We at Kuala Langat Power Plant, Malaysia, are facing some problem with ac interferences in the 125 Vdc digital signal of MK-V system. Some terminals that we measured with a multimeter could reach up till 8.5 Vac. Can this high ac voltage cause any damage to the other boards over a long period of time?

Also, may i know what are the specs of the diodes that can be used to filter out the AC interference? As i have found that the dc siganl only carries about 4mA, is this correct?

Thank you.

 

Check the filter capacitor of that supplies circuit. It must be leaking very bad. A DC circuit can only operate normal when riffle voltage is less that 1vac. Good luck.

Hien Nguyen

 

Thanks for the info.. will check it out when the GT is shut down.
Is there any other possibilities that can cause such interference?

Thanks

 

The source of low magnitude AC voltages measured at the contact input terminal boards of Mark Vs is usually induced from wires/cables in close proximity to wires/cables carrying AC voltage (110 VAC to 480 VAC).

AC voltages of the magnitude you are describing are not usually problematic. There are guidelines which should have been followed during construction regarding separation (distance) between wires/conductors carrying various DC and AC voltages. Practically speaking, these guidelines are not followed very closely.

On Mark V panels which use 125 VDC for contact input interrogation, when the magnitude of the AC voltage at the contact input terminal board exceeds approximately 40 VAC, nuisance 'Contact Input nn Failure' Diagnostic Alarms can be annunciated WHEN THE CIRCUIT IS OPEN (i.e., when the contact being monitored is open).

Another source of AC voltage on the DC supply to the Mark V is from the battery charger/DC supply. Some Mark Vs use 24VDC for contact input interrogation, not the typical 125 VDC source. Check the appropriate source depending on your panel's configuration. It's possible that lower values of AC voltage could be more problematic for such panels.