EDG Surveillance (Grid Sync) Question

[Bradtv]

I have an Ops question I'm hoping you all can answer for me.

Why exactly is the EDG synced to the electrical grid?


Some of my recent schoolwork has centered on electrical systems.  During a lecture on Emergency Diesel Generators, the instructor mentioned that the EDGs would be synced to the grid.  This seemed counter productive as the whole reason for them is to provide power independent of the grid in accident situations.

I did locate an NRC presentation concerning this (down by slide 29).  It does explain the possibility of turning the EDG into a motor and various trips.  The reason for it, however, is absent.

This brings me to my assumption (dangerous, I know) that the EDG sync to the grid is utilized for testing because grid operation is preferable to onsite equipment/load testing.


TLDR: Since the EDG is meant for in-house emergency supply, why is it even connected to the grid?

Edited for clarity.

[Higgs]

Simply for testing. There are various operational tests required, including full load runs periodically. We aren't going to simultaneously run all of our emergency loads just to load up the diesel for testing. You just synch to the grid, load it up, run it, divorce from the grid and shut it down. It isn't connected to the grid when it is sitting there in standby waiting to perform its safety function. When connected to the grid, it is considered "inoperable" for its safety function.

Hope that helps.

justin

[Bradtv]

Thanks Justin.

I almost PM'd you directly, but I think others may benefit from the info.

[Fermi2]

Simply for testing. There are various operational tests required, including full load runs periodically. We aren't going to simultaneously run all of our emergency loads just to load up the diesel for testing. You just synch to the grid, load it up, run it, divorce from the grid and shut it down. It isn't connected to the grid when it is sitting there in standby waiting to perform its safety function. When connected to the grid, it is considered "inoperable" for its safety function.

Hope that helps.

justin

Not true in all cases. It depends on the Diesel Logic. At my plant it is not considered Inop when connected to the Grid as if offsite power goes away after a LOCA we take credit for the overcurrent trip on the output breaker.

Also many plants have Surveillance criteria to prove the Synch Circuitry works post LOOP so you can restore Offsite power.

[Higgs]

Not true in all cases. It depends on the Diesel Logic. At my plant it is not considered Inop when connected to the Grid as if offsite power goes away after a LOCA we take credit for the overcurrent trip on the output breaker.

Also many plants have Surveillance criteria to prove the Synch Circuitry works post LOOP so you can restore Offsite power.

Ah didn't know that. Just going off of where I am.

Thanks!

Justin

[x633ro]

AT my plant it is purely for testing. Here it works like this: LOCA: diesels auto start but do not pick up safety busses (just in standby). LOOP sequencers strip the bus close the output breaker and sequence on loads. If paralelled for testing output breaker opens and either of the above happens (depending on LOCA or LOOP) only reason to synch to the grid is to load it down for testing (monthly) safety loads are only about half of the 6MW output per diesel

[Fermi2]

At what plant do LOOP Sequencers strip a bus? That's usually done by 27 devices or other under voltage relays...

Also are you ENTIRELY certain the Sequencer closes the output breaker? I bet if you check the schematics it is a combination of the diesel being up to speed combined with having a field... Sequencers usually need to see voltage on the bus to work.

Also it is very rare for sequences to start loads. They usually provide a permissive and if a standing start signal is present the load starts.

Examples:

You get a LOOP: The Undervoltage relays shed the bus loads. The diesel starts via Undervoltage relays, not the sequencer.
The diesel comes up to speed, the field flashes just below full speed.

When the diesel output breaker sees adequate speed and a field it closes.

When the diesel output breaker closes  the sequencer starts sending out signals in certain time groups. If an ECCS component does not have a signal to start it simply won't.

If a LOCA occurs the output  breaker opens this is not a sequencer generated event.

The output breaker opening causes another LOOP.

Loads shed.

Output breaker recloses

Sequencer or similar logic starts sending permissives to the loads.

Now that the LOCA has occurred the ECCS loads will start because they actually have a start signal.

I bet your plant works the same way though many do not have Load Sequencers, they use various TD Relays to perform said function within the breaker closing schemes themselves.

[Higgs]

At what plant do LOOP Sequencers strip a bus? That's usually done by 27 devices or other under voltage relays...

Also are you ENTIRELY certain the Sequencer closes the output breaker? I bet if you check the schematics it is a combination of the diesel being up to speed combined with having a field... Sequencers usually need to see voltage on the bus to work.

Also it is very rare for sequences to start loads. They usually provide a permissive and if a standing start signal is present the load starts.

Examples:

You get a LOOP: The Undervoltage relays shed the bus loads. The diesel starts via Undervoltage relays, not the sequencer.
The diesel comes up to speed, the field flashes just below full speed.

When the diesel output breaker sees adequate speed and a field it closes.

When the diesel output breaker closes  the sequencer starts sending out signals in certain time groups. If an ECCS component does not have a signal to start it simply won't.

If a LOCA occurs the output  breaker opens this is not a sequencer generated event.

The output breaker opening causes another LOOP.

Loads shed.

Output breaker recloses

Sequencer or similar logic starts sending permissives to the loads.

Now that the LOCA has occurred the ECCS loads will start because they actually have a start signal.

I bet your plant works the same way though many do not have Load Sequencers, they use various TD Relays to perform said function within the breaker closing schemes themselves.

Up to the bold is how ours works. Closing of the output breaker is T=0 for our sequencer. However, whether or not we have a signal to start loads, it all starts at the appropriate time except for aux feedwater, recirc and quench spray. Those are start permissives.

Our high/low heads and service water start even if there isn't a safety injection called for.

Last night in the sim we had a simple loss of E bus 4KV at power and when the EDG sequenced on, we had to eventually go through and stop the extra high and low head pumps.

Justin

[Fermi2]

Up to the bold is how ours works. Closing of the output breaker is T=0 for our sequencer. However, whether or not we have a signal to start loads, it all starts at the appropriate time except for aux feedwater, recirc and quench spray. Those are start permissives.

Our high/low heads and service water start even if there isn't a safety injection called for.

Last night in the sim we had a simple loss of E bus 4KV at power and when the EDG sequenced on, we had to eventually go through and stop the extra high and low head pumps.

Justin

And that is because they are designed to do so. An example at my plant would be the Charging Pumps, if you lose a bus, two seconds after the diesel starts the pumps start. It's how the system is designed. I bet it's not the sequencer that starts them, it's the LOP signal.

[Higgs]

And that is because they are designed to do so. An example at my plant would be the Charging Pumps, if you lose a bus, two seconds after the diesel starts the pumps start. It's how the system is designed. I bet it's not the sequencer that starts them, it's the LOP signal.

Oh I see what your saying now. I'll dig into it more tomorrow.

Justin

[thenuttyneutron]

AT my plant it is purely for testing. Here it works like this: LOCA: diesels auto start but do not pick up safety busses (just in standby). LOOP sequencers strip the bus close the output breaker and sequence on loads. If paralelled for testing output breaker opens and either of the above happens (depending on LOCA or LOOP) only reason to synch to the grid is to load it down for testing (monthly) safety loads are only about half of the 6MW output per diesel

I wish my plant had diesels this large.  We are only good for about 2600 KW for continuous running.  We are limited by what we can change due to margin on the diesels.

We consider the diesel INOP and unavailable while it is tied to the grid.  We did a mod on it recently to allow it to remain operable during testing but there are still issues that need to be resolved.  Anytime we do something with the diesel, I automatically perform a 3.8.1 Condition B surveillance check so we don't get hit for missing the 1 hour action.

If there is a LOOP while the diesel is tied to the grid, the output breaker and bus tie breaker open.  The undervoltage will strip all loads with the exception of the CCW pump and the 4160 V to 480 V transformer.  When the EDG output breaker recloses on the bus, the CCW pump instantly restarts if it were running before the undervoltage and 10 sec if it were not running before the undervoltage.  The Makeup Pump (similar to the Charging pump only centrifugal and higher capacity) restarts 2.5 sec later if it was previously running.  The service water pump (similar to a river water pump) restarts 21 seconds later.

If there is a LOOP and SFAS signal, the SFAS sequencer kicks in.  There is a start signal to the ECCS pumps depending on the SFAS Level that the SFAS sequencer will block, unblock for 3 seconds, and reblock the loads until the sequencer has finished its run time (about 25 seconds).  The sequencer only blocks HPI, LPI, Service Water, and CTMT spray (unblocks in this order).  The CCW pump will get an auto start on the SFAS and closes regardless of voltage.  The only thing that will stop the CCW breaker from closing is a Lockout.

We also have a Blackout diesel and it is bigger than the EDGs.  We have to manually start it and it has to go through a nonessential bus if we want to power an essential bus.

[HydroDave63]

You just gotta love how all of these relays and fancy logic have to be employed, simply because the utilities didn't pay a bit more money up front for larger diesels that could handle the starting current on safety-essential equipment 

[Fermi2]

I wish my plant had diesels this large.  We are only good for about 2600 KW for continuous running.  We are limited by what we can change due to margin on the diesels.

We consider the diesel INOP and unavailable while it is tied to the grid.  We did a mod on it recently to allow it to remain operable during testing but there are still issues that need to be resolved.  Anytime we do something with the diesel, I automatically perform a 3.8.1 Condition B surveillance check so we don't get hit for missing the 1 hour action.

If there is a LOOP while the diesel is tied to the grid, the output breaker and bus tie breaker open.  The undervoltage will strip all loads with the exception of the CCW pump and the 4160 V to 480 V transformer.  When the EDG output breaker recloses on the bus, the CCW pump instantly restarts if it were running before the undervoltage and 10 sec if it were not running before the undervoltage.  The Makeup Pump (similar to the Charging pump only centrifugal and higher capacity) restarts 2.5 sec later if it was previously running.  The service water pump (similar to a river water pump) restarts 21 seconds later.

If there is a LOOP and SFAS signal, the SFAS sequencer kicks in.  There is a start signal to the ECCS pumps depending on the SFAS Level that the SFAS sequencer will block, unblock for 3 seconds, and reblock the loads until the sequencer has finished its run time (about 25 seconds).  The sequencer only blocks HPI, LPI, Service Water, and CTMT spray (unblocks in this order).  The CCW pump will get an auto start on the SFAS and closes regardless of voltage.  The only thing that will stop the CCW from starting is a Lockout.

We also have a Blackout diesel and it is bigger than the EDGs.  We have to manually start it and it has to go through a nonessential bus if we want to power an essential bus.

Again essentially what I said. BTW that Blackout Locomotive you guys have in that "shed" is a monster!

[VTnuke]

I'd have to agree with what I see hear.  Where I work, we have to load the EDG to the grid at 10% of rated load immediately to prevent reverse powering the EDG.  We do have reverse power relays that protect the EDG from motoring, but you don't want to rely on a protective feature when it comes to your only source of power during a LOOP.

[GLW]

every now and then you guys rock this website,...

sincerely, it's the biggest reason I stick around,...

I learn almost nothing here in my discipline that I do not already know or have better sources for (with the exception of Mr. McGinnis),...

but when you guys start with this operator stuff I love it, partly because I remember enough that it still makes sense to me and mostly because, although I can follow it, it's just really interesting stuff and I don't have to take any tests on it to keep my job,...

most days test taking is fun, free money,...

but most folks can relate to the notion that on other days testing to keep your job is a total killjoy,...

BOT, this thread here: http://www.nukeworker.com/forum/index.php/topic,22374.0.html

and now this one are two of the gems here at NW.com,...

although I did like this one some years ago also: http://www.nukeworker.com/forum/index.php/topic,5516.0.html

thanks guys,...

peace,... nee 1575

almost forgot,...(sic)