LOP on/off? 1

[Mbrooke]

Do you enable LOP (loss of potential)? Is it worth it? And why?


I normally set it and revert to 50/51 elements.

 

[davidbeach]

Our line relay settings standard includes phase and ground overcurrents that are active during LOP conditions. If the line includes 87L, we require LOP and loss of 87L before enabling the OC elements.

 

[Mbrooke]

Have you ever had a transmission blink cause an issue? This was mentioned by another EE for his utility, which sparked my curiosity. His reasoning was also that an overwhelming number of potential failures were single phase rather than 3 phase.

 

[davidbeach]

The relays we use evaluate LOP by looking at changes in voltage without a corresponding change in current. A transmission event would cause changes in both voltage and current and wouldn't become an LOP event.

 

[Mbrooke]

I somewhat disagree- its possible to have a huge drop in voltage but little current flowing on a certain part of a very large interconnected system.

 

[111R]

The logic rules out LOP even with a minor change in current (>2%). It also analyzes other characteristics such as sequence component magnitudes and angles.

 

[Mbrooke]

What about one phase open? Better to initiate LOP or let MHO limp along?

 

[111R]

The change in zero sequence current magnitude would still rule out LOP.

 

[Mbrooke]

Even with one phase open?

 

[davidbeach]

One phase open where?

If it’s a phase of the VT there’s a huge change in voltage with no change in current. That a clear LOP condition.

One phase open on the primary conductors that causes zero volts on that phase of the VT secondary will also cause zero amps on the same phase. Clearly not an LOP condition.

One phase open out on the line doesn’t generally cause a loss of voltage. If sensing at the receiving end of a radial line there would be a loss of voltage and a corresponding loss of current.

I am not aware of our relays ever getting it wrong. May have happened, but never in a way that called attention to it. An LOP condition is alarmed back to the control center.

 

[Mbrooke]

Secondary VT circuit. (A failed VT can also do it) More often then not you loose one phase instead of all 3.

http://www.pes-psrc.org/kb/publishe...oltage_Considerations_for_Line_Protection.pdf

 

[davidbeach]

That’s the easiest case for the relay to get right.

 

[Mbrooke]

In terms of LOP?

 

[davidbeach]

I wonder if we’re missing what the other is saying...

Yes, that voltage going to zero with no change in current is cleanly and clearly an LOP condition. If you tell the relay to ignore it then two of the three phase distance elements get the wrong result and the memory voltage polarized elements all fail. The compliance standards are much more forgiving of elements that are only active during LOP conditions than they are of normally active settings.

 

[Mbrooke]

I understand, but I've been told by an EE that LOP has activated during recent hurricanes on transmission line blinks. His substation techs do not enable LOP for that reason. He would rather have 21 remain during those events, and would rather take the risk of erroneous step distance elements outside of that.


Trust me when I say I'm even more confused than the guests here. I do not know of a single case in which LOP has inadvertently activated but he is telling me that it can and does. I'll probe him more if he can hand me grid data... Or if there is something in his relay settings responsible for that preference...

 

[bacon4life]

We routinely have LOP alarms from radial parts of the system for faults elsewhere on the system. On radial portions of our system, the change in current for voltage depression is sometimes way less than 2%. Having a flood of momentary LOP alarms from several substations was confusing to dispatchers, so I contacted the relay manufacturer. The manufacturer claimed we were the first utility to report this issue to them. As a stopgap, so we added a timer to the LOP Status the requires it stay exerted for 10 seconds before passing the alarm onto dispatchers.

We have experienced blown PT fuses, so using LOP seems reasonable. I do not know of any instances where we had an incorrect trip due to LOP enabling backup settings. I would encourage Mbrooke to keep digging and pass the incorrect LOP events onto the relay manufacturer.

 

[Mbrooke]

Maybe the first to report it, but not the first to experience it IMHO.


The engineer I spoke with said the same thing: use a timer if you are going to enable it.

His LOP issues came during a hurricane where there were repeated transmission line faults and repeated "blinks" where voltage dipped significantly on one or more phases for many cycles. It enabled during, despite the secondary wiring checking out ok.

 

[bacon4life]

Just to clarify, we have the SCADA master performing the time delay on the LOP alarm to dispatch. The LOP function is so integrated into other relay functions that adding a timer in the protection logic section seemed pretty complicated. Relays are already complicated enough that I worry adding much custom logic dramatically reduces reliability.

At our sites with LOP issues, the backup settings are still secure under normal system conditions, so switching to the backup settings would only cause over tripping in the event of a second local fault occurring while there is a remote fault bringing down the voltage. Given the very rare case of a simultaneous fault causing a misoperation versus the common case of a blown PT fuse, we are using LOP in the protection functions without a custom timer.