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Impulse Voltage Measurements Using Voltage Dividers
If the amplitudes of the impulse voltage is not high and is in the range of a few kilovolts, it is possible to measure them even when these are of short duration by using CROS. However, if the voltages to be measured are of high magnitude of the order of megavolts which normally is the case for testing and research purposes, various problems arise. The voltage dividers required are of special design and need a thorough understanding of the interaction present in these voltage dividing systems. The voltage generator G is connected to a test object—T through a lead L.
These three elements form a voltage generating system. The lead L consists of a lead wire and a resistance to damp oscillation or to limit short-circuit currents if of the test object fails. The measuring system starts at the terminals of the test object and consists of a connecting lead CL to the voltage divider D. The output of the divider is fed to the measuring instrument (CRO etc.) M. The appropriate ground return should assure low voltage drops for even highly transient phenomena and keep the ground potential of zero as far as possible.
It is to be noted that the test object is a predominantly capacitive element and thus this forms an Oscillatory circuit with the inductance of the load. These oscillations are likely to be excited by any steep voltage rise from the generator output, but will only partly be detected by the voltage divider. A resistor in series with the connecting leads damps out these oscillations. The voltage divider should always be connected outside the generator circuit towards the load circuit (Test object) for accurate measurement. In case it is connected within the generator circuit and the test object discharges (chopped wave) the whole generator including voltage divider will be discharged by this short circuit at the test object and thus the voltage divider is loaded by the voltage drop across the lead L. As a result, the voltage measurement will be wrong. Yet for another reason, the voltage divider should be located away from the generator circuit. The dividers cannot be shielded against external fields. All objects in the vicinity of the divider which may acquire transient potentials during a test will disturb the field distribution and thus the divider performance. Therefore, the connecting lead CL is an integral part of the potential divider circuit. In order to avoid electromagnetic interference between the measuring instrument M and C the high voltage test area, the length of the delay cable should be adequately chosen. Very short length of the cable can be used only if the measuring instrument has high level of electromagnetic compatibility (EMC). For any type of voltage to be measured, the cable should be co-axial type. The outer conductor provides a shield against the electrostatic field and thus prevents the penetration of this field to the inner conductor. Even though, the transient magnetic fields will penetrate into the cable, no appreciable voltage is induced due to the symmetrical arrangement. Ordinary coaxial cables with braided shields may well be used for d.c. and a.c. voltages. However, for impulse voltage measurement double shielded cables with predominantly two insulated braided shields will be used for better accuracy.
During disruption of test object, very heavy transient current flow and hence the potential of the Ground may rise to dangerously high values if proper earthling is not provided. For this, large metal sheets of highly conducting material such as copper or aluminum are used. Most of the modern high voltage laboratories provide such ground return along with a Faraday Cage for a complete shielding of the laboratory. Expanded metal sheets give similar performance. At least metal tapes of large width should be used to reduce the impedance.
Voltages dividers for a.c., d.c. or impulse voltages may consist of resistors or capacitors or a convenient combination of these elements. Inductors are normally not used as voltage dividing elements as pure inductances of proper magnitudes without stray capacitance cannot be built and also these inductances would otherwise form oscillatory circuit with the inherent capacitance of the test object and this may lead to inaccuracy in measurement and high voltages in the measuring circuit. The height of a voltage divider depends upon the flash over voltage and this follows from the rated maximum voltage applied.
Now, the potential distribution may not be uniform and hence the height also depends upon the design of the high voltage electrode, the top electrode. For voltages in the megavolt range, the height of the divider becomes large. As a thumb rule following clearances between top electrode and ground may be assumed 2.5 to 3 meters/MV for d.c. voltages.2 to 2.5 m/MV for lightning impulse voltages. More than 5 m/MV rms for a.c. voltages. More than 4 m/MV for switching impulse voltage.The potential divider is most simply represented by two impedances Z1 and Z2 connected in series and the sample voltage required for measurement is taken from across Z2, FIG. 4.8.If the voltage to be measured is V1 and sampled voltage V2, then
The voltage V2 is normally only a few hundred volts and hence the value of Z2 is so chosen that V2 across it gives sufficient deflection on a CRO. Therefore, most of the voltage drop is available across the impedance Z1 and since the voltage to be measured is in megavolt the length of Z1 is large which result in inaccurate measurements because of the stray capacitances associated with long length voltage dividers (especially with impulse voltage measurements) unless special precautions are taken. On the low voltage side of the potential dividers where a screened cable of finite length has to be employed for connection to the oscillograph other errors and distortion of wave shape can also occur.
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