Applying vacuum switchgear at transmission voltages

March 1st, 2015, Published in Articles: Energize

 

Vacuum switchgear has been in extensive use in distribution systems for 30 years for the making and breaking of (fault) current and the switching of loads of all possible nature. The reliability and performance records of the vacuum switching technology are outstanding in the medium voltage range (up to 52 kV), having led to the domination of vacuum switching technology in distribution systems.

Since the 1960s, efforts have been made to extend the application of vacuum switching technology to the transmission voltage level. Around 1980, high voltage vacuum circuit-breakers (HV VCB), high voltage here defined as above 52 kV, were put in service in Japan and by 2010, around 10 000 HV VCBs were installed, mainly in industrial applications, but also in utility applications, see Fig. 1.

Fig. 1: Annual circuit breaker backlog order of 72/84 kV CB units (left vertical axis) and the ratio of VCB/ SF6 CB (GCB) (red, right vertical axis) in Japan.

Fig. 1: Annual circuit breaker backlog order of 72/84 kV CB units (left vertical axis) and the ratio of VCB/ SF6 CB (GCB) (red, right vertical axis) in Japan.

The reason for preferring vacuum technology in favour of SF6 was mainly the capability to switch very frequently and/or the lower maintenance costs compared to SF6. The voltage level of installed switchgear is (with few exceptions) at present limited to 72/84 kV and the technology is almost exclusively metal-enclosed. Reliability studies (on a limited population of HV VCB circuit breakers) show similar reliability for HV VCB and HV SF6 switchgear of the same rating. In the US, vacuum capacitor bank switches have been used for a few decades up to 242 kV. Around 2008, intense research and development programmes started in China and Europe in order to develop HV VCB, the main driver being the absence of sulphur hexafluoride (SF6) gas, recognised as a very strong greenhouse gas. This led to a number of products and applications with voltages up to 145 kV. In China a rapid growth of application in commercial operation is foreseen with at present (2013) several hundreds of HV VCB in service up to a voltage level of 126 kV. In Europe, field tests are being carried out on type-tested devices before the new products come to the market. For examples, see Fig. 2.

Fig. 2: Modern HV vacuum circuit breakers. From left to right: 72/84 kV dead-tank type (Japan);  72,5 kV live-tank type installed in low-temperature environment in China; one pole of a  145 kV live-tank type from Germany.

Fig. 2: Modern HV vacuum circuit breakers. From left to right: 72/84 kV dead-tank type (Japan);
72,5 kV live-tank type installed in low-temperature environment in China; one pole of a
145 kV live-tank type from Germany.

Differences between SF6 and vacuum switchgear

All HV VCB products are based on MV VCB interrupter technology. No essentially new technical features were necessary. The main extrapolation is in the geometry of the interrupter that has to be designed in order to deal with the higher voltage rating, e.g. by increasing the diameter and the contact gap length. In some cases for voltages above 126 kV, two vacuum gaps in series are applied.

Both vacuum and SF6 technology are equally well suited to handle the standardised duties related to fault- and load current switching. Nevertheless, because of the fundamentally different principles of current interruption there exist certain differences which are relevant to application in HV systems.

Apart from the medium itself, the main differences are:

Operational features

With regard to normal currents up to 2500 A, there are no significant differences, but above 2500 A it is challenging to realise in HV vacuum switchgear such normal current ratings. This is due to a number of reasons, but mainly because of the heat generation by VCB contact structure and the limited heat transfer capability of the interrupter.

It is easier to check the quality of the interruption medium in the case of SF6 circuit breakers. It is not practical to monitor the necessary degree of vacuum in service, if required. The number of possible switching operations is higher with vacuum than with SF6 due to the higher endurance of the VCB contact system to arcing. This makes vacuum attractive in applications requiring (very) frequent switching operations, such as daily operations. At a typical 72,5 kV rating, the drive energy of the vacuum circuit breaker may be as low as 20% of that of the equivalent SF6 circuit breaker. The sizes of the vacuum and SF6 devices are comparable.

HV VCB may need more than one interrupter in series above 145 kV, thus leading to nearly identical drive energies as compared to SF6 CB. In SF6 technology, single-break circuit breakers up to 550 kV have been put in service since 1994 and widely installed in many countries.

There is a large difference in physics between the technologies. For example the arc voltage of VCB is much lower than for SF6 CB, several tens of volts against several hundreds of volts. Also, the duration of the arc in fault switching is shorter in vacuum switchgear as the minimum arcing time in HV VCB is typically 5 to 7 ms against 10 to 15 ms for SF6 CB. The consequence of this is that the number of possible switching operations of VCB is generally significantly higher than for SF6 CB.

X-ray emission from HV vacuum circuit breakers up to and including a rated voltage of 145 kV are within the standardised limits of 5 µSv/h (microsieverts per hour) under normal operating conditions, and is thus not a practical issue. SF6 circuit breakers do not emit X-rays at all.

Electrical features

Interruption of fault current with an associated very steep rate of rise of transient recovery voltage is superior with vacuum interrupters because of their very fast dielectric recovery, compared to SF6. The breakdown statistics of vacuum and SF6 gaps differ. Although a vacuum gap in principle has a very high breakdown voltage, there remains a very small probability of breakdown at relatively moderate voltage.

Vacuum gaps are known to show spontaneous late breakdown, up to several hundreds of milliseconds after current interruption. However, the consequences of such an event are very limited because the vacuum gap immediately restores its insulation. Its system implications are not fully understood yet. In case of a late breakdown of a SF6 gap, which is extremely rare, the gap generally cannot recover.

In inductive load switching, notably shunt reactor switching, the number of repeated re-ignitions (at one power frequency current zero) is significantly higher in vacuum than in SF6, as illustrated in the direct comparison in Fig. 3.

Fig. 3: High-frequency interruption and re-ignition observed with an 84 kV VCB and an 84 kV SF6 CB  (GCB) during inductive load switching tests.

Fig. 3: High-frequency interruption and re-ignition observed with an 84 kV VCB and an 84 kV SF6 CB
(GCB) during inductive load switching tests.

This is due to the capability of vacuum to interrupt high-frequency current that follows re-ignition. The consequences of this for apparatus interacting with the re-ignition transients are presently unknown. For example, RC snubbers and MO arresters may be applied to mitigate this.
When switching capacitor banks with vacuum switchgear, very high inrush current must be avoided. This is because the contact system can be dielectrically deteriorated by the pre-strike arc. This duty is a challenge also for SF6.

Mitigation can be accomplished by insertion of a series reactor or controlled switching, although there is no field experience with the latter technology for HV VCB.

User inquiry

An inquiry was set-up to question users of HV switchgear on the assumed strengths and weaknesses of both technologies for application in their HV system. Around 120 replies from 28 countries were received.

The vast majority see the absence of SF6 as the main advantage of vacuum switchgear (provided that the external insulation is SF6-free). At the same time, the lack of service experience at the transmission voltage level was mentioned as a major hesitation for HV VCB application.
Generation of overvoltages by current chopping and the possibility of X-ray emission in switching in vacuum were referred to frequently by potential users. However, it is outlined in the brochure that chopping current levels of vacuum and SF6, also in HV applications, are similar. Up to and including a rated voltage of 145 kV X-ray emission (in single-break devices) remains below the standardised value of 5 µSv/h under normal operating conditions. Multiple-break devices have potentially lower levels.

A large majority of the respondents wish to start a pilot project with HV VCB in their system in order to gain experience with vacuum technology.

Conclusion

There is a general consensus in the Working Group that applying vacuum technology for high-voltage switchgear is already successful for applications involving fault-current interruption. The main bases for this are the excellent reliability and performance records of vacuum switchgear in distribution systems. High-voltage vacuum circuit breaker (HV VCB) technology is directly derived (up scaled) from MV switchgear technology. Wide scale application in Japan of thousands of HV VCB, some in operation for over 30 years, has shown performance reliability similar to that of SF6 switchgear.

Interruption of very high fault current even in combination with very high value of rate-of-rise of TRV by vacuum circuit breakers has been demonstrated and in this context may be even superior to SF6 circuit breaker. This can be particularly useful in the application of, for example, transformer limited faults and series reactor switching.

The main driving force for the development of HV VCB is the absence of SF6 gas, as well as the reduced maintenance (of the interrupter) and high electrical endurance. It must be noted, however, that in older designs of HV VCB, SF6 is still used for outside insulation. For SF6-free products, the volume and/or the withstand pressure of the enclosure may increase. The lack of methods to monitor the vacuum quality in service is seen as a disadvantage.

The main challenge for HV VCB is in the capacitive and inductive load switching duties. Capacitive switching is influenced by vacuum’s inherent wide variation in breakdown statistics, which becomes significant when frequently switching capacitor banks. Test-statistics show that capacitor bank switching, especially when large inrush current is experienced, is associated with late breakdown in an increasing occurrence at higher levels of rated voltages.

In the experience of one manufacturer, a special design of a high-voltage vacuum interrupter may reduce this effect and may sometimes be advisable for switching of single capacitor banks.

In shunt reactor (inductive load) switching, the number of re-ignitions (not the probability of re-ignition) can be large compared with SF6 circuit breakers. It is not completely clear at present what the impact is of this high repetition rate of re-ignitions on solid insulation. Some manufacturers recommend protective measures when switching small high-voltage reactors, especially when directly connected to the circuit breaker. Others promote the use of designs that are optimised for shunt-reactor switching.

The list of advantages and disadvantages of both SF6 and vacuum HV switchgear is fairly balanced. Therefore, no general statement, or even less any recommendation may be given on which of the two circuit breaker technologies – SF6 and vacuum – should be given preference. It obviously depends on technical requirements, overall economic as well as environmental considerations and regulations. Last but not least it also depends on the evolution of circuit breaker prices in the market. Right now, neither a general “must” for introducing VCBs in transmission systems, nor severe or non-solvable restrictions can be identified.

Our conclusion is that both technologies will, therefore, co-exist in the future, up to voltage levels of 245 kV, preferences to be decided case by case. For voltage above 245 kV there exists presently insufficient information to form any conclusion for application of VCB. Fig. 4 shows the wide gap in “breaking capacity” between vacuum and SF6 technology. In spite of the fact that significant efforts during the last 40 – 50 years were made for SF6 alternatives, no industrially viable alternative to SF6 technology covering all the voltage rating up to 550/800 kV has been found with the same performance. The manufacturers are continuously seeking for alternatives to SF6 technology and are committed to make available any industrially viable replacement. However, SF6 technology will remain essential for the transmission networks until a new technology that can cover all ratings is found.

Fig. 4: Development of interruption power per break of SF6 CB compared  to one design of vacuum CB (145 kV/40 kA).

Fig. 4: Development of interruption power per break of SF6 CB compared
to one design of vacuum CB (145 kV/40 kA).

Acknowledgement

This article was published in Electra, October 2014, and is republished here with permission.

Contact Rob Stephen, Eskom, Tel 031 563-0063, rob.stephen@eskom.co.za

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