3.2 Measurement of high voltages

Transcription

1 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY Part I- hapter 3: Insulation test techniques 3. Measurement of high voltages Instructor: Dr. Jian Li Lecture 7-

2 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3. Measurement of high voltages Major approaches of HV measurement in laboratory Measurement of peak values of A, D, and both types of impulse high voltages by using sphere gaps. Measurement of effective values of A and D high voltages by using electrostatic voltmeters. Measurement of A, D, and impulse voltages by using voltage dividing systems. Measurement of A high voltages by using peak voltmeters. Optical measurement systems of high voltages. Basic classification of HV measurement systems Reference measuring systems Approved measuring systems Lecture 7-

3 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3.. Peak voltage measurement by sphere gaps Two adjacent metal spheres of equal diameters, whose separation distance is limited, form a sphere gap for the measurement of the peak value of either A, D or both types of impulse voltages. Basic mechanisms of HV measurement by sphere gaps Breakdown voltage of air gap in uniform field is determined by gap distance. A quasi-uniform field between the two spheres Lecture 7-3

4 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING 3.. Peak voltage measurement by sphere gaps Vertical sphere gap. Insulating support. Sphere shank 3. Operating gear, showing maximum dimensions 4. High-voltage connection with series resistor 5. Stress distributor, showing maximum dimensions. P. Sparking point of HV sphere A. Height of P above ground plane. B. Radius of space free from external structures X. Item 4 not to pass through this plane within a distance B from P. Note: The figure is drawn to scale for a 00-cm sphere gap at radius spacing. Lecture 7-4

5 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING 3.. Peak voltage measurement by sphere gaps learance around the spheres Lecture 7-5

6 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3.. Peak voltage measurement by sphere gaps Remarks on the use of the sphere gap To avoid excessive pitting of the spheres, protective series resistances may be placed between test object and sphere gap. For A and D voltages, the value of the protective resistor may range from 0. to MΩ. For impulse voltages, it should not exceed 500 ohms and its inductance should be smaller than 30 μh. Influenced by irradiation and humidity. Lecture 7-6

7 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3.. Electrostatic voltmeters Electrostatic voltmeters are related to measurement of electrical field force generated by voltages between a pair of parallel plane disc electrodes. Electrostatic voltmeters are RMS-indicating instruments for A and D voltages. The measuring principle displays no upper frequency limit. The load inductance and the electrode system capacitance, however, form a series resonant circuit, thus limiting the frequency range. High-precision-type electrostatic voltmeters have been built for very high voltages up to 000 kv. Lecture 7-7

8 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..3 Peak voltmeters Disruptive discharge phenomena within electrical insulation systems or high-quality insulation materials are in general caused by the instantaneous maximum field gradients stressing the materials. The necessary calibration procedure and the limited accuracy of the sphere gap are hindering its daily application and call for more convenient methods. Peak voltmeters are based on a simple but accurate method for the measurement of peak values of A voltages, proposed by hubb and Fortescue. Lecture 7-8

9 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..3 Peak voltmeters FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING A.. peak voltage measurement by hubb and Fortescue I c t = i t dt T t c ( ) = T t t dv = V T max (a) Fundamental circuit. (b) Recommended actual circuit V max = Ic f Lecture 7-9

10 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..3 Peak voltmeters FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING Peak voltmeter for A measurements according to Davis, Bowdler and Standring Vd = Vm exp( t / V [ V + V d m m R) exp( T / R)]/ V m Vd = T R Lecture 7-0

11 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems. Basic concept of voltage dividers Voltage dividing ratio is as: k =V /V = (Z +Z ) / Z Basic requirement for voltage dividers No voltage distortion Steady voltage dividing ratio Not to influence measured voltages Lecture 7-

12 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems. High ohmic resistor voltage dividers Voltage dividing ratio k =(R +R )/R To keep i << i urrent flowing through voltage divider Upper limits of to ma Lower limit about 00 μa Very low temperature coefficients. Wire-wound metal resistors made from u Mn, u Ni, and Ni r alloys Down to about 0-5 /K Distributed stray capacitance to ground causes a strongly non-linear voltage distribution along a resistor column and overstresses individual elements during breakdown of a test object. FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING Mainly for D voltage measurement Lecture 7-

13 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems HV resistor element Wire-wound resistors are not only very expensive to produce, but also quite sensitive to sudden voltage drops. Sketch of cross-section of an HV resistor element 00-M, 00-kV standard resistor Lecture 7-3

14 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems 3. apacitor voltage dividers Voltage dividing ratio k =( + )/ : 00~00 pf Stray capacitors to surrounding objects and ground influence the voltage dividing ratio. Requirements for HV capacitors Independent of magnitude of voltage level and no ageing effects Very small temperature coefficient Small effective inductivity Mainly for A voltage measurement Lecture 7-4

15 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems 4. Generalized impulse voltage measuring circuit FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING. Voltage supply.. Lead to test object. 3. Test object. 4. Lead to voltage divider. 5. Voltage divider. 6. Signal or measuring cable. 7. Recording instrument. 8. Ground return Lecture 7-5

16 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems Resistor voltage dividers (for impulse voltages) HV resistor: ohms FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING Resistor voltage dividing circuit for impulse voltage measurement R + R3 = Z R4 = Z k = n[( R + R )( R + R ) + R R ]/ R R Equivalent circuit for resistor voltage dividers Lecture 7-6

17 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY Lecture Voltage dividing systems apacitor voltage dividers (for impulse voltages) apacitor voltage dividing circuit with a matched resistor at head terminal of cable apacitor voltage dividing circuit with matched resistors at both terminals of cable 0 ) / (, ) / (, 0 k t k t + + = > + = = + τ 0 3 ) / ( ) / (, ) / (, 0 k t k t + = = > + = = + τ

18 DEPT OF HIGH VOLTAGE AND INSULATION ENG, HONGQING UNIVERSITY 3..4 Voltage dividing systems FUNDAMENTALS OF HIGH VOLTAGE ENGINEERING apacitor voltage dividers (for impulse voltages) When R is a real resistor, the capacitor voltage becomes a series-damped capacitor voltage divider. t = 0 +, t, R > R k k = ( R = ( + R ) / R + ) / Lecture 7-8