Atec Biddle-27000 - Manual

PARTIAL DISCHARGE DETECTION SYSTEMS
BIDDLE 
Series 27000
• Key component of all
Biddle partial discharge
detection systems
• Includes multiple
interference control
features
• Five built-in types
of data readout
• RS-232C interface for
control and data export
Partial Discharge (Corona) Detector
INTRODUCTION
ASTM defines a partial discharge as a
type of localized discharge that results
from transient gaseous ionization in
an insulation system when the voltage
stress exceeds a critical value. The
ionization is localized over only a por-
tion of the distance between the
electrodes of the system. The result-
ant partial discharge signals appear as
very small magnitude, fast-rise pulses
with irregular waveshapes superim-
posed on the high voltage at the
terminals of the test sample. Partial
discharges cause deterioration of in-
sulation materials and are a primary
cause of insulation failure at moderate
and high voltages.
The accepted unit for measurement of
partial discharge magnitude is the
picocoulomb. The picocoulomb, a unit
of charge, is columbs x 10
-12 . Charge is
preferred to voltage as a unit of mea-
sure because it is proportional to the
destructive energy released at the
discharge site. Pulse voltage is unde-
sirable because it is dependent on
partial discharge pulse waveshape
which is irregular at best.
Partial discharge pulse waveshape is
dependent on the location of the dis-
charge site and changes as the pulse
propagates through the circuit. The
charge, proportional to the number of
ions formed at the discharge site, is
represented by the area under the
partial discharge pulse waveshape.
This area remains constant, indepen-
dent of discharge site location. The
energy released is proportional to the
product of the number of ions formed
and the critical voltage at the dis-
charge site. It is therefore apparent
that the response from a properly de-
signed partial discharge detection
system must be proportional to the
area under the partial discharge pulse.
The Biddle  Partial Discharge Detec-
tion System integrates the area under
any partial discharge signals detected
and displays an output pulse-signal on
the oscilloscope which is proportional
in height to the integrated area. The
ratio of proportionality, in pico-
coulombs per unit of deflection, is
established by using the built-in cali-
bration equipment.
DESCRIPTION
The Biddle Series 27000 Partial Dis-
charge (Corona) Detector is composed
of four functional units: the amplifier,
display, calibrator and evaluation unit.
Amplifier
The amplifier provides low-noise gain
and bandwidth limiting of the partial
discharge signals. The gain is continu-
ously variable over four decade ranges
and is determined by front-panel con-
trols. The input of the amplifier is
galvanically isolated from the source
of the signal to minimize grounding
problems and to provide protection
against common mode transients that
often occur during routine testing. Dif-
ferent partial discharge tests may
require amplifiers with different band-
width limiting. In these cases, the
detector can be equipped with two
amplifiers.
Display
The display circuitry includes partial
discharge pulse rectification and
brightening circuits, a display time
base and an electrostatic CRT display.
Pulse rectification is used to enhance
the clarity of the display and to make
the pulse polarity appear independent
of the phase of the test voltage.
The display time base provides a CRT
baseline upon which the partial dis-
charge pulses may be superimposed.
This baseline is phase-locked to either
the test voltage or detector line voltage,
providing a phase reference which is
essential for ac partial discharge pattern
recognition. The detector circuitry will
dynamically choose between these two
reference signals to see which
provides the more suitable phase refer-
ence. Front-panel indicators show which
phase reference signal is being used.
Three different baselines are available:
elliptical, linear and sinusoidal. Each
of the baselines is synthesized from
phase-locked-loop circuitry so that
test-voltage harmonics are completely
suppressed and do not appear on the
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