VLF AC Testing Waveshape

[saladhawks]

I am in the process of specifying a new VLF AC Test Set and was hoping to receive clarification on 2 vendor claims (HDW Electronics and High Voltage, Inc.) regarding the waveshape of their respective test sets:

HDW claims to use a AC VLF waveshape that features a flat top (looks like a chopped waveshape to me) that allows their unit to additionally measure leakage current as a result of the DC component that is present.

High Voltage, Inc. claims to use a true sinusoidal AC VLF waveshape that allows their test set to also be used as the excitation source for tan delta / pf testing. High Voltage includes "marketing language" claiming that a sinusoidal waveform is "better" than their competitors, but does not explain in detail why this is so.

Any thoughts on the above claims would be greatly appreciated.

 

[Zogzog]

"a sinusoidal waveform is "better" "

Makes sense, thats what the load will be

 

[ausphil]

Have a look at IEEE 400:2001 standard. It gives a good rundown on the different types of tests as well as their advantages and disadvantages.

Cheers

 

[benlanz]

Truthfully, there isn't much difference. Large scale studies show similar results. Both wave shapes can be used for an IEEE type 1 destructive withstand test. Both low frequency AC waveforms are capable of aggressively failing insulation defects much faster than DC. The two companies you forgot are:

- HV diagnostics (Baur)

http://www.hvdiagnostics.com/index.html.

- VON

http://www.voncorp.com/products/c1/xv_vlf.htm

Just remember, that an IEEE type 1 test may be able to improve reliability by failing a weak link but, they can not prove reliability. In order to prove reliability you need to measure partial discharge while applying the voltage stress. If you remember this point, you will never be disappointed with the outcome of your test.
.

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[edison123]

Benjamin Lanz

On the subject of partial discharges in HV rotating machines, I would appreciate your views on the following:

1. Are there any measuring equipment that measure the PD of individual coils/bars outside the machine prior to their installation ? If yes, can you suggest some manufacturers ?

2. Are there any measuring equipment that measure the PD of individual coils/bars inside the machine after their installation ?

3. Would one expect significant PD changes in coils/bars from outside the machine to inside the machine ?

Thanks.

 

[electricpete]

"1. Are there any measuring equipment that measure the PD of individual coils/bars outside the machine prior to their installation ? If yes, can you suggest some manufacturers ?"

Most electrical testing prior to installing coils in the machine will require you to construct a mock slot to provide a ground plane. Once you have a ground plane, observing proper electrical safety, you can do Doble testing, ac hi-pot testing, dc hi-pot testing, and off-line partial discharge testing (for example using Iris TGA-B and a coupling capacitor).

"2. Are there any measuring equipment that measure the PD of individual coils/bars inside the machine after their installation ?"
Assuming the coil connections have been made, your choices would be limited to:
1 - TVA probe / corona probe. Iris will perform this test and provide equipment. I think Adwell may sell the equipment. Also requires an ac source.
2 - Light-out test when powered with ac.


"3. Would one expect significant PD changes in coils/bars from outside the machine to inside the machine ?"
First remember there are two different types of tests you can do in-side the machine: off-line PD test and on-line pd test. On-line pd test is during normal operation, normal voltage stress highest at the line connections and goes down to near 0 volts to ground near the neutral. If you do an off-line test you will energize the neutral possible to normal line-to-ground voltage. Now another problem... generally you want to limit yourself to normal line-ground voltage for testing and ground the other two phases. That means the phase-to-phase paths have less voltage accross them then the would during on-line testing. Finally temperautre will be higher during on-line testing. Lot's of differences. I have seen off-line both higher and lower than what we got for on-line in the plant.







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[benlanz]

Sorry. I am not an expert in the area of PD measurements on rotating machines. I recommend talking to Eaton or IRIS.

If you have other cable questions I would be happy to help.

Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants