- 1. Measurement Of High Dc Voltage Contents •Series resistance micrometer •Resistance potential divider •Generating voltmeter •Sphere gaps •Conclusion and Reference 1Prepared By S ARUN M.Tech Measurements of high DC voltages
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- 3. Series resistance micrometer • Resistance (R) : – Constructed with large wire wound – Value: Few hundreds of Mega ohms –Selected to give (1-10μA) for FSD. – Voltage drop in each element is chosen to avoid surface flashovers and discharges (5kV/cm in air, 20kV/cm in oil is allowed) – Provided with corona free terminals. – Material: Carbon alloy with temperature coefficient of 10-4/oC . – Resistance chain located in air tight oil filled PVC tube for 100kV operation with good temp stability. • Mircoammeter – MC type • Voltage of source, V=IR 3Prepared By S ARUN M.Tech Measurements of high DC voltages
- 4. • Impedance of the meter is few ohms. i.e., very less compared to R so the drop across the meter is negligible. • Protection: Paper gap, Neon Glow tube, a zener diode with series resistance – Gives protection when R fails. • Maximum voltage: 500kV with 0.2% accuracy. • Limitations: – Power dissipation & source loading – Temp effects 4Prepared By S ARUN M.Tech Measurements of high DC voltages
- 5. • A very high resistance in series with a micrometer. • Current through R is measured using micrometer. • Voltage of source, V = IR • The resistance is constructed from a large no. of wire wound resistors in series. • Can be operated up to 500kV (D.C) • Accuracy = ±0.2% • Selection of R value: – Current allowed: 1 to 10A – Corona free termination – Temp. coefficient<10-4/0C : Carbon Alloy – Placed in airtight, oil filled PVC tube to maintain temp. stability 5Prepared By S ARUN M.Tech Measurements of high DC voltages
- 6. Resistance potential divider • It uses electrostatic voltmeter. • Can be placed near the test object which might not always be confined to one location • Let, V2-Voltage across R2 • Sudden voltage changes during transients due to: – Switching operation – Flashover of test objects – Damage due to stray capacitance across the elements & ground capacitance 2 21 21 21 2 12 R )R(R XVVmagnitude,voltageHigh )R(R R XVV 6Prepared By S ARUN M.Tech Measurements of high DC voltages
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- 8. Generating voltmeter • Generating principle is used where direct loading or direct connection is to be avoided. • Generating voltmeter: A variable capacitor electrostatic voltage generator. • It generates current proportional to voltage under measurement • This arrangement provides loss free measurement of DC and AC voltages • It is driven by synch. motor, so doesn’t observe power from the voltage measuring source • The high voltage electrode and the grounded electrode in fact constitute a capacitance system. • The capacitance is a function of time as the area A varies with time and, therefore, the charge q(t) is given as, 8Prepared By S ARUN M.Tech Measurements of high DC voltages
- 9. Schematic of generating voltmeter 9Prepared By S ARUN M.Tech Measurements of high DC voltages
- 10. • Fig. shows a schematic diagram of a generating voltmeter which employs rotating vanes for variation of capacitance • High voltage electrode is connected to a disc electrode D3 which is kept at a fixed distance on the axis of the other low voltage electrodes D2, D1, and D0. • The rotor D0 is driven at a suitable constant speed by a synchronous motor. • Rotor vanes of D0 cause periodic change in capacitance between the insulated disc D2 and the high voltage electrode D3. • Number and shape of vanes are so designed that a suitable variation of capacitance (sinusoidal or linear) is achieved. • The a.c. current is rectified and is measured using moving coil meters. If the current is small an amplifier may be used before the current is measured. 10Prepared By S ARUN M.Tech Measurements of high DC voltages
- 11. • Generating voltmeters are linear scale instruments and applicable over a wide range of voltages. • The sensitivity can be increased by increasing the area of the pick up electrode and by using amplifier circuits Advantages: – scale is linear and can be extrapolated – source loading is practically zero – no direct connection to the high voltage electrode. 11Prepared By S ARUN M.Tech Measurements of high DC voltages
- 12. Sphere Gaps • Applications: – Voltage Measurement (Peak) - Peak values of voltages may be measured from 2 kV up to about 2500 kV by means of spheres. • Arrangements: 1. Vertically with lower sphere grounded (For Higher Voltages) 2. Horizontally with both spheres connected to the source voltage or one sphere grounded (For Lower Voltages). 12Prepared By S ARUN M.Tech Measurements of high DC voltages
- 13. • The arrangement is selected based on the relation between the peak voltage, determined by spark over between the spheres, and the reading of a voltmeter on the primary or input side of the high-voltage source. This relation should be within 3% (IEC, 1973). • Standard values of sphere diameter are 6.25, 12.5, 25, 50, 75, 100, 150, and 200 cm. • The effect of humidity is to increase the breakdown voltage of sphere gaps by up to 3%. • Temperature and pressure, however, have significant influence in breakdown voltage. • Breakdown Voltage under normal atmospheric conditions is, Vs=kVn where k is a factor related to the relative air density (RAD) δ. 13Prepared By S ARUN M.Tech Measurements of high DC voltages
- 14. • The relation between the RAD(δ) and the correction factor k: • Under impulse voltages, the voltage at which there is a 50% breakdown probability is recognized as the breakdown level. 14Prepared By S ARUN M.Tech Measurements of high DC voltages
- 15. • Factors Influencing the Spark over Voltage of Sphere Gaps i. Nearby earthed objects, ii. Atmospheric conditions and humidity, iii. Irradiation, and iv. Polarity and rise time of voltage waveforms. • The limits of accuracy are dependant on the ratio of the spacing d to the sphere diameter D, as follows: – d < 0.5 D Accuracy = ± 3 % – 0.75 D > d > 0.5 D Accuracy = ± 5 % • For accurate measurement purposes, gap distances in excess of 0.75D are not used 15Prepared By S ARUN M.Tech Measurements of high DC voltages
- 16. • Conclusion From these we conclude that how to measure varies measurements of how to generate high DC voltage in power system engineering. • References “High voltage engineering ” by M S Naidu and V Kamaraju, Tata McGraw Hill Education, 5th edition. 16Prepared By S ARUN M.Tech Measurements of high DC voltages