Very useful info as usual from CSA,Djafer-QP,
We don’t know how old your GE-design Frame 5 heavy duty gas turbine is, only that it has a Mark VIe turbine control system. We also don’t know if your unit was packaged and sold by GE or by one of its licensed packagers (BHEL; John Brown; Nuovo Pignone; etc.).
In general, GE heavy duty gas turbine controls philosophy, especially for legacy units with an Accessory Gear drive, is to only alarm on electric motor (AC or DC) overload—not to trip the electric motor. The alarm alerts a conscious, trained operator to a motor overload which needs a quick investigation and resolution in order to prevent damage to the electric motor or worse, to the turbine and/or generator or auxiliaries. If the problem is serious, the operator can decide the course of action which might include shutting the unit down or tripping it. If the power being produced by the unit is critical at that time, then the operator can decide to let the motor burn up to keep producing power. (In the early days of GE heavy duty gas turbine production, there were very few electric motors required for normal running operation because the Accessory Gear drove the Main L.O. Pump and the Main Hydraulic Pump and the Cooling Water Pump and even the Cooling Water Fans!) And the DC L.O. Pump was considered to be critical but also “sacrificial” as a last means of protecting the unit bearings.
The Hydraulic Ratchet pump motor was also considered to be critical and sacrificial—and, again, the decision to stop the ratchet pump was left to the trained operator to determine if it was absolutely necessary to try to keep the ratchet working or open the motor’s main DC breaker to save the motor. (Yes—the ratchet is important, but if the rotor is hot or warm and it can’t be turned after a period of approximately 20-30 minutes then the ratchet should be stopped anyway, but the Auxiliary L.O. Pump should be kept running to supply L.O. to the bearings to cool them while waiting for the rotor to cool before re-starting the ratchet mechanism.)
Over decades GE has used more and more electric motors and relied less and less on the Accessory Gear drive, but the philosophy of only alarming on an electric motor overload and letting the operator investigate and decide if the motor should be stopped hasn’t changed much. The “problem” is that many people believe that whenever an electric motor overload occurs on any process or application the motor should automatically be stopped and they believe that should also be the same for electric power generation equipment as well. The cost of an electric motor might be nothing compared to what might happen if the power being produced is suddenly lost (think of people with ventilators keeping them breathing, or getting stuck between floors on an elevator or unable to use a bridge raised to allow ship traffic to pass that can’t be lowered without electricity).
Some purchasers of GE-design heavy duty gas turbines have demanded that electric motors be automatically stopped on overload. Some packagers of GE-design heavy duty gas turbines have decided to automatically shut down some, or even all, electric motors on overload. When there is a redundant motor/pump available it can automatically be started—but what if there isn’t a redundant motor/pump?
It’s all a matter of philosophy, and, really that involves operator (and operations supervisor and maintenance personnel and their supervisors and plant management and ownership!!!) training and understanding, as well. The operator should be capable of investigating and deciding the course of action given the operating circumstances and situation. That’s how the unit is most reliable. If the turbine control system, without a LOT of information it can’t process, makes the decision to shut the motor down then that can have very serious immediate and knock-on consequences. If power is critical, the cost of a motor or a relay (did you really mean contractor??) is nothing in the bigger picture. Damaged bearings—that’s a huge deal. But do you want the motor starter relay logic deciding to shut the motor down and possibly damaging the unit bearings when there wasn’t really an overload, just a loose connection???
Okay, so the relay/contactor was damaged or destroyed. Things like that are going to happen on occasion. Is the underlying problem the lack of spare parts, or the lack of training, or the lack of maintenance? What caused the overload—a damaged pump? What damaged the pump?
People want to rely on automation (the control “system”) to protect equipment under every condition and still be reliable all the time. (That’s what the sales people say the automated control system can do, right??!?!!) But the technology just isn’t really that “smart” yet (no matter what the sales people say or the marketing people write). We still need people. When the turbine control system can do everything, where will we people find work?
Tell us why the overload condition occurred. Tell us if the site had the spare part(s) to repair or replace the failed relay/contactor. (It would be great if you posted a picture of the failed part.)
A power plant can’t be reliably operated without people (though there’s at least one OEM who thinks autonomous power plants are coming soon!). People need to be trained and understand the equipment they are operating. It would be great if GE documented their heavy duty gas turbine control philosophy, also, but they didn’t and probably never will. But, thinking, reasoning experienced people (often more than one, working as a team...) should be able to discern these kinds of things based on training, familiarization, knowledge and experience.
All things a motor starter or even a digital control system can’t do—and won’t be able to do for at least another decade or two. Thinking, reasoning, rational human beings have to be involved in a reliable power plant.
Hope the helps.
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