Atec Biddle-27000 - Manual (Page 2)

PARTIAL DISCHARGE DETECTION SYSTEMS
Since partial discharge is often masked
by noise, internal digital filters may be
used to minimize false trips, and to vary
the sensitivity of the overlimit detector.
• Window-gating function: The win-
dow-gating function allows the user to
define a window period during which
partial discharge is measured. Any in-
terference signals occurring outside of
this window will be ignored by the sig-
nal measurement and display circuitry.
This feature may be used to blank spuri-
ous signals that would normally
interfere with the precision partial dis-
charge measurement or to isolate
signals of interest.
Additional Capabilities
• Partial discharge signal level graph-
ing: Some power cable testing spe-
cifications require a graph of the par-
tial discharge level as a function of
test voltage. This capability is pro-
vided by using the picocoulomb
channel analog output for one axis
and the Biddle kilovoltmeter analog
output for the second axis. The scale
of the plot may be calibrated to repre-
sent any picocoulomb value.
• The Biddle Series 27000 Partial Dis-
charge Detector with RS-232 port
permits creation of a computerized test
report. Microsoft

Windows

-based
software that will generate a one-page
test report is optionally available. Two
test report formats are available by
menu selection. One format is for rou-
tine testing of power cable; the other
format is for general-purpose use. The
test report includes an X-Y graph of
partial discharge level versus test volt-
age. Menu-driven software permits the
operator to enter descriptive informa-
tion, calibrate the system and initiate
data collection.
Upon completion of the test, the opera-
tor can view the test data on the monitor
to check for compliance with the test
requirements before printing or saving.
• Interference control: Window gating,
overlimit trip functions and digital fil-
tering all serve to minimize or eliminate
the effects of unwanted continuous and
sporadic signals. Additional shielding
and filtering techniques are optionally
available to augment this capability and
ensure accurate measurement of par-
tial discharge.
As an example, common interference
from single-phase SCR noise occurs
as a pulse on every half-cycle of the
test voltage. If the pulse is stationary
with respect to the phase of the test
voltage, its effects may be completely
eliminated by setting the window
phase angles so that the pulse is
excluded from any measurements. Of-
ten, however, the SCR pulse is not
stationary. In these cases, the interfer-
ence is suppressed simply by
selecting, via front-panel controls,
the criterion of a specified number of
pulse repetitions per half-cycle. Suit-
able settings for various types of
interference conditions must be deter-
mined on an individual basis.
• Multiple passbands: The Biddle
Series 27000 detector includes the
capability to operate with different
passbands. Some test specifications/
applications define the passband
needed for the detector system. Some-
times the interference to be expected
with a certain application will dictate
the pass-band requirement. Biddle
provides superior noise rejection by
designing specific passband amplifiers
with integrated filtering circuits for op-
timum noise suppression. Three
standard passband amplifiers are
available: one for cable testing, one for
general purpose use and one for trans-
former testing at test frequencies from
180 to 400 Hertz. Any two of the three
can be installed in the Series 27000
detector.
System Requirements
The Biddle Series 27000 detector is the
keystone of any partial discharge de-
tection system. A fully operational
system requires, in addition to the de-
tector, a power separation filter, a
bushing tap coupler or a bridge cou-
pling unit to couple the detector to a
test circuit. A properly rated noise-free
test voltage source is also required for
a complete system.
To graph any of the output channels
as a function of test voltage and to op-
erate the apparent power loss readout
channel, an optional Biddle kilovolt-
meter with recording output is needed.
CRT display. The rotation of the ellip-
tical time base may be altered from
the factory-set conventions to suit
user requirements.
Two phase-reference markers may be
manually activated by pressing a
pushbutton switch on the detector front
panel. These markers appear as pulses
on the display time base. One marker
indicates the 0
°
phase position and the
other indicates the 270
°
phase position
of the display time base. They may be
used to assist the user in determining
the phase angle of partial discharge sig-
nals and to remind the user of the
elliptical display conventions.
Calibrator
The calibrator provides a precision
signal that is used to calibrate the
desired readout in units of pico-
coulombs. The output of the calibrator
is continuously variable from 0.1 to
999 pC via a digital control. A digital
display with automatic decimal point
indicates the actual output level. Cali-
bration can be either direct or indirect
(in accordance with IEC 270, ASTM D-
1868, method 3 or ASTM D-1868,
method 4, respectively).
Evaluation Unit
The evaluation unit is used to measure,
process and display several variables
pertaining to the partial discharge sig-
nal. The variables measured are peak
partial discharge in picocoulombs,
average partial discharge in microam-
peres and apparent partial discharge
power loss in milliwatts. A front-panel
liquid crystal display as well as rear-
panel terminal block outputs are used
to convey the measurements to the
user. These outputs may be used to
drive strip chart or X-Y graphic record-
ers when this capability is required.
The evaluation unit also provides the
following two important functions:
• Overlimit trip function: The overlimit
trip function monitors the detector’s
peak partial discharge signal and indi-
cates when that signal exceeds the
overlimit level via a front-panel con-
trol. If the partial discharge exceeds
this level, then a trip will occur. A trip
is indicated by a front-panel LED and
relay contacts which are accessible on
a rear-panel terminal block.