Note: Descriptions are shown in the official language in which they were submitted.
1147839
A msthod for Qwitching in a three-phase high voltage circuit.
The invention relates to a method for interrupting an
inductive load in a three-phase high voltage network by
means of a switch having a quick dielectric recover~.
It has been found that upon interruption of an induct-
ive load in a three-phase high voltage networX, sometimes
high overvoltages occur on the load to be interrupted.
Since these overvoltages may cause considerably damage to
the network due to insulation breakdown and the like,
several investigations have been made in order to discover
the likely cau~es of these high overvoltages. ~rom these
investigations it has become apparent that these high ovçr-
voltages occur onl~ when in the pertaining circuit at least
one of a number of conditions is fulfilled, the most import-
ant ones of which being:
1. Switching should be performed by switches having
a quick dielectric recovery; this condition being fulfilled
by vacuum switches.
2. The requirement of a predetermined network con-
stellation; this implying among others the presence of a
sufficient capacitance between the phases on both the
source side and the load side of the switch.
3. ~he switching time should be such that upon
separation of the contact members one pole of the switch is
close to a zero current passage.
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U on interruption of a load under the above conditions,
in the first phase interrupted at zero current passage an
overvoltage will occur that may result in re-ignitions in
this phase. Under the prevailing circumstances the current
5 resultin~ from these re-ignitions and having a very high
frequency will be entirely or partially superi~posed on the
network current in both the other phases which although
alread~ being interrupted still carry current. When the
rssultant of the superimposed current and the network curre~
10 in these pha~es becomes about zero both the other phases
will also be interrupted. Since, however, at the re-ignition
moment the network current in the first phase causing the
re-ignitions is close to a zero current passage, the
momentary current value in both the other phases is relativ-
15 ely high, this resulting in a so-called high current chop-
ping. In the literature this chopping of the relatively
high current is generally called "virtual chopping". In
view of the fact that at this "virtual chop?ing" the di/dt-
value i9 high, very high overvoltages may occur in the
20 installation at "virtual chopping".
In a publication by M. Murano et al, "~hree-phase
simultaneous interruption in int~rupting inductive current
using vacuum switches", I.E.E.E. ~ran3ætions on Power
Apparatus Systems, Jan./Febr. 1974, pages 272-280, concern-
25 ing the phenomenon "virtual chopping" it has been proposedto include a resistance in series with a capacitance in
parallel to the load between the phases and ground. This,
however, entails the drawback that the resistance and
capacitance values have to be adapted to the pertaining
30 circuit and the load to be interrupted, this solution more-
over being rather expensive in case of high currents.
Another method referred to in the above publication
employs a non-linear resistance the high cost of which is
likewise of disadvantage.
A further method employs surge suppressors in order
to restrict the detrimental results of "virtual chopping".
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This method, however, also entails the drawback of relativ-
ely high expenses accompagnied by the fact that the surge
suppressors have to be installed as close as possible to
the load.
~he abovementioned methods have furthermore in common
that they are only operative under those network conditions
for which they have been designed. Upon a change in these
conditions one shall therefore have to adapt the method to
this change. Furthermore the abovementioned methods have in
c~mmon that they have not been directed to the prevention
of "virtual chopping" but only to a protection against the
result~ thereof.
A better method will therefore be complete prevention
of "virtual chopping". ~his may be achieved by the
provision of not fulfilling one of the abovementioned three
conditjons for the occurrence of "virtual chopping".
If one wishes to profit by the specific advantages of
a vacuum switch the first condition will ~et have to be met.
For eliminating the second condition the conductors connect-
ed to both the source side and the load side of the switchhav~ to be completely shielded with respect to each other.
In practice, however, such appears to be a difficult and
expen~ive task.
Finally the occurrence of condition 3 may be obviated
by synchronizin~ the switching time with respect to the zero
current passage in such a manner that the sep~ration of the
contact members does not happen close to zero current
passage. However, such likewise incurs the use of complicat-
ed and expensive devices.
Now the object of the invention is to provide a switch-
ing method for preventin the "virtual chopping" in a simple
and consequently inexpensive manner irrespective of the net-
work conditions. The invention therefore provides a switch-
ing method of the above mentioned type having the character-
istic that upon interruption two of the three switching
paths hsving a quick dielectric recovery open at least 1/3
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~147839
of a cycle of the network frequency later than the first
switching path augmented with the minimum arcing time in
the first switching path.
~ his switching me~hod guaranties that under no circum-
stances "virtual chopping" is possible anymore.
~ he invention further~ore provides a switch machanism
capable of preventing"virtual chopping". ~his switch
mechanism is characterized by means whereby upon inter-
ruption, the oprning of one switching path is initiated
earlier than both the other switching paths in such a
manner that between the time on which the first switching
path i5 opened and the time on which both the other switch-
ing paths are opened an interval is provided corresponding
to at least 1/3 of a cyclè of the network frequency.
The invention will now be described in detail with
referen¢e to the accompanying drawing.
Fig. 1 represents a simplified switching diagram for
elucidating the switching method according to the invention;
Fig. 2 represents an oscillogram of the situation in
which upon opening of one switching path or pole the current
in the pertaining phase is interrupted at the next zero
¢urrent pa~sage;
Fig. 3 represents an oscillogram of the situation in
which upon opening of a switching path or pole the current
in the pertaining phase is not interrupted at the next zero
current passage,
In ~ig, 1 the switch and the load have been indicated
by the reference letters K and L, respectivel~, the phases
by R, S and ~ and the accessory switching paths of the
switch by reference numerals 1, 2 and 3, re~pectively,
It is now assumed that the switching path 1 of the
phase R opens at an earlier time than the switching paths 2
and 3 of the phases S and ~, that is to say with an interval
~2 equa~ng or surpassing 1/3 of a cycle of the network
frequency augmented with the minimum arcing time of the
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switch at the current conduc~ive to "virtual chopping".
In ~ig. 2 a border-line case has been disclosed in
which "virtual chopping" might occur, that is to say the
situation in which the opening time t1 corresponding to the
5 opening of the switching path 1 has been selected such that
between this point t1 and the next æero current passage at
time t2 f the phase R the interval T1 is provided, said
interval equaling the minimum arcing time of the switch.
The switching paths 2 and 3 of the phases S and T are open-
10 ed at time t3 between which points t3 and t1 the above-
mentioned interval ~2 is provided.
~ow two situations are conceivable, i.e.:
a) ~he situation represented by Fig. 2 in which the
current in the phase R is interrupted at the first zero
currsnt passage after opening the switching path 1, that
is to say at point t2.
b) ~he situation represented by Fig. 3 in which the
current in the phase R is not interrupted at the first
zero current passage after opening the switching path 1.
In the situation a) no "virtual chopping" can occur
be¢ause at point t3 the other switching paths 2 and 3 of the
pha~e~ S and ~, respectively, are yet closed, this implying
accordingly, ~Jfulfilment of one of the conditions for the
"virtual chopping", i.e. the simultaneous three-phase
25 switchin~. Upon opening of the switching path~ 2 and 3 on
point t~ the currents in these phases will be interrupted
simultaneously in the known manner, that is to sa~ at point
t4. ~hus when the opening time t1 of the switching path 1
is moved to an earlier point, i.e. when the interval batween
30 t1 and t2 extends beyond the interval ~1' the current in the
pha~e R will be certainly interrupted at the point t2,
because the arcing time of the switching path 1 is longer
than the minimum arcing time T1 of the switch so that also
in these situations "virtual chopping" appears to be im-
~5 possible.
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If, however, as shown for situation b) the current inthe phase R is not interrupted at point t2 this current will
only be interrupted at the next zero current passage, that
is at the point t4 as indicated in ~ig. 3. The other switch-
ing paths 2 and 3 will then be opened at time t3, suchimplying for the switching path 2 that the opening occurs
~ust upon a zero current passage of the pertaining phase S.
Consequentl~ the phases S and ~ will in the known way
simultaneously o~ly be interrupted at point t5. If the
i~terruption of the current in the phase R will occur at
point t4 as de~cribed above, there will be no "virtual
chopping" because at that point the opening between the
contact members of the switching path 1 has become
sufficiently large to prevent re-ignition. Now when retardi~
the opening time t1 f the switching path 1, i.e. the
interval between t1 and t4 becoming smaller the switching
path R will always be interrupted at point t4 until this
intsrval doe~ become so short that in fact the course re-
presented by situation a) occurs again. It i9 evident that
in case of situation a) in similar manner, upon setting
point t1 at an accruing earlier time this situation a) i8
transposed in situation b). Where the situations a) and b)
delimit the borders of the area conduc-ive to "virtual
chopping", it has been demonstrated above that the switching
method according to the invention eliminates the "virtual
chopping" phenomenon.
Summarizing it may be remarked that in all the above-
mentioned situations the current in the switching path
opened first is also interrupted first a a time at which
either both the other switching paths are yet closed or the
switching path opened first has already been opened so far
that no "virtual chopping" is possible anymore. As a matter
of fact of course the above described situations also apply
when one of the two other switching paths is opene,d first
3~ whereas the result of the switching method is also ~epend-
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` 1147839
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ent from the phase order of the three phase current network.
The method according to the invention thus provides a-switching method conducive to switching under various situa-
tions without the occurrence of "virtual chopping" and without
requiring special measures apart from the simple adaptation
of the switch. In practice this switching method may be
realized by modifying the switching mechanisms of the switch
in such a manner that upon interruption one switching path
opens earlier at the above indicated time than both the other
switching paths. It is self-evident that the switching-on
may he performed in the normal way simultaneously for each of
the three switching paths.
In Figure 1, a control mechanism 4 is illustrated which
controls the opening of the switches 1, 2 and 3 so as to open
the switch 1 at a given time and to open the two remaining
swltches 2 and 3 at a time subsequent to said given time
equal to a greater than at least one-third of the period of
the three-phase high voltage network frequency plus the
minlmum arclng time of the switch 1. One mechanism well
~ulted for adaptation for use as the mechanism 4 has been
descrl~ed ln Dutch patent application 76.06848 published
December 15, 1977 and the corresponding United States patent
no. 4,119,820 dated October 10, 1978. The mechanism needs
only modification in such a manner that the cam disks con-
trolling the switching-on and interruption actuate two
switching paths simultaneously whereas the third switching
path is separately actuated by a separate or displaced cam
disk. This separate cam disk may have such a configuration
that the pertaining switching path is interrupted by means
thereof at the desired time before the interruption of the
- other two switching paths. The switching-on can remain
unchanged.
It goes without saying that the switch according to the
invention is not restricted to the said switch.
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