JPH0723526A - Method and device for suppressing excitation surge for transformer - Google Patents

Abstract

PURPOSE:To provide an inexpensive power supply for nullifying the remanence by applying an AC attenuation voltage and to facilitate the work for connecting the power supply with a transformer. CONSTITUTION:An AC single phase power supply 3 is connected across one 22 of three-phase windings 21, 22, 23 of a transformer 100 equipped with a tripod core 10. When the transformer 100 is exited with an AC attenuation voltage from the power supply 3, a flux attenuating with time while inverting the sign alternately can be superposed for two core legs 11, 13 as well as the core leg 12 applied with the voltage. Consequently, the remanence component decreases as the flux component attenuates and the AC attenuation voltage goes zero and thereby the remanence is nullified for three core legs 11, 12, 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transformer exciting-current suppressing method and apparatus for reducing the exciting-rush current when a transformer, particularly a three-phase transformer, is turned on to a power source. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for suppressing a transformer exciting magnetic current by reducing the residual magnetic flux, which is another factor, to reduce the exciting magnetic current.

[0002]

2. Description of the Related Art As is well known, when a transformer is turned on, an exciting rush current of several times to several tens of times the rated current flows, which varies depending on the closing phase, and then decays with time to reach a steady state. It is known that a transient phenomenon of The magnitude of the exciting rush is not limited to the input phase,
It is known that when the power supply is cut off before that, the amount of residual magnetic flux remaining in the iron core of the transformer also greatly changes.

In particular, in the case of a three-phase transformer, it is known that an exciting rush current is generated in almost all cases regardless of the cut-off phase and the closing phase. Most of the large-capacity transformers of several thousand kVA or more such as industrial transformers for supplying electric power to the furnace are three-phase, and the change in the system voltage due to the occurrence of the magnetizing surge is simply the target transformer. It affects not only the substations and factories where is installed but also a wide range.

For these reasons, various measures have been adopted in order to reduce the magnetizing surge when the transformer is turned on. One of the measures for reducing the excitation rush current is to reduce or substantially reduce the residual magnetic flux. Even if the residual magnetic flux is reduced to zero, the exciting rush current does not disappear, but it has the feature that it can be reliably and easily reduced. The method of reducing or reducing the residual magnetic flux is to apply three-phase alternating current to the windings of the transformer, and then gradually reduce the voltage until it becomes zero. The method used is the same. According to this method, the residual magnetic flux can be substantially reduced to zero by setting the voltage applied first to a certain value or more.

Since the three-phase power source used for this purpose needs to reduce the voltage with time, a dedicated power source different from the power system to which the transformer is connected is used.

[0006]

As described above, a dedicated three-phase power source is used to reduce or reduce the residual magnetic flux in the iron core of the three-phase transformer. The three-phase power source is more expensive than the single-phase power source, and even when the three-phase power source is connected to the transformer, it is necessary to connect three terminals, and there is a problem that connection work takes time.

An object of the present invention is to solve the above problems and to provide a method and an apparatus for suppressing a transformer magnetizing rush current which is inexpensive and can reduce the time required for terminal connection work.

[0008]

In order to solve the above-mentioned problems, according to the present invention, a transformer provided with a three-phase tripod iron core becomes zero while reducing its voltage with a single-phase AC power source over time. Also, a transformer equipped with a three-phase, three-leg iron core is excited by a single-phase AC power supply before it is applied to the system, and its voltage is attenuated with time until it reaches zero and the core remains. The magnetic flux shall be reduced and then applied to the system, and the transformer winding shall be connected in a triangular shape, and 2 out of 3 terminals of this triangular connection shall be used.
A single-phase AC power supply shall be connected between the two terminals, and the transformer winding shall be star-connected, and one of the three terminals of this star-shaped connection and the neutral point terminal shall be connected. A single-phase AC power supply shall be connected between these terminals, and the transformer winding shall be star-connected, and one of the three terminals of this star-connection shall be one of the single-phase AC power supplies. Then, the remaining two terminals shall be connected to the other.

[0009]

In the structure of the present invention, when a transformer provided with a three-phase, three-leg iron core is excited by a single-phase AC power source until it becomes zero while attenuating the voltage with time, even if the applied voltage is single-phase. The voltage is shared among the phases other than the directly excited phase, and the magnetic flux is shunted between the iron core legs, so that an alternating magnetic flux corresponding to the applied voltage is generated in all three phases.
The amplitude of this magnetic flux is attenuated and the residual magnetic flux is reduced, and the applied voltage becomes zero and the residual magnetic flux can be made zero in all three iron core legs.

When the winding to which the voltage is applied has a triangular connection, a single-phase AC power source is connected between two terminals of the three terminals so that the two-phase iron core legs sandwich the terminals to which no voltage is applied. The magnetic flux is divided into approximately one half of the magnetic flux flowing in the iron core leg of the phase to which the voltage is directly applied. When the winding to which the voltage is applied has a star connection, connecting a single-phase AC power supply between one of the three terminals and the neutral point
A magnetic flux flows in the iron core leg of the phase to which the voltage is applied, and this magnetic flux is shunted to the other iron core legs by about half. If the neutral point is not extracted, connect one of the three terminals of the star connection to one end of the single-phase AC power supply, and connect the other two terminals to the other end together. Can also achieve the same effect as described above, and this connection configuration can be applied even when the neutral point terminal is drawn out.

[0011]

EXAMPLES The present invention will be described below based on examples.
FIG. 1 is a schematic diagram showing an embodiment of the present invention. In this figure, a transformer 100 includes a core 10 and windings 2 wound around three core legs 11, 12, and 13 of the core 10.
It consists of 1, 22, 23. Of course, in addition to this set, at least another set is wound around each of the iron core legs 11, 12, and 13, and two sets of windings form the primary side and the secondary side. , The other windings are not directly related when the residual magnetic flux or the exciting magnetic rush current is a problem, so that the illustration thereof is omitted.

The windings 11, 12, and 13 are connected in a triangular shape, and their terminals are named u, v, and w from the left as shown in the figure. The single-phase AC power supply 3 is connected between the v terminal and the w terminal. Although it may be between terminals of other phases, it is best to connect between these terminals as described later. Since the attenuated AC voltage E applied by the single-phase AC power supply 3 is applied between the v terminal and the w terminal, the voltage E is applied to the v-phase winding 22, and u
The voltage E is applied to the windings 21 and 23 of the phase and the w phase connected in series. Therefore, the magnetic fluxes Φ _u , Φ _{v of} each iron core leg 11, 12, 13 induced by the voltage E,
The following equation holds between Φ _w . However, Φ _E is the magnetic flux induced by the applied voltage E.

[0013]

[Equation 1] Φ _v = Φ _E

[0014]

## EQU2 ## Φ _u + Φ _w = −Φ _E The shunt ratio between Φ _u and Φ _w is determined by the magnetic resistance of each iron core leg. When connecting so that the voltage E is applied to the v-phase iron core 12 located at the center as shown in the figure, since the u-phase and w-phase iron cores 11 and 13 have a symmetrical relationship with respect to the iron core 12, The magnetic resistances are almost equal. Therefore, since Φ _u and Φ _w are substantially equal to each other, the following equation holds. This is the reason why the connection of FIG. 1 is the best as described above.

[0015]

## EQU3 ## Φ _u = Φ _w =-(1/2) Φ _{E The} AC voltage E is a damped AC voltage whose peak value is attenuated with the passage of time after being applied. Therefore, Φ _u , Φ _v , and Φ _w in the above two equations are also damping alternating magnetic fluxes that attenuate with similar waveforms. However, since the time derivative of the magnetic flux Φ becomes the voltage E, the phase of the magnetic flux Φ differs from the voltage E by 90 °.

As is well known, when there is a residual magnetic flux, if the damping alternating magnetic flux is superimposed, the magnetic flux converges to zero and the residual magnetic flux disappears. Therefore, the residual magnetic flux remaining in the iron core legs 11, 12, and 13 before the AC damping voltage E is applied is
When the AC damping voltage E is applied and attenuated to become substantially zero, the state becomes zero. Therefore, the exciting rush current when the transformer 100 is subsequently put into the system becomes smaller than that in the case where the residual magnetic flux remains. As aforementioned,
In the case of a three-phase transformer, since there is a residual magnetic flux that makes the exciting magnetic surge large in at least one phase, it is possible to considerably reduce the exciting magnetic flux by making the residual magnetic flux zero as described above.

Now, when the saturation magnetic flux of each iron core leg is Φ _s , the peak value of the magnetic flux in the steady state corresponding to the system voltage is Φ _m , and the residual magnetic flux is Φ _r , the maximum exciting current I _EM that can be generated is It becomes the following formula. However, k is a proportional coefficient with respect to the magnetic flux Φ of the exciting current I _E when the iron core leg is saturated.

[0018]

I _EM = k (Φ _r + 2Φ _m −Φ _s ) If Φ _r = 0.7Φ _s and Φ _m = 0.8Φ _s , then
I _EM = k (1.3Φ _s ) Further, when Φ _r = 0, I _EM = k (0.6Φ _s ), and when the residual magnetic flux Φ _{r is set} to zero, the maximum exciting rush current value becomes about half. Since these approximate values are not so far apart from the actual values, it is safe to say that the maximum exciting rush current that can be generated by reducing the residual magnetic flux to zero will be about half.

FIG. 2 is a schematic view showing another embodiment of the present invention. Since it is similar to FIG. 1, the same members are designated by the same reference numerals and the description thereof will be omitted. The difference between FIG. 2 and FIG. 1 is winding 2
4, 25 and 26 are star-shaped connections, and the connection terminals of the single-phase AC power supply 3 are different accordingly. Figure 2
Then, the single-phase AC power supply 3 is connected between the V terminal and the neutral point O terminal, and the attenuated AC voltage E is applied between them.
Since the attenuated AC voltage E is directly applied to the V-phase winding 25, a magnetic flux Φ _E corresponding to this is generated. Although no voltage is directly applied to the U-phase and W-phase windings 24 and 26, the magnetic flux Φ _V (= Φ _E ) of the iron core leg 12 is shunted to the iron core legs 11 and 13 to become Φ _U and Φ _W , respectively. A voltage corresponding to this magnetic flux is induced in the windings 24 and 26. Similar to the case of FIG. 1, the residual magnetic flux becomes zero due to these magnetic fluxes.

In the star-shaped winding, the neutral point O terminal is not always drawn out. In the case of a transformer having a buried neutral point, the single-phase AC power supply 3 is connected between the V terminal and the terminals that combine both U and W terminals. In the case of such connection, the windings 24 and 26 are connected in parallel, which means that they are connected in series with the winding 25, so that the voltage sharing ratio between the winding 25 and the windings 24 and 26 is 2: 1. Therefore, the voltage (2/3) E, which is two-thirds of the attenuated AC voltage E, is applied to the winding 25 of the V phase by U
(1/3) E is applied to the phase-and W-phase windings 24 and 26, and the magnetic flux Φ corresponding to this is applied to the respective iron core legs 11, 12, and 13. Voltage E and magnetic flux Φ
Since the sharing ratio for each phase is the same as that in the case of FIG. 2, the magnetic flux generated in the iron core legs 11, 12, and 13 is also applied by applying a voltage 1.5 times the value of the attenuated AC voltage E of FIG. Will be the same as

[0021]

As described above, according to the present invention, a three-phase transformer is connected to a single-phase AC power source, and a damping AC voltage is applied to the three-phase transformer. Since the magnetic flux can be generated, the residual magnetic flux of each iron core leg decreases with time, and the residual magnetic flux also becomes zero when the attenuated AC voltage becomes zero. Since the AC power supply to be used may be a single phase, it is inexpensive and the effect of facilitating the connection work to the transformer can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an embodiment of the present invention.

FIG. 2 is a schematic diagram showing another embodiment of the present invention.

[Explanation of symbols]

 100 transformer 10 iron core 11, 12, 13 iron core leg 21, 22, 23 winding 24, 25, 26 winding 3 single-phase AC power supply

Claims (5)

[Claims] 1. A transformer-excited surge current suppressor characterized in that a transformer provided with a three-phase tripod core is excited by a single-phase AC power source until the voltage becomes zero while reducing the voltage thereof over time. 2. A transformer provided with a three-phase three-leg iron core is excited by a single-phase AC power source in advance before being put into the system while the voltage is attenuated until it becomes zero while reducing the residual magnetic flux of the iron core. Then, the method for suppressing the transformer excited magnetic current is characterized in that the current is applied to the system. 3. The transformer winding is triangularly connected, and a single-phase AC power supply is connected between two terminals of the three terminals of the triangular connection. Method and apparatus for suppressing transformer magnetizing rush current described. 4. A transformer winding is star-connected, and a single-phase AC power supply is connected between one of the three terminals of the star-connection and the neutral terminal. 3. A transformer excitation surge current suppression method and device according to claim 1 or 2. 5. A transformer winding is star-connected, wherein one of the three terminals of this star-connection is connected to one of the single-phase AC power supplies and the remaining two terminals are connected to the other. The method and apparatus for suppressing a transformer excited rush current according to claim 1 or 2, characterized in that.