10.1 Insulation coordination principles
10.1.1 The insulation level of equipment shall be determined according to the various voltages occurring in the system and the characteristics of protective devices. In other words, the equipment costs, maintenance expenses and failure losses shall be considered for insulation coordination to achieve high economic benefit.
The insulation level may vary for different structures and development stages of system.
10.1.2 Insulation coordination under power frequency voltage and temporary overvoltage:
a) The external insulation creepage distance of electric porcelain for electrical devices under power frequency voltage shall conform to provisions of specific creepage distance under corresponding environmental contamination level.
b) The electrical equipment in substation shall be able to bear a certain amplitude and time of frequency overvoltage and resonance overvoltage.
10.1.3 Insulation coordination under switching overvoltage:
a) When determining the insulation level of overhead line within scope II under switching overvoltage, the statistical method using overvoltage amplitude and insulation strength as random variables may be adopted, meanwhile, only unloaded line closing, single-phase reclosing and successful three-phase reclosing (such as the use in service) overvoltage shall be considered.
b) For the switching impulse insulation level of electrical equipment in substation within scope II and switching impulse insulation strength for insulator string and air gap in substation, the insulation coordination shall be carried out on basis of the corresponding protective level of lightning arresters. During coordination, conventional process is adopted for non-self-recovery insulation; for self-recovery insulation, only insulation strength is regarded as random variable.
c) For the insulation level of overhead line within scope I and that required by switching overvoltage of insulator string and air gap in substation, the insulation coordination is carried out on basis of the highest switching overvoltage for calculation. The insulation strength is regarded as random variable.
10.1.4 Insulation coordination under lightning overvoltage. The coordination shall be carried out on basis of lightning protective level of lightning arrester for the lightning impulse strength of electrical equipment, insulator string and air gap in substation. During coordination, conventional process is adopted for non-self-recovery insulation; for self-recovery insulation, only insulation strength should be regarded as random variable.
10.1.5 Waveforms used for insulation coordination under switching lightning overvoltage:
a) Switching impulse voltage wave. Time to the highest is 250μs and to wave tail is 2500μs.
Notes,
1 Excluding electrical equipment with winding.
2 If other waveforms are adopted, the insulation coordination margin shall meet the requirements of this standard.
b) Lightning impulse voltage wave. The wave head time is 1.2μs and the wave tail time is 50μs.
10.1.6 When carrying out the insulation coordination, the test data of emulational tower (frame) should be adopted for the insulation strength of insulator string and air gap (in transmission lines and substation within scope II) under each voltage.
