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This article proposes a novel capacitor-voltage balancing strategy for the double-Y modular multilevel converter static synchronous compensator operated under unbalanced current conditions of the system. The proposed method, based on zero-sequence voltage injection, manipulates the current distribution between the converter arms to minimize the converter ratings. The proposed balancing strategy is compared with the classical approach, based on controlling the circulating dc currents. For the same unbalanced conditions, the proposed strategy leads to lower energy rating for the converter as compared with the classical one, whereas the semiconductor ratings remain the same in both strategies. Theoretical findings are verified through experimental results.

The aim of this paper is to investigate the ability of cascaded H-bridge STATic COMpensator in star and delta configuration to exchange negative-sequence current with the connecting grid. Zero-sequence voltage for the star and zero-sequence current for the delta configuration are utilized to guarantee dc-capacitor voltage balancing. A general solution for the required zero-sequence voltage/current for any kind of unbalanced condition is provided. The obtained solutions show that a singularity for both configurations exists under specific conditions, leading to infinite zero-sequence requirement. In case of star configuration, this singularity occurs when the amplitude of the positive- and negative-sequence components of the current output of the converter are equal. Analogously, the singularity for the delta configuration occurs when the amplitude of the positive- and negative-sequence components of the voltage at the converter terminals are equal. This singularity impacts the ratings of the system and imposes a limitation in the utilization of the compensator for unbalance compensation purposes, both in industrial and utility applications. Experimental results are presented to validate the theoretical analysis.

The purpose of this paper is to investigate the impact of switching harmonics on the instantaneous power distribution in the cells of a cascaded H-bridge-based STATCOM when using phase-shifted pulse width modulation. The case of high- and low-switching frequency for the converter cells is investigated and the interaction between voltage and current harmonics is analyzed. It is shown that in both cases, this interaction results in an uneven power distribution among the cells in the same phase leg, leading to drifting of the dc-capacitor voltages and thereby the need for proper stabilization control loops. It is also shown that the selected frequency modulation ratio affects the active power distribution among the cells. In particular, the selection of a noninteger frequency modulation ratio helps in providing a more uniform power distribution among cells of the same phase leg, thus contributing to the capacitors balancing. A methodology for optimal selection of the frequency modulation ratio is given. Theoretical conclusions are validated through simulation and experimental results.

A controller for a hybrid converter for STATCOM applications, based on a cascaded H-bridge star in series with a generic converter connected at the star point, is proposed. The controller utilizes the generic converter in an optimized way to maximize the operating range of the converter, with focus on unbalanced operating conditions. So for each operating condition, the generic converter reference voltage minimizes the voltage rating of the star part. Theoretical investigations of the hybrid converter using the proposed control approach show that the hybrid converter allows to effectively extend the operating range of the converter under unbalanced conditions of the system, given that the generic converter is large enough to circulate the needed active power between the converter phase legs. This indicates that, depending on the requirements from the operator in terms of negative-sequence injection capability, the hybrid converter can be a valuable solution to overcome the limitations of the CHB star.

This paper presents a comparison between two DC-voltage balancing strategies for cascaded star-connected multilevel STATCOM: zero-sequence voltage, and negative-sequence current control. Different levels of unbalance and grid strengths are considered. The impact of the control strategies on the convert ratings in terms of voltage and current requirements are compared and the operating range of the converter using each control strategy is theoretically derived. The results show that for a given system ratings, the operating range of the converter using negative-sequence current control is wider than the zero-sequence voltage control when the STATCOM is connected to weak grids. On the contrary, in case of connection to strong grids the zero-sequence voltage control can provide wider operating range. PSCAD simulation results are provided to prove the theoretical results.

The purpose of this paper is to investigate the impact of switching harmonics on the instantaneous power that flows in the cells of a chain-link based STATCOM when using Phase-Shifted PWM. Two different cases are investigated for the converter cells: low, and high switching frequency. It is shown that any deviation from the ideal conditions lead to undesired harmonics, which will impact the charge of the dc capacitors. It is also shown that for low switching frequencies, cells voltage sideband harmonics interact with baseband harmonics of the current and causes extra source of unbalance. In order to provide more uniform power distribution among the cells, two different methods called, a) carrier swapping and b) non-integer frequency modulation ratio are studied. In particular, it is shown that the selection of a non-integer frequency modulation ratio helps in providing a more uniform power distribution among cells of the same phase leg, thus contributing to the capacitors balancing. Theoretical conclusions are validated through PSCAD simulation results.

The aim of this paper is to investigate the ability of star- and delta-connected H-bridge multilevel STATCOMs to exchange negative-sequence current with the grid. Zero-sequence injection is utilized for capacitor voltage balancing. It is shown that a singularity for these configurations exists, leading to limitations in the utilization of the compensator for asymmetry compensation purposes.

This paper provides an overview of multilevel converter topologies for grid connected applications. Different converter topologies such as Diode Clamp Converter (DCC), Capacitor Clamp Converter (CCC) and Chain Link Converter (CLC) are considered and compared from different aspects such as component sizing, complexity, efficiency and reliability. Thanks to its modular structure, the CLC appears as the most attractive topology for high power and high voltage applications. Single-Star Bridge-Cell (SSBC), Single-Delta Bridge-cell (SDBC), Double-Star Bridge-Cell (DSBC) and Double-Star Chopper-Cell (DSCC) are four basic subset topologies of CLC. A comprehensive comparison of these subset topologies for the CLC is provided for the particular application of grid connected converters. The problem of operation under unbalance grid conditions and the ability of this converter topology to inject negative sequence into the grid are discussed. It will be shown that due to the lack of a common DC link between phases, this converter topology presents limited negative sequence injection capability when the phase legs are connected in delta, while in case of star connection voltage balancing between the different cells of the converter becomes problematic due to the uneven current distribution among phases. This represents the main limitation of this converter topology, especially when used for load balancing and active filter applications.

This paper presents a hybrid PV-battery grid connected system suitable for high power PV power applications. High power PV systems have impressive voltage fluctuation with regards to their maximum power point voltage. Using DC/DC converters to regulate this voltage is inefficient in high power systems as it adds an extra power conversion stage. On the other hand switching frequency, voltage stress as inverter limitations and power quality issues should be considered in high power applications. Central PV structures because of low efficiency and high cost inverters are not suitable for high power systems. Maximum power extraction from PV together with active and reactive injected power control is another challenge in high power system. Z-source multilevel inverter for PV arrays together with battery energy storage systems is a suitable solution for high power application. This paper proposes a control strategy for this hybrid system. A simulation with PSCAD has been done to validate the control strategy. Simulation results show how proposed control and topology satisfies high power requirements for PV grid connected systems.

This paper reports on design of power electronics for photovoltaic (PV) energy system of an offshore remote sensing apparatus. Challenges in design of a PV energy system for a marine application are investigated and the design limitations compared to inland PV system are discussed. The designed system includes PV cells as the main source of energy, electric storage (battery), maximum power point tracking (MPPT) and protection circuitries. An MPPT algorithm based on measuring the slope of the PV power-voltage curves is presented which can be implemented with simple analog electronic circuits. The MPPT circuit uses Sepic converter as a core and it also includes a protection unit for maintaining the battery voltage in a safe range. The performance of the proposed MPPT algorithm in presence of measurement noises is verified using a circuit simulation software tool (PSCAD). Simulation results verify that the algorithm appropriately regulates the voltage of PV cells at MPP and it is robust against measurement noises for a signal-to-noise ratio above -2db.

This paper presents and demonstrates a simple and reliable electronic circuitry for photovoltaic energy harvesting which best suited for powering marine instrumentations. The main functions of the designed circuitry are: i) tracking the maximum power point of the photovoltaic cells which are subject to continuous movements due to sea waves, ii) power conversion and management of energy storages with a high efficiency; and iii) achieving a high level of reliability for operation of circuit in a harsh marine environment. The proposed circuitry is designed based on an analog controller which implements a sensorless algorithm for maximum power point tracking. The power converter unit includes a simple buck converter with minimum electronic components. Compared with DSP-based digital controllers, the simplicity of the proposed analog controller highly improves reliability of the power conversion unit. The total power consumption of the electronic circuit is in the range of a few milliwatts. This enhances the overall efficiency of power conversion especially in low power (a few watt) applications such as powering of marine and metrological sensors. The circuitry also includes a control unit for battery charging to protect batteries against overcharging. The performance of the designed circuitry is experimentally demonstrated using a test setup for power management unit of an ocean-graphical buoy.