These days mostly Numerical Relays are used for protection purpose instead of conventional Electromechanical Relays. In fact, Numerical Relays offer a variety of protection as well as trouble shooting features. A single Numerical Relay can provide many protection unlike conventional Electromechanical Relays. A single Numerical Relay can have provision of Overcurrent, Earth Fault, Over-voltage protection etc. whereas Electromechanical Relay mean for overcurrent protection can only provide overcurrent protection.
Apart from this, the best feature of Numerical Relay is Trouble shooting feature. Numerical Relay can log events and faults with date and time stamping. This really makes protection engineer’s life easy. Not only this, in case of fault, it registers the whole disturbance in its memory which can be downloaded in .comtrade format and can be played and shared. This feature is called Disturbance Recorder (DR). I know, you might be confused with these words. But don’t worry I am going to explain this with an example.
Suppose there happen a fault in the system which is protected by Numerical Relay. By opening the DR file, one can assess the exact time of fault, sequence of breaker opening and level of fault current in graphical presentation. One can see, how much time did the relay took to make its output contact high to give trip command to the breaker. The whole scenario becomes vivid and one can easily analyse the root cause. One can judge if it is relay malfunctioning, wrong setting or actual fault.
Normally a Numerical Relay can store such 50 DRs of 1.5 sec in its memory. When the number of fault increases, it deletes the last recorded fault i.e. 50th and stores the new one. This means relay works on First In First Out (FIFO) principle. Therefore it is very important to download the DRs when the number of DRs approaches near 50. A “Full DR” alarm is provided for this purpose to get the attention of operator.
But it may happen so that during the commission, the number of DRs exceed 50 or approaches near 50. In this case “Full DR” alarm will glow on relay interface. One may suggest to delete the DRs after completion of commissioning but every time it is not feasible as one may ask to see the commissioning data in the relay.
Due to this, it becomes annoying for operator to have “Full DR” alarm every time. So how to disable this “Full DR” alarm? I conducted many tests to get a solution of this problem and I observed that
The criteria of “Full DR” alarm is,
When the number of DRs exceeds the 70% of its totally capacity i.e. 50, relay shows “Full DR” alarm.
Thus what we can do is that, we can set the DR time to 6.5 sec or more so that relay will never register more than 35 DRs and hence “Full DR” alarm will never appear. Doing this does not really limit the disturbance registering capability of the relay as it works on FIFO principle. Thus the problem of always having “Full DR” alarm is solved. This solution is based on my personal tests conducted on Numerical Relay.