IEEE Power & Energy Magazine - May/June 2019 - 90

In 2016, a hybrid dc CB rated 200 kV was installed at the
Dinghai Station on the Dinghai-to-Daishan line, which made
the system more reliable and easier to control.
DC CBs Installed in China
Installed Hybrid dc CB in the Zhoushan
Multiterminal System
among the islands of the Zhoushan archipelago, the largest
power-consuming islands include the main island of Zhoushan, followed by the islands of Daishan, Qushan, yangshan, and Sijiao.
the annual load of the Zhoushan power grid reached 780 mw in
2010, and is estimated to reach 2,000 mw in 2020 as a result
of higher demand stemming from economic development.
a ±200-kV five-terminal VSc hVdc project, built by the
State Grid corporation of china, began commercial operation
in 2014. it transmits power from the mainland to the five islands.
this project uses a modular multilevel converter and connects
by submarine cables. During normal operation, the Dinghai
converter station on Zhoushan island provides dc voltage control, and the other stations operate in power control modes. as
a result, the power flows from the Dinghai Station to the other
stations. Due to the lack of hVdc cBs, clearing a dc fault was
originally achieved by slow-acting operations of ac system
breakers. the lack of rapid fault isolation and the inability to
switch circuits caused several technical problems, indicating a
need for improved system security, greater operating flexibility,
higher reliability, and faster system restoration capabilities.
in 2016, a hybrid dc cB rated 200 kV was installed at
the Dinghai Station on the Dinghai-to-Daishan line, which

Yangshan Station
100 MW

Sijiao Station
100 MW

32 km
39 km
dc
Breaker
46 km

17 km

400 MW
Dinghai Station
300 MW
Daishan Station

100 MW
Qushan Station

figure 6. A diagram showing the Zhoushan five-terminal
HVdc transmission system with the installed hybrid dc CB.
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ieee power & energy magazine

made the system more reliable and easier to control (Figure 6).
Figure 7 shows the topology and a photo of a prototype for 200-kV
hybrid hVdc cB. the three branches consist of the following:
1) the auxiliary branch carries nominal current and contains a series that combines an ultrafast mechanical disconnector (uFD) with an auxiliary commutation switch,
which consists of a full-bridge submodule (FBSm) using
high-power-rating iGBts. the FBSms appearing in the
auxiliary branch and main branch adopt the same fullbridge structure. this consists of four press-pack
iGBts and a snubber circuit. the auxiliary branch should
have as few FBSms as possible to reduce the on-state
impedance. therefore, a 2 × 3 FBSm matrix design is
used, featuring low conduction losses and high reliability. to make maintenance easier, each FBSm uses an
integrated modular unit connected by an inductance-less
busbar. a water-cooling system is needed because the
auxiliary branch conducts the long-term load current.
Because the uFD is opened with zero current, the contacts inside take up less space, and its driving mechanism
is lightweight. the compact design is achieved by using a
Vi with a voltage rating of 40.5 kV and current rating of
2 ka rather than an SF6 cB. the maximum separation
time of the breaker must be within 2 ms to meet the faultclearance time requirements of the Zhoushan project. as
a result, a series connection of six Vis is adopted, and
each Vi is equipped with voltage-sharing devices.
2) the main branch interrupts fault currents and consists of
four series-connected modular units. each modular unit
contains 36 FBSms in a series connection designed for
50-kV and 15-ka breaking capability. a cooling system
is not necessary for the main branch because it only experiences the fault current for several milliseconds. the
FBSm topology provides a bidirectional current path and
allows the hybrid cB to interrupt the fault current in either
direction. in addition, the capacitors in the FBSm enable
soft turn-off of the iGBts, significantly decreasing the induced voltage stress and balancing the voltage distribution
across the modular unit during fault interruption.
3) the energy absorber branch is made up of surge arresters
that limit transient interruption voltages (tiVs) and dissipate the magnetic energy stored in the system.
the installed hybrid hVdc cB offers the following advantages:
✔ a small footprint and low height, resulting from the
integrated and compact design (installed at a 200-kV
high-potential platform)
may/june 2019

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