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DNV GL, TenneT and ENGIE prove soft energisation system restoration for high-voltage grids

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Network Restoration
Reliable, safe and fully controlled method tested successfully on 220 kV network in north Netherlands
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ARNHEM, Netherlands - 15 September 2016 

DNV GL, TenneT and ENGIE have announced the successful completion of the trial of grid system restoration using soft energisation in the Dutch high-voltage grid. Fast and safe, soft energisation reduces the chances of triggering follow-up blackouts and ensures more reliable system restoration. It can be applied to networks of any size with a relatively small generator without electrical stress or risk of damage to components.

After a blackout it is critical that power is restored as quickly, reliably and safely as possible while minimising the stress on network components. Traditional processes for restoring power involve a dedicated black start power plant supplying electricity again to the various network components such as overhead lines, cables and transformers. However, this approach can lead to voltage oscillations that can damage network components or trigger protection devices initiating another shut down.

As part of an effort to increase the level of black start (system restoration) power in TenneT’s network in the northern Netherlands, TenneT, DNV GL and ENGIE developed a new system restoration method for the 220 / 380 kV high-voltage grid based on soft energisation. In this approach, once the network is prepared, the generator terminal voltage at the power plant is increased in a carefully controlled manner from 0 kV to the rated value. As a result, all network components are energised gradually and simultaneously, reducing the electrical stress on network components and substation assets.

The project was developed to use ENGIE’s black start facility at the Bergum power plant and involve ENGIE’s power plants in the Eemshaven area, 72 km to the north-east. Due to this distance and several complicating factors, the project faced several challenges to energise the 220 kV / 380 kV grid between Bergum and Eemshaven and start a large power plant. With an appropriate mitigation measure, soft energisation was found to be the most suitable method of system restoration for this grid.

“The trial showed that soft energisation is a reliable process to energise our network after a blackout, especially if limited generation capacity is available,” said Paul van den Heuvel, System Operations specialist at TenneT.

“The approach causes no stress on our assets of the black start generator and associated equipment, and allows us to consider using also rather small generators for system restoration. That gives us more flexibility,” added Siep Borneman, Electrical Equipment expert at ENGIE.

Evaluation of the soft energisation process included load flow calculations and transient simulations, followed by investigations into how to apply soft energisation in this specific network. Soft energisation can be used to energise any network with limited feasibility studies beforehand. Because it causes no transient / oscillation phenomena, complex transient studies and impact assessment are not needed. With this methodology, one rather small generator can safely energise the whole network as the process is fully controlled and oscillations are prevented. 

The approach can be downloaded here.