Operating a Distribution Circuit 12kV in an Islanded Case with a Generator - Protection 1

[Allen723]

Hi,

I work for an electrical utility as a protection engineer. Our job consists of setting protective relays for substation relays from 4kV to 500kV as well as field devices on distribution circuits i.e. reclosers.
We have some cases where the company wants to pick up a portion of a 12kV circuit in an islanded mode with a diesel generator(s) (i.e. 500-1000kW generator) without parallel operation with the utility. So it's the 480V diesel gen(s), then a step-up transformer, then the portion of the 12kV line the gen(s) are picking up. These are Permanent Magnet Generators (PMG) which can sustain their fault current for 10 seconds so from a protection standpoint, I am looking at providing an overcurrent element on a recloser device at the 12kV winding of the step-up XFMR to protect the line we are picking up.
The generator vendor provides the genset and/or xfmr. Do I need to be concerned with the gen's having over/under frequency elements and over/under voltage elements and what they should be set for or do the generator vendors typically set up all that in their gen breakers based on their experience? I figure they have Automatic Voltage Regulators (AVR's) and they are monitoring all these values? Like I mentioned previously, all I'm concerned about is whether we can protect the line with the fault current contribution from the gen's.. and am thinking the gensets come with their own protection. Is it safe to assume the generator vendor has the appropriate protection for their Tier 4 final gen's?

 

[wayne440]

Seems to me that asking the vendor about the supplied generator(s) protection would be a good idea. I would not assume anything.

 

[RRaghunath]

It is likely that generators as well as the step-up transformer (on its low side) have overcurrent protection. If true, these need to coordinate with the overcurrent protection that is set in recloser.
Apart from this, the under/over voltage protections as well as under/over frequency protections of generators shall not be sensitive to cause tripping of generators during overheadline faults / momentary overloads. Delaying these protections by about 2s is the norm and should help.
Generators of this size are unlikely to have out-of-step protection. Out-of-step protection is known to be sensitive to close-in overheadline faults. This and field failure protections need to be paid attention to avoid generator trips during overheadline faults.
Generally speaking, it is required for you to review the generator protection with the concerned engineer once the protection settings on overheadline side are ready.

 

[Allen723]

Thanks @RRaghunath, I typically provide settings to protect the overhead line using an overcurrent setting and check with the vendor if my overhead line overcurrent settings coordinate with their generator's overcurrent setting. Other than that, I leave the rest up to the vendor on how they want to protect their generator whether that be over/under voltage or over/under frequency.

 

[RRaghunath]

Many a time, the vendors don't care to look at your side settings unless there is something in the contract to force them to do. Also, how do you ensure they do a proper job of coordination unless you review their calculations/settings.
After all as clients, it is your loss if the generators trip unnecessarily for a overheadline/distribution side fault.