IET
Decrease font size
Increase font size
Topic Title: zero sequence transformer data
Topic Summary:
Created On: 03 June 2013 03:40 PM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 03 June 2013 03:40 PM
User is offline View Users Profile Print this message


Avatar for MAXMIRA.
MAXMIRA

Posts: 78
Joined: 25 January 2011

Hi can anyone explain why the zero sequence transformer data is required for a power study, it would also be useful to know exactly what the zero sequence transformer data relates to.

Thanks in advance
 03 June 2013 04:06 PM
User is offline View Users Profile Print this message



hpcompaq

Posts: 78
Joined: 27 September 2007

Hello.

This is the impedance to zero sequence currents - earth faults.

Best regards.
 03 June 2013 10:29 PM
User is offline View Users Profile Print this message


Avatar for timothyboler                                      .
timothyboler

Posts: 229
Joined: 25 July 2008

...or some phase to neutral faults :-)

If you stick a neutral earthing resistor between the transformer neutral and earth then this directly affects the the zero sequence impedance and therefore the earth-fault current. Interestingly it can be measured directly by shorting the 3 phases of a star winding together and applying a single-phase voltage at rated frequency between the neutral and the shorted phases and measuring the current. The zero seq. impedance of a delta winding is infinite. Obviously if you don't connect the star point neutral to earth then the zero sequence impedance of an earth fault is infinite also.

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 04 June 2013 10:04 AM
User is offline View Users Profile Print this message


Avatar for MAXMIRA.
MAXMIRA

Posts: 78
Joined: 25 January 2011

Originally posted by: timothyboler

...or some phase to neutral faults :-)



If you stick a neutral earthing resistor between the transformer neutral and earth then this directly affects the the zero sequence impedance and therefore the earth-fault current.


I know this may sound like a ridiculous question but would you expect the figure to be greater than that of the NER, the reason i ask is that we have commissioned a very well known company to carry out a power study and they have come back with assumed figures lower than the NER.
 04 June 2013 01:23 PM
User is offline View Users Profile Print this message



jimmyngcheong

Posts: 4
Joined: 23 December 2002

The zero sequence impedance data is required for unsymmetrical system studies particularly for unbalanced fault analysis. Refer to Fortescue theorem for positive, negative and zero sequence networks.

-------------------------
Jimmy Ng
 04 June 2013 04:24 PM
User is offline View Users Profile Print this message


Avatar for MAXMIRA.
MAXMIRA

Posts: 78
Joined: 25 January 2011

Thanks all
 04 June 2013 04:30 PM
User is offline View Users Profile Print this message


Avatar for timothyboler                                      .
timothyboler

Posts: 229
Joined: 25 July 2008

Depends if you are referring to just the transformer or the whole system.
The total zero seq. impedance of the system is the sum of the transformer zero seq. impedance of the windings plus the NER but normally the zero-sequence impedance of the transformer and any additional NER impedance are quoted separately and then summed together (at least on SKM PowerTools which is the calculator that I'm used to) as presumably the NER is a separate piece of equipment.

It's easy to check that it's been considered as the earth fault current will be significantly lower than the 3-phase short-circuit with an NER. In fact it works the other way in that they should have sized the NER value to limit the earth fault current to a specific value.

Regards, Tim

-------------------------
Everyone loves a fireman - but hates the fire inspector.
 04 June 2013 08:04 PM
User is offline View Users Profile Print this message



hpcompaq

Posts: 78
Joined: 27 September 2007

Hello.

Without knowing your network arrangment I cannot give you a definitive answer, but as Tim says, we choose a value of NER to limit the earth fault current to a low value compared to the symmetrical three phase fault current and so the NER value usually swamps rest of the network zero sequence impedances.

Best regards.
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.