I faced F26 fault on Simovert inverter, upon investigation, I found that the all four IGBTs of red phase were melted whereas IGBTs of yellow phase were found ok. As a precaution, I changed all the IGBTs of all three phases but fault still persisted. Then I changed firing card (6SE7036-5GK84-1JC2) of yellow phase. Fault F26 disappeared but inverter went on F11 fault.

For this, I replcaced the IVI, CUVC and PSU card but fault remained there. I checked the CT resistance and found it in range. As I did not had a reference of CT resistance, but on a similar inverter it was found to be 5.35 kohm.

After that I replaced the CTs and fault was cleared.

My question is that, what possibly can make the CTs faulty provided that there has been no fault reported on CTs for past 10 years. And what possible damage can a faulty CT cause?

CT connections are on the IVI card, which is further connected to firing card, and then to IGBTs... Is it possible that the fault may be transferred from CTs to IGBTs? I mean that if the life of the CTs is the main issue, then my remaining inverters are under threat as no CT has been replaced for past 10 years.
(CT Model: ES1000C, ABB)