Benoit Bletterie

On the Stability of Local Voltage Control in Distribution Networks With a High Penetration of Inverter-Based Generation

A stability study of distribution networks with a high penetration of distributed generators (DGs) actively supporting the network is presented in this paper. A possible way of mitigating the voltage rise caused by DGs is the local control of reactive power. Among the different possible options, the Q(U) control (reactive power as a function of the voltage) is one of the commonly suggested solutions. However, the Q(U) control can, under certain conditions, lead to instability. This paper summarizes the results of a stability study conducted on a single-inverter system and on a multiinverter system. It shows that the requirements for reaching a stable operation can easily be met for integrated systems but could be a significant constraint for systems relying on communication.

Unintentional islanding in distribution grids with a high penetration of inverter-based DG: Probability for islanding and protection methods

Unintentional islanding in distribution networks due to the presence of DG is one of the major safety concerns for the grid interconnection of generators. The present paper looks at the current protection and safety requirements and new activities in selected European countries with a focus on inverter based generation. By means of a field study carried out in a real LV network with a high DG penetration, occurrence and persistence of balanced load/generation conditions - which are the basic prerequisite for islanding - are analyzed and the according probability is calculated based on different penetration scenarios protection settings and load characteristics. The results show that under realistic conditions the probability to encounter an island is not negligible. Thus additional protection methods to the standard voltage and frequency monitoring are required in order to detect a loss of mains at the generator and ensure the safety of customers and maintenance personnel.

DG DemoNet validation: Voltage control from simulation to field test

Refinements of the voltage control algorithm for the DG DemoNet concept have been developed extensively over the last years. Consequently the next step will be field tests prior to the deployment. This paper describes the (re-)design of the existing prototype algorithm, offline and online simulation environments and testing of the implementation to prepare the central voltage control unit for the field test. The communication links - PLC and radio link - and the MV grids - ‘Großes Walsertal’ and ‘Lungau’ - impose different challenges for the validation of the voltage controller. During the porting of the prototype to the production implementation, the algorithm has run through a major code revision and re-design, to ensure a more general and modular approach for the voltage control algorithm.

Controlling active low voltage distribution grids with minimum efforts on costs and engineering

Intelligent control approaches for smart low voltage (LV) grids have to meet the future requirements caused by increasing penetration of decentralized generation (DG) from renewable sources and new network participators like electric vehicles. These technical requirements can be solved, but especially in LV networks it is necessary to consider the cost benefit ratio. This paper will therefore focus on active control approaches investigated within the research project “DG DemoNet - Smart LV Grid”. By using an existing Smart Grid metering infrastructure and pursuing the long-term objective of a “plug&automate” solution, the mentioned challenges should be solved with acceptable costs regarding investment, maintenance and operation. In this paper the proposed control concepts for the transformers on-load-tap-changer (OLTC) are in detail discussed along with first experiences from simulations of some of these concepts.

The Limitations of Digital Simulation and the Advantages of PHIL Testing in Studying Distributed Generation Provision of Ancillary Services

There is increasing interest in the evaluation of the capability of power-electronic-interfaced distributed generators (DGs) connected to weak medium-voltage (MV) feeders, to provide ancillary services. Classic simulations using simplified DG models have their limitations and may prove insufficient due to the complexity of adequate modeling of power electronic interfaces. Moreover, conventional testing does not allow the investigation of the real generator with the distribution system interactions. Therefore, a scaled-down physical DG (i.e., inverter and dc source) with exactly the same functionalities can be used to evaluate the network integration of the actual DG, by means of power hardware-in-the-loop (PHIL) testing. In this paper, suitable scaling of the power rating and voltage level of the hardware is performed, and an interfacing approach is proposed that achieves stability of simulations without compromising accuracy. The PHIL tests successfully demonstrate potential problems in the coordination of the on-load tap changer controlling the MV feeder with the voltage controller of the DG, such as recurring tap changes, increased reactive power flows, and opposing actions. Moreover, recurring oscillations of the voltage controller of the hardware model are observed at certain system configurations. These inverter control instabilities, which are not visible in purely digital simulations, demonstrate the added value of employing PHIL testing for current and future power system analysis and testing.

Active contribution of PV inverters to voltage control – from a smart grid vision to full-scale implementation

ZusammenfassungZur Demonstration zukünftiger Smart-Grid-Konzepte werden im Rahmen des Forschungsprojektes MetaPV, das von der Europäischen Kommission gefördert wird, die zusätzlichen Leistungen von Smart Photovoltaik(PV)-Anlagen zur Netzstützung und Erhöhung der Netzaufnahmekapazität untersucht. In dieser Veröffentlichung wird der Begriff der Aufnahmekapazität vorgestellt und erläutert. Die Auswirkungen der erwarteten PV-Entwicklungsszenarien auf Versorgungsnetze werden analysiert. Simulationstechnische Untersuchungen zeigten, dass die Aufnahmekapazität fast vollständig ausgeschöpft wird und dass Netzausbau unabdingbar ist, um die erwartete PV-Entwicklung bis 2020 im Versorgungsnetz aufnehmen zu können. Ferner wird das Konzept von Smart PV-Wechselrichtern eingeführt, die im Rahmen des Projekts entwickelt werden. Solche Wechselrichter stützen durch die Regelung von Wirk- und Blindleistung das Netz in sowohl Standalone- als auch fernüberwachungsgeregeltem Betrieb (übergeordnete Regelung), wodurch die Netzaufnahmekapazität erhöht und der Netzausbau vermieden wird. Eine kurze Übersicht über die nächsten Schritte zur Demonstration wird gezeigt.SummaryFor demonstrating future smart grid concepts the MetaPV project, funded by the European Commission, is investigating the additional services of smart photovoltaic (PV) systems for grid support and hosting capacity extension. In this paper the concept of hosting capacity is introduced and illustrated to analyze the impact of expected PV development scenarios on the distribution network selected for the demonstration. Investigations showed that the hosting capacity is currently almost fully exhausted and that network reinforcement would be necessary in order to host the expected PV generation by 2020. Also, the concept of smart PV inverters, which are under development in the frame of the project, is introduced. Such inverters should support network operation through their active and reactive power control possibilities for standalone and supervisory operations and enable the enhancement of the hosting capacity. A short overview on the next steps towards demonstration is provided.

Improving power quality with coordinated voltage control in networks with dispersed generation

Rapidly increasing share of distributed generation (DG) in distribution networks introduced the need for active distribution network operation. As current distribution networks were not designed to integrate the power generation, the DG introduced many technical challenges in sense of power quality, network planning, protection schemes, voltage stability.... This paper presents the advantages of active approach in distribution network operation. The paper focuses on a voltage quality problem and introduces the coordinated voltage control technique to increase the share of DG in distribution networks and at the same time supply the customers with the required voltage quality.

Active grid integration of distributed generation utilizing existing infrastructure more efficiently - an Austrian case study

This paper presents the technical and economical results for an Austrian case study for grid integration of distributed generation (DG). The current state of the grid segment is shown and its inability to host higher than present amounts of DG is discussed. Consequently, technical solutions to overcome this problem without reinforcing the grid are presented. Different strategies of active voltage control are studied on the basis of extensive simulations using the DIgSILENT PowerFactoryreg software. Detailed economic analyses for different grid segments as well as producer-driven strategies (e.g. local or coordinated voltage control) for voltage control are performed. These approaches are furthermore compared to conventional grid reinforcement measures, showing that investments in new smart grid technologies may even lead to lower overall cost for society.

Analysis environment for low voltage networks

Making the best use of low voltage networks will play an important role the in future, because of the increase of photovoltaic and electric vehicles connected to the households. For better understanding and modeling of low voltage networks, the Power Snap Shot method has been developed. This work describes the principles of the information and communication system to automatically manage the measurement data and analyze the grid models by means of automated load flow simulation.

Optimisation of LV networks with high photovoltaic penetration — Balancing the grid with smart meters

This paper presents the latest results of several research activities in the field of low voltage network optimization in regard to unsymmetrical power infeed from distributed generators. A method for balancing networks with a high number of single-phase generators is proposed. By applying the method, the voltage unbalance can be reduced, the available voltage band can be better utilized and the hosting capacity for distributed energy resources extended.