IGBT Modules - Technologies, Driver and Application (Second Edition) - page 124

Fig. 2.50
Principleofmeasuring insulation voltage
Depending on the voltage class of the IGBTmodule, there are different requirements for
the insulation voltage capability, according to the applicable standards. For example, for
industrial applications, EN50178 requires insulation voltages according t
5.1 U
Eq. 2.1
can also be used, on the basis of a different standard and application (e.g.
for traction applications):
V 1000
2 U
⋅ =
Eq. 2.2
Measurement of insulation voltage is deemed successful when there is no electrical
breakdown or flashover in the device under test or, for modules with a blocking voltage
> 1.7kV, when they pass a subsequent partial discharge test. The duration of the
test is defined in relevant standard
2.8.3 Partial discharge
Partial discharge may occur within an insulator due to voids in the insulating material,
junctions between insulating materials or other areas of high field strength due to non-
homogenous fields. These internal partial breakdowns cause erosive destruction or at
least minor damage to the defective surface. Repeated discharges over time increase
the damage (also known as partial discharge breakdown channels), taking it closer to
the adjacent electrodes and eventually leading to a full breakdown or flashover. The
partial discharges are caused by the local electrical field strength, which becomes
stronger than for the surroundingmaterial.
In systems operated with 1.5kV or more, it is advisable to check for the absence of
partial discharges, particularly in the area of traction applications with module voltages
of 3.3kV or more. Accordingly, when designing IGBT modules intended for these
applications, it is important to use only components and processes that are free of
partial discharges. This is why manufacturer's datasheets state the upper limit of the
partial discharge. The limit applies in connection with a specific partial discharge
Test duration during the qualification process is for instance 60s at nominal test voltage level. As a part of the
routine tests of series products the test duration can be reduced to for instance 1swith increased test voltage.
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