When to island industrial plant from utility? 1

[pnw_engineer]

Hello all,

I'm doing some work for an industrial plant that currently runs with their own generation: 3x 12.5 MW steam units, peak plant load around ~10 MW. This plant is connecting to a small, neighboring, islanded utility with peak load ~20 MW. Our job is to figure out what protection to add to the intertie that will isolate the industrial plant and keep them online if-and-when performance issues occur in the neighboring utility, such as large over/under-frequency or units going out-of-step. We're doing power flow and stability analysis of the two interconnected systems using PSS/E.

My question is, when should the industrial plant trip their intertie and isolate themselves? We've already though of adding over- and under-frequency protection to the intertie so large outages in the neighboring utility don't cause equipment in our industrial plant to trip, but what sort of other protection would be common for this type of situation? df/dt? Out-of-step tripping of the intertie? The last case is something I'm particularly worried about, since our transient stability simulations show some faults in the neighboring utility may cause units to go out-of-step and for blackout to occur... We definitely want to trip the intertie and keep our industrial plant online when that occurs.

Thank you for reading!

pnw_engineer

 

[cranky108]

I would suggest talking to the utility, because in some cases there stability may depend on the plant operations. No need to make a single concern into an area outage.
Besides, in this case the utility may make arrangements to dump the plant if they cause them an instability. It can go both ways.

 

[waross]

At a very large mine mill (Now defunct but at one time one of the largest open pit operations in the world) there were two under-frequency relays.
One set at 58 Hz and one set at 56 Hz.
At 58 Hz load shedding was initiated.
At 56 Hz the mill went offline.
The protocol to recover from a 56 Hz trip was much more stringent than the protocol to recover from s 58 Hz trip.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[wcaseyharman]

Voltage and Frequency are typical interconnection relay types for islanding scenarios. If there are close in faults on the utility that your relays could see you could apply directional overcurrent or distance relaying - I am doing sensitive reverse fault settings for an intentional island at the end of a long distribution line - if the gen is running a reverse fault on the feeder will cause the intestine breaker to open, islanding the little system.

Obviously you’d need to coordinate with the utility.

 

[crshears]

Intestine? Spell check choice unverified? Should be intertie, maybe?

I recall a synthetic fibre plant with on-site generation fully capable of islanding, normally connected to a pair of feeders looped through the plant, therefore had dual supply redundancy, outward looking impedance [ distance ] relaying, and remote tripping.

Plant would typically island at their own choice for perceived anticipated potential threats to grid stability, so only unanticipated events like motor vehicle accidents or line equipment failures with one supply out of service for maintenance or other reasons would cause unplanned plant islanding . . . and the probability of failure to smoothly island was empirically found to be quite low.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

[Anecdote Alert]The National Utility of a Central American country lost a significant portion of their generating capacity, to the point that the whole country was put on 6 Hour on, 6 Hour off rationing.
The utility offered to reimburse the extra cost of fuel for any business willing to go off grid and use standby generators.
And another similar case;
Our little island micro grid had one of our main generators out of service.
When the loading was approaching peak, our plant operator would call a major user and request them to go off grid and use their stanby generators.
Some different reasons for islanding.[/AA]

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

I think there are efforts for the same thing on some of the utilities that are on the major grids. Not because of capacity, but because of cost to purchase power.