PRD Spurious actuation is a case in which transformer tripping is initiated by PRD contacts even when transformer is healthy. Ideally PRD is the last line of defense and actuates when the pressure of oil increases beyond the setting value of PRD. But its spurious actuation results in transformer tripping and may lead to power outage. Thus possibility of its spurious actuation may not be overlooked.
PRD stands for Pressure Relief Device. It is a protective device mounted on the Main Oil Tank of Transformer and protects it from over pressure of oil. Thus, tripping from PRD actuation is inevitable. But there are many cases of PRD spurious actuation which results into transformer tripping. Prevention of transformer tripping from such spurious actuation of PRD is necessary. But any compromise from PRD protection should not be done. The method of prevention of tripping from spurious PRD actuation must not be a compromise from the intended protection offered by PRD.
Cause of PRD Spurious Actuation
Before going into the method, it is imperative to know why PRD actuates spuriously. There may be many reasons. I am here going to describe one reason. If you see the Junction Box (JB) of PRD, you will notice how compact and cramped it is. The contacts are very close to each other and if the wiring is not done properly in this JB, then it may lead to shorting of contacts in near future. The PRD contacts may also actuate due to some kind of foreign material (like dust,moisture etc.) in JB.
Prevention of Transformer Tripping from PRD Spurious Actuation
The method for prevention of Transformer tripping due to PRD spurious actuation is quite simple but reliable. One NO (Normally Open) and One NC (Normally Close) contacts of PRD is wired to Opto Input of Numerical Relays (This may be for differential protection of Transformer).
In the above figure, PRD NC contact is assigned as Opto Input L8 and PRD NO as L11. Now, these opto inputs are configured in PSL (Programmable Scheme Logic) as shown in figure above. You can see, the logic has been designed in such a way that output to master trip relay 86 is only initiated when both PRD NO and NC contacts changes its status.
If only NC contact changes to NO, the output of first AND will be 0. Why? To understand, let us assume if the status of contact assigned as opto input to the numerical relay is Close, the corresponding input is 1 else it is 0. So when NC contact changes to NO, corresponding input L8 will be 0. As NO contact has not changed its status to NC, L11 will also be 0. Therefore, both the inputs to the first AND gate is 0, hence its output will be 0.
Since NO contact has not changed its status, the input of second AND will be 0 and 1(output of first AND gate when inverted), hence its output will be 0. Thus master trip relay won’t operate.
Similarly, if only NO contact changes its status to NC, the output of first AND gate will be 1. This output when inverted will give 0 which is input of second AND gate. Thus the output to lock out relay 86 will be still zero.
When both NC & NO contact changes their status then output of first AND gate will be 0. This output when inverted will give 1 which is input of second AND gate. Since NO contact has changed its status to close, its input to second AND gate will be1. Thus both the inputs are 1 and hence master trip relay will be actuated in this case.
Thus we observe that, the lock out relay (86) will only operate if both NO and NC contact changes their status which is possible only during genuine actuation of PRD. There is a very remote possibility of both the contacts getting high (change their status) spuriously. Thus the above scheme prevents transformer tripping due to spurious actuation of PRD without compromising with the PRD protection.