Ehsan Behrouzian

Investigation of Negative-Sequence Injection Capability of Cascaded H-Bridge Converters in Star and Delta Configuration

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.

An overview of multilevel converter topologies for grid connected applications

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.

Investigation of negative sequence injection capability in H-bridge multilevel STATCOM

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.

Impact of Switching Harmonics on Capacitor Cells Balancing in Phase-Shifted PWM-Based Cascaded H-Bridge STATCOM

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.

Individual capacitor voltage balancing in H-bridge cascaded multilevel STATCOM at zero current operating mode

Individual capacitor voltage balancing is one of the challenges in the field of multilevel converters, especially when the phase legs of the converter are connected in star configuration. This issue can be even more problematic when the converter is operating close to zero average current operating mode. This paper first shows the issue related to the capacitor voltage balancing at zero average-current mode and second proposes a novel algorithm to overcome this problem. The method proposes a modulation technique for individual balancing in H-bridge cascaded multilevel converters operated at zero average-current mode. The proposed algorithm modifies the conventional sorting algorithm based on current ripple. The individual DC-link voltage control method is applied to a 19-level star-connected cascaded converter in PSCAD and simulation results verify the ability of the proposed method in maintaining the balancing of the capacitors voltages at zero current injection mode. Several practical limitation such as forward voltage drop over semiconductor elements, noise in measured data, delay in the digital controller, switching deadtime, and grid voltage harmonics are also considered in the model.

A hybrid photovoltaic and battery energy storage system for high power grid-connected 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.

Power electronics for photovoltaic energy system of an Oceanographic buoy

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.

Impact of frequency modulation ratio on capacitor cells balancing in phase-shifted PWM based chain-link STATCOM

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.