Ever since the first Gotland HVDC link in 1954, islands have always been an exceptional testbed for HVDC technologies demonstration. The Zhoushan district in Zhejiang province, with more than 1300 islands and excellent wind resources, offers such a testbed in China to demonstrate new HVDC technology.
Back in July 2014, the island complex become the location for the first five-terminal multiterminal HVDC (MTDC) system, the Zhoushan ±200kV MTDC with Modular Multilevel Voltage Source Converters at each of the terminals.
In order to provide a truly multiterminal operation as a network it is important for the MTDC to have the ability to operate under dc and ac side faults and appropriately isolate faulted terminals. The key device to provide such functionality is the dc breaker.
In the end of 2016, the Zhoushan MTDC became the ground for another breakthrough in HVDC tehnologuy with the installation, commissioning and operation of the first 200kV circuit breaker with fast isolation. The new circuit breaker is based on the combination of a high-speed mechanical switch and a hybrid cascaded full bridge module which allows fast extinction of bidirectional fault currents,on the dc side of the HVDC networks. The hybrid dc breaker is capable of isolating and disconnecting the dc transmission line for fault currents up to 15kA at 200kV within 3 ms.
In order to provide a truly multiterminal operation as a network it is important for the MTDC to have the ability to operate under dc and ac side faults and appropriately isolate faulted terminals. The key device to provide such functionality is the dc breaker.
In the end of 2016, the Zhoushan MTDC became the ground for another breakthrough in HVDC tehnologuy with the installation, commissioning and operation of the first 200kV circuit breaker with fast isolation. The new circuit breaker is based on the combination of a high-speed mechanical switch and a hybrid cascaded full bridge module which allows fast extinction of bidirectional fault currents,on the dc side of the HVDC networks. The hybrid dc breaker is capable of isolating and disconnecting the dc transmission line for fault currents up to 15kA at 200kV within 3 ms.
Location of the five-terminal in google map : https://www.google.com.au/maps/place/Zhoushan,+Zhejiang,+China/@30.0124857,122.0165304,9.25z/data=!4m5!3m4!1s0x3452c5c61b6c087f:0xa95ffbbca9c810a6!8m2!3d29.985295!4d122.207216
The Zhoushan MTDC project connects the five MMC stations in five different islands by 134 km undersea cable, the overall capacity is 1000 MW. The MTDC has transmitted nearly 500 MWh since its first operation, and enlarges the ability of
connecting distributed wind farms throughout the islands and improve the reliability of power supply
for all islands.
‘Although VSC-HVDC as a technology has
vast advantages, HVDC circuit breaker technology is a bottleneck in the electric engineering field’ said Jiejie Zhang, the vice director of department
of HVDC, World Power Grid Research Institution, China. VSC-based HVDC transmission networks, especially those based on the half-bridge MMC topology has no
reliable ability to clear faults on the dc-side. Especially in the current implementation of the MTDC project, a partial fault in one converter station results in
blackout in all converter stations leading to a reduced ability in avoiding cascading faults.
VSC-HVDC has significant advantages in MTDC systems as it enables independent control of active reactive power, and is expected to play a significant role in enabling bulk
renewable energy integration, enhancement of flexible transmission systems. The
application the hybrid breaker in the Zhoushan MTDC is an first step towards future DC transmission networks.