Multifunctional Smart Grid Control based on Power Electronic Systems

Abstract

There is a worldwide switch to electricity generation plants based on renewable energy sources(RES) to decarbonise electricity generation. In contrast to the fossil fuel based traditionalpower plants, power plants based on RES are widely distributed and often connected to thedistribution system operator (DSO) grid level, which leads to a structural change of theelectricity network. The rising numbers of installed RES and the high fluctuation of powergeneration increase the stress on this grid level. To improve the stability, reliability andefficiency of the DSO grid level, it is necessary to transfer and adapt ancillary service functionsknown from the transmission system operator (TSO) to the DSO grid level.To provide ancillary services at the DSO grid level under high fluctuations, unbalanced gridconditions and harmonic distortions, a new Multifunctional Energy and Power Server (MEPS)based on modern power electronic is introduced in this research. The system topology consistsof a series- and a parallel-connected inverter branch. This structure is known as the UnifiedPower Quality Conditioner (UPQC) and the Unified Power Flow Controller (UPFC), whichare used in active power filters and in power flow control in electrical grids. The systemapproach developed in this work for implementing grid service functions aims to combine thevarious approaches for this in a single system.The series branch consists of an inverter system connected by a transformer in series to theupstream network and is able to compensate for asymmetrical and harmonic distorted voltages.The parallel branch consists of a second inverter system, which is connected in parallel to thegrid and is able to compensate for asymmetrical and harmonic distorted currents. Incombination with a battery system, the parallel branch can also provide active power-basedfunctions, such as primary control and power fluctuation compensation. All these grid-specificdynamic control functions are implemented based on symmetrical components (SC) withindividual controller loops for the positive, negative and zero sequences in the fundamentaland harmonic frequency range. To use the SC for real-time control, all measured voltages andcurrents are separated into different harmonic components using the heterodyne method. Thecombination of the heterodyne method with the SC transformation allows for the individualand decoupled control under asymmetrical and harmonic distorted conditions.The simulation and application tests carried out during the research show, that unbalanced andharmonic distorted voltages and currents can be controlled selectively and in a decoupledmanner. By considering the effective impedance of the grid connection point individually, forevery harmonic frequency under control, allows for a stable operation and good transientresponse - also at difficult impedance characteristics, such as found at a 3-leg, 4-wire splitcapacitor inverter topology.Finally, several inverters were connected in parallel to increase the output power forexperiments under real grid conditions. The successful operation of a whole system consistingof several inverters demonstrates the flexibility and scalability of the approach. The control hasa positive impact on the capacity and stability of the examined grid area by reducing powerfluctuations and unsymmetrical and harmonic voltages and currents. The experiments confirmthe effectiveness of the decoupled control also under real grid conditions