Three winding transformer Dyn5yn5 interpretation of zero sequence impedance Z0% from data sheet 1

[EBDRUP]

Hi all

This is my first post her I have tried to search for the topic of this post but have not found a answer to my question so hopefully some you cleaver guys can help me find an answer.

So my problem is as follows:

I doing relay setting in an offshore Windfarm. The Windfarm is operating at 33 kV and the relay I am working with are located on the offshore platform which collects the power generated by the wind turbines.

The relays I working with is the primary protection in the 33 kV array system but is required to provide backup protection on the low voltage side of the 33/0.64 KV turbine transformers.

So to find the correct relay setting I must be able to calculate the 3 phase, 2 phase and single line ground fault currents on LV side of the turbine transformers and refere them to the 33 KV side as that’s the currents my relay will see.
As indicated in the name of this post the 33/0.64 turbine transformer is a three winding transformer which have the following specifications:

HV winding: 33kV 10 MVA
LV1 winding 0.64kV 5 MVA
LV2 winding 0.64kV 5 MVA

The positive sequence short circuit impedance Z% e.g (HV-LV1) is 9.79% and is based on the MVA rating of LV1 winding - 5 MVA so calculating 2 and 3 phase fault currents on the LV side and referring them to the HV side is not a problem!

In the data sheet it is stated that the zero sequence impedance Z0% (HV-LV1 and HV-LV2) is 97.6% each but it is not stated what MVA rating the Z0% is based on.
Do any one have any experience in - or any literature about what the 97.6% relates to?

Is it based on the 10 MVA or 5 MVA rating?
Or is the Zero sequence short circuit impedance Z0= Positive short circuit impedance • 0.976?

Any sparring or assistance is much appreciated

Best regards
Endrup

 

[waross]

Are you planning for a possible simultaneous line to ground fault on both 0.64 kV windings or i just not understand the question?
Yours
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[RRaghunath]

Endrup,
For a Dyn transformer, Zo is considered to be almost equal to %Z. To be precise, manufacturers provide the Zo value as a percentage of %Z.
So, answer to your questing "is the Zero sequence short circuit impedance Z0= Positive short circuit impedance • 0.976" is "Yes, it is".

Rompicherla Raghunath

 

[prc]

It means zero sequence impedance is 97.6 % of the positive sequence impedance. Please see table 1 of IEC 60076-8 Transformer application guide.

 

[EBDRUP]

@Warross
The two LV windings are connected to two separat LV systems in the turbine and therefore I will only consider faults on one LV winding.

@RRaghunath and prc
Thank you very much for you good answers they were very useful

To anybody else that might be interested - I managed to get in contact with the manufactures test engineer for this specific transformer and he also confirmed the interpretation of the zero sequence short circuit impedance as so correctly stated by RRaghunath and prc!

Furthermore, I found that the same is stated in the ABB Switchgear manual - if anybody needs a different reference than IEC60076-8


Best regards

Ebdrup

 

[waross]

Thanks for the information.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

From IEC 60909-4 Examples for the calculation of short-circuit currents
3.3 Determination of the zero-sequence impedances
3.3.1 Transformers
For the transformers T1 and T2 with the vector group Dyn5 the following relations are given by the manufacturers:
R(0)T = RT and X(0)T = 0,95 XT

 

[stevenal]

The question seems to be about referring low side short circuit currents to the high side. The zero sequence impedance is important for calculating the low side current, but I0 will be trapped in the delta and not show up on the 33 kV lines. There you will see positive and negative sequence current only.

 

[7anoter4]

You are right, stevenal.

 

[waross]

A single phase to ground load on on A phase on the secondary will be fed by a line to line current on the primary. A phase will see a current in phase with the load current. B and C phases will act as an open delta and will also support the load current on A phase, however the load currents on B phase and C phase will also be in phase with A phase.
Thus for a resistive load, The currents in B phase and C phase will be at 50% PF.
This is similar to the action of the double delta connection used to derive true single phase from a three phase generator.
Given the typical low power factor of a fault current, I imagine that while the PF of the individual phases are skewed by a fault current as compared to an in phase current on one phase, that there will still be currents in each primary phase and the currents will be in phase with the fault current.

Bill
--------------------
"Why not the best?"
Jimmy Carter