Lightning affecting Frame 6 Gas Turbine


Thread Starter


Is there any recommendation from GE regarding earthing & lightning protection for MarkVI system. Should the earth pit be separate from DCS & PLC. Should the value be less than 1 ohms.

Even though there is huge covering shed for the whole GT with separate earthing (for metal body), lightning is affecting cards in MarkVI panel.

The Mark VI System Guide, GEH-6421, has a section on Installation which talks about proper grounding (earthing).

The Mark VI (and Mark VIe, as well) is designed to have two grounds: one call "protective" earth, and one called "functional" earth. This is explained in the System Guide(s).

"Protective" earth is just what it sounds like: it's function is to protect humans against shock should something untoward happen with a wire or some electrical circuit. It's the same ground (earth) as that found on a 110/220 VAC outlet, or on a wye-connected three-phase motor.

The "functional" earth is intended to be used <b>ONLY</b> for the control system and it's field devices and components. The two earth "pits" are to be electrically separate, and sized properly (that's up to the plant designer to size and construct them properly).

It's also very common for electricians building a power plant to not understand the differences between the two ground systems and to improperly connect instruments to the protective earth grid, and vice versa.

That's for a new unit Mark VI--a Mark VI provided with a new turbine. If the Mark VI was supplied as an upgrade (retrofit) to an older control system, most older plants were NOT built with two separate earth pits (grounding systems), and so there is a strap that connects the protective earth in the Mark VI to the functional earth in the Mark VI--and if the plant is struck by lightning then the Mark VI will likely be affected.

Most Mark VI panels are/were shipped with the strap that connects the functional and protective earths installed in the panel, and the commissioning engineer was supposed to remove the strap <b>if there were separate earth pits at the plant where the Mark VI was being installed.</b>

So, you need to understand what earthing (grounding) system(s) are in place in your plant. Then you need to make sure the Mark VI is properly connected to them (if more than one is available). And, then you need to make sure all devices in the plant are connected to the proper earthing systems (if more than one is available).

If this problem has been getting progressively worse since commissioning, then it's very likely that there is a motor or some other device that is developing a serious ground that is not connected to the proper earth (presuming there is more than one earthing system).

Also, some soils are very poor for earthing systems, and can, when wet, cause electrical current to flow between the earthing pits/systems. When being built, the designer/builder is supposed to analyze the soil and use amendments as necessary to ensure isolation--and this rarely happens, unfortunately, in most parts of the world.

There are companies that specialize in lightning protection--some are better than others, so do your homework before choosing one if you decide to go that route.

If you're looking for a specification from GE for exact earthing resistances and potentials, that doesn't exist anywhere that I know of. Earthing is kind of a unique science which requires a good deal of experience and training to properly design and install proper systems. And, once the installation is done, there's usually little which can be done to fix any deficiencies.

So, it's not a simple answer--but the biggest things you can do is understand if the system was provided with a new turbine, how many earthing pits are available in the plant and what their functions are, and if the Mark VI is properly connected to the earthing systems. Again, there are usually two bars in the Mark VI--one for protective earth, and one for functional earth. If there is only one earthing system at the plant, then the two MUST be tied together with a strap or suitable conductor. If both a protective and functional earth system are available, then the strap which connects the two bars in the Mark VI must be removed. And the potential between the two must be very low (as measured with a voltmeter). AND--this is the hardest part--if there are multiple earths in the plant, someone needs to audit/inspect all connections to the earths and be sure they are connected to the proper earth pit.

And, hope the earth pit(s) were properly designed and constructed.

Syed Affan Mustafa

Yes GE has certain recommendation based on IEEE-142-1991 and IEEE-1100-1991 and NEE-Article 250. but before that i would like to know your problems in details. similar problems we have successfully rectified in a refinery CPP Area in India some days before and gave them implementable solutions. they were having Mark V protection system and their GTs were tripping on Spurious signals of Seismic and Vibration sensors, and there control cards were burning in day in day out basis. we did the Adequacy study and gave them a solution.

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