6.11 Three-level NPCcircuits
Controlling an inverter with a three-level NPC topology (chapter
, taking into
account the necessary isolation voltage, is exactly the same as for one with two-level
topology. The only difference iswith regard to turning off a short circuit.
In general, the outer IGBTs of a half-bridge should always be turned off first in a three-
level NPC inverter, if thegatedrive does not haveU
clamping protection circuitry. This
protects the internal IGBTs from excessive blocking voltages.
In a three-level NPC topology, there are three different types of short circuit:
Bridge short circuit
Short circuited load
Short circuit at the phaseoutput against earth or DC-bus potential
Bridge short circuits do practically not occur in three-level NPC inverters because all
four IGBTs arenever turnedon at the same time.
Short circuited load is usually a SC2 short circuit, because there is high inductance in
the path. This type of short circuit should be detected via current measurement in the
output phases. If, however, the inductance is small, this short circuit is equal to a short
circuit in the phase output. A distinction is made between a short circuit to earth and a
short circuit to theDC-bus
shows a short circuit to theDC-bus.
Exampleof a short circuit ina three-level NPC design
are turned on and a short circuit current I
flows through both switches to
DC-. If T
were turned off first, the T
emitter would haveDC- potential and the collector
would have DC+ potential. The full DC-bus voltage U
would appear across this