Battery ground alarm

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Thread Starter

KHAIRI

hello teachers
I have battery ground alarm on our turbine mark vi what does it mean this alarm and what effecting and how could i fix that
 
KHAIRI,

Battery ground alarms have been covered several times before on control.com. (There is a 'Search' function cleverly hidden at the far right corner of the Menu bar at the top of every control.com page. It's highly recommended that you use the Search 'Help' first.)

The Mark VI is typically powered by a (nominal) 125 VDC battery and the battery supply is to be ungrounded so that if a ground should develop on one leg or the other of the 125 VDC system the turbine will continue to run until such time as the ground is resolved <b>OR</b> if a ground develops on the other leg of the 125 VDC system then the turbine will trip and there will be lots of sparks and blown fuses and tripped circuit breakers.

Think about a DC system where one leg, let's say the negative leg, was grounded. If one or more grounds developed on the negative leg of any circuit connected to the battery they would not matter and would never be detected--because the negative leg is already grounded. HOWEVER, if a ground were to develop on the positive leg of any circuit connected to the battery that would be the equivalent of a short (a "dead" short if the ground were severe enough) across the battery terminals--and all sorts of arcs and sparks and blown fuses and tripped breakers would quickly ensue (along with a turbine trip).

So, GE control system designers use an ungrounded battery to power the Mark VI. (Sometimes there is an AC-DC converter which is used as a back-up to the battery, or even as the primary source for some applications (usually steam turbines, not gas turbine). But the Mark VI power supplies require 125 VDC for operation.

125 VDC is also used by most Mark VI gas turbine control systems for powering the discrete input circuits (contact inputs; from temperature switches, limits switches, etc.), and for powering 125 VDC solenoid circuits (most of the solenoid-operated devices on a GE-design heavy duty gas turbine use 125 VDC for solenoid coil power). So, this means that a ground could exist anywhere on any contact input circuit or any solenoid output circuit.

Further, most of the fuel stop (trip) valves are controlled by 125 VDC solenoids which are connected to the <P> core through the TREG and TRPG cards, so a ground in any of those circuits can also cause a 125 VDC Battery ground alarm.

Still further, the same 125 VDC battery is usually used for the Emergency L.O. Pump, and any other DC motors on the gas turbine and its auxiliaries. So a ground in any of those circuits would also show up as a 125 VDC battery ground alarm on the Mark VI.

Even further still, often the 125 VDC battery is used for the fire detection and protection circuit(s) so any ground in that circuit will be detected by the Mark VI and annunciated as a 125 VDC battery ground.

Lastly, there can be other uses of the 125 VDC battery in a plant--the same 125 VDC battery that powers the Mark VI and the discrete inputs and the solenoid outputs and the fuel trip solenoids and the DC auxiliary motors. And, any ground in any of those other circuits will also be sensed by the Mark VI and will cause a 125 VDC battery alarm.

By now it should be clear: The Mark VI is monitoring the entire 125 VDC system that is being used to power the Mark VI--not just devices connected to the Mark VI! So, a ground in the transformer protection panel--if it's powered by the same 125 VDC battery as the Mark VI--will be detected by the Mark VI and annunciated as a 125 VDC battery ground alarm.

The other HUGE misconception--in addition to the one above that ONLY devices connected to the Mark VI that develop a ground will be annunciated as grounds by the Mark VI--is that ANY device connected to the Mark VI can also cause a 125 VDC battery ground. That's not true. The thermocouple inputs to the Mark VI can be grounded or ungrounded; the RTD inputs can be grounded or ungrounded; milliamp inputs to the Mark VI can be grounded or ungrounded; servo-valve outputs can be grounded (they shouldn't be--but if the are, it won't result in a 125 VDC battery ground alarm); even LVDTs and speed pick-ups can be grounded (they shouldn't be--but they can be) and it won't result in a 125 VDC battery ground. ONLY devices which are connected to the 125 VDC battery supply (either through the <PD> Power Distribution core, or via breakers in the 125 VDC Distribution Pane) can cause 125 VDC battery ground alarms when they become grounded.

You can take a voltmeter (set to DC, minimum 200 VDC), and measure the voltage of the incoming 125 VDC to the <PD> core at PDTB-1 with respect to ground, and also at PDTB-3 with respect to ground. When there is NO ground on any circuit connected to the 125 VDC battery the voltages will be nearly equal in magnitude (but opposite in polarity). If there is a ground on either leg of the 125 VDC system then the voltage with respect to ground which is LOWEST is the leg of the system which has or is developing a ground.

The Mark VI usually doesn't alarm until the magnitude of either leg of the 125 VDC with respect to ground drops below approximately 35 VDC. The "harder" the ground, the closer to 0 VDC the voltage of that leg with respect to ground will be. The voltage on the other leg (which doesn't have a ground), with respect to ground, will be very high--almost equal to full battery terminal voltage. (Grounds can be "hard" or "soft"--a soft ground is usually caused by moisture or dust and humidity, and will result in voltages not so close to 0 VDC on the leg which is developing the ground.)

The next thing to note is that there can be multiple grounds on either leg of the 125 VDC system--and it will only show up as one ground on that leg. BUT, if a ground develops on the other leg of the 125 VDC supply, then arcs and sparks are gonna start flying, and fuses are gonna blow, and circuit breakers are gonna trip. And, if the turbine is running, it's gonna trip.

Because there are <b>SO</b> many places where grounds can develop it's <b>CRITICAL</b> to troubleshot and resolve a battery ground as soon as it is annunciated. Usually, when a battery ground is annunciated it's the result of an overtemperature in a compartment that has caused wire insulation to melt, or rain water has entered a junction box or conduit and made its way to a junction box (or even oil can cause grounds under the right circumstances). The point is: There is usually some indication (high compartment temperature) or a heavy rain storm which precedes a battery ground. Also, improper termination of wires after a maintenance outage can also cause grounds. Sometimes junction boxes are not properly sealed after maintenance outages, or conduit connections are not properly made up after a maintenance outage, so water and/or oil makes its way into the conduit and to a junction box.

But, again--there is usually something which can ultimately be pointed to which will help one determine where a ground might be located. BUT, if one waits for days or even weeks or even months or years (yes--people have waited years to troubleshoot grounds!) then finding them (because the longer one waits, the more grounds can--and usually do--develop) becomes VERY difficult.

MOST grounds occur in exterior junction boxes, or in boxes which conduit penetrations were not properly made or were not properly sealed. So, exterior (outside) junction boxes are usually the most suspect.

Also, I've learned the very hard way that a lot of grounds can and do occur in "odd" circuits that are powered by the 125 VDC battery--such as fire detection/protection circuits, or transformer protection circuits, or cooling water fan vibration switches, or similar circuits which are not part of GE's normal circuits.

The important thing is that when a battery ground is annunciated you take immediate action by asking, "What has changed?" What other alarms were recently annunciated which might help me understand where the ground might be? Has there been heavy rain recently? Was there work recently done in some compartment on some auxiliary that involved 125 VDC circuits and wiring? And start your investigation there.

But, don't just power the Mark VI down and start using a voltmeter to check every terminal with respect to ground. Because, again only those devices which are powered directly by 125 VDC are going to cause a 125 VDC battery ground alarm if they are grounded. Thermocouples will not; RTDs will not; milliamp inputs will not; servo-valve outputs will not--cause a 125 VDC battery ground alarm if they develop a ground, so it's a senseless waste of time and effort to even check those circuits.

And, a lot of devices that are NOT connected to the Mark VI can also cause 125 VDC battery ground alarms if they develop a ground. And it's a lot easier to go an do a visual inspection of those circuits than to start checking every wire coming into the Mark VI.

And, no; using a voltmeter to check each wire while it is energized will not tell you which wire is grounded--because the positive and negative wires are each common to each other, so a ground on any negative wire will look like a ground on all negative wires. ONLY IF YOU ARE VERY LUCKY will you be able to use the resistance mode of a multimeter to find a grounded wire when there is no power on the circuit--and if you are THAT lucky, you should be buying lottery tickets and Irish Sweepstakes tickets!

Hope this helps! Look at exterior junction boxes; think about what has changed recently--before the ground occurred. Finding grounds can be very frustrating, but not impossible. Vigilance and timeliness are key.

 
i really useful from these information which you told me about and we have in one of our turbine battery charge alarm and we force the alarm to start the turbine is it effect on the turbine and how could we fix the alarm
 
KHAIRI,

I'm beginning to be VERY distressed by all of the issues you are raising.

If the battery charger was not provided by GE or the packager of the turbine and auxiliaries there is a very good chance that it has an internal battery ground detection circuit. This is VERY BAD as it will interfere with the ground detection circuit of the Mark VI (unless someone disabled the Mark VI ground detection circuit--which is not very likely).

The two ground detection circuits (the one in the battery charger and the one in the Mark VI) will be in parallel with each other and this will result in nuisance battery ground alarms on BOTH systems (the battery charger and the Mark VI).

If the same battery/charger is also used for other systems and purposes in the plant it is very common for the supplier to provide a charger with ground detection circuitry not realizing that if the battery will be supplying the Mark VI that the Mark VI also has a battery ground detection circuit. Two ground detection circuits on the same battery are not recommended and will lead to lots of problems.

I'm only GUESSING that the battery ground you referenced previously and the battery charger trouble alarm are related (usually, all battery charger problems--including a battery ground--are grouped into a single output which is connected to the Mark VI as "Battery Charger Trouble"). It's not typical for a Battery Charger Trouble condition to be used as a turbine start-check permissive, but, these days, just about anything is possible.

Forcing any logic to start a turbine without fully understanding what the cause of the problem is can lead to serious problems. If the battery isn't capable of driving the emergency L.O. Pump during a turbine trip and loss of AC in the turbine, then the cost of repair (and lost production) is going to be VERY high.

 
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