Generator sizing doubts 1

[eeprom]

Hello,
I'm on a project where we will be using a 410 kw prime diesel generator to run three motors: a 30 Hp, 30 Hp, and 335 Hp. All motors start across the line, and the 335 will start first. The generator can produce 617 peak amps. The big motor has an FLA of 400. I have doubts that the generator will be able to start the big motor. The generator dealer disagrees. Does anyone have some information to convince me that the generator can produce enough current to start the big motor?

thanks

 

[waross]

I haven't run into that problem (YET) Lionel. Were you able to reduce the voltage drop with a better quality AVR? I may run into this myself someday.
Thanks
Bill

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

 

[davidbeach]

as long as the generator uses a suitable regulator

No doubt, but when every last kW is being pinched out of the generator size, what are the odds of a suitable regulator?

 

[waross]

Hi David. With a soft start I would be concerned with the regulator possibly misinterpreting the voltage distortion due to the part cycle conduction combined with the relatively high impedance of a generator and pushing peak voltages too high.
Another issue may be a d'Arsonval based meter giving an inaccurate indication on a distorted wave form.
On the other hand, if the frequency is being dragged down I want to see a voltage drop.
But, I won't challenge Lionels experience.

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

 

[maxcontrol]

Most VFDs and soft starters do not tolerate fluctuations in incoming frequency very well. Frequency regulation is only as good as the governor on the diesel engine. Unless the engine is substantially oversized to the load then anticipate problems with the VFD or soft start faulting. When the diesel engine is oversized enough to maintain steady frequency on the generator output, you don't need a soft starter or vfd.

 

[waross]

Not always. I experienced a group of soft starts at a remote sawmill. The soft starts had trouble on the grid as we had unbalanced voltages and a displaced neutral on the grid. The soft starts would trip out on phase angle error from the neutral issues.
They started almost well on the generators.
By that I mean that we had two generators in parallel and needed both generators to start the large motors even with the soft starts.
One generator had a hydraulic governor and the other had an electronic governor.
When the motors started tripping out on phase angle error we would start the generators but if we tried to start immediately the hydraulic governor could not respond fast enough to prevent the electronic governed machine from hogging the load and tripping the main breaker on the gen-set. After warming up for 15 or 20 minutes the oil would be thin enough that gen-sets would share the load well enough to avoid tripping the breaker.
Al;though we had phase angle issues on the grid we never had frequency issues when running on the gen-sets.
A VFD with a passive front end is just rectifying the AC. It should be even more forgiving of frequency excursions than a soft start.

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

 

[eeprom]

Given the application of two generators in parallel, how do you prevent backfeeding? And does it matter if there is backfeeding in small amounts? Suppose the AVRs were not perfectly similar, and gen 1 output 482V and gen 2 output 479V. At a glance I'd say that some small percentage of gen 2 would become a load to gen 1. This imbalance seems inevitable. Does it cause any damage?

 

[waross]

People write pages and chapters on these subjects but quickly;
Generators in parallel.
1> The load (KW) that each generator will accept depends on the throttle setting of the engine or prime mover. This may cause a slight interaction with the frequency.
The Classic control scheme is to run the governors in droop mode. At no-load the generators run 3% fast and drop to nominal speed/frequency at full load.
2> The VAR sharing depends on the voltage setting. There is a scheme called cross current compensation or the quadrature circuit which biases the AVRs to compensate for small inequalities in AVR settings. In practice, with a normal load, the set in your example that has the higher voltage setting will take a larger share of the VAR demands. If your example voltages are stand-alone voltages, the cross currrent compensation will work to reduce the difference when the sets are in parallel.
3> Do the imbalances cause damage? Usually small imbalances cause one set to take more than its share of the load. Governor issues cause KW sharing issues. Voltage setting issues cause VAR sharing issues.
4> One set may take more than its share of the KW load while the other set takes more than its share of the VAR demand. There is very little interaction if any between speed settings and voltage settings on paralleled sets.
BUT
If the sets are heavily loaded, sharing issues may cause one set to be overloaded. This may be a current overload (with heating issues), a mechanical overload with prime mover issues or both.
Governor issues may cause mechanical overloading which may lead to overcurrent issues.
Voltage setting issues generally lead to overcurrent and heating issues.
And, a severe unbalance under light loading may cause the unloaded set to "Motor". This does not generally cause immediate damage but if the condition persists may lead to wet stacking or slobbering. This is a carryover of lube oil into the exhaust system. Some piston rings particularly when old, do not seal well without the force of combustion. Also, any lube that passes the rings will eventually be pushed out into the exhaust system instead of being burned with the fuel in the combustion chamber. When the engine is again put under load, the wasted lube oil may be ignited in the exhaust system, it may be expelled, (messy) burning lube oil may be expelled from the exhaust or an explosion inside the exhaust system may result.
This used to happen when the operator tried to stop a generator but forgot to open the circuit breaker first. A common design feature is a reverse power relay/trip to avoid this event. The last time I saw this happen I replaced a faulty reverse power relay.
Another possible issue may be damage to the fuel injection system. Some injection pumps depend on the flow of fuel for lubrication and extended operation with no fuel flow may cause damage. While most diesel engines will run on jet fuel, it will also lead to damage to some injector pumps due to insufficient lubricity.

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

 

[X49]

A soft start that I commonly use has a rated frequency range of +-20% of 60Hz. I'm not sure if this is typical. Frequency dip on a generator should be 10% at worst.

 

[anythingwithawire]

Beware of where you control power (contactor) comes from - you may have enough generator power but the volt drop can drop out the contactor (around 80%) if it is powered from the one phase and neutral, often the case for larger contactors.

 

[waross]

X49; Your question regarding PMG equipped generators;
These hold the voltage up very well as long as the frequency holds up. When you are starting large motors on a gen set you often have a frequency drop. Depending on the size ratio between the generator and motor this may be until the governor opens up enough to pick up the load or it may be sustained longer.
If the generator is lightly loaded and turbo charged, you may have a frequency drop while the turbo spools up.
When the frequency drops more than 3 Hz the UFRO function of the AVR will drop the voltage regardless of the voltage available from the PMG exciter supply.
Note: The Permanent Magnet Generator supplies power to the AVR to avoid voltage collapse on faults. The main generator is not changed and may be excited with a conventional AVR in an emergency while waiting for parts should the PMG add-on fail. There may be protection and block loading issues but the set will generate and supply most loads.

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