Relay CT requirement for TPY class 1

[sadgol]

Hi everybody
I would like to know that for protection relay CT requirement,is there any limitation for Rct in TPY class?

 

[randyman]

Rct is only a portion of the total secondary circuit impedance and is a controlling factor which drives the CT size. Limits set on Rct must be reasonable with respect to the CT ratio (turns). Too low a number can force the CT supplier to use a larger copper wire size than practical, which can lead to dimensional issues. Often it is best to leave that parameter up to the CT supplier as he will optimize the design while satisfying (hopefuly!!) all other parameters.

 

[sadgol]

Thank you for your reply.In IEC 6044-6 standard there is an example for TPY class design,in this example mentioned that Rct should be less than 50% connected R to the secondary of CT.I would like to know that should we do this in our design or Rct is defined only by manufacturer and is not related to secondary connected burden.

 

[slavag]

Hi Sadgol.
Toady TPY class is not so used.
Don't design Rct, trust to CT mnf., them know what needed.
Please check requerements to TPY class in manual of relay, what do you want to use.

Good Luck.
Slava

 

[scottf]

sorry, but I disagree with the comments on leaving the Rct to the manufacturer based on them knowing the requirements.

When defining a TPY core, Rct must be specified, as the CT manufacturer has no way of knowing the relaying performance requirement, and Rct is integral to the relay performance of the CT. Often Rct is defined as a function of the number of turns and/or the accuracy limiting voltage is defined as a function of the Rct (which is the best way to do it in my opinion).

TPY cores are very complicated to specify and to calculate/build. They shouldn't be used unless they are absolutely needed by the application. Typically, those applications are ones with multiple reclosings and a high level of DC offset, where normal relay performance classes may not be adequate.

 

[sadgol]

Dear scottf,so grateful for your help.I thing we should use TPY class for distance protection in 400kv lines because of DC offset of the current.I don't know that should I do the rule mentioned in IEC 44-6 for Rct or not.I asked CT manufacturer about this but they said that there is no need to do that rule.

 

[randyman]

In IEC 6004-6, Table 2, Rct is NOT a required parameter to be given by the user. There are certain parameters that MUST be provided regarding the fault conditions, duty cycle and burden.

Scottf, Yes, it is an integral part of determining Ual, however it is the CT design that will ultimately determine this value. In some cases it is like a dog chasing its tail when working through these designs and trying to balance the Rct to keep the Ual at a reasonable value. I have been in situations where compromises with other parameters had to be made in order to keep the Ual at a reasonable level and keep CT size so it can be manufactured. I did not say that the CT supplier will know these requirements, but has limited control on the final design size.

Slavag, you would be surprised how much TPY class is used, I know that I am. I see them more than I care to and often ask myself the questing about protection philosphies - why do you never see them in the USA market but always encounter them in Europe and Asia?? They are extensively used in China.

 

[slavag]

Hi Randyman.
Thanks a lot to you and Scottf for explanations.
I don't see TPY class in newer system.
We work for big projects with voltage level:
500, 400, 330, 220, 170, 132 KV with full range of protection relays in few parts of world. NO TPY class.
In ME and south America lot of companies request class X.
But in lot of cases is 5Pxxx.
About China..is new for me.

Best Regards.
Slava