Atec 27000 User Manual

AVO INTERNATIONAL

BRINGING RELIABILITY TO AMPS • VOLTS • OHMS

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 por­tion of the distance between the
electrodes of the system. The result­ant partial discharge signals appear as
very small magnitude, fast-rise pulses
with irregular waveshapes superim­posed on the high voltage at the
terminals of the test sample. Partial
discharges cause deterioration of in­sulation 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
preferred to voltage as a unit of mea­sure because it is proportional to the
destructive energy released at the
discharge site. Pulse voltage is unde­sirable 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 dis­charge 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

-12

. Charge is

partial discharge pulse waveshape.
This area remains constant, indepen­dent of discharge site location. The
energy released is proportional to the
product of the number of ions formed
and the critical voltage at the dis­charge site. It is therefore apparent
that the response from a properly de­signed partial discharge detection
system must be proportional to the
area under the partial discharge pulse.

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The Biddle
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 pico­coulombs per unit of deflection, is
established by using the built-in cali­bration equipment.

DESCRIPTION

The Biddle
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 continu­ously variable over four decade ranges
and is determined by front-panel con­trols. The input of the amplifier is
galvanically isolated from the source
of the signal to minimize grounding

Partial Discharge Detec-

Series 27000 Partial Dis-

problems and to provide protection
against common mode transients that
often occur during routine testing. Dif­ferent partial discharge tests may
require amplifiers with different band­width 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 dis­charge 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 refer­ence. 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

PARTIAL DISCHARGE DETECTION SYSTEMS

CRT display. The rotation of the ellip­tical 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 sig­nals 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 pico­coulombs. 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. Cali­bration can be either direct or indirect
(in accordance with IEC 270, ASTM D­1868, 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 sig­nal. The variables measured are peak
partial discharge in picocoulombs,
average partial discharge in microam­peres and apparent partial discharge
power loss in milliwatts. A front-panel
liquid crystal display as well as rear­panel 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 record­ers 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 indi­cates when that signal exceeds the
overlimit level via a front-panel con­trol. 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.

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 in­terference signals occurring outside of
this window will be ignored by the sig­nal measurement and display circuitry.
This feature may be used to blank spuri­ous signals that would normally
interfere with the precision partial dis­charge measurement or to isolate
signals of interest.

Additional Capabilities

• Partial discharge signal level graph­ing: Some power cable testing spe­cifications require a graph of the par­tial discharge level as a function of
test voltage. This capability is pro­vided 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 repre­sent any picocoulomb value.

• The Biddle Series 27000 Partial Dis­charge Detector with RS-232 port
permits creation of a computerized test
report. Microsoft

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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 rou­tine 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 volt­age. Menu-driven software permits the
operator to enter descriptive informa­tion, calibrate the system and initiate
data collection.

Upon completion of the test, the opera­tor 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 fil­tering 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 par­tial 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. Of­ten, however, the SCR pulse is not
stationary. In these cases, the interfer­ence is suppressed simply by
selecting, via front-panel controls,
the criterion of a specified number of
pulse repetitions per half-cycle. Suit­able settings for various types of
interference conditions must be deter­mined 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. Some­times 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 op­timum noise suppression. Three
standard passband amplifiers are
available: one for cable testing, one for
general purpose use and one for trans­former 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 de­tection system. A fully operational
system requires, in addition to the de­tector, a power separation filter, a
bushing tap coupler or a bridge cou­pling 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 op­erate the apparent power loss readout
channel, an optional Biddle kilovolt­meter with recording output is needed.