© by SEMIKRON
/ 2017-09-07 /
Technical Explanation
/ SKiiP
®
4
Page 56/73
It is recommended to ramp up the isolation voltage with 10kV/s. Faster ramp up leads to capacitive
leakage currents and could cause an erroneous activation of the isolation test control unit. A slower ramp
up leads to a longer testing time. The start of the test time 1s as specified in the data sheet begins when a
ramp up of the full isolation voltage is reached. The isolation voltage could be switched off without a ramp
down. After finishing the test it must be checked that the DUT is not charged anymore.
Please note:
The isolation test voltage should not be higher than necessary for the application and
stipulated in the corresponding standards.
The isolation measurement is performed in two steps:
•
high voltage isolation test
•
repeated isolation test
The high-voltage isolation test and repeated test of an isolation barrier can degrade the isolation capability
due to partial discharge. During the IVT since the isolation voltage is applied the partial discharge starts
after the voltage goes beyond the partial discharge inception voltage. The higher and the longer the
voltage stress is applied, the stronger the damage of the isolation through the partial discharge will be.
Thus each IVT leads to a potential weakening of the isolation. Partial discharge in the DCB doesn’t
necessarily lead to a weakening of the isolation, because the ceramic substrate tends to be robust against
partial discharge. First of all organic materials (plastic) e.g. circuit boards and compound of transducers will
be damaged.
Since every isolation test may cause a premature damage to the module as a result of partial discharge,
the number of tests should be kept low. If they cannot be avoided, however, a regeneration time of at least
10 minutes must be kept between 2 subsequent tests and the repeated isolation voltage tests should be
performed with reduced voltage. The test voltage must be reduced by 20% for each repeated test.
Please note:
The F-option must be removed during the IVT (mounting instruction on request). Then
the normal test procedure as above described should be performed.
7.7
FRT (Fault Ride Through) - Function
A typical SKiiP
®
4-application is a converter for wind turbines. It is state of the art that turbines must fulfill
grid code requirements for the connection to the electric power system. Parts of this requirements are
known as Fault Ride Through (FRT) or Low Voltage Ride Through (LVRT).
