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Protection Riddle No.44 - Bad designing of protection circuit and disastrous accident in power plant
The lubrication oil pumps of turbo generator bearing are fed by outgoing of an emergency panel in power plant as illustrated in below figure. The control circuit of mentioned AC panel is supplied by power station safe (of course not very safe) uninterruptable DC system. Due to an unknown reason, the DC power supply interrupts for a little time and restore after many seconded. This accident was the beginning of many interval problems in systems. After restore of dc supply, suddenly the incoming circuit breaker of emergency panel trips due to mal-function of relevant under voltage relay without any distortion of AC supply. This outage of ac supply in emergency panel caused a disastrous in power plant. When we lost ac supply of lubrication oil pumps, you can guess bad condition of turbine and generator bearings.
You can see three various schematic diagrams which could be used by protection system designers as following figures.
How can you choose the best wiring design for prevention of accident from below three submitted plans?






Author : Hamid - From: Iran
 
#1
Mon, August 31st, 2009 - 23:55
The quickest acting trip in order to protect the turbine/generator bearing is best.  

I'd choose #1.  As soon as 27 relay senses the under frequency, according to the diagram it will let the dc travel through it to activate the trip circuit.  Depending upon what 27 relay you use, I'd strongly recommend testing this relay in order to maximize response time.  It should also have some internal settings to adjust the response time/voltage that should be calibrated.

Hope that helps.

~From Technician to Engineer

D.Priebe  
Author : D. Priebe - From: America
 
#2
Tue, September 1st, 2009 - 14:10
I'd chose #3 in regard to mentioned problem. why? 
Author : Hamid - From: Iran
 
#3
Sat, September 12th, 2009 - 08:12
Answer #3 is correct as this scheme does not initiate a trip during interruptions to 110VDC tripping supply.

Also it is bad practise to utilise only an AC lube oil pump.  I usually see a DC lube oil pump fed from a battery supply.  In the event the AC LO pump fails the turbine is tripped and the DC lube oil pump starts up to provide lubrication during run down of the turbine. 
Author : Scoota - From: Australia
 
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