Dynamic braking systems for offshore HVDC transmission
Options and challenges
Company / Unternehmen: Alstom Grid – Power Electronics Activity
Speaker / Referent: Colin Davidson
Position: R&D Director, HVDC
HVDC Systems for connecting off-shore wind farms to the land-based AC grid are becoming an important market sector. Such systems have some unusual constraints which are typically not present in conventional, land-based HVDC systems. One such constraint is that since such HVDC systems are normally arranged to be the only means of evacuating power from an offshore wind farm, the offshore wind turbines may be vulnerable to over-speed damage in the event that the receiving (on-shore) AC system suddenly becomes unable to accept the power infeed, for example because of a fault. Although wind turbines normally include protections to deal with such an event it is becoming common for purchasers of the HVDC systems which connect wind farms to the shore, to specify that the HVDC system itself must be capable of dissipating the entire generated power of the wind farm for a short period of time so that the offshore wind turbines can ride through an onshore AC system fault with minimal disturbance. Such systems usually take the form of a braking resistor connected to a power electronic switch, located on the DC side of the onshore converter station, and are frequently referred to as a DC chopper. However, this name is misleading and Dynamic Braking System is considered preferable. A classic buck DC-DC converter can be used for such a dynamic braking system, but when scaled up to the levels required for HVDC, this requires an IGBT valve and a diode valve which each contain hundreds of semiconductor devices in series. The current and voltage in the circuit exhibit very fast step changes which can result in high levels of conducted and radiated EMC. In addition, the IGBT valve requires special IGBTs of limited availability. However, there are many other alternatives to the chopper circuit, which are capable of achieving better EMC performance. This paper will discuss the advantages and disadvantages of various possible alternatives.