New ride-through technology for converter stations

Siemens has now released its 'full bridge technology' as applied to the power electronics of AC-DC converters, and it is to be employed in the latest generation of its converter stations, including two that were ordered in October. This is a contract worth €900 million to build two converter stations, each with a transmission capacity of 2000 MW, for the ULTRANET direct current project.

Transmission system operators Amprion and TransnetBW will implement this, the first of three planned high-voltage direct current transmission (HVDC) links between northern and southern Germany - an important milestone for the country's transition to a new energy mix.

Full bridge circuitry employs full wave rectification as opposed to the half wave rectifier arrangement usually found in AC-DC converters. Its use, says Siemens, makes it possible to resolve faults in the DC sections quickly and flexibly with no need to switch off the system. It can also stabilise the AC grid at the same time.

The former characteristic confers a major benefit by increasing power transmission availability. The use of full bridge technology means that the system can ride through line faults. It allows faults on an overhead DC line to be resolved within the converter, whichkeepsthefaultanditsrepercussionsto a minimum and helps fulfil a basic principle of grid cntrol - that faults in the grid must be repaired as quickly as possible to prevent them from spreading.

Another advantage is what is known as black start capability. This refers to the ability to quickly re-supply a part of the grid with electricity, following a power failure for example, and so avoid longer outages. The insulated gate bipolar transistors (IGBTs) used by Siemens can also act as a generator to help a grid segment that has lost power to recover autonomously, which makes them in effect black start-capable. These add to the fundamental advantages of DC compared to AC transmission, that the transmission capacity can be better controlled and losses on overhead lines are lower than with AC.

“The full bridge technology resolves faults extremely quickly and can reliably prevent grid faults from spreading to a blackout," commented Jan Mrosik, CEO of Siemens' Energy Management division. "With this innovative development, Siemens is providing the technology needed to successfully expand the grid and thus also [help] ensure the success of the transition to a new energy mix."

(Originally published in MPS January 2016)