3.3Kv cable Hipot test 2

[Rizgar]

Hello,
we have two medium voltage (3.3Kv)cables of size (3/C-185sq mm + 3/C-70 sq mm) feeding same load from a generator that we want to Hipot test them. Is it right to short the three cores (phases A, B and C) and connect it to ground in both ends of first cable and the 3 cores (phases A, B and C) of the sceond cable (that we want to test) are short and connected to the supply terminal of Hipot test equepment, the other end (phase A, B and C) of the same cable are shorted. The return path and ground terminal of the test equipment are connected to the ground.
Knowing that the sheath of cables are grounded and the 7.6Kv DC supplied . Is this right or there is another wayes to connect the the cable phases to the hipot device.
Thanks,

 

[mpparent]

My first question would be: How old is the cable?

Mike

 

[Rizgar]

mpparent, The cable is new not energized yet.
Does the cable age make diffrence.

Rizgar

 

[54494060]

Does the cable age make diffrence? Yes. HiPot testing, like Megger tests are evaluating the insulation of the cable. The age of the installation and the environment it resides in has great significance in the insulation passing or failing. Especially when HiPoting. If I read you right, you are trying to HiPot all conductors at the same time? Why not test the cables individually?

12 fish
As Iron sharpens Iron so does a man sharpen the countenance of his friends.

 

[Rizgar]

54494060 you read me right.
I wonder is it right to HiPot all conducters of a multi conducter cable at the same time? If it's right I can save time because I am working in a water project that contain many medium voltage multi conducter cables, medium voltage motors and transformers.

 

[benlanz]

The first question I ask all my clients is, "What do you expect to get from the test"? The answer to this question will direct how you should test. If the point of your test is to limit liability and assure reliability connecting or disconnecting the cable will not make a difference because you are using the wrong test. You may not be aware that IEEE 400-2001 no longer supports the use of a DC HIPOT as an acceptance test for MV shielded cable! The reason is that a DC HIPOT misses >95% of all defects on new cable systems. I recommend an AC withstand or, better yet, a PD stress test per IEEE 400-2001. However, if you are just trying to fulfill some internal ‘insulation resistance test’ standard and wash your hands of the project, you will need to have all the cables completely prepared and the disconnected from the bolt-on terminals. For best results test each cable separately and look for a tip-up in the current. I welcome your questions.

Kind regards,

-Ben


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

 

[applekor]

Rizgar:

If you are not familiar with hipot testing it is my STRONG advice to find someone who is to look over your shoulder (or even better show you how it's done a few times) before you perform the test yourself. Interpreting the results is only half of the issue. Just performing the test and knowing what to watch out for in order to perform the task safely and eliminate false data is more critical.

benlanz:
What about VLF methods?

Mike

 

[benlanz]

Mike (applekor)

Which VLF method are you asking about? Very Low Frequency (VLF) is a voltage source. VLF hipot is a low freqency AC hipot withstand method. An AC hipot is much more meaningful than a DC hipot, however, no withstand test can predict future performance. An AC hipot can fail defects that a DC hipot can not. If a defect fails during the hipot withstand, great, you have found a defect that probably would have cause a failure in service. However, if no failure occurs, this is not proof that a defect does not exist and the withstand just made it worse! A VLF hipot is the best hipot, but be prepared to fail cable with defects.

Does this help? More than this may require a phone conversation.

Regards,


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

 

[applekor]

benlanz:

I was refering to VLF hipot.

Thanks,

Mike

 

[camilaf72]

You may want to try partial discharge test to evaluate the insulation integrity of medium voltage (MV) power distribution circuits. Early detection of partial discharge can evaluate the integrity of the cable, help prioritize cable replacement or other corrective action and avoid an unplanned power interruption.
I think a guide is offered in IEEE 400-2001.
Benlanz, could you tell us if this method could provide the information needed?
Thanks,
Camila

 

[benlanz]

Camila,

I see that you have been keeping up with the latest technology and standards. You are correct. Partial Discharge is actually one of the best options for testing shielded power cables. However, it is also one of the most expensive options.

One note:
If Rigar’s cables are not shielded, he is out of luck. This isn’t a good solution for unshielded MV cables except for replacing them with shielded cable.

There are only a few companies that offer the test as a service and fewer who can do a reliable job. You can purchase equipment but, the good equipment is too expensive and complex for most and the cheap equipment is very limited a typically overrated.

One of the keys to using PD technology is being able to prove the reliability of the test results. Some tests are not verifiable and can not prove the reliability of a cable system but, are useful for picking up some types of problems while the cable is in service. Other tests require the cable system to be taken off-line but, can predict future performance with variable confidence. The type of test you choose depends on your needs. The key to any technology choice is to understand the limitations and capabilities. I recommend talking to more than one vendor. Solid vendors can supply statistically significant evidence to support their claims and case studies to compare their offering with competing services.

Regards,

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