Unlike MOSAs being
used for overvoltage protection conventional
AC and systems, where they suppress over
voltages and direct current ground, MOSAs
in MVDC circuit breakers must absorb much
higher energy from the network after fault current
interruption.
§ The energy absorption branch typically
uses metal oxide surge arrester for energy
absorption during the current breaking
process. MVDC applications, this
is usually based single vacuum interrupter.
§ The current commutation branch uses a
pre-charged capacitor, magnetic coupling
commutation module solid-state switch to
realize current commutation and establish
the breaking voltage. This requires much greater heat
dissipation. Following fault clearance, no-load
disconnectors mechanical breakers can isolate
the faulted section.33
MVDC technologies and operational considerations
Achieving these outcomes requires more
complex design than circuit breakers.3.
These converters block limit the fault current
contribution from the power source the fault
location, thereby protecting the semiconductor
switches within the converter well the
associated grid.
For MVDC applications, modular multilevel
converter (MMC) topology particular interest
due its scalability and handling harmonics
and faults.
4.1 AC-DC voltage conversion
As shown Figure 4-2, for conversion from AC
to (or vice versa), there are wide variety of
options.
4.
Figure 4-1 Generic MVDC circuit breaker sub-
systems
.1.
Such converters require additional power electronic
components the submodule level, leading to
increased costs and conduction losses.2 Breaker-less fault current protection
The need for MVDC circuit breakers can be
avoided utilizing fault-blocking converters. While conventional MOSA applications
for overvoltage protection are very mature and well
standardized, the design, selection, and testing
of MOSAs for circuit breakers still require
further investigation leading the development of
appropriate standards.2.2 Voltage conversion
One the key components MVDC power
system the converter, which changes voltage
levels and/or switches between and DC.
MVDC converter stations equipped with fault
ride-through (DC FRT) capabilities can withstand
and clear faults within tens milliseconds without
the need for energy absorption dissipation
branches, thereby achieving fast fault isolation.
Almost all proposed MVDC circuit breaker
technologies use metal oxide surge arrester
(MOSA) technology realize the energy absorption
branch shown Figure 4-1. Broadly,
MVDC circuit breakers are based around three
individual sub-systems, shown Figure 4-1:
§ The continuous current branch, which
carries the load current and (other than
in semiconductor breakers) consists a
mechanical switch.
4
