für ANNA angelegt 2004-06-25 für ANNA aktualisiert 2004-06-26
Description of a measuring method
Uz-stat , Uag
Uag D.C. sparkover voltage
Uz-stat static sparkover voltage
The Procedure
Recording of sparkover voltage at first ignition of gas discharge
arresters with linear ramp.
The test voltage constantly rises up to set voltage limit.
A trigger current sensor ascertains, when the arrester sparkes over
switches off the voltage immediately.
The trigger voltage and the ignition state are recorded.
Voltage according to layout of the device up to 2kV or 3kV.
Switch between two measuring ranges:
Range NV: 800V resolution 0.2V
Range HV: 2kV/3kV resolution 1V
Tmax: timeout - when arrester doesn't spark over
Umax = 0..2000V / (optional 3200V )
voltage limit - when arrester doesn't spark over.
>
Imax = 0..2mA / (optional 10mA) current limitation
Two Ramps to your choice:
Ramp CCITT: 100V/s
Rampe fast: 2kV/s
According to equipment of the device another two adjustable ramps are possible:
Ramp variably: 1. 10 kV / s
Ramp variably fast: 10. 100 kV / s
Imax = approx. 1.5 mA current limitation
When the arrester sparks over the shutdown occurs very fast (5-40µs).
Thus only little current flows through the arrester and the measurement
is well reproduceable. At some gas discharge arresters dispersions was
found less than 0.3 V in the sparkover voltage with 100 repetitions
of the measurement.
The ramps are precisely linear about the whole measuring range.
The usual standards of gas discharge arrester prescribe a ramp of 100 V/s,
however, admit also quicker ramps if is guaranteed that the quicker ramp
does not falsify the measured data.
Arresters with 90 V and 150 V of nominal voltage can be often checked
only with 100 V / s.
With arresters with rated voltage from 230 V one can apply pp easily
the quicker ramp with 2 kV/s as long no dark effect appears.
The dark effect appears if the arrester
lies in the darkness and causes that the first measuring value is found
to be higher than the immediately next ones or the measured data with
the illuminated arrester. Formerly to avoid this effect one has added
a small radioactive dose to the arresters to get a sufficient ionization
of the noble gas in the arrester also in the darkness. Today one has
gone off of it and accepts rather the dark effect. According to arrester
type and manufacturer the dark effect appears differently as strong and
quick. With some arresters the effect already appears after few seconds
in the darkness and with other after hours. Typical distinguishing mark
is always that with measurements following shortly on each other the first
value is essentially higher than the second one.
In most cases the dark effect rises with quicker ramps strongly.
Y-circuit
One can also test the series circuit of Varistor and gas discharge
arrester (Y-circuit,power net protection):
Requirements:
- Gas discharge arrester high-isolating (typ. 10G)
- Varistor not charged = about 30s for self discharge
- Varistor voltage below tolerance range of the sparkover voltage
Under the Requirements above the D.C. sparkover voltage of the gas
discharge arrester can be measured with the quick ramp (2kV/s).
If the measurement is repeated immediately afterwards, a higher
D.C. sparkover voltage arises in the identical polarity,
with reverse polarity an accordingly lower value.
The Varistor with its relatively high capacity (nF) drags the
gas discharge arrester with its low capacity (typ. 2pF) up to
sparkover voltage. The arrester sparks over and builds up the
trigger current through the Varistor.
In an Y-circuit mostly there are two identical Varistors (L,N)
which can be measured undependent on gas discharge arrester
(between l and N) with Uvdr .
Overview about the measuring methods.
End of Doc