With increased environmental considerations, power line designers turn to concepts such as Externally Gapped Line Arresters (EGLAs) that reduce the footprint of structures while fortifying networks to perform at the highest level of reliability.
The article appeared in ESI Africa Issue 1-2021.
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The use of metal-oxide surge arresters with external series gaps improves the reliability of transmission systems. These smart solutions help eliminate lightning damage of transmission line equipment and slight power interruptions after trip-outs caused by lightning. This solution is ideal to meet the demands of our growing societies’ needs for stable power supply.
The integrated external series gap eliminates the need for countermeasures against switching overvoltage or the ZnO disks’ deterioration. This means transmission line arresters can be made compact and lightweight and significantly ease installation workload and maintenance of the arresters.
Transmission line arrester with external series gap
Lightning striking power transmission lines cause very slight power interruptions, despite transmission lines’ ability to successfully recharge. These interruptions may seriously damage IT equipment, which is unacceptable in today’s online society. Incorporating a design with lightning protection is crucial for ensuring a stable power supply.
The transmission line arrester is the optimal method of lightning protection. Toshiba can offer a much smarter lightning protection solution; namely a transmission line arrester with an external series gap, otherwise known as an Externally Gapped Line Arrester (EGLA).
The EGLA solution prevents trip-out from lightning and contributes to ensuring quality and stability on the transmission of electricity. EGLAs also help eliminate lightning damage of transmission line equipment and slight power interruptions after trip-outs.
Furthermore, EGLAs can protect insulator assemblies from flashovers caused by lightning. Overall, the application of EGLA will improve transmission system reliability.
How will the EGLA prevent power service interruptions?
To answer this question, before fitting The Toshiba EGLA features an EGLA, it is worth noting that when
lightning strikes, a flashover occurs at the insulator strings, resulting in a slight interruption of the power service. Whereas, after fitting EGLAs, the following is noted:
- Sparkover occurs across the series gap of the arrester.
- The lightning-induced current flows through the ZnO disks (the non-linear resistors in the arrester unit).
- The voltage across the insulators is suppressed sufficiently by the arrester unit and it will prevent any flashover at the arc horn.
- The subsequent current will be reduced and immediately removed.
The circuit breaker need not be tripped, and the power supply remains reliable and uninterrupted. Refer to Graph 1 for a visual on the comparison.
Do you install EGLA or Gapless Line Arresters?
One of the countermeasures would appear to be installing full-size arresters specified with IEC or ANSI classification. However, the application of EGLA is a much smarter solution to protect the transmission line from lightning.
Usually transmission systems are designed to withstand internal surges such as switching surges and power frequency overvoltage. Therefore, it may be practical to design the arrester unit to handle lightning but not switching surge and power frequency overvoltage by ensuring it is equipped with an external series gap and having insulation coordination between the arrester unit and insulator strings.
Consequently, EGLA has advantages compared with Gapless type transmission line arresters. EGLA can be downsized compared to station type surge arresters and hence handled very easily. Moreover, ZnO disks are not normally energised and will not deteriorate when used under normal circumstances, eliminating the need to maintain and monitor current leakage.
How can the compact type be applicable?
In the case of a shielded system, when lightning strikes the grounding wire or tower structure, the lightning current will decline. It traverses the grounding wire and tower structure, thus significantly reducing the load to which the EGLA body is exposed. EMTP analysis has shown that the subsequent current through the arrester unit is decreased to approximately 10% or less of the lightning current (refer to Diagram 1).
The shielding distance from the grounding wire depends on the lightning current. Smaller lightning current may pass close to the grounding wire and strike a phase conductor. When such shielding failures occur, the lightning current is not significantly high, and the load generated by such lightning is not considered so severe.
Consequently, to sum up, while larger lightning events may strike the shielding wire, only smaller lightning can impact the phase conductor, meaning the current following through the arrester unit is expected to be small.
With an analysis of the lightning phenomena and long-term experience of EGLA applications in Japan, the compact type EGLA (the same as IEC Class Y3) became a reality and achieved a satisfactory experience.
The lightning current and subsequent current following through the arrester unit may be significantly larger under certain transmission system conditions, such as shielding conditions and footing resistance. In these cases, arresters must provide much larger lightning withstandcapability to protect such systems. Toshiba can also supply heavy-duty type EGLA with sufficient lightning withstand-capability.
What are the ratings?
The compact type TMLRG series units are suitable for shielded systems of up to 245kV. For unshielded systems or those up to 550kV, the heavy-duty type RVLRC series are applicable.
The wave shape of 2/20 μs for the discharge current is based on practical applications in Japan. The high current impulse with a wave shape of 2/20 μs has much larger energy.
The impulse current of 25kA having the wave shape of 2/20 μs is equivalent to the energy as high as 65kA if the wave shape is 4/10 μs. Concerning the EGLA type selection, please contact Toshiba for information on EGLA applications.
What are the structure dynamics?
The Toshiba EGLA arrester unit consists of the ZnO column, which is directly moulded with silicone rubber, reducing the arrester unit’s size and weight.
The silicone material has excellent weatherproof and hydrophobic characteristics, meaning the Toshiba EGLA boasts an extended service life.
Toshiba EGLA is mounted to the hardware of insulator strings, rather than directly to the power conductors.
Direct installation to the power lines could damage the conductor, due to the load, vibration and torsion caused by the wind to which the line arrester is exposed, or arcing movements along the conductor.
Why is setting the series gap during installation important?
The Toshiba EGLA can easily be installed to the hardware of the insulator strings. Typical installations are shown in the figures. Mounting the arrester unit to the horn holder is typical and the easiest installation method. When mounting the arrester unit to other hardware, the mounting parts can be modified to fit the latter.
Setting the external series gap distance is vital for EGLA because of the need to consider the following items:
- The series gap must withstand switching overvoltage and power frequency.
- The series gap must be designed to coordinate the arcing horn against any lightning impulse.
- The gap horn is designed to be adjustable to fix the series gap distance correctly.
How are verification tests undertaken?
Toshiba has excellent testing facilities and tested the EGLA based on newly revised testing methods. Its excellent performance has been confirmed in all test areas.
The insulation coordination between the complete arrester and the insulator assembly was tested repeatedly under dry and wet conditions and it proved the high reliability of Toshiba EGLA.
A follow current Interruption test was conducted under wet and polluted conditions. The hydrophobicity of the tested arrester unit was eliminated via a method based on long experience. Such test conditions were far more severe than the other test methods and effectively simulated practical conditions.
A short circuit test was conducted, and the superior safety of the Toshiba EGLA was confirmed. The test proved that no violent scattering of fragments harmful to the human body occurred. The tests were conducted under circumstances where a fuse wire shorted the arrester unit, and the arrester unit was pre-failed due to power frequency overvoltage. The test also confirmed the self-extinguishing capability of open frames.
The vibration test is crucial because when a transmission line arrester is placed on top of the power towers the effects of vibration caused by wind must be verified. The Toshiba EGLA was vibration tested with more than one million oscillations, confirming its ability to withstand the vibration stress. ESI
The Toshiba EGLA features
- Easy installation: Compact and lightweight design thanks to the superior performance of ZnO disks
- Maintenance-free: No deterioration of ZnO disks and no need to disconnect devices because of series gaps. The gap arrester units are not energised thus less electrical stress occurs and less maintenance is needed
- Public safety: No violent scattering or burning ensuring public safety
- Improving transmission system reliability: Successful re-closing even in the arrester failure
- Long term performance: Based on advanced silicone technologies
About the company
Toshiba Group is a world-leading, electronic manufacturing and energy solutions provider focussed on creating advanced electronics, electrical products and solutions. Toshiba Africa is a subsidiary of Toshiba Corporation Tokyo responsible for energy, infrastructure and electronic products and services within Africa. www.toshiba.co.za