Modular Multilevel Cascaded Flying Capacitor STATCOM for Balanced and Unbalanced Load Compensation

Voltage and current unbalance are major problems in distribution networks, particularly with the integration of distributed generation systems. One way of mitigating these issues is by injecting negative sequence current into the distribution network using a Static Synchronous Compensator (STATCOM) which normally also regulates the voltage and power factor. The benefits of modularity and scalability offered by Modular Multilevel Cascaded Converters (MMCC) make them suitable for STATCOM application. A number of different types of MMCC may be used, classified according to the sub-module circuit topology used. Their performance features and operational ranges for unbalanced load compensation are evaluated and quantified in this research. This thesis investigates the use of both single star and single delta configured five-level Flying Capacitor (FC) converter MMCC based STATCOMs for unbalanced load compensation. A detailed study is carried out to compare this type of sub-module with several other types namely: half bridge, 3-L H-bridge and 3-L FC half bridge, and reveals the one best suited to STATCOM operation. With the choice of 5-L FC H-bridge as the sub-module for STATCOM operation, a detailed investigation is also performed to decide which pulse width modulation technique is the best. This was based on the assessment of total harmonic distortion, power loss, sub-module switch utilization and natural balancing of inner flying capacitors. Two new modulation techniques of swapped-carrier PWM (SC-PWM) along with phase disposed and phase shifted PWM (PS-PWM) are analyzed under these four performance metrics. A novel contribution of this research is the development of a new space vector modulation technique using an overlapping hexagon technique. This space vector strategy offers benefits of eliminating control complexity and improving waveform quality, unlike the case of multilevel space vector technique. The simulation and experimental results show that this method provides superior performance and is applicable for other MMCC sub-modules. Another contribution is the analysis and quantification of operating ranges of both single star and delta MMCCs in rating the cluster dc-link voltage (star) and current (delta) for unbalanced load compensation. A novel method of extending the operating capabilities of both configurations uses a third harmonic injection method. An experimental investigation validates the operating range extension compared to the pure sinusoidal zero sequence voltage and current injection. Also, the superiority of the single delta configured MMCC for unbalanced loading compensation is validated

Reactive power compensation by modular multilevel flying capacitor converter-based STATCOM using PS-PWM

This paper presents a simulation study into the implementation of a modular multi-level flying capacitor converter as a STATCOM. The converter modulation scheme applied is based on Phase Shifted PWM and the two scenarios which require compensation are investigated to verify this topology. The two scenarios are PCC voltage regulation through reactive power compensation and power oscillation damping through reactive current compensation. Simulation results verify the performance of the chosen topology

Effective Design of STATCOM Considering Fundamental Frequency Current, Active Harmonic Filtering and Zero Sequence Current

The main objective of this thesis was to investigate the effect of parallel reactive power compensation (RPC) and active harmonic filtering (AHF) operation on a STATCOM design, in terms of needed number of submodules (SMs), DC link voltage capacity, MV busbar voltage, zero sequence current demand, transformer and coupling inductor reactance. To achieve this objective, two design scenarios were carried out. In the first scenario, fundamental reactive current of studied STATCOM was prioritized over its current for active harmonic voltage filtering. In the second scenario, studied STATCOM was required to produce the nominal fundamental reactive power and perform active harmonic voltage filtering simultaneously. The problem was studied in PSCAD based simulation environment. In all simulations, q-component current was supplied manually to enable the RPC operation of studied STATCOM. To enable AHF operation, harmonic current control mode was used, and the reference value of the desired harmonic filtering current was supplied accordingly. However, before proceeding with any simulation, first, the system limitations based on the studied STATCOM technology were studied and adequate majors were placed inside the simulation mode accordingly. Thereafter, simulations providing information on the basic STATCOM design operating in RPC mode only (and without AHF functionality) were carried out so that it can be compared later with the aforementioned scenarios of parallel RPC and AHF operations. In the first design scenario, it was found out that additional AHF operation affects the STAT-COM design in three ways. First was the magnitude of AHF current where an increment in the needed number of SMs w.r.t basic design was noticed with increasing magnitude of AHF current. The second was the phase angle references of AHF current where if phase angle references of AHF current are chosen such that peaks of produced voltage source converter’s (VSC’s) fundamental and harmonic voltages are aligned then the amount of needed SMs to produce the same VSC voltage was increased. But, if phase angle references of AHF current are such that the peaks of VSC voltages are opposite to each other, then fewer SMs are required to produce the same VSC voltage. The third effect on STATCOM design was based on the harmonic order of AHF current produced. It was noticed that when harmonic order of AHF current was high, then the amount of needed SMs to produce the same magnitude of AHF current was increased. In the second design scenario, it was found that maximum fundamental reactive current and maximum filtering current cannot be achieved at the same time with a geometrical summation principle of these currents, but possible with an arithmetical summation principle with a trade-off between optimum utilisation of current capacity and extra hardware cost. Hence, an optimum design to achieve the maximum of RPC and AHF current simultaneously exists between economical (based on the geometrical summation principle) and conservative (based on the arithmetical summation principle) design, but rather close to the economical one. In last, it was also noticed that the maximum demand of zero sequence current occurred when STATCOM was producing fundamental reactive current and negative sequence AHF current simultaneously in the maximum capacitive operation point, with an unbalanced network. And, peaks of positive and negative sequence network voltage and peaks of produced VSC voltages (fundamental and harmonic) were aligned

Control of a Modular Multilevel Flying Capacitor Based STATCOM for Distribution Systems

Voltage fluctuation and power losses in the distribution line are problems in distribution networks. One method to mitigate these problems is by injecting reactive power into the network using a Static Synchronous Compensator (STATCOM). This can be used both for regulating the voltage and reducing the losses. A STATCOM is critically dependent on a grid synchronisation scheme that can accurately track the changes occurring in the grid phase and frequency. The Modular Multilevel Converter (MMC) is a promising topology for STATCOM applications because of its simple modular circuit structure that allows for higher voltage ratings, and conventionally uses a stack of sub-modules which are either two-level half or H-bridge converters. As a novel alternative, the thesis investigates the practicality of a STATCOM based on a three-level flying capacitor (FC) converter. Two variants of this topology are presented; the FC Half-bridge and FC H-bridge. A comprehensive study is undertaken to compare these with the Half and H-bridge sub-module under STATCOM operation. Most importantly, an FC H-bridge-based STATCOM is investigated for reactive power compensation. The challenges of multilevel, multi-module PWM control schemes achieving good waveforms at low switching frequency, whilst maintaining module capacitor voltage balance, are thoroughly addressed. Simulation results validate the operation for both line voltage regulation and power factor correction. An experimental power system with an FC-based STATCOM rig is designed and built, and validates the simulation results for power factor correction. It demonstrates correct operation of a control scheme that includes a system for maintaining capacitor voltage balance. Another new contribution is the investigation of a phase locking technique based on the Energy Operator (EO). The method, combining two different EO computations, is shown to achieve fast and accurate detection of frequency and phase angle when combined with an appropriate filter, and crucially operates well under unbalanced voltage conditions. The technique is compared with two other well-known phase locked loop (PLL) schemes, showing that it outperforms the others in terms of speed and accuracy. A hardware implementation of the EO-PLL validates the principle, showing the simplicity of the metho

Enhanced decoupling current scheme with selective harmonic elimination pulse width modulation for cascaded multilevel inverter based static synchronous compensator

This dissertation is dedicated to a comprehensive study and performance analysis of the transformer-less Multilevel Cascaded H-bridge Inverter (MCHI) based STATic synchronous COMpensator (STATCOM). Among the shunt-connected Flexible AC Transmission System (FACTS) controllers, STATCOM has shown extensive feasibility and effectiveness in solving a wide range of power quality problems. By referring to the literature reviews, MCHI with separated DC capacitors is certainly the most versatile power inverter topology for STATCOM applications. However, due to the ill-defined transfer functions, complex control schemes and formulations were emerged to achieve a low-switching frequency high-bandwidth power control. As a result, adequate controller parameters were generally obtained by using trial and error method, which were practically ineffective and time-consuming. In this dissertation, the STATCOM is controlled to provide reactive power (VAR) compensation at the Point of Common Coupling (PCC) under different loading conditions. The goal of this work is to enhance the performance of the STATCOM with the associated proposed control scheme in achieving high dynamic response, improving transient performance, and producing high-quality output voltage waveform. To evaluate the superiority of the proposed control scheme, intensive simulation studies and numerous experiments are conducted accordingly, where a very good match between the simulation results and the experimental results is achieved in all cases and documented in this dissertation

Operation and control of cascaded H-bridge converter for STATCOM application

In the last decade, particular attention has been paid to the use of Modular Multilevel Converters (MMC) for grid applications. In particular, for STATCOM applications the phase leg of the converter is constituted by a number of single-phase full-bridge converters connected in cascade (here named Cascaded H-Bridge, CHB, converter). This multilevel converter topology is today considered the industrial standard for STATCOM applications and has replaced other converter topologies, mainly due to its small footprint, high achievable voltage levels (allowing transformer-less operation), modularity and reduced losses. However, there are still areas of research that need to be investigated in order to improve the performance and the operational range of this converter topology for grid-applications. The aim of this thesis is to explore control and modulation schemes for the CHB-STATCOM, both under balanced and unbalanced conditions of the grid, highlighting the advantages but also the challenges and possible pitfalls that this kind of topology presents for this specific application. The first part of the thesis is dedicated to the two main modulation techniques for the CHBSTATCOM: the Phase-Shifted PulseWidthModulation (PS-PWM) and the Level-Shifted PWM (LS-PWM) with cells sorting. In particular, the focus is on the impact of the adopted modulation on the active power distribution on the individual cells of the converter. When using PS-PWM, it is shown that non-ideal cancellation of the switching harmonics leads to a non-uniform active power distribution among the cells and thereby to the need for an additional control loop for individual DC-link voltage balancing. Theoretical analysis proves that a proper selection of the frequency modulation ratio leads to a more even power distribution over time, which in turns alleviates the role of the individual balancing control. Both PS-PWM and cells sorting schemes fail in cell voltage balancing when the converter is not exchanging reactive power with the grid (converter in zero-current mode). To overcome this problem, two methods for individual DC-link voltage balancing at zero-current mode are proposed and verified. Then, the thesis focuses on the operation of the CHB-STATCOM under unbalanced conditions. It is shown analytically that regardless of the configuration utilized for the CHB-STATCOM (star or in delta configuration), a singularity exists when trying to guarantee balancing in the DC-link capacitor voltages. In particular, it is shown that the star configuration is sensitive to the level of unbalance in the current exchanged with the grid, with a singularity in the solution when positive- and negative-sequence currents have the same magnitude. Similar results are found for the delta configuration where, in a pure duality with the star configuration, the system is found to be sensitive to the level of unbalance in the applied voltage. The presence of these singularities represents an important limit of this topology for STATCOM applications

Stability control and protection of power systems with VSC HVDC and VSC FACTS

The recent progress of high-voltage high-power fully controlled semiconductor technology laid the foundation of VSC technology, which continues to advance the developments of HVDC technology and FACTS., However, the high penetration of VSC based systems may introduce certain risks to existing power systems in two primary aspects: dynamic stability and protection. This thesis investigates the impacts of VSC HVDC and VSC FACTS on system dynamic stability and protection. An integrated small-signal stability model for the study of interactions between CFC and VSC is established. Modal analysis results are verified by simulation results from RTDS under both small and large AC/DC disturbances. The impacts of control parameters of CFC on the integrated AC/DC system and the interactions between VSC and CFC are investigated using both modal analysis and time-domain simulations. The mathematical representation of the apparent impedance measurements of distance relay is derived considering the infeed current from VSC HVDC and VSC FACTS at different locations. A RTDS-based HIL testing platform is established. The impacts of VSC HVDC and VSC FACTS on feeder distance protection are investigated, based on different types of internal/external fault test simulation occurred at various locations

Design and implementation of hybrid series compensators for smart grid applications

The vision of future modern grids goes through the increase of renewable énergies penetration while providing an efficient, reliable and sustainable power supply to consumers. According to the recent report on climate challenging the way electrical energy is produced and because of the rapid emerging of power electronics based equipment; some serious actions should be engaged. In order to achieve such promoting visions, all power grids are required to become smarter especially at the distribution level. Increasing the application of renewable energy sources and distributed generations assist these vision in the development of a modern power grid where modern equipment are becoming highly sensitive to the supplied voltage quality. Moreover, in this paradigm of design, the traditional power systems based on large concentrated power plants should be able to deal with these unpredictable sources of energy at distribution level. Under these circumstances, considerable activities were carried out aiming to render the grid more flexible and intelligent while taking the power efficiency and its environmental impacts into account. In this way, the power quality issues should be considered for the development of new type of smart grids which are more efficient and sustainable with regards to environmental constraints. Available active and passive compensators are widely involved to improve major power quality issues. Recent trends towards realization of multitasking devices which can solve several power quality issues simultaneously, propose Hybrid active filters or Unified power quality conditioners. These versatile devices should threaten both voltage and current related issues in one place for compensation. They can significantly improve power quality issues, such as voltage distortions, voltage sags, voltage swells, voltage unbalances, and ensure a constant and reliable voltage supply to the load. On the other hand, they compensate for current problems of linear and non-linear loads, such as current harmonics, unbalances, neutral current, and load reactive power. The Hybrid series active filter (HSeAF) is among the most versatile and efficient power electronics based active power compensators. Without the shunt passive filter, the active part could operate solely to rectify for voltage problems and is commonly known as Dynamic voltage restorer. A conventional HSeAF, targeting three-phase system, consists of a three separate series isolation transformer connected to a three-phase converter sharing a common DC link bus. The device is controlled as a variable voltage source in similar but duality manner as of Shunt active power filter. A shunt passive filter tuned for harmonic frequencies is installed to produce an alternative path for load current harmonics and reducing voltage distortions at the load terminals. The existing literature suggests utilizing the hybrid active power filters to compensate for load current related issues only, while due to the complexity and implementation outlays of such devices, it shows a significant drawback of under usage of series compensation to address such power quality problems. The present doctoral research is based on the philosophy of optimal utilization of the available resources in the most efficient way to enhance the product efficiency and to reduce the overall cost. This work proposes a novel control approach for three-phase system in which both the grid’s voltage and load current issues are treated in a co-ordination between the series active and the shunt passive filters without affecting the basic voltage or current compensation capabilities. This eventually results in a better utilization of the series active filter, reduction of the shunt passive filter rating to some extent, and ultimately in the reduction of the overall cost for a unified compensator. Moreover, this thesis also introduces a novel transformerless topology in which the threephase configuration is split into separate devices. It is then possible to extent the Series active power compensation based for three-phase systems with three or four wires to single-phase or bi-phase systems. This newly transformerless hybrid series active filter (THSeAF) is first hosted for single-phase system where appropriate developed controllers ensure adequate operation under low profile power quality systems. The developed single-phase THSeAF concept is successfully validated through digital simulations as well as real-time extensive experimental investigations. The experimental results show that for a given laboratory test conditions with highly polluted nonlinear loads, the active compensator ride of the bulky transformer is capable of compensating load current and correcting the power factor. Moreover, the performance of the THSeAF under polluted grid supply with voltage harmonics, sags, and swells, demonstrates regulated and reduced voltage distortions at the load’s terminals. Following this successful transformerless configuration, and to integrate the series compensation concepts dedicated for power quality improvement of distribution network, the three-phase configuration is anticipated. Three-phase control strategies developed previously for the HSeAF are applied to the proposed topology to make the point of common coupling (PCC) smarter and to decentralize the control of the distribution network. This affordable solution increases the efficiency and sustainability of modern smart power systems and help higher penetration of renewable fluctuating power into the network. The off-line simulations demonstrate that the three-phase THSeAF is capable of healing voltage problems and load current issues simultaneously. The real-time experimental results, carried out on a laboratory prototype, validate successfully the proposed configuration