A Silicon Carbide Based Solid-State Fault Current Limiter for Modern Power Distribution Systems

The fault current limiter represents a developing technology which will greatly improve the reliability and stability of the power grid. By reducing the magnitude of fault currents in distribution systems, fault current limiters can alleviate much of the damage imposed by these events. Solid-state fault current limiters in particular offer many improved capabilities in comparison to the power system protection equipment which is currently being used for fault current mitigation. The use of silicon carbide power semiconductor devices in solid-state fault current limiters produces a system that would help to advance the infrastructure of the electric grid. A solid-state fault current limiter utilizing silicon carbide super gate-turn off thyristors (SGTOs) and silicon carbide PiN diodes was designed, built, and tested as a technology demonstrator. The impact of using silicon carbide (SiC) devices in this application was assessed, as well as the associated design challenges. The feasibility of implementing SiC based solid-state fault current limiters for 15 kV class distribution systems was investigated in order to determine the practicality of wide-scale deployment

Study of a current limitation strategy for grid-forming inverters in case of short-circuit faults

The use of renewable energies and their participation in the electricity market has been increasing in recent years with the aim of achieving a future where 100% of the generation comes from renewable sources. In this new scenario, in which most of the synchronous generators with high inertias will no longer participate, the main reference of the network will have to be formed in an alternative and robust way. Given that renewable energy generation and storage systems require power converters to adapt to an AC grid, they already now shall have power distribution control algorithms based on the network voltage and frequency. On the other hand, a power converter can behave and be modeled as a controlled voltage source or controlled current source according to the variable that is been regulated. In the absence of synchronous generators in the future, the formation and responsibility of a stable grid must be distributed among different converters working as voltage controlled sources with power distribution algorithms. When a converter works as a controlled voltage source, it presents a new challenge in the field of the corresponding control algorithm when facing a fault scenario at the grid side. In the event of a short circuit, the output current of the inverter must be saturated to prevent possible damage of the inverter. This limitation can have consequences for the upper control loops if they are not consistently adapted. This master thesis presents a new control strategy for dealing with short-circuit scenarios by saturating the amplitude of the reference voltage from the droop power distribution loop. In this way, possible windup effects are avoided, which would cause the instability of the system.El uso de las energias renovables y su participación en el mercado eléctrico viene en aumento durante los últimos años con el objetivo de alcanzar un futuro donde el 100% de la generación provenga de fuentes renovables. En este nuevo escenario, en el cual los generadores síncronos con grandes inercias dejarán de participar en gran medida, la red principal de referencia deberá formarse de una manera alternativa y rebusta. Teniendo en cuenta que los sistemas de generación renovables y almacenamiento de enegia eléctrica requieren de convertidores de potencia para adaptarse a una red AC, estos en la actualidad ya deben contar con algoritmos de control de distribución de potencia en función de la tensión y frecuencia de la red. Por otro lado, un convertidor de potencia puede comportarse y modelarse como una fuente de tensión controlada o fuente de corriente controlada según la variable a monitorizar. En un futuro ausente de generadores síncronos, la formación y responsabilidad de una red estable debe estar distribuida entre los distintos convertidores trabajando como fuente de tensión controlada con algoritmos de distribución de potencia. Cuando un convertidor trabaja como fuente de tensión controlada presenta un nuevo reto en el ámbito del correspondiente algoritmo de control frente a un escenario de falta en el lado de la red. En caso de cortocircuito, la corriente de salida del convertidor debe saturarse para evitar posibles daños de este. Esta limitación, puede tener consecuencias en los lazos de control superiores si estos no son adaptados de forma coherente. Este trabajo presenta una nueva estrategia de control para abordar escenarios de cortocircito mediante la saturación de la amplitud de la tensión de referencia proviniente del lazo de repartición de potencias "droop". De esta forma, se evitan posibles efectos de "windup", los cuales causarian la inestabilidad del sistema.L'ús de les energies renovables i la seva participació en el mercat elèctric ve en augment durant els darrers anys amb l'objectiu d'assolir un futur on el 100% de la generació provingui de fonts renovables. En aquest nou escenari, en el qual els generadors síncrons amb grans inèrcies deixaran de participar-hi en gran mesura, la xarxa principal de referència haurà de formar-se d'una manera alternativa i robusta. Tenint en compte que els sistemes de generació renovables i emmagatzemament d'energia elèctrica requereixen de convertidors de potència per a adaptar-se a una xarxa AC, aquests en la actualitat ja han de comptar amb algoritmes de control de distribució de potència en funció de la tensió i freqüència de la xarxa. Per altra banda, un convertidor de potència pot comportarse i modelarse segons una font de tensió controlada o font de corrent controlada segons la variable a monitoritzar. En un futur absent de generadors síncrons, la formació i responsabilitat d'una xarxa estable ha d'estar distribuida entre diferents convertidors treballant com a font de tensió controlada amb algoritmes de distribució de potència. Quan un convertidor treballa com a font de tensió controlada presenta un nou repte en l'àmbit del corresponent algoritme de control davant un escenari de falta al costat de la xarxa. En cas de curtcircuit, la corrent de sortida del convertidor s'ha de saturar per a evitar possibles danys d'aquest. Aquesta limitació, pot tenir conseqüènices ens els llaços de control superiors si aquests no són adaptats coherentment. Aquest treball presenta una nova estrategia de control per a afrontar escenaris de curtcircuit mitjançant la saturació de l'amplitud de la tensió de referència provinent del llaç de repartició de potència "droop". D'aquesta forma, s'eviten possibles efectes de "windup", els quals causarien la inestabilitat del sistema

An Assessment of PIER Electric Grid Research 2003-2014 White Paper

This white paper describes the circumstances in California around the turn of the 21st century that led the California Energy Commission (CEC) to direct additional Public Interest Energy Research funds to address critical electric grid issues, especially those arising from integrating high penetrations of variable renewable generation with the electric grid. It contains an assessment of the beneficial science and technology advances of the resultant portfolio of electric grid research projects administered under the direction of the CEC by a competitively selected contractor, the University of California’s California Institute for Energy and the Environment, from 2003-2014

Design and Implementation of High-Efficiency, Lightweight, System-Friendly Solid-State Circuit Breaker

Direct current (DC) distribution system has shown potential over the alternative current (AC) distribution system in some application scenarios, e.g., electrified transportation, renewable energy, data center, etc. Because of the fast response speed, DC solid-state circuit breaker (SSCB) becomes a promising technology for the future power electronics intensive DC energy system with fault-tolerant capability. First, a thorough literature survey is performed to review the DC-SSCB technology. The key components for DC-SSCB, including power semiconductors, topologies, energy absorption units, and fault detection circuits, are studied. It is observed that the prior studies mainly focus on the basic interruption capability of the DC-SSCB. There are not so many studies on SSCB’s size optimization or system-friendly functions. Second, an insulated gate bipolar transistor (IGBT) based lightweight SSCB is proposed. With the reduced gate voltage, the proposed SSCB can limit the peak fault current without the bulky and heavy fault current limiting the inductor, which exists in the conventional SSCB circuit. Thus, the specific power density of the SSCB is substantially improved compared with the conventional design. Meanwhile, to understand the impact of different design parameters on the performance of SSCB, an analytical model is built to establish the relationship between SSCB dynamic performance and operating conditions considering the key components and circuit parasitics. Simulation and test results demonstrate the accuracy of the proposed model. To limit the fault current with the proposed SSCB without a current limiting inductor, power semiconductors need to operate in the active region temporarily. During this interval, a severe voltage oscillation has been observed experimentally, leading to the DC-SSCB overstress and eventually the failure. A detailed MATLAB/Simulink model is built to understand the mechanism causing the voltage oscillation. Three suppression methods using enhanced gate drive circuitry are proposed and compared. Test results based on a 2kV/1kA SSCB prototype demonstrate the effectiveness of the proposed oscillation mitigation method and the accuracy of the derived model. Meanwhile, when the system fault impedance is close to zero (e.g., high di/dt), the influence of the parasitic inductance contributed by interconnection (e.g., bus bar, module package, etc.) cannot be neglected. To study the influence of the bus bar connections on SSCB with high di/dt, a Q3D extractor is adopted to extract the parasitic parameters of the SSCB and understand the influence of different bus bar connections. A vertical bus bar is proposed to suppress the side effect and verified by the Q3D extractor and experimental results. Finally, a system-friendly SSCB is demonstrated. The proposed gate drive enables the SSCB to operate in the current limitation mode for the overcurrent limitation. The current limitation level and limitation time can be tuned by the gate drive. Then, this dissertation provides an all-in-one solution with integrated circuitries as the fault detector, actuator for the semiconductor’s operating status regulation, and coordinated control. This allows the developed SSCB to limit system fault current not exceeding short-circuit current rating (SCCR) and also take different responses under different fault cases. The feasibility and the effectiveness of the proposed system-friendly SSCB are validated with experimental results based on a 200V/10A SSCB demonstrator

Contribution for the Study of the Integration of Inductive Superconducting Fault Current Limiters in Electrical Distribution Grids

A wider adoption of distributed generation sources and an increased interconnection of networks tend to increase the complexity of electric power grids, thus causing a surge in failures, especially short-circuits. The conventional solution against short-circuit currents, for example, the construction of new substations, splitting of busbars, even updating the technology of the existing current limiters may prove either economically or technically unfeasible. Fault current limiters, mainly the superconducting fault current limiters, have already demonstrated their viability in electric power grids. Fault current limiter devices at normal operation are invisible to the grid, acting almost instantly upon a fault, returning to their normal state upon its correction. To disseminate these technologies, the development of straightforward design tools is required. These tools must consider the properties of the available constitutive elements of the devices. Behind these design tools, the integrity of the fault current limiter should be assured during its operation. Problems regarding the electrodynamic forces developed under short-circuit events must be properly characterized because they can damage windings, causing device breakage and affecting the power grid. In this thesis, a design methodology that intends to model and optimise saturated cores superconducting fault current limiters is presented. This methodology considers the characteristics of each constitutive element of the limiter while addressing utility requirements and power grid characteristics. Genetic algorithms are used both to optimise the constitutive elements of the limiter and its performance in the power grid. In order to validate the present methodology, a three-phase superconducting fault current limiter is designed/optimised, built and tested. The electrodynamic forces analysis developed in superconducting tapes of an inductive transformer type superconducting fault current limiter, under short-circuit conditions is performed.A crescente adoção de fontes de geração distribuída e o aumento das ligações internas entre redes de energia levou ao aumento da complexidade das redes elétricas, causando um provável aumento do número de falhas, especialmente os curto-circuitos. Soluções convencionais para lidar com curto-circuitos, como por exemplo, a construção de novas subestações, a divisão dos barramentos, ou a atualização tecnológica dos limitadores de corrente existentes, podem se mostrar muito dispendiosas ou tecnicamente inviável. Os limitadores de corrente de defeito, principalmente os dispositivos supercondutores, têm vindo a demostrar a sua viabilidade em redes de energia elétrica. Estes dispositivos são considerados invisíveis para a rede, quando em operação normal. Quando uma falha na rede ocorre, estes agem instantaneamente, retornando ao seu estado normal após a falha terminar. De modo a disseminar estas tecnologias, é necessário o desenvolvimento de ferramentas de projeto e modelação, de fácil uso. Essas ferramentas devem considerar as propriedades dos elementos que constituem os dispositivos de proteção. Por detrás dessas ferramentas de projeto, a integridade do limitador deve ser assegurada durante todo o seu funcionamento. Problemas relacionados com forças eletrodinâmicas desenvolvidas sob eventos de curto-circuito devem ser devidamente caracterizados, pois podem danificar os enrolamentos, e por sua vez o equipamento e afetar a rede elétrica. Nesta tese, é apresentada uma metodologia de projeto, que visa modelar e otimizar limitadores de corrente de defeito supercondutores, do tipo núcleos saturados. Esta metodologia considera as características de cada elemento constitutivo do limitador enquanto aborda os requisitos da concessionária da rede de distribuição de energia e as características da rede elétrica. Algoritmos genéticos são usados para otimizar os o limitador e o seu desempenho na rede elétrica. A fim de validar a metodologia atual, um limitador trifásico é projetado/otimizado, construído e ensaiado. É ainda realizada a análise das forças eletrodinâmicas desenvolvidas em fitas supercondutoras de um limitador de corrente de defeito, do tipo transformador, em condições de curto-circuito

Investigation into Photovoltaic Distributed Generation Penetration in the Low Voltage Distribution Network

Significant integration of photovoltaic distributed generation (PVDG) in the low voltage distribution network (LVDN) could potentially pose threats and challenges to the core activity of distribution system operators (DSO), which is to transport electrical energy in a reliable and cost-effective way. The main aim of this research is to investigate the active planning and operation of LVDNs with increased PVDG integration through steady state power system analysis. To address the impacts of voltage profile fluctuation due to power flow modification, this research proposes a probabilistic risk assessment of power quality (PQ) variations and events that may arise due to significant PVDG integration. A Monte Carlo based simulation is applied for the probabilistic risk assessment. This probabilistic approach is used as a tool to assess the likely impacts due to PVDG integration against the extreme-case scenarios. With increased PVDG integration, site overvoltage is a likely impact, whereas voltage unbalance reduces when compared with no or low PVDG penetration cases. This is primarily due to the phase cancellation between the phases. The other aspect of the work highlights the fact that the implementation of existing volumetric charges in conjunction with net-metering can have negative impacts on network operator’s revenue. However, consideration of capacity charges in designing the existing network tariff structure shows incentivising the network operator to perform their core duties under increased integration of PVDG. The site overvoltage issue was also studied and resolved in a novel way, where the active and reactive power of the PVDG inverters at all the PV installed premises were optimally coordinated to increase the PV penetration from 35.7% to 66.7% of the distribution transformer rating. This work further explores how deficiencies in both reactive power control (RPC) and active power control (APC) as separate approaches can be mitigated by suitably combining RPC and APC algorithms. A novel “Q” or “PF” limiter was proposed to restrict frequent switching between the two droop characteristics while ensuring a stabilizing (smoothened) voltage profile in each of the PV installed nodes. This novel approach not only alleviates the voltage fluctuation but also reduces the overall network losses

Small-signal Analysis of Active Loads and Large-signal Analysis of Faults in Inverter Interfaced Microgrid Applications

Rectifiers and voltage regulators that have characteristics of constant power loads may form a significant percentage of a microgrid's total loads. The real part of the input impedance of a constant power load is negative and it may have control loop dynamics in a similar frequency range to the inverters that are supplying the microgrid. This thesis examines the interactions between an active constant power load and a microgrid for the impact on stability. Participation analysis of the eigenvalues that result from the model of the combined microgrid and active load identified that the low-frequency modes are associated with the voltage controller of the active rectifier and the droop-controllers of the inverters. The analysis also revealed that when the active load dc-voltage controller is designed with large gains, the voltage controller of the inverter becomes unstable but the low frequency modes associated with the droop controller of the inverter remain stable. The transient stability of a microgrid may require that the inverter-interfaced generation remain connected during a fault and return to normal power export once a fault is cleared. For an inverter to supply fault current, the controller of the inverter must current-limit the output and the fault strategy chosen must ensure that the current and voltage limiter do not latch-up and that the controller integrators do not wind-up. This thesis analyses different limiting and reset strategies and concluded that that it is not possible to successfully reset a limiter when using a reset signal from a closed-loop controller within the inverter. In a system where there are cascaded limiters, successful operation is obtained when the inner limit is a saturation limiter and the output limiter is a set-reset limiter. It was found that the transient stability of an inverter interfaced microgrid using a droop control algorithm is dependent on the current limiter and inductance of the network.Open Acces

A New Protection System Design of Active MV Distribution Network

The increasing implementation of renewable energy sources (RES), along with the diversity of energy source types, has additionally imposed significant operational and management problems in distribution networks. These problems are manifested in voltage regulations, system stability and coordination of protection, both in the distribution and transmission networks. For the medium voltage (MV) network, this includes new energy sources and higher amounts of fault currents, invisibility of several faults in the existing protection scheme, reduction of the range of protection devices and reduction of the possibility of detecting small fault currents with existing protection relays. Such changes significantly reduce the possibility of proper distribution system protection, both in subordinate and superior networks. The subject of this paper is the presentation of a new concept of the use of automation in the management and arrangement of power system protection dependent on the scheme and configuration of an active MV network. The goals of this analysis and research are to find and define the necessary architecture in which the scheme and appearance of the MV network should be automatically detected, and based on network topology to establish new settings of protection devices (ground fault, overcurrent and short circuit protection). The contributions of generation units of RES in the MV network must be considered. This paper specifically analyses the problems of power system management with simultaneous harmonization of protection systems both in the transmission and in the radial distribution network, offering optimization algorithms that have the ability of achieving the optimal solution. The implementation of the proposed technique was tested on a radial connection integrated with a microgrid (MG) which has the possibility of two-way power supply. The obtained results indicate that the proposed technique can solve described problems in the coordination of protection system and network management, even with the dynamic character of operation mode of the networks