European Spallation Source (ESS) - Design and Simulation of the Electric Power Supply System
ESS is a research facility under construction in Lund, Sweden, for scientific research by use of particle accelerators. Currently there are 17 European countries participating in the Steering Committee. In this facility, protons will be accelerated in a pulsed pattern and collide with the heavy metal target. Due to the impact of the protons, intense pulses of neutrons are emitted. These neutrons are gathered and used for experimental stations, where research on materials is done. A total power of 35 - 45 MW is needed when the beam is used, heavily impacting the regional power grid. The 14 Hz pulse pattern of the beam yields a high risk to the stability and security of the transmission grid. To support the design and construction of ESS, the institutes ACS and PGS are evaluating mutual impacts of ESS and the regional grid and innovative solutions for the internal ESS grid and the charging converters. Partners in the project are ESS, E.ON and Kraftringen.
As shown in Figure 1, the real-time simulators RTDS and Opal RT are coupled in a co-simulation framework. The electromagnetic transient model of the grid surrounding ESS is implemented in RTDS. On the other hand, the internal electrical model of ESS, including real-time models for the modulators and pulsers, is implemented in Opal RT. Data exchange between the two simulators is executed with the analog input and output ports of the two systems.
Alike the concept of racks in RTDS, a core is the basic unit for describing the computational resource limitation of Opal RT. Due to the complexity of the ESS grid with to the large number of components and power electronics, the model of ESS is split onto four cores, each being the unit of computational power. Thus parallel technology is applied to accelerate the computation for real-time simulation. The power electronic parts of the linear accelerator, which were modeled using PLECS in the first stage of the project, have been successfully migrated to Opal RT without losing high fidelity. To represent the concurrent behavior of about 100 power converters to supply the accelerator, a controllable dynamic load is utilized.
The models and corresponding real-time co-simulation platform allow an analysis of the mutual impact of ESS and surrounding grid. Moreover, the Opal RT system is open to include models of thermal systems. Thus, electricity and the use of waste heat can be taken into consideration jointly to improve the overall energy efficiency by including the district heating system in Lund in subsequent steps.
One of the main interests of the grid impact evaluation is the flicker problem that arises from standard accelerator topologies available on the market today. The charging process of the main capacitor inside the electric supply system has to be interrupted when a pulse is fired, resulting in a 14 Hz flicker with a worst-case power difference of around 30 MW. Different approaches with passive and active rectification and different power equalization schemes are simulated with the real-time simulation setup to analyze the Total Harmonic Distortion (THD), the flicker in the supply and the power factor.
Within the project also different topologies for the supply of the main capacitor bank were investigated. This includes the detailed simulation of the power electronic charger, where mainly resonant dc-dc converters are used. The main focus of the topology and control analysis is to find converters that directly suppress the 14 Hz flicker internally so that additional external measures are not required.