Schering Bridge

The Schering Bridge is an AC voltage bridge and is used to determine the capacitance and loss factor of capacitors , primarily in high-voltage technology. It was first used by Harald Schering in 1920 .

functionality

Schering circuit diagram of the Schering bridge

In the Schering bridge the unknown capacitor C is _X with the lossless capacitor C _N compared. Due to the low-resistance design of the branches R ₃ , R ₄ , C ₄ on the earthed side of the bridge circuit, R ₃ and C ₄ can be set safely despite the high voltage. R ₃ is used to adjust to the capacity of the test item. By changing C ₄ the loss fraction is determined. The loss portion results from the angular deviation of the current through C _X from a 90 ° lead compared to the applied voltage U ₀ due to a certain conductivity. ${\displaystyle \delta }$

The following then apply to the capacitance C _{X of} the real capacitor to be determined :

${\displaystyle C_{\mathrm {x} }=C_{\mathrm {N} }{\frac {R_{4}}{R_{3}}}}$

for his share of loss : ${\displaystyle R_{\mathrm {x} }}$

${\displaystyle R_{\mathrm {x} }=R_{3}{\frac {C_{4}}{C_{\mathrm {N} }}}}$

and for the loss factor : ${\displaystyle \tan \delta }$

${\displaystyle \tan \delta _{\mathrm {x} }={\frac {R_{\mathrm {x} }}{\frac {1}{\omega C_{\mathrm {x} }}}}=\omega R_{\mathrm {x} }C_{\mathrm {x} }\,}$

${\displaystyle \tan \delta _{\mathrm {x} }=\omega R_{4}C_{4}=2\pi fR_{4}C_{4}\,}$

use

The Schering Bridge is used to determine unknown capacities. The principle is not only used in high voltage technology, but is also used for monitoring and control in the manufacture of capacitors. A non-destructive quality test of a dielectric is also possible with this bridge circuit . The start of a glow in a dielectric or air inclusions lead to unrest in the display and then a steep rise in the curve. Another application is the determination of corona losses in high-voltage lines. ${\displaystyle \tan \delta =f(U)}$

The advantage of the Schering bridge is that the necessary settings (R3, C4) can be made on a low potential (several 100 V). The accuracy can be further improved by auxiliary devices such as guard ring electrodes and Wagner's auxiliary branch .

Nowadays, quantities are usually determined from high-precision, phase-resolved current and voltage measurements using special diagnostic devices . ${\displaystyle \tan \delta }$

literature

• Andreas Küchler: High voltage technology . 3. Edition. Springer, 2009, ISBN 978-3-540-78412-8 .