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Three Questions about Diesel Generator
Power generation equipment control. topic
Posted by azman on 16 January, 2009 - 11:09 pm
We have 2 sets of diesel generator 5MW, 6.6KV and used in our plant. My questions:

1) Can we synchronise 2 generator without load and, synchronise first and then loading this two sets later (due to we have to start a 6MW motor)

2)When try to adjust voltage trimmer (manually) during manual mode there is no increment & decrement of voltage and voltage maintain at 6.4 KV and why.

3)Why during synchronising (other plant case) only one set take load and another set still 0MW and not sharing the load with other unit.


Posted by jojo on 17 January, 2009 - 12:20 pm
1. Yes you can start the generators synchronise then and then start your 6MW motor. However:

a. It all depends on how the 6MW motor starts. I would expect that there is some sort of soft starting facility on the motor. This however depends on how much this soft starting facility is capably of softly starting the motor, applying load to the generators and give them time to pick up the load. Having said that, given that the motor rating is very close to the combined rating of the gensets, I would presume that the soft starting facility of the motor is capable of controlling the load demanded by the motor, and if the driven process permits, one can start the motor on one genset, slowly load it, and then when the demand from the motor is close to the 1st genset rating synchronise the 2nd genset and continue loading the motor.

b. If you want to synchronise your 2 gensets before starting the motor, you have to be sure that the gensets share the motor load pretty evenly. Otherwise the 1st genset that goes in overload when the motor load is applied, will trip out immediately, tripping the 2nd genset with it. Given you stated that the gensets do not share loads equally, then there is no point of synchronising the gensets before starting the motor. The above indicated will happen.

c. You stated that when you adjust the voltage trimmer nothing happens on the genset. The voltage does not change.

Well in view of all the above, you have to be sure that:

1. you have a clear understanding of the generators' control systems.

2. you have to ensure that the speed droop settings of the generators are set as far as is practically possible the same.

3. you have to ensure that the generator AVR droop setting are set close to each other.

Unless the above is performed, I am afraid that you will find it very difficult to run your 6MW motor on your two 5MW generators.


Posted by CSA on 17 January, 2009 - 3:24 pm
1) It's possible, but usually difficult. Requires some well-tuned governors. It would probably be best if you had some load on one generator, then synch'ed the second to the first, then tried to start the large load.

2) Impossible to say without looking at the exciter regulator schematics.

3) I'm not sure I really understand the question, but here goes, anyway. The answer to this question, and also somewhat to the first, lies in how the two unit's governors are being operated: the governor modes.

In general, there are two basic modes: Isochronous Speed Control and Droop Speed Control. The reason one unit is picking up all the load and the second one (we presume the two are synch'ed together) is probably because the one that's picking up the load is being operated in Isochronous mode and the other is being operated in Droop mode. (Note: Many governor manufacturers have different names for these modes, but most generator prime mover governors should have modes equivalent to Isochronous and Droop, especially ones being provided for a site being operated as you describe.)

As load is added to the "system" the frequency of the grid would tend to decrease but the prime mover being operated in Isochronous mode will increase the power output of the unit to maintain the frequency.

If the other unit is in Droop mode, it will just maintain it's power output constant, especially if the Isochronous unit is doing a good job of maintaining frequency.

If you want the Droop unit to pick up some of the load, you need to manually tell it to "share some of the load." This whole concept of "sharing the load" is very misleading. Sharing the load *DOES NOT* mean the unit will automatically increase or decrease its power output to try to accept some "equal" amount of load. It just means that it won't try to control the grid frequency and will "behave" and provide power to the grid in a stable manner. If two Isochronous units are synchronized together (without some kind of special load-sharing control scheme) the grid frequency will usually be very unstable and difficult to control.

If you manually increase the power output of the Droop unit, the grid frequency will try to increase but the Isochronous unit will reduce it's power output to maintain the grid frequency. The amount of load will not change, just the division of the load between the two units. If the power output of the Droop unit is increased by 0.5 MW, the power output of the Isoch unit will decrease by 0.5 MW, this is necessary to maintain grid frequency. This is the "sharing" part. Two Isochronous units will not generally share load; they will fight to control the load and as a result the grid frequency will be vary unstable.

I have seen cases where people have detuned the Isochronous mode of units to try to make them behave as if they were Droop units, but while that may work in one condition it may not work in all conditions. And, people usually get "surprised" when it doesn't work in all conditions.

A 6 MW load suddenly being added to a grid with no single generator capable of supplying 6 MW "instantaneously" will usually result in problems (frequency dips at a minimum). Hopefully, there is some kind of soft starter on this load, and the power output of the motor can be increased gradually to allow the operators to control the frequency properly.

Let's just say that there is some kind of soft start mechanism or the load can be gradually increased on a 6 MW motor on a grid with only two 5.5 MW generators. Let's say one generator is operating in Isochronous mode and the other is operating in Droop mode and the present load on the system (grid) is 1.5 MW, with the Isoch unit producing 1.0 MW and the Droop unit producing 0.5 MW. The motor's operator(s) should notify the generator operator(s) before they start the motor, and the generator operator(s) should probably increase the load on the Droop unit to about 1.25 to 1.40 MW. The motor operator(s) can now start the motor and as it starts and is loaded, the Isoch unit will increase its power output to pick the load of the motor *and* maintain grid frequency. The Droop unit won't do anything; it's power output should remain at 1.25-1.40 MW. As the power required by the motor reaches 5.0 MW (near the limit of the generator's prime movers), the generator operator(s) will have to start to increase the load of the Droop unit by at least 0.5 MW, probably more, which will cause the Isoch unit's load to decrease by the same amount (presuming the system load is stable when this is happening). When the Isoch unit has sufficient available output to provide the power required to load the motor fully to 6 MW, the motor operator(s) can then load the motor to maximum.

In this scenario, the two generators are "sharing" the load of the motor which is at 6 MW. If the Isoch unit is at 5.5 MW and the Droop unit is at 1.75 MW, then the Isoch unit has 5.5 MW of the motor's load and the Droop unit has 0.5 MW of the motor's load, plus the remainder of the system load.

The generator's operators should then probably increase the power output of the Droop unit (which will cause the output of the Isoch unit to decrease by an equal amount) in order for the Isoch unit to be able to respond to any further load increases on the system to maintain system frequency.

So, the two generators are "sharing load", but not "automatically". There are load sharing control systems to do that, but they're not always set up properly and that may be the case at your site. But, it's been my (painful) experience over the years that operators just don't understand Isoch and Droop and "load sharing" and that even with automatic control systems there must still be careful monitoring of system operation with conditions such as you're describing.

Operators and their supervision just think the control systems should do it all and while they might be able to, they generally have to be configured and/or programmed to do so and that usually requires proper testing to demonstrate desired operation, and that usually just doesn't happen. While systems can usually be operated successfully without complicated load sharing schemes, it takes trained and knowledgeable operators, and as in the example above, coordination. And that usually is difficult to achieve.

Best of luck with your situation. I would suggest some review of your system configuration by a firm that can make some recommendations and maybe even provide some training to really make the best of the equipment and the conditions at your site.


1 out of 1 members thought this post was helpful...
Posted by Phil Corso on 17 January, 2009 - 6:28 pm
Azman, is this a problem you have now? Or one you anticipate?

If the latter, then, there are several approaches. But, they depend on what is known about your diesel, generator, motor, load, and system characterstics!

If you can provide the details, I can suggest several schemes to study!

Regards, Phil Corso (cepsicon@aol.com)


Posted by blackstoneboy on 18 January, 2009 - 1:27 am
Sir,
1. Yes it is really possible to do synchronization without load. I agree with CSA that you need fine tuned governors. Why? During synchronization without load, generators must maintain very low current at all times to avoid reverse power. When synchronizing automatically, you will notice that there is current going in and out of your genset(called circulating current). This is due to the reaction of governor.

When the generators are synch'ed without load, all the genset are in the same frequency, but the speed is different. Some goes slower and some goes faster. Why the same frequency? We have a sensing coming from the generator, and we have a sensing coming from the synch bus or grid. When the breaker is closed, the genset terminal and bus or grid terminal is already common, thus, your sensing from genset and bus is also common(clear??). Governor reactions should have the greatest consideration at this times. The less circulating current, less fuel intake at no load.

2. Trimming the voltage. There are lots of reasons why the voltage can't be adjusted.
a. Check if your avr is efficiently working.
b. Check control lines if all is ok.
c. During synchronization, same as the frequency, the output voltage of your generator and bus or grid is the same once your sensing is attached to a common point when the breaker is already closed. But as you trim your voltage, you will see that your power factor is changing. =). If your gensets have the same power factor, it means that you have a good reactive load sharing. But do not attempt to trim the voltage if your avr is in droop mode. This might burn your rectifiers as i have experienced when synchronizing 6 engines in an industrial plant.


Posted by anil on 2 April, 2010 - 4:06 am
The answer to the first question is yes,u can synch 2 gensets without load. in fact you can synch on a dead busbar also.

Your second query about not able to raise voltage, could be a reason of overload or probably some issues in AVR controls.

Your third question about the generator not sharing load, could be this unit is in droop mode with minimum fuel or if it is isosynchronous mode. the there would be some problems with the load sharing lines (wiring) or with some set points concerning load sharing.

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