Protection Engineers must learn the new technologies to stay in the field . Otherwise protection Engineer will be cornered as a mere testing engineer or a setting calculation engineer . But the fact is that , the the field of protection has gone a long way by the latest digital technology ,Digital signal processing and protection communication ( remote as well as local communication ) .
With the advent of numerical relaying which uses Digital Signal processing as heart and brain for achieving the relay functions , have even changed even the requirement on the CT sizing . traditionally , the transient diamensioning which was met by the 1+x/R factor but now with the present numerical technology , the CT saturation is allowed after some time after the fault , thereby reducing the requirement on CT sizing . Every relay manufacturer with numerical relays will publish the transient dimensioning factor for their relay operation based on a range of X/R ratio . It may be noted that that the transients occures in the system due to X?R ratio of the system and henec the transients vary based on the X/R ratio . This is why themanufacturere i s giving the transient diamansioning factor based on the x?R ratio.
The basic function block of the numerical relays are
1, input unit which will convert the voltage and current input to an equivalent voltage signal suitable to the digital circuit in the relay hardware
2. A low pass filter which will filter the high frequencies in the input signal .There are different type of filters including fourier filter which will give out sine and cosine functions of different frequencies
3. The sampling of the filtered waveform
4, The processing of the sampled values usually called relay algorithm based on the different requirement.
The filtering requirement
As the numerical relay process its algorithm based on the different frequency component present in the waveform . Generally we are interested in the fundamental wave form and the some higher order harmonics . Hence suitable cut off frequency is selected based on the frequency requirement for processing .
Sampling the filtered signal and anti aliasingg \
In order to avoid the anti-aliasing ( high frequency component will appear as low frequency ) , the minimum sampling frequency shall be selected so that , it is at least double that of the highest frequency component of interest. Suppose we are interested in finding out the 5th harmonic component in 50 nHz system , we should have minimum of 500 Hz as sampling frequency. A frequency of 800 Hz or more is usually set by different manufactures to sample the data
Processing of the data
The sampling and processing of the data is done in the DSP ( Digital signal processor ) in case of numerical IEDs . This processing of such sampled data is called Relay Algorithm,
Relay Algoritham
Relay Algorithm is pre programme done inside the Relay so that different protection functions can be processed according to the requirement . For example , in a relay that uses over current protection as one of its function will have an over current algorithm running to check whether an over current situation has arisen in the system . Similarly when a distance or differential protection is activated inside the relay , it starts executing the algorithm for the same
( to be continued .. )
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