Checking Ripple Voltage AC and DC Voltage to MKV


Thread Starter


Dear Friends

I would like to know how to check ripple voltage of AC (110V) over DC (125) Voltage of Mark V panel. By Fluke (RMS) Multimeter and by Oscilloscope. what is allowable range?

What are the problems arising if there is more ripple voltage in supplied DC voltage?

Power Island

I have successfully used both a Fluke True AC RMS multimeter (in AC mode with respect to ground) and an oscilloscope to detect AC ripple on the 125 VDC.

The effect of excessive AC ripple on the 125 VDC being used in the Mark V is that, usually, there will be a LOT of 'Contact Input' failure Diagnostic Alarms, particularly on older Mark V turbine control panels with older TCDA cards.

The ripple can come from several places. The output filter capacitors of the 125 VDC battery charger fail over time and need to be replaced. AC ripple is a sign of failing battery charger output filter capacitors.

Incorrect cable installation (running cables/wires sensing contact changes of state (even though the excitation/wetting voltage is 125 VDC!) in cable trays or conduits for long distances with AC motor power wiring is another. Most wires/cables used for contact sensing are NOT shielded, and ripple can be induced when high-current motor leads are alongside these sensing cables/wires for long distances (in cable trays, cable trenches, or conduits). A lot of time contractors tried to take shortcuts by running the exhaust frame blower pressure switch wiring in the same conduit as the exhaust frame blower motor power leads, for example. Same for turbine compartment vent fan pressure switches.

Another possible cause of AC ripple on the 125 VDC power is if GE <DACA>(s) is(are) used. They are horrible for magnifying any spikes or ripple or dips on their incoming AC power on the outgoing DC leads. Many sites used inexpensive inverters instead of clean UPS power sources to supply AC to the <DACA>s and that has caused LOTS of problems over the years.

Finally, many Mark Vs were supplied with <CPF> modules (Conditioning Power Filter) in a VERY difficult place to get to--under <QD1> and behind all the field wiring in the bottom of the Mark V panel. The filters and varistors (MOVs) in the <CPF> module can be damaged by voltage spikes (from the <DACA> even!) or lightning strikes or battery charger problems, or even severe 125 VDC ground problems.

The general maximum limit for AC ripple on the 125 VDC wiring is 40 VAC; anything around or above that will usually cause nuisance Diagnostic Alarms, particularly the 'Contact Input Failure' Diagnostic Alarms.

Hope this helps!

Power Island

Dear CSA,

I would like to say "Thanks" for your quick and detailed reply and giving your precious time to my question. I am sure now I will try to find best solution at one of our turbine site.

Best Regards
Power Island