Low capacitance cascaded H-bridge StatCom

The application of Cascaded H-Bridge (CHB) multilevel converter StatCom is well established in the industry. In a conventional CHB StatCom, low frequency ripple on the dc side is limited to 10% by utilizing large capacitance. Having a smaller capaci-tance is advantageous because the system will be smaller, cheaper, and more reliable. However, reducing the capacitors will increase the ripple’s magnitude and causes problems such as, control and filtering issues, and high voltage stress on the semicon-ductors. In this thesis, the next generation of CHB StatCom i.e. Low Capacitance StatCom (LC-StatCom) is developed which is able to operate with ripple magnitudes close to the theoretical limit (smallest capacitors’ size). A feed-forward filtering scheme is developed that is able to effectively filter out large ripples without imposing any delay to the control loop and facilitates design of higher bandwidth capacitors’ voltage controllers. A decoupling theory which outlines requirements for completely decoupling the individual capacitors’ voltage controller from the rest of the control system is introduced. The decoupled control system has a linear cluster capacitors’ voltage controller which is essential for operation of the LC-StatCom The proposed LC-StatCom utilizes this linearity to have a variable capacitors’ voltage reference in order to limit the maximum voltage stress on the semiconductors. The proposed LC-StatCom and innovative solutions are evaluated by simulation and experimental case studies. It is shown that the LC-StatCom can achieve 80% reduction in the capacitors’ size, improve current quality, and reduce the maximum voltage stress on the semiconductors com-pared to a conventional StatCom. The LC-StatCom has a reduced operating area in the inductive region compared to conventional StatComs. In this thesis, to overcome this drawback, a hybrid LC-StatCom that utilizes a series thyristor bypassed inductor is developed. The compensated LC-StatCom developed in this thesis, provides approxi-mately 75% reduction in overall energy storage capacity of passive components

Grid-Connected Energy Storage Systems: State-of-the-Art and Emerging Technologies

High penetration of renewable energy resources in the power system results in various new challenges for power system operators. One of the promising solutions to sustain the quality and reliability of the power system is the integration of energy storage systems (ESSs). This article investigates the current and emerging trends and technologies for grid-connected ESSs. Different technologies of ESSs categorized as mechanical, electrical, electrochemical, chemical, and thermal are briefly explained. Especially, a detailed review of battery ESSs (BESSs) is provided as they are attracting much attention owing, in part, to the ongoing electrification of transportation. Then, the services that grid-connected ESSs provide to the grid are discussed. Grid connection of the BESSs requires power electronic converters. Therefore, a survey of popular power converter topologies, including transformer-based, transformerless with distributed or common dc-link, and hybrid systems, along with some discussions for implementing advanced grid support functionalities in the BESS control, is presented. Furthermore, the requirements of new standards and grid codes for grid-connected BESSs are reviewed for several countries around the globe. Finally, emerging technologies, including flexible power control of photovoltaic systems, hydrogen, and second-life batteries from electric vehicles, are discussed in this article.This work was supported in part by the Office of Naval Research Global under Grant N62909-19-1-2081, in part by the National Research Foundation of Singapore Investigatorship under Award NRFI2017-08, and in part by the I2001E0069 Industrial Alignment Funding. (Corresponding author: Josep Pou.

Low-capacitance StatCom with modular inductive filter

In this paper, a cascaded H-bridge low-capacitance StatCom (LC-StatCom) with a modular inductive filter is introduced. Replacing the fixed-value filter inductors with thyristor-switched reactors, a well-developed member of the static VAR compensators family, reduces the overall energy storage capacity and voltage rating of the LC-StatCom without violating standard limits on current quality. The proposed modular inductive filter takes advantage of LC-StatCom's ability to synthesize high-quality ac voltage to reduce the overall energy storage requirement of the filter inductance. The proposed modular filter inductor concept adds an additional degree of freedom to control the maximum voltage drop on the filter inductor by changing the inductance value. This additional capability decreases the overall voltage rating of the converter. The effects of adding the modular inductor unit on the I-V characteristic of the LC-StatCom are evaluated. The effectiveness of the proposed solution is demonstrated by experimental results on a seven-level 0.7-kVA LC-StatCom.MOE (Min. of Education, S’pore

A power ramp rate control strategy with reduced energy storage utilization for grid-connected photovoltaic systems

Fluctuations in photovoltaic (PV) power generation are inevitable due to intermittent irradiance transients. These power fluctuations, when injected into the grid in grid-connected PV systems, can have a negative impact on grid stability. This issue becomes more prominent in grids with high PV penetration. Therefore, this article proposes a hybrid power ramp rate control (PRRC) strategy that mitigates the fluctuating PV power at the dc-side before transmitting it to the grid during positive and negative irradiance changes. The proposed hybrid PRRC aims to reduce the energy storage system (ESS) utilization and its required capacity by employing a flexible power point tracking control, which limits the increment in PV power generation during positive irradiance fluctuations. A supercapacitor (SC)-based ESS is only used to smoothen the power reductions during negative irradiance fluctuations, and the SC state-of-charge is regulated afterwards to prevent saturation. The transient performance and efficacy of the proposed hybrid PRRC are investigated using simulations in MATLAB/Simulink.Submitted/Accepted versio

Balancing average capacitor voltages in neutral-point-clamped converters using band-limited three-level modulation

In this paper, a band-limited three-level modulation scheme is presented which aims to balance the average capacitor voltages in neutral-point-clamped converters under all load operating conditions. Compared to previous three-level modulation schemes, the proposed scheme provides a trade-off between voltage balancing performance and switching losses. Consequently, helping in optimizing the system’s footprint and its operating cost. A variable current band is calculated in order to provide the necessary control action for balancing the average (as opposed to instantaneous) capacitor voltages. The proposed modulation scheme is tested under both linear and nonlinear loads. A comparison is also performed between the proposed band-limited three-level modulation and other modulation schemes presented in the literature.Accepted versio

Reduced battery usage in a hybrid battery and photovoltaic stand-alone DC microgrid with flexible power point tracking

A hybrid control strategy for photovoltaic (PV) system and energy storage system (ESS) in a dc microgrid is proposed in this paper. The proposed control strategy minimizes the utilization of ESS in order to prolong the battery lifetime, reduce the total system power losses, and maintain the state-of-charge (SoC) of the battery within a desired range as long as the required power from PV is available. In contrast to the conventional control strategies that regulate the dc bus voltage with the ESS, the proposed control strategy exploits the use of both PV and ESS in regulating the dc bus voltage. It also employs an advanced flexible power point tracking algorithm for the PV system to further enhance the dynamic performance of the system by adaptively adjusting the PV output power according to the load profile.Ministry of Education (MOE)Submitted/Accepted versionThis work was supported by the Singapore Ministry of Education Academic Research Fund Tier 1 under Grant No: 2019-T1-001-168 (RG 80/19)