High Voltage Schering Bridge_ HV engineering.pptx

chapter 9 in Kamarajar Naidu book

1. 22EERB0 HIGH VOLTAGE ENGINEERING HIGH VOLTAGE SCHERING BRIDGE

2. INTRODUCTION Schering bridge- to find unknown capacitance or dielectric loss factor. HV Schering bridge- testing of bushings, cables insulators as they have low capacitance and dielectric loss . Uses HV supply at power frequency reduces error has high sensitivity reducing errors in results.

3. HV SCHERING BRIDGE- UNEARTHED SPECIMEN Test object: Cx , Rx Standard capacitor: Cs (50 to 500pF), Detector- Vibration Galvanometer Variable arms: R3, R4, C3 Where C3- 3 terminal capacitor Upon balancing,

4. Balanced equations Loss angle Usually, δx will be small at power frequencies for the common dielectrics so that

5. METHOD OF BALANCING Fix R3= (1000/ π ) ohms for ω = 100 π . Then, tan δ = 0.1 C 3 Adjust C3, R4. R4 of range- 10 4  Ω and 0.01 Ω. The range takes care of the errors due to contact resistances as well as the stray capacitance effects across R4. The low voltage arms of the bridge (R 4  and R 3  C 3 ) and the detector are enclosed in grounded shielded boxes to avoid stray capacitances during the measurements. The protective gaps G – the low voltage arms are protected from high voltages in case the test objects fail. The voltage drop across EB or FB does not exceed 10 to 20 V. The arms will be usually rated for a maximum instantaneous voltage of 100 V. For a very accurate measurement of the dissipation factor at power frequency, the stray and grounded capacitances (from E and F to ground) should be eliminated. It is eliminated by Wagner earthing device .

6. GROUNDED CAPACITANCE Two methods, Inverted bridge and Grounded detector arrangement. The hv schering bridge is earthed. For grounded objects like underground cables or bushings with flanges grounded to the tank of a transformer, one of the detector terminals is grounded.

7. INVERTED BRIDGE OPERATION Used when the test object is grounded. The entire bridge circuit is placed inside a Faraday cage at the high-voltage terminal. A standard capacitor is mounted on insulating supports. The operator adjusts the bridge using insulated rods, sometimes from inside the cage.

8. GROUNDED DETECTOR ARRANGEMENT One of the detector terminals (E or F) is grounded, introducing 𝐶𝑔- stray capacitance from the high-voltage terminal and leads. Parallel to test object . Balancing methods, The test object is disconnected and the capacitance C g  and tan ( δ g )   are measured. The test object is connected and a new balance is obtained.

9. BALANCE EQUATIONS From the second balance gives, the actual capacitance and dissipation factor of the test object are Where, Cx - Actual capacitance of the test object 𝐶𝑥′- Measured capacitance with test object connected 𝐶𝑔- Ground capacitance (stray capacitance from leads, terminals, etc.) tan⁡𝛿𝑥- Actual loss tangent of the test object tan⁡𝛿𝑥′- Measured loss tangent with test object connected tan⁡𝛿𝑔Loss tangent of ground capacitance

10. MEASUREMENT OF HIGH LOSS FACTOR If the loss factor of the specimen is large, R2 is modified. R2 is made parallel with C2. Suitable for loss factor upto 1.0. For 1<tan δ <10, R2 is connected in series with C2.