Introduction to Lightning Impulse Testing

Lightning impulse voltage testing is essential for evaluating the dielectric strength of power system equipment such as transformers, insulators, and switchgear. These tests simulate the effects of transient overvoltages caused by lightning strikes, ensuring equipment can withstand real-world electrical stresses. According to international standards including IEC 60060-1, both full and chopped lightning impulse voltages must be applied to newly produced high-voltage power transformers to validate their insulation systems[citation:1].

Key Components of Impulse Test Systems

The core component of a lightning impulse test system is typically a Marx Impulse Generator (MIG), which generates high-voltage impulses through a series-parallel arrangement of capacitors. When triggered, these capacitors discharge through sophisticated wave-shaping networks. The test circuit includes several critical components:

• Front and tail resistors: These resistors control the rise time (front) and decay time (tail) of the impulse waveform[citation:1]

• Wave-shaping capacitors: The generator capacitance (C1) and load capacitance (C2) collectively determine the energy storage and delivery characteristics[citation:5]

• Circuit inductance: The total inductance of the system (including generator internal inductance and connection leads) significantly impacts waveform quality[citation:9]

• Chopping gap: For chopped wave tests, a chopping circuit abruptly terminates the impulse to simulate insulation breakdown scenarios[citation:1]

• Measurement systems: Precision measurement instruments compliant with standards such as GB/T 16896.1-2024 are essential for accurate waveform recording and analysis[citation:2]

Standard Impulse Waveform Parameters and Tolerances

The standard lightning impulse waveform is defined as a 1.2/50 μs wave, meaning it reaches its peak in 1.2 microseconds and decays to half-value in 50 microseconds. International standards permit limited tolerances on these parameters: ±30% for the front time (T1) and ±20% for the time to half-value (T2)[citation:5]. For equipment rated above 800 kV, the tolerance for front time has been extended to 100% (2.4 μs) in the latest edition of IEC 60060-1[citation:6].

One critical consideration in impulse testing is overshoot management. Overshoot occurs when a damped oscillation at the peak of the impulse causes the voltage to exceed the target value. According to standards, the relative overshoot magnitude (β′) must not exceed 10%[citation:5]. Overshoot typically results from excessive circuit inductance, often caused by long connecting leads between the impulse generator and test object, particularly when testing high-voltage equipment requiring substantial clearances[citation:9].

Test Procedures and Methodologies

Lightning impulse testing follows a structured methodology to ensure reliable and reproducible results. Before applying full-level impulses, transformers are subjected to reduced-level impulses to establish reference waveforms. The test sequence then includes both full wave (FW) and chopped wave (CW) impulses at standard test voltages. Any significant deviation between reduced and full-level impulses, or between FW and CW impulses up to the chopping instant, indicates potential insulation issues[citation:1].

For chopped impulse tests, the chopping time must occur 3-6 microseconds after the voltage peak, and the circuit should introduce minimal impedance to ensure rapid voltage collapse. However, if the subsequent voltage oscillation with opposite polarity exceeds 30% of the peak voltage, controlled impedance must be added to maintain it below this limit[citation:1].

High-frequency performance considerations are crucial during impulse testing. The transformer's behavior in the frequency range from 10 kHz to 10 MHz significantly influences the resulting waveform shape. Various modeling approaches, including Multi-Transmission Line (MTL) models and lumped parameter models, are employed to predict this high-frequency response accurately[citation:1].

Technical Challenges and Solutions

Implementing effective lightning impulse testing presents several technical challenges that require careful engineering solutions:

Overshoot and Oscillation Control

Circuit inductance remains a primary cause of waveform overshoot and oscillations. This inductance comprises both the internal inductance of the impulse generator and the inductance of external connections. As test voltage levels increase to accommodate UHV (Ultra High Voltage) system components, the physical size of test loops necessarily grows, resulting in higher inductance[citation:9]. Modern solutions include optimizing generator internal design to minimize inherent inductance and carefully routing connecting leads to reduce inductive loops.

Parameter Selection and Waveform Shaping

Selecting appropriate values for front and tail resistances represents another practical challenge. Available resistance values are often limited, making it difficult to achieve standardized waveforms without considerable experimentation. Rather than relying on trial-and-error approaches, modern testing facilities employ computerized simulation methods that model the complete test circuit, including the high-frequency characteristics of the test object[citation:1][citation:5].

Standardization and Compliance

With ongoing revisions to international standards, testing laboratories must continuously update their practices and equipment. The recent introduction of a defined front time for switching impulses (170/2500 μs) in IEC 60060-1 exemplifies such changes that affect test system configuration and operation[citation:6]. Compliance with updated standards requires both equipment adjustments and revised calculation methodologies for waveform parameters.

Industry Applications and Importance

Lightning impulse testing serves critical quality assurance and research functions across the high-voltage equipment industry. Power transformer manufacturers rely on these tests to verify insulation integrity, while utilities use them for acceptance testing of newly installed equipment. The growing emphasis on grid reliability and the integration of UHV transmission systems have further increased the importance of precise impulse testing methodologies.

As test requirements evolve toward higher voltage levels and more complex equipment designs, the industry continues to refine testing techniques. Recent research focuses on accurate determination of generator internal inductance, improved modeling of circuit parameters, and development of more sophisticated measurement systems compliant with the latest editions of international standards[citation:9].

Properly implemented lightning impulse test systems provide invaluable data on equipment performance under transient conditions, helping to ensure the long-term reliability and safety of electrical power systems worldwide. Through continued refinement of testing methodologies and adherence to international standards, the high-voltage testing industry supports the development of more robust and efficient power infrastructure.