HIGH VOLTAGE ENGINEERING UNIT 4 Measurements of AC

HIGH VOLTAGE ENGINEERING UNIT 4 – Measurements of AC High Voltages 1

Unit 4 Measurements MEASUREMENT OF HIGH DC VOLTAGES ØSeries Resistance micro ammeter Ø Resistance potential divider Ø Generating Voltmeters Ø Sphere Gaps 2

Unit 4 Measurements MEASUREMENT OF HIGH AC VOLTAGES Ø Series impedance voltmeter Ø Potential Transformers (CVT) Ø Electrostatic Voltmeters Ø Potential Dividers Ø Sphere gaps 3

Unit 4 Measurements MEASUREMENT OF HIGH AC HIGH FREQUENCY & IMPULSE VOLTAGES Ø Potential Dividers (R & C) Ø Peak voltmeters Ø Sphere gaps 4

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 1. Series Resistance micro ammeter Circuit diagram: 5

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 1. Series Resistance micro ammeter Construction: v. A large value of resistance (few hundreds of mega ohms) is connected in series with u. A v. Protective device (Zenor diode, neon glow tube)connected across the u. A 6

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 1. Series Resistance micro ammeter operation ØR should be high ØHigh DC voltage is applied ØVoltage drop across the resistance ØThe current flowing thro’ R is measured in u. A 7

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 1. Series Resistance micro ammeter Note: ØVoltage V=IR ØDrop in Ammeter is negligible ØR should be chosen such that 1 to 10 u. A is allowed for full scale deflection Ø 500 kv can be measured ØAccuracy : 0. 2% 8

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 1. Series Resistance micro ammeter Need for protective device üIf R fails, heavy current will flow thro’ u. A üTo divert protective device is used 9

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 1. Series Resistance micro ammeter Drawbacks üMore power dissipation üTemperature effects 10

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 2. Resistance potential divider Need ØTo avoid high loading of R ØAnother resistor is added 11

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 2. Resistance potential divider Circuit diagram 12

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 2. Resistance potential divider Construction ØR 1 R 2 connected in series ØVoltage is measured across R 2 13

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 2. Resistance potential divider Operation ØDC voltages are measured using resistance voltage divider ØR 2 <<< R 1 ØHigh DC voltage applied. ØDrop across R 1 ØMeasure the voltage directly across R 2 14

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 2. Resistance potential divider Note üPower Dissipation in resistor R 1 üNeed for cooling üSeries connection of resistors 15

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 3. Generating Voltmeter Principle Variable electrostatic generator which generates a current proportional to the applied external voltage 16

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 3. Generating Voltmeter Time variant capacitance system can be developed between the high voltage electrode and earthed electrode, then current flowing to earth electrode will be a measure of the voltage 17

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 3. Generating Voltmeter ØConnect High voltage source to disc electrode H ØL 1, L 2, L 3 are earthed electrode ØThe L 1 has vanes and its rotated at constant speed ØThe rotor vanes of L 1 periodically cover and uncover the static sensing electrode L 3 ØCapacitance between H & L 3 changes periodically 18

Unit 4 MEASUREMENT OF HIGH DC VOLTAGES 3. Generating Voltmeter ØThe shape and No of vanes of L 1, L 2 are designed that they produce sinusoidal variation in the capacitance ØThe generated ac current is rectified and read by a moving coil instrument 19

Unit 4 Measurements MEASUREMENT OF HIGH AC VOLTAGES Ø Series impedance voltmeter Ø Potential Dividers ØElectrostatic Voltmeters Ø Potential Transformers (CVT) ØSphere gaps 20

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Potential Dividers 1. R Dividers ü Power loss ü Temperature Effects 2. Series impedance 3. C dividers 21

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES C dividers Series capacitance Dividers ü Apply the voltage ü Drop Across C ü Charging Current s measured using Micro ammeter 22

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES C dividers capacitance potential Dividers 23

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES C dividers capacitance potential Dividers Need to measure 1. Withstand-RMS voltage 2. Breakdown-Peak voltage RMS = Peak/√ 2 for only sine wave 24

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak value measurement 1. Chubb-Frotscue method ( Series capacitor peak voltmeter) 2. Peak voltmeter with potential voltmeter(Davis , Bowdler , stand ring circuit) 3. Digital peak voltmeter 4. Sphere gap 25

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES RMS value measurement 1. Peak voltmeter with potential voltmeter(Davies , Bowdler , stand ring circuit) 2. Electrostatic voltmeter 26

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak value measurement 1. Series capacitor peak voltmeter: Principle Measure of charging current Ic=VὡC Construction Stand. Capacitor Diode & Meter 27

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES 1. Series capacitor peak voltmeter: Operation ü Apply voltage ü Charging current is rectified by D 1 and measured ü Other half cycle no conduction 28

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak value measurement 1. Series capacitor peak voltmeter: Current leads the voltage 29

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak value measurement 1. Series capacitor peak voltmeter: Drawbacks ü Voltage wave shape is not pure Sinusoidal. Contains oscillations ü Current may not be uniform ü Calibration is wrong 30

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak voltmeter with potential voltmeter(Davis , Bowdler , stand ring circuit) 31

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak voltmeter with potential voltmeter(Davis , Bowdler , stand ring circuit) ü The voltage to be measured is applied across C 1&C 2 divider. ü The voltage appears on C 2 is rectified into DC and charge the Cs. ü Meter read the peak value of C 2 ü Multiplying Divider ratio we can get the voltage to be measured 32

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Peak voltmeter with potential voltmeter(Davis , Bowdler , stand ring circuit) v Discharge Error v Rest Charge Error 33

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES RMS value measurement 1. Peak voltmeter with potential voltmeter(Davis , Bowdler , stand ring circuit) 34

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Electrostatic Voltmeter Principle Force between the parallel plate electrodes is given by F=1/2 €V 2 A/D 2 35

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES RMS value measurement Electrostatic Voltmeter 36

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Electrostatic Voltmeter Construction ü HV Electrode insulated from earth ü LV electrode – central point is movable ü Light beam ü Scale 37

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Electrostatic Voltmeter Operation ü High voltage is applied ü Due to electrostatic forces ü deflection of rotating Element ü Light focused on the mirror ü Gets reflected & measured 38

Unit 4 MEASUREMENT OF HIGH AC VOLTAGES Electrostatic Voltmeter Drawback § For a Constant distance d, the sensitivity is small Advantage § No source loading § Continuous Reading also possible 39