Testing of air-cored HV reactors 1

[AusPowEng]

Hi All,

I have a client who has been advised to replace some 33kV air-cored reactors that are part of a high-pass filter (L/R-C) on the basis that the measured inductance was significantly less than the design value (<1/4). The filters have been out of service for several years and are being checked prior to reinstatement. From the concentric construction of these reactor coils and the lack of any obvious damage of the expoxy coating, I suspect the measurements were taken incorrectly.

It would appear that the measurements were taken using either a handheld inductance meter or a motor winding analyser, but that is all the information I have. With consideration of the low flux levels in air-cored reactors, what is the correct way to test the inductance? In operation, the voltage across the reactor is quite low (1kV), so would a variac and ac ammeter be the best way to test (say at 110V ac?).

Thanks & regards,

AusPowEng

 

[scottf]

Did you check to see at what frequency the inductance of the reactor was rated. For filter reactors, it may be something, for example, like 10 mH at 1000 Hz, etc...

The reactors also have inherent capacitances, so the total inductance changes as a function of frequency.

Other than that, I agree that it sounds like a measurement error, there is not much that can change in an air-core reactor, especially if you don't see signs of any damage.

 

[AusPowEng]

Scottf,

Thank you for your reply. The concentric shells making up each phase of the reactor reactor are separated ~25mm air-gap then connected externally at the spiders. Therefore there is no way that I can see at inter-turn fault shorting any sufficent turns such that overall inductance is reduced to any appreciable extent.

I am not at the office at the moment, but I recall that the inductances for the two reactor stacks (113mH and 361mH) were the stated values at 50Hz.

Having said that, I thought there would be good indcutance linearity for the reactor right up to the 50th on the basis of the air-core. Is this the case?

Regards,

AusPowEng

 

[scottf]

The inductance is linear with respect to voltage due to the air core, i.e. no saturation.

Induction, or better yet reactance, is always a finction of frequency. Remember that just about any device has inherent inductances and capatances and the total reactance changes based on frequency, i.e. Xl = 2*pi*f*L and Xc = 1/(2*pi*f*c)and Xt = Xl - Xc. When Xt is positive, it's considered inductive and when it's negative, it's considered capacative.

 

[AusPowEng]

Scottf,

My query about the lineararity related to your first post, where I got the impression you were hinting a non-linear inductance...which seemed a bit strange for a air-core.

Understand your point that inherent (ie. stray) capacitance will play a part, however there is no chance that stray capacitances will influence the overall inductive reactance of these puppies within the power freq range...these reactors are rather large (1.5m diameter, 450kg each).

Thanks again for your input...I'll post the results of the test later in the week.

Regards,

AusPowEng