Recently we faced a tripping due to operation of cable differential relay (Alstom PQ741). This is an end to end protection for a 220KV XLPE underground cable which runs from a generator step up power transformer to GIS.

Post the tripping, Cable IR was checked & found well within limits. all protection related CTs checked & found ok. PQ741 relay at both ends checked with secondary injection. only differential threshold was checked as end to end checking was not possible. BDV for oil filled cable boxes checked & found ok. After ensuring all above checks, cable energized & loaded up to maximum, and it is still operational after 3 weeks.

What could be the reason for this tripping? Was it actual due to some momentary fault or was it due to possible malfunctioning of relay(s)?
What further checks should be made? Please advise & share your thoughts/experience.

Please note that this cable is in service since more than a decade & we faced this issue very first time.

 

Sohaib... can you furnish more details, such as:

a) Type of cable.

b) Shielding technique.

c) Grounding (earthing) method.

d) Charging current.

e) Type of CT's used.

f) LV interconnecting wiring between CT groups; 4-conductors or 6, single or multi-conductors, shielded, or not.

g) CT grounding (earthing) at one end or both ends.

h) How charged... increasing voltage via alern'r, or at full voltage.

Regards,
Phil Corso

 

Dear Phil,

I have tried to answer your questions below, please let me know if any further info is required.

a) Type of cable.
220KV, 630mm2, Cu/XLPE/LA/PVC (Underground)

b) Shielding technique.

c) Grounding (earthing) method.
At transformer end, cable sheath is connected to ground through SVL surge arrestors while at GIS end it is directly/solidly connected to ground through disconnecting links. I believe this is called "single point bonded system".

d) Charging current.
440A

e) Type of CT's used.
Ring type 400/1A single core CTs which are immersed in transformer oil at one end while are under SF6 pressure at GIS side

f) LV interconnecting wiring between CT groups; 4-conductors or 6, single or multi-conductors, shielded, or not.

I am not sure how to answer this, I will try to explain.
2 wires/conductors coming out of each CT(as always). So total 6 wires out of which 3 directly going to the relay while other three shorted together & to the ground and then a single wire going to relay as a neutral. (I can send a picture to your email if you want)

g) CT grounding (earthing) at one end or both ends.
CTs are grounded at both ends

h) How charged... increasing voltage via alern'r, or at full voltage.
Charged at once on 220KV

Please note that there was no fault at the system while tripping happened.

 

Sohaib... can you correlate events:

o Day or night.

o Good or inclement weather.

o Hot or cold periods.

o Particular loading.

o Shift-change.

o Recent change in operation.

o Recent maintenance on circuitry.

o Presuming 3x1c, have shield interconnections been altered?

Yes! Please send Power and protective relaying sketch to: [email protected]

Phil