Differential Protection on 600V generators not required? 1

[mbk2k3]

IEEE Buff Book recommends protection schemes based on:

The recommended protection schemes for generators are given by machine sizes.
a) Small (i.e., 1000 kVA maximum up to 600 V; 500 kVA maximum above 600 V)
b) Medium (i.e., from small machine sizes up to 12 500 kVA regardless of voltage)
c) Large (i.e., from medium machine sizes up to approximately 50 000 kVA)

Why is differential protection not listed as a requirement for 600V <1kVA Units?
Just curious about the mechanics of a 600V generator that dictate why or why not 87 differential protection would be needed or not.....

 

[waross]

It has nothing to do with mechanics.
It has to do with the cost of protection versus the cost of repairs or replacement.
Also, smaller machines often do not have trained operators.
Even full time operators may not be adequately trained.
Example #1. A small power plant, 2 x 600 kW and 3 x 350 kW. An operator on all three shifts.
At about 2:00 A.M. a rad hose broke and the overheat alarm went off. The shift left the plant and went to his supervisor's nearby apartment for instructions. By the time that the supervisor woke up and answered the door, the engine had seized and shut itself down.
Example #2. A new standby set for a business. The rings were not properly "seated". The unit pumped all of the lube oil into the exhaust in a short time and the unit shut down on no oil pressure. A stock boy pushed the reset button and the set started and ran without oil until the start delay timed out and the set shut down. Three times. Then I was called.
I serviced a couple of dozen small machines.
Example #3. A standby set was installed and then ignored for years. I was called and told that the set had not worked for about a year.
Every filter was a couple of years past replacement time. The oil had never been changed. The battery had finally failed due to advanced age.
After an oil change, a change of all filters and a new battery the set started and ran well.

I found that such neglect and lack of operator knowledge was common on smaller sets.
Differential protection would be a waste of time and money.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

You often can't get the neutral end of the windings of small machines brought out to allow the installation of the CTs necessary for differential protection.

 

[waross]

Very true for motors and also true for very old Delco 10 lead generators.
The 10 lead Delcos were typically telephone exchange standby sets. These machines dated back to the days when DC generator exciters and slip rings were common. The machines that I saw had all been retired due to age and reold on the surplus market. Almost all of them had been upgraded to brushless exciters.
However with the exception of two old, surplus Delcos, all the three phase generators that I have installed in the last 20 years were 12 lead machines with both ends of each winding brought out.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

It's been more than 10 years since I dealt with machines that size, but had made inquiries and understood at the time it would require a special order. May have not asked around enough. Given what's available today for protection, I'd throw in differential anytime a generator relay is being applied. Just using a low voltage circuit breaker doesn't leave many options.

 

[waross]

Hi David. I think that it is more a cost issue. Most of the machines in this size range are bought from the lowest bidder.
The closest that I ever came to differential protection was for a 1500 kW machine.
I wanted to add differential protection but could not get the cost of CTs and a relay approved.
The vendor could have supported me but chose to remain silent on the issue.
Don't forget these are less than 1000 kW, not 1000 MW.
When I was doing a lot of generator sizing, I spent many hours going over specifications of diesel generators.
All of the three phase machines were 12 lead machines.
All of the sets above about 15 or 25 KVA were three phase with 12 leads.
These sets were marketed and suitable for use world wide. The base machines could be configured for 50Hz or 60 Hz.
All of the machines could be connected as star or delta.
Above about 15 KVA or 25 KVA (depending on speed, 1800 RPM or 3600 RPM) all were suitable for conversion to single phase.
Both delta connections and single phase conversions require access to the neutral leads.
Your comments are accurate for motors and transformers, but not for small generators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

I agree with waross, its primarily a cost issue.

Not all locations will consider incidental costs (i.e. replacement of a containerised generator isn't that expensive, but the logistics and resultant outage can be) but in almost all cases I've seen, adding differential is a significant expense, whereas smaller machines aren't that expensive. I've seen it in use once on an LV machine, but this was a 6.6kV station with a single LV unit with its own step up transformer covered by differential, all the other machines were 6.6kV and also had it.

I've never seen a unit under 2MW that has used a generator protection relay, its certainly correct that a circuit breaker doesn't offer that much protection, but most of the controllers these days will also cover Undervoltage, Overvoltage, overload and most of the functions that would normally get covered by a protection relay (and the controllers are compliant with relay standards such as IEC60255 anyway). Since the controller also covers most of the functions that are needed to manage the generator, adding in an extra relay for differential is just extra cost and complexity that doesn't gain much in terms of equipment protection.

If we're talking unit step up generators and covering the transformers as well, then that is probably a different argument, as above LV there's a much higher need for trained operators as well as ensuring that the transformers are protected as well.



EDMS Australia

 

[catserveng]

The beauty of a forum like this is that there are a number of different experiences here. In my case the engine/generator distributor I worked for applied differential protection to a large number of "small" generators, mostly in prime or cogeneration service paralleled to grid, or multiple unit island mode systems. Size ranges typically 500 to 2000kW. Not easy to get neutral side CT's as pointed out above, but with 6 or 12 lead machines not really all that hard either if well planned and executed.

I guess it always comes back to "it depends". If you have a system where getting the generator out if a failure occurs, like in a parking garage, on a roof or in a building that has been modified to make getting the tail end out a real PIA, then what you spend for improved protection may be worth it.

I just recently added neutral side CT's and upgraded the protective relay on a 1.5MW standby unit at a sewer plant, the protection engineer wanted to add differential to include the step up transformer and a section of LV outdoor busduct after a rather spectacular failure. So there may be cases where cost is offset by access, safety and risk, at least from what I've seen.

MikeL.

 

[waross]

Hi Mike; What brand or manufacture were the sets with buried neutrals?
Back when the Olympian sets were manufactured for Cat by F. G. Wilson I went over a couple of hundred spec sheets for small generators and never encountered a buried neutral. All were 12 lead machines.
Or is the issue the physical space to mount the neutral CTs. That I can understand.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Bill,

We used CAT. KATO, Marathon, and Ideal mostly, Stamford and Avk were also specified quite a bit, especially for units going overseas. Also Baylor (now NOV) and Kato oil field generators, all of those were 12 lead. Most had 12 leads, some 6 lead machines in smaller, less than 500kW from CAT and Marathon. Main issue was room to mount CT's, but we usually developed extension boxes and mounts, and in many cases had to extend the neutral leads to make it all work.

Kato, Stamford and Avk had factory options to ship with Neutral CT's, but most were not a match to the switchgear CT's. When newer relays like the SEL300G came out this became less of an issue.

Olympian sets were a challenge, and we never used them in prime applications when I was there, they were a "low cost" alternative to some of our product lines, but never performed as we liked for many applications.

I guess something we should all ask the OP is, what application are you looking at for the generators in question? If these are single unit standby's or small prime units then differential probably makes no sense, but in other cases maybe should be considered.

MikeL

 

[Hoxton]

On small low voltage ac generators I have seen 'Restricted Earth Fault' protection used. This is a halfway house to full differential, since it relies on one ct in the generator neutral and three on the line side - one in each phase. Perhaps one ct in the earth, depending on layout.

The application is on mains - parallel cogeneration and biogas gas engine sets. Continuous operation so protection important.

The scheme is not usually sensitive for phase - phase faults (they end up as earth faults fairly quickly...) or for earth faults near the neutral.

One problem in small generators is accommodating all the cts, since terminal boxes can be retaltively small. Remote cts (in switchboard) can be used, but the same problem of ct size and pilot wires can be a problem.

Again, like others have said, it's a question of cost - benefit. The low voltage generating set market is extremely competitive and trying to sell the benefit of additional protection is rarely successful. Remember a 1000 kVA ac generator can often be sourced from stock, a 5000 kVA ac generator is unlikely to be in stock.

 

[waross]

Thanks MikeL
I suspected that it was a space issue rather than a lead issue.
The old ten lead Delcos dated back to when the GM 71 series of two cycle diesels were state of the art in prime movers.
The last ten lead Delco that I encountered was about 20 years ago and it was an old surplus unit then.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[krisys]

Usually the 1500 kW and below DG sets are used as emergency generators. for the emergency generators, the resumption of supply and ensuring the power supply availability is more critical than just saving the alternator from the repair.
As most of the faults in the LV from the alternator side is related to earth fault. Hence providing an reasonably sensitive earth fault protection, generally serves the purpose. For the LV breakers, Built in LSIG with provision for the neutral CT input is adequate.

Yes, the differential protection makes sense, if you are operating multiple sets. Let us say 10 nos. of prime power duty DG sets operating in parallel. The whole network should not collapse, due to one alternator winding fault. So provide differential protection for these alternators, thus the power supply reliability would drastically improve.