Chopped Wave Lightning Impulse Test (LIC) for Transformers

Introduction

Terms and definitions related to lightning impulse (LI) and lightning impulse chopped (LIC) tests and general requirements for test procedures are given in IEC 60060-1. General definitions of terms related to performance tests and routine checks on approved measuring devices are given in IEC 60060-2. The test voltage levels and test sequence are given in IEC 60076-3 and further information is given in IEC 60076-4.

Impulse tests on distribution and power transformers are mandated by major international standards like IEC and IEEE, and are widely adopted by national regulatory bodies and standards organizations. These tests play a crucial role in assessing the impact strength of both the main and longitudinal insulation within transformers. Their primary purpose is to ensure compliance with stringent national and international standards, while also facilitating the investigation and improvement of the overall insulation structure of transformers.

Among these tests, the lightning impulse test is particularly essential. It replicates the intense shock wave experienced during actual lightning strikes on transformer winding terminals. This rigorous assessment simulates lightning shocks to evaluate the transformer's ability to withstand such extreme conditions. It focuses on detecting potential failures, notably in paper insulation and windings, to determine the transformer's capacity to endure high-stress situations.

Chopped wave impulses, characterized by a sudden cessation of the lightning wave due to protection gap or air insulation flashover when lightning enters a substation, represent a rapid drop in voltage to zero. This waveform's truncation can occur either at the wave's front or tail. The cutoff test serves as a diagnostic tool for transformers, ensuring their resilience and reliability when subjected to lightning-induced stresses. The Figure 1 shows positive polarity full wave (FW) and full chopped wave (FCW) lightning impulses.

This paper delves into the relevant IEC standards and emphasizes the significance of the LIC test while scrutinizing its aspects. It covers the standard lightning impulse chopped wave and its variations - front chopping, tail chopping - along with an explanation of associated parameters. Lastly, the paper discusses the methodology of conducting the chopped wave lightning impulse test.

Importance of Impulse Testing

Insulation stands as a critical structural component within a transformer, serving to guide electrical currents along intended paths while preventing their diversion into harmful channels. Any compromise in insulation integrity can lead to transformer failure. In the case of distribution and power transformers, the primary test for evaluating insulation is the Impulse Test. This examination involves subjecting the insulation to a series of impulses, tailored to match the specified limits of the insulation class. The goal is to mimic the impact of a lightning discharge on the transformer's insulation.

These impulses generate capacitive and oscillatory currents that flow through the transformer's insulation. Analyzing the shapes and distortions of these currents can reveal potential failures or weaknesses within the insulation. Additionally, the traveling nature of these impulsive waves creates an irregular distribution of voltage across the windings, placing particular stress on their initial sections.

Examining Full and Chopped Wave Impulses

A full impulse signifies a lightning discharge stress on transformer insulation without effective activation of surge protection. Its short duration induces significant oscillations in the windings, generating higher stresses compared to power frequency stresses. These stresses span across a considerable portion of the windings and between the windings and the ground.

In the context of network insulation coordination, overvoltage levels might surpass the insulation capacity of components like insulators and bushings, even if they are shielded by surge arresters. This scenario occurs mainly during direct lightning strikes or strikes near the transformer. Flashovers on insulators or bushings can cause a tail-chopped impulse to reach the transformer, mimicking this stressful event.

To simulate such stress effects, reduced and full-level chopped impulses are incorporated into the test sequence. The amalgamation of full and chopped impulses intentionally overstresses the transformer's insulation, revealing weaknesses that could compromise its long-term performance. Each type of applied impulse serves a distinct role in evaluating failure within the test sequence.?

Standard Chopped Lightning Impulse Voltage?

This is a standard impulse chopped by an external gap with a time-to-chopping value between 2 μs to 5 μs.?

IEC 60060-1 states that the duration of voltage collapse should be much faster than the front time of the impulse and limits may be set by the relevant Technical Committee. The requirements for measurement and the associated uncertainties are given in IEC 60060-2.?

Chopped Lightning Impulse Voltage?

Lightning-impulse voltage during which a disruptive discharge causes a rapid collapse of the voltage, practically to zero value (see Figure 2 to Figure 4).

Linearly rising Front Chopped Impulse Voltage

Voltage rising with approximately constant steepness, until it is chopped by a disruptive discharge.

Generation of the Test Voltage?

The impulse is usually produced by an impulse generator consisting essentially of a number of capacitors that are charged in parallel from a direct voltage source, then switched into series and discharged into an impulse-forming circuit that includes the test object.?

Explanation of Chopping Related Parameters

Instant of Chopping

This refers to the moment when the extrapolation of the line between points C (70%) and D (10%) on the voltage collapse curve crosses the level just before the collapse (refer to Figure 2 and Figure 3).?

Time to Chopping TC

A virtual parameter defined as the time interval between the virtual origin O1 and the instant of chopping (refer to Figure 2 and Figure 3).

Characteristics Related to Voltage Collapse during Chopping

Points C and D represent 70% and 10% of the voltage immediately before the collapse (refer to Figure 3).

Regarding the duration of the voltage collapse, it's defined as 1/0.6 times the time interval between points C and D. The steepness of the voltage collapse is determined by the ratio of the voltage at the instant of chopping to the duration of the voltage collapse.

The impulse is further defined by:

• Maximum voltage Ue
• Front time T1
• Virtual steepness S: S = Ue * T1

This virtual steepness S is the slope of the straight line drawn through points A and B, expressed in kilovolts per microsecond.

To classify the chopped impulse as approximately linearly rising, the front, from 30% amplitude up to the instant of chopping, should be entirely enclosed between two lines parallel to line AB but displaced in time by ± 0.05 T1 (refer to Figure 4).

Chopped Wave Lightning Impulse Tests

The chopped wave lightning impulse tests are composed of full and chopped impulses. To understand how the test works, Figure 5 depicts the test circuit for a three-phase transformer. In this case, each high voltage winding is tested individually while the others are grounded along with the tank and low voltage windings. The grounding is made in such a way that there is only one path for the current, allowing its measurement to be made by a low impedance shunt or a wide-band current sensor. The test sequence, according to IEC 60076-3 is as follows:

· One reduced level full impulse (RFW), 50% to 75%

· One full level full impulse (FW), 100%

· One or more reduced level chopped impulse(s) (RCW), 50% to 75%

· Two full level chopped impulses (FCW), 110%

· Two full level full impulses (FW), 100%

The specified test sequence provides the generation of a current fingerprint when the reduced impulses are applied to the test sample. For the reduced impulses, the peak value is 50 to 75% of the equipment BIL. The test assumes that these impulse levels are harmless to the transformer. If the transformer is sound, the following full level impulse will generate a current that, unless by a scale factor - linear behavior, is equal to the current generated by the previous reduced level impulse.

Usually, if the transformer is defective, differences on the current fingerprint can be observed. An impulse shape of 1.2/50 μs, representing a lightning surge, is considered as a high frequency wave, as it presents a high dV/dt, mainly during the front time. For this kind of impulse, the windings modeled by inductances, present high impedance. The insulation, on the other hand, modeled by capacitances, presents very low impedance. In this case, there is the predominance of capacitive currents, through the insulating elements of the transformer towards the grounding point. Basically, the measured current consists on this current through the insulation.

The LI and LIC tests conducted on transformers employ negative polarity 100% FW and 110% FCW voltages respectively. The choice of negative polarity is made to prevent unpredictable flashovers in both the external insulation and the test circuit.

An example of a full level full impulse and corresponding measured current is seen on Figure 6. It is interesting to note that the first microseconds of the current record are critical, due to severe oscillations as consequence of the larger increasing rate of rise of the corresponding applied voltage.

After the full impulses, chopped ones are applied. First, a reduced impulse to generate a fingerprint for evaluation, then, two full level chopped impulses. As seen, in Figure 7, the chopped impulse allows the shape to be acquired in a smaller time scale, with a more time detailed sweep, resulting in a better evaluation of insulation behavior during the front of the impulses, without missing the information that would be observed from possible failures on the tail of a full impulse.

Interpretation of Test Results

Assessment of test results is primarily based on the comparison of wave shapes of voltages and currents between reduced and full impulse voltage levels or between successive records at rated test voltage.

REFERENCES

[1] IEC 60060-1:2010, High-voltage test techniques - Part 1: General definitions and test requirements.

[2] IEC 60060-2:2010, High-voltage test techniques - Part 2: Measuring systems.

[3] IEC 60076-3:2013, Power transformers – Part 3: Insulation levels, dielectric tests and external clearances in air.

[4] IEC 60076-4:2002, Power transformers – Part 4, Guide to the lightning impulse and switching impulse testing – Power transformers and reactors.

[5] Estácio Tavares Wanderley Neto et al., High Voltage Laboratory Federal University of Itajubá Itajubá, Brazil, “Chopped Impulses for Distribution Transformers Their Importance for Dimensioning and Performance Evaluation”, 2014 International Conference on Lightning Protection (ICLP), Shanghai, China.

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