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PT fuses, static excitation, and multiple failed PTs

PT fuses, static excitation, and multiple failed PTs

(OP)
We recently upgraded a dynamic exciter to a static exciter. (We ran a temporary setup of static excitation that pulled from our 480 station service we had 0 issues with this other than the amps being pulled were bordering on a bit much for the transformer the temporary was a rental) We got our new permanent setup which we tapped the 13.8kv bus to run the exciter and field flash from a battery bank. After running for about 3 weeks two fuses blew on the instrument PTs. I initially changed the PT fuses figuring they were quite old and maybe we just had an event. The fuses immediately blew again this pointed to a failed PT.

There are four PTs A, B, and C phase PTs are wired line to ground. The fourth PT is a sync PT it is wired phase to phase. They are 120:1 14400 PTs. The blowing of two fuses at once pointed to the sync PT because it was the only one wired phase to phase on the two phases where the fuses blew. The meggar came out okay it built voltage. For reference we used a meter to ohm across the high side bushings h1 to h2 all showed about 1.5ohms, while the sync PT showed 8.7 gigaohms. I removed the sync PT from the circuit replaced fuses and started the unit(after meggaring the stator to ensure there were no issues). The unit came up no problems, replaced the sync PT then two weeks later it went out again. The PTs are all wired wye wye except the sync whose phase to phase creates a partial open delta. The high side of the A,B,C is tied to the neutral isolated from ground. The low side is grounded.

My suspicions now are we are from somewhere getting ferroresonance so I am going to put a 100ohm resistor 500watt onto across the secondary side of the sync PT to create a load as in general the sync pt has no load on it. The only change to our system was adding the static exciter, the vendor says they have seen numerous installations with phase to phase wired sync PTs with 0 issues. Has anyone experienced anything similar, or have suggestions?

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