Automatic Impulse Voltage
Routine Testing of
Distribution Transformers
Reprint
L. Furrer
S. Kurz
M. Loppacher
ISH 99
E 1-91
HIGH VOLTAGE TEST
Automatic Impulse Voltage Routine Testing of Distribution Transformers
L. Furrer, S. Kurz, M. Loppacher
Haefely Trench AG
Basel, Switzerland
Abstract
Distribution transformers are often produced by
highly automated production lines. Overall factory throughputs for small transformers
sometimes reach one unit per minute. This
throughput requires a fully automatic impulse
test system capable of generating and analysing
an impulse every few seconds. In addition
production information has to be integrated and
verified by a host computer retrieving
instantaneous pass / fail information from the
impulse test system. High speed, high system
reliability and correct pass / fail decisions are
the challenges in implementing such an
automated test system.
In future sophisticated analysis tools such as
coherence and transfer functions with tolerance
bands could be implemented. This allows a further refinement of pass / fail decisions which
have to be investigated.
Introduction
Distribution transformers are exposed in their
daily operation to transient over-voltages: impulses, which are due to atmospheric lightning
or switching operations. Field experience has
shown that these transients have certainly the
most strenuous impact on transformers’ insulation and cause perhaps approx. 50 % of all premature failures ! [1]. Thus the international
standards have for a long time adopted impulse
tests in their scope of tests.
Depending of the rated voltage, lightning and
switching impulse tests must be carried out on
each terminal of the test object. Since 1993, the
L.I. test has also become mandatory for distribution transformers being tested according to
ANSI / IEEE standards.
Especially US manufacturers are faced with the
problem of testing each distribution transformer
with impulse voltages (ANSI/IEEE Standard
C57.12.90 [2]) to check the manufacturing
quality. Often running 3 shifts and 7 days a
week, special features are required from the
production line test equipment..
To meet these requirements, Haefely Trench
AG has developed and delivered customised
test systems designed to meet these
specifications. A particular emphasis has been
laid on the high transformer throughput rate,
integration of state of the art software tools and
efficient
communication
between
the
customer’s production line and the test system.
Automatic impulse test system
An automatic impulse test system for performing the complete impulse voltage test on distribution transformers (block diagram see figure
2) has to fulfil the following requirements [3]:
•
•
•
•
•
Figure 1
Distribution transformer (400 kVA,
20 kV ; from TESAR)
•
•
•
•
•
Long life expectancy
Lowest cycle time
Lowest maintenance
Automatic report & test certificate
generation
Customised design according to client’s
specification and applicable standards
Integration into manufacturing chain
Automatic calibration feasibility [4].
Safety features
Design suitable also for unskilled operators
Possibilities for off-line-analysis of faults
[5]
Test Object
Charging Rectifier
Impulse Generator
Divider
V
A
Shunt
mm
Impulse Measuring
System
Control Unit
H.V. Connections
Test System Control and Measuring Connections
Data Exchance Connections
Pass
ü
Fail
û
Handling Device
Figure 2
Chopping Gap
:
Host Computer
Automatic impulse voltage test system - block diagram
Figure 3 shows an impulse voltage test system
that fulfils the above mentioned requirements
(total charging voltage 300 kV / 15 kJ). A powerful charging rectifier, a fast sphere gap drive
system and resistors with a high energy receptivity allow to reach a cycle time between 3 10 seconds between the single impulses. A
special design of the components grants a
continuous operation of the system 24 hours
per day over years with minor maintenance inbetween.
The control and measuring systems are customised and prepared for integration into the production line (figure 4). A design with a central
host computer allows off-line analysis of the
measurements on test objects with faults. Network connections enable the integration into
quality systems.
Figure 3
Impulse voltage generator with
charging rectifier and divider
Test Procedure
According to the standards, the basic method
for judging the results of impulse tests is the
comparison between 2 test wave shapes, a
reduced and a full level impulse. Figure 5
shows a typical test procedure for a comparison
measurement on distribution transformers. The
test object has to be identified (e.g. scan of a
bar code), the control system inquires test
levels, tolerances and pass-fail criteria from a
database for the impulse set-up. At first a
reduced level
Figure 4
Control and measuring system
Host Computer/
Control System
Measuring
System
Action
Start
Setup for 50%
BIL impulse
Charging/
Trigger for
50% BIL imp
Setup for 50%
BIL impulse
U, i
measurement/storage
Upeak50 ; tfront;
ttai l; ipeak50 within
tolerances
No
Failed
message
A/B/C or D
Yes
Setup for
100% BIL
(using x)
Charging/
Trigger for
100% BIL imp.
calculation of
efficiency
factor x
U, i
measurement/storage
Upeak100 ; tfront;
ttail; ipeak100 within
tolerances
No
Failed
message
E/F/G or H
No
Failed
message I
Yes
calculation of
voltage normalisation factor y
impulse will be generated (e.g. 50%). Records
of voltage and currents are taken. A check of
the wave shape regarding tolerances is
performed. The efficiency is calculated, a
correction factor is used for the calculation of
the charging voltage for the full level impulse
test. After the wave shape check of the full
level impulse a normalisation and difference
calculation of voltage and current traces is
carried out. The differences are the criteria for
the pass or fail decision.
Pass / Fail Analysis
At present the maximum difference for voltage
and current record are the relevant quantities
for pass / fail decisions. The sensitivity to
failures is generally different for beginning,
centre and end of a voltage or current record,
the same is valid for the sensitivity to
disturbances. For those reasons it is in most
cases wise to select a certain time window
where the sensitivity is optimal for fault
detection and minimal for influences of
disturbances. A typical comparison of a 50%
with a 100% L.I. measurement is shown in
Figure 6.
normalization
of 50% impulse
voltage shape
(using factor y)
diff.calculation
U100 - U50*y (for a
spec.time window)
Upeak diff < 3%
of BIL
Yes
normalization
of 50% impulse
current shape
(using factor y)
Figure 6
diff. calculation
i100 - i50*y (for a
Comparison of L.I. 50% with 100%
showing both impulses and the
maximum deviations numerically
spec. time window)
ipeak diff.<10%
of (i peak50*z)
No
Failed
message J
Yes
Passed
message
storage of
shapes U, i
(50 and
100%
impulse)
next DUT
Figure 5
Automatic impulse voltage test
- flow chart
Up to now pass / fail analysis are mostly performed by comparison of measurements
recorded with the same impulse shape but
different voltage level. Such a comparison is
sensitive to voltage non-linearities of the test
object. An example for such a non-linearity
would be an internal "flashover" occurring at
75% B.I.L.. This failure is not effecting the
50% but strongly effecting the 100% amplitude
measurement and therefore leading to nonlinearities - clearly visible in the normalised
difference
calculation.
Reporting
Depending
on
customers
requirements
reporting is performed very detailed for all
transformers or just a pass / fail list is printed.
Failed transformers are often of higher interest
as knowledge about the actual occurring errors
can help the manufacturer to improve his
production accordingly. However quality
standards
nowadays
require
detailed
documentation which might in future lead to
more detailed reports in general. An example of
a distribution transformer test report is given
below (figure 7).
tool for fully automatic transformer testing.
Still though field experience is required and
reliability has to be checked before
implementing this tool.
Conclusions
Fully automatic impulse voltage test fields
suitable for high throughput production lines
have been implemented successfully. The
required throughput, reliable pass / fail
decisions and - most important - the desired
reliability has been reached. Several of these
automatic test systems are continuously in
operation for months generating and analysing
thousands of impulses without interruption.
References
[1]
[2]
[3]
[4]
[5]
Figure 7
Example of a fully automatic test
report. Shown are for all three
phases the two voltage recordings,
the voltage difference and the
current difference calculation.
[6]
Outlook
Insulation faults generally also show a very
specific behaviour in the frequency domain for
which sophisticated analysis tools are available.
For large power transformer testing Transfer
Function [6] and Coherence Function [7]
analysis have become well accepted already.
However the judgement of differences between
two Transfer Functions is still requiring skilled
people and, so far, is not ready for automatic
testing. A Tolerance Band Method for the
Transfer Function [8] might allow using this
[7]
[8]
D. Ballard; „Routine Impulse Testing on
Distribution Transformers“; High
Voltage Seminar 3rd October 1995
ANSI/IEEE Standard C57.12.90
A. Claudi; „Neue Entwicklungen in der
Impulsprüftechnik“; Haefely Symposium
1998
A. Claudi, J. Schramm; „Calibration of
Digital Impulse Measuring Systems “;
Paper 53.02; 8th International Symposium
on HV Engineering, Yokohama; Japan,
1993
R. Malewski, K.Feser, A. Claudi, E.
Gulski; „Digital Techniques for Quality
Control and In-Service Monitoring of
HV Power Apparatus“; Paper 15/21/3303, International Conference on Large
High Voltage Electric Systems; CIGRE;
Session 1996; Paris; France; 26.8.30.8.1996
R. Malewski; B. Poulin; „Impulse testing
of power transformers using the transfer
function method“; IEEE Transactions on
Power Delivery Vol. 3; No. 2; April
1988
R. Malewski; A. Claudi, Ch. Josephy; St.
Jud; „Checking electromagnetic compatibility of a HV impulse measuring circuit
with coherence functions“; ERA Technology Conference H.V. Measurements
and Calibration; Arnhem, 1994
T. Leibfried; K. Feser; „A new Method
for Evaluating Transfer Functions of
Power Transformers“; 10th International
Symposium on HV Engineering,
Montréal; Canada 1997
Haefely Test AG
High Voltage Test Division
CH-4028 Basel/Switzerland
Phone +41.61.373 41 11
Fax
+41.61.373 49 12
www.haefely.com
e-mail: sales@haefely.com
HIGH VOLTAGE TEST