100% capacitive reactive power exported to grid by an induction (asynchronous) generator

[ebarba]

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.

[ul]
[li]The grid operator installed a bi-directional meter (net meter)[/li]
[li]There are no power factor correction items in place[/li]
[li]The starting inductance is shunted 10 seconds after the main breaker closes (so the generator was directly connected to the grid during the test)[/li]
[/ul]

100% reactive power being exported really puzzles and worries me...

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.

 

[Skogsgurra]

It is not possible to make an induction generator (motor, actually) produce reactive power at all. But it is possible to make it absorb 100% reactive power. All you need to do is to run the machine at synchronous speed - or a wee bit above - to cover the losses.

I think you have to check the polarities once more.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

You are probably carefully running the engine at too slow a speed. The exported power will be closely related to the power supplied by the prime mover. As you increase the speed, you will increase the exported power.
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

 

[ebarba]

I checked all datalogs and I think that what Bill says is what happened (running at too slow a speed, just above sync).

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.

 

[Skogsgurra]

As I said: I think you have to check the polarities once more.

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ebarba]

Gunnar,

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.

 

[Skogsgurra]

If there's software involved (you didn't mention that), then it is about the definition. Wo is exporting? Wo is importing? But you can rest assured that vars are never generated by an induction generator/motor. It just doesn't happen.

Why "Thanks!"

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ebarba]

I meant the software running inside the meter. The meter is electronic and performs net metering besides sending the billing information over the grid to the utility company... a nice piece of technology, but if the software running inside has a bug, then all the right connections just don't count.

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!

 

[Skogsgurra]

OK. If there's nothing wrong with the installation and nothing wrong with the software. And your observations are right. Well, then you have some kind of a new and very unique machine - an induction motor that actually produces vars! What about that?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[ebarba]

my guess is that

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.

 

[Skogsgurra]

Sure. The load is not a problem. That is trivial.

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[davidbeach]

100% reactive power being exported from the utility to the generator is entirely possible. Import and export; forward and reverse are not unambiguous definitions.

 

[waross]

I think that it is what it is. I read it as the grid exporting capacitive VARs to excite the motor. We know the characteristics of VAR flow associated with an induction motor. what we don't know is the convention that the power company is using. Whatever convention the utility chooses to use, I am sure that it will be consistent.

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

 

[wroggent]

These are the conventions I'm familiar with:

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?

 

[ebarba]

About the convention: this electronic meter has two different sections to read what comes from the grid (the grid charges the user for it) and what goes into it (the user sells this energy to the grid).

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".

 

[ebarba]

little update: we did not make the test yesterday because the plant that produces the fuel (wood gas) on site was down. We'll run the generator again on Tuesday.

 

[waross]

It is what it is. But consider, reactive energy is an imaginary concept that elegantly simplifies the description and calculation of the effect of a phase shift between the current and the voltage in an electrical circuit.
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