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FB Elektrotechnik und Informationstechnik

Institut fr Elektrische Energiesysteme

Fachgebiet Hochspannungstechnik

7th

Exercise on High Voltage Technology 1, WS 08/09

Solution Example

Task 1: Measurement of high voltages - Voltage divider

a) Divider types:

Ohmscher Teiler: resistive divider

Kapazitiver Teiler: capacitive divider

Ohmsch-kapazitiver Teiler: mixed resistive-capacitive divider

Gedmpft kapazitiver Teiler: damped capacitive divider

Gleichspannung: direct voltage

Wechselspannung: alternating voltage

Stospannung (Stospg.): impulse voltage

b)Resistive divider:

Capacitive divider:

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c) The time constants must be equal: R1'C

1' = R

1C

1= R

2C

2

d)

Measurement of direct voltage with resistive divider

The cable is connected directly to the low voltage connection of the divider, and its

other end is terminated with a termination resistance equal to the characteristic wave

impedance of the cable (50 for RG58 cable) in order to avoid reflections. The

oscilloscope which is connected in parallel to the termination impedance is highlyresistive (gig ohm-range).

Measurement of alternating voltage with capacitive divider

There must not be a resistive load connected to a capacitive divider, as this would cause

a frequency dependence of the divider ratio. So it is not possible to connect the cable to

the divider and terminate it with a resistance, as it is shown above! Here is a resistance

equal to the characteristic wave impedance of the cable (50 for RG58) connected in

series to the cable, which forms a voltage divider and divides the measured voltage by 2.

The cable is terminated with the highly resistive oscilloscope, so that the travelling wave

along the cable is completely reflected and therefore the voltage is doubled again. The

voltage measured by the oscilloscope is now the original voltage taken from the

capacitive divider. Of course the reflected travelling wave travels back to the series

resistor, but as its resistance is as big as the characteristic impedance of the cable, thetravelling wave is completely absorbed in the resistor and does not cause any further

reflections.

Measurement of impulse voltages with damped capacitive divider

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The low voltage part of the damped capacitive divider consists of a capacity and a

resistance R2. To form a voltage divider in the cable connection that divides the

voltage by (approximately) 2, a series resistor (Z R2) is necessary. The voltage wae

that is travelling in the cable is now divided by the ratio Z/(2ZR2). The methodical

error is usually negligible, in the case of RG58 (50 Ohm) and R2 = 1 it is

approximately 1 %. Here again, the cable termination is the highly resistive

oscilloscope which doubles the travelling wave and delivers the (almost) original

voltage value. The reflected travelling wave is absorbed in the resistance (ZR2

+ R2)

and cannot cause further reflections.

e) When a direct voltage is applied to a capacitive voltage divider, the capacitors are firstloaded according to the capacitive divider ratio. But every real capacitor does not only

represent a capacity but a very high parasitic resistance which is connected in parallel

to the capacitance in the equivalent circuit. This forms a resistive divider that is

connected in parallel to the capacitive divider. When direct voltage is connected for alonger time, this resistive divider causes the capacitors to charge according to the

resistive divider ratio! As the parasitic resistances are totally unknown, the divider

ratio is not known, and the measured voltage is complete nonsense, or single

capacitors or the whole divider can even be destroyed!

The time constants resulting from RC are so high that the methodical error for the

measurement of alternating voltages of normally used frequencies is negligible. For

extremely low frequencies or direct voltage, measurement this error becomes so high,

that measurement is not possible any more.