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Z. Lu and G. Zhu
In China, flexible DC transmission projects have been constructed in Nanhui
(±30 kV, 18 MW, 2011), Nan’ao (±160 kV, 200 MW, 2013), 5 terminals of Zhoushan
(±200 kV, 400 MW, 2014) and Xiamen (±320 kV, 1000 MW, 2015), which have
been running in a stable manner and gained valuable experience in construction and
operation. The Zhangbei DC power grid (±500 kV, 3000 MW) that has been fully
connected also represents the multi-terminal flexible DC transmission network with
the highest voltage class and the largest capacity in the world. The design of the
hybrid DC network under construction in Wudongde (±800 kV, 5000 MW) has also
been upgraded. Both projects are expected to be commissioned in 2020.
Compared with traditional HVDC transmission, flexible DC boasts the following
advantages:
(1) Reactive power is not required on the AC side with no commutation failure.
For traditional HVDC transmission, conversion stations need to absorb tremendous reactive power, which calls for many more devices for compensation;
what’s more, traditional DC transmission relies on voltage support of AC system
during commutation. Inadequate support might result in commutation failure—a
problem that would never occur in flexible DC.
(2) Flexible DC transmission technology can operate in 4 quadrants and control
active and reactive power independently, providing power supply to passive
networks. Flexible DC transmission can be used as STATCOM application to
compensate reactive power on AC side and stabilize its voltage. But traditional
DC transmission can operate in 2 quadrants only without independent control
of active and reactive power.
(3) Minor harmonic content requires almost no filters due to the fairly high switching
frequency of flexible DC transmission.
In view of these advantages, flexible DC transmission technology is mainly
applied in the following scenarios:
(1) Massive power transmission and networking of AC/DC systems.
(2) Grid connection of distributed power.
(3) Grid capacity expansion and DC power supply. The less harmonic content
during the operation of flexible DC transmission system enables quick control
of system-wide power to upgrade power quality; less floor space is required for
flexible DC transmission conversion stations than traditional ones, hence more
land saving and less waste; furthermore, the ability of the system to control the
current on AC side as needed makes control of short-circuit capacity possible.
(4) Power supply to weak systems or isolated islands. No voltage is needed from
the outside for flexible DC transmission systems during commutation, and the
system could be run in passive inversion and the passive network can be used as
a receiving system to ensure stable power supply to remote areas. But reliability
remains a challenge for overhead flexible DC transmission lines.
The development of high-voltage and large-capacity flexible DC transmission
projects entails urgent measures to enhance the capacity, voltage level and reliability
of power electronic devices. More efforts are needed to slash the cost and facilitate
Z. Lu and G. Zhu
In China, flexible DC transmission projects have been constructed in Nanhui
(±30 kV, 18 MW, 2011), Nan’ao (±160 kV, 200 MW, 2013), 5 terminals of Zhoushan
(±200 kV, 400 MW, 2014) and Xiamen (±320 kV, 1000 MW, 2015), which have
been running in a stable manner and gained valuable experience in construction and
operation. The Zhangbei DC power grid (±500 kV, 3000 MW) that has been fully
connected also represents the multi-terminal flexible DC transmission network with
the highest voltage class and the largest capacity in the world. The design of the
hybrid DC network under construction in Wudongde (±800 kV, 5000 MW) has also
been upgraded. Both projects are expected to be commissioned in 2020.
Compared with traditional HVDC transmission, flexible DC boasts the following
advantages:
(1) Reactive power is not required on the AC side with no commutation failure.
For traditional HVDC transmission, conversion stations need to absorb tremendous reactive power, which calls for many more devices for compensation;
what’s more, traditional DC transmission relies on voltage support of AC system
during commutation. Inadequate support might result in commutation failure—a
problem that would never occur in flexible DC.
(2) Flexible DC transmission technology can operate in 4 quadrants and control
active and reactive power independently, providing power supply to passive
networks. Flexible DC transmission can be used as STATCOM application to
compensate reactive power on AC side and stabilize its voltage. But traditional
DC transmission can operate in 2 quadrants only without independent control
of active and reactive power.
(3) Minor harmonic content requires almost no filters due to the fairly high switching
frequency of flexible DC transmission.
In view of these advantages, flexible DC transmission technology is mainly
applied in the following scenarios:
(1) Massive power transmission and networking of AC/DC systems.
(2) Grid connection of distributed power.
(3) Grid capacity expansion and DC power supply. The less harmonic content
during the operation of flexible DC transmission system enables quick control
of system-wide power to upgrade power quality; less floor space is required for
flexible DC transmission conversion stations than traditional ones, hence more
land saving and less waste; furthermore, the ability of the system to control the
current on AC side as needed makes control of short-circuit capacity possible.
(4) Power supply to weak systems or isolated islands. No voltage is needed from
the outside for flexible DC transmission systems during commutation, and the
system could be run in passive inversion and the passive network can be used as
a receiving system to ensure stable power supply to remote areas. But reliability
remains a challenge for overhead flexible DC transmission lines.
The development of high-voltage and large-capacity flexible DC transmission
projects entails urgent measures to enhance the capacity, voltage level and reliability
of power electronic devices. More efforts are needed to slash the cost and facilitate