HVAC pressure test

[eejacky]

As advised by our testing department, HVAC pressure test was not recommended for a 132kV cable circuit. The circuit included a 1km underground cable and a long OHL. The reason for this was that the OHL has a high resistance due to the presence of lightning arrestors and insulators. My question is as follows:

1. What is the principle of the HVAC test set?
2. Why the resistance of the lightning arrestor and insulators would affect the HVAC test?

I hope the expert can help to explain the reasons.

 

[davidbeach]

Usually a test of this type is conducted at a voltage well above normal operating voltage. Lightning arresters will conduct at voltages closer to the normal operating voltage than the test voltage, so you can never achieve the test voltage while the arresters are connected.

 

[RalphChristie]

David is correct.

A pressure test is done to detect insulation failures - normally on new installations.

I would suspect you have XLPE-cable if you want to do an AC pressure test.
For a 132kV system the test voltage between phase & earth is 132kV for 60minutes.
For an effective grounded system (132kV systems will be solidly grounded) the rated voltage of surge arresters need not to exceed 0.8 x Um - with the pressure test you will exceed this value, the arresters will conduct and finally be damaged.


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[benlanz]

This advice only pertains to your shielded power cable, not your OHL:

HV and EHV shielded power cable tests are performed using frequency resonant transformers which can provide up to 400kV AC when used in tandem. One unit that we use can produce 260kV at 83 amps.

As RalphChristie stated arrestors must be removed during the test. When the arrestor goes into conduction mode the transformer will be kicked out of resonance and the voltage will not increase beyond the arrestor turn-on threshold voltage.

We recommend the cable be tested according to international standards to at least 2Uo (approximately 152kV). This is a tradeoff between 1.5Uo for accessories and 2.5Uo for new cable insulation. By international standard definition, a voltage withstand does not prove reliability and is likely to make defects worse without failing them!!! A longer test time does not improve a withstand test’s ability to predict future performance.

Depending on the type of cable insulation, you must perform a general condition assessment, partial discharge test, or both to predict future performance. The 2Uo test does not have to last more than a few seconds to just capture the data.

Regards,

-Ben


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

 

[davidbeach]

One unit that we use can produce 260kV at 83 amps.

I don't think so. Even single phase, that would be over 21.5MW.

 

[benlanz]

Davidbeach,

It is true that one unit we use can produce 260kV at 83amps. Perhaps you are not familiar with resonant transformer technology. Resonant transformers coupled to HV and EHV cable (a reasonably pure capacitive load) can attain a Q factor up to 80, and thus only have to supply 3.25kV at 83amps (or ~270kVA) to achieve 260kV. The effective transformer output is, as you have stated, about 21MVA.

We have tested over a dozen HV and EHV lines this way in both Europe and the USA. We will be performing a standardized partial discharge test on two 3.5km, 345kV lines in Connecticut this fall at 1.25Uo (250kV, 40amps, @49Hz). If this is not enough evidence of the truth of my statements, further references are available upon request.

I hope this clarifies the confusion. ;-)

Kind regards,

-Ben





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

 

[davidbeach]

Ok, 21.5MVA at close to zero W, if you in fact have 83A at 260kV. But that 21.5MVA still has to come from somewhere. If you are supplying 83A into a resonator at 3.25kV, I have no problem with the idea of reaching 260kV, but you won't also have 83A, you'll have something closer to 1A. Otherwise you have a violation of all sorts of conservation laws, and we can't have that. MVA can't just be created in the resonator.

 

[ausphil]

davidbeach,

Its quite a proven technology, and based on the resonant frequency of a tuned circuit. When Ben describes the 260kV and 83Amps, he is decscribing the capacitive current that is being supplied to the cable, and in no way reflects the power drawn from the power supply.

The power supply to the unit basically supplies the resistive losses of the system. The capacitive reactance is balanced (ie tuned in either inductance or with frequency or a combination of both) with the inductive reactance, and a Q factor is esentially determined by the resistance of the unit.

The following paper describes the concept and one of the test units around that does the work. It is a series resonant test set, but it can also be done with parallel resonance as well.

http://www.hvtechnologies.net/images/cigre400k.pdf

Ben, i'm jealous of yours!!

 

[benlanz]

Davidbeach,

If I can help you in any way with the theory resonant systems please feel free to contact me.

Kindest regards,

-Ben

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

 

[davidbeach]

I think the issue is the 83A at 260kV; which says to me that you can supply 83A across a voltage drop of 260kV, or 21.5MVA. What is more likely is that you are pushing 83A of line charging current into a system that has a much smaller voltage difference between the ends of the cable under test. Should that cable fault to ground, you would not be pushing 83A across 260kV.