RMS versus fundamental sensing for high impedance busbar protection

[submonkey]

Hi Everyone,

I'm looking for opinions or experience in the use of RMS-sensing
numerical overcurrent relays in high impedance busbar protection
schemes (i.e. relay in series with a stabilising resistor, connected
in parallel with the secondaries of a number of summed CTs).

The original 1948 paper on high impedance busbar protection by
H.T. Seeley and F. Von Roeschlaub proposes a series-tuned LC
filter in series with the overcurrent element for rejection
of the DC component.

Various manufacturers whose relays include 50 Hz fundamental
filtering state that their relays are safe for use in high
impedance schemes (Areva, Siemens, Schweitzer).

I have an application at hand using a Reyrolle DCD415A (now
rebranded as Siemens 7SG11) which provides only true-RMS
sensing.

The IOC elements in the DCD415A claim to be free (better than 5%)
from transient overreach on systems with high X/R. I'm trying to
decide if the above claim makes their relays suitable for high
impedance use.

Any thoughts greatly appreciated.

Submonkey

 

[stevenal]

The setting procedure for high impedance differential generally assumes a through fault that fully saturates one CT so there is zero contribution from that one. If using rms values, though, you might see a sub-cycle increase from this CT instead since the saturated waveform spikes before dropping. Fundamental filtering is one way to remove these spikes. Perhaps there is another method.

 

[submonkey]

Thanks for the reply stevenal!