Mitarbeitende

Design of the PQ-controller for the Grid-Side Converter

The transformation of the electrical energy supply in Germany, commonly referred to as the energy turnaround, is characterised by the transition from a system with central generation, within large power plant units with approx. 1 GVA of electrical power output, to a system in which the generating units are low-powered and decentralised. These small and medium-sized generating units (wind power, photovoltaics, CHPs) and storage systems with connected loads of up to approx. 10 MVA are almost exclusively connected to the medium and low-voltage grid via power electronic systems (inverters, in particular „Active Infeed Converter“).

The design of the controller for the grid side converter(GSC) in a way to enable fast dynamics and efficient coordination between flywheel storage system (Schwungmassenspeichern-SMS) and the grid is one of the main design challenges of the KoREV-SMS2 project (See below for details). My work is focused on the design and cost-optimal control of the grid side converter that ensures the stability and the stationary accuracy of the references in all possible grid conditions while mitigating the total harmonic distortion of grid current (THD)Ig to a minimum; and the load flow coordination between the grid and SMS to ensure efficient operation of the SMS.

The PQ-controller for the GSC will be designed to work:

1- In worst-case scenario of up to 5% of the THD of the grid voltage

2- Up to ±10% of the fluctuation of the magnitude of the grid voltage

3- Such that injected current into the grid does not exceed beyond the THD limit set by VDE-AR-N 4100.

4- Such that the controller is able to synchronize the generated voltage to the grid voltage within ±0.2 of the frequency fluctuation

Different control schemes will be investigated and compared against each other for the most optimal and energy efficient operation of the GSC. Simulations and experiments will be carried out for the potential control schemes. The 400V distribution grid will be connected to the DC link via two 3-Level converters fed by three phase wye-delta-wye three winding transformer with 30 or 60 degree phase shift between the two corresponding secondary windings.

Controller Implementation on SoC Board

The GSC controller will be implemented on Zynq UltraScale+ MPSoC ZCU102. Real-time execution capability of the controller cannot be compromised as security and robustness of the controller is to be ensured in grid applications. Model-based development workflow of MATLAB will be utilized to implement the controller inside FPGA. The design technique allows you to develop more complex software in less time with a disadvantage that the generated code might contain overheads.

Project KoREV – SMS2

Cost Reduction with simultaneous increase in availability and efficiency of flywheel storage (SMS) in outer rotor design

The energy content of the system is linearly dependent on the inertia of the flywheel and quadratically on the angular frequency [E_kin=1/2 J_m ω^2]. Therefore, the energy content can be increased in a targeted manner via high speeds and centrifugal masses with a large moment of inertia. The efficiency of the flywheel kinetic storage system in the KoREV-SMS2 is increased using multiple different techniques that are summarized below;

Using carbon-fiber/fiber reinforced plastic composites which have specific energy [Wh/kg] around 5 to 7 times greater than steel and titanium and tensile strength [M Pa] around 1.5 and 2 times greater than steel and titanium respectively. Therefore an increase in the load on the flywheel and higher circumferential speed of the outer rotor is possible.

The use of Active Magnetic Bearings (AMB) and enabling vacuum between the outer rotor and stator will decrease the mechanical losses and the ageing of the system. The maintenance cost will be drastically reduced. Speed limits of the flywheel will also increase.

The use of AMB to accurately sense the rotor position is researched. It will enable the sensorless control of the motors thus decreasing the costs of the SMS.

PMSM and SynRM (Sync. Reluctance Machines) which have higher efficiencies will be utilized. Developing specialized converter concepts in control and design topology, for the motors, to enhance the coordination for optimal operation and lower the overall power losses including the CM/harmonic losses.

Design and selection of the optimal topology for the GSC and the design of the PQ-controller for achieving lower harmonic losses, stable and stationary accurate reference active and reactive power tracking.

Load flow coordination between the grid and flywheel storage system controller to minimize the system losses and cycle-dependent/calendar ageing.

Project focus at the Institute of Power Electronics and Control of Drives

LEA is leading 3 of the 11 sub-projects which are elaborated below;

Dynamic Load Flow Coordination

For the provision of highly dynamic power for grid control, the existing energy management system for energy exchange between the SMS and grid via the DC link has to be fundamentally revised. The goal is to identify a suitable control structure that performs coordination between the grid-side and machine-side setpoints in an optimized manner. Instead of the grid-side and machine-side converters to react on the voltage of the DC link, the flywheel control system will coordinate with individual converters to react faster to the power demands of the grid. The provision of requested active and reactive power shall take place within a few milliseconds, which will contribute to the primary control reserve and therefore stabilization of the grid.

Design and construction of machine and grid converters

Investigation of how losses in the electrical machine can be minimized by selection and design of the power electronics. Potential topologies for the machine side converter and the GSC will be selected and simulatively designed, investigated and compared for overall efficiency and cost.

Sensorless control of the electrical machine

An encoderless control method is being developed for the electric machine in the flywheel mass storage system. This method shall be applicable for a synchronous machine with permanent magnets as well as for a synchronous reluctance machine.

Project partners and funding

The joint partners are the Institute for Mechatronic Systems in Mechanical Engineering (IMS), the Institute for Power Electronics and Drive Control (LEA), both TU Darmstadt, Adaptive Balancing Power GmbH (ABP), compoScience GmbH (CS) and KEBA Industrial Automation Germany GmbH (KEBA).

The project is funded by the Federal Ministry for Economic Affairs and Energy.