Keeping capacitor banks on while running fron diesel generators

None that I am aware of, as long as the diesels are replacing the supply in general and are not simply standby or emergency equipment.

Power factor correction is used to maintain an acceptable and economical electrical load condition - not a supply condition and as long as your PF is close to BUT not running at 1, then your load installation is running at its most effective condition.

If your diesel generators are not supplying the whole load and are standby or emergency sets supplying only essential equipment, then leaving manually selected capacitors in circuit will cause a heavy and unnecessary leading power factor load if the inductive load they are correcting has been disconnected.

Guest #1 is correct.

However, the initial question uses the plural word "generators", indicating that there is more than one generator running is parallel.

If a sinigle generator is the sole supplier of the load, little or no difference (except higher voltages) will be observed.

If the generator is synchronized on a network, at unity or near unity power factors, severe instability conditions can occur with resulting power swings that will destroy the network (Blackout).

In the power industry, running a generator connected to a power grid at unity pf can cause major power swings that will jeopardize the system and should be avoided at all cost.

The exception is when running a synchronous motor at high field excitation as a VAR generator. Note I mentioned MOTOR. ( energy is being extracted from the shaft)

Gents

Please check engine manafacturers name plate ans relevent ISO refered diesel engines for genset application is manafactured to operate with PF 0.8 as per ISO by all engine manafacturers world wide .if operated with capaciter bank on with higher PF say 0.9 or 1 you would be overloading the engine and reduce engines mechenical life ,and asking for trouble .Your alternator will accept PF of 1 many a times we oversize the engine Example 2000Kva engine coupled to 1500Kva alternator whenever we come across inductive furnaces and Heating loads where Pf will be 1 due to nature of load.one has to keep in mind AC Generator is a diffrent animal and cant compare with grid power.

This is interesting, you size your generators on a KVA basis rather than a KW base and your engine horsepower is sized accordingly.

I guess this makes sense because in practice, most user loads are going have a reactive component, and the current can become the limiting factor and will be determined by the winding copper size.

Using this criteria, I assume that a gen set supplying a pure incandescent load at rated KVA would run out of horsepower and load down or overheat the engine.

Come to think of it, all large generators (10 Megawatts and up) have a PF rating on the nameplate but the KVA rated current is above the KW current and takes the pf into consideration. I assume that you rely on engine inertia for starting heavy loads.

Again, my comment, very interesting information.

Snaker is correct to reply No. 2 you cannot calculate or express the output of the mechanical prime mover in KVA - only watts calculates a direct comparison to horse power.

Further you cannot dictate what the alternator output PF will be on a fixed parameter Gen set, this can only be done if you have control over the rotor field excitation.

The load will dictate the PF.

Running at a low power factor increases the loading on the alternator, PF correction is therefore equally desirable with a diesel set as with the power company supply.

The rating plate specifies the loading output of the set at the stated PF as a performance output indicator. If the load is not at the power factor stated on the plate then the stated output is not valid

If the name plate states 40 KVA at 0.8 PF(46A at 400V), and in case you are operating at a PF less than 0.8, using PF correctors will help. But you have to be careful that the load doesnot exceed 46A. For instance, at 0.75 PF you can load only upto 42A at 400V, if the PF is improved to 0.8 you can load upto 46A at 400V and if you improve PF to 0.9 your operating current can be upto 51A to deliver 40KVA. But your genset winding is designed only for 46A.

So, in this case, as long as you use PF correctors to improve upto 0.8PF it has a positive effect. But beyond that no.

To Quote you: "If the name plate states 40 KVA at 0.8 PF(46A at 400V)"

Whoa... 40,000 / 400 = 100 amps (reactive) .... I have never seen a alternator rated in KVA.

Note KVA is wattless power. What you are describing is not a generator, its a large capacitor or reactor.

Generator size is rated in KW not KVA... This determines the horsepower required to turn it.

Your generator should be able to deliver the full rated KW current Plus the additional 20 percent reactive current to allow for the poor power factor. (at 0.8 pf)

I think your Engineering Power Lab instructor just flunked you.