Synchronising generators 2

[maqhawe]

Hi,

I need some advise please, please see the attached SLD

I have three standby gensets running in parallel, 400V, from the 400V bus bar to the two transformers, I am going to keep the breakers at a normally close position. I don't expect any problem in terms of voltages as my transformers are identical.
my master controller will only control the utility incomer, and do a soft mains return, that is when there was load shedding or something.

my only worry is when I want to do a test with load, whereby the Utility will be live, which means the 400V bus bar will be live, my gensets will be synchronizing to the live bus bar,
I am worried about the phase shift that will be imposed by the start-delta transformer's, can the 30 Deg phase shift affect the synchronizing of each gensets?

 

[nkn5]

Hello,

The phase shift by transformers shouldn't be a problem as the generators would be synchronizing to the voltage on your 400V bus. Is is just than the generators will be running 30[sup]0[/sup] lagging from that of utility which is corrected by the transformers (assuming dYN11).

If you're still designing the system, I have a piece of advice. In the case of running the generators in parallel with utility, the fault current at the 400V busbar will be dangerously high. Infact, it might be hard for you to procure a type tested switchboard for the ratings there. So if you have a chance, please consider using 11kV diesel generators or split up the generator-transformer units to decrease the fault levels.

 

[maqhawe]

Thanks NKN5,

Unfortunately the project is in the commissioning stage now, I don't like the design as well, but I must make it work, I was involved later on this project.
in my designs I make sure I avoid such, if I don't use a 11KV alternator I use a step up transformer for each genset and sync on 11 KV,

in terms of fault level, the units will be in parallel with the utility for a very short time during the change over period only, not on peak shave mode.

 

[waross]

The best arrangement is to sync check on both sides of the breaker that will be closed to parallel. This avoids a lot of problems.
You should check the fault current levels with the generators in parallel with the grid.
The fault current contribution of co-generators is generally much less than the grid levels. You may be safe.

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

 

[ScottyUK]

The fault level with two paralleled 3MVA transformers is going to be very high without the contribution from the three 1.5MVA diesels, and the fault level when paralleled is likely to be well above 100kA.

It is a peculiar arrangement in so many ways, and very likely would have been better served by captive GSU transformers for each generator. It would almost certainly be cheaper too, trading an entire LV switchboard for an extra 11kV breaker and a transformer.

 

[maqhawe]

Thanks a lot, much appreciated,

Each genset has a maximum fault level of 20KA, the 5000A n/c breakers to feed the transformers are rated at 100KA at 1 sec,
if I keep this breakers N/C it means the 400V bus bar will always be Live and will be subjected to the full fault level of the utility,
Scott you are on point, working with a 11KV design was going to be easy and cheaper, with less complications,

I am trying this option of keeping the transformer feeder breakers closed to avoid the use of much more expensive master controller to open and close those breakers,
I asked for sync checks on the LV 5000A breakers, but the client couldn't procure them, and if I could get them, now it will mean that I will have to keep those breakers N/O

 

[maqhawe]

Hi all,

since I am keeping the 4 breakers closed, should I be worried about the inrush currents from energizing two transformers? because this means that the first gensets to close to the Bus will energize both transformers before the other two gensets close their breakers.

By the way, thanks for all your information, it was quite helpful, on Wednesday I commissioned the site, test 1 and test 2 were successful, I still have to get time to work on test 3 which is to synch to a live utility, and change over the load to genesets.

 

[crshears]

Hello maghawe, I can't tell from your SLD [a] where the grid connection is, or where the load is connected...maybe I'm not clear on your terminology...

CR

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

 

[maqhawe]

Hi crshears,

thanks for your time,
The relocated incomer 1 cable connection is the grid connection, new breaker number 9 is going to a distribution board for all the plant loads

 

[waross]

If you ever need to do a black start:
Breakers-
One breaker from a generator to the 400V bus closed.
Both 400V breakers to the transformers closed.
#7, New loads. OPEN
#8, #9, Transformers. OPEN
#10, Incomer, Grid Connection, OPEN
Now the first generator may be started. It will bring up the voltage on the two transformers as if they are Generator Step-up Units, without any energization surge.
Note: These transformers should be energized from the grid, or if this is not possible, they should be brought up as GSUs by a starting generator.
Once the 400V bus and the 3.0 MVA transformers are energized, the second and third generator may be started and sync'ed to the 400 Amp bus.
Now breakers #7, #8 and #9 may be closed to pick up any loads on the 11 kV bus and the new loads.

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

 

[maqhawe]

Thanks waross,

I have a challenge with my master controller, when I got to site I found out that I cant control most of the breakers, breaker #9 WILL ALWAYS BE CLOSED,
and breakers 5000A, #7 and #8 WILL ALWAYS BE CLOSED ASWELL,
I am not worried about the initial plant load if I have to do a black start, the aux power is around 300 KW, and the system is automatic,
when the master controller senses mains failer, it sends a start signal to all 3 gensets, they start and the first to reach 90% of its rating, will close its paralleling breaker to the 400V bus, which at the same time will be supplying the plant with aux power, and also energize the 2 transformers, that's where I am worried about the inrush current, over the long run of the units, does it have an effect to the first genset to close its breaker?

 

[ScottyUK]

You need to reconsider your black start control strategy. Avoiding the switching transient caused by closing onto a de-energised transformer is well worthwhile because of the substantial voltage disturbance it will cause. If there is already automation in place then the hardware capability is probably available - you need to look at the software. Bill has given you some good starting points.

 

[waross]

Good luck energizing two 3 MVA transformers with one 1.5 MVA generator.
Now is the time to be writing the CYA ("Cover Your Assets") memos in regards to the system's inability to do a black start as currently configured.


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

 

[mher1]

Hello guys,
Sorry for posting a late reply to this thread.
I have a similar project like Maghawe's.
In my design there is NO paralleling with Mains.
I have 4 generators synchronized at LV (400 Volts)then Voltage will be stepped up to 33 KV.
The SLD is similar to what Maghawe had posted but our transformers are off the shelf Delta-Star Dyn11.
I would appreciate your comments on the attached SLD.

Thanks.

 

[iceworm]

Give us some context:
Comments about what in particular?
Is this existing equipment and you are stuck with it??
Is most all in place and you are making a modification?
You get to spec all the equipment?

Disclaimer: Medium voltage distribution is not an area of my expertise. I've worked on a few, and around a few, but MV design is definitely not an area of expertise. My comments will all be biased toward IEEE and NEC. However, I have a lot of time around off-grid industrial generation.

There are a few things that caught my eye:
Gen cb ratings

Gen CB look too small. For 1MVA I would expect:

1MVA/415V/sqrt(3) x 1.25 ---> 17400A
For 1.5 MVA --> 2600A
For 2MVA --> 3500A​

The CB sizes selected are close, but with industrial, marginal is rarely long term successful. The customers are usually happiest at WFO + 10%. Unless the CBs are 100% rated - In which case you are golden.

Why two cb's in series?
That first CB next to the gen adds little. One common method I've seen is to have the Gen mounted CTs connected to differential/Over Current protection that shuts down the driver as well as opens the CB.

Are the gens solidly grounded, or perhaps HRG? If solidly grounded, there is and issue with paralleled gens where if one gens gets an internal ground fault, the others can deliver enough short circuit current to damage the faulted gen beyond rewind repair. I have not seen this happen. A few of the systems I have worked (1970s design) incorporated a small reactor (small impedance, large wire) as the NG bond in the switchgear to limit the available fault current. There are a few IEEE papers on this.

Later design systems went with HRG. Definitely my favorite. Ground faults are not an issue.

Gen Maintenance Safety:
Solid grounded or HRG doesn't matter - although it does matter where the HGR is located. Say a gen is off-line, CB open, Techs are inside the alternator, and a system GF occurs. Neutral voltage can easily rise to 231V. And the techs get really pissed. One solution I like is to install a neutral disconnect at each gen. Use a second pole to open the start circuit for the driver.

Next Installment - Moving right to the transformers (Y-D!! yuck

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[mher1]

Thanks Iceworm,

The comments I meant to receive are related to the choice of step up transformer and if it would work fine.
The generators will be solidly grounded.
The step up transformers will be off the shelf Dyn11 (originally designed as step down)connected in reverse feed mode (star on LV side).
Can we connect the LV Switchgear common neutral bus to the transformer star points?
Should we solidly ground the transformer star points?
My concerns are based on the fact that a lot of transformer makes do not recommend reverse feeding step down units...

 

[maqhawe]

If your transformers are identical or at least close to identical, then I don't see a problem,

which breakers you are keeping close and which ones are controlled, especially the transformer feeder breakers?
you have to consider inrush currents, the first genset that comes online may be affected by high inrush current due to energizing 3 transformers.

Why are you having low rated transformers as compared to the total gensets capacity?

on the earthing part, you must control your earthing, the only option I can think of in this design since you don't have a star point on your HV transformer,
you can have an NEC something like a zig-zag transformer, connected from your 33KV Switchgear, and of course have a NER to limit the fault current to a desired value.

 

[mher1]

Thanks maghawe,
Yes, my transformers are identical 33/0.415 KV Dyn11 Step down (connected in reverse feed mode).
I intend to keep the transformer feeder breakers closed. These will provide upstream protection for each feeder.
The controlled breakers are the 4 incoming breakers on the LV switchboard (synchronization panel).
To solve the Inrush Current problem (on the first gen that closes on the dead bus), delayed time closing can be a solution for the feeder breakers.
The transformer capacity is not matched with the generating capacity as not all 4 gens will run at the same time.
Only 2 gens will be powered to supply the load. The provision of having 4 gens in the design is to provide redundant power supply (When Gas supply is interrupted the diesel Gens will supply the load).
Can you please supply more details on the earthing part?

 

[waross]

A four wire wye:delta connection will transfer power between phases.
In the event of a line to line short on A-B. B-C-A will act as an open delta and feed current to the line to line fault on A-B.
The actual fault current will be twice the result calculated by considering only the single phase Available Sort Circuit Current of one phase.
For the same reason a single phase load will be fed by all three phases. A load on A-B will have 50% of the load current flowing in A-B, 50% of the load current at 50% PF flowing in B-C and 50% of the load current flowing in C-A at 50% PF.
This is for a four wire primary connection.
Rather than a Zig-Zag grounding transformer I prefer three distribution transformers in Wyeelta. The calculations are easy and the transformers are generally available "Off the shelf". In the event of a grounding transformer failure distribution transformers may be more readily available than a replacement Zig-Zag transformer.

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

 

[mher1]

Good morning waross,

Thanks for your comments.
So generally speaking the design is acceptable and no need to introduce any changes.
I am a bit worried about the reverse feeding of the step down transformers (to step voltage up).
As a matter of fact all the suppliers here in Nigeria are against the idea. They even went on stating that warranty will not be granted...