High Voltage Schering Bridge
Hello, friend Today we are discussing High Voltage Schering Bridge. High Voltage Schering Bridge is a Is A interesting Topic So let start, The bridge is widely used for capacity and dielectric. loss measurement of all kinds of capacitances, for instance, cables, insulators, and liquid insulating materials. This bridge is suitable for measurement of small capacitance equipment. as the bridge uses either high voltage or high-frequency supply. If measurements for such low capacity equipment is carried out at low voltage, the results so obtained are not accurate. Fig. 1 shows a high voltage Schering bridge where the specimen has been represented by a parallel combination of Rp and Cp.’
The Special Features of The High Voltage Schering Bridge are
1. High voltage simply that consists of a high voltage transformer with regulation, protective circuitry and special screening The Input voltage is 220 volt and output continuously variable between 0 and 10 KV. The maximum current is 100 mA and it is of 1KVA capacity.
2. Screened standard capacitor Cs of 100 pF+-5%, 1O KV max. It is a gas-filled capacitor having negligible loss factor over a wide range of frequency.
3. The impedances of arms I and ll are very large and, therefore, current drawn by these arms is small from the ‘ source and a sensitive detector is required for obtaining balance. Also, since the impedance farm l and H are very large as compared to II] and IV, the detector and the impedances in am 111 and IV are at a potential of only a few volts (10 to 20 volts) above earth even when the supply voltage is 10 RV, except of course, in case of breakdown of one of the capacitors of arm I or ll in which case the potential will be that of supply voltage. Spark gaps are, therefore, provided to spark over whenever the voltage across arm all or IV exceeds 100 volts so as to provide personal safety and safety for the null detector.
4. Null Detector: An oscilloscope is used as a null detector. The Y-plates are supplied with the bridge voltage Vab and the x-plates with the supply voltage V. If Vab has a phase difference with respect to V, an ellipse will appear on the screen (Fig. 2). However, if magnitude balance is not ‘ reached, an inclined straight line will be observed on the screen. The information about the phase is obtained from the area of the ellipse and the one about the magnitude from the inclination angle. Fig.2(a) shows that both magnitude and phase are unbalanced. Fig.2(b) shows that both magnitude and phase ‘ are balanced and this represents the null point condition. Fig. 2(c) and 2(d) show that only phase and amplitude ‘ respectively are balanced.
5. Automatic Guard Potential Regulator: While measuring capacitance and loss factors using bridges, the detrimental stray capacitances between bridges junctions and the ground adversely affect the measurements and are the source of error. Therefore, arrangements should be made to shield the measuring system so that these stray capacitances are either neutralized, balanced or eliminated by precise and rigorous calculations. F tg.3 shows various stray capacitances associated with High Voltage Schering Bridge;
Ca, Cb, Cc, Cd are the stray capacitances at the junctions A, B, C and D of the bridge. if point D is earthed during the measurement capacitance Cd is thus eliminated.
Since Cc comes across the power supply for the earthed bridge, has no influence on the measurement. The effect of other stray capacitances Ca and Cb can be eliminated by use of auxiliary arms, either guard potential regulator or auxiliary branch.