PT broken delta winding should be earthing? 3

[fayehooengtips]

Some PT's broken delta secondary winding is earthing,but some is not. Why ?Which one is right? The earthing is for safety,why not earthing? And how to earthing?
Please help me.

http://files.engineering.com/getfil...26d-19551edcec6d&file=IMG_20110610_171301.jpg

http://files.engineering.com/getfil...a2f-733ae2a58be6&file=IMG_20110610_172058.jpg

 

[waross]

With higher voltage primaries there may be capacitive coupling between the primary winding and an ungrounded secondary resulting in a hazardous common mode voltage floating on the secondary winding.
In the ungrounded circuit, the primary voltage may be low enough that the hazard is minimal and/or the ungrounded winding may be shielded by the grounded windings on the same core. Then again there may be a capacitively coupled common mode voltage present on the ungrounded delta.

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

 

[scottf]

Secondary windings of instrument transformers (CT and VTs) should always have reference to ground. I know of no reason for an exception to this rule.

 

[davidbeach]

What scottf said. Always, no exceptions.

 

[fayehooengtips]

Tnanks everyone.
But why it should always have reference to ground?
I think earthing is for safety as what waross said, not for reference.
PT can work well without secondary winding reference to groud if there's no capacitive coupling voltage from primary winding.
I am confused.

 

[davidbeach]

And how, exactly, do you not have capacitive coupling? Do you have some magic ability that isn't available to the rest of us mere mortals?

 

[waross]

I am not talking about a capacitively coupled PT. I am talking about a capacitor formed by the primary winding, the insulation and the secondary winding. If the secondary circuit is not earthed, the transformer may act as a small capacitor connected between the primary and the secondary. On large power transformers the capacitive charge has been great enough to kill copper thieves who attempted to steal the ground cable between the ground grid and the transformer case. (There used to be some graphic pictures of the body of a copper thief on this site.) On smaller PTs there is extremely little energy but there may be enough voltage to damage instrument circuits. Lightning strikes may be more destructive if the secondaries are not grounded.
There are small transformers made for isolation that are used in patient care areas and operating theaters that have a grounded metallic screen between the primary and secondary windings. In the event of a transformer burnout, the screen prevents the primary winding from shorting to the secondary winding. In normal operation the primary winding is capacitively coupled to ground and the secondary winding is capacitively connected to ground. There is no transfer of energy between the primary winding and the secondary winding by capacitive coupling.
But as David says, you cannot avoid capacitive effects when conductors are separated by an insulator.
As far as I know the special screening feature is not available in potential transformers.

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

 

[scottf]

I used the term "reference" because in a scheme where the secondary windings are connected in delta, broken-delta, or open-delta, only one point needs to be grounded. This gives all of the windings ground "reference".

Ground "reference" is needed to prevent the secondary winding(s) from floating up to a higher potential due to capacitive coupling between the primary and secondary. Also, grounding/ground reference provides safety in the event of a transient.


As an example, for a VT without ground reference on the secondary, the voltage between the secondary terminals (e.g. X1-X2) would be per the nameplate ratio (e.g. 120V), but the entire winding could float up to 5 kV due to capacitive coupling from the primary winding.