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DC Hi-Pot testing a Transformer & Cables

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DC Hi-Pot testing a Transformer & Cables

DC Hi-Pot testing a Transformer & Cables

SparkyLarks

(Electrical)

(OP)

4 Sep 10 08:07

I need to test the insulation resistance of a 3-Core cable, but one end is still attatched a 3.3kV/400V 1.5MVA oil filled trandsformer.

I'm planning to connecct 5kV to all three cables and the earth to the Cable Armour, which is connected in turn to the Transfomer frame.

That will check the insulation of all three cores to the steel armour.

I can't check from a single core to the other two cores earting one of the cores would mean path from the live cable through the transformer windings to earth, but is ther any other tests availible to me?
 

Replies continue below

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RE: DC Hi-Pot testing a Transformer & Cables

Zogzog

(Electrical)

4 Sep 10 10:29

DC hipot testing of MV cables >5 years old is not recommended by IEEE, ANSI, NETA, IECA, or any other standard, it is considered a destructive test and not very effective anyways besides finding gross installation errors. VLF, PD, and Tan Delta are all acceptabel tests that each have thier pros and cons.

This has been discussed here many times in depth, you can catch up on the discussions by searching the forum for previous threads on the subject.  

RE: DC Hi-Pot testing a Transformer & Cables

Marmite

(Electrical)

4 Sep 10 13:39

You are stuck with testing all three phases to earth unless you disconnect the cable from the transformer winding. If the HV termination is dry then you should be able to unbolt the cable cores and bend them away from the transformer connections to do the test. If its compound filled then you would need to take the lid off the transformer to disconnect the winding from the cable. This situation is common in the utility environment I work in, and the testing is usually in reality a combined cable/transformer test to earth, rather than going to the effort of disconnecting the cable. The use of DC overvoltage is common, but utilities are generally interested in go/no go test prior to making live, rather than condition assessment. There is a school of thought that in this situation the connections should be left as they are and not disturbed to do testing, as you may inadvertently reconnect something wrongly, or even leave a spanner somewhere you shouldn't.
Regards
Marmite

RE: DC Hi-Pot testing a Transformer & Cables

SparkyLarks

(Electrical)

(OP)

4 Sep 10 17:48

Marmite
The cable terminations are in a compound filled cable box.
and it is a Utility customer. WE discussed removing the compound but a concern was raised that heating the compound may result in damage to the Transformer bushings.

It is exactly a go/no go test that is wanted prior to energising.
it's a 3.3kV Transfomer. Is testing at 5KV too high for an existing cable & transformer. Normally each cable or winding would only see a voltage of 1.9kV above earth. Voltage of 3kV  to ewarth be more appropiate>
 

RE: DC Hi-Pot testing a Transformer & Cables

k1y9l8e7

(Electrical)

5 Sep 10 04:47

Regards

Kyle

• http://files.engineering.com/getfile.aspx?folder=da1fe122-d319-451c-8be7-7f

Page 13 on the attached document suggests a 3.3kV system should be tested at 5 kV phase to phase (if your cables arn't screened) and 3 kV phase to earth/armour, although this is for new cables. Hope this is of help.RegardsKyle

RE: DC Hi-Pot testing a Transformer & Cables

Zogzog

(Electrical)

5 Sep 10 09:11

You won't find a recommended test voltage for DC on service aged cables in any standard because all standards agree you should not DC hipot service aged cables.

 

RE: DC Hi-Pot testing a Transformer & Cables

Zogzog

(Electrical)

5 Sep 10 09:15

http://www.eng-tips.com/viewthread.cfm?qid=
 

I suggest you read this thread

The company is the world’s best DC hipot test kit for transformers supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.

RE: DC Hi-Pot testing a Transformer & Cables

dpmac

(Electrical)

5 Sep 10 10:40

Assuming it;s strictly a GO/No Go test to insure no grounds exist.   Assuming un-screened cables rated 3.3 kV a simple V insulation resistance (Megger) test will tell you its ok to energize or not.

And you get the added bonus of using a Megger test set at VDC instead of a DC hipot at VDC which will prevent the DC Hipot Police from coming to your house late one night.

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News

The electrical grid network is oftentimes classified as critical infrastructure. For electrical utilities, any disruption which leads to major downtime is a major source of revenue loss. Furthermore, policymakers today expect network operators to deliver 24/7 electricity to the retail consumers. As a result, it is important that electrical equipment have the highest standards of quality and reliability while also ensuring user safety. As a leading manufacturer of electrical test and measurement equipment in India, SCOPE takes great pleasure in providing our customers with products with the above-mentioned criteria. 

In this blog, we will discuss the technique of Hipot testing, a commonly used safety test which ensures the safety of the user and the insulation healthiness of the test object.

What is Hi-pot Test?

Hipot test, shorthand for high-potential test, is also known as the Dielectric-Withstand test. The test is used to evaluate the insulation condition of the test object (such as transformers, switch-gear, relays, generators lightning arresters, bus-bar, cables, DC high voltage capacitors, etc).  The Hipot test ensures that current will not flow from one point to another and indicates the ability of the insulation to withstand high voltage and over voltage transient. Therefore, the Hipot test is an excellent method to detect any defect in an electrical product on account of poor workmanship.

In order to prevent the breakdown of the insulation of electrical products during operation potentially causing catastrophic damage to man and machine, this test is usually performed during the manufacturing process and during installation, acceptance, routine repair and maintenance activities. Through such evaluations, managers can decide on corrective action to be taken to ensure quality of electrical equipment.

Hipot tests are useful in determining:

1. Nicked or Crushed Insulation
2. Stray wire stands or braided shielding
3. Conductive or corrosive contaminants
4. Terminal spacing problems
5. Reduction in creepage and clearance
6. Improper cable joints and terminations

How is Hipot Testing Done?

Hipot tests involve the application of overvoltage to a device under test (DUT) for a short duration to verify the dielectric integrity of the system. Hipot testing can be done using AC, DC or Very Low Frequency (VLF) test voltage. In this blog, we will discuss AC & DC Hipot testing and the parameters that need to be considered while selecting the ideal AC/DC Hipot test kit for a particular test object.

Fig 1 : Hipot Testing

Guidelines for Hipot Testing

IEC is the applicable standard for Hipot testing. As per this standard:

Sr.noParametersStandard1Basic test voltage2*(operating voltage) + 1 kV2Test duration1 minute

Furthermore:

• As a thumb rule, if DC Hipot test is used on AC circuit, Hipot voltage should be two times the peak (2 * 1.4 * RMS) + 1kV
• Extra 1 kV voltage is to be applied to check whether the DUT insulation is able to withstand daily transient overvoltage
• DUT is considered to have passed the test if either the leakage current measured is less than the maximum allowable current or no breakdown occurs, i.e., there is no sudden and uncontrolled flow of current

Pre-Testing Arrangements for Hipot Testing

• Ensure that the body of the instrument is grounded properly
• Collect previous testing data, logs for reference
• Ensure the test location / area has sufficient light / illumination
• Wooden bench / table to place the instrument
• Mains power supply with proper earthing
• Instrument with proper test lead set without any wear and tear
• Multimeter should be available for continuity testing of test Leads
• Discharge rod must be used before physically touching the test object
• Wear HV Gloves, safety glasses, helmet and safety shoes for personal safety
• Operator should be standing on HV insulated rubber mat while operating the kit
• Ensure proper earthing, earth spikes required for additional earthing
• Maintain safe distance from the HV Terminals while applying HV
• Audio- visual indications and barricades can be used for warning people while injecting HV

Steps of Hipot Testing

• Step -1 : open circuit breakers or switches to isolate the DUT from both the ends
• Step -2: discharge all the conductors using a discharge rod
• Step -3: put CAUTION labels / Barricades around the HV testing area
• Step -4: connect Master Earthing Terminal of Hipot to a proper System Earthing
• Step-5: connect Operational Earthing Terminal (LV terminal of the kit) to safety ground (Body Earth)
• Step -6: ensure proper connection
• Step -7: the HV terminal of the kit should be connected to the conductive part of the test obejct
• Step-8: test all circuit phase conductors testing each phase to ground as per pre-determined voltage, time and current setting

Once these steps are followed, the below points are to be ensured:

1. As a result of the voltage difference between the conductors, current flow would be observed. No breakdown of either the air insulation or the solid insulation should take place
2. Furthermore, current should not be more than 5mA peak (3.5mA rms)
3. To PASS the test, the test object must be exposed to a minimum stress of pre-decided Voltage for 1 minute (or other test duration depending on the DUT and its test standard) without any indication of breakdown

Fig 2 : Connection of Hi-pot to the Device Under Test (DUT)

Conclusion

In this manner, Hipot testing can ensure the safety, quality and reliability of critical electrical equipment. The parameters which should be considered while selecting the ideal Hipot test kit are:

• Application: AC/DC/AC-DC depending on the DUT
• Combined or separate control unit as per the voltage level
• Required output voltage of the Hipot test kit as per the DUT&#;s rated voltage
• Required leakage current range of the Hipot test kit as per the DUT&#;s recommended current range 
• Timers
• Trip current settings
• Volt-meters & Amp-meters for output parameters
• Add on safety features include external interlocks, beacon/buzzer

Hipot test kits provided by SCOPE are fit for purpose and the features mentioned above make our test kits stand apart from kits offered by our competitors

To know more about SCOPE Hipot testing capabilities, please visit https://www.scopetnm.com/test-and-measurements/other-products/hi-pot-series or write to us at .

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