Demonstration of sustained and useful converter responses during balanced and unbalanced faults in microgrids

In large power grids where converter penetration is presently low and the network impedance is predominantly reactive, the required response from converters during faults is presently specified by phrases such as “maximum reactive output”. However, in marine and aero power systems most faults are unbalanced, the network impedance is resistive, and converter penetration may be high. Therefore a balanced reactive fault current response to an unbalanced fault may lead to over-voltages or over/under frequency events. Instead, this paper presents a method of controlling the converter as a balanced voltage source behind a reactance, thereby emulating the fault response of a synchronous generator (SG) as closely as possible. In this mode there is a risk of converter destruction due to overcurrent. A new way of preventing destruction but still providing fault performance as close to a SG as possible is presented. Demonstrations are presented of simulations and laboratory testing at the 10kVA 400V scale, with balanced and unbalanced faults. Currents can be limited to about 1.5pu while still providing appropriate unbalanced fault response within a resistive network

Stability Analysis of Fully Power Converter-based Microgrids

Due to rising energy demand and climate crisis, distributed energy generation utilizing the renewable energy resource is constantly evolving. Generation near the electrical loads within a defined boundary forms a microgrid. It can be operated by connecting with utility-grid or as a stand-alone system. Solar and wind energy resources use a power-electronic converter to interface with the load or grid. The fast dynamics of the converter is very different from the inertial dynamics of the grid with large synchronous machines. Furthermore, low short-circuit capacity, more resistive network and unbalanced loading are few inherent characteristics concerning the operational reliability of the microgrid. This thesis aims to present the various issues with fully power converter-based microgrids in terms of stability and protection. High-power converters with LCL-filter are simulated under various operating conditions in typhoon real-time simulator. A relay function is used to detect the unstable operating points. High resistance-to-reactance ratio in the low-voltage line forms active-reactive power coupling, making the conventional droop control inaccurate. For a disturbance, droop control allows a steep voltage or frequency deviations which lead to unnecessary protection tripping. Use of virtual inertia control avoids the steep change in the system variables and preserves the stability. Parallel droop-based converters with non-identical parameters or output impedance induce circulating current or reactive power oscillations. Use of virtual impedance control minimizes the circulating current and enhances power-sharing. Phase-locked loop synchronized with a weak grid (high-impedance grid) is unstable on large-signal disturbances. Current-reference saturation limits the converter current for a three-phase balanced fault condition. For higher fault-impedance, the fault current is nearly equal to the load current, which possibly blinds the microgrid protection

Operation and Control Strategies of Solid State Transformer for a Stable MV-LV Interaction. Strategie di funzionamento e controllo di trasformatori a stato solido per un interfacciamento stabile tra reti MT e BT

Il trasformatore a stato solido (SST)è un elemento chiave del sistema elettrico futuro basato su generazione distribuita da fonti rinnovabili. In questa tesi sono analizzati i possibili utilizzi del SST,è studiata la sua topologia edè sviluppato un controllo basato su tecniche allo stato dell'arte. Le diverse topologie e controlli sono infine simulati in ambiente Matlab Simulink

30kW, 200V/900V, thyristor LCL DC/DC converter laboratory prototype design and testing

ACKNOWLEDGMENT The authors would like to acknowledge significant input from Aberdeen University technician, A. Styles, in building this converterPeer reviewedPostprin

Development of a converter for grid-tied and isolated operation of an interior permanent magnet synchronous generator, coupled to a twin-shaft gas turbine

South Africa’s overreliance on coal fired power generation has led to the government’s commitment to diversifying the country’s energy mix. Gas turbine generators are poised to play a larger role in South Africa’s energy mix, due to the country’s abundance in natural gas reserves. Therefore, there is a need to developed gas turbine emulation systems to investigate how this transition is to be implemented and to discover new efficient ways to generate power through gas turbines. This thesis presents the development of a twin-shaft gas turbine emulator. A DC-machine that accepts both torque and speed references is used to emulate the behaviour of the gas turbine according to a modified Rowen gas turbine model. The emulator is coupled to a 1.5kW interior permanent magnet synchronous generator (IPM). The power density of a DC-machine is significantly lower than that of a gas turbine of the same rating. Thus, the DC-machine is rated at double the rating of the IPM to overcome the high inertia it has when compared to a gas turbine of the same rating. This means that the DC-machine can produce large toques to successfully emulated the dynamic behaviour of the gas turbine. A maximum error 2.5% in the emulation of the gas turbine’s speed is reported. A two-level active converter is used to compare control strategies for an IPM. Ninety-degree torque angle (NTA) control, maximum torque per ampere (MTPA) control and unity power factor (UPF) control are compared for performance. The UPF and MTPA control result in the lowest and second lowest DC-link utilisation respectively when compared to NTA control. This is due to a negative d-axis current component as opposed to a zero d-axis current component in the case of NTA control. It is also concluded that to achieve a high power factor and torque development, a negative d-axis current component is required. UPF and MTPA control perform well in both categories, with UPF control and MTPA control resulting in the highest power factor and developed torque respectively. A fourth control strategy that maximises the efficiency of the IPM is developed experimentally. The maximum efficiency (ME) control strategy minimises mechanical, core, windage and conduction losses. It also results in near unity power factor and near maximum developed torque. A nonconventional control structure that involves control of the DC-link from the generatorside converter is presented. This frees the outer-loop control of load-side converter to regulate voltage across the load when the system is supplying power to an isolated load. This control structure also allows the grid-side converter to employ reactive power compensation, without having to regulate the DC-link voltage at the same time. In doing so, large grid currents are avoided. A recursive least squares (RLS) algorithm is used to separate negative and positive sequence current components during grid voltage unbalance. A method to minimise the presence of negative sequence components in the load current is presented and implemented successfully in an experiment

Stabilised Control of Converter Interfaced DERs for Reliable Operation of Microgrid and Microgrid Clusters

This thesis aims to achieve a stabilised control of converter interfaced DER for the reliable and resilient operation of microgrid and microgrid clusters. The suitability of voltage and current control for VSCs is evaluated and corrective measures are proposed to stabilise converter operation. Furthermore, the accurate power demand distribution in islanded MGs and interconnected MGs are ensured by advanced control strategies. The proposal presented in the thesis is verified both through simulation and experimental work

Contributions to impedance shaping control techniques for power electronic converters

El conformado de la impedancia o admitancia mediante control para convertidores electrónicos de potencia permite alcanzar entre otros objetivos: mejora de la robustez de los controles diseñados, amortiguación de la dinámica de la tensión en caso de cambios de carga, y optimización del filtro de red y del controlador en un solo paso (co-diseño). La conformación de la impedancia debe ir siempre acompañada de un buen seguimiento de referencias. Por tanto, la idea principal es diseñar controladores con una estructura sencilla que equilibren la consecución de los objetivos marcados en cada caso. Este diseño se realiza mediante técnicas modernas, cuya resolución (síntesis del controlador) requiere de herramientas de optimización. La principal ventaja de estas técnicas sobre las clásicas, es decir, las basadas en soluciones algebraicas, es su capacidad para tratar problemas de control complejos (plantas de alto orden y/o varios objetivos) de una forma considerablemente sistemática. El primer problema de control por conformación de la impedancia consiste en reducir el sobreimpulso de tensión ante cambios de carga y minimizar el tamaño de los componentes del filtro pasivo en los convertidores DC-DC. Posteriormente, se diseñan controladores de corriente y tensión para un inversor DC-AC trifásico que logren una estabilidad robusta del sistema para una amplia variedad de filtros. La condición de estabilidad robusta menos conservadora, siendo la impedancia de la red la principal fuente de incertidumbre, es el índice de pasividad. En el caso de los controladores de corriente, el impacto de los lazos superiores en la estabilidad basada en la impedancia también se analiza mediante un índice adicional: máximo valor singular. Cada uno de los índices se aplica a un rango de frecuencias determinado. Finalmente, estas condiciones se incluyen en el diseño en un solo paso del controlador de un convertidor back-to-back utilizado para operar generadores de inducción doblemente alimentados (aerogeneradores tipo 3) presentes en algunos parques eólicos. Esta solución evita los problemas de oscilación subsíncrona, derivados de las líneas de transmisión con condensadores de compensación en serie, a los que se enfrentan estos parques eólicos. Los resultados de simulación y experimentales demuestran la eficacia y versatilidad de la propuesta.Impedance or admittance shaping by control for power electronic converters allows to achieve among other objectives: robustness enhancement of the designed controls, damped voltage dynamics in case of load changes, and grid filter and controller optimization in a single step (co-design). Impedance shaping must always be accompanied by a correct reference tracking performance. Therefore, the main idea is to design controllers with a simple structure that balance the achievement of the objectives set in each case. This design is carried out using modern techniques, whose resolution (controller synthesis) requires optimization tools. The main advantage of these techniques over the classical ones, i.e. those based on algebraic solutions, is their ability to deal with complex control problems (high order plants and/or several objectives) in a considerably systematic way. The first impedance shaping control problem is to reduce voltage overshoot under load changes and minimize the size of passive filter components in DC-DC converters. Subsequently, current and voltage controllers for a three-phase DC-AC inverter are designed to achieve robust system stability for a wide variety of filters. The least conservative robust stability condition, with grid impedance being the main source of uncertainty, is the passivity index. In the case of current controllers, the impact of higher loops on impedance-based stability is also analyzed by an additional index: maximum singular value. Each of the indices is applied to a given frequency range. Finally, these conditions are included in the one-step design of the controller of a back-to-back converter used to operate doubly fed induction generators (type-3 wind turbines) present in some wind farms. This solution avoids the sub-synchronous oscillation problems, derived from transmission lines with series compensation capacitors, faced by these wind farms. Simulation and experimental results demonstrate the effectiveness and versatility of the proposa

Development of a converter for grid-tied and isolated operation of an interior permanent magnet synchronous generator, coupled to a twin-shaft gas turbine

South Africa’s overreliance on coal fired power generation has led to the government’s commitment to diversifying the country’s energy mix. Gas turbine generators are poised to play a larger role in South Africa’s energy mix, due to the country’s abundance in natural gas reserves. Therefore, there is a need to developed gas turbine emulation systems to investigate how this transition is to be implemented and to discover new efficient ways to generate power through gas turbines. This thesis presents the development of a twin-shaft gas turbine emulator. A DC-machine that accepts both torque and speed references is used to emulate the behaviour of the gas turbine according to a modified Rowen gas turbine model. The emulator is coupled to a 1.5kW interior permanent magnet synchronous generator (IPM). The power density of a DC-machine is significantly lower than that of a gas turbine of the same rating. Thus, the DC-machine is rated at double the rating of the IPM to overcome the high inertia it has when compared to a gas turbine of the same rating. This means that the DC-machine can produce large toques to successfully emulated the dynamic behaviour of the gas turbine. A maximum error 2.5% in the emulation of the gas turbine’s speed is reported. A two-level active converter is used to compare control strategies for an IPM. Ninety-degree torque angle (NTA) control, maximum torque per ampere (MTPA) control and unity power factor (UPF) control are compared for performance. The UPF and MTPA control result in the lowest and second lowest DC-link utilisation respectively when compared to NTA control. This is due to a negative d-axis current component as opposed to a zero d-axis current component in the case of NTA control. It is also concluded that to achieve a high power factor and torque development, a negative d-axis current component is required. UPF and MTPA control perform well in both categories, with UPF control and MTPA control resulting in the highest power factor and developed torque respectively. A fourth control strategy that maximises the efficiency of the IPM is developed experimentally. The maximum efficiency (ME) control strategy minimises mechanical, core, windage and conduction losses. It also results in near unity power factor and near maximum developed torque. A nonconventional control structure that involves control of the DC-link from the generatorside converter is presented. This frees the outer-loop control of load-side converter to regulate voltage across the load when the system is supplying power to an isolated load. This control structure also allows the grid-side converter to employ reactive power compensation, without having to regulate the DC-link voltage at the same time. In doing so, large grid currents are avoided. A recursive least squares (RLS) algorithm is used to separate negative and positive sequence current components during grid voltage unbalance. A method to minimise the presence of negative sequence components in the load current is presented and implemented successfully in an experiment