NESC Violation And Sag Mitigation 1

[Mbrooke]

What is typically the best way to go about mitigating post contingency sag in overhead lines from a relaying perspective? Would operator intervention alone be sufficient or is some type of scheme also required as a back-up and what the preferred way to go about? Boolean logic with a timer, 51 or thermal modeling of the conductor in the relay?

Do I need two station batteries since conductor sag is (from my view) technically a hazard to life and property?

Any tips or experience welcome.

 

[jghrist]

The best way to mitigate post contingency sag is in planning. The line capacity should consider sag and if there is excessive sag, then the line is overloaded and the contingency should be handled by other lines.

 

[Mbrooke]

I take that to hint beyond N-1 and N-1-1? As of right now N-1 is covered and taken into account, but N-2 and definitely N-3 brings about excessive sag. Conductor annealing and hardware thermal limits are not exceeded.

Ideally this section of line would either be re-conductored or the towers themselves replaced with elevated structures.

 

[stevenal]

I think perhaps that protective relaying is the wrong tool. Relays operate quickly to remove faulted lines from service. Operators and remedial action schemes deal with contingencies and overloads that are not as time sensitive.

 

[Mbrooke]

Noted.

I don't have much experience with RAS. How is it typically accomplished hardware wise?

 

[bacon4life]

The general idea in PRC-023 is that protection settings should allow at least 15 minutes at a load of 115% of the 15-minute Facility Rating in order to give operators a chance to take action. Setting an overly simple thermal model to trip a line at 100% of the NESC sag limits might not align with the spirit of PRC-023. Several of the recent regional blackouts have been exacerbated by equipment tripping on overly restrictive protection.

When compared to relay protect for faults, the risk for failure to detect excess sag seems less severe. In order for excessive line sag to cause a physical problem there has to be several simultaneous occurrences:
1) High flows due to N-1-1 conditions.
2) Low wind speeds and/or high ambient temperatures.
3) An object physically present at the NESC boundary line.
4) The high flow condition to exist for a period of time. Transmission conductors have thermal time constant on the order of 15 minutes.
5) The NESC clearance include addition buffer. In order to actually flash over, the line also needs to traverse the buffer distance.

If this is not needed for NERC compliance, you have lots of flexibility on how much complication to put into the mitigation.

Implementing a RAS under NERC standards is quite complicated, and usually involves redundant hardware and communication paths. There are numerous compliance requirements around RAS, so it can be simpler to add dedicated RAS equipment rather than integrate the RAS into hardware that also performs normal protection/control functions. Some regions (i.e. WECC) also allow for simplified non-redundant RAS if the impact is localized.

Keep in mind that tripping a line on overload often exacerbates rather than solves a problem. Having either automation or operator action to solve the overload can be a much better approach. Generic SCADA/EMS/PLC/PCS automation may be able to solve the problem in practical terms, but since these systems would not meet the requirements to be considered a RAS, the automation may not be directly applicable to TPL planning requirements. If you specifically need to meet TPL planning requirements, considering installing both automation and a RAS, such that the RAS only operates if the automation fails.