When conducting lightning impulse tests, each test report will state "1.2/50µs lightning impulse full wave." What do these two numbers signify, what is the allowable deviation, and how are the waveform parameters adjusted—only by understanding these can one truly interpret the test results.

Waveform Parameter Definition

The standard lightning impulse full-wave waveform is represented by "T₁/T₂"

T₁ (wavefront time, 1.2µs): The time from the waveform's starting point (actually the intersection of the line extending from 10% to 90% of the peak with the time axis) to the peak, representing the rate of voltage rise. The actual wavefront of a lightning strike typically lasts only a few microseconds, while 1.2µs is a standardized parameter for simulating natural lightning strikes.

T₂ (wave tail time, 50µs): The time from the waveform starting point (as defined above) to when the voltage drops to 50% of its peak, representing the duration of energy persistence.

The permissible deviations shall comply with the provisions of GB/T 16927.1: the wavefront time of 1.2µs allows a deviation of ±30% (i.e., 0.84 to 1.56µs), and the wavetail time of 50µs allows a deviation of ±20% (i.e., 40 to 60µs). The waveform shall be verified against this standard during the acceptance test of HZCJ-400kV.

How to adjust waveform parameters

The output waveform of the impulse generator is determined by the wavefront resistor (R₁), the wavetail resistor (R₂), and the load capacitor

The wavefront time is primarily controlled by the wavefront resistance R₁: a larger R₁ results in a slower wavefront (increased T₁), while a smaller R₁ produces a steeper wavefront

The tail time is primarily controlled by the tail resistor R₂: the larger R₂ is, the slower the energy decay (T₂ increases); the smaller R₂ is, the faster the decay

The load capacitance also affects the waveform: the larger the capacitance of the test specimen, the greater the equivalent output capacitance, resulting in a slower rise time

The HZCJ-400kV is equipped with three sets of lightning wavefront resistors and two sets of wavetail resistors. Each set can be connected in parallel with four units, allowing waveform adjustment under various load conditions through combinations of different resistance values. For extremely large-capacity loads (>5000pF), an appropriate combination of adjustable wave-shaping capacitors and resistors can be added to achieve waveform adjustment.

Voltage utilization factor

The theoretical peak voltage when the nominal energy is fully discharged is 400 kV. However, due to the voltage drop across the wavefront, wavetail resistance, and the spark gap itself, the actual output peak is slightly lower. The voltage utilization coefficient of the HZCJ-400kV system is >85% (over 90% at no load with 300 pF), indicating that the actual maximum peak voltage ranges above 340–360 kV, meeting the requirements for lightning impulse tests of 35 kV-class equipment.