IEEE Power & Energy Magazine - May/June 2019 - 88

assuming a dc fault at t = 0 s. a 240-mh series inductor (L dc)
is used. Figure 3(a) shows that it takes approximately 8 ms for
the Vi1 contacts to fully separate. at that instant, Vi3 closes,
injecting an oscillating current, I S3, which facilitates a zero
crossing of I VI1 . when the arc is interrupted in Vi1, the dc
current I dc is transferred to the energy absorber, and voltage
VdcCB abruptly rises to the clipping voltage of the surge arresters, which is commonly around 1.5 p.u. (600 kV). this high
voltage brings a negative voltage across inductor L dc (400-
600 kV), enabling fault current suppression. the fault current
is fully extinguished after an additional 20 ms [Figure 3(b)].
manufacturers commonly specify the time to voltage recovery (8 ms in Figure 3) as the key indicator of dc cB performance.
the peak fault current is specified as the single indicator of interrupting capability (15 ka at 8 ms), which is a key parameter for
sizing the series inductor L dc . Sizing could vary for different dc
cB technologies because of different opening speeds.
the expected energy dissipation by the arresters in this
case (E SA) is around 83 mJ, as seen in Figure 3(d). Figure 3
illustrates fault current interruption in the positive direction,
but this dc cB topology can similarly interrupt dc fault current
in the negative direction. however, because the charge of capacitor C 1 is unipolar, the responses in the negative direction will
be slightly different, and component stresses might be higher.

Hybrid dc CB
Topology
Figure 4 shows a representative topology for a hybrid dc cB,
which consists of the following.
1) included is a main branch that has two 400-kV semiconductor valves, T2A and T2B (one for each direction). For protection in only one direction, just one
valve wou ld b e ne e de d. e a ch valve is similar to
one arm of a (six-arm) VSc hVdc converter. this
branch can conduct load current in a closed state, but
losses would be high with prolonged operation.

2) an auxiliary branch has low resistance and conducts full
load current in the closed state. this branch includes
✔ a low-voltage (lV) semiconductor valve T1, also
called the load commutation switch, which should
have a voltage rating comparable to the closedstate voltage drop of the valve T2 (i.e., on the order
of 10 kV)
✔ an ultrafast disconnect switch S 1, which cannot support arcing (it can open only at zero current) but has
an extremely fast opening speed (significant technological advances have been made recently using
thomson coil drivers, and the opening speed for
320-kV units has been demonstrated as approximately 2 ms)
3) energy absorber Sa, which consists of banks of surge
arresters
4) residual breaker S 2, which interrupts only the arrester
leakage current and whose opening speed is not critical.
L dc is a current-limiting inductor.
the lV valve t1 continuously conducts load current, and it
must withstand fault current in the period before the protection relay sends the trip signal. in practice, this valve consists
of several parallel branches, with each branch having a few
insulated gate bipolar transistors (iGBts) in series. Because
of the continuous conduction stress, this valve requires a
forced-liquid cooling system. the main valve T2 conducts
the fault current for only around 2 ms. therefore, it does
not need liquid cooling. however, the whole hybrid dc cB
should be located indoors (in a valve hall) similar to any
large converter.

Operating Principles
Figure 5 illustrates a dc fault on a 400-kV dc system interrupted by a hybrid dc cB that uses a 76-mh series inductor
(L dc). the fault occurs at t = 0 s , and the current peaks at
15 ka. Figure 5(b) shows that S 1 begins to open when the
current is transferred to T2 and I T1 drops to zero. it takes

IT1

Idc
dc
Source

Ldc

T1

S1

Residual
Breaker
IT2

Auxiliary Branch
dc Cable
T2A

+
-

Vdc

S2
VdcCB

Main Branch
SA

T2B
Fault

dc
Load
+
-

Energy Absorber

figure 4. A diagram of a hybrid dc CB.
88

ieee power & energy magazine

may/june 2019



IEEE Power & Energy Magazine - May/June 2019

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