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WG C4.48 Overvoltage Withstand Characteristics of Power System Equipment 35-1200 kV

Background:

Currently there are no overvoltage withstand characteristics (OVWC) of power system equipment available that cover all the spectrum of the phenomena. Instead, we have several stand-alone standards and recommendations that cover only parts of the spectrum, e.g: 

  • IEC Standard 60071 defines insulation level for HV equipment, as a set of standard values that include: lightning impulse withstand voltage (LIWV) and short duration power-frequency withstand voltage (SDWV) for range I; and LIWV and switching impulse withstand voltage (SIWV) for range II.
  • CIGRE WG 33.10 defines characteristics of temporary overvoltages (TOVs) versus time for EHV equipment from 345 kV and higher, and
  • CIGRE TB 542 “Insulation Coordination for UHV AC systems” of WG C4.306 has a collection of data on overvoltage limits for UHV systems, but not as continuous time characteristics.    

Power system equipment insulation design has been traditionally carried out based on the comparison of estimated lightning overvoltages (LOVs) and switching overvoltages (SOVs) with LIWV and SIWV accordingly. TOV characteristics, on the other hand, have been used in power system planning and operation for estimation of overvoltages that can arise on the system due to such phenomena as ferroresonance, and/or when a large amount of harmonics is present in the system voltage.

As new types of equipment have been continuously introduced in power systems, such as gas-insulated circuit breakers with very fast switching transients, or modern zinc-oxide arresters designed not only to reduce LOVs and SOVs, but also to withstand TOVs, it has become clear that assessment of overvoltages needs to be carried out in the entire time frame from microseconds to hundreds of seconds. Furthermore, advancements in high-speed measurements (e.g. phasor-measurement units – PMUs), and in electromagnetic transients (EMT) modelling of the complex transmission and distribution systems, indicate that higher overvoltages can exist in those time periods that are not covered by the traditional classification. With existing approach, overvoltages are partially defined in the following time frames:

  • LOVs –microseconds up to tens of microseconds;
  • SOVs –milliseconds up to half a cycle of power frequency;
  • TOVs –hundreds of milliseconds to hundreds of seconds;

Time gaps clearly exist between each of these time frames and they need to be addressed.

Scope:

It is proposed to define the “Overvoltage withstand characteristics” of power system equipment that operates in the nominal voltage range 35 kV – 1200 kV versus time throughout the range  microseconds to hundreds of seconds. The following stages of work are proposed:

  • Collect real-world requirements for the overvoltage withstand characteristics for power system equipment rated from 35 to 1200 kV in transmission and distribution systems from around the world.
  • Collect information about best practices on measurements, assessments, and EMT analyses of the overvoltage withstand characteristics of such equipment.
  • Summarise the expert information from the above and propose a universal overvoltage withstand characteristics for power system equipment rated 35 to 1200 kV in a timeframe from microseconds to hundreds of seconds.

Deliverables:

 Technical Brochure and Executive summary in Electra

 Electra report

 Tutorial

Time Schedule: start: October 2017                                  

Final Report: October 2020

Convener: Ivan Dudurych (Ireland)