Download Discussion on lightning impulse test procedure

Discussion on lightning impulse test procedure
Problematic:
- A standard waveshape (1,2 µs ± 30% x 50 µs ± 20%) is specified. For most of the cases,
there is no problem to meet this required waveshape with the actual test laboratory equipment;
- For some transformers (large MVA and/or relatively low winding voltage and/or low leakage
impedance), the tail time can not be obtained easily and for some cases, impossible to
achieve even with the most powerful impulse generators ( 400 kJ).
- The tail time achieved during the impulse tests is an important factor to demonstrate that the
transformer is capable to withstand its rated BIL level;
- The tail time is function of the capacitance used in the impulse generator together with the
leakage impedance (for low leakage impedance); t2  (LTCG). To get the required waveshape,
one of the options is to have the proper impulse generator capacitance or to use
alternative methods as stated in IEEE C57.98.
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Problematic (continued):
- IEEE C57.12.90, clause 10.3.1.1 does permit the use of a shorter waveshape provided that
the impulse generator capacitance is equal to or greater than 0,011 µF; How short can be the
waveshape to get a meaningful test (30 µs, 20 µs, 10 µs etc…)?
- During previous meetings, it has been a general consensus that the capacitance value
(0,011 µF) given in IEEE C57.12.90 is totally inadequate and shall be replaced by
something else.
- The concept of a "minimum impulse generator energy" has been proposed and
discussed in several occasions since Leon (Mexico) meeting (November 1998).
- Some manufacturers are equipped with more powerful impulse generators than others;
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Problematic (continued):
- Two identical transformers made by two different manufacturers may be tested with
different severities depending of the manufacturer laboratory capabilities.
- This is not giving a fair market competition (manufacturer concern);
- Some transformers may be not adequately tested (user concerns).
- It is important to define minimum test equipment capabilities in order to have a common
minimum testing severity within the transformer industry.
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Last meeting proposal:
- We proposed to change the first paragraph of indent b) of clause 10.3.1.1 of C57.12.90 as
follows:
"The impedance of some windings may be so low that the desired time to the 50% voltage point
on the tail of the wave can not be obtained with available test equipment. For such cases,
shorter waves are acceptable provided that:
1- The optimum impulse generator connection is used (use of parallel stages,
largest available capacitance);
2- The available energy from the impulse generator with the actual test
connection and charging voltage is equal to or higher than the values given in
the following table:
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Discussion on lightning impulse test procedure
Last meeting proposal (continued):
Transformer
category
Single phase
(kVA)
Three phase
(kVA)
Minimum available
impulse generator
energy during tests1,2
(kJ)
I
II
III
IVa
IVb
5 to 500
501 to 1667
1668 to 10000
10001 to 80000
 80000
15 to 500
501 to 5000
5001 to 30000
30001 to 240000
 240000
12,5
25,0
25,0
37,5
100,0
Notes:
1- Applicable only if the impulse tail time parameter ("t2") is outside the prescribed tolerances (50 s ± 20%).
2- Any generator configurations can be used during tests if the impulse waveshape "t 2" parameter is within
the prescribed tolerances."
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Oklahoma City meeting decisions:
1- To revise the proposed minimum impulse generator energy values to cover not more than
80 % of the studied cases (actual values did cover from 83% to 92% of the studied cases).
2- Transformer categories to be aligned to the those defined in C57.12.00 for short-circuit
performance (minimum nameplate rating i.e. ONAN where applicable).
3- An informative Annex has to be written explaining how the values were derived.
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting:
From Arthur Molden:
1- "As I believe we may have discussed during the last meeting, the value of inductance we are using in our
calculations is based on the 60 Hz parameters of the transformer. If we were able to calculate the effective
value of this inductance in the impulse frequency range I believe it would be a higher value. It would be quite
easy to estimate the value of this inductance if we could observe the natural frequency obtained when a
particular transformer was connected to a particular IG capacitor in an underdamped test circuit. Knowing the
IG capacitance and the natural frequency we could easily calculate the effective inductance.
And so for this reason I believe the value of IG energy we obtain from a calculation using the 60 Hz
inductance value, is on the high side."
Answer: By definition, the inductance is theoretically independent from frequency. Practically, due
to other phenomena, the inductance can slightly increase at higher frequency. It is suggested to use
the inductance value calculated from the power frequency leakage impedance as used by IEC 60076-4
for the calculation of the impulse generator components. This is giving impulse generator energy
values slightly on the high side.
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting (continued):
From Arthur Molden (continued):
2- "This "over estimation" is compounded by two other parameters that are used in the calculations.
1) The use of the nominal value of tail time, 50 us.
2) The use of 80% efficiency for the IG voltage efficiency.
If we were to use the minimum tail time of 40 us, the required IG energy would be reduced by a factor equal
to (40/50)2.
If we were to assume the IG efficiency was 100 %, the required IG energy would be reduced by a similar
factor again (80/100)2.
In other words our estimate of the required IG energy is using other estimated parameters, the values of
which are all erring in the same direction.
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting (continued):
From Arthur Molden (continued):
If we could obtain a better idea of how the 60 Hz based inductance value relates to the "effective" inductance
during impulse testing, we could possibly determine our required energy with a little more precision.
I believe the manufacturers would have a case for arguing that they only need to guarantee a 40 us tail. Also,
I believe that the "optimistic" results that would be obtained by using a 100 % voltage efficiency in the
calculations, would be somewhat offset by the fact that the inductance value we are using is based on a 60
Hz calculation.
If there are a few occasions when these reduced energy requirements do result in a short tail, the
manufacturer and customer can still elect to use "alternative means" of extending the tail."
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting (continued):
From Arthur Molden (continued):
Answers:
1- We tend to agree that the calculated "minimum energy levels" should be based on a minimum tail
time of 40 µs instead of 50 µs as calculated previously since the values are called "minimum energy
levels". This will effectively reduce the values proposed during the Oklahoma City meeting (64% of
the original proposal).
2- We did review several impulse test parameters used on our recent transformers orders and we
have found that the impulse generator efficiency was generally ranging from 70% to 90%. Only few
cases were outside that window. We found only few cases were the impulse generator efficiency was
as low as 50% and for few other cases, the efficiency was close to 100%. The original hypothesis
using a generator efficiency of 80% was not so pessimistic. We do recommend to continue to use a
mean value of 80%.
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting (continued):
From Loren Wagenaar:
1- "I plotted the minimum generator energy (MGE) vs. MVA and BIL rating of the winding. The result is that
for a given BIL, MGE is linear with MVA. The significance of this fact is that we can, if we wish to, specify a
minimum MGE at a given MVA (e.g., 100 MVA) for a given BIL and then ratio accordingly for the particular
MVA of interest."
Answer:
Yes, the MGE is linear with the MVA and inversely proportional to the leakage impedance. By using your
suggestion, we did derive a formula giving the minimum impulse generator energy as a function of tail
time, BIL, power rating, winding voltage and leakage impedance (see the revised proposal). This
formula gives only the minimum impulse generator energy level for a particular transformer design which can
be lower or higher than the values proposed originally. If lower, every body (manufacturers and users) will be
happy because you do not need a so powerful impulse generator and the required waveshape tail time will be
achieved. On the other hand, if the number computed is higher than the limits proposed, you need an
impulse generator having an higher capacity. For some cases, even the most powerful impulse generators (
400 kJ) will not be able to produce the required waveshape. Thus, we need to propose a certain upper limit
for which manufacturers can design their test plants.
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting (continued):
From Loren Wagenaar (continued):
2- "I added a couple of BIL levels (200 and 250 kV) at the bottom end of the range. These correspond to
34.5 and 46 kV system voltage, respectively. These were added because AEP often has tertiary windings
with 34.5 kV and 200 kV BIL ratings."
Answer:
Of course, these cases have to be covered and to my opinion implicitly covered in the proposal. We
have computed (by using the formula) some of our actual cases and it appears that the required energy level
to obtain a tail time of 40 µs is generally huge and generally exceed the values proposed. Moreover, in a lot
a cases, no manufacturers have the capability to test them with an appropriate waveshape without the use of
a resistor added on the other terminals. For our large auto-transformers with tertiary winding, they normally
fall into "Category IVb".
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Discussion on lightning impulse test procedure
Inputs received since Oklahoma City meeting (continued):
From Loren Wagenaar (continued):
3- "The standard allows for a minimum time to half value of 40 us. I have therefore added a spreadsheet for
8 % impedance for 40 us and compared it with the MGE values at 50 us. The result is that the MGE's for the
40 us waveshape are about 64 % of those for 50 us. This method of defining the MGE seems more straight
forward to me, rather than to define an arbitrary percentage of cases where the MGE can be met. It is
preferable to have something that already falls within standards, and the 40 us (50 us minus 20 % tolerance)
does."
Answer:
See Art's comment no. 2. Calculation have been revised using a minimum tail time of 40 µs.
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Discussion on lightning impulse test procedure
New calculations:
New sets of calculations have been performed by using the following assumptions:
The common input data used for calculations were:
- single phase, two windings transformers;
- system voltages and impulse levels taken from IEEE C57.12.00;
- generator efficiency: 80% (practical value);
- time to 50%: 40 s (minimum tail to be met);
- minimum impulse generator capacitance calculated by using equation A.8 of IEC 60076-4.
A total of 1044 cases were computed.
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Discussion on lightning impulse test procedure
New calculations (continued):
Studied cases
Transformer
Range of power
category
(minimum nameplate
rating, single phase
ONAN)
I
II
III
IVa
IVb
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(kVA)
5 to 500
(5, 10, 20, 40, 80,
160, 320 and 500)
501 to 1667
(501, 1000 and 1667)
1668 to 10000
(1668, 3200, 6400
and 10000)
10001 to 80000
(10001, 20000, 40000
and 80000)
80001 to 600000
(80001, 160000,
320000 and 600000)
Range of rated system
voltage
Range of leakage
impedance
(kV)
1,2 to 69
(1,2, 2,5, 5,0 8,7,15,0, 25,0,
34,5, 46,0 and 69,0)
1,2 to 69
(1,2, 2,5, 5,0 8,7,15,0, 25,0,
34,5, 46,0 and 69,0)
8,7 to 230
(8,7, 15,0, 25,0, 34,5, 46,0,
69,0, 115, 138, 161 and 230)
69 to 765
(69,0, 115, 138, 161, 230,
345, 500 and 765)
69 to 765
(69,0, 115, 138, 161, 230,
345, 500 and 765)
(%)
1,0 to 4,0
(1,0, 2,0, 3,0 and 4,0)
15
Number of
cases
studied
288
3,0 to 6,0
(3,0, 4,0, 5,0 and 6,0)
108
4,0 to 8,0
(4,0, 5,0, 6,0, 7,0 and 8,0)
200
8,0 to 14,0
(8,0, 9,0, 10,0, 11,0, 12,0,
13,0 and 14,0)
8,0 to 14,0
(8,0, 9,0, 10,0, 11,0, 12,0,
13,0 and 14,0)
224
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
New calculations (continued):
- The proposed energy levels were chosen in order to cover 70%, 80% and 90% of the
cases studied (80% has been agreed upon during the Oklahoma City meeting);
- A maximum of 100 kJ has been used because this is the upper limit that can be
obtained from today's impulse generator;
- Transformer power ratings have been categorized according to the classification used
in C57.12.00 for short-circuit withstand capability (minimum nameplate rating; ONAN; as
agreed during the Oklahoma City meeting);
. Existing category IV has been split in two (IVa and IVb) because this range is too large
for testing circuit requirements.
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Discussion on lightning impulse test procedure
New calculations (continued):
Studied cases
Transformer
category
Range of power Minimum available impulse generator energy during tests to obtain a tail time of
(minimum
40 µs
nameplate rating,
single phase
ONAN)
(kVA)
Covering 90% of
the studied cases
I
II
III
IVa
IVb
5 to 500
501 to 1667
1668 to 10000
10001 to 80000
80001 to 600000
(kJ)
Covering 80% of
Covering 70% of
the studied cases the studied cases
7,0
30,0
21,0
30,5
See note
3,0
15,0
13,5
22,0
See note
1,6
8,0
9,5
16,5
See note
Original proposal
based on a tail
time of 50 µs
12,5
25,0
25,0
37,5
100,0
Note:
For a minimum limit of:
100 kJ, 61% of the studied cases will be covered
75 kJ, 51% of the studied cases will be covered
50 kJ, 35% of the studied cases will be covered
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
New calculations (continued):
Observations from the results obtained:
1- The new values are:
- 24% of the original value for Category I transformers;
- 60% of the original value for Category II transformers;
- 54% of the original value for Category III transformers;
- 59% of the original value for Category IVa transformers.
The reduction for Categories II to IVa is mainly due to the change from a tail time of 50 µs to 40
µs while the reduction for Category I is also due to the fact of covering only 80% of the studied
cases instead of 90%.
2- The fact of using a "t2" time of 40 µs instead of 50 µs reduces the required energy by 32%.
3- The extra impulse generator energy required to cover the last 10% (i.e. from 80% to 90% of
studied cases) is quite significant. Numbers are approximately doubled.
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4- A value of 100kJ for Category IVb seems to be still valid because only 61% of the studied
cases are covered.
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Modified proposal:
- We propose to change the first paragraph of indent b) of clause 10.3.1.1 of C57.12.90 as
follows:
"The impedance of some windings may be so low that the desired time to the 50% voltage point
on the tail of the wave can not be obtained with available test equipment. For such cases,
shorter waves are acceptable provided that:
1- The optimum impulse generator connection is used (use of parallel stages,
largest available capacitance);
2- The available energy from the impulse generator with the actual test
connection and charging voltage is equal to or higher than the values given in
the following table:
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Proposal (continued):
Transformer
category
Single phase
(kVA)
Three phase
(kVA)
Minimum available
impulse generator
energy during
tests1,2,3
(kJ)
I
II
III
IVa
IVb
5 to 500
501 to 1667
1668 to 10000
10001 to 80000
 80000
15 to 1500
1501 to 5000
5001 to 30000
30001 to 240000
 240000
3,0
15,0
15,0
25,0
100,0
1- Applicable only if the impulse tail time parameter ("t2") is outside the prescribed minimum tolerance (40 s).
2- Any generator configurations (available energy) can be used during tests if the impulse waveshape "t 2" parameter is within
the prescribed tolerances.
3- For windings rated below 95 kV BIL, a minimum impulse generator capacitance value of 4,0 F may be used instead of the
minimum energy level given in the table. This minimum capacitance value may be used in conjunction with the alternative
methods described in IEEE C57.98.
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
The minimum impulse generator energy required to meet the minimum tail time (40 µs) during
an impulse test on a particular transformer design and connection can be estimated by using
the following equation:
2    f  t2   U BIL 

 
  VA
2
z U
  
2
Emin .
2
where:
Emin.: minimum energy required from the impulse generator (joules);
f: power frequency, 60Hz;
t2: tail time (second); t2 equal 40 µs;
z: impedance in p.u. seen from the impulsed terminal;
U: winding rated voltage (volts, phase-to-phase);
UBIL: rated BIL of the tested winding (volts);
: impulse generator efficiency in p.u.;  = 0,8;
VA: three-phase power rating in volt-ampere for which the impedance "z" is
defined.
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
200
250
350
450
550
125
650
100
750
1050
75
1550
50
2050
25
200
0
250
8
9
10
11
12
13
14
350
450
Impedance, %
550
Figure YYY: Example showing the variation of minimum generator energy for a 100
MVA as a function of
650
Minimum Generator
kJ
Generator Capacity as a Function of
Impedance & BIL to Obtain 40 us Tail on a
100 MVA 1-Phase Transformer
BIL and impedance value.
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Pierre Riffon P. Eng. 03/03/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
If the calculated impulse generator energy for a particular connection is such that the minimum
time to 50% (e.g.: 40 µs) can not be achieved, the manufacturer shall mention it during the
bidding stage. The manufacturer shall also state the strategy that will be taken to obtain an
acceptable waveshape.
For cases where the minimum tail time (40 µs) can not be achieved, even by using the
minimum energy level given in the previous table, resistors can be added on the non tested
terminals as described in clause 10.3.2.1."
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Pierre Riffon P. Eng. 03/03/06