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 traditional
power plants, power plants based on RES are widely distributed and often connected to the
distribution system operator (DSO) grid level, which leads to a structural change of the
electricity network. The rising numbers of installed RES and the high fluctuation of power
generation increase the stress on this grid level. To improve the stability, reliability and
efficiency of the DSO grid level, it is necessary to transfer and adapt ancillary service functions
known from the transmission system operator (TSO) to the DSO grid level.
To provide ancillary services at the DSO grid level under high fluctuations, unbalanced grid
conditions and harmonic distortions, a new Multifunctional Energy and Power Server (MEPS)
based on modern power electronic is introduced in this research. The system topology consists
of a series- and a parallel-connected inverter branch. This structure is known as the Unified
Power Quality Conditioner (UPQC) and the Unified Power Flow Controller (UPFC), which
are used in active power filters and in power flow control in electrical grids. The system
approach developed in this work for implementing grid service functions aims to combine the
various approaches for this in a single system.
The series branch consists of an inverter system connected by a transformer in series to the
upstream 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 the
grid and is able to compensate for asymmetrical and harmonic distorted currents. In
combination with a battery system, the parallel branch can also provide active power-based
functions, such as primary control and power fluctuation compensation. All these grid-specific
dynamic control functions are implemented based on symmetrical components (SC) with
individual controller loops for the positive, negative and zero sequences in the fundamental
and harmonic frequency range. To use the SC for real-time control, all measured voltages and
currents are separated into different harmonic components using the heterodyne method. The
combination of the heterodyne method with the SC transformation allows for the individual
and decoupled control under asymmetrical and harmonic distorted conditions.
The simulation and application tests carried out during the research show, that unbalanced and
harmonic distorted voltages and currents can be controlled selectively and in a decoupled
manner. By considering the effective impedance of the grid connection point individually, for
every harmonic frequency under control, allows for a stable operation and good transient
response - also at difficult impedance characteristics, such as found at a 3-leg, 4-wire split
capacitor inverter topology.
Finally, several inverters were connected in parallel to increase the output power for
experiments under real grid conditions. The successful operation of a whole system consisting
of several inverters demonstrates the flexibility and scalability of the approach. The control has
a positive impact on the capacity and stability of the examined grid area by reducing power
fluctuations and unsymmetrical and harmonic voltages and currents. The experiments confirm
the effectiveness of the decoupled control also under real grid conditions.