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Synchronization of Small DG and Big GTG
Synchronization of small DG and big GTG in a Captive Generation System

In my plant (Oil & Gas), there is this scenario:

This is a captive power generation system (no grid connection). There are 3 GTGs (Gas Turbine Generators, rated 10 MVA each) supplying power to the plant in normal operation. And there is one small DG (Diesel Generator, rated 1 MVA) to supply power to facilitate black-start of GTGs and to power-up utilities like HVAC, lighting etc, when GTGs are not working.

During black-start, the DG is started and it supplies all the consumers (GTG auxiliaries) required for black-start and utilities like HVAC, lighting etc. Then, one of the GTG is started. Next, two sources of power (GTG and DG) need to be synchronized and plant load need to be transferred from DG to GTG so that DG can be isolated and switch-off and plant can resume normal operation using power from GTG.

Now, my query is: During synchronization of DG and GTG, which one of them should be considered as fixed power source and which one shall be regulated (V & f)? If we assume that running source shall be considered as fixed and incoming source shall be regulated then DG will be fixed source and GTG will be regulated. Is it acceptable to assume small 1 MVA source as a fixed reference and regulate & synchronize big 10 MVA source with it? I am not feeling comfortable with this because in my personal experience, I have always seen smaller source (DG) being regulated and synchronized with bigger source (GTG) without considering which one is running source and which one is incoming source. In fact, regulating GTG and trying to synchronize it with small DG seems like a mouse is being regulated and synchronized with an elephant.

Please share your experiences and knowledge on the matter to help me out.

Thanks & Regards,
Sk

2 out of 3 members thought this post was helpful...

skbarc,

So, the DG, when running by itself supplying the "house" loads for the plant and auxiliaries for the GT it is, or should be, in Isochronous Speed Control mode.

And, technically, when synchronizing a second unit to a running unit it should be in Droop Speed Control Mode.

And, yes, you are right--it would be possible for the tail to wag the dog in some cases.

A 10W turbine-generator should be capable of light load; is there a minimum load which the unit is to be operated at?

What is the house load when the DG is supplying all of the house load and a gas turbine-generator is being started and run up to rated speed for synchronization?

Is the load stable when the DG is supplying all of the house load and the auxiliaries for a GTG?

I would say that after synchronization of the GTG with the DG--presuming the DG was operating in Isoch mode and the GTG was in Droop mode--it should be possible to switch the DG to Droop mode momemtarily, then switch the GTG to Isoch mode.

But, something tells me that there is some kind of "PMS" (Power Management System) for the plant that is used to control the sharing of load between the three GTGs, and isn't connected to the DG.... Is that correct? Can it be enabled and disabled? Is it disabled when synchronizing a GTG to the DG? Can the GTGs be run in Isochronous Speed Control Mode?

What you definitely DON'T want to happen is for both the DG and the GTG to be in Isoch mode at the same time--that will be bad. Very bad. VERY bad. Blackout bad.

If it was my plant, the DG would be operating in Isoch mode when supplying the house load, and when starting a GTG. When synchronizing the DG and the GTG, the load should be stable at the time of synchronization, and the GTG should be in Droop Speed Control mode at the time of synchronization and for a short period afterwards--not under the control of any external load control system (PMS). Then, I would switch the DG to Droop mode--which would be okay as long as the load was, and remained stable--and then switch the GTG to Isoch mode. I wouldn't switch the GTG out of Isoch mode or to the control of any external load control system (PMS) until a second GTG was synchronized to the running GTG and the plant load was stable, but expected to increase.

But, again--there's a LOT we don't know about the configuration of the plant at your site. There's probably a PMS or external, over-riding load control system for the GTGs. And, I'll wager there are people who wouldn't ever consider running any of the GTGs without the PMS/external load control system enabled and controlling the GTGs. And there might even be other things we don't know about the plant and the nature of its load(s).

Hope this helps!

Thanks a lot for taking time to reply me. I am excited to see the response as this is my first post on control.com.

I would like to take this discussion a little further right upto its conclusion and I seek your valuable guidance on this.

>So, the DG, when running by itself supplying the "house"
>loads for the plant and auxiliaries for the GT it is, or
>should be, in Isochronous Speed Control mode.
>And, technically, when synchronizing a second unit to a
>running unit it should be in Droop Speed Control Mode.
>And, yes, you are right--it would be possible for the tail
>to wag the dog in some cases.
>
>A 10W turbine-generator should be capable of light load; is
>there a minimum load which the unit is to be operated at?

As of now, I do not have value available for minimum load but I believe this is not a concern for short time parallel operation of DG and GTG; just during synchronization and load transfer from DG to GTG.

>What is the house load when the DG is supplying all of the
>house load and a gas turbine-generator is being started and
>run up to rated speed for synchronization?

House load is approx. 0.8MW.

>Is the load stable when the DG is supplying all of the house
>load and the auxiliaries for a GTG?

Yes, load is stable at 0.8MW. Once load is transferred to GTG and DG is taken out from circuit, we will gradually increase the load on GTG.

>I would say that after synchronization of the GTG with the
>DG--presuming the DG was operating in Isoch mode and the GTG
>was in Droop mode--it should be possible to switch the DG to
>Droop mode momemtarily, then switch the GTG to Isoch mode.

I can change the mode of GTG from droop to Isoch but I cannot change the mode for DG (I am not able to find any provision).

>But, something tells me that there is some kind of "PMS"
>(Power Management System) for the plant that is used to
>control the sharing of load between the three GTGs, and
>isn't connected to the DG.... Is that correct?
>Can it be
>enabled and disabled? Is it disabled when synchronizing a
>GTG to the DG?

Yes, it is correct. PMS is there for load sharing between GTGs and it is not connected to DG. And yes, it is disabled during synchronization between DG and GTG. It comes into picture only when synchronization and load sharing between GTGs is needed.

>Can the GTGs be run in Isochronous Speed Control Mode?

Yes.

>What you definitely DON'T want to happen is for both the DG
>and the GTG to be in Isoch mode at the same time--that will
>be bad. Very bad. VERY bad. Blackout bad.
>
>If it was my plant, the DG would be operating in Isoch mode
>when supplying the house load, and when starting a GTG. When
>synchronizing the DG and the GTG, the load should be stable
>at the time of synchronization, and the GTG should be in
>Droop Speed Control mode at the time of synchronization and
>for a short period afterwards--not under the control of any
>external load control system (PMS). Then, I would switch the
>DG to Droop mode--which would be okay as long as the load
>was, and remained stable--and then switch the GTG to Isoch
>mode. I wouldn't switch the GTG out of Isoch mode or to the
>control of any external load control system (PMS) until a
>second GTG was synchronized to the running GTG and the plant
>load was stable, but expected to increase.

Great! So I understand that considering running DG of 1MVA as a FIXED REFERENCE and trying to regulate V & f of a BIG 10 MVA GTG to synchronize it with DG, is not really a matter of concern as far as I do not put both of these generators in Isoch mode simultaneously. Is my understanding correct?

Now, is it possible (and preferable) if we consider incoming GTG as a FIXED REFERENCE and regulate V & f of already running DG set to synchronize it with GTG? In fact, I want to pursue this approach because I have always seen it this way in all my earlier plants. And I somehow believe that it is easy and better to consider BIG size generator as FIXED REFERENCE and regulate a small size generator with it. Can you enlighten me if my belief has got some basis Or is it just biased opinion due to past experience? I am not really an expert or anything on matters of generators and synchronization so your detailed explanations and guidance is welcome.

>But, again--there's a LOT we don't know about the
>configuration of the plant at your site. There's probably a
>PMS or external, over-riding load control system for the
>GTGs. And, I'll wager there are people who wouldn't ever
>consider running any of the GTGs without the PMS/external
>load control system enabled and controlling the GTGs. And
>there might even be other things we don't know about the
>plant and the nature of its load(s).

As I explained above, PMS does not come in picture until GTG is synchronized with DG and then DG is taken out of service.

1 out of 1 members thought this post was helpful...

skbarc,

>Great! So I understand that considering running DG of 1MVA
>as a FIXED REFERENCE and trying to regulate V & f of a BIG
>10 MVA GTG to synchronize it with DG, is not really a matter
>of concern as far as I do not put both of these generators
>in Isoch mode simultaneously. Is my understanding correct?

Yes; if you put both generator prime movers into Isoch mode, that would be bad. I suspect the DG is only capable of Isoch mode, since it is basically only for operation when no GTG is available. Operating for a "brief" period of time to switch all load to the GTG should be no problem. The way to unload the DG if it can only be operated in Isoch mode is to load up the GTG slowly (in Droop mode); as the GTG load increase towards 0.8 MW the DG load will decrease to zero. Once the DG is isolated from the GTG, then you enable the PMS to let the GTG control frequency and voltage.

>Now, is it possible (and preferable) if we consider incoming
>GTG as a FIXED REFERENCE and regulate V & f of already
>running DG set to synchronize it with GTG?

I don't understand what you mean by "FIXED REFERENCE". I only know Droop and Isochronous. And, if the DG is running and you want to bring the GTG "on" to the bus (which is being powered by the DG), you have to synchronize the GTG to the DG. You will be closing the GTG generator breaker onto the bus with the DG, after all. If the load is stable when you're doing this, there should be no problem. And you don't want the GTG to be in Isoch during synchronization if the DG can only be in Isoch.

I have been at sites where they operate in "island" mode when the grid is unavailable or something causes a separation from the grid. When they want to re-synchronize (re-connect) to the grid, many need to remain in Isoch (to maintain frequency) during synchronization--but as soon as the tie-line breaker closes they have to switch to Droop mode. Some sites actually switch from Isoch to Droop just before synchronizing--as long as the load is stable, and then re-synchronize to the grid.

Again, the concept is the same--only one unit can be in Isoch at a time on a grid with other generators and their prime movers. EVERY time I've seen a generator synchronized to a grid in Isoch mode, the load--and voltage--just does haywire and usually gets disconnected on reverse power. The same thing happens when trying to synch a unit in Isoch to a unit already running in Isoch--load, and voltage, swings usually result in one or both units tripping, either on reverse power or overload/over-temperature.

Hope this helps! You are free to try synchronizing however you think it might work--and if the two governors can "share" load when both are Isoch and the load is fairly stable, well, you would have a very unique site and set of circumstances.

Thanks CSA.

Now, concept is clear to me. In fact, I also read your response to other posts on similar topics and it really helped a lot.

skbarc,

Thanks for the feedback! "Feedback is the most important contribution!"(c) here at control.com.

I am happy to have been of help.