Gas-insulated substations (GIS) represent a key element of high-voltage electrical transmission networks thanks to its reliability, low maintenance requirements and compact dimensions. Here is a brief history of its development and pioneers.

[1966 - The first 245 GIS installed at Plessis-Gassot, near Paris in France]

The roots of HV encapsulated substations go back to the metal enclosed concept of the 1920s when oil was used as the insulating medium. Compressed air and different gases were the focus of much research work, and the first Freon-based solution at 33 kV appeared in 1936. The following decades brought new versions until developments in industrial processes, chemistry and physics led the switchgear industry, towards the end of the 20th Century, to the use of SF₆ (Sulfur Hexafluoride) for arc extinguishing and insulation as the main GIS technology.

SF₆ gas was already known during the 1940s. Westinghouse holds the original patent for the use of SF₆ as an interrupting medium, and their engineers developed the first applications for switches and circuit breakers in the early 1950s. In the 1960s major manufacturers such as BBC-Calor Emag, Siemens, Magrini, Merlin-Gerin, NEI-Reyrolle, the Japanese and Delle-Alsthom had started intensive developments on the basis of SF₆. The dual-pressure SF₆ circuit breakers of the early GIS systems were soon replaced by single pressure, while circuit breakers were adopting puffer and combined thermal-puffer arc extinguishing chambers. The GIS focus was on the benefits of a compact indoor solution, protected from the environment and closer to users, whereas some markets preferred outdoor rugged solutions using hybrid GIS solutions.

GE Vernova's Contribution

GE Vernova's Grid Solutions business has a rich GIS development history through its ancestor companies, Delle-Alsthom, Sprecher & Schuh, GEC, and AEG.
 
Delle-Alsthom France started GIS development in 1958 and in 1966-1967 delivered a world's first with its “Fluobloc” at 245 kV in several Paris substations, demonstrating the benefits of underground GIS to supply bulk power close to city users. Achievements in the higher voltage ranges were subsequently marked by the deliveries of the first substations for 420 kV in 1976 and for 550 kV in 1977. Another “world-first” was the completion of AEP’s 800 kV GIS in Joshua Falls in 1979.
 
Sprecher & Schuh studied compact metalclad installations as early as 1954 with oil insulation systems, but soon concluded that SF₆ gas insulation offered greater advantages. Their first GIS for 220 kV was delivered in 1970 and the 145 kV, 40 kA in 1971. The original circuit breakers with double-pressure SF₆ systems (220 kV, 50 kA), developed together with ITE USA, were operated by the well-known Sprecher motor-wound spring operating mechanisms, which contributed to the success of subsequent GIS families. The exclusive third-generation FK mechanism today serves all Grid Solutions' GIS products in applications around the world.
 
AEG in Germany has also long been involved in GIS and SF₆, with its first GIS substation delivered in 1971.
 
Meanwhile, GEC in England was collaborating with Siemens, and their first GIS was a 145 kV substation in London in 1982. As GIS systems developed and their extensive use in HV networks grew, Grid Solutions became the manufacturer of complete GIS ranges of 72.5-800 kV in which single-phase and three-phase encapsulation was and continues to be used.

Looking ahead

Clearly, the most significant development factor was the adoption of SF₆ as an insulation medium. This boosted the development of smaller switchgear requiring less operating energy and reduced materials and resources, leading to higher performance. So far 420 kV, 63 kA with a single break is possible with the spring mechanism.



After 50 years in the making, GIS development is accelerating thanks to the availability of simulation tools and the capability to integrate environmental needs into the design. Future trends could be influenced by the substitution of SF₆ technology, which, however, is likely to be a very complex task. Other steps have already been taken. Moving HV substations closer to consumers results in reduced transmission losses. Indoor GIS reduce the environmental influences on the switchgear, reducing maintenance needs and increasing lifetime. More and more “intelligence” is integrated into the GIS using electronic devices, forming part of digital substations. Ecological and economic considerations, together with ongoing technological developments have made even further optimisation of GIS conceivable.

[2018 - GE Vernova's first 145 g³-GIS installed at Axpo's Etzel substation, in Switzerland]

As customers increasingly push to adopt condition-based maintenance for Gas-Insulated Substations (GIS), new opportunities are arising for periodic or permanent measurement of partial discharge.

Traditionally, high-voltage substations are air insulated. But the clearances required between phases and between phases and earth are huge. This results in rather large installations, making them difficult to house in urban environments where space is at a premium. To overcome this constraint, a parallel technology was developed, the Gas-Insulated Substation (GIS), using a gas, for example sulphur hexafluoride (SF₆), at high pressure. SF₆  has excellent dielectric properties and is used as the insulating medium between the phases and between the phases and earth. As a consequence, a GIS is much more compact. In fact, gas-insulated substations can be down to one-tenth the size of their air-insulated cousins, depending on the voltage level.

The use of gas insulation in the power system network has developed rapidly due to its compact nature, low maintenance requirements, and reliable operation. But the reliability of the GIS equipment can be undermined by the presence of free particles that originate mainly from the mechanical vibrations, from moving parts in the system such as breakers or disconnectors, or even from the manufacturing process.

According to David Gautschi, electrical engineer with GE Vernova's Grid Solutions business, “they are rare, but can locally generate high electric fields exceeding the structure’s design limits and initiate partial discharges (PD) forming free electrons and ions in the insulation. Repeated partial discharges are capable of triggering a progressive carbonisation of spacers that can slowly build up over years until they produce a flashover, or failure of the switchgear insulation structure resulting in the entire installation, or parts of it, being shut down.” Repairs – often involving the manufacture of specific parts – can take several weeks to complete.

Measuring partial discharges

When partial discharges occur (resulting in voltage drops of less than a nanosecond), they generate electromagnetic waves that propagate through the switchgear. These waves can be measured by means of different technologies operating in a variety of frequency ranges. Detecting partial discharges in lower frequency ranges can be carried out by taking measurements with acoustic sensors. Says Gautschi, "In the medium frequency range, between a few kHz and a few MHz, measurements are usually made by means of a coupling capacitor. The disadvantage of using this device is that it is large and not suitable for online monitoring. However, partial discharges in pressurised gas can be measured in the Ultra High Frequency (UHF) range between 100 MHz and 2 GHz. The added advantage here is that this allows the whole substation to be permanently monitored and the location of PD activity can also be pinpointed.” Demand for this level of monitoring is particularly high in the Middle East, though less pronounced in Europe, where utilities are more hesitant to make the additional outlay required.

UHF range measurements

Different types of equipment are available to carry out measurements in the UHF range. GE Vernova's Grid Solutions business has developed its own solution, called PDwatch. The center of competence for the PDwatch product is located in the BHT unit in Aix-les-Bains, France. The PDwatch system can be used either for periodic measurement (PDwatch portable) or for permanent (online) condition monitoring. The second method has the obvious advantage of tracking all partial discharge activity over time and therefore offering a better basis on which to decide when maintenance is required rather than relying only on spot checks using a portable system. “The benefit of measurements in the UHF range is the effective avoidance of external noise,” explains Jean-François Penning, PDwatch project manager.

The PDwatch Portable offers frequency spectrum and time analysis.

The frequency range can be chosen to measure in a band with low external noise. The suppression of external noises, for example in the GSM mobile phone range, can be achieved in the following way: the measurements made by the sensors fitted in the GIS are compared with the results of those installed in other compartments or those of an additional external antenna. This method avoids using additional band stop filters on the input ports, as generally required by standard wide band monitoring systems. It also maintains a good signal level. Once the partial discharge activity has been measured, the next task – and the more complicated one – is to interpret the partial discharge patterns and classify them into degrees of severity.

“Part of the complexity is that partial discharge patterns will vary according to the switchgear design,” notes Gautschi. “So it is essential to have access to the manufacturer's database to make sure that partial discharge information will be accurately interpreted. Grid Solutions is going to make its databases available to customers.”

PDwatch online partial discharge monitoring

The PDwatch Online UHF monitoring system records and displays the UHF signals generated by partial discharges in a gas insulated substation. It is permanently fitted into the substation and can be interrogated remotely at any time. This makes it possible to detect and eliminate emerging dielectric faults before a flashover can occur. Used with suitable sensors, this system can detect critical defects such as particles, coronas, free potentials and insulator voids. It can also be programmed to generate alarms at specified absolute value and time thresholds. “The latest system is very advanced,” points out Gautschi, “and uses fast algorithms to provide very high accuracy.”

PDwatch portable UHF detector

PDwatch Portable is designed to measure campaigns in substations at the commissioning stage or periodically in the course of the substation's life. It is a two-in-one device, offering frequency spectrum analysis and time analysis. By using this equipment at regular intervals, developing dielectric faults can also be detected and eliminated before complete breakdown occurs. The portable UHF detector and its laptop PC are fitted into a travelling case and supplied with all necessary cables and accessories.

PDwatch Manager

This software tool enables event records to be managed while at the same time facilitating defect recognition. It can be used locally on the central unit's human machine interface (HMI) PC or run from a remote PC via the Internet. It includes a constantly updated library of partial discharges that helps to identify PD patterns. It has the added advantage of saving users a considerable amount of time by generating test reports automatically.

Sensors for measuring partial discharge activity 
Different types of sensor can be used to measure partial discharges in a gas-insulated substation. Grid Solutions' latest design uses a conical antenna with a small footprint. Its sensitivity has been tested under laboratory conditions in different calibration cells as well as after having been installed in the switchgear. It offers an extremely high degree of accuracy and high linearity. Furthermore, the cost of the device has been dramatically reduced and its small footprint now allows it to be retrofitted to older substations. Its output has an integrated low frequency cut-off so that no power frequency voltage is visible on the sensor connector. The output can also be adapted to meet customer needs, and it can, for example, be used as a conventional voltage detector to detect whether a particular phase is energized or not.

The new sensor has been fitted in all types of Grid Solutions gas-insulated substations and tested for use in retrofit projects. These latest developments have resulted in an adapted version of the sensor being used in large power transformers to monitor partial discharges in oil. This version has been installed in 800 MVA transformer poles of the Swiss utility Alpiq. The transformers have been in service since 2011.

During the development of the sensor, the existing calibration methods for GIS sensors were tested, and a new high performance calibration cell has been developed to carry out tests when no bays are available to carry out this procedure in situ.

Taking advantage of latest design techniques as well as innovative concepts, the newly developed 800 kV GIS substation succeeds in combining compactness, high reliability and performance in compliance with the latest IEC standards.

In large countries such as India, China or Brazil, conventional and new sustainable power sources are often located in regions remote from load centres and need large-scale AC transmission systems. One solution to reduce the power losses and improve the transmission capacity of AC links is to increase the system voltage up to 800–1,200 kV AC. For these high-voltage transmission systems, the use of gas-insulated substations (GIS) is extremely advantageous, as this substation type is highly reliable and requires less maintenance than air-insulated substations (AIS) because all active parts are protected from environmental hazards. In addition, a GIS’s inherent compactness (owing to the superior insulating properties of SF₆ compared with air) reduces bay dimensions and overall substation footprint and height, which is very important in minimising seismic impact.

However, “at 800 kV, existing GIS are based on technologies from the 1990s, so dimensions are still relatively large and 800 kV GIS are mostly installed outdoors,” explains Mathieu Bernard, GE Vernova R&D project manager for bay apparatus and substations. “Responding to a growing demand for greater compactness (notably from the Indian utilities), our main objective was therefore to develop an 800 kV GIS compact enough to be installed in a small building. One key point was the circuit breaker architecture, as it represents a large part of the substation.”

Innovative circuit breaker architecture with smaller footprint and reduced height

“We tried various circuit breaker configurations combining two 420 kV breaking chambers in series,” says Nicolas Garbi, GE Vernova's R&D project manager for the circuit breaker. “The solution was found by positioning the two chambers vertically, side by side, in a single tank, with an oblique conductor in between. This enables us to have the two breakers as close as possible to each other – the shortest distance between them is less than 5 cm. The chambers can also be equipped, where necessary, with closing resistors without greatly increasing the dimensions of the enclosure.”

Circuit breaker section view

With this innovative circuit breaker design, improved substation architecture and other innovations (see below), GE Vernova's new 800 kV GIS not only achieves the reduced footprint required but also, with a maximum height of 5 meters as for the standard architecture, can be easily installed inside a building. Moreover, even though it is the most compact 800 kV GIS, the unit still offers “exceptional access to all components and viewports: the highest drive position is at 3 meters, readily viewable from the floor without requiring specific – and heavy – catwalks.”

Typical diameter arrangement inside a building

Another important objective was to ensure reliability under any and all service conditions, particularly with respect to earthquakes. “As India – a major developing network using 800 kV GIS – can be subject to serious seismic events, one of our R&D missions was to take this risk into consideration in the very early stages of the design of our new substation,” says Bernard. Having a reduced height is already a good point where a high level of seismic withstand is required. In addition, all the most massive equipment is close to the floor (hence a low centre of gravity), and seismic calculations have been conducted jointly with design studies to ensure optimum behaviour of the substation. “As a result, Alstom’s 800 kV GIS offers top-class safety with regard to seismic constraints of 0.3 G and more.” 

Performance and reliability

Development of equipment for such high-voltage ratings cannot be made by simply applying a size-factor ratio from lower voltage products. The characteristics of UHV overhead lines and substation schemes demanded the continuation of fundamental studies and the application of innovative solutions to achieve maximum reliability for the equipment. 

For instance, when the service voltage rises, the bus charging current switching (BCCS) capability of a disconnector has to be increased. As a consequence, managing BCCS implies a better understanding of the complete phenomenon. “When the circuit breaker opens, the load current is interrupted and only a small capacitive current can flow through the closed disconnector,” Bernard explains. “During the opening operation, multiple restrikes can be observed between contacts. The main issue in disconnector development is to ensure that no flashover between the two electrodes will propagate and reach the enclosure. An innovative solution has been found, applied and validated: a specific characteristic of the electrode, which includes mobile parts, allows the gap to be reduced during the closing operation. The reliability of the disconnector in terms of very fast transient overvoltage (VFTO) phenomenon is therefore increased.” At the same time, bus transfer performance has also been studied in depth, for all voltage levels, in order to be able to comply with IEC standard requirements (and even beyond, as some customers may demand). “To do so, we developed an innovative concept of mobile arcing contact, combining fast translation and rotating displacement.” This new solution, which is patented, has been tested and validated on a prototype 800 kV GIS rolled out for the production systems.

Disconnector section view

The kinematics of the equipment was also a topic of concern. A multi-domain simulation programme was used to model many possible kinematics combinations and to enhance connecting rods, crank handles and hydraulic drive to have minimum energy consumption for the required opening and closing speeds. “The kinematics of the circuit breaker is driven by a single hydraulic command, even in the case of a circuit breaker connected with pre-insertion resistor (PIR). The actuation system is installed at the bottom of the circuit breaker tank and moves two different shafts, one for the chambers and one for the PIR,” adds Garbi.

Fully IEC compliant, even for the most constraining performance

Particular efforts have been made to ensure that the 800 kV GIS meets foreseeable reliability requirements. It has been successfully subjected to all IEC-type tests: dielectric testing and temperature rise, bus charging current switching and bus transfer for the disconnector, making test for high speed earthing switch, terminal faults, short line faults and capacitive switching for the circuit breaker. The result of this complex development project, which required enhanced international collaborative R&D work in France, China, and India, is a cutting-edge 800 kV GIS that is super compact, highly reliable and easily maintainable. Manufacturing and after-sales service will mainly be ensured by the GIS manufacturing site in Chennai, India.