Recommended method of parallel operation of Gen sets after Islanding


Thread Starter


Hi Tech gurus,

I have a query on parallel operation of generators post islanding from grid. Say, we have 3 Generators of rating 20MW each runnning in parallel with Grid with a total load of 45MW, each sharing 15MW (15MW exported with 30MW Home load). In case the grid trips, what should be the recommended mode of governors: 1) One machine in isochronous with other two in Droop, 2) All three in Droop. Please state advantages and dis-advantages of the said methods.

Please help...

Without some kind of PMS (Power Management System) or over-riding frequency control scheme when operating in island mode it's best to have one unit in isochronous control, which means all others would need to be in droop control. While this <b>DOES NOT</b> relieve the operators from monitoring island load changes and "moving load" around to keep from overloading or "underloading" the isochronous unit, it does generally mean the frequency of the island should remain fairly stable as island load changes because the isochronous machine will respond to island load changes in order to maintain stable island frequency without too much operator intervention. (This, of course, presumes the isochronous governor is properly tuned and matched to expected load changes and even unexpected (but anticipated) load changes.)

There ARE sites/plants that operate multiple generators and their prime movers all in droop control, but I think you will find there is some kind of over-riding PMS or frequency control scheme that is adjusting load(s) on the individual units as electrical load increases or decreases in order to maintain stable frequency. If left to operators they will be constantly having to make adjustments (unless the island load is relatively stable) in order to maintain a stable frequency.

If all three gensets are operating in droop control in island mode with no PMS or frequency control scheme you will find that every time the island load changes the frequency will change and the operators will have to change load on one or more of the droop machines to respond. Depending on the stability of the island load, this could mean a lot of work for the operators--and they don't like doing much more than leaning back in their chairs, eating biryani, reading the newspaper, and telling the outside operators to check this or that.

The important thing for operators to know in any case when operating as an island is that they need to monitor the load even on an isochronous machine and adjust the load(s) on any other machine(s) to prevent the isochronous machine from being overloaded (and causing the frequency to decrease) or from going to zero power and causing frequency to increase. The operators CANNOT change load on the isochronous machine directly--they can only change the load on the isochronous machine by changing load on one or more of the droop machines. (Increasing the load on a droop machine will decrease the load on the isochronous machine by the same amount; decreasing the load on a droop machine will increase the load on the isochronous machine by the same amount.)

When re-synchronizing to the grid, the operators use the isochronous machine to vary the island frequency to match the grid frequency and then, usually, switch the isochronous machine to droop immediately before closing the tie breaker, or an automatic switch changes the isochronous unit to droop at the instant the tie breaker closes.

There are even sites that operate with multiple units in isochronous control, called isochronous load sharing, but in those cases there is usually some kind of PMS or overriding frequency control and/or the isochronous governors are de-tuned to prevent fighting, and the operators have to be very vigilant.

Hope this helps!
@CSA : Sir, let me first thank you for such a nice and easily understandable reply.

I have earlier read many of your threads and the analogies you give are simply superb. Hats off to you sir. There will be only very few like you who share their knowledge.

Coming to my question, i have clearly understood the Isochronous and Droop mode of operations as briefed by you. One thing i want to ask/recheck with you is, if during re-synchronisation with the grid when speed raise / low signals are given to the isochronous machine to match with grid what would be the scenario?

i.e say the grid is at 49.5Hz and our system is at 50Hz, now we have to lower the speed of our system to 49.5Hz. For that if the speed of isochronous machine is reduced what would be the impact of load on other Droop machines and Isochronous machine? Will the load vary or the decrease in speed doesn't have any impact on load of Droop and Isochronous machines?

Similarly if all the machines are in Droop mode and the frequency has to be raised to 50Hz from 49.5Hz, should we give speed raise signal to all Generators at the same time to match 50Hz? Since raise signal is given to all generators at same time i don't think there will be any impact on load. Please correct me if i am wrong.

Also please clarify the same on variation of voltage (Reactive power). These small doubts are haunting me since a long time, if you could help me understand this i would be very grateful to you sir.

You are most welcome, sir.

When operating as an island (separated from the utility) <b>EVERY</b> synchronous generator is running at the same frequency because they are <b>synchronized</b> together. That's the definition of synchronism--all generators are operating at the same frequency (synchronous speed). No single generator, or even two out of three generators, can operate at any other frequency than the other generators are operating at--they all locked together in synchronism at all times (except during split-second transient conditions). Even if the frequency is higher or lower than nominal/design, all synchronous generators are synchronized together--electromagnetically locked in synchronism--at the same frequency (synchronous speed).

In fact, even when synchronous generators are connected (synchronized!) to a large grid, <b>EVERY</b> synchronous generator is operating at the same frequency. It's simply not possible for any single synchronous generator or group of synchronous generators (two, three, or twenty-nine, or two thousand two hundred twenty-nine) to operate in parallel <b>synchronized together</b> at different frequencies. All generators synchronized together, whether physically right next to each other or 2229 km apart from each other, are operating at the same frequency because they are <b>synchronized</b> together.

When operating as an island with one unit in isochronous mode, when a raise speed (frequency) signal is given to the isochronous machine then the frequency of all synchronized generators will increase by the same amount--including that of the isochronous machine and any droop machines <b>synchronized</b> to/with the isochronous machine.

During resynchronization of the island to the grid, if the grid frequency is 49.5 Hz then the island frequency has to be 49.5 Hz or slightly greater than 49.5 Hz during synchronization, and when the tie breaker is closed the frequency of all the generators of the "island" will now be equal to, or the same as, the frequency of all the generators on the grid, and if that's 49.5 Hz, then all generators synchronized to the grid will be at 49.5 Hz, including the newly synchronized units from the island. (This presumes the grid is very large, or "infinite", and that the entity or organization responsible for controlling grid frequency is doing a good job of monitoring and controlling grid frequency, which, if a 50 Hz grid is operating at 49.5 Hz it's obviously not doing a very good job.)

When operating in island mode, the isochronous machine controls the frequency of all synchronized generators--they all have to operate at the same speed as the isochronous machine. That's what isochronous mode does--changes the energy flow-rate into the prime mover in response to changes in island load which would tend to change frequency (speed) but the isochronous governor senses the changes and adjusts the energy flow-rate into the prime to keep the frequency (speed) constant as island load changes.

Since the droop machines are synchronized to the isochronous machine the frequency (speed) of the droop machines is controlled by the isochronous machine. That's what isochronous mode does: keeps speed (frequency) constant as load changes (which would tend to change speed (frequency)). And any generators connected to (<b>synchronized to/with</b>) the isochronous machine have to operate at the same frequency (speed) as the isochronous machine.

The load of the droop machines does not change as the load of isochronous machine changes; they are operating in droop mode, so their energy flow-rate is remaining constant, but the energy flow-rate of the isochronous machine is changing so it's load changes.

If you are resynchronizing your island to a grid which is at 49.5 Hz, you have to lower the frequency of your island (by lowering the speed (frequency) setpoint of the isochronous machine) to 49.5 Hz--but that does NOTHING to change the island load (the number of motors and lights and computers and computer monitors). The island load remains the same, but the frequency will have to change in order to resynchronize to a grid operating at 49.5 Hz.

Actually, what I think will happen is that as the isochronous machine energy flow-rate is lowered the droop machines will pick up some load (because the error between the speed reference and the actual speed will increase), and the load on the isochronous machine will drop--but the total number of lights and motor and computers and computer monitors will not change (the island load). If the island was operating at a stable 30 MW at 50.0 Hz, then to resynchronize to a grid operating at 49.5 Hz the island will have to operate at 30 MW and 49.5 Hz. And after resynchronization the island will remain at approximately 49.5 Hz--or whatever the grid frequency is. Immediately after synchronization, the load of the three synchronous generators that were re-synchronized to the grid will remain at 30 MW until someone or something increases their load (by increasing their speed setpoint, because they will be operating in droop speed control mode). But, their frequency will be the same as every other synchronous generator on the grid they are <b>synchronized</b> to.

I want to go back to my bicycle analogy. If there is a tandem bicycle with two riders and two cranksets (sets of pedals) and the cranksets are chained together such that both cranksets have to spin at the same speed, they are "synchronized." Both riders have to pedal at the same speed.

Now, if the bicycle is operating on a flat road and is supposed to maintain a constant speed then the crankset speed of both riders has to be the same. If one rider applies more torque to his crankset than the other the bicycle will speed up and if the other rider does not alter the torque he is applying to his crankset the bicycle speed will remain above desired.

Multiple synchronous generators--and their prime movers--<b>synchronized together</b> on a grid are EXACTLY the same. They all operate at the same frequency (synchronous speed). Regardless of the number of generators and prime movers. That's the nature of AC electrical systems. That's why a synchronous generator must be "synchronized" to the grid--it's frequency (synchronous speed) has to be very nearly equal to the grid frequency when connecting (synchronizing) to the grid. And once connected (synchronized) to the grid, its frequency (synchronous speed) is the <b>SAME</b> as the grid frequency--and of all the other generators <b>synchronized</b> to the grid. Regardless of the size of the "grid" (island, or infinite, or anything in between). If one rider is continually adjusting his torque in order to maintain speed as the other rider changes his torque then speed of the bicycle will remain relatively constant. But, if the two riders do not coordinate their efforts (torque) the speed of the bicycle will be erratic. But, because the two cranksets are chained together, neither can go faster or slower than the other.

I prefer not to get into the whole reactive power thing at this time; it's very draining. And it's so misunderstood by so many people (possibly even myself) that it's becoming clearer to me each time I write about it that I don't want to write about it any more.

There have been a LOT of threads on about reactive power. If you can explain what you do and don't understand about reactive power, we can try to help, but to just "explain" reactive power is to invite a lot of ... speculation. Since we can't draw vectors here, it's even harder to explain.

I wish I'd paid more attention to power factor and VArs on my shipboard experiences ("islanded"). Unfortunately, the ship I sailed on had a DC system, not an AC, for the majority of the ship. AC was only used on the bridge for navigation electronics, and in some classrooms (where computers were located). So, our turbogenerators were DC, and we didn't have to deal with AC very much (there were DC-AC motor-generator sets that supplied the AC power for the bridge and classrooms, and I didn't really get involved that much with them).

I've often wondered what would happen on an islanded power system with, say, a 0.87 lagging power factor, if someone inadvertently increased the generator voltage of one of, say, three generators by a large amount. I think the island voltage would go up, and the reactive power flowing in that generator would go very high in the lagging direction, and the reactive power flowing in the other generators would go to or towards the leading direction. The reactive power "load" of the island wouldn't change (because the number of motors and lights and computers and computer monitors wouldn't have changed), but the reactive power "splits" between the generators would change, and likely the island voltage would change as well. In my experience, once generators are synchronized together their voltages are usually pretty equal as well--just like their frequencies (which are exactly equal, though). Trying to change the voltage of one generator with respect to the others (the island- or grid voltage) is what causes reactive power to flow in one direction or the other of the generator whose voltage is being increased or decreased with respect to the island- or grid voltage.

I think reactive power management of an island is as important as the real power management. But, I haven't spent a lot of time analyzing or troubleshooting it. Usually when I'm at an islanded plant, they doing everything they can to re-synchronize as quickly as possible, everything else is secondary.

But, unless you can tell us what you do and don't understand, then I'm not going to venture into that territory any more.