Electricity: electrical systems and devices – Safety and protection of systems and devices – Arc suppression at switching point
Reexamination Certificate
1999-01-11
2001-09-25
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
Arc suppression at switching point
C361S006000, C361S115000
Reexamination Certificate
active
06295188
ABSTRACT:
The invention relates to an improvement to causing a hydraulically-controlled circuit breaker to close synchronously, in particular a very high voltage circuit breaker placed in an AC network.
BACKGROUND OF THE INVENTION
In most cases when such a circuit breaker is closed on a load placed in an AC network, transient current and/or voltage conditions are observed.
The amplitudes of such conditions depend on the nature of the load (line, reactance, transformer, bank of capacitors, . . . ) and also on the instant in the voltage period when connection actually takes place.
In order to minimize such transient conditions which give rise to voltage and/or current surges in the network, so-called “synchronous” closure controls have already been designed to close each pole of the circuit breaker in a manner that is controlled to coincide with an instant in the voltage period when the transient conditions are zero for voltage or for current. By way of example, in
FIG. 1
, applying voltage to a shunt reactance is optimum in terms of transient current conditions if voltage is applied at the instant marked t
2
which coincides with an instant when the voltage is at an extremum Vmax: under such conditions, the current that is established in the load at instant t
2
does not have any transient conditions and obeys the sinewave I=I
max
·sin &ohgr;(t−t
2
). This result can be obtained by applying two appropriate successive time delays starting from the instant marked t
0
at which the control system receives the order to close a pole:
a time delay K
1
measured from an instant when the voltage is zero; and
a time delay K
2
which is theoretically constant, corresponding to the time lapse required for closing the pole at an instant t
2
, as measured from instant t
1
. This time delay is associated essentially with the time required K
2
′ mechanically to drive the contacts of the circuit breaker, and to the prestriking time for the electric arc K
2
″ which is struck between the contacts of the pole (which time is always constant).
Successful synchronous closure of a circuit breaker pole therefore depends particularly on the duration K
2
′ required for the circuit breaker to operate under hydraulic control remaining constant over time. By way of example, for a network operating at 60 Hz, and when closing on a reactance, the maximum acceptable current under transient conditions requires closure to take place at a maximum voltage value Vmax with precision that is better than or equal to 1 ms.
It has been observed that in hydraulically-controlled circuit breakers using “very cold weather” oils, the repetitiveness of the mechanical time K
2
′ is reliable over the temperature range −40° C. to +50° C. with the required accuracy, providing switching operations are very frequent. However, if the circuit breaker has remained open-circuit for a long period of time (e.g. several months), experience shows that the mechanical time K
2
′ required on first closure following said standby period is subject to an amount of error that is incompatible with synchronous closure. The origin of this error is associated with the phenomenon known as “gumming” that occurs in the synthetic rubber sealing rings that serve to provide sealing around the outlet shafts of the hydraulic control system. This difficulty is particularly significant in that the parameters that influence this error, in addition to the standby time, are numerous and poorly known.
OBJECT AND SUMMARY OF THE INVENTION
The object of the invention is to remedy that difficulty in simple manner.
To this end, the invention provides a method of synchronously closing a hydraulically-controlled circuit breaker connected in an AC network, the method consisting, for each pole of the circuit breaker, in using a microcontroller connected to the hydraulic control system via static switches to perform the following steps:
on receiving a closure order, controlling the static switches to close said pole;
waiting for a signal indicating that said pole has moved a certain distance between its open position and its closed position;
on receiving said signal, controlling the static switches to cause said pole to open;
detecting when the network voltage passes through a zero value; and
waiting for a certain amount of time to elapse measured from said detection prior to controlling the static switches to cause said pole to close, said time lapse being such that full closure of said pole takes place at a moment that coincides with the instant at which the network voltage reaches a characteristic value depending on the nature of the load to be connected, e.g. an extremum when a shunt reactance is involved.
Advantageously, if a high degree of synchronous closure accuracy is required, the sequence—controlling the static switches to close said pole; waiting for a signal indicating that said pole has moved a certain distance between its open position and its closed position; on reception of said signal, controlling the static switches to open said pole—is repeated a plurality of times. Accuracy can be further improved if the microcontroller measures the time between the instant when it controls the static switches to close said pole and the instant when it receives said signal indicating a certain displacement of said pole between its open position and its closed position, and repeats said sequence until said measured time is equal to a prescribed value.
REFERENCES:
patent: 4306263 (1981-12-01), Gray
patent: 0 574 300 A1 (1993-12-01), None
patent: 2 711 279 A1 (1995-04-01), None
Alstom T&D SA
Jackson Stephen W.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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