The growing penetration of variable wind and solar energies fast approaching very high levels in some areas in Europe has been raising concerns over dynamic stability issues and resultant degradation of power quality on the region’s transmission system.
The MIGRATE (Massive InteGRATion of power Electronic devices) project was formed to investigate this issue in 2016, with 11 transmission system operators (TSOs) and other partners from 12 countries and four years on, is now drawing to a close.
The project, with Horizon 2020 support, identified five key objectives, to develop in depth understanding of power system stability issues, develop new real-time monitoring and forecasting solutions, formulate recommendations for the design of protection systems, investigate cost effective mitigation options, and propose new control strategies and management rules for a 100% renewables-based system.
“Power system stability is one of the most important tasks for TSOs as the systems of the different synchronous areas in Europe supply millions to hundreds of millions of customers and instabilities can impact a substantial number of them,” says project coordinator Hannes Munzel, Asset Manager HVDC System and Control at the German TSO TenneT.
Power system stability
Munzel explains that modelling in the project has identified several stability issues that can propagate through networks with an increase of power electronic devices. Among them are a decrease of inertia, lack of reactive power and the interactions of power electronic controllers both with one another and with passive grid components.
For example, analysis of frequency variations has indicated that the main impact from intermittent supply of windfarms is on primary regulation, rather than on system inertia.
“We see these effects becoming an issue when a minimal share of 40% synchronous machines is present or there is 60% power electronic penetration,” he says.
While understanding of power instabilities is one issue, being able to forecast and monitor them is key for managing the grid. Munzel says that wide area inertia monitoring systems and forecasting methodologies have been developed and incorporated in certain demonstration frameworks. In addition, tools for monitoring of sub-synchronous resonances and visualisation of power quality propagation as well as guidelines for warning procedures are available for use by TSOs.
Another impact of the growth of power electronics on the transmission system is a change in short circuit dynamics with the existing protection systems, developed for short-circuit currents generated by synchronous machines, no longer responsive. New protection schemes proposed are based on under frequency load shedding which utilises local frequency measurement and adaptively changes the size of load shedding. Modelling has shown that such protection schemes are able to protect the system from frequency instability, Munzel says.
In addition, new power swing detection methods based on the monitoring of voltage angle differences are proposed.
High renewable penetration
Countries are increasingly experiencing periods of running solely on renewables, i.e. equivalent effectively to 100% renewable penetration, and both the length and frequency of such events will increase as the penetration and the means to effectively utilise it grows. System operation for very high penetrations of power electronics have been addressed for UPS, radial topologies and microgrids, but the transposition of this research work to the transmission system is not possible due to its specific characteristics.
This topic has formed a key pillar of the MIGRATE project. This has resulted in the application of grid-forming converters, in addition to the standard ‘grid following’ devices in use today, which are able to replace the synchronous generators, with recommendations on numbers and placement.
“The large inertia historically provided by synchronous generation is not a goal in itself although a minimum level is needed driven by the stability conditions. These grid forming converters provide a fast response and the flexibility required and such control reduces the control effort compared to ‘virtual inertia’,” comments Munzel.
The economic impact of the MIGRATE outcomes have yet to be released but Munzel says the project provides a clear way forward. “Grid forming converters and the monitoring and forecasting methods we have developed will help to operate the grid in a way which supports security of supply. And with the continuing integration towards a pan-European grid, strong collaboration among the TSOs is necessary.”
Detailed results from the various work packages will be presented for TSOs and others at a roadshow during October and November with the final wrap-up event in Brussels on 4 December.