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 most of the actual test laboratory
equipment available;
- 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;
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Problematic (continued):
- The tail time is function of the capacitance used in the impulse generator together with the
leakage impedance (for low leakage impedance); t 2  (LTCG). To get the required waveshape,
options are to have the proper impulse generator capacitance and to use alternative
methods as stated in IEEE C57.98.
- 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…)?
- 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.
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Discussion on lightning impulse test procedure
Problematic (continued):
- 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;
- 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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Discussion on lightning impulse test procedure
Raleigh meeting conclusions:
- Category IV (10 MVA to 600 MVA, single phase rating) has to be split in at least three
sub-categories (IVa, IVb and IVc) because this is a too wide power range. This will
give lower energy requirements for mid-band of that power range.
- Transformers classified as Class I (highest system voltage rating  69 kV) has to be
treated separately. Lower requirements have to be specified for that transformer
class.
- Reconsider the minimum energy value for the class covering the highest MVA ratings
(part of the actual class IVb). The proposed value of 100 kJ seems to be too
restrictive.
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Inputs since Raleigh meeting
1- Dirk Russwurm (HV Technologies)
"After observing the discussions and in personal talks, I come to the conclusion that
there is a conflict of interest with the transformer manufacturers that can not be
solved.
In my believe the main problem is the mandatory requirement of the minimum
energy.
I think a solution we all could live with would look like this:
- On your last slide you wrote: "If the calculated impulse generator energy is such
that the minimum time... can not be achieved, the manufacturer shall mention it
during the bidding stage. The manufacturer shall also state the strategy...." I would
make this mandatory.
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
1- Dirk Russwurm (HV Technologies), continued
"- Based on this information, the customer (utility) can make an informed decision. If
they think that a restricted test and increased risk of lower reliability is worth the
money they save with a lower bid, so be it.
On the other hand, a manufacturer with a powerful enough impulse generator has a
competitive advantage. And if manufacturers with weak generators are loosing too
much business, it would give them an incentive and justification for upgrading their
generator.
- The minimum energy tables could be used to explain the limitations in the test to
the customer. It is certainly easier for a manufacturer to argue and explain why he
cannot fulfill the tail time requirements if he has a generator with IEEE recommended
energy level than for a manufacturer without. In the later case the customer would
have a better bargaining position to reduce the price further."
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Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
2- Dr. Gustav Preininger
"Wave-shapes for impulse tests are standardized and deviations of obtainable shapes are
object of discussions. Proposals have been made to introduce correction factors to increase the
test voltage and so to obtain the internal stresses in transformers as they would have been
caused by a standard wave-shape. But whereas deviations of a wave-shape have a different
impact on different harmonics of transformer windings, a simple increase of the test voltage
would overstress certain parts of the insulation system. At least two factors, one for the
front and one for the tail, are necessary. They have to be harmonized in each special case, a
troublesome procedure with a doubtful result.
Therefore a standardized minimum energy shall be defined for impulse generators in
test-fields of transformer manufacturers which allow to obtain at least the minimum
standardized tail-time (time to 50% of the crest, 40 µs). Depending on the circuit, either a
double exponential wave or a damped co-sinus oscillation will be obtained as test voltage. In
the latter case, the first negative peak should be less than a given value, e.g. -0.5."
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
2- Dr. Gustav Preininger (continued)
"From this two limits, a maximum frequency and a necessary damping can be calculated. For
the (simple) theoretical part see file impgen01.doc. Examples are shown in file impgen03.xls.
Dependent on a given inductance L, the necessary Capacitance C of the impulse generator and
the minimum damping resistor Rmin can be calculated. The factor is used to calculate the
voltage drop in the circuit. Data of an usual impulse generator are used to check the
possibilities. Emax is the theoretical available energy, when the generator would be fully loaded,
Eav is the available energy under actual conditions, depending on the required full wave crest.
The right part of the table shows the obtained frequencies and damping constants.
There is no problem to obtain the required wave-shape in the HV-range from 765 kV
down to 161 kV, even for extreme ratings (P = 1500 MVA, z = 10%). Problems occur for
low voltages, especially of course for low voltage windings of GSUs."
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
2- Dr. Gustav Preininger (continued)
"For those windings generally the necessary frequency cannot be obtained and tailtimes down to 20 µs can be expected. Generally an additional grounding resistor RG is used.
It is then no problem to obtain a sufficient wave-length, but the voltage across the inductance
is very short. This is only a good test of the insulation strength to ground. On the other
hand it must be considered that already low order harmonics of low impedance windings
show a high frequency and are not essentially influenced by the tail (see example file
impgen04.doc). No influence can also be observed on gradients, they are mainly influenced
only by the front (see the same file). Therefore a test with the winding grounded at one
terminal is in the most cases meaningful, even when the tail-time is short."
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
2- Gustav Preininger (continued)
Conclusion:
"General guide-lines for the necessary minimum energy of impulse generators are
certainly a valuable tool, although they cannot cover every case. Tests should be
performed in a way that they simulate stresses in service as close as possible. How meaningful
a test is depends on the type of winding and/or transformer and has to be found out in the
course of design reviews. Low impedance windings may require a combination of tests. I
believe that a guide should point out these facts and encourage user and manufacturer
to find the optimum by close cooperation."
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Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
2- Gustav Preininger (continued)
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Low Impedance Winding
1.2
Influence of Tail on Voltage to Ground
Wave Shape and Response of Winding Center
1.0
00
0.8
40
0.6
0.4
20
0.2
0
0
10
20
µs
30
40
50
kette01
Discussion on lightning impulse test procedure
Inputs since Raleigh meeting (continued)
2- Gustav Preininger (continued)
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Pierre Riffon P. Eng. 03/09/06
Low Impedance Winding
1.2
Influence of Front-Time on Gradient
Wave Shape and Response at HV-Terminal
0
1.0
1.2
2.0
0.8
0.6
0.4
0.2
0
0
0
2
4
6
8
10
µs
kette02
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Discussion on lightning impulse test procedure
Changes made to the former proposal (Raleigh proposal):
1- Category IV has been spilt into 3 categories
Transformer
category
IVa
IVb
IVc
Single phase
(kVA)
10001 to 40000
40001 to 160000
 160000
Three phase
(kVA)
30001 to 120000
120001 to 480000
 480000
2- Transformers classified as Class I (highest system voltage rating  69 kV) and having
small power ratings (categories I and II) are now treated separately since a impulse fault in
service of such transformers have generally minor impacts compared to power
transformers having higher rated power (transmission substations). The requirement
regarding a minimum impulse generator energy is not any more mandatory. For these
transformers ratings, a minimum impulse generator capacitance is now given.
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Discussion on lightning impulse test procedure
Changes made to the former proposal (Raleigh proposal):
3- The minimum impulse generator energy for category IVc (highest power range) has been
reduced to 62,5 kJ since several transformer manufacturers have no impulse generators
rated more than 250 kJ. This will reduce the number of cases covered but could be a
valuable compromised (to be discussed)
4- The minimum impulse generator energy suggested for transformers belonging to Class II
et to medium and large power ratings (categories III and IV) of transformers designated
Class I is not anymore mandatory but only recommended.
5- The use of resistors on the non impulsed terminals is now considered fully acceptable as
the mitigation method to achieve the tail time.
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Pierre Riffon P. Eng. 03/09/06
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 1100 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
IVc
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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 40000
(10001, 20000 and
40000)
40001 to 160000
(40001, 80000 and
160000)
160001 to 600000
(160001 320000 and
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)
69 to 765
(69,0, 115, 138, 161, 230,
(%)
1,0 to 4,0
(1,0, 2,0, 3,0 and 4,0)
17
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)
8,0 to 14,0
(8,0, 9,0, 10,0, 11,0, 12,0,
168
168
168
Pierre Riffon P. Eng. 03/09/06
600000)
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345, 500 and 765)
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13,0 and 14,0)
Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
New calculations (continued):
- The proposed energy levels were chosen in order to cover 80% of the cases studied
(80% has been agreed upon during the Oklahoma City meeting);
- A maximum of 62,5 kJ has been used because this is the practical and available upper
limit that can be obtained from most impulse generators;
- 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);
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Discussion on lightning impulse test procedure
New calculations (continued):
Studied cases
Transformer category
I
II
III
IVa
IVb
IVc
Range of power
(minimum nameplate
rating, single phase
ONAN)
Minimum available impulse generator energy
during tests to obtain a tail time of 40 µs
(kVA)
(kJ)
Covering 80% of the
Raleigh proposal
studied cases
based on a tail time of
40 µs
3,0
3,0
15,0
15,0
13,5
15,0
12,5
25,0
50,0
25,0 and 100,0
See note
100,0
5 to 500
501 to 1667
1668 to 10000
10001 to 40000
40001 to 160000
160001 to 600000
Note:
For a minimum limit of 62,5 kJ, 26% of the studied cases will be covered
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
New calculations (continued):
Observations from the new results obtained:
- In order to optimize and to reduce the size of impulse generators, the limits applicable to each
sub-categories of Category IV, have been changed.
- For transformers belonging to Category IVc, the number of cases that will be covered by the
new impulse generator limit (62,5 kJ instead of 100 kJ) has decreased from 47% (Raleigh's
proposal) to 26% (new proposal with reduce energy).
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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 considered acceptable provided that:
1- The optimum impulse generator connection is used (use of parallel stages,
largest available capacitance);
2- A minimum recommended impulse generator capacitance of 1,0 F is used
for tests on transformers classified "Class I", as defined in clause 5.10 of IEEE
C57,12,00, and belonging to categories I and II.
Moreover, if by using this minimum recommended capacitance value, the
minimum tail time specified (40 s) can not still be achieved, the following
alternative methods have to be used in the following preferred order:
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
a) use of resistor(s) on the non-impulsed terminals of the non-impulsed
winding(s). The resistor value shall be minimal for achieving the minimum
required tail time of 40 s and shall not exceed the values given in Table 3
of clause 10.3.2.1. If the transformer is foreseen to be connected to a
cable or to a gas-insulated switchgear or line (GIS or GIL), the resistor
value shall not exceed 75 .
b) use of resistor on the grounded terminal of the impulsed winding. Here
also, the resistor value shall be minimal for achieving the minimum required
tail time of 40 s and shall not exceed the values given in Table 3 of clause
10.3.2.1. If the transformer is foreseen to be connected to a cable or to a
gas-insulated switchgear or line (GIS or GIL), the resistor value shall not
exceed 75 .
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
3- For tests on transformers classified "Class I" and "Class II", as defined in
clause 5.10 of IEEE C57,12,00, and belonging to categories III and IVa, IVb
and IVc the available energy from the impulse generator with the actual test
connection and charging voltage should be equal to or higher than the
minimum recommended energy values given in the following table:
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
Minimum recommended impulse generator energy
Transformer
category
Single phase
(kVA)
I
II
III
IVa
IVb
IVc
5 to 500
501 to 1667
1668 to 10000
10001 to 40000
40001 to 160000
 160000
Three phase
(kVA)
Minimum available
impulse generator
energy during tests1,2
(kJ)
15 to 1500
Not applicable3
1501 to 5000
Not applicable3
5001 to 30000
15,0
30001 to 120000
15,0
120001 to 320000
50,0
62,5
 320000
1- Applicable only if the impulse tail time parameter ("t2") is lower than the prescribed minimum tail time (40 s).
2- Any generator configurations (available energy) can be used during tests if the impulse waveshape "t 2" parameter is within
the prescribed tolerance (40 s to 60 s).
3- These transformer categories generally belong to transformers classified "Class I" in IEEE C57.12.00. For such
transformers, a minimum impulse generator capacitance of 1,0 F should be used in combination with alternative methods.
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
Moreover, if by using this minimum impulse generator energy value, the minimum tail time
specified (40 s) can not still be achieved, the following alternative methods have to be used in
the following preferred order:
a) use of resistor(s) on the non-impulsed terminals of the non-impulsed
winding(s). The resistor value shall be minimal for achieving the minimum
required tail time of 40 s and shall not exceed the values given in Table 3
of clause 10.3.2.1. If the transformer is foreseen to be connected to a
cable or to a gas-insulated switchgear or line (GIS or GIL), the resistor
value shall not exceed 75 .
b) use of resistor on the grounded terminal of the impulsed winding. Here
also, the resistor value shall be minimal for achieving the minimum required
tail time of 40 s and shall not exceed the values given in Table 3 of clause
10.3.2.1. If the transformer is foreseen to be connected to a cable or to a
gas-insulated switchgear or line (GIS or GIL), the resistor value shall not
exceed 75 .
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Pierre Riffon P. Eng. 03/09/06
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. 03/09/06
Discussion on lightning impulse test procedure
Modified proposal (continued):
If the calculated impulse generator energy for a particular connection and transformer design is
such that the minimum time to 50% (e.g.: 40 µs) can not be achieved, the manufacturer shall
notify it during the bidding stage. The manufacturer shall also state the strategy that will be
taken to obtain an acceptable waveshape. In case that the minimum recommended impulse
generator energy or capacitance value can not be achieved for a particular transformer
design and test laboratory limitations, shorter waveshapes may be agreed upon the
manufacturer and the user at the time of an order."
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Pierre Riffon P. Eng. 03/09/06
Discussion on lightning impulse test procedure
Next step(s):
- Is the latest proposal mature enough to be surveyed within the Dielectric
Tests Subcommittee?
- How to obtain more comments on it? A survey or a ballot seems to be the
only effective way.
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