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Erlangen, 20.04.99EV MNK V/4521.90/Sä/Kt

3AH vacuum circuit-breakers in comparison with SF6 circuit-breakers

Major features of Siemens vacuum circuit-breakers

• Constant dielectric

• Constant, minimal contact resistance

• No decomposition products

• High total current switched

• Freedom from maintenance

• High reliability

• Minimal use of plastic

• No explosion in the event of a fault in the vacuum interrupter

• Suitable for all switching duties

- Minimal chopping currents

- No restrikes

- High short-circuit currents

These factors – in conjunction with economic advantages – have further popularisedthe vacuum quenching principle. The vacuum circuit-breaker is the world’s mostfrequently purchased switching device.

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Details of the vacuum / SF6 quenching principles

The advantages offered by the vacuum switching principle make it the most eco-nomically efficient at present and the breakers superior to other designs:

• Constant dielectricThere are no gas decomposition products in the vacuum, and the hermeticallysealed vacuum interrupter keeps out all environmental effects:

In the case of SF6 circuit-breakers, decomposition products occur in the breakerpole as a result of arcing. These have to be disposed of.

• Constant contact resistanceIn vacuum, the contacts cannot oxidise, a fact which ensures that their very smallresistance is maintained throughout their life.

SF6 circuit-breaker contacts erode to such a degree that separate main andarcing contacts are necessary.

• High total current switchedThe total current switched (short-circuit current plus normal current) by a vacuumcircuit-breaker is considerably in excess of that applying to an SF6 breaker.

For example, a 3AH1 standard vacuum circuit-breaker for 12 kV / 20 kA / 1250 Acan interrupt a current of

20 kA 140 times4,5 kA 4,000 times1250 A 10,000 times

The mechanical service life is 10,000 switching cycles.

3AH vacuum circuit-breakers are therefore particularly suitable for frequent auto-reclosing in overhead line systems.

If, for special applications (e.g. as furnace breakers) these figures are not ade-quate, the 3AH2 respectively 3AH4 vacuum circuit-breakers with a mechanicalservice life of 60,000 resp. 120,000 switching cycles may be used.

• Maintenance3AH vacuum circuit-breakers are maintenance-free up to 10,000 operating cycles.This means that, under normal operating conditions, no relubrication or readjust-ment is required throughout the entire service life of more than 20 years. With ap-plications which call for higher operating cycles or under unfavourable ambientconditions easy maintenance has to be carried out, e.g. greasing.

All component parts of the interrupter are soldered or welded together. Such con-nections are not subject to aging. The vacuum is therefore maintained throughoutthe entire service life.

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Interrupters not switched remain in working order for more than 20 years. Thevacuum is even improved by the switching of current. There is therefore no time limiton their fitness for use.

The maintenance required for all breakers operating on other arc-quenchingprinciples is considerably higher. Medium-voltage SF6 circuit-breakers, for example,require annual inspection and a minor or major overhaul after five or ten yearsrespectively. A major overhaul in some cases necessitates complete refitting of theentire arc control device, even if the breaker has been operated only seldom over theten-year period.

• ReliabilityThe most important function for the circuit-breaker is to interrupt short-circuit faultcurrents in order to protect connected equipment from high stress resulting from theshort-circuit. The breakers must perform this function with maximum safety andreliability.

A measure of the reliability of the unit is the mean time between failures (MTBF),expressed statistically as the interval between two faults. This in turn is known as theMTTF (mean time to failure), applying to parts which cannot be repaired. The MTBFfor Siemens vacuum circuit-breakers is approximately 1,000 years, the MTTF forSiemens vacuum interrupters 24,000 years. The inverse value of the MTBF is thefailure rate. Experience gained over many years by analysing faults occurring inoperation shows that the failure rate is roughly proportional to the number ofcomponents in a unit.

A comparison of vacuum and SF6 circuit-breakers shows that the pole column of thelatter type incorporates substantially more parts than that of a vacuum CB.

The number of moving parts in the pole area is an important point in the comparisoninsofar as they naturally have a higher failure rate than fixed parts. On some SF6 CBsthese parts are inaccessible once the breaker is in operation, as the poles are pottedor cemented up. Repairs can therefore only be done at the manufacturer’s works.

This comparison of the two breaker systems disregards the operating mechanism,even though one of the advantages of the vacuum breakers over the SF6 types lies inthe fact that less energy is required to operate them. The comparison moreover showsthat higher reliability may be expected of the vacuum breakers because they havefewer parts than breakers operating on other arc-quenching principles.These statements are based on the experience gained from more than 200,000Siemens vacuum circuit-breakers manufactured in the Berlin Works and worldwide bya total of more than 20 authorised manufacturers and licenses.

If the above results of various independent investigations are collated, a conclusion isreached that the vacuum switching principle is superior in terms of reliability. By virtuelargely of their high degree of availability, German power supply utilities have decidedto use vacuum circuit-breakers in nuclear power plants. In Germany the vacuumcircuit-breaker has reached a market share of 99 % and thus has the leading positionof the medium voltage quenching principles.

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• Use of plasticOn the Siemens 3AH vacuum circuit-breakers, the use of plastic has been minimised:- The pole columns are insulated to earth by means of standard post insulators and

operating rods.- Plastic struts (parallel to the interrupter assembly) provide the necessary mechani-

cal strength.

SF6 breakers incorporate a considerable amount of plastic, particularly when all threepoles are arranged in a common housing. This leads to insulation between phaseswith high dielectrical stressing, as the insulation paths are very short and the phase-to-phase voltage is constantly being applied. The use of large amounts of plastic has thefollowing disadvantages:

- Increased risk of partial discharges during operation,- More plastic which can burn in the event of a fault arc,- In the event of quenching medium loss, risk of explosion as a result of failure to

quench the breaking arc, which in turn leads to a three-phrase fault arc andsudden burning of the plastic in the insulating vessel.

• Loss of quenching mediumIf on an interrupter which has lost its vacuum the current is not interrupted in anopening operation, the interrupter will, depending on the magnitude of the current,burst after a certain period of time. As experiments have shown, this never involvesan explosion.

In contrast, if an SF6 circuit-breaker suffers gas loss, an explosion is to be reckonedwith under similar circumstances.

In the event of quenching medium loss, the vacuum circuit-breaker is clearly superiorto types operating on other principles.

• Switching dutiesBesides their main function of interrupting short-circuit currents, circuit-breakers mustalso be capable of handling any other switching operations that may occur. Widelydifferent demands are thus made of the breaker which are difficult to fulfil in onecommon design. The individual solutions mean in effect that a compromise has to befound. The following explains why modern vacuum circuit-breakers represent anoptimal solution in this respect.

If short-circuit fault interruptions are discounted, switching operations may besubdivided into three groups. These are:- Switching of capacitive currents- Switching of very low inductive currents (up to about 20 A),- Switching of low inductive currents (from about 20 A up to the rated normal

current).

- Switching of capacitive currentsThis covers the switching out of capacitors, unloaded cables and overhead linesas well as the switching-in and paralleling of capacitors.

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The vacuum circuit-breakers disconnect these loads safely without restrike andthus without derivative overvoltages. SF6 circuit-breakers are likewise generallyrestrike-free.

When capacitor banks are switched in – and especially when they are paralleled– equalising currents with a very high rate of rise and high amplitude occur. SF6

circuit-breakers with sliding or tulip contacts may consequently suffer fromretardation of the moving contact. Additional measures (reactors) are thereforeusually needed to reduce such effects. The short pre-arcing times and flatcontact surfaces of vacuum breakers give rise to fewer stresses.

- Switching of very low inductive currentsThis mainly entails switching out transformers at no-load. High overvoltages mayoccur here through current chopping.

Owing to their low chopping currents (below 5 A) the 3AH vacuum circuit-breakers are capable of safely switching unloaded transformers (the most difficultinductive loads) so that overvoltages remain low and consequently no surgelimiters are required.

Those SF6 breakers which function with current-dependent arc quenching(selfblast/rotating-arc types) have roughly the same chopping currents as modernvacuum CBs.

SF6 breakers with additional piston (for reliable interrupting of capacitive currents)and above all puffer-piston breakers have substantially higher chopping currentswhich lead to high switching overvoltages.

- Switching of low inductive currentsTwo main applications fall within this category. They comprise compensatingreactors and switching off of motors – up to a certain size – under stalled rotor orstarting conditions.

When vacuum circuit-breakers are used to switch motors with starting currentsbelow 600 A, switching off during start-up may give rise to high overvoltages dueto multiple reignition and virtual current chopping. 3EF surge limiters can restrictthese overvoltages to safe values. The reliability of the system consisting ofvacuum circuit-breaker, 3EF surge limiter and motor (starting up) has beenverified in the course of comprehensive experiments, has proved its worth manytimes over in operation and has been used in nuclear power plants in numerouscountries without any problems occurring.

SF6 circuit-breakers with current-dependent quenching do not tend to multiplereignition or virtual current chopping.

SF6 puffer-piston circuit-breakers behave on the other hand like vacuum circuit-breakers when switching off motors during start-up. It must also be noted thatpuffer-piston circuit-breakers, owing to their high chopping currents, can evenendanger motors in normal switching-off operations.

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In summary it can be stated that, where the switching of motors is concerned,vacuum circuit-breakers (owing to their high permissible number of switchingcycles and to their reliability of operation) are the most suitable option, evenunder special circumstances (switching off of small motors during start-up).

When compensating reactors are switched off, both vacuum circuit-breakers andSF6 puffer-piston circuit-breakers suffer overvoltages due to multiple reignitionand, at rated currents up to 600 A, virtual current chopping too. (On account oftheir rating data, SF6 breakers with current-dependent quenching are generallynot suitable for this application; typical ratings are 36 kV/31.5 kA). Here too thevacuum circuit-breaker (with adapted protective circuits) enjoys advantages byvirtue of its high permissible number of switching cycles.

Finally, 2 further special categories are worthy of mention, namely tractionsystem circuit-breakers and arc furnace transformer circuit-breakers.

Switching in single-phase traction systems entails for the breakers the followingdifficulties in comparison with three-phase breakers for 50 Hz or 60 Hz:

- At low frequencies (16 2/3 Hz or 25 Hz) fewer current zeros occur than at50/60 Hz; the average arcing times are therefore correspondingly longer,

- On three-phase breakers the three poles "help each other": for example, on a50 Hz three-phase circuit-breaker a current zero occurs in all three polesalternately every 3.3 ms, which leads to a further reduction in the averagearcing time. Traction-system circuit-breakers cannot profit from such a facility.

When the arc is quenched in the three-phase system, SF6 circuit-breakersrequire considerably more energy across the contact gap than do vacuum circuit-breakers (by a factor of about 10). In view of the arcing times described above,they are therefore hardly suitable as traction-system breakers.

When arc furnaces are switched, currents between the figure for no load andtwice the current rating of the transformer have to be handled up to 100 times perday. This entails extremely high electrical and mechanical stressing of thebreakers, something to which the vacuum circuit-breaker (with adapted protectivecircuits) is best suited.

In view of their limited permissible number of switching cycles and their rateddata (frequently 36 kV / 31,5 kA), SF6 circuit-breakers can be used only inexceptional cases as furnace breakers.

In conclusion it may definitely be stated that vacuum breakers have the edgeover other breakers, no matter what the application may be. This also applies toso-called critical cases.