Energy systems are undergoing a major transformation driven mainly by higher shares of distributed generation. With millions of small and fluctuating generators feeding into voltage levels below 132kV, the need to increase the capacity of distribution grids to host distributed generation requires new solutions.
Capacity of distribution grids
The capacity of distribution feeders is defined by national or local grid codes and current practices of distribution system operators (DSOs). However, several factors, such as thermal rating, voltage regulation, fault levels, power quality, reversal power flow and islanding, and protection schemes limit hosting capacity. Many countries have proposed possible methods of overcoming this limitation:
- Changing the topology of the grid, grid enforcement and/or new installations
- Short-circuit current as an ancillary service
- Voltage regulation and reactive power compensation
- Power control of distributed generators
- Adaptation of protection schemes
- Future options such as wide-area control, storage, load management and active elements.
In Germany, the electricity system has been designed with high reserve capacities, meaning many grids can host additional generation. However, for most grids a limiting factor concerning grid capacity is voltage level. On top of this, fluctuations in wind speed and solar irradiation lead to fast voltage changes.
Under these conditions, keeping the voltage within defined boundaries and avoiding flickers becomes quite a challenge. To stabilize voltage and provide reactive power from distributed generators, grid operators in Germany mainly consider two guidelines for compliance to their local grid code:
- The technical guideline from the German Association of Energy and Water Industries (BDEW) concerning the connection of plants to the medium-voltage network; the guideline is applicable to all generators with a capacity of 100kW or higher.
- Compliance with the VDE network-connecting regulation, VDE-AR-N 4105, is mandatory for all generators with an installed capacity below 100kW.
The German Renewable Energy Act of 2012 requires all distributed generators with a capacity higher than 30kW to participate in the feed-in management of the distribution system operator, who can then reduce active power by remote control in case of grid stability problems. In August 2014, the new Renewable Energy Act became effective, enhancing participation of distributed generation in the market and encouraging a reliable forecast of generation. New European network codes prepared by the European Network of Transmission System Operators for Electricity (ENTSO-E) are currently in the process of becoming European law. In its “Ancillary Services Study 2030” the German Energy Agency, dena, says that the very high penetration of distributed and renewable resources requires a new systemic approach for the development of the whole energy system over all voltage levels.
Pilot projects with grid operators and academia have resulted in innovative solutions from ABB to operate and control distribution grids with high shares of distributed generation in Germany, starting with smart planning.
Smart planning in Aachen and Duisburg
Despite the fact that voltage regulation is widely acknowledged as an economic solution to modernize the grid, implementing it in standard planning and operation is not so straightforward. For many DSOs, knowing when their grid will reach its operating limit is a challenge because they do not know the time, size and type of requests made to their grids. After the introduction of the Renewable Energy Act in Germany, many grid operators were overrun by a very high number of private requests to connect generators with a short response time.
To overcome this barrier and to enable quick decisions, ABB has developed the “smart planning” approach, which essentially transforms an existing low-voltage grid into a smart grid step by step according to the current requirements . The grids are first classified using a few structural features, such as the number of housing units and points of common coupling, the radius of the secondary distribution grid, and penetration of photovoltaic systems (PV) in the grid.
If distributed generation doesn’t reach a critical point, the request for connection can be granted without further network calculations. A grid is classified as potentially critical, then proceeds to the observation phase where the voltage level in the secondary substation is measured. By using the grid’s fingerprint (taken by measurement determination or a grid calculation) as reference, the voltage level of the local grid is estimated. It has been validated in various real grids that the estimated (fingerprint-based) voltages at the critical point in the feeder and the actual measured values in the various distribution grids only differ by a maximum of ±2V (less than 1%). If, during this phase, the grid reaches the maximum permitted voltage limit, the respective secondary substation has to be extended in the next phase with, for example, a voltage regulator or a voltage-controlled distribution transformer.
This then leads on to other solutions that have been developed, which will be outlined in the second part of this article.
 A. Slupinski et al., “Neue Werkzeuge zur Abschätzung der maximalen Spannung im Niederspannungsnetz” (New tools to estimate maximum voltage in the low-voltage grid), in Proceedings of VDE-ETG Congress, Berlin, 5–6 November 2013, ISBN 978-3-8007-3550-1 VDE Verlag.
This article was prepared by Britta Buchholz, Martin Maximini, Adam Slupinski and Leyla Asgarieh from ABB Power Systems Consulting in Mannheim, Germany.