reported Model Predictive Control (MPC) methods for MMC aim to control the grid current but MPC suffers from computational burden and difficulties in adjusting of weights. Space Vector based Hysteresis Current Controllers (SCHCC) have been well established for drive applications. In this work, an SVHCC has been developed for a grid connected MMC system. Simulation studies show fast current control for cases of sudden change of active power reference and current reference. The controller exhibits quick tracking of the current reference in all cases. The proposed method has been discussed for a MMC system with three submodules, but it may be extended to any number of submodules and can be easily implemented in a digital controller with less computational effort.

This paper presents a novel synchronous-frame repetitive controller for three-phase UPS inverters. Distinguished from conventional repetitive control techniques, the proposed synchronous-frame approach minimizes the repetitive control time delay to one-sixth of the fundamental period such that the dynamic response is significantly improved. In order to overcome the harmonic distortions under severe load conditions (e.g., unbalanced and nonlinear), in this paper, three synchronous rotating frames are deliberately selected, in each of which the repetitive controller is incorporated. Resultantly, the (6n ±1)th harmonics as well as the triplen harmonics are compensated. Moreover, a high-performance fourth-order linear phase infinite-impulse-response filter is applied to further enhance the accuracy of steady-state tracking. The proposed controller is programmed on the 16-bit fixed-point digital signal processor (TI TMS320LF2407) and eliminates high-resolution current sensors for cost effectiveness. Simulations and experimental tests have been carried out based on an 18-kW three-phase UPS system. Low total harmonic distortion (<;0025;) has been achieved under heavily distorted nonlinear load and unbalanced load. Fast dynamic response has been demonstrated during step load transients.

This study is to suppress the cell voltage and arm current in modular multilevel converters (MMCs) based on power balancing and current tracking control law. MMCs have been widely applied to direct-current transmission, high-voltage grid power injection, back-to-back power transfer, energy storage systems and renewable energy systems. In particular, the renewable energy systems have been depicting a lot of prospect due to low pollution, natural replenishment, large capacity, and sustainable development. The dimension of each cell capacitors and the rating of switches play a critical part in designing an MMC, and cell voltages highly interact to arm currents, dc circulation currents and grid-injection currents. Normally, higher arm current require lower cell-voltages when adopting a zero-mean current injection technique. This paper presents unique approaches to deriving cell-voltage expression in terms of arm currents, injected zero-mean current and current tracking control laws, on which cell-voltage ripple reduction and arm-current swing suppression can be achieved. Simulation results obtained from a 10 kW single-phase MMC have verified the analysis and discussion.

Model predictive control (MPC) is considered one of the best non-linear control methods for controlling the modular multilevel converter (MMC) due to various advantages, such as no need for direct modelling, high dynamic response, and the ability to manage multiple control targets simultaneously. In this paper, a current closed-loop controller (CCLC) based on the current predictive strategy (CPS) is used to control the circulating and output currents in an MMC. A bilinear mathematical model of the MMC is derived and discretized to predict the states of the MMC for one step ahead. In addition, to reduce the practical costs and reduce the converter sensors, a sensor reduction algorithm has been used to reduce the number of voltage sensors and increase the reliability of the system. The proposed method is implemented in a single-phase MMC with four sub-modules (SMs) in each arm. The accuracy and performance of the proposed method are evaluated with simulation and experimental results.

The arm inductors utilized in a modular multilevel converter (MMC) can suppress the MMC circulating current, fault current, and output current, and circulating current ripples. This paper presents a four-step design and optimization method for the MMC arm inductor. The first step is the define the requirements and specifications of the arm inductor based on the MMC ratings and detailed modeling, in order to define the core material and select the appropriate winding wire as a second step. The third step is to develop a multi-objective optimization for the core design of the arm inductor, while the considered constraints are area product, maximum magnetic flux density, and window area, which is here done based on a Non-dominated Sorted Genetic Algorithm (NSGA-II). The objective functions are inductor total mass, total volume, total power losses, total core length, and air gap length. Finally, using the average ranking (AR) method, the best optimal solution is selected from the Pareto solution provided by NSGA-II.

Modular multilevel converter (MMC) has been widely used in high-voltage direct current (HVDC) transmission system due to its advantages of modularity, low switching frequency and excellent output voltage waveforms. Proper operation of MMC is based on capacitor voltage balancing. Conventional MMC capacitor voltage balancing control needs a large number of voltage sensors and sorting algorithm which increases computing burden on the processor. In this paper, a capacitor voltage self-balancing MMC with DC fault ride-through capability is proposed. Capacitor voltage balancing control of this proposed MMC is significantly simple due to its self-balancing auxiliary circuit. Sorting algorithm is not needed, each phase leg only needs one voltage sensor for top SM. So control complexity and computing burden are reduced greatly. Furthermore, DC fault ride-through capability is also reserved. Simulation results validates capacitor voltage of this new MMC topology can be well balanced and DC fault ride-through capability is effective.

In view of the surplus torque and complexity of controlled plant in passive electrical load system, a novel approach based on single neuron PID adaptive control and repetitive control for repetitive periodic load control system is proposed. Radial basis function (RBF) neural network is used to identify the system on-line for the single neuron PID controller to adjust its weights and PID parameters by self- learning and self-adapting based on the desired output. The dynamic state performance can be improved by the single neuron adaptive PID control and the steady state performance is also improved by modified repetitive control. Computer simulation results show that the force/position hybrid control system can effectively reduce the surplus torque and improve the loading precision, and also it has fine dynamic and steady state performance and good robustness. The reliability of whole system is further improved.

The modular multilevel converter (MMC) is a highly efficient multilevel converter topology suitable for high and medium voltage applications. However, one of the negative traits of MMC is the existence of circulating current. The detrimental effects of circulating currents include distortion in arm voltages, converter power loss and high switching stresses on semiconductor devices. This paper aims to focus on different circulating current mitigation techniques and their comparative analysis. This paper also proposes a circulating current suppressing controller (CCSC) which depends upon the repetitive control (RC) to minimize the inner circulating current in an MMC. The proposed model is simulated in MATLAB/Simulink and the results are quite in proximity with theoretical analysis.

This paper develops an enhanced harmonic voltage compensator which is implemented with the aid of two repetitive controllers (RCs) in order to improve the output voltage performance of stand-alone distributed generation (DG) systems. The proposed harmonic voltage compensator is able to maintain the DG output voltage sinusoidal regardless of the use of nonlinear and/or unbalanced loads in the load side. In addition, it can offer good steady-state performance under various types of loads and a very fast dynamic response under load variations to overcome the slow dynamic response issue of the traditional RC. The feasibility of the proposed control strategy is verified through simulations and experiments.