Direct connection of renewables to an HVDC grid
DC lost against AC a century back in the “war of currents” – one of the most popular debates in the history of power transmission and distribution technologies. At that point in time, there was a choice to be made between AC and DC – the generation and load centers were at favorable location. AC transmission and distribution (T&D) ultimately emerged due to transformers that allow seamless change in voltage levels for T&D.
Today, the ever growing demand of energy, with large distance between generation and load centers, is poised to change the nature of T&D – Ironically DC seems to be the favorable option recognizing growing need of cable based transmission instead of overhead lines for transporting power. The way power is generated has changed in the past few years due to rapid penetration of renewables located far from the load. There is an upward growing trajectory of the % of renewable generation added compared to overall generation added to the power system. This trend is expected to continue which means furthering the transmission distance and hence HVDC transmission to be the economically feasible option.
Given the geographical location of the renewable resources – it is envisaged that they will be connected to the load center through an HVDC grid based on voltage-source converter (VSC). One example is HVDC grid studies in the North Sea to tap the rich wind resource of the region and to interconnect the U.K. and Nordic pool with continental Europe. Other popular example in North America is Mid Atlantic offshore wind integration in the east coast. The HVDC grid would require a step-up HVDC converter to connect distributed renewable sources. For higher availability and reliable operation, a “dc breaker” would connect the HVDC converter to the HVDC grid since state-of-the-art HVDC converter is uncontrollable during dc short circuit fault. System studies shows that for a 320kV HVDC grid, this dc breaker is required to perform open and reclose operation in less than 5 milliseconds with the capability to break current up to 10 kilo-amps. A commercial dc breaker with such requirement didn’t exist until recently; the research on dc breaker however is a 40 year old topic. Very recently in 2012, one commercial dc breaker with the aforementioned requirement is released. However, a closure look at this product offering reveals its size and weight to be more than one-third of the HVDC converter valve – let alone its additional cost to the entire system. For offshore generation, the real estate on the offshore platform is very expensive. Combining the cost of HVDC breaker and severe penalty due to increase in offshore real estate and weight, may offset the advantage of an HVDC grid compared to a system with multiple point to point HVDC link in which dc breaker is not required.
GE Global Research with the support of ARPA-E (Advance Research Program Agency – Energy) is developing a dc fault resilient HVDC converter system, to enable direct connection of renewable generation farm to the HVDC grid, without the need of a dc breaker. This dc fault resilient technology is a modular solution. It embeds high speed electronic isolation feature within the HVDC converter. Following a dc short circuit fault on the HVDC grid, this technology would allow the HVDC converter to disconnect itself from the HVDC grid in less than 50 microseconds. Reclose operation in few milliseconds would enable continuous power generation without major impact on the load centers. Unlike the state-of-the-art HVDC converter connection to a HVDC grid with a dc breaker, in which dc short circuit fault causes electronic component stress to 6 kilo-amps or higher for few milliseconds, the high speed isolation feature in the dc fault resilient HVDC converter limits the stress level to fractionally higher than the rated current. The dc fault resilient HVDC converter by virtue of its power conversion method shrinks the overall system weight down to about half of the state-of-the-art HVDC power conversion system – meaning huge platform structure cost savings for offshore applications.
GE Global Research has collaborated with North Carolina State University and Rensselaer Polytechnic Institute and currently developing fault resilient modules, components, scaled down HVDC converter and system control methods in a three year project contract started in 2012.
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