Coordi Coo rdinac nación ión del del Aisla Aislamie mient nto o en UHV UHV - AC
Una introducción a los conceptos
Juan Baut Bautist ista a – Pro Profes fesor or Princ Principal ipal
Bibliografía
Bibliografía
Stan St anda dard rd ins insul ulat atio ion n le leve vels ls fo forr UH UHV V (IE (IEC C 600 60071 71-1 -1 Ed. Ed.8) 8)
Standard rated switching impulse withstand voltage and minimum phase-to-phase clearance for UHV ( I E C 6 0 0 7 1 - 1 E d. d. 8 . 1 )
Standard voltage for UHV (IEC 60038)
UHV insulation coordination concept
Flow chart for the determination of rated or standard insulation level in IEC60071-1
NOTE In brackets the subclauses reporting the definition of the term or the description of the action
OVERVOLTAGES SPECIFIC TO UHV AC TRANSMISSION SYSTEM Representative maxima of amplitude of overvoltages Um (pu) Table siguiente Classes and shapes of overvoltages, Standard voltage shapes and Standard withstand
Classes and shapes of overvoltages, Standard voltage shapes and Standard withstand voltage tests (IEC 60071)
TEMPORARY OVERVOLTAGES (TOV) TOV includes healthy phase overvoltages due to transmission line ground faults and load rejections. In the case of sudden load rejection on a heavily loaded, long line, such as a UHV system, the overvoltage is about 1.3 - 1.5 p.u. This TOV is required not only to cover the peak voltage in the system, but also to cover the overvoltage generated during their operation. Therefore power frequency withstand test was verified in both long time range and short time range, because the voltage stress is different from both range as described in Chapter 8.4.
SLOW-FRONT OVERVOLTAGES (SWITCHING OVERVOLTAGES)
Relation between air insulation and switching impulse withstand
voltages
Size reduction of 1100kV tower
Size reduction of 1100kV tower
FAST-FRONT OVERVOLTAGES (LIGHTNING OVERVOLTAGES)
Lightning strokes terminating on UHV transmission lines can generate overvoltages of several MV depending on the front-steepness of the overvoltage and the height of the tower. Shielding failures as well as back-flashovers have to be taken into account. Lightning overvoltage is the predominant factor for substation equipment design. Therefore, lightning overvoltages in the UHV substation are highly suppressed for size reduction within a rational level by installing several higher performance surge arresters at adequate locations.
VERY FAST TRANSIENT OVERVOLTAGES (VFTO)
The GIS disconnector, when switching a charging current, repeats restriking and generates VFTO, which can reach up to approximately 3.0 p.u. At a UHV substation, lightning overvoltages are effectively suppressed by higher performance surge arresters. Disconnector switching overvoltages are likely to exceed the lightning overvoltage if no measures are taken to limit them. Therefore, the resistors can be a suppression measure for the VFTO.
SELECTION OF INSULATION LEVEL Insulation coordination of substations and transmission lines can be achieved to set a reasonable insulation level voltage without sacrificing supply reliability by installing higher performance surge arresters on specific locations in substations, adopting resistor-fitted switching schemes of disconnectors and circuit breakers, and comprehensive simulations and analysis of assumed overvoltage phenomenon. To select an appropriate insulation level and insulation requirements for equipment, it is necessary to evaluate technical data of equipment and set reasonable design margins to secure supply reliability while satisfying each project’s design constraints, such as substation type: open-air/Hybrid-IS/GIS type. Figure 2.1.6 shows substation designs and corresponding insulation levels (LIWV and SIWV) of Chinese and Japanese projects.
Substation designs and corresponding insulation levels (LIWV and SIWV) of China and Japanese projects (TR: Transformer, SW: Switching equipment in above captions) LIWV = Lithning Impulse withst and voltage SIWV = Switc hing ov ervoltage withst and voltage
Reduction of insulation levels using overvoltage suppression measures
The higher performance surge arresters, high voltage shunt reactors, resistor-fitted switching schemes of disconnectors and circuit breakers have been utilized to suppress the overvoltages peculiar to UHV systems, and to reduce insulation design level of each project.
O V E R V O LT A G E S U P P R E S S I O N W I T H H I G H E R P E R F O R M A N C E S U R G E A R R E S TE R S
La Tabla adjunta muestra los LIWVs and protective performance of arresters in recent projects. Although each project adopts different insulation levels due to differences in location of arresters and substation types, all projects succeeded in reducing insulation voltage level ranges: 1950 kV-2250 kV for transformers and 2250 kV2400 kV for switchgears.
Protective performance of surge arresters in substation projects
RESISTOR-FITTED CIRCUIT BREAKERS
To suppress the switching overvoltage, pre-insertion resistor is employed for UHV circuit breakers. Chinese and Indian UHV projects introduce resistor-closing technique, while Japanese project introduces resistor-closing / opening technique. Both techniques suppress switching overvoltages of transmission lines to below 1.7 p.u. The resistance of this switching scheme is usually between 500700 Ω depending on the size of UHV system and its characteristics. Table 2.2.2 shows the insulation coordination of several UHV projects: (a) Closing overvoltage in Indian project and, (b) Opening overvoltage in Japanese project Figure 2.2.4 shows the study example of the relation between switching resistance and overvoltage suppression effects in Indian and Japanese project, and Figure 2.2.5 is an example of GCB with pre-insertion resistors.
Example of relation between resistance and overvoltage
Suppression methods and insulation designs of international projects
T H E D A M P I N G E F F E C T O F T H E R E S I S T O R - F I T T E D D I S C ON N E C T O R S E M P LO Y E D I N G I S A G A I N S T TH E S W I TCH I N G OV E R V OLTA G E S
In gas insulated substations, the resistor-fitted disconnectors are commonly utilized to suppress switching overvoltages. Examples of applications of resistor-fitted disconnectors are shown in the Table 2.2.3 below. The GIS system with fast-operating disconnectors can suppress the disconnectors’ overvoltage levels from 2.8 p.u. without the resistors to less than 1.3 p.u. with pre-insertion resistors.
Application of pre-insertion resistor in international projects
MUCHAS GRACIAS
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