Converter Based Power Grids

The nowadays control in the European interconnected grid is based on the physical properties of the synchronous generators in large-scale conventional power plants. With their rotating masses, these generators provide the necessary electrical stability for the power supply. Therefore, a sufficiently high proportion of these so-called »must-run units« must be connected to the grid at any time to ensure a stable system operation. In an energy system based on 100 % renewable energy, power converters must assume all necessary system services, for example, in photovoltaic and wind power plants or in large-scale battery storage systems.

Various types of power plants connected by power electronics, such as PV plants, wind turbines, battery storage systems, but also HVDC converters, STATCOMs and electrolyzers, are suitable for grid-forming. However, since these converter-based plants, in contrast to electro-mechanical synchronous generators, do not inherently possess the corresponding electrical behavior, suitable new control structures must be developed, implemented, and tested. This is the only way to ensure stable grid operation at any time in the future, without large synchronous generators. For this purpose, we are developing novel grid-forming control methods for power converters, which can provide the grid with all system services and stabilize it not only during normal operation but also during major disturbances.

Furthermore, the structure of the power grid will change fundamentally due to the decentralized installation of renewable generators. Therefore, we develop methods for the control and stability analysis of such decentralized converter-based grids of the future. In doing so, we investigate interactions between different power converters as well as between power converters and other grid components.

The verification of these new system services and behaviors is central to a stable, reliable, and resilient grid operation. We develop methods for the verification of the grid-forming behavior of power converters as well as new grid control methods. Furthermore, we test the new behavior and grid services as well as the verification methods at the Multi-Megawatt Lab.

Our R&D-Services in the field of »Converter-based Grids« include:

• Development of Control Algorithms for Grid-forming Inverters
• Dynamic Grid and Plant Simulation
• Development of Generation Units and Plants According to Grid Codes
• Dynamic Reactive Power Supply
• Network Support in the During Failures or Faults
• Design of Protection Systems
• Characterization and Measurement of Grid-forming Inverters
• Grid Codes and Test Guidelines

The control algorithm mainly defines the electric behavior of a converter in response to the grid.

Today a converter measures the grid voltage and synchronously feeds in a corresponding current, which yields the required active and reactive power. This behavior is achieved by a current-based control, which is synchronized with the grid voltage. In order to provide additional system services, further functions based on system parameters are used to generate new setpoints for the current feed-in, such as voltage-supporting reactive current during grid faults, a reduction of the active power at increased grid frequency, or reactive power for static voltage maintenance.

To realize grid-forming properties of a converter, fundamentally new control strategies are necessary. The converters are no longer programmed to behave as a current source but as a sinusoidal voltage source with an internal impedance. As a result, these grid-forming converters provide the required harmonic currents to the non-linear loads in the grid, which leads to an improvement in voltage quality.

Moreover, the control algorithms provide the output voltage with an inertial response that behaves in a manner similar to a synchronous generator. This applies to both the phase angle and amplitude of the voltage signal. Since several parallel-connected voltage sources with inertial behavior result in a very oscillation-prone system, damping elements are also taken into account in order to guarantee stable grid operation.

Since converters have a limited output current due to the semiconductors used, the current must also be limited, otherwise the current can reach many times the nominal current in the event of a fault. The controls are also specially designed to react to different fault conditions in the grid, such as undervoltage and overvoltage, frequency faults (due to a large generation imbalance) or phase jumps, and to stabilize the grid especially in these cases.

We develop these control methods from hardware-based control of the semiconductors, through fast-voltage control with current limitation, up to algorithms for inertia emulation, and, if needed, superimposed active and reactive power. Besides this, we also develop communication interfaces to the grid operator and to other parties, if required.

With the expansion of renewable energy generation, more and more attention is being directed towards these decentralized energy generation plants and their impact on the stability of interconnected grids.

The experts at Fraunhofer ISE create new methods for the generation of dynamic models as well as the dynamic simulation for power converters in the grid, with the aim of accurate simulations with optimized runtime. Both real-time methods and purely software-based methods are applied to achieve the best results to fulfill the objective of the investigation. The simulation tools implemented include, among others, DIgSILENT PowerFactory®, PSCAD™, MATLAB®/Simulink®. For Hardware-in-the-Loop simulations, systems from OPAL RT® are available. These can be used as controler-Hardware-in-the Loop (c-Hil), i.e., using the real control hardware of the power converters, but also as Power-Hardware-in-the-Loop (P-HiL), i.e., using a power amplifier and the complete power converter hardware. We are developing new methods to simulate the harmonics and their propagation in the network and are working on new methods to analyze the stability in converter based grids, such as using impedance-based stability evaluation. We also consider future behavior of plant and systems , such as grid-forming control methods and decentralized generation. Our in-house expertise relies on in-depth knowledge and long-term experience on the structure and behavior of power electronic devices.

In addition, simulations of complete power plants and network sections are carried out to investigate and optimize the converter performance. We perform stability analyses, sensitivity analyses, parameter optimizations and studies on grid integration.

According to various international grid codes, models for grid-connected power generation units (PGUs) must undergo a multi-stage certification process. In Germany, for example, the first step involves type tests in accordance with FGW TG3, in which the system’s electrical characteristics are tested. The second step a model that simulates the dynamic behavior is created. The third step is the model validation, in which the model behavior is compared with the measured data.

The aim of the modeling is to reproduce the interaction between the generation unit and the power grid. On the one hand, the dynamic behavior during faults (Fault Ride Through) is analyzed for symmetrical and asymmetrical voltage dips. In addition, the active power control during changes in grid frequency, the active power limitation, the reactive power provision, and grid protection are simulated.

At Fraunhofer ISE we use our expertise to support manufacturers of power electronic converters such as photovoltaic and battery inverters as well as combustion engines, with the model development. We hold long-standing expertise in simulation, are well-acquainted in the grid-connection guidelines and with testing the guidelines' requirements in the laboratory. We also hold in-depth knowledge of power electronics and the generator-grid system. For these reasons, we are able to develop appropriate models for our customers quickly and cost-effectively, while accompanying the entire certification process.

The power plant controller is used to control the exchange of active and reactive power between the plant and the public utility grid at the grid connection point. Based on voltage and current measurements at the connection point, it sends active and reactive power setpoints to the plant's control system. The evaluation of the operating and control behavior of the plant controller is based on international grid-connection guidelines.

Manufacturers of power plant controllers must create simulation models that reflect their device’s behavior for the grid-connection of photovoltaic, wind, storage or hybrid power plants .  At Fraunhofer ISE, we develop such models for our customers, supporting them throughout the certification process.

Reactive power occurs in overhead lines, underground cables, transformers and motors. On the other hand it can also be applied specifically to maintain voltage bands in power grids. The decentralization of the power system changes the provision of reactive power. As large-scale power plants are shut down, they are no longer available for performing the role of centralized reactive power balancing. According to studies, the reactive power demand will generally increase and at the same time will be subject to greater fluctuations, due to the  variable feed-in from wind and PV power plants.

With the help of inverters in wind and PV power plants and in battery storage systems, reactive power can be provided in a decentralized and highly dynamic manner. In principle, this offers the opportunity to carry out reactive power management in the grid more locally, more quickly and more precisely. However, this requires the development of new and innovative methods for grid planning, grid operation management and power plant control.

We conduct research in this area to create the necessary conditions for innovative reactive power management. To this end, we create simulation models of power grids in cooperation with research and industry partners and use them to perform grid simulations. In addition, we use our metrological expertise to characterize inverters, generators, and other sources of reactive power. In this way, we contribute to maintaining or even increasing the grid quality in an energy system dominated by renewable energies.

The stability of the power grids is of central importance for society and the economy. In order to avoid a large-scale or even European-wide blackout as a result of a local fault in the electricity grid, photovoltaic systems, and, in principle, all distributed generation systems, must remain connected to the grid during short-term voltage dips, continuing to feed in short-circuit current in order to support the grid. At Fraunhofer ISE, we have been dealing with the topic of dynamic grid support for over ten years. We carry out voltage dip tests in our Multi-Megawatt Lab and record and analyze the behavior of inverters and generators with our high-precision measurement technology. We also develop requirements for generation and storage facilities, based on the grid's needs. Increasingly there are other grid faults arising that are becoming important issues to be tackled for grid operators and researchers: These include frequency deviations and intermittent overvoltage events in particular.

Although the hazard due to such faults is not new, the problem itself may become acute due to the increasing decentralization of the power supply system. Thus, our research work focuses on investigating the causes, frequency, and consequences of various grid-fault events. For this purpose, we use our excellent laboratory infrastructure which contains our own high-voltage transformer, various testing facilities and grid simulators as well as our profound modeling and simulation experience in the field of short-circuit and grid modeling.

The expansion of renewable energy is changing the power flow and dynamics in the grid. This also affects the grid's protection systems, or so-called »secondary technology«. Both existing and future protection systems must be able to reliably distinguish between normal operation and fault conditions, even during the highly dynamic and fluctuating feed-in of the distributed generation units. The new challenges concern, among other things, the decentralized provision of reactive power, as well as the changed short-circuit powers and the risk of the occurrence of additional harmonics.

We conduct research on the secondary and protection technology required for the transformation of the energy system. In addition to detailed grid simulations, we have a laboratory center where the results can be validated, and grid situations can be simulated in reality. In this way, we help to ensure that protection technology keeps pace with new challenges and thus ensures reliable grid operation.

At the Multi-Megawatt Lab in the Center for Power Electronics and Sustainable Grids, we measure grid-forming inverters up to the multi-megawatt range.

Here, we investigate the voltage source behavior of the devices, i.e., the provision of a sinusoidal voltage source behind an internal impedance. Other topics that we investigate are harmonic currents for the supply of nonlinear loads and of asymmetric loads.

Furthermore, we investigate the inverter's contribution to system inertia by studying the effective inertia and damping behavior.

Grid faults can cause inverters to reach their current limits and therefore current limitations must be imposed. We investigate the behavior during undervoltage and overvoltage events, phase jumps, fast frequency changes as well as different combinations of these events. In addition to the grid-supporting behavior, stability is also evaluated.

We also investigate possible interactions between the devices. Beside other methods use the differential impedance spectroscopy to characterize the devices.

The unique infrastructure of the Multi-Megawatt Lab also enables us to investigate the behavior of complete microgrids and inverter clusters. Here, we investigate the interactions between different grid-forming inverters and also interactions between grid-forming inverters with the grid itself or other generation technologies, e.g., synchronous generators.

As a result, we can develop and optimize methods used to verify the properties of grid-forming inverters, thus actively contributing to their standardization.

Grid codes are an essential instrument for ensuring secure grid operation with decentralized feed-in. All connected systems must comply with the specified connection requirements. These regulations have undergone major changes in recent years - in Germany and internationally -in order to reflect the increasing decentralization of our energy systems.

During this process, new test guidelines were developed, which define the verification procedure (tests and processes) for inverters and generators.

Through our research at Fraunhofer ISE, we have contributed and continue to contribute our scientific expertise and support to these guidelines. We see ourselves as an independent partner working interactively between grid operators and plant manufacturers. With our measurement work on inverters and generators in our Multi-Megawatt Lab as well as with the modeling of electrical systems, we offer direct assistance to system manufacturers and serve as competent partners for all questions in this field.