Impact of reduced rotating mass on the grid operation

Project state
What is the effect on the grid operation if the rotational mass from large generators disappears? Depicted: An air-cooled generator that delivers up to 350 MVA. © Siemens

The electrical power supply in Germany is being increasingly dominated by renewable energy generation systems. This is causing conventional power plants to be displaced. But how is this impacting on the grids? Renewables are being granted a legally secured, priority infeed, which as a result is displacing conventional power plants. In addition to the geographical distribution of the generation centres and the associated power flows, this is also changing the grid dynamics.

Unlike the synchronous generators in mostly fossil-fuelled power plants, renewable generators are often not synchronised with the grid in terms of their frequency. In particular, the electricity produced by decentralised photovoltaic systems and wind turbines is fed in using converter technology. This is reducing the mass rotating synchronously with the grid and, consequently, is also reducing the instantaneous reserve. This reserve describes the rotational energy stored in the rotating mass in the grid. It is converted into electrical energy in the event of a power deficit between the production and consumption. With excess power, electrical energy is in turn converted into rotational energy. The instantaneous reserve is used for providing rapid frequency support and has a significant impact on the grid stability.

Another factor for the grid stability is the voltage behaviour, which is controlled locally by providing positive or negative reactive power. Especially the dynamic voltage control in the transient area is currently carried out by synchronous generators, so that their increasing displacement is in particular expected to have a significant impact in ensuring voltage stability.

Identifying the impact of reduced rotating masses on the grid

The aim of the planned research project is to provide a detailed analysis as to how the reduced rotating mass impacts on the grid stability in the German control block. Here the researchers are incorporating the entire Interconnected Network of Continental Europe with its synchronous frequency. The research project is focussing on investigating the impacts on the frequency and voltage behaviour and on developing and analysing various solutions. Here it needs to be clarified how the frequency and voltage behaviours are influenced by new technologies and how they can contribute to ensuring grid stability. It is intended that the findings shall enable transmission grid operators to identify possible future problem areas and their potential solutions, and at the same time serve as a basis for future necessary developments, requirements and solutions for stable grid operation.

Using a detailed model of the continental European interconnected system, the engineers are therefore initially investigating the expected changes in the grid dynamics behaviour caused by the reduction in rotating mass in the German control block as well as the impact that this reduction will have on the grid. For this purpose, they are selecting characteristic grid usage cases and creating various future scenarios for the development of renewable and conventional infeeds. The scenarios will be based on existing grid development studies and coordinated with the four German transmission system operators.

In a further step, they will develop new control systems and techniques for providing frequency and voltage support, which will supplement the dynamic model for the entire interconnected power system. In particular, the scientists will be analysing and assessing the contributions from wind turbines connected to converters as well as the high-voltage direct current (HVDC) transmission. For the analysis, they are developing reliable criteria for evaluating the transient frequency and voltage behaviour. These include critical limits for the frequency amplitudes and oscillations as well as for the maximum permitted transient voltage deviations. They are also focussing on the rate of change of frequency (ROCOF). Using selected system proposals, they will also identify problem areas and analyse existing solutions. If required, they will also develop new solutions.

The research project was launched in January 2015 and will be coordinated with the German transmission system operators – which are acting as associated partners – for the entire duration of the project. As the first milestones, discussion rounds have therefore been scheduled for the middle and end of 2015 where it is intended to develop concepts for sensible scenarios. In addition, it is also planned to hold a workshop about half way through the project, which will be used to discuss the interim results with representatives from various energy technology fields.

Project duration

01/2015 - 12/2017


Univ.-Prof. Dr.-Ing. Hendrik Lens
Pfaffenwaldring 23
70569 Stuttgart
+49 711 685-66213
+49 711 685-63491

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