Capacitor RMS Current Reduction for Back-to-Back Three-Level Converters Based on Zero-Sequence Voltage Injection

Modular multilevel converter (MMC) is drawing more and more attention in medium-voltage microgrid applications. Large-scale switching-off loads and grid faults may lead to the voltage swell in microgrid, where MMC should have the ride-through ability. In this paper, a simple operation approach with the fundamental and irregular zero-sequence voltage (ZSV) injection is proposed for MMC under grid voltage swell condition. First, the fundamental ZSV injection is applied to adjust the modulation margin of three phases where the amplitude values of modulation waves in each phase can be the same. Then, if the amplitudes with fundamental ZSV injection are still in over-modulation status, the new irregular ZSV injection is added to further enlarge the operation region. In addition, the limitation of the irregular ZSV injection is analyzed. Simulation and experimental results of MMC under different grid voltage swell conditions verify the effectiveness of the proposed operation approach.

High frequency and high amplitude common-mode voltage (CMV) can induce a number of problems on the reliable operations of back-to-back two-level converters, such as shaft voltages and bearing currents. To restrain the CMV for back-to-back two-level converters, a simple and effective CMV reduction method based on zero-sequence voltage (ZSV) injection is proposed in this article. By injecting the optimal amount of ZSV on either the rectifier-side or the inverter-side, the corresponding pulse overlapping areas that result in high CMV amplitudes can be eliminated with the proposed method, then the amplitude of CMV can be reduced by half, and the pulse edges of CMV can be reduced by one-third. Simulation and experimental results are presented to verify the effectiveness of the proposed method in CMV reduction, while some comparisons are also conducted to demonstrate the superiority of the proposed method.

Modular solid-state transformers (SSTs) are a promising technology in converting power from a 10kV three-phase medium voltage to a lower DC-voltage in the range of 100…400V to provide pure DC power to applications such as electrolyzers for hydrogen generation, data centers with a DC power distribution and DC micro grids. Modular SSTs which can be interpreted as modular multilevel converters with an isolated DC-DC output stage per module, are designed with redundant modules to increase reliability. Usually, each of the three arms operates independently, and therefore, only a fixed number of faulty modules can be compensated in each arm, even if all modules are operational in the remaining two arms. With the proposed zero-sequence voltage injection, up to 100% more faulty modules can be compensated in an arm by employing the same hardware. In addition, module power imbalances are nearly eliminated by utilizing a fundamental frequency zero-sequence voltage. A dominant 3rd harmonic zero-sequence voltage injection in combination with the 5th, 7th and several higher order harmonics with adaptive (small) amplitudes minimize the required arm voltages at steady-state. For nominal operation or symmetrical faults, the proposed technique is equivalent to the well known Min-Max voltage injection, which already reduces the peak arm voltage by 13.4% (to $\frac{{\sqrt 3 }}{2}$) compared to a constant star point potential. A statistical analysis proves, that the expected number of tolerable faulty modules of the 1MW SST increases by 12% without the need for additional hardware.

Cascaded H-bridge converter is a promising candidate for large scale grid connected solar PV plant. However, non-uniform power among the phases of the converter occurs due to non-uniform irradiation and temperature of the cells. This causes unbalanced grid currents which is not acceptable according to the standard grid codes. A fundamental zero sequence voltage is therefore injected to balance the grid currents. But, high degree of unbalance in power generation among the phases leads to over modulation in the cells. This can happen during failure of PV modules. To avoid the over modulation, a novel zero sequence voltage is proposed in this paper. It increases the power balancing capability of the converter. The zero sequence voltage injection is implemented through a novel PWM clamping method. For obtaining the novel zero sequence voltage, a control scheme is also presented. The proposed technique is verified through simulation in MATLAB Simulink and through experiment in the lab prototype.

DC voltage balance based on zero sequence voltage is commonly used for star-connected cascaded H-Bridge (SCHB) STATCOM to compensate negative sequence current. However, zero sequence voltage injection brings about side-effect of increasing STATCOM output voltage and thus the Negative sequence Current Compensation Range (NCCR) is significantly limited to avoid overmodulation. To extend the NCCR, improving the dc voltage utilization by injecting third harmonic component is a typical method. Compared with third harmonic injection (THI), square wave features lower peak-to-average ratio. Unfortunately, the capability of square wave injection (SWI) for the NCCR extension has not been well explored. In this article, to further extend NCCR, not only the SWI is employed for SCHB STATCOM, but also it is compared with THI in term of NCCR by establishing the relationship between the maximum STATCOM voltage and negative sequence current. The comparison results reveal that the THI and SWI both have their own strengths and weaknesses in NCCR, which highly depends on the amplitudes and phases of positive and negative sequence currents. Furthermore, the criteria associated with the amplitudes and phases of the positive and negative sequence currents are derived to determine the harmonic injection method with larger NCCR by using symbolic-graphic combination method and MATLAB tools. As a result, a new harmonic injection method is proposed by selecting the THI or SWI with larger NCCR flexibly according to the amplitudes and phases of positive and negative sequence currents. Finally, the proposed method is verified by experimental results on a 400 V/7.5 kVar SCHB STATCOM prototype.

Hybrid transmission systems associated with line commutated converters (LLC) and voltage source converters (VSC) have gradually become a particular solution for high voltage direct current (HVDC) transmission. The full-bridge submodule (FBSM) based modular multilevel converter (FB-MMC) has gained significant attention for its dc voltage reversal and fault ride through capabilities. However, large capacitance is essential for high power transmission of FB-MMC, increasing cost and size rapidly. This paper analyzes the FBSM voltage fundamental frequency and second-order harmonic fluctuations of the FB-MMC with zero sequence voltage injection. The design principles of the FBSM capacitance are also presented compared with the one without zero sequence voltage injection under equivalent voltage ripple conditions. Finally, comparative simulation results on a FB-MMC system verify the correctness of the analyses.

The parallel three-phase grid-connected converters (GCCs) with common dc and ac buses can increase the system power level effectively. However, zero sequence circulating current (ZSCC) will occur inevitably. In this paper, the mechanisms and characteristics of ZSCC in two parallel three-phase GCCs are first investigated based on three typical cases. From the analysis, it was found that not all ZSCC can be suppressed. Generally speaking, ZSCC caused by inconsistent modulation voltages between modules cannot be effectively handled. And ZSCC caused by different injected zero sequence voltages (ZSVs) or asynchronous carriers can be suppressed. Based on the theoretical analysis, identical ZSV injection and carrier synchronization methods without communication are, thus, proposed to suppress ZSCC at the source, which are easily implemented without adding any hardware. Finally, the theoretical analysis and the effectiveness of the proposed methods on ZSCC suppression are well-verified by experiments.

A significant problem of three-level neutral-point clamped (NPC) converters is the fluctuation of the neutral-point voltage. A SVPWM strategy with neutral point voltage balancing for single phase three-level converters adopted widely in railway traction drive system is proposed in this paper. The relationship between the adopted SVPWM and carrier-based pulse width modulation (CBPWM) with a zero-sequence voltage injection (ZSVI) is analyzed in details. Theoretical analysis shows that SVPWM is a special type of CBPWM-ZSVI methods. Performance of the proposed SVPWM scheme is verified by simulation and experimental test.

The full-bridge submodule (FBSM)-based modular multilevel converter (FBSM-MMC) offers promising performance due to its ability to output negative FBSM voltage. This paper studies this ability of the FBSM-MMC under zero-sequence voltage injection (ZSVI) and second-order harmonic current injection (SHCI). The focus of the research is to redistribute the FBSM powers by injecting an additional power degree of freedom, resulting in a smaller FBSM capacitor voltage ripple, while keeping the maximum AC output voltage for a given fundamental frequency component of the arm voltage reference. Accordingly, a control strategy was developed, based on non-sinusoidal ZSVI, and SHCI is proposed for further improving the performance of the FBSM-MMC. The proposed non-sinusoidal ZSVI contains a higher sinusoidal third-order harmonic component than that of pure sinusoidal third-order voltage injection (THVI) when operating under the same maximum AC output voltage. By implementing this solution, a smaller amplitude of the injected second-order harmonic current can be achieved, producing a lower power loss in the FBSM-MMC. Considering the proposed solution, the relationship of the arm powers and FBSM capacitor voltages are also discussed. Finally, the simulation results and experimental results are presented to verify the effectiveness of the theoretical analysis of the proposed method.

Harmonic analysis currently plays an important role for the power quality analysis because the non-linear loads, as source of the harmonic distortion, have significantly increased. The harmonic distortion influences power quality in power systems such as overheating of electrical equipment and aging of electrical equipment. Therefore, non-linear loads should be monitored in real time. This research proposes the design and implementation of the prototype current harmonic analysis that can be applied in real time based on S-transform Method. The equipment of this prototype can be explained as follows: CT-235, TZ2L9 and CT for KWH meters as current sensor, microcontroller AT mega 8535 as data acquisition, and Visual Basic software as displaying current waveform, current spectrum, rms current value, and total harmonic distortion (THD) for current. S-transform method is used to obtain the THD for current. The experimental result is validated by FLUKE 43B and the THD for current is accurately measured.

Three phase Shunt Active Power Filters (SAPF) are widely used to inject required amount of compensating current to eliminate current harmonics generated by Non-Linear Loads (NLL) and restore sinusoidal waveform of the supply. This paper analyze the development of a three phase grid connected PV array generating a P&O algorithm based MPPT tracked DC power, converted to AC power by PLL controlled VSI supplying a NLL connected with SAPF constituting of series inductor, ANN based self-supported DC link capacitor connected to IGBT based Current controlled VSI compensating for load induced current harmonics in the line. The developed SAPF works on instantaneous active and reactive power measurements to calculate the reference current for the HBCC to produce the gate pulse for the inverter. The proposed model is developed in MATLAB 17a Simulink and Levenberg Marquardt training algorithm based ANN is implemented to regulate DC link capacitor voltage. Simulated results approves the proposed control technique for a fast and flexible dynamic response of the solar to three phase compensated grid system and Total Harmonic Distortion of supply current is found to be satisfactory as per the IEEE-519 standards.

In this article, a new method is introduced for operation of three-phase symmetric cascaded H-bridge (CHB) multilevel inverter after failure of power switches. When a switch fails, the faulty leg of the H-bridge is bypassed and the remaining leg is kept in use. Even with multiple faults in different H-bridge cells, the CHB is optimally reconfigured to continue its operation with its maximum healthy capacity. Based on the remaining operational part of the converter, a fault tolerant control scheme with zero-sequence voltage injection is proposed to synthesize a balanced line-to-line voltage. Using this method, the postfault line-to-line voltage of CHB is maximized with the minimum common mode voltage (CMV). Reduction of CMV is of importance in motor drive application. Also, fundamental component of CMV affects the real power of CHB phase legs. Through reducing the fundamental CMV, unidirectional power transfer from the dc side to the ac load is achieved in a wide operation region. The performance of the presented fault tolerant technique is verified in MATLAB/Simulink environment and validated on a three-phase five-level hardware prototype.

This paper proposes an inter-cluster balancing of an input-parallel output-series modular multilevel cascaded solid-state transformer (SST) based on a feedforward negative-sequence power control. An imbalanced energy flow into each cell of the SST would cause the dc-capacitor voltage of each cell to diverge if the outflow of energy were not regulated to be equal to its inflow. The proposed method distributes the DC-input power to the three clusters corresponding to the AC output power imbalance among the three clusters. This enables the SST to prevent the divergence of the dc-capacitor voltage of each cell without the zero-sequence voltage injection. Simulation results are presented to validate the effectiveness of the proposed control.

This paper presents the modelling and simulating of a general wind power system with double-fed induction generator controlled by a back-to-back converter, in order to manage power delivery to or absorbed from the grid through reactive compensation and active filtering of electrical current. The system modelling and the controller design was developed in Matlab®/Simulink® software, where the rotor-side converter control regulates torque and controls active and reactive power. The grid-side converter control maintains the DC link voltage constant and compensates reactive power demanded from the load, besides this converter acts as an active power filter, which improves the total harmonic distortion of the grid current. The main difference of the proposed methodology with respect to others in the literature is the implementation of grid side converter control as an active power filter using proportional integral resonant to mitigate the 5ª, 7ª, 11ª and 13ª harmonics instead of simple Proportional-Integral (PI) controller, which reduces the accomplishment difficulties since the decoupled term is eliminated. The results obtained shows the correction of the power factor and the extraction of the maximum power from the wind. Finally, it is shown harmonic compensation for nonlinear loads and improvement of the total harmonic distortion of the grid current.

The Unified Power Quality Conditioner (UPQC) is a technology that has successfully addressed power quality issues. In this paper, a photovoltaic system with battery storage powered Unified Power Quality Conditioner is presented. Total harmonic distortion of the grid current during extreme voltage sag and swell conditions is more than 5% when UPQC is controlled with synchronous reference frame theory (SRF) and instantaneous reactive power theory (PQ) control. The shunt active filter of the UPQC is controlled by the artificial neural network to overcome the above problem. The proposed artificial neural network controller helps to simplify the control complexity and mitigate power quality issues effectively. This study aims to use a neural network to control a shunt active filter of the UPQC to maximise the supply of active power loads and grid and also used to mitigate the harmonic problem due to non-linear loads in the grid. The performance of the model is tested under various case scenarios, including non-linear load conditions, unbalanced load conditions, and voltage sag and voltage swell conditions. The simulations were performed in MATLAB/Simulink software. The results showed excellent performance of the proposed approach and were compared with PQ and SRF control. The percent total harmonic distortion (%THD) of the grid current was measured and discussed for all cases. The results show that the %THD is within the acceptable limits of IEEE-519 (less than 5%) in all test case scenarios by the proposed controller.