During the tests described in 8.2, the voltage is set to Un and the current is set to 0,1 Imax for a direct connected meter or to 0,5 Imax for a CT operated meter.
These values of voltage and current may not reflect correctly the profiles of voltage and current that the meter will meet at normal use conditions.
IEC 1695/08
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For example, if in the case of a direct connected meter the internal temperature rises significantly when the meter is operated at a current above 0,1 Imax, and the normal use profile of the meter shows that the meter is typically operated at higher currents, then this shall be taken into account when evaluating life characteristics, to avoid significant errors in the estimation.
To avoid this possible error, the yearly average temperatures (for each fault mode) estimated in 9.2.1 have to be corrected by the following procedure:
• Step A: Define the normal use profile of voltage and current;
• Step B: Measure the variations of the internal temperature of the meter for each voltage and current of the normal use profile;
• Step C: Calculate the average internal temperature of the meter corresponding to the normal use profile of voltage and current (this calculation uses the acceleration factor parameter Ea obtained from 8.2 step 6 for each fault mode). Then apply to the yearly average temperature estimated in clause 9.2.1, for each fault mode, a correction equal to the difference between the average internal temperature and the internal temperature measured in step B at Un and 0,1 Imax for a direct connected meter, or 0,5 Imax for a CT connected meter.
These steps are described with more details in the following clauses.
9.3.1 Definition of the normal use profile of voltage and current
The normal use profile of voltage and current defines the proportion of time (in %) at which the meter will be used inside the following ranges of voltage and current:
• voltage ranges: 0,85 Un < U < 0,95 Un , 0,95 Un < U < 1,05 Un , 1,05 Un < U < 1,15 Un;
• current ranges: 0 < I < 0,1 Imax , 0,1 Imax < I < 0,2 Imax , … , 0,8 Imax < I < 0,9 Imax , 0,9 Imax < I < Imax .
If the normal use profile of voltage and current is not available, the same proportion of time (3,33 %) will be set by default for each combination of voltage and current range.
An example is given in 9.3.3 Table 10 (columns 1 to 3).
9.3.2 Measurement of the meter internal temperature at each current and voltage For all values of voltages equal to 0,85 Un , Un and 1,15 Un, and for all values of current equal to 0,1 Imax , 0,2 Imax , 0,3 Imax , 0,4 Imax , 0,5 Imax , 0,6 Imax , 0,7 Imax , 0,8 Imax , 0,9 Imax and Imax , the temperature inside the equipment is measured. This measurement shall be done in a room where temperature is maintained at 23 °C +/- 2 °C.
An example is given in 9.3.3 Table 10 (columns 4 to 6).
NOTE The spot temperature measurement inside the meter will vary depending on exactly where the sensor is placed relative to local internal hot spots and also dependent on the mounted position of the meter. The rate of change of temperature is usually slow and time to settle is required.
9.3.3 Calculation of the meter average internal temperature
For each value of voltage and current as described in 9.3.2, the Arrhenius acceleration factor, compared to temperature measured at Un and 0,1 Imax for a direct connected meter or 0,5 Imax for a CT connected meter, is calculated with parameter Ea obtained from 8.2 step 6. The Arrhenius acceleration factor is calculated from the following formula:
1) (1
i n a
T T k E
e onFactor
Accelerati = −
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Where Ea is obtained from 8.2 step 6, Tn is the temperature measured at Un and 0,1 Imax for a direct connected meter (or 0,5 Imax for a CT meter) and Ti is the temperature measured for other values of voltage and current. Tn and Ti are in K.
Then the average acceleration factor is calculated from the following formula:
100
)
∑ ( ×
= Accelerati onFactor Proportion OfTime actor
elerationF AverageAcc
Where ProportionOfTime is obtained from the normal use profile of voltage and current (see 9.3.1).
The average internal temperature of the meter is calculated then from the following formula:
) 1 ln(
1
actor elerationF AverageAcc
E k T rature ernalTempe AverageInt
a n
−
=
Table 10 below gives an example of the calculation of acceleration factor for each value of voltage and current for a direct connected meter. For this example, Ea is assumed to be obtained equal to 0,9 from 8.2, step 6.
Table 10 – Example of procedure for temperature correction
Voltage Range Current range % of time U
(x Un) I
(x Imax) T (°C)
Acceleration factor
0,85 Un < U < 0,95 Un 0 < I < 0,1 Imax 3 0,85 0,1 25,7 0,97 0,85 Un < U < 0,95 Un 0,1 Imax < I < 0,2 Imax 2 0,85 0,2 26,2 1,02 0,85 Un < U < 0,95 Un 0,2 Imax < I < 0,3 Imax 1,5 0,85 0,3 27,1 1,14 0,85 Un < U < 0,95 Un 0,3 Imax < I < 0,4 Imax 1 0,85 0,4 28,3 1,31 0,85 Un < U < 0,95 Un 0,4 Imax < I < 0,5 Imax 0,9 0,85 0,5 29,8 1,55 0,85 Un < U < 0,95 Un 0,5 Imax < I < 0,6 Imax 0,6 0,85 0,6 31,7 1,92 0,85 Un < U < 0,95 Un 0,6 Imax < I < 0,7 Imax 0,4 0,85 0,7 33,9 2,46 0,85 Un < U < 0,95 Un 0,7 Imax < I < 0,8 Imax 0,3 0,85 0,8 36,5 3,27 0,85 Un < U < 0,95 Un 0,8 Imax < I < 0,9 Imax 0,2 0,85 0,9 39,5 4,52 0,85 Un < U < 0,95 Un 0,9 Imax < I < Imax 0,1 0,85 1 42,7 6,35 0,95 Un < U < 1,05 Un 0 < I < 0,1 Imax 24 1 0,1 26 1,00 0,95 Un < U < 1,05 Un 0,1 Imax < I < 0,2 Imax 16 1 0,2 26,5 1,06 0,95 Un < U < 1,05 Un 0,2 Imax < I < 0,3 Imax 12 1 0,3 27,4 1,18 0,95 Un < U < 1,05 Un 0,3 Imax < I < 0,4 Imax 8 1 0,4 28,6 1,35 0,95 Un < U < 1,05 Un 0,4 Imax < I < 0,5 Imax 7,2 1 0,5 30,1 1,60 0,95 Un < U < 1,05 Un 0,5 Imax < I < 0,6 Imax 4,8 1 0,6 32 1,99 0,95 Un < U < 1,05 Un 0,6 Imax < I < 0,7 Imax 3,2 1 0,7 34,2 2,54 0,95 Un < U < 1,05 Un 0,7 Imax < I < 0,8 Imax 2,4 1 0,8 36,8 3,38 0,95 Un < U < 1,05 Un 0,8 Imax < I < 0,9 Imax 1,6 1 0,9 39,8 4,67 0,95 Un < U < 1,05 Un 0,9 Imax < I < Imax 0,8 1 1 43 6,55 1,05 Un < U < 1,15 Un 0 < I < 0,1 Imax 3 1,15 0,1 26,3 1,04 1,05 Un < U < 1,15 Un 0,1 Imax < I < 0,2 Imax 2 1,15 0,2 26,8 1,10 1,05 Un < U < 1,15 Un 0,2 Imax < I < 0,3 Imax 1,5 1,15 0,3 27,7 1,22
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Voltage Range Current range % of time U
(x Un) I
(x Imax) T
(°C) Acceleration factor 1,05 Un < U < 1,15 Un 0,3 Imax < I < 0,4 Imax 1 1,15 0,4 28,9 1,40 1,05 Un < U < 1,15 Un 0,4 Imax < I < 0,5 Imax 0,9 1,15 0,5 30,4 1,66 1,05 Un < U < 1,15 Un 0,5 Imax < I < 0,6 Imax 0,6 1,15 0,6 32,3 2,06 1,05 Un < U < 1,15 Un 0,6 Imax < I < 0,7 Imax 0,4 1,15 0,7 34,5 2,63 1,05 Un < U < 1,15 Un 0,7 Imax < I < 0,8 Imax 0,3 1,15 0,8 37,1 3,49 1,05 Un < U < 1,15 Un 0,8 Imax < I < 0,9 Imax 0,2 1,15 0,9 40,1 4,82 1,05 Un < U < 1,15 Un 0,9 Imax < I < Imax 0,1 1,15 1 43,3 6,76
This example gives an average acceleration factor of 1,45, an average internal temperature of 29,2 °C and a correction of 3,2 °C to be applied to the yearly average temperature.