UFLS 1

[Fulbert]

hi,

Could someone explain me the undervoltage protection function in UFLS Relay?
I worked as an protection electrical maintenance engineer in both transmission and distribution.
Recently i faced with 1 issue regarding our UFLS Areva P923 Tripped on all stages (Stage1-6)without any fault on the network grid, i have checked the grid voltage systems and frequency with the operator found both are okay before the incident occured(1 minute before the incident).
We also have checked the VT's ratio & insulation and found okay. On the configuration relay P923 i found that the UV Blocking set to 5V where our nominal voltage is 63.5V(P-G)/110V(P-P). Can someone explain me the function?

TQ

 

[davidbeach]

In general (I don't know the Areva relay specifically) the frequency elements are supervised by an undervoltage blocking element such that below some voltage (the blocking setting) the frequency elements are turned off. At some point the voltage is too low for the frequency calculation to provide decent results. Depending on what you're doing, or what your philosophy is, that block might occur relatively high, as much as 0.8pu, with the intent that you only want to respond to frequency while the system is reasonably normal. Or the block might be quite low, as little as 0.2pu, with the intent that the frequency elements will function down to the point where the raw data begins to become suspect.