GENERATOR LOAD SHARING

[cvirgil]

Anyone know where to get some literarture on why load sharing is required when generators are run in parallel?

 

[davidbeach]

Why would you even consider not having them share the load?

 

[byrdj]

I didn/t know it was required. A unit could be run in "load limit". If only two units were parrallel and both load sharing, they would fight each other trying to control frequency. Putting the unit on "load limit" would prevent it from swinging with the load/frequency and force the other one to do all the swinging

Why not operate load sharing? what if the generation is needed for the system, but its governor is unstable.

 

[rbulsara]

cvirgil:

In fact the term parallel implies that they will be sharing the load, whether or not equally is other question.

Ideally they should share the load in proportion of their ratings. And when do that they will not be 'fighting' for frequency or in other words they will run at same speed with least trouble when sharing the load properly and not otherwise. So I am not sure where you are coming from.

There are other issues other than load sharing, like maintaining power factor (reactive power flow)and all those issues get much more easier to set and control when load sharing is done properly.

 

[SteveFehr]

They should never be "fighting" for frequency- one will be the master and the other will parallel to it, decided on by the paralleling logic. We usually set it up that the first to come up to 90% becomes the "master" and the other(s) parallel(s) to it.

Since they're both connected to the same load, wouldn't the load sharing percentage be determined by Ohms law? Each generator is (hopefully!) outputting a very similar voltage- as both see the same load, I=V/R, current will be split pretty much evenly, with small fluctuations just due to individual voltage output as well as whatever differences in manufacture and wiring.

 

[motorspert]

Stevefehr
they will only be generating the same voltage if at the same power factor, if one is overexcited and one under then there will be a few % difference in their voltages. The AVR's will sort this out though, if set up correctly.

Remember - the governor controls power and frequency only.
Whereas, the excitation (AVR) controls voltage and powerfactor only.
Fundemental, i know but surprising how many people forget this

Richard

 

[cvirgil]

Good information listed here but I guess I should have been more specifcic. Anyone know of good literature on the baiscs of generator paralleling and load sharing?

 

[byrdj]

you might find what you are looking for in IEEE or API standards.

 

[niallnz]

As Motorspert points out the governor controls the speed/frequency of the prime-mover generator. In an isolated load (no grid connection) single generator system, as the load changes, the frequency changes and the governor acts appropriately to maintain the speed/frequency at the desired setpoint (50/60Hz). It is important to note that from the governor's perspective it has no knowledge of the load, only the speed.

So if you add a second generator into this isolated system (and imagine that at some instant the load and speed are stable), both governors will see a speed change and react. However, they will not react identically, and the load supplied from each generator will be different. This is a real time process so the load will possibly oscillate, but more likely one generator will be take all the load and the other will go into reverse power.

To provide a stabilising influence on this system feedback from the governor output can be used, this feedback is called droop. Typically it is set up so that speed is allowed to drop, proportionately, by 5% for a 100% governor output. This is how simple mechanical governors on diesel engines work. As governors have moved into the electronic age the feedback can be from an actuator or real power output of the generator, and the govenors can communicate between each other to balance out the load and maintain the frequency at the setpoint.

To cvirgil I haven't provided any literature, but I hope I have explained why/how load sharing is needed and is done.

Niall

 

[tladson]

Both Basler and Woodward have excellent reference material on their websites.

 

[Jari]

Hello Everybody!
Load sharing lines are needed when we run isochronous mode with motor controller and VDC or cross current with AVR. Basler have that cross current system and ABB VDC. Baslers cross current is for user little more complicated because if you have system where you are normally parallel with grid and if grid disapear your AVR:s must have sychronized all units with same voltage or you must go to AVR and set the values same by hand (practically all have little different values).ABB Unitroll had beginning problems with VDC but they have now upgaded the sofwares and it is easy also upgrade at site. One thing for these load sharing lines, you must remember when you trouble shooting from one engines function the problem can be the other unit. Also the cabling for load sharing lines must be done very carefully.
Merry Christmas happy new Year

 

[jtkirb]

loadsharing can help keep engines operating in more effective ranges. Woodward makes products that can help here.

For example, Cat engines like to be 50% or more loaded to reduce oil consuption and be in the turbo "happy range". Two engines online sharing load would be better than one loaded base and the other only partially loaded.

JTK

 

[e5fornow]

Generally load sharing is required to prevent the prime mover from having a reverse power condition. You also need to consider VAR sharing. If you do not take these items in consideration good luck trying to keep your breakers closed. The protective relaying will trip everytime. You might want to look at the Woodward site and do a search on a 2301A Load share Module. I think you should also check out the protective relay requirement such as a GE489 Generator Protection Relay.

Hope this helps