Method for monitoring an on-load tap changer

Abstract

The invention relates to a method for monitoring an on-load tap changer that is used to switch between winding taps of a tapped transformer without interrupting the load current and is actuated using a motor drive. According to the invention, prior to the actual load switching process, the on-load tap changer is initially moved a short distance, preferably against the direction in which the load is subsequently switched. In said initial process, it is verified whether the on-load tap changer has moved, i.e. has left its previous stationary position, and/or whether the torque on the motor drive has exceeded a predefined threshold value. The actual load switching process takes place only once the test has yielded a positive result.

Claims

1. A method of operating and monitoring operation of a tap changer actuated by a motor drive for, only when the tap changer is operating properly, switching between winding taps of a tapped transformer, the method comprising the steps of: after receipt of a setting command for load switching but prior to starting load-switching, prophylactically checking whether the tap changer is functioning correctly by activating the motor drive such that its contacts will not shift all the way to another tap, subsequently checking whether the tap changer has moved or torque at the motor drive has exceeded a predetermined limit value, and, if the tap changer has moved or the torque at the motor drive has not exceeded the limit value, starting the actual load switching to a new winding tap, or if the tap changer has not moved or the torque at the motor drive has exceeded the limit value, generating a fault signal or stopping the tap changer.

2. The method according to claim 1, wherein after generation of the fault signal a predetermined number of load switchings is still permitted.

3. The method according to claim 1, wherein the motor drive comprises a step motor that for brief activation of the motor drive is excited by only a predetermined number of pulses.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The sole FIGURE of the drawing is a schematic flow chart of the method according to the invention.

SPECIFIC DESCRIPTION OF THE INVENTION

(2) In the starting position the tap changer is in the position n, i.e. it connects a (any) winding tap of the tapped transformer. Through a voltage regulator or the like it receives a command for a load changeover, here by way of example to the next tap n+1. The direction of the load changeover, i.e. from n to n+1 or alternatively from n to n−1, derives from the information of the setting command to increase or reduce the voltage.

(3) Before the start of the actual load changeover the tap changer is initially moved by a small amount in the opposite direction to the later load changeover, thus here in the direction n−1. The movement in opposite direction to the later load changeover is advantageous, but not necessary for the method according to the invention. A movement in this direction has the advantage of a higher acceleration for the subsequent load changeover due to the lengthened switch-over path.

(4) The small amount of the explained movement before the actual load changeover can be realized, in the case of use of a step motor in the motor drive, in that the step motor receives only a few pulses. In the case of a usual drive motor this can be effected by application of current over only a very short period of time of, for example, 100 ms. The small amount of the movement is to be dimensioned in all cases in such a manner that actuation of individual components and contacts of the tap changer, which takes place in a specific switching sequence for the load changeover, is still not commenced.

(5) On the basis of evaluation of the explained movement of the tap changer by a small amount the functional capability thereof can be determined even before the actual load changeover.

(6) Thus, it can be detected whether, for example, the tap changer moves at all. This can be determined by checking whether it begins to leave its previous stationary position n, in that the corresponding reporting contact is interrogated.

(7) The movement can also be determined by interrogation of an AOW sensor, a Hall sensor or an optical sensor.

(8) If the tap changer does not move, a fault situation is present: the drive train is interrupted, the drive motor is defective or the like.

(9) Alternatively or additionally it can be detected whether the torque at the drive motor in the case of the brief movement exceeds a limit value. This torque detection can be carried out mechanically, for example by a dynamo hub, or by evaluation, which is known from the prior art, of the load current. If the limit value is exceeded, a blockage, heavy motion or the like is present, thus a further fault situation.

(10) If these evaluations have a positive result the tap changer is in order; subsequently, the actual load changeover begins until, in the example illustrated here, the new position n+1 is reached.

(11) If a fault is recognized, there are basically two possibilities which the user can select: The tap changer can be stopped. Thus, no load changeover is possible until the fault is eliminated and a system reset undertaken. It is also possible to merely generate a warning report, but nevertheless permit further load changeovers, in certain circumstances limited to a specific number.

(12) Overall, the method according to the invention enables functional monitoring not retrospectively after a load changeover, but prophylactically before a load changeover. This increases operational safety and reliability of the energy supply in that even prior to a possible fault in a load changeover process this is recognized and the tap changer can be stopped as a precaution. As a consequence, until elimination of the fault then further voltage regulations are indeed impossible, but consequential damage or breakdown of the voltage supply will not be expected to happen.