Hong Kong Utility Expands Program to Install Line Arresters at 400 kV

Hong Kong Utility Expands Program to Install Line Arresters at 400 kV

UTILIT Y PRACTICE & EXPERIENCE

Hong Kong Utility Expands Program to Install Line Arresters at 400 kV CLP Power is among the largest utilities in SouthEast Asia, supplying some three-fourths of the electricity needs of the Hong Kong Special Administrative Region. The particular area served covers Kowloon, the New Territories as well as Lantau Island, site of the airport as well as a large amusement park.

Hong Kong, with its population density, depends heavily on high-rise residential and commercial buildings and therefore has especially stringent standards when it comes to reliability.

This, in turn, has influenced the policies and investment programs at CLP Power when it comes to finding additional ways to reduce unplanned outages.

INMR meets with engineers in the Asset Management Department to update a recent project aimed at improving power quality and reliability through selective installation of more line arresters along the 400 kV overhead network.

Bustling, non-stop, Hong Kong is one place where uninterrupted electricity supply is more-or-less taken for granted. With a virtual forest of high rises and a continuous reliance on elevators and escalators, maintaining a very reliable power supply is seen as critical. As such, reliability is viewed by the local government as well as by the utility itself as one of the key measures of its overall performance.

exposed to a mostly difficult operating environment with high ambient temperature and humidity. In addition, the system is subject to frequent pollution drifting in from the neighbouring areas as well as occasional heavy wind loads from seasonal typhoons.

Lightning is yet another important factor affecting line performance.

Hong Kong typically experiences over 40 lightning days per year.

According to engineers at CLP Power, maintaining such a high reliability has always been a challenge because of the great dependence on its overhead network. While distribution within the urban service area is mainly underground, the backbone of transmission still involves double circuit 400 kV lines that move power between the utility’s various EHV substations. In addition, a smaller system of double circuit 132 kV lines is used to supply other important substations.

The more than 900 towers that comprise this overhead network are

Hong Kong lies in a sub-tropical location and experiences an average of more than 40 lightning days per year, most often from April to September, typically the period of peak demand in summer season.

Moreover, due to the hilly terrain in the New Territories and much of Kowloon, many transmission towers are exposed to strokes in spite of protective groundwire systems. Yet another factor in the operation of the overhead transmission system is the relatively high footing resistance of towers running across the mountainous landscape. This means that back flashovers can also occur

HONG KONG UTILITY EXPANDS PROGRAM TO INSTALL LINE ARRESTERS AT 400 KV

Q3 2009

INMRÂŽ 5 3

UTILIT Y PRACTICE & EXPERIENCE

Hong Kong Utility Expands Program to Install Line Arresters at 400 kV CLP Power is among the largest utilities in SouthEast Asia, supplying some three-fourths of the electricity needs of the Hong Kong Special Administrative Region. The particular area served covers Kowloon, the New Territories as well as Lantau Island, site of the airport as well as a large amusement park.

Hong Kong, with its population density, depends heavily on high-rise residential and commercial buildings and therefore has especially stringent standards when it comes to reliability.

This, in turn, has influenced the policies and investment programs at CLP Power when it comes to finding additional ways to reduce unplanned outages.

INMR meets with engineers in the Asset Management Department to update a recent project aimed at improving power quality and reliability through selective installation of more line arresters along the 400 kV overhead network.

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Bustling, non-stop, Hong Kong is one place where uninterrupted electricity supply is more-or-less taken for granted. With a virtual forest of high rises and a continuous reliance on elevators and escalators, maintaining a very reliable power supply is seen as critical. As such, reliability is viewed by the local government as well as by the utility itself as one of the key measures of its overall performance.

exposed to a mostly difficult operating environment with high ambient temperature and humidity. In addition, the system is subject to frequent pollution drifting in from the neighbouring areas as well as occasional heavy wind loads from seasonal typhoons.

Lightning is yet another important factor affecting line performance.

Hong Kong typically experiences over 40 lightning days per year.

According to engineers at CLP Power, maintaining such a high reliability has always been a challenge because of the great dependence on its overhead network. While distribution within the urban service area is mainly underground, the backbone of transmission still involves double circuit 400 kV lines that move power between the utilityâ&#x20AC;&#x2122;s various EHV substations. In addition, a smaller system of double circuit 132 kV lines is used to supply other important substations.

The more than 900 towers that comprise this overhead network are

Hong Kong lies in a sub-tropical location and experiences an average of more than 40 lightning days per year, most often from April to September, typically the period of peak demand in summer season.

Moreover, due to the hilly terrain in the New Territories and much of Kowloon, many transmission towers are exposed to strokes in spite of protective groundwire systems. Yet another factor in the operation of the overhead transmission system is the relatively high footing resistance of towers running across the mountainous landscape. This means that back flashovers can also occur

HONG KONG UTILITY EXPANDS PROGRAM TO INSTALL LINE ARRESTERS AT 400 KV

Q3 2009

INMR ÂŽ 5 3

All circuits on 132 kV towers in the CLP Power network are now fully equipped with gap type line arresters.

outages from lightning as well as voltage dips lasting up to 0.1 seconds. In order to best deal with this problem, CLP Power relied on both its lightning mapping as well as fault location systems to determine where the lightning protection capabilities of towers most needed improvement. “We do not employ surge counters,” says Chan, “but rather depend on a lightning location system and fault location system.”

According to Chan, the lightning location system, which was first put into place during the mid 1980s and then upgraded, can detect cloud-toground lightning strokes with an accuracy of ± 300 meters.

during lightning events and result in unwanted voltage dips.

With the specific goal of reducing the number of trip outs due to lightning and also improving power quality, CLP Power’s Asset Management Department initiated a program in 2001/2002 to install line arresters at selected high-risk locations within its overhead network. The project began by identifying those towers that, due to their location and exposure, experienced the highest number of lightning strokes. This helped prioritize where line arresters should be installed first so as to have the greatest immediate impact.

Asset Utilization Engineer, Steven Chan, explains that prior to 2001, the utility periodically experienced both double circuit and single circuit

54

INMR® Q3 2009

CLP Power relied on both its lightning mapping as well as fault location systems to assist the analysis of where the lightning protection capabilities of towers most needed improvement.

The very first installation of line arresters began in 2001 and involved three double circuit 132 kV lines where historic trip out rates due to lightning were especially high.

Although arresters were placed on all phases of every tower, this initial work was still considered only a trial evaluation. Explains Chan, “we first wanted to make sure that the arrester design and other factors fully met our expectations. A 132 kV line was selected since the impact of any unsatisfactory experience would be less than at 400 kV.” Basically, Chan indicates that a 100 per cent success rate was achieved in eliminating lightning induced faults on the fully protected 132 kV lines over the years following installation of the arresters. Based on this, the installation program was expanded to other lines and, today, all towers in the 132 kV network are equipped with line arresters on every phase of both circuits.

Given the success at 132 kV, another installation program was soon begun, this time at 400 kV. By 2004, of the circa 740 towers in the 400 kV network, around 7 per cent had

(left) Typical configuration of early arresters at 400 kV. New design employing silicone V-string insulators (right)

already been equipped with line arresters. Since then, the program has continued to the extent that just over half of all CLP Power’s 400 kV towers are now also protected by arresters.

Again, as at 132 kV, the initial goal was to identify which specific lines and towers should receive priority based on past lightning trip out statistics.

The particular design of line arrester selected from the start by CLP Power engineers was the gap type whereby an air gap is placed in series with the metal oxide arrester. This is the design that has been widely used in Japan for more than 20 years and which has recently been receiving growing attention from arrester standards setting committees at IEC and elsewhere.

The length of the air gap, which in the case of 400 kV is 1.620 meters, was established so as to prevent flashover when re-closing the circuit following a switching surge. Moreover, the air gap in series allows the line to be re-energized quickly should any arrester be overwhelmed by excessive lightning energy.

Chan states that the selection of this gap type design over gapless units was made based on the expectation of longer service life. “We demand

“We demand extremely high reliability for arresters and we expect them to perform for around 50 years without need for maintenance.” extremely high reliability for arresters,” he notes, “and we expect them to perform for around 50 years without any need for maintenance. Since these units are not under continuous voltage, they should last longer. Even in the event the arrester stops functioning, the air gap is still there to maintain the operation of the line at its service voltage.”

The gap type arresters selected by CLP Power all have silicone housings for superior pollution performance. Hong Kong is regarded as a Class IV area,

even though pollution here is still primarily coastal and not as severe as in some industrial parts of China.

The most recent additions of line arresters at 400 kV have seen some important changes over the first installations back in 2002/2003. For one, the V-string insulators that used to be porcelain longrods are now silicone. Indeed, Chan points out that these silicone insulators are the first ever used on the utility’s 400 kV system and therefore will offer engineers an opportunity to also assess their service performance. In the past, CLP Power has been conservative toward composite insulator technology and preferred to rely on porcelain and glass.

A second important change over early installations is that new suppliers have been qualified and approved to ensure more competitive bidding. The arresters now being used, for example, differ somewhat from the first models in that they are from another manufacturer and equipped with larger corona rings.

Yet another change over the very first installations is the typical mounting configuration for each 400 kV tower.

While in the early installation program every phase on both circuits was protected by an arrester, the new

HONG KONG UTILITY EXPANDS PROGRAM TO INSTALL LINE ARRESTERS AT 400 KV

Q3 2009

INMR® 5 5

All circuits on 132 kV towers in the CLP Power network are now fully equipped with gap type line arresters.

outages from lightning as well as voltage dips lasting up to 0.1 seconds. In order to best deal with this problem, CLP Power relied on both its lightning mapping as well as fault location systems to determine where the lightning protection capabilities of towers most needed improvement. “We do not employ surge counters,” says Chan, “but rather depend on a lightning location system and fault location system.”

According to Chan, the lightning location system, which was first put into place during the mid 1980s and then upgraded, can detect cloud-toground lightning strokes with an accuracy of ± 300 meters.

during lightning events and result in unwanted voltage dips.

With the specific goal of reducing the number of trip outs due to lightning and also improving power quality, CLP Power’s Asset Management Department initiated a program in 2001/2002 to install line arresters at selected high-risk locations within its overhead network. The project began by identifying those towers that, due to their location and exposure, experienced the highest number of lightning strokes. This helped prioritize where line arresters should be installed first so as to have the greatest immediate impact.

Asset Utilization Engineer, Steven Chan, explains that prior to 2001, the utility periodically experienced both double circuit and single circuit

54

INMR ® Q3 2009

CLP Power relied on both its lightning mapping as well as fault location systems to assist the analysis of where the lightning protection capabilities of towers most needed improvement.

< PREVIOUS PAGE NEXT PAGE >

The very first installation of line arresters began in 2001 and involved three double circuit 132 kV lines where historic trip out rates due to lightning were especially high.

Although arresters were placed on all phases of every tower, this initial work was still considered only a trial evaluation. Explains Chan, “we first wanted to make sure that the arrester design and other factors fully met our expectations. A 132 kV line was selected since the impact of any unsatisfactory experience would be less than at 400 kV.” Basically, Chan indicates that a 100 per cent success rate was achieved in eliminating lightning induced faults on the fully protected 132 kV lines over the years following installation of the arresters. Based on this, the installation program was expanded to other lines and, today, all towers in the 132 kV network are equipped with line arresters on every phase of both circuits.

Given the success at 132 kV, another installation program was soon begun, this time at 400 kV. By 2004, of the circa 740 towers in the 400 kV network, around 7 per cent had

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PREVIEW

(left) Typical configuration of early arresters at 400 kV. New design employing silicone V-string insulators (right)

already been equipped with line arresters. Since then, the program has continued to the extent that just over half of all CLP Power’s 400 kV towers are now also protected by arresters.

Again, as at 132 kV, the initial goal was to identify which specific lines and towers should receive priority based on past lightning trip out statistics.

The particular design of line arrester selected from the start by CLP Power engineers was the gap type whereby an air gap is placed in series with the metal oxide arrester. This is the design that has been widely used in Japan for more than 20 years and which has recently been receiving growing attention from arrester standards setting committees at IEC and elsewhere.

The length of the air gap, which in the case of 400 kV is 1.620 meters, was established so as to prevent flashover when re-closing the circuit following a switching surge. Moreover, the air gap in series allows the line to be re-energized quickly should any arrester be overwhelmed by excessive lightning energy.

Chan states that the selection of this gap type design over gapless units was made based on the expectation of longer service life. “We demand

“We demand extremely high reliability for arresters and we expect them to perform for around 50 years without need for maintenance.” extremely high reliability for arresters,” he notes, “and we expect them to perform for around 50 years without any need for maintenance. Since these units are not under continuous voltage, they should last longer. Even in the event the arrester stops functioning, the air gap is still there to maintain the operation of the line at its service voltage.”

The gap type arresters selected by CLP Power all have silicone housings for superior pollution performance. Hong Kong is regarded as a Class IV area,

even though pollution here is still primarily coastal and not as severe as in some industrial parts of China.

The most recent additions of line arresters at 400 kV have seen some important changes over the first installations back in 2002/2003. For one, the V-string insulators that used to be porcelain longrods are now silicone. Indeed, Chan points out that these silicone insulators are the first ever used on the utility’s 400 kV system and therefore will offer engineers an opportunity to also assess their service performance. In the past, CLP Power has been conservative toward composite insulator technology and preferred to rely on porcelain and glass.

A second important change over early installations is that new suppliers have been qualified and approved to ensure more competitive bidding. The arresters now being used, for example, differ somewhat from the first models in that they are from another manufacturer and equipped with larger corona rings.

Yet another change over the very first installations is the typical mounting configuration for each 400 kV tower.

While in the early installation program every phase on both circuits was protected by an arrester, the new

HONG KONG UTILITY EXPANDS PROGRAM TO INSTALL LINE ARRESTERS AT 400 KV

Q3 2009

INMR ® 5 5

Early installation on 400 kV towers (top) saw every phase on both circuits protected. New installations (bottom) use a 3 plus 1 configuration.

design sees all phases on one circuit but only the middle phase on the second circuit protected. Explains Chan, “a consultant study recommended this particular 3 plus 1 configuration for optimal cost and protection against shielding failure, since the middle phase is typically the most exposed to lightning strike.”

According to Chan, all arresters are installed dead in a process that typically takes about 3 to 4 days per 3 phases for one circuit since there is also some pre-installation work putting in portable earth leads after a Permit-to-Work is issued. Also, there is a need for additional metalwork on the tower cross-arm in order to support the jumper since it was not originally designed to carry a V-string and arrester. This work, he says, is carried out mainly during the winter months when load demand is lower and it is easier to schedule an outage.

Chan states that the only maintenance of the line arresters so far has involved routine visual inspections carried out by helicopter or tower climbing. Based on these, there have been no problems or malfunctions reported such as damage to any of the arresters’ polymeric housings. Still, he says that there are plans to use a corona camera to assist future such inspections of the arresters as well as their V-string insulator assembly.

Looking back on the 400 kV line arrester project to date, Chan feels that all available evidence suggests it has been as successful as the experience at 132 kV. “Our lightning location system allows us to identify the location of any faults on the network. When we have linesmen climb the affected tower they can

56

INMR® Q3 2009

also see evidence of any lightning strike by flashover marks on the insulator glaze. Up to now, all such cases have been only on those towers not equipped with arresters.

This proves that protected towers are performing as expected.”

Chan also says that while lightning faults still occur from time to time, management is generally satisfied with the gains that have already been achieved thanks to existing line arresters. With this experience in mind, there are now plans to increase the average coverage of protected 400 kV towers beyond 50% over the future years. As in the

past, engineers will try to pinpoint those remaining unprotected towers most exposed to possible shielding failure and also use historical lightning data to identify where the next installations of line arresters should ideally take place for maximum effect.

This means that, at the end, some 400 kV towers will still be unprotected. But, notes Chan, these will be the ones that benefit most from shielding either by nearby mountains or from passing other lines. This means that they will be less vulnerable to lightning strike. ?

Photo: INMR ©

“When we have linesmen climb 400 kV towers affected by any lightning trip they see evidence of the strike by the flashover marks on the insulator glaze.

Up to now, all such cases have been only on those towers not equipped with arresters and this proves that protected towers are performing as expected.”

Chan (left) and Engineer W.K. Cheung inspect arresters on 400 kV line near Shatin.

HONG KONG UTILITY EXPANDS PROGRAM TO INSTALL LINE ARRESTERS AT 400 KV

Q3 2009

INMR® 5 7

Early installation on 400 kV towers (top) saw every phase on both circuits protected. New installations (bottom) use a 3 plus 1 configuration.

design sees all phases on one circuit but only the middle phase on the second circuit protected. Explains Chan, “a consultant study recommended this particular 3 plus 1 configuration for optimal cost and protection against shielding failure, since the middle phase is typically the most exposed to lightning strike.”

According to Chan, all arresters are installed dead in a process that typically takes about 3 to 4 days per 3 phases for one circuit since there is also some pre-installation work putting in portable earth leads after a Permit-to-Work is issued. Also, there is a need for additional metalwork on the tower cross-arm in order to support the jumper since it was not originally designed to carry a V-string and arrester. This work, he says, is carried out mainly during the winter months when load demand is lower and it is easier to schedule an outage.

Chan states that the only maintenance of the line arresters so far has involved routine visual inspections carried out by helicopter or tower climbing. Based on these, there have been no problems or malfunctions reported such as damage to any of the arresters’ polymeric housings. Still, he says that there are plans to use a corona camera to assist future such inspections of the arresters as well as their V-string insulator assembly.

Looking back on the 400 kV line arrester project to date, Chan feels that all available evidence suggests it has been as successful as the experience at 132 kV. “Our lightning location system allows us to identify the location of any faults on the network. When we have linesmen climb the affected tower they can

56

INMR ® Q3 2009

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also see evidence of any lightning strike by flashover marks on the insulator glaze. Up to now, all such cases have been only on those towers not equipped with arresters.

This proves that protected towers are performing as expected.”

Chan also says that while lightning faults still occur from time to time, management is generally satisfied with the gains that have already been achieved thanks to existing line arresters. With this experience in mind, there are now plans to increase the average coverage of protected 400 kV towers beyond 50% over the future years. As in the

past, engineers will try to pinpoint those remaining unprotected towers most exposed to possible shielding failure and also use historical lightning data to identify where the next installations of line arresters should ideally take place for maximum effect.

This means that, at the end, some 400 kV towers will still be unprotected. But, notes Chan, these will be the ones that benefit most from shielding either by nearby mountains or from passing other lines. This means that they will be less vulnerable to lightning strike. ⌧

Photo: INMR ©

“When we have linesmen climb 400 kV towers affected by any lightning trip they see evidence of the strike by the flashover marks on the insulator glaze.

Up to now, all such cases have been only on those towers not equipped with arresters and this proves that protected towers are performing as expected.”

Chan (left) and Engineer W.K. Cheung inspect arresters on 400 kV line near Shatin.

HONG KONG UTILITY EXPANDS PROGRAM TO INSTALL LINE ARRESTERS AT 400 KV

Q3 2009

INMR ® 5 7