IET
Decrease font size
Increase font size
Topic Title: Transformer/Generator Neutral Earthing cable size
Topic Summary:
Created On: 27 December 2013 07:20 AM
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.
 27 December 2013 07:20 AM
User is offline View Users Profile Print this message



jackdaniel

Posts: 69
Joined: 29 November 2012

I'm confused how to determine the cable size of neutral earthing of transformer and generator, please guide me.
 28 December 2013 08:28 AM
User is offline View Users Profile Print this message



statter

Posts: 124
Joined: 06 February 2013

....glibly, the same way you determine a phase cable size. .... reduced neutrals are very common but you really need to understand the loads on the t/f. Balanced loads like three phase motors will not contribute towards neutral currents. The more diverse and disaggregated the non balanced loads are the lower their overall contribution to neutral current at the source is likely to be (though as you go down the network and approach the load the network carries the loads full neutral current simply because there is likely to be less equipment on other phases helping to reduce the neutral current).

In the past I have used both 50% neutral (up to 1MVA) and 66% neutral (up to 1.5MVA) (50% one neutral cable, two phase cables, 66% 2 neutral cables 3 phase cables) but you need to be confident about the loadings and do the checks.

It tends to be poor network design or large single phase loads that screw things up especially when they end up on the same phase so watch out for these carefully otherwise careful design of the network and / or monitoring of an existing network over time will help
 28 December 2013 10:26 AM
User is offline View Users Profile Print this message



jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: statter

....glibly, the same way you determine a phase cable size. .... reduced neutrals are very common but you really need to understand the loads on the t/f. Balanced loads like three phase motors will not contribute towards neutral currents. The more diverse and disaggregated the non balanced loads are the lower their overall contribution to neutral current at the source is likely to be (though as you go down the network and approach the load the network carries the loads full neutral current simply because there is likely to be less equipment on other phases helping to reduce the neutral current).



In the past I have used both 50% neutral (up to 1MVA) and 66% neutral (up to 1.5MVA) (50% one neutral cable, two phase cables, 66% 2 neutral cables 3 phase cables) but you need to be confident about the loadings and do the checks.



It tends to be poor network design or large single phase loads that screw things up especially when they end up on the same phase so watch out for these carefully otherwise careful design of the network and / or monitoring of an existing network over time will help



I don't think reduced neutral is accurate way to determine the size, the calculation shall be based on earth fault level for transformer and generator solidly grounded, i'm looking for calculation or examples...
 28 December 2013 11:25 AM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

That is was Statter was saying....you need to know the loads and maximum fault level.

The maximum fault level for transformer is restricted by its impedance, which is measured in a simple short circuit test at the factory, and printed on the TX name-plate. The secondary is shorted and the voltage increased at the primary until full load current flows in the primary. The impedance is this applied voltage over the rated voltage expressed as a percentage.

i.e.
A TX rated at 1MVA with a measured impedance of 5% will have a maximum fault level of:

1MVA * 100 / 5% = 20MVA

Large power TXs often have resistors to reduce earth fault current.


If the neutral is the same size as a phase conductor, you can't go wrong. You can reduce the CSA to save money if the load specifically allows it, as Statter explained.

Edited: 28 December 2013 at 12:53 PM by Zuiko
 28 December 2013 12:56 PM
User is offline View Users Profile Print this message



jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: Zuiko

That is was Statter was saying....you need to know the loads and maximum fault currents.



If the neutral is the same size as a phase conductor, you can't go wrong. You can sometimes reduce this to save money if the load specifically allows it.



Large power TXs often have earthing resistors to reduce the fault current.


So you were saying the calculation exactly same like NGR calculation?, do we need to consider zero sequence impedance?

please check my calculation

transformer voltage level (11kv/433v)
max fault current
= 230/( 2 x cable impedance from transformer to ground termination)
 28 December 2013 01:02 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

Max Fault current = Max Fault Level / sqrt3 * VL

Edited: 28 December 2013 at 02:40 PM by Zuiko
 28 December 2013 01:18 PM
User is offline View Users Profile Print this message



jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: Zuiko

Use the 3 phase power equation to give you the maximum fault level



Max Fault current = Max Fault Level / sqrt3 * VL


How to get max fault level? is it same as 3 phase fault level for the transformer calculation? =FLA/transformer impedance
 28 December 2013 02:34 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

The maximum fault level is restricted by the impedance of the transformer, as described above. You would only find this sort of level close up to the transformer.


i.e.
A TX rated at 1MVA with a measured impedance of 5% will have a maximum fault level of:

1MVA * 100 / 5% = 20MVA


Distant earth faults generate very low currents, sometimes just a few amps.



Most networks have a rule of thumb that you can use. For example, the 11 kV network where I live has a maximum fault level of 250 MVA.

Beware that transformers (and generators) in parallel decrease the impedance of a fault, with a corresponding increase in fault level. Network operators have to consider this due to the increased micro generation coming online
 29 December 2013 04:11 AM
User is offline View Users Profile Print this message



jackdaniel

Posts: 69
Joined: 29 November 2012

Originally posted by: Zuiko

The maximum fault level is restricted by the impedance of the transformer, as described above. You would only find this sort of level close up to the transformer.





i.e.

A TX rated at 1MVA with a measured impedance of 5% will have a maximum fault level of:



1MVA * 100 / 5% = 20MVA





Distant earth faults generate very low currents, sometimes just a few amps.







Most networks have a rule of thumb that you can use. For example, the 11 kV network where I live has a maximum fault level of 250 MVA.



Beware that transformers (and generators) in parallel decrease the impedance of a fault, with a corresponding increase in fault level. Network operators have to consider this due to the increased micro generation coming online


Most of utility companies have their own fault level declaration, please correct me if i'm wrong, the fault level declared by them is represent 3 bolted fault or ground fault?, as far as I know, it's represent 3 bolted fault value.
Statistics

See Also:



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