9th International Symposium on H.V. Engineering - ISH'95 Graz, Austria

DR. STRAUSS

Abstract

The first calibration laboratory in Europe for electrical impulse parameter was founded in 1994 by the holder DR. STRAUSS SYSTEM-ELEKTRONIK GMBH in Germany. The accreditation was performed by the Physikalisch-Technische Bundesanstalt (PTB) represented in the Deutschen AkkreditierungsRat (DAR). The paper describes the calibration system to establish traceability to national standards and the accreditation procedure as also the calibration facilities and procedures of the laboratory and gives an overview of calibration results.
The requirements for the overall uncertainty of reference standard and approved measurement systems are discussed.

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) installes calibration laboratories which are accredited and supervised by the PTB.
The first accredited calibration laboratory for electrical impulse parameter inside the WECC in Europe is founded now which may calibrate impulse measuring and calibration systems for high voltage impulse tests with traceability to national standards and issues calibration certificates recognized.
The paper describes the accredidation procedure, calibration facilities and gives an overview of calibration results performed.

Calibration system to establish traceability
PTB:
The Physikalisch-Technische Bundesanstalt at Braunschweig is the metrological institute of the Federal Republic of Germany and has by law the responsibility for the national standards of measurement. PTB develops the national standards of measurement for representating the international units SI and is responsible for the necessary international comparisons.

DKD:
In the German Calibration Service (Deutscher Kalibrierdienst) calibration laboratories of industry and other institutions are jointed, which are accredited and supervised by PTB according to fixed criterions.
Calibration laboratory:
The calibration laboratories are installed within the organisation and hold the reference measuring systems calibrated by DKD to calibrate the measuring and test systems used in the test laboratories (approved measuring systems).

Picture : Accreditation procedure of the DKD

The German Calibration Service (DKD)

Accreditation procedure of the DKD
The calibration laboratory must meet the requirements of DIN EN 45001 "Common criterion for working of an test laboratory". This includes the installation of a quality management system documented in the respective quality management handbook according to DIN ISO 9001 and EN 29001.

Picture : Calibration system to establish traceability

The picture show the hierarchy of the calibration system to establish traceability to national standards.
The PTB Calibration Certificate is the supposition for the DKD calibration laboratory to issue the DKD Calibration certificate.
The DKD Calibration Certificate is the supposition for the calibration laboratory of organisation to issue the Works Calibration Certificate.
The Works Calibration Certificate is the supposition for the test laboratory of organisation to issue the Works Test Record.
Accredidation by PTB
The calibration laboratory was supplied with the necessary standard reference measuring equipment by the holder DR. STRAUSS SYSTEM-ELEKTRONIK GMBH, 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.
After the examination of these documents the assesment 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.

Measuring uncertainty
The measuring uncertainty for the calibration is as follows:

voltage recorded were determined.

Calibration procedure for digital recorder
The calibration impulses were generated with help of an impulse calibration generator KAL 1000 according to IEC 1083-1, table 3, calibrated by the PTB. The connection of the calibration generator to the digital recorder under calibration followed with help of a coaxial cable with 50 Û impedance and a capacity of 100 pF/m. After setting the d.c. voltage at the calibration generator, 20 impulses of the selected wave shape were released in automatic or single operation.
The data recorded by the digital recorder were evaluated according to IEC 1083-1, Item 2.2.4, 2.2.6.1 or 2.2.6.2 with help of the calibration software. At least from the 20 records of each impulse shape the mean value, their standard deviation and their maximal deviation were calculated and stored in a protocol file for generating the calibration certificate.
Before the step-voltage was evaluated, the raw data were smoothed with a smoothing interval of 20 samples to eliminate the internal noise of the digital recorder.
Before the impulse voltages were evaluated, the raw data were smoothed adaptively with an interval of 20 % of T1, limited to 20 samples, to eliminate oscillations at the front and to eliminate the internal noise of the digital recorder.
For determination of T1 and T2 the events at 30 %, 90 % und 50 % were calculated by interpolation between adjoining samples.
For determination of T_c, a straight line was calculated fitting the first sample behind the peak and the sample at 50 % of the breakdown of the raw data. The crossing point of that line with a horizontal line through the peak determinates the instant of chopping T_c /3//4/.
The determination of the time-to-peak Tp at SI is replaced by determining the front time T1 and multiplied with factor 2,40336 (time-to peak T1p, double-exponential lapse assumed), because the direct determination with help of the raw data is not possible with the necessary accuracy.
For determination of the time interval the levels = 50 % of two rising edges of the square

Measuring Conditions
The impulse calibration generator KAL 1000 with calibration mark 2576 PTB 94 comprises the following modules:

The desired value of the impulse voltage is calculated from the loading voltage displayed on the calibration generator multiplied with the efficiency eta specified by the manufacturer for each impulse shape.
The desired values of the time parameters are the respective nominal values, corrected with the systematic deviations determined by the PTB as follows:

Europe's 1^st accredited calibration laboratory for electrical impulse parameter

The first accredited calibration laboratory for electrical impulse parameter inside the WECC in Europe is founded now which may calibrate impulse measuring and calibration systems for high voltage impulse tests with traceability to national standards and issues calibration certificates recognized.
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).
Reference standard of measurement
The Reference Standards of measurement neccessary for furnishing the calibration laboratory, comprising a digital recorder type TR-AS 100-10 and an impulse calibration generator type KAL 1000, are traced to the national standards at the PTB.

Calibration procedure for impulse calibration generators
The output voltage of the calibration generator under calibration was measured with a digital recorder TR-AS 100-10 calibrated by the PTB. The connection followed with help of a coaxial cable with 50 Û impedance and a capacity of 100 pF/m. The input impedance of the digital recorder amounts to 1 MÛ in parallel with 30 pF.
The calibration was performed for the following two cases of load of the calibration generator:
without load: with coax cable 1 m length and the input impedance of the digital recorder
with load: with coax cable 1,5 m length and the input impedance of the digital recorder and resistance 300 kO in parallel with capacitor 100 pF
The further calibration and evaluation procedure is equivalent to the obove descibed for digital recorder.
Measuring Conditions
Data and settings of the digital recorder TR-AS 100-10 with calibration mark 2577 PTB 94:

deviation max for impulse calibration generators is shown.

The impulse calibration generators calibrated fulfill the requirements of IEC 1083-1 and can be used in calibration laboratory of organisation for calibration check of the reference measuring system and for works calibration of digital recorders in the test laboratories.

Conclusion
The DKD calibration laboratory started in June 1994 and performed inside the first half year more than 20 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 laboratorium in their company and to document the quality measures inside the quality management system of the organisation according to DIN/ISO 9001.
The IEC-Standard 1083 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, low internal noise and suitable software will meet all accuracy requirements.
The calibrated digital recorders TRAS 100-8 and TRAS 100-10 fulfil all requirements for measurements during high voltage and high current tests and for reference measurements.

The digital recorders calibrated fulfill the requirements for reference measuring systems (picture 2.2.1) according to IEC 60-2 and can be used twice, in calibration laboratory of organisation for reference measurements together with a reference voltage divider and in test laboratory for works tests, too.

The sample rate of 500¨10⁶ s^-1 at LIC results in a special sampling procedure supplied by the TR-AS 100-10 digital recorder whereby seven single impulses are recorded sequentially with 100¨10⁶ s^-1 sample rate. After calculating the phase shift of these impulses relative to the sample events, one resulting impulse with 500·10⁶ s^-1 equivalent sample rate is built.

Measuring results
The following tables show calibration results for several digital recorders type TR-AS 100-10 respective impulse calibration generators type KAL 1000 calibrated in the calibration laboratory, exemplary here for positive impulses with peak voltages of approx. 500 V. The complete calibration results are documented in the respective DKD calibration certificates and not shown here (space).
For the evaluated impulse parameter the mean errors Ù of 20 recorded pulses are listed, additional their standard deviation s for digital recorder calibration and their maximal

References

/1/ IEC Publication 1083: 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/ 7th ISH Dresden 1991, 62.03, W. Strauss: Calibration of digital recorders for h.v. impulse tests
/4/ W. Strauss: Automation of i.v. tests online with microcomputers and digital recorders, thesis TU Berlin 1983.