CLC/TR 50462:2008

SLOVENSKI STANDARD
01-november-2008
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Rules for the determination of uncertainties in the measurement of the losses on power
transformers and reactors
Regeln zur Bestimmung der Messunsicherheiten von Verlusten in
Leistungstransformatoren und Drosselspulen
Ta slovenski standard je istoveten z: CLC/TR 50462:2008
ICS:
29.180 Transformatorji. Dušilke Transformers. Reactors
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT
CLC/TR 50462
RAPPORT TECHNIQUE
July 2008
TECHNISCHER BERICHT
ICS 29.180
English version
Rules for the determination of uncertainties in the measurement
of the losses on power transformers and reactors

This Technical Report was approved by CENELEC on 2008-03-07.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2008 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. CLC/TR 50462:2008 E
Foreword
This Technical Report was prepared by the Technical Committee CENELEC TC 14, Power
transformers.
The text of the draft was submitted to vote in accordance with the Internal Regulations, Part 2,
Subclause 11.4.3.3 (simple majority) and was approved by CENELEC as CLC/TR 50462 on
2008-03-07.
__________
– 3 – CLC/TR 50462:2008
Contents
Introduction. 5
1 Sc op e . . 6
2 Normative references . 6
3 Definitions . 6
4 Symbols . 6
4.1 General symbols . 6
4.2 Symbols for uncertainty . 7
5 Power measurement, systematic deviation and uncertainty . 8
6 Procedures for no-load loss . 8
6.1 General . 8
6.2 Model function for no-load losses at reference conditions . 9
6.3 Uncertainty budget . 10
7 Procedures for load loss . 11
7.1 General . 11
7.2 Model function for load loss at reference conditions . 11
7.3 Uncertainty budget for measured power P referred to rated current . 12
7.4 Uncertainty budget for reported load loss . 14
8 Three-phase calculations . 14
8.1 Power . 14
8.2 Reference voltage (current) . 15
9 Reporting . 15
9.1 Unc e r t a i nt y . 15
9.2 Traceability . 15
10 Estimation of corrections and uncertainty contributions . 15
10.1 Ratio error of instrument transformers . 15
10.2 Phase displacement of instrument transformers . 17
10.3 Power meter . 21
10.4 Voltage measurement in no-load loss . 22
10.5 Ampere meter in load loss measurement . 22
10.6 Correction to sinusoidal waveform . 23
10.7 Winding temperature θ at load loss test . 23
10.8 Winding resistance . 24
Annex A (informative) Example of load loss uncertainty evaluation for a large power
transformer . 26
A.1 Introduction . 26
A.2 Transformer rating . 26
A.3 Measuring method and instrumentation used . 26
A.4 Model of the measurand (see 7.2) . 27
A.5 Results of the measurements . 27
A.6 Uncertainty of load loss . 28
A.7 Estimates of the single contributions to the uncertainty . 30
Annex B (informative) Example of load loss uncertainty evaluation for a distribution
transformer . 34
B.1 Introduction . 34
B.2 Transformer rating . 34
B.3 Measuring instrumentation . 34

B.4 Model of the measurand (see 7.2) . 34
B.5 Results of the measurements . 35
B.6 Uncertainty of load loss . 36
B.7 Estimate of the single contributions to the uncertainty formation . 37
Annex C (informative) General rules for the uncertainty estimate . 40
C.1 The basic concepts . 40
C.2 Measurements, estimates and uncertainties . 40
C.3 Evaluation of the input quantity uncertainties. 41
C.4 Evaluation and expression of the expanded uncertainty . 44
Annex D (informative) Sensitivity coefficients for uncertainty contributions due to phase
displacement correction of measurements at low power factor . 45
D.1 Introduction . 45
D.2 Sensitivity factors . 46
Annex E (informative) Model function for load loss temperature correction . 51
E.1 General . 51
E.2 Model function . 51
E.3 Sensitivity coefficients . 52
E.4 Estimation of temperature during load loss test . 53
E.5 Simplified analysis. 53
Annex F (informative) Measurement of winding resistance . 55
F.1 Description of the measurement . 55
F.2 Inductive voltage drop . 56
Bibliography . 58
Figures
Figure D.1 – Sensitivity coefficient for uncertainty in power, current and voltage . 48
Figure F.1 – Equivalent circuit . 55
Tables
Table 1 – No-load loss uncertainty, general case . 10
Table 2 – No-load loss uncertainty without correction for phase displacement . 11
Table 3 – Uncertainty in the general case . 13
Table 4 – Uncertainty without correction for phase displacement . 13
Table 5 – Standard and expanded uncertainty for load loss . 14
Table 6 – Procedures for uncertainty analysis . 18
Table A.1 – Uncertainty contribution . 29
Table A.2 – Calibration of voltage and current transformers ratio error. 30
Table A.3 . 31
Table A.4 – Calibration of voltage and current transformer phase displacement . 32
Table B.1 . 35
Table B.2 – Uncertainty contribution . 36
Table B.3 . 38
Table B.4 . 38
Table C.1 – Combined uncertainties for uncorrelated quantities . 43
Table E.1 . 53

– 5 – CLC/TR 50462:2008
Introduction
Although the efficiency of a power transformer is very high, the losses (no load and load losses) are
object of guaranty and penalty in the majority of the contracts. As a matter of fact, considering the long
power transformer life (20 years and more) the cost of the losses play an important role in the
evaluation of the total (service) costs and therefore in the investments involved.
A further reason that justifies the attention paid to the losses is that from the generation to the final
user, the energy is passing through a number of transformers: step up transformers of generation
power stations, interconnecting units for transmission systems, distribution transformers for primary
systems (from 100 kV to 400 kV), medium voltage to low voltage transformers in small distribution
substations (from 10 kV to 20 kV feeders).
The sum of the losses accrued in the transformer chains may be significant and therefore of
importance in nationwide efforts to save energy. A large number of European Countries have
instituted measures to conserve energy where losses in electric transmission are an important part.
In power transformers the direct measurement of the efficiency is not recommended because of the
uncertainty of this method.
The indirect method based on the measurement of the losses is largely preferred even if the
conditions in which such losses are measured differ a little from those that occur in operation.
EN ISO/IEC 17025 requires that the result of any measurement shall be qualified with the evaluation
of its uncertainty. A further requirement is that known corrections shall have been applied before
evaluation of uncertainty.
This document deals with the measurement of the losses that from a measuring point of view consist
of the estimate of a measurand and the evaluation of the uncertainty that affects the es
...