Distance Operation Due to inrush

[SMB1]

Hi,

I have distance protection relay used for 33KV radial line around 80 KM long with more than 150 Distribution transformers.

this relay have been operating by Zone-1 many times at energization due to high inrush current 800 A while the maximum feeder load is 200 A.

to avoide this situation we usually open at the middle of the line and energize then we close.

my question: I understand the effect of inrush current in overcurrent operation, can any one explain its effect on distance relay?

thanx

 

[davidbeach]

The distance relay knows nothing of inrush. If it sees a lower apparent impedance than its setting it trips. Inrush can cause a low apparent impedance. There is no particular way to block operation on inrush other than raising your setting.

 

[JJHorak]

Maybe if the events are always apparent 3ph inrush, and if the inrush we are talking about is just magnetizing inrush that only peaks for a couple cycles, you can add some logic like, "If all three phases are above ## per unit, and breaker just closed, delay 21Phase for 5 cycles." Or maybe just leave the phase overcurrent out of the logic, "If breaker just closed, delay 21Phase for 5 cycles." Leave the ground distance with no delay.

 

[cranky108]

An other method to avoid inrush on zone 1 is the same thing you do with zone 2, add a time delay.

Or if you know the inrush dosen't exceed 800A you can set the fault detector above that level.

 

[RRaghunath]

Delayed the protection at the time of switching on has one disadvantage.
Many times after OHL maintenance, it happens that the earth is not removed and the OHL is switched on. This is called 'Switchon to fault' condition.
The protection would not be effective for this as it is delayed.
Instead, to make the relay stable against transformer inrush currents, it could be good idea to inhibit the protection operation on sensing 2nd harmonic component in the OHL current.
This should be easy to configure in case of numerical relays.

 

[shalhoob109]

there is a feature in 'Transformer Diff.' ABB Typre RET541
Called Deblocking ! ( removing blocking of 2nd harmonic in
case of 'Switching on to fault'.but if there is no exist
of fault will continue blocking 2nd harmonic after energization.I don't know if new numerical relays solved
that in OHL or not with distance relays but that should be
treated anyway.

 

[davidbeach]

The typical line protected by distance protection wouldn't have 150 transformers tapped from the line. Distance works best for a two-terminal line, reasonably for a three-terminal line. For a 150-terminal line I think you need to stick with overcurrent protection.

 

[xxjohnh]

I think the inrush current is much higher than 800 amps. What is the type of relay? Maybe it needs a cold load pick up timer. What is the reach of the relay set at?

 

[SMB1]

summary of responses:

1. time delay can be added to zone 1 . But how much? Inrush period & magnitude is un predectable.

2. Second harmonic detection in series with tripping signal. i think this can be implemented in configuration file.

3. stick with overcurrent : Good solusion

thanks everyone

SMB

 

[rasevskii]

This must be a rather exotic system with 80km of 33kV on one circuit. Do you mean distribution transformers of 33000/240/415 actually or are we talking down to 6.6 KV or similar? Is there not an intermediate switching station along the line? I suppose that this was a line that just kept getting longer without any planning...

A misapplication of distance protection obviously. See davidbeach comment above.

Zone 1 has to be instantaneous in principle, unless you want to burn down something.

A protection study is indicated/recommended.

Must be far out in the bush someplace.

regards, rasevskii

 

[cranky108]

Actually zone 1 dosen't have to be instantaneous. If you need a fast operation you can use a 50H element.

 

[marks1080]

Newer IED relays can be programmed with several zones (i've see up to 4) and 'blinders' can be used on the mho settings so you can avoid having to use a delay.

 

[JJHorak]

How long? Well, load inrush can last awhile, but I doubt that is what is causing tripping. My guesstimate (derived from a minor understanding of inrush to large substation xfmrs, distribution xfmrs may be different) is that the apparent impedance of manetizing inrush will appear as roughly 2-3 times the the xfmr through fault impedance for 1-3 cycles, and then the apparent impedance rapidly quickly rises. Delay only needs to be something like 5 cycles.

The issue of slow tripping for cases of closing into a fault is one I did not think about. That is something to consider. One way to partially address it for the first ## cycles after closing a breaker, enable an impedance element with high settings that will not trip for inrush. After that time delay, enable a more sensitive impedance element.

 

[nightfox1925]

Some modern distance relays have 2nd harmonic restraint logically built-in to the relay via digital comparators like the AREVA Micom P442 relays.

Some relays have a 50 element that is used to supervise the distance element such that it is set higher than the maximum line in-rush but below the minimum line-line fault. This 50 element is used to arm and disarm the distance element (like in the case of SEL 311B).

If your relay does not have one, then you may (IMHO) provide a 50 relay to supervise the trip.