100% capacitive reactive power exported to grid by an induction (asynchronous) generator
100% capacitive reactive power exported to grid by an induction (asynchronous) generator
(OP)
Hi all,
we are starting up a 100 kW power plant meant to export 100% power to the grid. The plant consists of an internal combustion engine driving an asynchronous (induction) generator. The generator has a rated power factor of 0,85.
After a short test (50% rated power, about 10 minutes), the meter recorded ONLY reactive (capacitive) energy being exported, no real energy at all.
Meter readings follow:
Power import prior connecting the generator to the grid: 1.7 kW
Power import after connecting the generator to the grid: 0.0 kW [amp reading at generator was 77A]
Active energy exported: 0 kWh
Reactive (capacitive) energy exported R+C: 6 kVARh
Reactive (inductive) energy exported R-L: 0 kVARh
Not having any power factor correction I was expecting high reactive energy readings, but not 100%! Anyone knows what's going on here?
Thanks and regards.
we are starting up a 100 kW power plant meant to export 100% power to the grid. The plant consists of an internal combustion engine driving an asynchronous (induction) generator. The generator has a rated power factor of 0,85.
After a short test (50% rated power, about 10 minutes), the meter recorded ONLY reactive (capacitive) energy being exported, no real energy at all.
- The grid operator installed a bi-directional meter (net meter)
- There are no power factor correction items in place
- The starting inductance is shunted 10 seconds after the main breaker closes (so the generator was directly connected to the grid during the test)
Meter readings follow:
Power import prior connecting the generator to the grid: 1.7 kW
Power import after connecting the generator to the grid: 0.0 kW [amp reading at generator was 77A]
Active energy exported: 0 kWh
Reactive (capacitive) energy exported R+C: 6 kVARh
Reactive (inductive) energy exported R-L: 0 kVARh
Not having any power factor correction I was expecting high reactive energy readings, but not 100%! Anyone knows what's going on here?
Thanks and regards.





RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
I think you have to check the polarities once more.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
An example:
An induction motor is rated for a speed of 1760 RPM. The slip is 1800 RPM minus 1760 RPM equals 40 RPM slip. Running at 1800 RPM plus 40 RPM or 1840 RPM is a good starting speed for 100% export power.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
What still puzzles me is that the meter registered EXPORTED kVARs: if I'm operating the generator at a very low power it will have a lousy PF, so it will draw lots of kVARs from the grid, that's clear even for me... but how on earth will it export capacitive reactive power to the grid? This isn't even a synchronous machine, which can become a rotating capacitor of sorts...
Any and all enlightening to this wannabe-electrician mechanic is highly appreciated.
Thanks!
Ernesto.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
Polarities of the CT:s and how U and I are connected to the instrument, that is. There are endless possibilities to hook it up wrongly and only ONE correct connection. I once had to visit a ship twice (in different harbours) to help the electrician wire the W and var instruments correctly. He never understood how important it is to define and connect correcttly.
It may even be that the export/import definition is wrong.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
the meter connections were done by the utility company. The POD has a plate where the utility plugs their meter. The utility company supplies both the plate (to which we connected the 3 phases, neutral and ground) and they installed the meter and applied their seals.
I'm assuming that there's only one way to plug the meter to the backplate (socket) and that they know how to do it... if there's no physical explanation for what the meter registered, then I'm assuming that there's something wrong with the software running the meter.
Thanks!
Ernesto.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
Why "Thanks!"
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
Tomorrow we'll do a longer test and will ramp up the power to 80% rated output, let's see what the meter records this time.
"Thanks" is for helping a mechanic understand electricity... and for using your time for my problems!
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
1) Running at a higher load will show active power exported
2) there is a bug on the meter software... there's no way we're exporting vars
Tomorrow we'll see.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
I wanted to point out the incompatibility in the assumptions about the vars. (100% reactive power being exported really puzzles and worries me...)
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
Generator/Active:
+P = Real power delivered
-P = Real power absorbed (not a very good generator)
+Q = Reactive power delivered
-Q = Reactive power absorbed
Load/Passive:
+P = Real power absorbed
-P = Real power delivered
+Q = Reactive power absorbed
-Q = Reactive power delivered
Delivered meaning, from the device to the utility/network/grid, and absorbed meaning, from the utility/network/grid to the device.
A +Q load would be an inductor and a -Q load would be a capacitor.
Also remember:
S=(V<a)(I<b)* where * denotes the complex conjugate and F<d means Fe^(jd) and j=(-1)^0.5
Thus,
S=(V<a)(I<-b)
or,
S=((V*I)<(a-b))
or,
S=VIcos(a-b)+jVIsin(a-b)
or,
S=P+jQ
P is + for:
270<(a-b)<90 in degrees
P is - for:
90<(a-b)<270 in degrees
Q is + for:
0<(a-b)<180 in degrees
Q is - for:
180<(a-b)<360 in degrees
I would guess that a meter from a utility for a typical customer would be configured for a Load/Passive convention.
If you can uncouple the prime mover and just run the motor off of the utility and you get the same Q reading on your meter as when you have the the prime mover driving it, then you will know for which convention your meter is configured.
A question for the other posters: Could an induction generator deliver reactive power if one phase had DC current supplied to it?
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
The two sections are different and separate and there is no question that the 6 kVARh where registered as flowing from into the grid. Now, we know that the generator was running at low load/high PF. The generator manufacturer showed us that the 6 kVARh are consistent with the operating conditions of the generator during test, but I would have expected these being registered in the section of energy going from grid to customer.
There is no way that the user could change the configuration of the meter, so my opinion is that there is a software error in the meter itself.
There's another more twisted explanation: maybe the meter is configured in such a way as to show VARS consumed by customer during active generation in the section of energy flowing from plant to grid, while VARS consumed during passive mode in the section "grid to plant".
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
RE: 100% capacitive reactive power exported to grid by an induction (asynchronous) generator
If the meter is set up to measure power exported from your site, then Capacitive VARs exported equals Reactive VARs imported.
There is the same phase relationship between the current and the voltage for capacitive VARs in one direction as there is for Reactive VARs in the other direction. See Gunnars comments re the polarity of the CTs. If the CTs are connected so as to show export power, then imported reactive VARs will be shown as exported Capacitive VARs.
Bill
--------------------
"Why not the best?"
Jimmy Carter