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Bulk is beautiful

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Rene Smeets René Smeets
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Bulk is Beautiful
There is no doubt about the rapid increase in renewable energy generation in the world. Mostly, the generation of it is concentrated in remote areas, such as off-shore (wind power in Germany) or in outbacks (wind power in US), desert-like environments (solar power in the Sahara?) or in mountainous regions (hydro power in China). There is no automatic link between renewable and local or de-centralized generation.
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  • Keywords: Energy, Laboratories

Centralized renewable sources are being developed at a very high rate, calling for the need of bulk energy transmission to the load centers of the world. Somebody already coined the one-liner “Bulk is Beautiful”. Indeed, many companies and authorities are studying plans for "overlay" grids, new power systems having very large transmission capacity. In the LCE5 framework of the EU Horizons 2020 program, DNV GL is managing the PROMOTioN project on meshed offshore DC transmission networks. In this project, KEMA Laboratories will demonstrate the testing of DC circuit breakers, needed for such networks. In another example, the European Climate Foundation presented its Roadmap 2050 in which a large amount of new trans-European transmission up to 170 GW of inter-regional transmission capacity is required to guarantee reliability in any of the de-carbonized pathways.
In the US, transmission planning is changing in response to the need, mandate and opportunity to integrate large amounts of renewables into the energy mix. The Joint Coordinated System Plan produced conceptual transmission expansion plans for various scenarios for the Eastern US. In China, several 1100 kV AC backbone in the world are in commercial operation, and India has well-defined plans for a 1200 kV AC grid. Japan has plans to revive its – now dormant – 1000 kV system.

At the other side of the spectrum, there is the "smart grid" development in low-voltage and distribution networks, serving locally generated energy distribution needs.

At the high voltage end, the impact on testing will be significant. Stresses on equipment will increase, because of maximum exploitation of capacity. Short-circuit currents will increase along with greater transmission capacity, interconnectedness and density of generation. Nowadays, short-circuit currents are increasing up to 80 kA, and test-laboratories have to cope with this. New devices, such as current limiters (often based on high-temperature superconductors) that have to limit fault current within the withstand capability of the installation are rapidly coming onto the market – and they need to be tested. In AC systems, typical "long distance" components such as series and shunt capacitor/reactor banks provide new challenges for the operation of circuit breakers. Unlike what is often thought, very small current in capacitive (and inductive) load switching applications are tough challenges to circuit breakers. Especially at the highest voltages, these switching duties cause tremendous transients, potentially dangerous for components, not the least for the circuit breaker itself. Unlike faults, these stabilizing components have to be switched at an often daily basis.
Standards have to be adapted to include the new ultra-high voltage equipment. KEMA Laboratories has demonstrated high-power testing of 1100 kV class circuit breakers by applying a double stage synthetic circuit to reach a recovery voltage of over 2000 kV, right after interruption of current up to 50 kA. This was the first time in the world such breaking power was applied.

At the same time, requirements regarding reliability of equipment are rising. One way to increase reliability is monitoring and/or diagnostics, but questions have been raised on the reliability of the supervising electronic equipment, the interpretation of data and the threshold above which to intervene. Another approach is reliability testing. In order to accommodate for this, IEC has introduced various endurance classes in its standards, e.g. in the circuit breaker standard different classes for mechanical and electrical endurance are defined now. In case of higher than usual stresses, the standard provides test-programs simulating life-time ageing of equipment in an affordable test program.
Users of equipment should be aware of the do's and don'ts of such an approach: it makes no sense to test for electrical endurance in cable systems (with far fewer faults than in overhead line systems) or test a standard circuit breaker for 10.000 mechanical operations. Care must be taken not to inflate requirements beyond reality, which would lead to unnecessary costs.
Standards are like a menu to choose from and not every course should be taken. Too much appetite leads to indigestion.

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