Yzn earthing transformers

[Electrical1948]

Hi

In IEC 60076-8 it is stated that YNd and Yzn earthing transformers has the same functionality. I understand the broken delta solution (YNd), but I am struggling with understanding the Yzn solution.

For instance, if you have a 11 kV delta connected system and introduce a 11/0.69 kV Yzn earthing transformer to restrict the earth fault current to 20 A on the 11 kV system, how to you size the resistor connected to the zigzag neutral?

Thanks for Your help

 

[7anoter4]

In IEC 60076-8 ch. 4.7. it is stated:
"The Z-connected transformer is referred to as an earthing transformer or neutral coupler; see section 6 of IEC 60289."
Since IEC 60289 is withdrawn I think IEC 60076-6 it is instead. See:

http://webstore.iec.ch/preview/info_iec60076-6{ed1.0}b.pdf

IEC 60076-6 Power transformers – Part 6: Reactors
ch.8 Current-limiting reactors and neutral-earthing reactors
The zig-zag transformer [ or Neutral Grounding Resistors ] manufacturer could instruct you how to calculate the resistance. For instance:

http://www.postglover.com

http://www.eskom.co.za/tenderbulletin/File_Show.asp?ID=116748

and other.

 

[LiteYear]

I haven't read 60076-8, but for what it is worth a YNd is generally considered equivalent to a Zigzag grounding transformer, not a Yzn transformer. In particular the Y primary is not necessary. A typical Zigzag grounding transformer has no secondary - it simply has a zigzag wound primary that creates a neutral point. The resistor sizing is easy in that case - it should be sized as if it was connected to the primary system's neutral point.

 

[prc]

A) The relevant standards for Earthing Transformers(Americans call it Grounding Transformer) are as below:
1) IEC 60076-6 :ed1.0 2007 Power Transformers Part 6 Reactors -Section 10 covers Earthing Transformers (Neutral Couplers) Earlier Standard was IEC 289 ed2.0-1968, 1988
2) ANSI/IEEE Std 32-1972( Reaffirmed 1990) Requirements, Terminology and Test procedure for Neutral Grounding Devices
3) Indian Standard IS 5553 (Part 6) -1990 Reactors -Part 6 Earthing Transformers (Based on IEC 289-1988) Earlier Standard IS 3151-1965,1982

B) As per 60076-8, YNd and ZN has same functionality ie provide an earthing neutral to an otherwise ungrounded system. ZN has only one auto connection. With YNd, you can use d for connecting a resistor either internal or external to delta. With ZN advantage is less copper used in windings and the resistor is connected to the neutral of Z winding.Earthing transformers are some times provided with a star secondary to load auxiliaries (ZNyn).60076-6 section 10 covers both Ynd and ZN connections.

YNzn,YNynd(ie delta stabilising connection) and Dyn has same functionality ie distribution transformers feeding four wire distribution system. Advantage of YNzn over YNynd is less copper compared to a star/star connection with stabilising delta winding.Some parts of the world Ynzn is still used for sub-transmission step down.(125 MVA 220/34.5kV YNzn11 in Mumbai)