Diesel generator connected to grid absorbing reactive power 6

[Carlos Melim]

Good evening everybody.

Can a diesel generator, connected to the grid, absorb reactive power while producing active power?

A small grid, with underground power cables and frequency converters everywhere, can have a surplus of capacitive reactive power. That raises the grid voltage, mainly during low demand periods.

I have seen hydro power plants performing exactly how I have described.

But some colleagues tell me that a diesel generator cannot be used in the way.

I think that a 10MVA generator will produce 8MW if the load imposes a 0.8 power factor, either capacitive or inductive.

Am I missing something here?

Thanks for your replies.

 

[waross]

Theoretically an underexcited generator will draw vars from the grid.
In practice it may be well to check the capability curve of the generator before prolonged operation at low excitation levels.
When running underexcited the generator may be more susceptible to pole slip.
Induction generators draw all of their exciting current from the grid.

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

 

[RRaghunath]

Generally speaking, the capability of generators to produce VARs is higher than that for absorption of VARs.
If generator is rated at 0.8pf that is for supplying VARs to grid, means lagging. The rated power factor on leading side will not be 0.8.
I have seen rated pf of 0.95 on leading side.

 

[argotier]

Always theoretically speaking, gens are able to draw or produce VARs as long as they are of synchronous type, but I guess most of diesel generators are.

The other theoretical condition, if I remember correctly, it's that the generator must be connected to an already powered grid. If the gen works islanded, the excitation regulation will only change the output voltage.

It's the "fixed" grid voltage that "forces" the generator to deliver reactive power when you change it's excitation. This depends on the relationship between the grid and gen relative powers (ideal case: gen connected to an infinite power grid).

All this from what I can remember from my days in college. I would be glad for someone more experienced to correct me if I'm wrong.

Edit addition: For better correspondence to OP question, I forgot to clarify that in the cases I mentioned the gen is able to produce also active power at the same time. The total output must be within the gen's capability curve, as waross mentioned.

 

[che12345]

Dear Mr. Carlos Melim

Q. " Can a diesel generator, connected to the grid, absorb reactive power while producing active power?"
A1. Yes, but running it at [Leading power factor], which is [undesirable].
A2. a) If you tune the [voltage regulator], you would see the (power factor) swings from lagging to leading or from leading to lagging. Adjust it to say [0.85-0.9 lag]. You will NOT see (any changes in the grid voltage) as your gen set is very small in comparison to the grid.
b) If you toggle + or - the [speed regulator], the kW will increase or decrease. Load the kW based on your gen set rating. Again, you will NOT see (any changes in the grid frequency), as your gen set is very small in comparison to the grid.
Che Kuan Yau (Singapore)

 

[edison123]

Any synchronous generators, when connected to grid, can absorb or supply VAR's within its capability curve. The prime mover is irrelevant. Running the generator at unity power factor gives the lowest copper loss and hence lower winding temperature for the same KW.

Muthu

http://www.edison.co.in

 

[waross]

Again, you will NOT see (any changes in the grid frequency), as your gen set is very small in comparison to the grid.

Any detectable change that an individual set is able to make to the grid frequency will be cancelled out by the swing set.
Any detectable change that an individual set is able to make to the grid voltage and/or power factor will be cancelled out by the swing set.
However, line impedance between the set and the swing set will tend to diminish the voltage effect seen at the swing set.


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

 

[catserveng]

When larger renewable projects came online in may areas this subject became more active, while all of the projects I have been involved with used natural gas fueled RICE units, as pointed out above, what the prime mover is really isn't the issue. This experience is with dispatchable standby systems, most in conjunction with large area wind farms, largest plant was 110MW using twelve 20 cylinder gas fueled engines, one of the operation modes was to bring on units when voltage was rising on grid and to run at low real power loads and leading power factor as part of a voltage management scenario by the local utility. Usually used during periods of varying wind conditions.

Take a look at this,

https://r5.ieee.org/houston/wp-cont...-Leading-Power-Factor_white-paper_24Feb14.pdf

In the projects I worked on we did not run at leading power factors for extended periods of time, but did operate near the limits of the capability curve in both leading and lagging power factors depending on what was going on with the local grid and what the system operator wanted the plant to do. To properly do this a lot effort went into selecting appropriate generator design, AVR selection and programming, control system interface and programming and multiple levels of protections.

Hope you find the article helpful.

MikeL.

 

[waross]

Good article Mike. Thanks.

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