Note: Descriptions are shown in the official language in which they were submitted.
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2~365-2728
ARRANGEMENT FOR CONTROLLING SWITCHGEAR
sackground of the Invention
The invention relates to an arrangement for control-
ling switchgear, preferably remote-controlled switchgear, in
dependence on state data generated by feeding an external volt-
age into a load and using an evaluating circuit connected
thereto to effect a comparison of reference and actual values.
The load may comprise a motor and its associated power lines.
In one known arrangement of the type mentioned a~ove
(DE-OS 33 47 209), a measuring voltage is applied to the load,
to which a current sensor is connected in series. By switching
to the individual phases, the impedance between the individual
phases and to ground is determined and the determined values
are fed to a processing stage. If a small voltage is used in
the arrangement, voltage flashovers and also shorted turns
(which occur only at higher voltages) cannot be detected. If,
however, a higher voltage is used, a relatively costly switch-
ing device for switching to the individual phases is necessary
since the full power must be switched.
OBJECTIVES A~D SUMMAR~ OF THE INVENTION
It is an object of the present invention to improve
the above-described arrangement in such a manner that the
detection of shorted turns becomes possible with a small amount
of circuitry. This is achieved in a simple manner by providing
a pulse generator for providing the external voltage. In order
to realize an even more reliable detection of shorted turns
which is insensitive to interference voltages, it is advanta-
geous if the evaluation circuit evaluates power. A relatively
simple and inexpensive evaluation of the power is obtained if
the evaluation circuit evaluates the peak values of the instan-
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taneous power. It is advantageous if the pulse generator
applies a pulse to all phases of a multi-phase group simulta-
neously, or alternately. In order to make use of one pulse
voltage generator and one evaluation circuit, the pulse voltage
generator may be equipped with a random generator for selecting
a single phase to be stressed. So that non-linear effects such
as turns shorted by partial arcs in the windings can also be
detected, it is advantageous if the protection or peak value of
the pulse voltage corresponds approximately to that of the
nominal peak voltage for the load. However, -the voltage should
also be chosen not substantially higher than the peak values of
the line voltage so as not to stress the windings excessively
and to thereby cause shorted turns. In order to minimize the
effect of stray inductances and the magnetic properties of the
lamination stack, it is advantageous if the amplitude spectrum
of the voltage pulse is in a frequency range of 50 to 1 kHz.
If the internal impedance of the pulse generator is large as
compared to the line and in particular a motor load, a voltage
as well as a current change results in the presence of shorted
windings. For realizing the pulse generator with only a few
components in a compact design, an arrangement is provided
wherein the pulse generator consists of a capacitor which can
be charged through rectifiers and which can be discharged
through an electronic switch into the load lines and in parti-
cular into the motor windings. In order to avoid a damped
transient which decays only after a few milliseconds, it is
advantageous if the output terminals of the pulse generators
are shunted by a bypass diode. Thereby, the pulse is cut off
after the first voltage zero crossing. A simple coupling of
the evaluation circuit is provided if the bypass diode is
shunted by a voltage divider and a shunt is
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connected into the line. A simple evaluation circuit is further
obtained if the evaluation circuit consists of a circuit for
sensing the current and the voltage, and an amplifier connected
thereto as well as of a subsequent multiplier circuit which is
i followed by a peak-voltage rectifier with a holding circuit. If
the amplifiers are provided with adjustable gain, the evaluating
circuit can be matched to the pulse generator and the motor
parameters in a simple manner. In order to simplify the circuit
further, it is of advantage if the evaluating circuit contains a
microprocessor which triggers the pulse generator and performs the
comparison with reference values as well as delivers the blocking
command. A state of the art use of digital techniques with
appropriate reliability is presented if an arrangement is provided
in which current and voltage values are digitized and evaluated
after multiplication corresponding to power waveform power, and
used to generate a blocking command. For this purpose, a method
has been found to be advantageous which consists of the provision
that aEter the pulse generator is triggered, the peak values of
the instantaneous power of the individual phaRe windings are
compared with previous measured values and the blocking command is
delivered in accordance with a predetermined deviation. If such
values are stored in the arrangement, -the measured peak values of
the instantaneous power can be corrected in dependence on the
rotor position.
In accordance with a broad aspect of the lnvention there
is provided an arrangement for controlling a preferably remote-
controlled switchgear used for feeding power to a load, comprising
a pulse generator connected to said load for feeding pulses to
said load and an evaluating circuit connected to said pulse
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generator to evaluate at least one response signal Erom said load
indicative of the instantaneous peak power supplied to said load
by said pulses from said pulse generator, said circuit including
comparison means Eor comparing said response signal to preselected
reference values, said comparison means generating a blocking
signal for blocking the switchgear when said response signal
exceeds said reference values.
In accordance with another broad aspect of the invention
there is provided a method of controlling a switchgear used for
providing power to a load comprising the steps of:
applying a pulse to said load;
evaluating an instantaneous peak power delivered to said load
through said pulse;
comparing said instantaneous peak power with a reference
value derived from previous evaluations; and
generating a blocking command to block said switchgear if
said instantaneous peak power exceeds said reference value by a
predetermined deviation.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will be described with
reference to the drawings wherein
Fig. 1 shows a preferred embodiment of circuit of the
pulse generator and the evaluation circuit,
Fig. 2 shows the peak power as a function of the number
of pulse stresses, and
Fig. 3 shows an alternative embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
The pulse generator with the evaluating circuit shown in
Fig. 1 consists of the isolating transformer 1, the secondary side
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2 of which is connected to a capacitor 5 via a rectifier 3 and a
resistor 4. The capacitor is connected via the thyristor 6 to the
windings 7 of the motor 8. The firing electrode of -the thyristor
6, shown by line 9, is connected to a control circuit 25. A
bypass diode 10 is connected in parallel to the output terminals
of the circuit and to the winding 7 of the motor. A voltage
divider 11 serves for taking off -the voltage and a shunt 12 serves
for sensing the current for the evaluating circuit. The voltage
divider 11 is shunted by a further capacitor 13. The current and
voltage values sensed by circuit 26 are amplified via amplifiers
14, 15 and fed to a multiplier circuit 16. A peak value rectifier
17 rectifies the multiplier output. The peak rectified value is
stored in a capacitor 18, and fed by an amplifier 19, to an
indicating instrument 20 and a comparison circuit 21 which
delivers a blocking signal. Because of the adjustable gains of
the amplifiers 14, 15, the evaluating circuit can be matched to
the pulse generator and the motor parameters in a simple manner.
Since the peak value reading is held, the holding member must be
reset before the pulse genera-tor is triggered for a new operator.
The firing electrode 9 of the thyristor 6 can be controlled by a
microprocessor in a manner not shown in detail. After the
thyristor is fired, the capacitor 5 is discharged via the
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! motor. The frequency of the undisturbed transient is
determined by the capacity and the inductance of the cable
motor wiring. The evaluating circuit measures the peak value
of the instantaneous power. In this connection, the number of
pulses, the design of the phase winding and the rotor position
~as well as, possibly shorted turns must be considered as
parameters. The stator and rotor laminations are either
¦demagnitized before the first pulse stress or are premagnetized
in some direction. Since the magnetizing curve of the iron has
a steep slope for small magnetic-field intensity, the
inductance of the motor in this region is large (L~d~/di
dB/DH). The current flowing during the pulse is thereby
limited. Upon being stressed by further unipolar voltage
pulses, the current and thereby, the power p increases since L
becomes smaller by the increasing magnetization. After about
the fourth stress, or pulse the ualue of p no longer increases,
see Fig. 2. With a possible repetition rate a final value can
therefore be obtained after 82 milliseconds. No significant
¦difference of the phase windings can be detected.
,¦ The peak values of the instantaneous power p show a
dependence on the rotor position which can be determined by
measurements or motors with known characteristics to
established reference values. By comparing the peak values of
the measured power with a threshold value which is based on the
reference values, conclusions can be drawn as to the state of
;the motor and its phases. If the peak values exceed the
reference values, a blocking signal is sent to block the
,switchgear.
i For all possible shorted turns on'winding higher p
,'values occur than with a motor free of defects. In the case of
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very large values, a terminal short or a short in the feeding
cable is indicated.
In the embodiment of Figure 1, the pulses from the
pulse generator are applied simultaneously to the three motor
windings as shown. In the alternative embodiment of Fig. 3,
three individual switches are provided, one for each phase
winding, and a random generator may be used to close arbitarily
any two of the three switches to test the windings. In both
figures 1 and 3, the main switches to the power generators a-e
not shown for the sake of clarity. Figure 2 shows the curves
obtained for three different phases of a motor. The curves are
not identical because of differences in measurement inaccura-
cies, as well as inherent asymmetries within the motor.
Obviously numerous modifications may be made to the invention
without departing from its scope as defined in the attached
claim.
