the following is what happened to our steam generator:

Excitation minor trouble alarm and over excitation limit operated alarms came up. and the north and south breakers show opened position, but in actual sense it is still closed.

the unit downloaded itself and eventually tripped due to uncontrollable drum level with a load of 20mw. Initial load was 70mw.

Turbine speed failed to come down even after manual trip push button was initiated. it later found that loss of DC power to protection relay was responsible.

Please can you explain why this type of trip will occur. All this took place in Lagos, Nigeria

 

>Please can you explain why this type of trip will occur.

Based on the information provided, the immediate thing which jumps out of this question (without a question mark) is: There were multiple failures, at least one in a critical protection system.

For multiple failures like this to occur in a running plant there were probably multiple alarms <b>which were overlooked <i>(ignored)</i> by the operators <i>AND</i> their supervisors</b> who were under some kind of impression that as long as the plant continued to run that any alarm was unimportant and could be overlooked (ignored).

It would also appear that when the turbine started unloading that the boiler fuel control and/or the boiler feedwater control systems malfunctioned leading to the "uncontrollable" drum level. Quite often when something like this occurs I find that the operators have over-ridden the automatic controls (for example, the boiler feedwater control) for whatever reason and have forgotten, or they have over-ridden several automatic controls and have too many "fires to fight" at one time because multiple systems are in manual control when an event occurs to trigger a problem like this.

I have rarely found a turbine trip circuit that, one, wasn't de-energized to trip so that a loss of power would result in a turbine trip; or, two, didn't have an alarm on the trip circuit power supply to indicate an inability to trip because of a loss of power to the tripping circuit (fuse failure, or undervoltage, or something similar). When operators don't understand the implications of these alarms and fail to take appropriate action then situations like this occur.

The upshot of all this is: Poor operator training leads to poor plant operation. And it's poor training--and lack of expectation and accountability--that allows operators and their supervisors to overlook (ignore) alarms. And this leads to questions like this (with or without a question mark).

Regardless of where the scenario takes place in the world.

For some reason alarms at power plants just seem to be considered nuisances, and if the alarm message isn't explicit or the operator hasn't encountered the alarm before and the turbine didn't trip, well, then, it can wait.

The exact sequence of events isn't the critical issue in this question. It's what led up to the culmination of several failures that led to this question.