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Topic Title: Voltage dependant overcurrent
Topic Summary: Electrical Protection
Created On: 17 November 2012 09:13 AM
Status: Read Only
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 17 November 2012 09:13 AM
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I have recently found that the protection relay my company have been looking to use has a strange quirk, the overcurrent needs to see voltage from the VT to operate. I understand this would be standard for voltage dependant protection, however this relay has both voltage dependant and non-voltage dependant but still needs to see a voltage from the VT regardless of the function active. The VT voltage does not need to be the nominal, but >1V needs to be present. I have approached the manufacturer who sends documents telling me they comply to IEC requirements for the time / current curve but I cant seem to get a senible answer on what standards this device as a whole was built to. I suppose the question is; what standards does the relay functionality need to comply with? For obvious reasons I am less than satisfied with the proposed product, but people counting beans are involved so i need a sound argument.
 17 November 2012 12:59 PM
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IEC 60255 is one of the principal standards applying to protection relays.

You could go onto Alstom or Schneider, Siemens protection web sites to see what comparable standards they're quoting for similar protections.

 17 November 2012 04:33 PM
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A polarising voltage is also needed for directional overcurrent. Is there any settings for direction or torque angle? If so, this may explain the need for voltage reference to be present.

 17 November 2012 07:01 PM
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What you have is a crude form of distance protection. If the fault is close in then the voltage will be depressed and the over current element will opperate quicker, If the voltage stays up then it assumes a down stream fault and the overcurrent is delayed to allow for grading. I have seen these relays most often in site that have their own generation, such as offshore platforms. There are two basic flavours voltage controlled and voltage dependant, one changes the overcurrent settings in a single step the other changes the overcurrent settings gradually dependent on the voltage.
Some of the solid state relays use the voltage supply to power the circuit board. You could put a voltage from another source onto the VT terminals to make it work as a simple overcurrent, but you would have to check why rhese relays were fitted in the first place and are they still required. Do you have an up to date grading study?
 20 November 2012 07:26 AM
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Thanks all for the help, I understand the principals for the protection types. I was looking to find the standards that governed the manufacture / operation so I could verify the relays compliance.

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