×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

HVAC pressure test

HVAC pressure test

HVAC pressure test

(OP)
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.

RE: HVAC pressure test

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.

RE: HVAC pressure test

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.

Failure seldom stops us, it is the fear for failure that stops us - Jack Lemmon

Make the best use of Eng-Tips.com
Read the Site Policies at FAQ731-376

RE: HVAC pressure test

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

RE: HVAC pressure test

Quote (benlanz):

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.

RE: HVAC pressure test

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. winky smile

Kind regards,

-Ben


  

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

RE: HVAC pressure test

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.

RE: HVAC pressure test

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!!

RE: HVAC pressure test

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

RE: HVAC pressure test

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources