We are facing a DC supply problem at our power plant. The DC power of turbine control panel and generator relay panel is going and coming back within a split second which is causing the turbine to trip. We then provided the dc source from a different battery charger and changed the cables.Five days later the same thing again. We have checked the voltages, connection tightening, any looseness or cuts in the wires of the DC circuitry, IR test of cables but all were found good. Even the individual mcbs are not tripping neither the incomer mcb. Everyone is just going nuts. Has anyone ever faced this type of problem?? Any sort of information or help regarding this will be highly appreciated.

 

I've faced this type of problem. Intermittent, short-duration failures are the hardest to troubleshoot for obvious reasons. The best I can suggest from here is to connect a multi-channel recorder at several key points throughout the power supply chain, including the supply of the DC supply. Also re-evaluate your thoroughness. Often what seems to be thorough initially proves not to be later.

 

The following information would be helpful:

1. What manufacturer/model of turbine?

2. This sounds like a battery bank with a trickle battery charger. Is this correct? Have you checked all the cells of your batteries?

3. How does the DC power supply trip the turbine? Powers the trip circuit or component of the trip circuit?

4. What voltage?

5. Is your DC power system grounded? If so, how?

6. Is this a new problem (e.g. after a change in the system)?

7. How often is this occurring?

8. Have there been any changes to the system recently?

9. Do your cables run next to any AC cables?

10. Is lightning occurring in the area when this occurs?

11. Have you placed a monitor on the DC supply (like a storage scope) to look at what the DC voltage is doing during these events?

12. Has the event been visually witnessed? Were the lights on the relay/control panel observed to blink?

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

 

> The DC power <snip> is going and coming back within a split second.

How do you know this is the case? Do all 24V powered controls 'blink' momentarily as power drops?

 

Thank you for your reply. Yes, we have also thought of connecting a recorder.A nd Sir you are absolutely right about the thorough part. We will need to check everything again as we are also not satisfied yet, and I'm very sure that the problem will come again.

 

> The following information would be helpful:

> 1. What manufacturer/model of turbine?
The turbine manufacturer is Triveni turbines. As for the model I cannot recall it right now.

> 2. This sounds like a battery bank with a trickle battery
> charger. Is this correct? Have you checked all the cells of
> your batteries?
Yeah it's a fcbc battery charger. We have checked the voltages of the 55th cell,53rd cell with the negative. It's showing approx 123vdc and 120vdc respectively which is OK. Moreover there are other panels which are running good on this battery charger.

> 3. How does the DC power supply trip the turbine? Powers
> the trip circuit or component of the trip circuit?
The turbine trip ckt is of DC supply. Moreover the turbine vms and governor controller are rebooting when the turbine is tripping.

> 4. What voltage?
> 6. Is this a new problem (e.g. after a change in the system)?
> 8. Have there been any changes to the system recently?
It&#8217;s 120Vdc ungrounded DC system. As for change in system is concerned we had shutdown activities two months before this problem started happening from last week. During the shutdown we installed a new redundant avr panel for this turbine's generator.

> 5. Is your DC power system grounded? If so, how?
The problem has happened twice till date. And yes there are ac cables running next to this DC supply.

> 7. How often is this occurring?
Sir we haven't placed any recorder or monitors as of yet but we have thought of this and we will place it on each of the DC circuit mcb's outgoing,next time the problem comes again. This is the only solution we find right now to pinpoint the problem.


> 9. Do your cables run next to any AC cables?

> 10. Is lightning occurring in the area when this occurs?
Sir in these part of the world where I'm working right now, lightning and rainfalls are a distant dream. It's Saudi Arabia, but no there has been no lightning issues.

> 11. Have you placed a monitor on the DC supply (like a
> storage scope) to look at what the DC voltage is doing
> during these events?

> 12. Has the event been visually witnessed? Were the lights
> on the relay/control panel observed to blink?
The event has been visually witnessed. When the turbine trips, the Bently Vibration monitoring system and Woodward 505E governor controller on the turbine control panel is rebooting. It only happens when power recycling is done. On the grp,80DC supervision relay for DC supply failure is creating alarm and in SOE,DC supply failure is the first alarm to get logged. Even micom P343 on grp is not able to log anything during the trip, which suggests that its power is also getting recycled.

 

>> The DC power <snip> is going and coming back within a split second.

>How do you know this is the case? Do all 24V powered
>controls 'blink' momentarily as power drops?

Sir, in turbine control panel the Bently Vms and Woodward governor controller is rebooting (seen by me during the trip) which are 120Vdc supply powered. On the generator relay panel 80dc supervision relay is creating alarm. Even micom P343 on grp is not logging any alarm. All these suggests that there is a momentary power drop and then power normal again. 24v DC which is through a 110vdc to 24vdc psu are only for signal isolators and Transmitters (on the turbine gauge panel outside in field) which stays fine during and after the trip.

 

The DC power of turbine control panel and generator relay panel is going and coming back within a split second which is causing the turbine to trip.

Do you know this is what is actually happening or is that what you think is happening. It may be that there is nothing wrong whatsoever with the actual DC supply, but might be a problem with the trip circuit.

Does this DC supply get checked periodically with a serious load to make sure the battery stack is OK?

 

First- Check your DC solenoid valve. The coil could be AC rated including the the plunger lift. Make sure the coil is DC rated. Good luck.