Hardware, Software and Calibration Results of Digital Recorders for H.V. Impulse Tests

W. Strauss

Table of Contents:

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Abstract

The requirements for individual errors and the overall uncertainty of reference standard and approved measurement systems for measurement during high voltage and high current impulse tests are specified in the Standards IEC 1083-1 and IEC 1083-2, which are found as practicable standards.

The paper describes hard- and software for a digital i.v. measuring system with a modular digital recorder optimized for applications according to IEC 1083, to fulfil all requirements necessary for accreditation of laboratories for full and chopped lightning impulses.

The first calibration laboratory in Europe for electrical impulse parameter was founded in 1994 in Germany, the accreditation was performed by the Physikalisch-Technische Bundesanstalt (PTB) represented in the Deutschen AkkreditierungsRat (DAR).

The paper describes the calibration facilities of the laboratory and gives an overview of calibration results of digital recorder performed in the last three years.

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1 Introduction

The IEC-Standard 1083-1 /1/ "Digital recorders for measurements in high-voltage impulse tests, Part I" specifies the requirements for digital recorders and calibrations required to meet the measuring accuracies for measurement during high voltage and high current impulse tests in accordance with IEC 60-2 /2/. In this respect the overall uncertainty of the digital recorder specified in IEC 1083-1 must have smaller limits than the limits for the complete measuring system comprising i.v. divider and digital recorder as specified in IEC 60-2.

The user in the test laboratory is obliged to use measuring equipment according to these standards. To document the compliance this equipment must be calibrated accordingly with traceability to national standards. For this task the German Calibration Service (DKD) installs calibration laboratories which are accredited and supervised by the PTB.

According to IEC 1083-1 digital recorders shall meet the accuracy requirements regarding the limits on overall uncertainty given in 2.1. In order to stay within these limits, the limits for individual uncertainties given in 2.1.2 should be met. In some cases, one or more of these limits may be exceeded provided the overall uncertainty is not exceeded.

Care must be taken using digital recorders with a sampling rate below 100 MS/s, accreditation of laboratories will not include measurement of chopped waves in this case.

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2 Hardware of digital recorders for h.v. tests

2.1 Design optimized for h.v. application
The integration of all components like screen, keyboard, printer and the measuring system with directly coupled personal computer, designed in 19" rackmount technic and built into an EMC-tested measuring rack with common line filter, yields an excellent shielding. The requested tests of electromagnetic interferences of digital recorders according to IEC 1083 were fulfilled without any additional measures /5/7/10/.

2.2 Measuring rack mount with AT-Computer
The measuring rack mount is formed by an EMC-designed and tested housing with excellent shielding. In individual shielded separate cabinets the measuring electronic and the computer are included, which are connected via an interface cable. The high-speed interface taking one slot is designed in a way, that address-, data- and controlsignals of the AT-computer are available at the backplane of the measuring system and direct access is possible without any special transfer protocol.


Picture 2.2.1: Measuring Rack Mount

To solve the problems of electromagnetic interferences and interfacing, the AT-compatible control- and evaluating computer was built into the well-shielded housing of the measuring system and coupled electrically directly. The direct computer access to the programmable registers of the digital recorder and to the recorded measuring data minimizes the access times. The time duration for evaluating and storing of measuring data are considerably decreased.

Measuring Channels: modular design
The measuring electronic is specially designed considering the application in the h.v. testfield with extremly high electromagnetic distortions. The individual measuring channels comprising of
  Tei  input divider   Ver  measuring amplifier
  A/D  A/D-converter   MEM  recorder memory
are constructed modular and independently from each other, so easy assembly of 1 up to 4 channels is possible including a later upgrade depending on the application.

The 1600 V input divider as the critical component is designed for high performance and long term stability /16/ including an overvoltage protection tested with 7 kV peak voltage. The analogue bandwidth and risetime of the input divider as also of the direct input could be increased and amounts to 75 MHz and 5 ns respectively, so the analogue bandwidth and risetime overall for both corresponds to 50 MHz and 7 ns.

The direct input feeds a fully programmable measuring amplifier comprises 24 stages, which allows a very fine adaption in 80%-steps according to a factor of 1.26 to the amplitude values to be measured. In this respect, signal amplitudes can be measured with optimal deflection from 1600 V to 8 V using the input divider while signal amplitudes from 10 V to 50 mV can be measured using the direct input.

The EMV-oriented design of the A/D-converter and memory system determines beside the sampling rate and resolution the overall performance of the digital recorder with respect to ripple, noise and interference.

The digital impulse voltage measuring systems TR-AS can therefore mostly be used without an additional measuring cabine also for testing of vacuum and SF6-insulation in the high-voltage testfield /10/.

2.3 Intercomparison test by HUT
12 laboratories in Europe participated the intercomparison test attended by the Helsinki University of Technology HUT between September 1995 and June 1996. The circulating measuring system included a 400 kV resistive divider and a digital recorder TR-AS 100-10 with the above mentioned design. The digitizer was found to be stable by HUT and PTB /16/.

2.4 Requirements acc. to IEC 1083
According to Item 2.1, accuracy requirements for impulse measurements, the overall uncertainty of the digital recorder shall not be more than:
 Approved System
required limits		    Reference System
TR-AS 100-10
peak value 2%    1%
time parameters 4%    2%

In order to stay within these limits, the limits for individual uncertainties given in IEC 1083-1, Item 2.1.2, should be met. Some of these uncertainties are very important with respect to measurement and diagnostic accuracy and for meeting future standards and will be discussed as follows:
Sampling Rate versus measured time interval: fs > = 30/Tx
LI 1.2/50 required limits TR-AS 100-8/-10
full wave, Tx=0.5µs 60 MS/s 100 MS/s
Tc=0.5µs, Tx=0.3µs 100 MS/s 100 MS/s
Sampling Rate versus linearity < = 2% in the time interval
LIC: Tx=0.5µs required limits TR-AS 100-8/-10
0.02*Tx=10ns 100 MS/s 100 MS/s

2.5 Accredidation for chopped waves
In case the required limits were not met by the digital recorder an official accreditation of the laboratory with respect to the failing condition is not possible, e.g. if the sampling rate is below 100 MS/s, accreditation only for full waves is possible.

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3 Software of digital recorders for h.v. tests

3.1 Control and graphic software
The menu-oriented operation, function-keys programmed with user-specific test sequences as also the load facility of pre-defined setup-files from the harddisk for the respective tests allow a quick and error-free test performance. The measuring ranges of the individual channels were displayed considering the respective high-voltage divider ratio and shunt values, any dimension as kV, kA can be added. At the setting of the independent timebases and the trigger event automatically the record interval as also the pretrigger and posttrigger time interval is calculated and displayed in s or ms.

3.2 Evaluating software
For the calculation of the impulse parameters of standard and non-standard lightning impulses, switching impulses, oscillating impulses and impulse currents an evaluating program is implemented that determinates the respective values, e.g. for a lightning impulse the peak value, the front time and the time-to-halve-value and for chopped impulses the time-to-chopping, too. The calculation is done automatically for each shot, whereby the results can be displayed on screen together with the impulse shape. Additionally a normalized amplitude grid with respect to the actual peak value can be displayed. Furthermore measuring lines for front time, time-to-halfe-value or time-to-chopping can be displayed as also a time grid normalized to virtual origin.

Actual recorded or from harddisk reloaded measuring data and mathematical combinations of them, e.g. a difference curve or the transfer function during transformer tests, simultaneously can be displayed in up to 32 virtual channels and allow a fast and improved fault diagnostic.

3.3 Automatic protocol generation
Immediately after finishing the test a test report is printed on a laser printer. The protocol-modul implemented gives an easy way to generate any protocol to the user. The protocol form, built by a header sequence, a schedule with the impulse parameters and a bottom sequence as also the requested language is largely user-definable. It is possible to get test reports or certificates with one or more pages with or without the waves in any desired language. The printout of the waves can be done in one ore two columns, with one channel as continuous printout with selectable vertical offset is possible.

3.4 Performance test with IEC 1083 TDG
The TR-AS evaluation software Version 5.58 was tested /12/ with respect to the reference waveforms generated by the IEC 1083-2 - Testdata Generator Release Date 03/09/1994.

TDG4
Picture 3.4.1: IEC-TDG No. 4, LI with f>500kHz

All test results from the standard and non-standard waves listed were evaluated automatically regarding overshoot and meancurve calculation /15/. Picture 3.4.1 shows exemplify shape No. 4 together with the calculated meancurve, the complete set of shapes can be found in /12/. The test results for LI for a resolution of the generated data of 10 Bit are listed in the following test record, picture 3.4.2:

PIC342
Picture 3.4.2: Test Record of IEC-TDG reference waveforms

The TR-AS Software passed all tests for all groups of impulses and for all parameters specified according to IEC 1083-2 Part 2, table 2 - specified limits of the parameters of the reference waveforms (1996).

For generation of the IEC 1083-2 TDG reference waveforms and loading them into the TR-AS digital recorders refere to /12/.

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4 Calibration of digital recorders for h.v. tests

4.1 Traceability to national standards
The first calibration laboratory in Europe for electrical impulse parameter was founded in 1994 and supplied with the necessary standard reference measuring equipment comprising digital recorder, impulse calibration generator and digital voltmeter calibrated by the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig for the required comparison with the national standard.

The calibration laboratory was integrated into the Quality Management System of the holder company according to DIN-ISO 9001 resp. EN 29001. The calibration laboratory must meet the requirements of DIN EN 45001 "Common criterion for working of an test laboratory". After the examination of these documents the assessment of the calibration laboratory and several comparison tests were performed by the PTB. Finally the accreditation was performed on 06/06/1994 by the German Calibration Service (DKD), supervised by the PTB and represented in the Deutschen AkkreditierungsRat (DAR) as calibration laboratory for electrical impulse parameter with DAR registration number DKD-K-11701.

The paper /6/ describes the calibration system to establish traceability to national standards and the accreditation procedure as also the calibration facilities and procedures of the laboratory. The requirements for the overall uncertainty of reference standard and approved measurement systems are discussed.

4.2 Calibration facilities
The DKD Calibration laboratory offers the calibration of
  • impulse calibration generators
  • unit step voltage generators
  • digital recorders
  • impulse voltmeters
and issues DKD-Calibration Certificates which document the traceability to national standards which realize the physical units of measurement according to the international system of Units (SI). The DKD calibration laboratory started in June 1994 and performed about 100 calibrations in the meantime with traceability to national standards. These calibrated digital recorders and impulse calibration generators are the basis for many users around the world to etablish an accredited test laboratory for full and chopped waves in their company and to document the quality measures inside the quality management system of the organisation according to DIN/ISO 9001.

4.3 Reference standard of measurement
The Reference Standards of measurement neccessary for furnishing the calibration laboratory, comprising a digital recorder type TR-AS 100-10 /10/ and an impulse calibration generator type KAL 1000 /11/, are traced to the national standards at the PTB.

4.4 Measuring results
The following pictures show calibration results with full lightning impulse LI 0.84/60 and chopped lightning impulse LIC with a time to chopping of Tc=0.5 s, summarized for all digital recorders type TR-AS 100-10 calibrated in the calibration laboratory since 1994.

The pictures show for the respective impulse parameter the mean values ( ) of the deviations of all calibrations depending on the input measuring range, as also the positive ( ) and negative ( ) maximum values of these deviations. All recorders met the limits of 2% for peak values and 4% for time parameters.

PIC441
Picture 4.4.1: LI peak value

PIC442
Picture 4.4.2: LI front time

PIC443
Picture 4.4.3: LI time-to-half-value

PIC444
Picture 4.4.4: LIC peak value

PIC445
Picture 4.4.5: LIC time-to-chopping

More than half of the calibrated recorders met the limits of 1% for peak values and 2% for time parameters and so the requirements for reference measuring systems according to IEC 60-2. They can be used economical twice, in calibration laboratory of organisation for reference measurements for full and chopped waves together with a reference voltage divider and in test laboratory for works tests, too.

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5 Conclusion

The IEC-Standard 1083-1 is an applicable instruction for the tests and procedures to find the relevant errors of digital recorders (and impulse calibration generators) used for measurements during high voltage and high current impulse tests.

Only digital recorders with a special design for h.v. application with calibrated and stable input stage, 100 MS/s and 10 Bit digitizing characteristic, low internal noise and suitable and tested software will meet all accuracy requirements for accreditation for full and chopped waves.

The IEC-Standard 1083-2 including the TDG is an applicable method for the test of evaluation software to find the relevant errors with respect to standard waves and some typical non-standards waves, too.

The DKD calibration laboratory for impulse measurement performed inside three years about 100 calibrations with tracebility to national standards.

These calibrated digital recorders and impulse calibration generators are the basis for many user to etablish an accredited test laboratory in their company and to document the quality measures inside the quality management system of the organisation according to DIN/ISO 9001.

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6 References

/1/  IEC Publication 1083-1: Digital recorders for measurements in high voltage impulse tests, Part 1 - Requirements for digital recorders, 1991
/2/  IEC Publication 60: High Voltage Test Techniques, Part 2: Measuring Systems, 1994
/3/  W. Strauss: Automation of i.v. tests online with microcomputers and digital recorders, thesis TU Berlin 1983.
/4/  W. Strauss: Calibration of digital recorders for h.v. impulse tests, 7th ISH Dresden 1991, paper 62.03.
/5/  H. Bachmann, et al: Field and EMC-tests of a digital h.v. impulse control and measuring system, 7th ISH Dresden 1991, paper 62.04.
/6/  W. Strauss: Calibration of digital recorders and impulse calibrators with traceability to national standards, 9th ISH Graz 1995, paper 4495.
/7/  W. Hauschild, et al.: A 4 MV outdoor impulse test system for the Korean 765 kV transmission project, 9th ISH Graz 1995, paper 4487.
/8/  W. Hauschild, et al.: Computer-aided performance tests and checks for high-voltage measuring systems, ERA-Report 94-0776, paper 3.3.
/9/  IEC Publication 1083-2: Digital recorders for measurements in high voltage impulse tests, Part 2 - Evaluation of software used for the determination of the parameters of impulse waveforms, 1996.
/10/  Type Test Report: Digital i.v. measuring system TR-AS 100-10, information brochure of DR. STRAUSS GmbH, 1993.
/11/  Type Test Report: Impulse calibrator system KAL 1000, information brochure of DR. STRAUSS GmbH, 1994.
/12/  Type Test Report: Control and evaluating software for digital recorder TR-AS, information brochure of DR. STRAUSS GmbH, 1996.
/13/  A. Marinescu, V. Onu: Accreditation by Relar of high voltage and high power laboratories of Craiova, ERA-Report 96, MILAN, paper 2.3.
/14/  W. Hauschild, et al.: Calibration of hv measuring systems for the mutual recognition of hv test results in eastern and western Europe, ERA-Report 96, MILAN, paper 3.3.
/15/  W. Strauss: Selfadaptive evaluation software for high-voltage impulse tests, ERA-Report 96, MILAN, paper 5.4.
/16/  M. Aro, et al.: Intercomparison of h.v. impulse measuring systems with digital recorder, 10th ISH Montreal 1997.