JPS5923454B2 - On-load tap changer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an on-load tap changer, and more particularly to an on-load tap changer equipped with a protection device that uses a reactor as a circulating current suppressing element during the tap changing process.

First, the structure of a conventional on-load tap changer equipped with the above-mentioned protection device will be explained in accordance with its operating order with reference to FIGS. 1a-c.

In Fig. 1a, numeral 1 indicates a tap winding of a transformer, etc.
2, 3, 4, and 5 are tap position N of tap winding 1.
and fixed contacts of the tap selector connected to the taps pulled out from tap position N-1, 6 and 7 are movable contacts of the tap selector that select the taps alternately, 8 is a current limiting reactor, 9 , 10 is a load switch.

Further, 11 and 1012 are current transformers, and 13 and 14 are respectively opened after a certain period of time after the load switches 9 and 10 are opened, and closed before the load switches 9 and 10 are respectively closed. configured short circuit switch, 1
Reference numeral 5 denotes a current detection device that detects the output of the current transformer 11 or 12 when the short circuit switch 13 or 14 is opened. The above structure is already known from, for example, Japanese Patent Laid-Open No. 46/5024, so only the parts necessary for explaining the present invention will be explained, and the rest will be omitted. 20 FIG. 1a shows the state in which the on-load tap changer is in operation at tap position N.

At this time, the load current flows through the electrical path shown by the chain line, and since the outputs of the current transformers 11 and 12 are short-circuited by the short-circuit switches 13 and 14, no signal is given to the current detector. In order to switch to 25-P-N-1, the load switch 9 is opened as shown in FIG. 1b. At this time, the load switch 9 commutates the load current to the load switch 10 with an arc. Next, the shorting switch 13 is opened as shown in FIG. 1c. At this time, if the arc of the load switch 9 is extinguished in the process shown in FIG. Therefore, no signal is given to the current detector 15. However, if the load switch 9 fails to interrupt the above-mentioned arc 35- for some reason, the arc continues as shown by the broken line in the load switch 9 in Figures 1b) and 1c. If so, a current signal of a magnitude corresponding to the transformation ratio of the current transformer 11 and the current value flowing through the load switch 9 at that time is generated as shown in Fig. 1c.
The current is sent to the current detector 15 as shown by the broken line. If the current detector 15 detects the current, it is possible to issue a signal to stop the operation of the tap selector configured to subsequently operate and to disconnect the transformer etc. from the power supply circuit. In FIG. 1c, if there is no abnormality in the load switch 9, the tap switching operation continues. The following steps are well known in the above-mentioned publications, and will therefore be omitted. However, the conventional on-load tap changer as described above has the following drawbacks.

That is, in the explanation of c above, if the load switch 9 successfully interrupts the arc, the primary current becomes zero at the commercial frequency natural zero point of the arc current, and the current detection connected to the secondary circuit of the current transformer 11 Although we have written that no current flows through the transformer 15, strictly speaking, the secondary current of the current transformer has a phase difference with the primary current due to its excitation current, so at the same time, the It is impossible to reach the zero point. FIG. 2 is a diagram showing waveforms of current flowing through the current transformers 11, 12 and the current detector 15. Since current transformers 11 and 12 are not ideal current transformers, their secondary currents (FIG. 2b) lag their primary currents (FIG. 2a) by a phase φ, as shown. (
This is detailed in "Instrument Transformer" P20-24 published by Denkishoin in 1955. ) Therefore, even if the primary current reaches the zero point, the secondary current has not yet reached the zero point. When the primary current is suddenly cut off to zero, the secondary current slowly decays from its value at the time of the cutoff.

That is, if the short circuit switch 13 is opened at such a point, a current containing a DC component as shown in FIG. 2 flows through the current detector 15. In addition, during the tap switching process, there is a process in which the current limiting reactor 8 is connected to the circuit by the load switch 9 or 10, but at this time, a direct current component transiently flows into the primary circuit of the current transformers 11 and 12. Superimposed currents may flow. Such a current is equivalent to DC exciting the iron cores of the current transformers 11 and 12. If this DC excitation is repeated, in the worst case, the operating points of the current transformers 11 and 12 may become near the saturation magnetic flux density, but in any case, DC excitation further reduces the phase difference φ of the secondary current. It becomes something that increases.
In other words, the primary current of the current transformer reaches zero point at a certain point,
After that, when the current is interrupted, the above-mentioned current remaining in the secondary circuit of the current transformer acts as a DC electromotive force in the secondary circuit of the current transformer, even though the primary current is not flowing. , a DC transient current as shown in FIG. 2b flows. As shown in FIG.
If the DC current is not sufficiently attenuated by the time it opens, this DC current will be commutated to the current detector 15, resulting in malfunction. The above phenomenon can be expressed mathematically as follows. FIG. 3a shows a Laplace-transformed equivalent circuit from when the negative current of the current transformer reaches a zero point until the short-circuit switch 13 or 14 is opened. L1・i(-o)=(
R+SLl)・i(S) Inverse Laplace transform, i(t)
However, L1: inductance seen from the secondary terminal of the current transformer R1: current transformer} and resistance 1 (-
o): Instantaneous value of the secondary current at the primary current zero point of the current transformer FIG. 3b shows the Laplace-transformed equivalent circuit after opening the short-circuit switch 13 or 14.

The equivalent circuit of a current detector is treated as a capacitive and resistive load. In the same manner as in FIG. 3a, the current flowing through the current detector in FIG. 3b is determined by the following equation. However, R.O.
Rl+r1+R2 2,: Stray inductance of the electric line for connecting the current detector R,: Resistance c of the electric line for connecting the current detector
: It is based on the capacitance seen from the human terminal of the current detector.

Hereinafter, one embodiment of the present invention will be described in detail based on the drawings.

FIG. 4a shows the first embodiment of the present invention, and FIG. 4b shows the second embodiment.
An example is shown below. In the figure, reference numeral 16 denotes rectifying elements connected in antiparallel, and the other configurations are the same as the conventional one shown in FIG. 1A. As shown in Figure 5, the voltage-current characteristics of a rectifying element generally causes almost no current to flow below a certain voltage (threshold level), then passes through a non-linear resistance characteristic region, and then very little current flows. It can be expressed as a curve leading to a linear resistance characteristic region with differential resistance. Since the residual magnetic energy in the current transformer core after the primary current zero point mentioned above is weak, by connecting the rectifying element 16 as shown in Figure 4a, it can be used as a back electromotive force to limit the problematic transient DC current. When a threshold level voltage acts on the rectifier and therefore the current decreases, a large differential resistance in the nonlinear resistance region of the rectifying element becomes R as shown in equation (1) above.
The same effect can be obtained by increasing the value, and the attenuation can be accelerated. In addition, the threshold level is usually less than 1V, so it is 15 to 40A, which is usually used for electric power.
If a similar current transformer is used, the increase in load on the alternator due to the insertion of the rectifying element 16 will be almost negligible, and the economical effect will be great. According to experiments conducted by the inventors, it has been confirmed that the decay time constant of the DC transient current can be reduced to 1/10 to 1/100 of that of the conventional method using the above method. In FIG. 2b, the dot-dash line shows the state of the transient direct current when the improvements according to the present invention are completed. Next, in the second embodiment shown in FIG. 4B, a rectifying element 16 is inserted in series with the current detector 15 and connected in antiparallel thereto.

In this case, it is not possible to reduce the transient DC component to zero before opening the short-circuit switch 13 or 14, but the current detector 15
Depending on the operating sensitivity of the current detector 15, the transient DC current may be
It is possible to limit the operating sensitivity to a value less than or equal to the operating sensitivity of will occur. Hey, Figure 4 a} and Figure 4
Figure BVC. }, although an example using a rectifying element has been described, it goes without saying that the same effect can be obtained by using an element having similar non-linear resistance characteristics, such as a varistor. Furthermore, other embodiments are shown in FIGS. 6a to 6c.

In these figures, 17 is a resistor, 18 is a reactor,
All other components are the same as those in FIG. 1a above.
By inserting the resistor 17 at the position shown in FIG. 6a, R in the above-mentioned equation (1) becomes larger, so that the attenuation becomes faster, and the same effect as that of a rectifying element can be obtained. In addition, by inserting a reactor in series with the current detector as shown in Figure 6b, the current flowing to the current detector after opening the short switch is limited, as predicted from equation (2) above. You can. Further, if this reactor is a saturable reactor, the same effect as in the above embodiment can be achieved without increasing the load on the current transformer. Furthermore, by inserting a resistor in series with the current detector 15 as shown in FIG. 6c,
This can accelerate the attenuation of the problematic transient DC current, and may be effective depending on the operating sensitivity of the current detector.

As described above, according to the present invention, it is possible to sufficiently attenuate the DC transient current flowing in the secondary circuit of a current transformer, which was a drawback of the conventional method, and to reliably prevent malfunctions due to transient current commutation. I can do it.

[Brief explanation of drawings]

Figures 1a-c are electrical circuit diagrams showing the circuit configuration of a conventional on-load tap changer according to its operating order, and Figures 2a-c are disadvantages of the configuration shown in Figure 1 and the effects of the present invention. 3A and 3B are equivalent circuit diagrams for mathematically explaining the drawbacks of the conventional example, and FIG. 4A is a current waveform diagram for explaining the disadvantages of the conventional example.
Electrical circuit diagram of the on-load tap changer according to the embodiment of
Figure b is an electric circuit diagram of the second embodiment, Figure 5 is a graph showing the characteristics of the rectifier elements in Figures 4A and b, and Figure 6a is
is an electrical circuit diagram of an on-load tap changer according to a third embodiment of the present invention, FIG. 6b is an electrical circuit diagram of an on-load tap changer according to a fourth embodiment of the present invention, and FIG. FIG. 5 is an electrical circuit diagram of an on-load tap changer according to a fifth embodiment of the present invention. 1... Tap winding, 9, 10... Load switch, 11, 12... Current transformer, 13, 14...
... Short circuit switch, 15 ... Current detector, 16 ... Rectifying element, 17 ... Resistor, 18 ... Reactor.

Claims (1)

[Claims] 1 Load switching that is connected to a tap winding of a transformer via a current limiting reactor and commutates the load current from a contact associated with a first tap position of the tap winding to another contact adjacent to this first tap position. a current transformer that detects the current of the load switch, a current detector that detects the current flowing in the secondary circuit of the current transformer at a time when the load current should be commutated, and the load switch. The secondary circuit is connected to the secondary circuit of the current transformer through a short circuit switch that is controlled to open after a predetermined time after the switch is opened and to close before the load switch is closed. An on-load tap changer consisting of a current limiting element that limits the current flowing to the next circuit.