Download DC circuit breakers

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DC circuit breakers:
an important role in the continuity and
quality of service
The development of DC circuit breakers boosts confidence
in the evolving HVDC market.
High voltage direct current (HVDC)
links are of particular interest to large
networks because they provide an
efficient means of transmitting power
MRTB SCHEME
30
Think T&D///Winter 2008-2009
over long distances without capacitive
or inductive losses. They also make it
possible to improve network stability by
coupling with neighboring networks with
simultaneous decoupling
of frequency and phase.
However, an important
issue with HVDC is current
interruption, a much more
challenging task than
with alternating current.
Direct current interruption
cannot be performed by
a conventional circuit
breaker.
In AC networks the
source drives the current
periodically through zero
(every one-hundredth of a
second for a 50 Hz current), and current
zero is the ideal instant to interrupt:
while the arc power decreases, the
circuit breaker can flush the contact
area from conducting plasma and the
current is permanently interrupted within
20 milliseconds. An analogy of the AC
principle could be represented by a
child’s swing: current zero corresponds
to the highest point where the swing
stops rising and starts to return in the
opposite direction. At this instant there
is no kinetic energy, which is equivalent
to the absence of magnetic energy in
the cables and lines of the AC network
(only electrostatic energy remains). For
DC networks the analogy is somewhat
different, as there is no natural current
zero. It is more like a train that runs at
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FOUR DIFFERENT TYPES OF DC
BREAKER DEVICE
As in any AC substation, commutating
switching devices are needed on the DC
side of HVDC stations. The main current
commutating switches include up to four
different types of DC circuit breaker device
(see figure):
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in case of monopolar operation, it is
used to commutate direct current from
the ground return in the metallic return
(for corrosion and safety reasons, it is
often not permitted to operate an HVDC
scheme continuously at a high ground
current);
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this is used to allow reconfiguration from
monopolar to bipolar operation with
metallic return to monopolar operation
with ground return without interrupting
power; this step is required before the
scheme is allowed to return to bipolar
operation;
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to clear an earth fault in a pole converter
by commutating the healthy pole current
into the electrode line;
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if there is an open-circuit fault on the
electrode line, the NBGS is closed to
provide a temporary earth connection;
it allows the scheme to continue
transmitting power.
MRTB:
Metallic Return
Transfer Breaker;
GRTS:
Ground Return
Transfer Switch;
NBS:
Neutral Bus Switch;
NBGS:
Neutral Bus
Grounding Switch.
constant speed: the only way to stop
the train is to have it roll up a slope
and then block it at still point. In the
DC network, a voltage—opposing the
current flow—has to build up in order
to reach current zero.
Another challenge
DC current commutation is also
challenging. This difficulty can be
illustrated through another mechanical
model: ramping up the DC current to
3,000 A in an overhead line that is, say,
1,000 km long, implies exerting 4.5 MJ.
“This amount of energy is equivalent to
lifting a 1 ton mass up to a height of 450 m,”
explains Wolfgang Grieshaber, research
engineer at AREVA T&D. “When dropped
from this height, the mass impacts the
ground at a speed of about 100 m/s. A
switching action could be compared to
catching this falling mass before it hits the
ground.” In AC networks, typical arcing
times are of the order of 20 milliseconds;
this time elapses when decelerating
the mass over the last meter of fall to
deposit the mass gently onto the ground.
Obviously a breaker alone cannot be used
to commutate the direct current.
So the absence of naturally occurring
current zeros in DC networks means
3
“These devices are arranged in various
ways to cope with every situation,
depending on the customer’s
requirements; they can perform a variety
of tasks such as reconfiguration of the
scheme and protection against faults,
short circuits, etc.,” explains Bruno
Kayibabu, Principal Engineer. The common
duty for all these switches is to commutate
up to full load direct current from one
circuit into an existing parallel circuit;
what differentiates them is the circuits
involved. Though modest components,
they play an important role in the
continuity and quality of service.
DC BREAKER TYPES
Think T&D///Winter 2008-2009
31
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TESTING OF DC CIRCUIT BREAKERS:
ARC VOLTAGE IS THE KEY
The oscillations between circuit-breaker
and LC circuit branches are crucial for
ensuring direct current interruption.
“The faster the current oscillation grows,
the faster the oscillating current is able
to reach the value of the DC current
and create a current zero suitable for
interruption,” says Grieshaber. These
oscillations strongly depend on the arc
voltage behavior, which is related to the
current and the arc length.
3
32
the breaker has to be upgraded with
elements that are able to create an
artificial current zero. “This was achieved
by adding an LC 1 oscillating circuit
in parallel to the AC breaker,” says
Bruno Kayibabu, Principal Engineer,
AREVA T&D.
The DC circuit breaker scheme on page
30 is just such an example with its two
parallel branches, the first being a series
LC circuit and the second a Surge
Arrestor using a Metal Oxide Varistor
(MOV). The switching scenario is a twostep process:
s3TEP)NTHECLOSEDPOSITIONTHECURRENT
flows through the breaker, LC and MOV
branches playing no role. On a trip order,
the breaker draws an arc of increasing
length between its contacts, but the
current flowing through the breaker
(Iarc) remains almost unchanged from
the initial value when the breaker was
closed. As the arc voltage increases,
the arc starts interacting with the
LC circuit: the naturally occurring
fluctuations of the arc voltage initiate
current oscillations in the loop formed
by the breaker and LC branch. The
oscillating current (Ic) increases and
finally exceeds the DC current (IDC) that
is to be interrupted. Now Iarc has zero
crossings and the breaker can interrupt
the current. Arcing time is in the order
of 20 milliseconds.
Think T&D///Winter 2008-2009
s3 TEP 7HEN THE ARC CURRENT IS
interrupted, the energy in the line (or
cable) is still too high, and the line will
continue charging up the capacitor.
When the knee point voltage2 of the
MOV is reached, it starts conducting,
absorbs energy and clamps the voltage
to UMOV. This voltage opposes the
current flow through the MRTB and will
slowly commutate the current into the
metallic return. The duration for this
action is at least an order of magnitude
higher than for the arc extinction.
Consequently, the energy dissipated in
the MOV is the dimensioning factor.
The DC circuit breaker concept that
AREVA T&D has developed is simple
and reliable as there are no active
components included, only passive
ones. “It will allow AREVA T&D to
provide its customers with a complete
set of solutions suitable for any HVDC
transmission grid with high confidence
and reliability,” concludes Kayibabu.
r
An LC circuit consists of an inductor, represented by the letter
L, and a capacitor, represented by the letter C, in series.
1
2
Knee point voltage: The voltage above which the surge
arrester becomes highly conductive.
TO KNOW MORE
@ [email protected]
Wolfgang Grieshaber
Test campaigns were performed to clearly
define the physical fundamentals of
AREVA T&D’s DC circuit breaker solution,
i.e., gain knowledge of the arc voltage
and current as a function of time.
“Then, in order to be able to correctly
dimension our components, the goal was
to configure the system and verify, on
the whole range of current, that the arc’s
dynamic resistance (dUarc /dIarc ) stays negative
enough so that oscillation amplitudes are
sufficient to rapidly interrupt the current
under any circumstances,” explains
Grieshaber.