Master of Science / Department of Electrical and Computer Engineering / Ruth D. Miller / This thesis presents the design of a multilevel D-STATCOM inverter for small to mid-size (10–20 kW) permanent-magnet wind or solar installations. The proposed inverter can actively regulate the reactive power on individual feeder lines at a programmable output while providing the variable output power of the renewable energy source. The aim is to provide utilities with distributive control of VAR compensation and power factor correction on feeder lines. The designed inverter utilizes a 5-level hybrid-clamped multilevel voltage-source converter topology and uses the optimized harmonic stepped waveform technique for harmonic elimination. The topology allows for the separation of active and reactive power control and the ability to operate under any load conditions. Reactive power is controlled by the modulation index and active power is controlled by the phase angle. Simulations and validation of the proposed inverter were carried out using MATLAB/Simulink and SimPowerSystems toolboxes.

Because renewable energy sources are environment-friendly and inexhaustible, more and more renewable energy power plants have been integrated into power grids worldwide. To compensate for their inherent variability, STATCOMs are typically installed at the point of common coupling (PCC) to help their operation by regulating the PCC voltage. However under different contingencies, PCC voltage fluctuations in magnitude and frequency may impede the STATCOM from tracking the grid frequency correctly, hence worsening its overall compensation performance, and putting at risk the operation of the power plant. Further, the virtual synchronous machine (VSM) concept has recently been introduced to control grid-connected inverters emulating the behavior of rotating synchronous machines, in an effort to eliminate the shortcomings of conventional d-q frame phase-locked loops (PLL). In this dissertation, the VSM concept is extended by developing a STATCOM controller with it, which then behaves like a fully-adjustable synchronous condenser, including the adjustment of its "virtual" inertia and impedance. An average model in two D-Q frames is proposed to analyze the inherent dynamics of the VSM-based STATCOM controller with insight into impacts from the virtual parameters and a design guideline is then formulated. The proposed controller is compared against existent d-q frame STATCOM control strategies, evincing how the VSM-based approach guarantees an improved voltage regulation performance at the PCC by adjusting the phase of its compensating current during frequency fluctuations, in both simulation and experiment. Secondly, the dynamics of the VSM-based STATCOM controller in large signal sense is studied, especially its capability to ride through faults. Analysis is carried out with phasors to obtain a fundamental understanding at first and followed by state space equations to predict the transients analytically, which is validated by matching both simulation and experiment. The effects of two outer loops are also reviewed and some possible solutions are suggested and evaluated. Moreover, the relationship between the virtual inertia and the actual inertia is established and the dc capacitor sizing is discussed in a possibly more economical way. The start-up process of a VSM-based STATCOM is presented to implement a practical prototype as well. / Master of Science

To increase the allowable capacity of wind generation and system reliability of distribution system for islanding operation, an actual Taipower feeder has been selected for computer simulation. The voltage enhancement of distribution feeders with fixed speed wind generator has been obtained by using the static synchronous compensator(STATCOM). The model of Doubly-Fed induction generator(DFIG) in Matlab/Simulink has also been applied in the feasibility study of islanding operation for Taipower feeder. The critical clearing time of fault contingency is solved by considering the low voltage ride through (LVRT) capability of wind generators (WG) and the CBEMA curve of sensitive loads. In this way, feeder circuit breaker(CB) can be tripped in time to achieve islanding operation of distribution feeders without causing the tripping of WG due to voltage disturbance for the system fault contingency. To restore the stable operation of distribution feeders after being isolated from the rest of the power system, an adaptive load shedding scheme has been presented to disconnect the proper amount of system loading to achieve the balance of wind power generation and load demand according to the variations of feeder load profile and wind speed. It is found that system voltage sag problem due to severe fault contingency can be mitigated effectively by using the STATCOM for the support of low voltage ride through capability of WG. With the proper design of protection relay settings for feeder CB tripping and load shedding scheme, the WG and critical loads can be prevented from tripping during transient disturbance to ensure the successful islanding operation of distribution system.

Vindkraftverk ansluts idag i snabb takt till elnät runt om i världen. Elnäten är ofta inte tillräckligt dimensionerade på platser där vindkraftutbyggnad är lämplig vilket leder till ett behov av nätförstärkning. Förstärkning sker konventionellt genom att övergå till en högre spänning, något som kan vara förenat med stora kostnader och ledtider. Bland annat därför har en filosofi med beteckningen smarta elnät uppstått, som handlar om hur elnätsystemet ska se ut när det är mer anpassat till de nya energikällornas karaktär och samtidigt medför ett mer effektivt totalutnyttjande. För vindkraftverk kan det då handla om att i högre grad än idag reglera produktionen och understödja nätet, t.ex. via intelligenta kontrollsystem, kraftelektronik och energilager. I den här rapporten undersöks hur mycket vindkraft som kan anslutas till ett svagt elnät då principer för smarta elnät tillämpas och för att se om kostnaden blir lägre än anslutning via konventionella nätförstärkningsmetoder. Det svaga elnätet som studien bygger på ligger i Tidaholm. Vindkraftseffekten som ska anslutas är på 62 MW. Begränsningar i befintliga regionnätets ledningar gör att 60,3 MW kan anslutas med principer för smarta elnät, fast det finns flera skäl till att anta att den fulla mängden vindkraft går att ansluta. Detta till en kostnad som väsentligt understiger kostnaden att förstärka nätet på konventionellt sätt, kostnadsbesparingen uppgår till ca 58 % eller ca 95 Mkr.

Magíster en Ciencias de la Ingeniería, Mención Eléctrica / Ingeniero Civil Electricista / En los últimos años, los problemas climáticos asociados a las emisiones de gases de efecto invernadero (GEI), han aumentado significativamente el interés por diversificar las fuentes primarias para la generación de energía eléctrica. De hecho, las tecnologías de generación como la fotovoltaica han reducido considerablemente sus precios, llegando incluso a que en ciertos lugares la energía solar fotovoltaica sea más económica que conectarse a la red, aumentando su incorporación a los sistemas eléctricos en general. Es factible pensar que en un futuro no muy lejano los consumidores residenciales o pequeñas industrias opten por aprovechar las bondades de este tipo de energía para satisfacer sus requerimientos de electricidad, lo que ocasionará un aumento significativo de generación en los puntos de consumo. Lo anterior se traduce en desafíos técnico-operacionales para la operación de las redes de distribución. Dentro de las problemáticas que se producirán, está el aumento del desbalance en las redes, dado que ya no bastará con estudios estadísticos de demanda, sino que también entrarán a regir factores climáticos y de mantenimiento de los generadores en la operación, dificultando la tarea de despacho para una operación segura de la red. Estos desbalances tanto de carga como de tensión pueden ser atenuados si se utilizan tecnologías de control de flujo para redes de transmisión y aplicando estratégias de control adecuadas. Consecuentemente, desde el punto de vista del operador de la red de distribución, es factible operar una red balanceada que cumple con los criterios de la norma técnica. Se propone el diseño de un D-Statcom con capacidad de almacenamiento, basado en un convertidor tipo VSC de 3 piernas. Su sistema de control está constituido por el módulo sintetizador de tensión, que tiene como función calcular y generar las señales de encendido y apagado de los interruptores de potencia; El módulo sincronizador, que mediante un PLL (del inglés Phase Locked Loop) permite que las tensiones generadas por el conversor y las de la red estén en sincronía; El módulo de control de corriente, el cual por medio de controles PI genera referencias de tensión para el bloque sintetizador de tensión y así se inyecten las corrientes de referencia al sistema en el punto de conexión; Por último, el módulo de cálculo de referencias de control, el cual se encarga de generar las referencias de corriente para el resto del equipo. Lo anterior se implementa mediante el uso de MATLAB /Simulink y el módulo PowerSystem. Los resultados de los casos de estudios simulados revelan la factibilidad y funcionalidad esperada del equipo diseñado en diversas situaciones. Con el control propuesto, y a partir de las simulaciones realizadas, se pudo disminuir un desbalance de tensión de un 3.29% a sólo un 0,53%, es decir un 84% de mejora con el alimentador normal, en cambio con la red más débil se logró compensar de un 4.60% a sólo un 1,99%, o sea, un 57% de mejora del indicador. En los casos estudiados, el incluir la operación de este equipo compensador y su control mejora el indicador de desbalance a lo menos un 77%, salvo en el escenario más complejo que se obtienen mejoras en torno a un 56%. Dada la efectividad del equipo de compensación y su control, se propone finalmente extender el trabajo a otras fuentes de generación distribuida, pues al ser una solución modal, será de gran ayuda en la introducción masiva de nuevas fuentes de energía en las futuras Smart grid que se instalen en el mundo.

Η παρούσα διπλωματική εργασία έχει ως αντικείμενο μελέτης την ανάπτυξη ενός υβριδικού συστήματος που περιλαμβάνει έναν HVDC μετατροπέα (με θυρίστορ μεταγωγής γραμμής) και ένα στατικό αντισταθμιστή και χρησιμοποιείται για τη μεταφορά ενέργειας σε δίκτυα που δε διαθέτουν σύγχρονη παραγωγή. Η μελέτη θα περιλαμβάνει όλα εκείνα τα στοιχεία που είναι απαραίτητα για τη διασύνδεση σε ένα κεντρικό δίκτυο. Το προτεινόμενο σύστημα συνδυάζει καλή απόδοση, χαμηλό κόστος και μικρή απώλεια ισχύος από τον HVDC μετατροπέα με τη γρήγορη δυναμική απόδοση ενός συστήματος μεταφοράς στηριζόμενο σε VSC (μετατροπέας πηγής τάσης). Η μελέτη περιγράφει τις αρχές και τις στρατηγικές ελέγχου του προτεινόμενου συστήματος. Επίσης παρουσιάζονται προσομοιώσεις στο πρόγραμμα PSCAD που καταδεικνύουν τη δυναμική συμπεριφορά και αξιολογούν το προτεινόμενο σύστημα κάτω από διάφορες συνθήκες λειτουργίας, όπως ψυχρή εκκίνηση, αυξομειώσεις φορτίου ή παραγωγής και συνθήκες ac σφαλμάτων. / The current diploma thesis deals with the development of a hybrid system comprising an HVDC converter (line commutated thyristor) and a STATCOM which is proposed for the connection of networks with no synchronous generation to a main grid. The proposed system combines the robust performance and low capital cost and power loss of a line-commutated HVDC converter, with the fast dynamic performance of a VSC system. The thesis describes the principles and control strategies of the proposed system. PSCAD/EMTDC simulations are presented to demonstrate the robust performance and to validate the proposed system during various operating conditions such as black-start, variations of load or generation and ac fault conditions.

This thesis presents methods for voltage management in distribution systems with high photovoltaic (PV) power production. The high PV penetration leads to both new challenges such as voltage profile violation and reverse power flow, and also new opportunities. Traditionally, the voltage control in the distribution network is achieved by common devices in the networks such as capacitor banks, static synchronous compensators (STATCOMs) and on-load tap changers (OLTCs). This thesis has considered existing reactive power capable solar PV inverters together with STATCOMs to provide voltage support for the distribution network. In this thesis, two effective coordination methods using the STATCOM and PV inverters are developed in order to study their interaction and how they together can stabilize the voltage level. Data from existing low-voltage (LV) and medium-voltage (MV) networks are used for a case study. The first control method is developed for LV network’s voltage control by means of PV inverter and STATCOM. The second control method is developed for both LV and MV networks’ voltage control, where reactive power control in PV inverters and STATCOMs are used in the LV network and only STATCOMs in the MV network. The control methods follow a hierarchical structure where reactive power compensation using PV inverters are prioritized. The STATCOMs, first in the LV and thereafter in the MV network in the second control method, are used only when the PV inverters are not able to provide or consume enough reactive power. This is beneficial due to the significant reduction in numbers of STATCOMs and their operation. The simulation results indicate that the proposed method is able to control both the over- and undervoltage situations for the test distribution networks. It is also shown that reactive power supply at night by the PV inverters can be an important resource for effective voltage regulation by using the proposed coordinated voltage control method.

Power generation of the future will be dominated by renewable energy sources. This is a positive trend as society aims to improve sustainability, energy security, and reduce CO2 emissions. This progress however bears challenges for the power systems of the future. One area of particular concern is the blackstart recovery capability of the power systems following blackout events that disrupt electricity supply, as traditional blackstart suppliers have originated from unsustainable power sources. As new sustainable technologies are explored to fill the void, offshore wind power has been identified as a potential suitor to address future blackstart deficiencies. However, following a blackout event that leaves an offshore wind farm isolated, the current technology does not allow for start-up of the wind farm in an islanded mode without assistance from the onshore grid. Hence, a STATCOM with active power storage, known as an ES-STATCOM, is the proposed solution to energize the offshore wind farm to operate in an islanded mode, in preparation for assisting the onshore grid with blackstart. An EMT model and simulation of an ES-STATCOM with grid forming control capabilities and the surrounding offshore wind farm export system were developed in this thesis to assess the viability of the STATCOM solution in energization. The results are promising and show that the ES-STATCOM is in fact able to energize the offshore system to a stable voltage, while avoiding high order harmonics and large overvoltages that have the potential to cause damage to expensive subsea power system components. Ultimately, this thesis serves as a proof-of-concept for blackstart technology, providing useful insights towards the maturity of offshore wind-based blackstart providers of the future. / Framtidens elproduktion kommer att domineras av förnybara energikällor. Detta är en positiv trend eftersom samhället strävar efter att förbättra hållbarheten, tryggheten gällande energiförsörjning samt minska koldioxidutsläppen.. Denna utveckling innebär dock utmaningar för framtidens kraftsystem. Ett område som är särskilt oroande är kraftsystemens förmåga att återhämta sig efter blackout som stör elförsörjningen, eftersom de traditionella leverantörerna av ett strömavbrott har sitt ursprung i ohållbara kraftkällor. I takt med att man utforskar ny hållbar teknik för att fylla tomrummet har havsbaserad vindkraft identifierats som en potentiell kandidat för att åtgärda framtida brister vid svartstart. Efter ett strömavbrott som gör att en havsbaserad vindkraftpark blir isolerad från kraftnätet kan vindkraftparken inte startas upp i ett ö-läge utan hjälp från landnätet med nuvarande teknik. Därför föreslås en STATCOM med aktiv energilagring, en så kallad ES-STATCOM, som lösning för att ge energi till havsbaserade vindkraftparker så att de kan fungera i ö-läge, som en förberedelse för att hjälpa landnätet med svartstart. En EMT-modell och simulering av en ES-STATCOM med nätbildande kontrollfunktioner och det omgivande exportsystemet för havsbaserade vindkraftparker utvecklades i denna avhandling för att bedöma lönsamheten hos STATCOM-lösningen för energitillförsel. Resultaten är lovande och visar att ES-STATCOM kan försörja det havsbaserade systemet med den en energi som krävs för en stabil spänning, samtidigt som man undviker harmoniska övertoner av hög ordning och stora överspänningar som har potential att skada dyra komponenter i det havsbaserade kraftsystemet. I slutändan tjänar denna avhandling som ett proof-of-concept för blackstart-teknik och ger användbara insikter om hur mogna framtida leverantörer av svartstart-teknik för havsbaserad vindkraft är.

This dissertation is dedicated to a comprehensive study of static synchronous compensator (STATCOM) systems utilizing cascaded-multilevel converters (CMCs). Among flexible AC transmission system (FACTS) controllers, the STATCOM has shown feasibility in terms of cost-effectiveness in a wide range of problem-solving abilities from transmission to distribution levels. Referring to the literature reviews, the CMC with separated DC capacitors is clearly the most feasible topology for use as a power converter in the STATCOM applications. The controls for the CMC-based STATCOM were, however, very complicated. The intricate control design was begun without well-defined system transfer functions. The control compensators were, therefore, randomly selected. The stability of the system was achieved by trial and error processes, which were time-consuming and ineffective. To be able to operate in a high-voltage application, a large number of DC capacitors are utilized in a CMC-based STATCOM. All DC capacitor voltages must be balanced in order to avoid over-voltages on any particular link. Not only do these uneven DC voltages introduce voltage stress on the semiconductor switches, but they also lower the quality of the synthesized output waveforms of the converter. Previous researches into DC capacitor voltage-balancing techniques were very straightforward, in that individual voltage compensators were added into the main control loop. However, the compensator design for these individual loops is very problematic because of the complexity of the voltage-loop transfer functions. Basically, the trial and error technique again provides the simplest way to achieve acceptable compensators. Moreover, the greater number of voltage levels, the more complex the control design, and the main controller must perform all of the feedback control procedures. As a result, this approach potentially reduces the reliability of the controller. The goal of this dissertation is to achieve high-performance, reliable, flexible, cost-effective power stages and controllers for the CMC-based STATCOM. Major contributions are addressed as follows: 1) optimized design for the CMC-based STATCOM power stages and passive components, 2) accurate models of the CMC for reactive power compensations in both ABC and DQ0 coordinates, 3) an effective decoupling power control technique, 4) DC-link balancing strategies; and 5) improvements in the CMC topology. To enhance the modularity and output voltage of the CMC, the high-switching-frequency, high-power H-bridge building block (HBBB) and the optimized design for its power stage and snubber circuits are first proposed. The high-switching-frequency feature is achieved by utilizing the Virginia Tech-patented emitter turn-off (ETO) thyristor. Three high-power HBBB prototypes were implemented, and their performance was experimentally verified. To simplify the control system design, well-defined models of the CMC in both ABC and DQ0 coordinates are proposed. The proposed models are for the CMC with any number of voltage levels. The key system transfer functions are achieved and used in the control design processes. To achieve independent power control capability, the control technique, called the decoupling power control, is proposed. By applying this control technique, real and reactive power components can be controlled separately. In order to balance the DC capacitor voltages, a new, effective pulse width modulation (PWM) technique, which is suitable for any number of H-bridge converters, is proposed. The proposed cascaded PWM algorithm can be practically realized into the field programmable gate arrays (FPGA), and its complexity is not affected by the number of voltage levels. In addition, the complexity of the main controller, which is essentially based on the digital signal processor (DSP), is no longer a function of the number of the output voltage levels. The basic structure of the cascaded PWM is modular, which, in general, enhances the modularity of the CMC power stages. With the combination of the decoupling power control and the cascaded PWM, a CMC with any number of voltage levels can be simply modeled as a three-level cascaded converter, which is the simplest topology to deal with. This significantly simplifies and optimizes the control design process. To verify the accuracy of the proposed models and the performance of the control system for the CMC-based STATCOM, a low-power, seven-level cascaded-based STATCOM hardware prototype is implemented. The key control procedures are performed by a main controller, which consists of a DSP and an FPGA. The simulation and experimental results indicate the superior performance of the proposed control system, as well as the precision of the proposed models. / Ph. D.

Multiple STATCOM units in proximity have been adopted in power transmission systems in order to obtain better voltage regulation and share burdens. Throughout stability assessment in this dissertation, it is shown, for the first time, that STATCOMs could interact with each other in a negative way in the small-signal sense due to their control, causing voltage instability, while loads and transmission lines showed small effects. Since this voltage stability problem is induced by STATCOMs, d-q frame impedance-based stability analysis was used, for the first time, to explore the inherent power system instability problem with presence of STATCOMs as it provides an accurate understanding of the root cause of instability within the STATCOM control system. This dissertation first proposes the impedance model in d-q frame for STATCOMs, including dynamics from synchronization, current and voltage loops and reveals the significant features compared to other types of grid-tied converters that 1) impedance matrix strongly coupled in d and q channel due to nearly zero power factor, 2) different behaviors of impedances at low frequency due to inversed direction of reactive power and 3) coupled small-signal propagation paths on the voltage at point of common coupling from synchronization and ac voltage regulation. Using the proposed impedance model, this dissertation identifies the frequency range of interactions in a viewpoint of d-q frame impedances and pinpointed that the ac voltage regulation was the main reason of instability, masking the effects of PLL in power transmission systems. Due to the high impedance of STATCOMs compared to that of transmission lines around the frequency range of interactions, STATCOMs were seen to interact with each other through the transmission lines. A scaled-down 2-STATCOM power grid was built to verify the conclusions experimentally. / Ph. D. / STATCOMs have been proven a type of effective power electronics device for reactive power compensations and people are trying to install multiple STATCOMs in proximity in power systems in order to have better performances. This dissertation, for the first time, evaluates the operation of multiple STATCOMs in proximity and finds out that they could interact with each other in a negative way in the small-signal sense due to their control, causing voltage instability, while loads and transmission lines showed small effects. Since this voltage stability problem is induced by STATCOMs, d-q frame impedance-based stability analysis was used, for the first time, to explore the inherent power system instability problem with presence of STATCOMs as it provides an accurate understanding of the root cause of instability within the STATCOM control system. To this end, an impedance model of STATCOMs is proposed, which accurately explains the terminal behaviors of STATCOMs. Using the model, this dissertation identifies the frequency range of interactions in a viewpoint of d-q frame impedances and pinpointed that the ac voltage regulation was the main reason of instability, masking the effects of PLL in power transmission systems. All the above is validated experimentally in a scaled-down 2-STATCOM power system.