Electrostatic Ground Shield

[k0412d]

Hi all,

My question is a bit wide, and I apologize for that.

Often, Electrostatic ground shield are used in Rectifier transformer, I manage to get the following explanation from :

Electric power transformer engineering / edited by James H. Harlow. // ISBN 0-8493-1704-5

"2.4.13 Electrostatic Ground Shield
It is usually desirable to have an electrostatic ground shield between the primary and secondary windings.
The electrostatic ground shield provides capacitive decoupling of the primary and secondary windings.
Generally, the winding connected to the rectifier circuit is ungrounded. Without the presence of the
electrostatic ground shield, transients on the primary side transfer to the secondary side of the transformer.
These may be approximately 50% of the magnitude of the primary transient if there are no
grounds in the system. This is high enough to fail secondary windings and core insulation or to cause
rectifier-circuit failures. The other normally considered advantage to the system is the minimization of
high-frequency disturbances to the primary system due to the rectifier ^{second part}."

Ok for the second part about high-frequency disturbance from the rectifying process.

But I dont get why ungrounded secondaries systems are more vurnerable to disturbance (from primary) than grounded systems.

Could anyone clarify it for me?

Thank you in advance.

 

[davidbeach]

Because they have no reference to ground to keep the secondary voltage to ground at a known value. Without the shield, transient overvoltages are capacitively coupled from the primary to the secondary.

 

[waross]

Consider a 50 Volt circuit fed by a 500:50 Volt transformer.
If the secondary is grounded the maximum voltage will be 50 Volts to ground. The impedance of the transformer secondary will shunt most capacitively coupled transients. If the secondary is floating, then a high voltage transient on the primary may induce a high voltage to ground on the secondary circuit through capacitive coupling.
Basically what David said.

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

 

[k0412d]

Ok, thanks for the explanation,

Ok, considering that transient overvoltage are mostly capacitive it makes sense.

One last question before I draw a equivalent scheme to fully grasp the concept.
So electromagnetic induced differential overvoltage is negligeable because it is shunt by the transformer?

Again thanks for the support.

 

[Skogsgurra]

"So electromagnetic induced differential overvoltage is negligeable because it is shunt by the transformer?"

Not quite. If the coupling mechanism is inductive which, after all, is the main coupling mechanism in a transformer, then it is mostly the stray inductance (the Xr of the transformer) that reduces coupled transients because of the high frequency components involved. The stray inductance is particularly effective if the secondary load is resistive or capacitive.

The electrostatic shield has no real influence on normal mode inductively coupled transients.

Gunnar Englund

http://www.gke.org

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