A 9.8 Corona shielding
10. Lightning impulse and switching impulse test
10.1 References / Standards
• IEC 60060-1(1989), High-voltage test techniques – Part 1: “General definitions and test requirements” [21]
• IEC 60060-2 (1994), High-voltage test techniques – Part 2: “Measuring systems” [22]
• IEC 60060-3 (w.i.p.*), High-voltage test techniques – Part 3: “Definitions and requirements for on-site tests” [23]
• IEC 60076-3 (2000), Power Transformers –
Part 3: “Insulation levels, dielectric tests and external clearances in air” [3]
• IEC 60076-4 (2002), “Guide to lightning impulse and switch-ing impulse testswitch-ing of power transformers and reactors” [4]
• IEEE Std C57.12.90-1999, clause 10: “Dielectric tests” [51]
• IEEE Std C57.98-1993, “Guide to impulse testing techniques, interpretation of oscillograms and failure detection criteria” [58]
* work in progress
Although the two main standards [3], [51] are harmonized today;
there are several differences, especially in the way insulation criteria and requirements are specified. IEC specifies the ratings Lightning Impulse Withstand Level, (abbreviated LI) and Switching Impulse Withstand Level (abbreviated SI) for impulse withstand.
IEEE specifies the rating Basic lightning Impulse insulation Level (abbreviated BIL) and BSL (Basic Switching impulse insulation Level).
IEC allows free selection of standardized values for LI and SI, while a given BIL value specifies the lightning impulse as well as the switching impulse insulation requirements.
Note:
The lightning impulse test is a routine test for transformers where Um > 72,5 kV according to IEC [3] or ≥115 kV accord-ing to IEEE [50]; the switchaccord-ing impulse test is a routine test for transformers where Um > 300 kV according to [3] or 345 kV and above according to [50]. For any other value of Um, these tests are considered to be design tests or special (“other”) tests, see section 2, tables 1 and 2.
10.2 Purpose of the test
The purpose of the test is to verify the insulation integrity for
10. Lightning impulse and switching impulse test 10. Lightning impulse and switching impulse test
10.3 General
When the Standards and/or a customer specification request an impulse test, the following tests may apply:
• the lightning impulse test at one or all terminals
• the switching impulse test at the terminal with the highest rated voltage
If not otherwise specified, the impulse tests precede the voltage insulation test at power frequency (induced voltage test and separate source test). The IEC requires that a switching impulse test precede any of the lightning impulse tests.
A transformer is generally considered to have passed an impulse test when there is a close similarity between the traces for a calibration impulse and those of the impulses at specified voltage levels. Because the impulse tests precede all other dielectric tests at power frequency, it is also possible to later identify non-conformances or hidden damage caused by the impulse tests.
Lightning impulses as well as switching impulses have standardized shapes that are specified in the applicable Standards, see section 10.4. A switching impulse has only one characteristic, identified as a normal full wave. A lightning impulse on the other hand may have three different shapes:
• Full wave
• Chopped wave on the tail
• Front chopped wave (front-of-wave)
In a full wave the applied voltage has a monotonous decay to zero after the voltage crest has been reached. In a chopped wave the applied voltage is short-circuited to zero voltage after a pre-set time.
The polarity of the impulse voltages is generally negative to prevent flashovers on the air-side of the transformer bushings or any other external flashovers.
Test sequence
If not otherwise specified by the customer the test sequence for an impulse test should be:
• Switching impulse test, if applicable
• Lightning impulse test
10.3.1 Lightning impulse test
The test sequence depends on the test code, IEC [3] or IEEE [51], and customer requirements. Generally the test sequence starts with reduced level impulses and ends with full wave test impulses at specified amplitude.
IEC specifies:
• One reduced level full impulse (calibration impulse)
• One full level full impulse (LI)
• One or more reduced level chopped impulse(s) (only if specially requested)
• Two full level chopped impulses (LIC), only if specifically requested
• Two full level full impulses (LI)
If not otherwise requested, IEC [3] test requirements specify that the chopped wave should have an amplitude of 110% of the LI value.
IEEE specifies:
• One reduced full wave impulse (calibration impulse)
• Two front-of-wave impulses at specified amplitude (only if specifically requested)
• Two chopped wave impulses at specified amplitude
• One full wave impulse at rated amplitude (BIL)
The chopped wave should be at 110% of the BIL value and the front-of-wave should have an amplitude according to table 5 of [50] (only for Class I transformers).
The chopped wave calibration impulse for the applicable wave shape may immediately precede the chopped wave test itself, and it need not be carried out as the initial part of the test sequence.
The integrity of the transformer is confirmed when there is a close similarity between the voltage traces for the applied calibration impulse voltage and all of the applied full test voltages.
10.3.2 Switching impulse test
The switching impulse test generally consists of:
• A calibration impulse at about 60% of the specified insulation level
• Two [51] or three [3] impulses at the specified insulation level
10. Lightning impulse and switching impulse test 10. Lightning impulse and switching impulse test
To the transformer, the switching impulse represents a time integral of the applied voltage. It may force the transformer core into saturation and in a saturated condition the impedance will drop to such a low value that the applied voltage can no longer be maintained. It is therefore important that the transformer has a remanence condition opposite to the flux generated by the applied impulse. This is normally achieved by applying a number of switching impulses of opposite polarity (positive) and reduced voltage prior to each test impulse.
10.4 Impulse shape
10.4.1 Lightning impulse
The basic impulse is used to verify the lightning withstand voltage in accordance with IEC 60076-3 [3] and IEEE Std C57.12.90 [51].
The waveform of the impulse is aperiodic as shown in figure 10.1.
It has a specified rise time of 1,2 µs and duration of 50 µs to half value. The specified shape is the same for both IEC and IEEE.
The waveform is characterised by:
front: T1 = 1,2 µs, tolerance ± 30% (0,84 to 1,56 µs) tail: T2 = 50 µs, tolerance ± 20% (40 to 60 µs) polarity: negative (for oil insulated transformers)
Because impulse generating circuits are not perfect, the definitions of rise time as well as crest value need additional clarification. The slope of a straight line passing through the voltage values 0,3 and 0,9 times the crest value gives the rise time. The rise time will then be 1,67 times the time between these two voltage values.
See clause A 10.1 for a definition and assessment of crest value.
10.4.2 Impulse voltage wave shape, chopped on the tail
IEC states:
The chopped wave peak should be 10% higher than the full wave. The time to chopping is Tc= 2 to 6 µs, see figure 10.2.
Triggered chopping equipment is preferred but untriggered spark gaps are permitted, see clause A 10.2. The overshoot Us of the test amplitude after the first zero crossing should not be more than 30 % of the test amplitude.
IEEE states:
The lightning impulse chopped on the tail shall always be carried out together with the lightning impulse full wave test.
The test voltage and the time to chopping are defined in table 5.6 (IEEE C57.12.00) [50] and lie approximately 10 % above the full wave values, see figure 10.1. The overshoot Usmay not exceed 30 % of the test amplitude.
Figure 10.1: Lightning impulse full wave;
IEC and IEEE
Figure 10.2: Wave chopped on the tail;
IEC and IEEE
T1 = 1,67 · T = 1,2 µs ± 30 % – front T2 = 50 µs ± 20 % – tail
TC = 2…6 µs – time to chopping