Gustav Lammert

Dynamic grid support in low voltage grids — fault ride-through and reactive power/voltage support during grid disturbances

Faults in medium and high voltage grids lead to voltage sags in the low voltage level. Due to the voltage dip, distributed generators disconnect from the low voltage grid. Depending on the amount of disconnected generation, system stability could be compromised. With a dynamic grid support, distributed generators remain connected to the grid during faults, also called fault ride-through. Moreover, the feed in of reactive power supports the voltage during the fault. According to the present state of the art, no requirements exist for a dynamic grid support in low voltage grids. However, with a high penetration of distributed generation these requirements might change in the future. This paper investigates the dynamic grid support in low voltage grids. The impact of a dynamic grid support on a distribution grid is studied with IEEE/CIGRE benchmark models for low and medium voltage grids. Models of inverters, directly-coupled synchronous and induction generators are implemented. Furthermore, a present and a future protection system with fault ride-through capability are added to the grid models. The inverters are equipped with a reactive current controller to give a voltage support during faults. To determine the effect of a dynamic grid support, faults in high and medium voltage grids with and without dynamic grid support are simulated and evaluated. The results have shown that the effect of the voltage boost through dynamic grid support in order to recover the voltage is marginal. However, with a high penetration level of distributed generators the voltage boost increases and the impact of faults can be further limited. This investigation has demonstrated the potential of dynamic grid support in low voltage grids to avoid the loss of a large amount of power and to improve the voltage recovery.

Implementation and validation of WECC generic photovoltaic system models in DIgSILENT PowerFactory

This paper presents the implementation and verification of generic PhotoVoltaic (PV) system models, developed by the Western Electricity Coordinating Council (WECC), in the commercial software simulation tool DIgSILENT PowerFactory. The scope of this work is large-scale PV plants connected to the distribution or transmission grid. The general model structure is described and the required modifications made during the implementation process are explained in detail. The implemented PV models were extensively tested and validated against the EPRI written Renewable Energy Model Validation (REMV) tool, that represents the WECC specifications. Moreover, the REMV tool has been validated against real measurements. The dynamic performance of the models was investigated in response to voltage and frequency deviations. The simulation results have verified that the generic PV system models, as implemented in DIgSILENT PowerFactory, match perfectly the REMV tool, and therefore correctly represent the WECC model specifications. Furthermore, the implemented generic PV system models in DIgSILENT PowerFactory can be downloaded from the authors website.

Case study on primary frequency control with wind-turbines and photovoltaic plants

The transition to sustainable power systems with a high penetration of renewable energy sources and distributed generation is one of the major challenges of the future energy supply. So far mainly conventional power plants provide ancillary services such as frequency and reactive power control. With the increasing penetration of distributed generation systems this has to change. In this paper, it is assumed that all generation units in the power system participate on the primary frequency control with a certain percentage of their current generation. For this case, this investigation presents possible approaches to participate wind-turbines and photovoltaic plants on primary frequency control and shows the results of the transient behavior by this control for several scenarios.

Impact of fault ride-through and dynamic reactive power support of photovoltaic systems on short-term voltage stability

This paper investigates the impact of the Fault Ride-Through (FRT) capability and the dynamic reactive power support of large-scale PhotoVoltaic (PV) systems according to the newly proposed German Grid Code (GGC) on short-term voltage stability. The PV system is based on a generic model and realistic parameters are used that are determined in consultation with a manufacturer. In this context, improvements of the generic PV system model are stated. The GGC requirements are compared with several other control schemes for dynamic reactive power support, such as the adjusted control mode, i.e., the active and reactive current is calculated according to the grid impedance angle. The results show that the GGC requirements help the power system to avoid short-term voltage instability. Moreover, the requirements lead to an enhanced dynamic performance in terms of voltage support and recovery. Only the adjusted control mode shows a slightly better performance than the GGC. However, as this control scheme needs on-line information about the grid impedance angle, and therefore, additional infrastructure for the measurements, it is also more expensive. Hence, it can be concluded that the GGC requirements, with respect to FRT and dynamic reactive power support, are reasonable from a technical and economical point of view.

Implementation and validation of the Nordic test system in DIgSILENT PowerFactory

This paper presents the implementation of the Nordic test system in DIgSILENT PowerFactory. The implemented system is a variant of the Nordic32 network and is based on the technical report “Test Systems for Voltage Stability Analysis and Security Assessment” prepared by the IEEE Power System Dynamic Performance Committee, the Power System Stability Subcommittee, and the Test Systems for Voltage Stability and Security Assessment Task Force. This work deems all of the implemented controllers and validates the PowerFactory implementation against the original model presented in the aforementioned technical report. It is shown through RMS simulations that the action of load tap changers and over excitation limiters are leading forces through a long-term voltage collapse and therefore, their accurate modeling is crucial. Furthermore, the implemented system can be whether downloaded from the IEEE PES Power System Dynamic Performance Committee website or requested directly to the authors.