TEN350 The waveform that hijacked the IEC lightning standards
IEC 61643-11
NAME:
Low-voltage surge protective devices – Part 11:
Surge protective devices connected to low-voltage power systems – Requirements and test methods
AUTHORS:
SC37A (Subcommittee 37A Low Voltage Surge Protective Devices) of Technical Committee 37 Surge Arrestors.
INFO:
Cancels and replaces IEC 61643-1. This standard sets out the Class 1 Test for surge protective devices.
PARAMETER TIMELINE:
The parameters on which the Class 1 Test is based can be traced back as follows:
1995: IEC 61312-1 (parameters come directly off the Hasse 10/350 chart)
1998: IEC 61643-1 (parameters taken from IEC 61312-1)
ANOMALIES IN 61643-11
Charge transfer. The determination of the amount of charge transferred by a lightning stroke is a theoretical endeavor. It's measured in C (coulombs) or ampere-seconds (As) but you cannot actually count the lightning-caused electrons moving past a point in an electrical circuit. You can only "derive" the amount of lightning charge through formulas. The formula used for determining lightning "charge" makes it a function of the Ipeak and the length of the tail of the chosen waveshape. The longer the tail, the greater the charge "derived." In the Hasse 10/350 chart, you'll see that the Ipeak (in kilo amps) has twice the value of the charge transferred (in coulombs or ampere-seconds.) For example, 200kA comes with 100 coulombs. Without attributing its source, IEC 61643-11 has adopted Hasse's 2:1 relationship. The math is not difficult -- Hasse computed this based on a 350-500 microsecond long waveform. CIGRE's 2013 Technical Brochure 549 has 1) Invalidated the basis for using the 350µs tails, 2) attributed the long tails imputed to Berger to the phenomenon of continuing currents, 3) failed to substantiate any 2:1 relationship between Ipeak and charge transfer and 4)provides a Table of "recommended lightning waveshapes" which gives 4.65C as median charge value. These should be sufficient reasons to fully re-evaluate the Q values used in the 61643-11 standard.
Inclusion of W/R parameter for SPDs. Even though IEC 61643-1 used the Hasse 10-350 lightning parameters as the basis of its SPD tests, that standard did not use the specific energy (W/R) parameter in its tests because in its own words: "W/R is not a parameter of primary interest for testing SPDs." In that, it was entirely correct. When describing the parameters of the first stroke, specific energy may be a significant factor for the design of the conductors of a lightning protection system. However, when it comes to the SPDs – typically nonlinear devices – the concept of specific energy based on a constant value of the circuit resistance becomes irrelevant.
Curiously, IEC 61643-11 has reverted, without any explanation whatsoever, to including a W/R parameter for SPDs. The CIGRE 2013 Report found that the large W/R figures imputed to Berger's data in fact came from continuing currents. By including this irrelevant parameter in the tabulation of Class I test parameters, IEC 61643-11 has introduced a "red herring" which will cause nothing but confusion and useless discussions. W/R should be omitted from any tests of SPDs.
Contrived relationship between maximum current (Ipeak), charge (Q) and specific energy (W/R). Table 6 in this standard is named "Preferred parameters for Class 1 Test." Here is a quote from the text which strives to "explain" the values in the table: "The impulse discharge current passing through the device under test (SPD) is defined by the crest value Iimp, the charge Q and the specific energy W/R...Table 6 gives values of Q (As) and W/R (kJ/Ω) for example values of Iimp (kA). The relationship between Iimp, Q and W/R is as follows: Q=Iimp × a where a = 5 × 10-4 s. W/R=Iimp 2 x b where b=2,5×10-4s."
These formulas are based on air. The CIGRE 2013 report finds no relationship whatsoever between Iimp, Q and W/R. The monitoring factor in these "explanatory formulas" for both Q and W/R is the length of the tail. The 350 μs long wavetail skews the values for both Q and W/R to unrealistic and unusable levels.
Long stroke and short stroke mix-up. Here is a quote from 61643-11: "The impulse current shall show no polarity reversal and shall reach Iimp within 50 μs. The transfer of the charge Q shall occur within 5 ms and the specific energy W/R shall be dissipated within 5 ms. The impulse duration shall not exceed 5 ms."
"Short strokes" are measured in microseconds. "Long strokes" are measured in milliseconds. When we talk about the impulse of the first stroke, we are talking about microseconds. (IEC 62305-1 Fig. A1 stipulates that the short stroke part of a lightning flash is less than 2 milliseconds. Yet this standard (61643-11) has included the charge (Q) for the entire flash in the impulse test for 1st strokes. As mentioned above, the way that the 62305 series calculates charge is already flawed. But to be talking about a charge transfer continuing for 5 milliseconds has gone way beyond any charge of the first stroke and is irrelevant to the impulse test.
Simple solution. The source of all the above anomalies is the imposition of the arbitrary and unhelpful 10/350 waveform into standards. If it were simply removed, all the above problems would vanish. In its place base tests on the lightning parameters identified in the CIGRE 2013 Technical Brochure 549 shown here.
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