High direct current measurement
- 1. P R E P A R E D B Y M D I B R A H I M K H A L I L C O U R S E : E E E - 5 2 0 7 G R O U P : B Measurement of High Direct Current 1
- 2. Outline Measurement of high DC High Direct Currents Hall generators for DC current measurement Measurement of High Power frequency alternating Currents Measurement of High Frequency and Impulse Currents Resistive shunts Resistance Shunt Type Cathode ray Oscillographs for Impulse Measurement . 2
- 3. Measurement of high DC 3 High currents are used in power system for testing circuit breakers, cables lightning arresters etc. and high currents are encountered during lightning discharges, switching transients shunt faults. These currents require special techniques for their measurements
- 4. High Direct Currents 4 Low resistance shunts are used for measurement of these currents. The voltage drop across the shunt resistance is measured with the help of a milli voltmeter. The value of the resistance varies usually between 10 micro ohm and 13 milli ohm. This depends upon the heating effect and the loading permitted in the circuit. The voltage drop is limited to a few millivolts usually less than 1 V. These resistances are oil immersed and are made as three or four terminal resistances to provide separate terminals for voltage measurement for better accuracy
- 5. Hall generators for DC current measurement
- 6. Hall generators for DC current measurement 6 Hall effect is used to measure very high direct current. Whenever electric current flows through a metal plate placed in a magnetic field perpendicular to it, Lorenz force will deflect the electrons in the metal structure in a direction perpendicular to the direction of both the magnetic field and the flow of current. The charge displacement results in an e.m.f. in the perpendicular direction called theHall voltage. The Hall voltage is proportional to the current I, the magnetic flux density B and inversely proportional to the plate thickness d
- 7. Hall generators for DC current measurement 7 where R is the Hall coefficient which depends upon the material of the plate ,temperature of the plate and high magnetic field strengths, For metals the Hall coefficient is very small and hence semiconductor materials are used for which the Hall coefficient is high. When large d.c. currents are to be measured the current carrying conductor is surrounded by an iron cored magnetic circuit .So that the magnetic field intensity H is produced in a small air gap in the core. The Hall element is placed in the air gap (of thickness d ), and a small constant d.c. current is passed through the element . The voltage Developed across the Hall element in the normal direction is proportional to the d.c. current .
- 8. Measurement of High Power frequency alternating Currents 8 1. EHV conductor 2. Current sensing transformer 3. Power supply CT. 4. Power supply PT. 5. Analog-digital signal converter 6. Insulator for EHV 7. Electro-optical glass Fiber 8. Series high ohmic resistance 9. Digital-analog converter 10. Indicating or recording Unit
- 9. Measurement of High Power frequency alternating Currents 9 High Power frequency currents are normally measured using current transformers as use of low resistance shunts involves unnecessary power loss . Also the current transformers provide electrical isolation from high voltage circuits in power systems. Current transformers used few extra high voltage (EHV) systems are quite different from the conventional designs as they have to be kept at very high voltages from the ground . Electro-optical technique is introduced in current transformer measurements .
- 10. Measurement of High Power frequency alternating Currents 10 A voltage signal proportional to the measuring current is generated and is transmitted to the ground side through an electro-optical device. Light pulses proportional to the voltage signal are transmitted by a glass-optical fiber bundle to a photodetector and converted back into an analog voltage signal. Accuracies better than ±0.5% have been obtained at rated current as well as for high short circuit currents. The required power for the signal converter and optical device are obtained from suitable current and voltage transformers as Shown in Figure .
- 11. Measurement of High Frequency and Impulse Currents 11 In power system applications as well as in other scientific and technical fields, it is often necessary to determine the amplitude and waveforms of rapidly varying high currents. High impulse currents occur in lightning discharges, electrical arcs and post arc phenomenon studies with circuit breakers, and with electric discharge studies in plasma physics. The current amplitudes may range from a few amperes to few hundred kilo amperes. The rate of rise for such currents can be as high as 𝟏𝟎 𝟔 to 𝟏𝟎 𝟏𝟐 A/s, and rise times can vary from few microseconds to few nano seconds. In all such cases the sensing device should be capable of measuring the signal over a wide frequency band. The methods that are frequently employed are (1) resistive shunts, (2) magnetic potentiometers or probes, and (3) the Faraday and Hall effect devices.
- 13. Resistive shunts 13 The current through the resistive element R produces a voltage drop v(t)=i(t)R. The voltage signal generated is transmitted to a CRO Through a coaxial cable of surge impedance Zo . The cable at the oscilloscope end is terminated by a resistance Ri = Zo . The inductance L may be neglected at low frequencies , but becomes appreciable at higher frequencies Normally L and C become significant above a frequency of 1 MHz. The resistance value usually ranges from 10 micro ohm to few milliohms, and the voltage drop is usually about a few volts.
- 14. 14 The voltage drop across the shunt in the complex frequency domain may be written as: where s is the complex frequency or Laplace transform operator and V(s) and I(s) are the transformed quantities of the signals v(t) and i(t). With the value of C neglected it may be approximated as:
- 15. Resistance Shunt Type 15 The stray inductance and capacitance should be made as small as possible for better frequency response of the shunt. The resistance shunt is usually designed in the following manner to reduce the stray effects. (a) Bifilar flat strip design, (b) coaxial tube or Park's shunt design, and (c) coaxial squirrel cage design
- 16. Bifilar Strip Shunt 16 1. Metal base 2. Current terminals (C1 and C2) 3. Bifilar resistance strip 4. Insulating spacer (teflon or bakelite) 5. Coaxial UHF connector Pi, P2— Potential terminals
- 17. Bifilar Strip Shunt 17 The bifilar design consists of resistor elements wound in opposite directions and folded back, with both ends insulated by a teflon or other high quality insulation. The voltage signal is picked up through a ultra high frequency (UHF) coaxial connector. The shunt suffers from stray inductance associated with the resistance element, and its potential leads are linked to a small pan of the magnetic flux generated by the current that is measured. To overcome these problems, coaxial shunts are chosen.
- 18. Coaxial Tubular or Park's Shunt 18 1. Current terminals 2. Coaxial cylindrical resistive element 3. Coaxial cylindrical return conductor(copper or brass tube) 4. Potential pick up lead 5. UHF coaxial connector Fig : Schematic arrangement of a coaxial ohmic shunt
- 19. Coaxial Tubular or Park's Shunt 19 The voltage drop across the resistive element is measured between the potential pick-up point and the outer case. The space between the inner and the outer cylinder is air and hence acts like a pure insulator. With this construction, the maximum frequency limit is about 1000 MHz and the response time is a few nanoseconds. The upper frequency limit is governed by the skin effect in the resistive element.
- 20. Coaxial Tubular or Park's Shunt 20 The inductance 𝐿0 shown in Fig. 7.48 may be written as: 𝐿 𝑜 = 𝜇 𝑑𝑙 2𝜋𝑟 where, 𝜇 = 𝜇0 𝜇 𝑟; the magnetic permeability, d = thickness of the cylindrical tube The equivalent circuit of the shunt Fig. 7.48.
- 21. Coaxial Tubular or Park's Shunt 21 Exact equivalent circuit L0 —Inductance R0 - d.c. resistance n — Number of sections per unit length
- 22. Coaxial Tubular or Park's Shunt 22 The step response and the frequency response are shown in Fig. 7.49.
- 23. Squirrel Cage Shunts 23 In certain applications, such as post arc current measurements, high ohmic value shunts which can dissipate larger energy are required. In such cases tubular shunts are not suitable due to their limitations of heat dissipation, larger wall thickness, and the skin effect. To overcome these problems, the resistive cylinder is replaced by thick rods or strips, and the structure resembles the rotor construction of double squirrel cage induction motor. The equivalent circuit for squirrel cage construction is different, and complex.
- 24. Squirrel Cage Shunts 24 Rise times of better than 8 n s with bandwidth more than 400 MHz were obtained for this type of shunts. A typical R- C compensating network used for these shunts is shown in Fig. 7.51.
- 25. Cathode ray Oscillographs for Impulse Measurement 25 When waveforms of rapidly varying signals (voltages or currents) have to be measured or recorded, certain difficulties arise. The peak values of the signals in high voltage measurements are too large, may be several kilovolts or kilo amperes. Therefore, direct measurement is not possible. The magnitudes of these signals are scaled down by voltage dividers or shunts to smaller voltage signals.
- 26. Cathode ray Oscillographs for Impulse Measurement 26
- 27. Cathode ray Oscillographs for Impulse Measurement 27 Modern oscillographs are sealed tube hot cathode oscilloscopes with photographic arrangement for recording the wave forms . The cathode ray oscilloscope for impulse work normally has input voltage range from 5 mV/cm to about 20 V/cm. In addition, there are probes and attenuators to handle signals up to 600 V (peak to peak). The bandwidth and rise time of the oscilloscope should be adequate. Rise times of 5 n s and bandwidth as high as 500 MHz may be necessary.
- 28. Cathode ray Oscillographs for Impulse Measurement 28 Sometimes high voltage surge test oscilloscopes do not have vertical amplifier and directly require an input voltage of 10 V. They can take a maximum signal of about 100V (peak to peak) but require suitable attenuators for large signals . Oscilloscopes are fitted with good cameras for recording purposes.
- 29. Cathode ray Oscillographs for Impulse Measurement 29 With rapidly changing signals, it is necessary to initiate or start the oscilloscope time base before the signal reaches the oscilloscope deflecting plates, otherwise a portion of the signal may be missed. Such measurements require an accurate initiation of the horizontal time base and is known as triggering . Oscilloscopes are normally provided with both internal and external triggering facility.
- 30. Cathode ray Oscillographs for Impulse Measurement 30 When external triggering is used, as with recording of impulses, the signal is directly fed to actuate the time base and then applied to the vertical or Y deflecting plates through a delay line. The delay is usually 0.1 to 0.5 micro seconds. Thedelay is obtained by: ( I ) A long interconnecting coaxial cable 20 to 50 m long. The required triggering is obtained from an antenna whose induced voltage is applied to the external trigger terminal.
- 31. Cathode ray Oscillographs for Impulse Measurement 31 (2) The measuring signal is transmitted to the CRO by a normal coaxial cable. The delay is obtained by an externally connected coaxial long cable to give the necessary delay. This arrangement is shown in Fig. 7.54. (3) The impulse generator and the time base of the CRO are triggered from an electronic tripping device. A first pulse from the device starts the CRO time base and after a predetermined time a second pulse triggers the impulse generator.