JPH0315406B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an excitation current suppression device for a transformer, and in particular to a device for reducing or preventing excitation inrush current that occurs when power is turned on in a transformer that has a load that requires frequent turning on and off, such as an electric furnace for steel manufacturing. Regarding auxiliary equipment. General power transformers are normally connected to the power supply system at all times, and there are few opportunities to turn them on or off, and only once every three to five years for maintenance, etc., so they are particularly important. Countermeasures against magnetizing inrush current are not a problem. However, transformers that load industrial electric furnaces such as steelmaking electric furnaces need to be turned on and off more than 30 times a day, and therefore, measures to suppress the magnetizing inrush current at the time of turning on are an important issue. becomes. The magnitude of this magnetizing inrush current varies depending on the transformer capacity, etc., but it is said to be 4 to 6 times the primary rated current for a 40 [MVA] class transformer, and this large current is applied to the transformer windings and It acts as an electromagnetic mechanical force on conductive parts such as circuit breakers, shortening their lifespans and causing unexpected accidents, and can also cause malfunctions of protective relays, resulting in a situation in which they cannot be turned on. In other words, when a transformer is put into a circuit, the exciting current does not immediately reach a steady state, but a transient phenomenon occurs;
The peak value may exceed five times the rated load current. This current is called magnetizing inrush current, and it acts on the differential relay that protects the transformer in a similar way to the current that occurs when there is an internal failure in the transformer, so if the current value becomes excessive, it can cause the relay to malfunction. A situation may arise where the transformer cannot be inserted into the circuit. The magnitude of this rush current varies depending on the point in the circuit voltage phase at which the transformer is turned on, the state of the residual magnetic flux in the iron core, and other factors. The largest inrush current occurs at P in Figure 1.
This is the case when the voltage is turned on at the moment when the voltage is zero. In this case, the maximum magnetic flux in the steady state is defined as φ _n
Then, during 1/2 cycle after turning on, the magnetic flux is 2φ _n
change. However, at the starting point of the magnetic flux, the residual magnetic velocity in the iron core before the transformer is turned on is φ _r , so in 1/2 cycle after the transformer is turned on, the magnetic flux becomes 2φ _n + φ _r ,
If φ _r is in the same direction as the magnetic flux change direction, the saturation magnetic flux of the iron core will be far exceeded, resulting in a large excitation inrush current. FIG. 1 shows this relationship, and shows a state in which the inrush current peak value increases from the point where the transient magnetic flux exceeds the core saturation magnetic flux. FIG. 2 shows temporal changes in magnetic flux and inrush current. The DC component included in the magnetic flux attenuates with time, and the peak value of inrush current also decreases accordingly. The duration of the inrush current is determined by the circuit resistance and inductance, and becomes longer as the capacity increases, but actual measurements show that it lasts more than 10 cycles for small capacity capacitors, and 5 to 10 seconds or more for large capacity capacitors. It may extend. Transformers using grain-oriented silicon steel strips tend to have a smaller ratio of saturation magnetic flux to steady-state magnetic flux and larger residual magnetic flux than non-directional ones, so the magnetizing inrush current generally increases. Figure 3 shows the capacity of the transformer and the calculated maximum inrush current that can occur. In reality, such a large inrush current is unlikely to occur for the following reasons. (1) There is little possibility that the circuit breaker will close at a point where the voltage is 0. (2) When inrush current flows, the voltage of the external circuit itself drops. (3) The residual magnetic flux is not necessarily the same as the voltage change direction,
Also, depending on the state of the external circuit, it may decrease. By the way, in a three-phase bank, a transient phenomenon always occurs in one of the three phases, so inrush current cannot be avoided no matter what moment the power is turned on. In general, in a core type transformer, the air core reactance of the inner winding is smaller, so the inrush current will be larger if the inner winding is excited. If this inrush current is large, the differential relay may malfunction as mentioned above, and to prevent this, there are methods of disconnecting the relay for a certain period of time after the transformer is turned on, and methods of inrush current and fault current. A method using a harmonic suppressing differential relay that discriminates between waveforms is used. However, such relay malfunction measures alone are insufficient as a countermeasure against magnetizing inrush current for electric furnace transformers that are frequently turned on and off, and for this reason, conventional measures have been taken to prevent the effects of voltage fluctuations caused by furnace transformers. to separate the power supply system,
A special four-winding transformer was used, but separating the power supply system would increase equipment costs, and even if a four-winding transformer was used, the transformer itself was special. Therefore, there is a need for a simple measure to suppress the excitation inrush current using a normal two-winding transformer. The present invention was made in view of the above situation, and
The object of the present invention is to provide an auxiliary device that can reduce or prevent the magnetizing inrush current of a transformer that is frequently turned on and off without using a special transformer. In order to achieve this object, in the excitation inrush current suppressing device of the present invention, before the main transformer connected to the load such as an electric furnace is connected to the main power supply circuit, the main transformer connected to the load is Auxiliary excitation in the same phase as the main circuit is applied to the winding directly or through the primary winding of the main transformer by a high-impedance input auxiliary circuit, and then the transformer is connected to the main circuit. be. The closing auxiliary circuit flows a limited in-phase excitation current for about 5 seconds to the secondary winding connected to the load in connection with the operation of the main circuit closing/opening circuit breaker, thereby causing the main transformer to This system synchronizes the residual magnetic flux of the transformer core with the power supply voltage to prevent magnetic flux saturation of the core, and prevents inrush current from occurring when the main circuit is turned on by closing the circuit breaker. It has the capacity to supply the no-load excitation current of the device. In one aspect, the input auxiliary circuit includes:
a contactor connected to the secondary winding of the main transformer;
and a high-impedance auxiliary transformer that, when connected to the secondary winding by the contactor, causes a limited current in the same phase as the main circuit to flow through the secondary winding, and connects the auxiliary transformer to a low voltage power source. After the secondary winding is auxiliary excited by a current synchronized with the main circuit phase, the main transformer is turned on by the main circuit breaker. In this case, a combination of a normal transformer and a reactor may be used as the high impedance auxiliary transformer. In this manner, the secondary windings of the main transformer are energized via high impedance circuits with in-phase limited currents prior to input into the main circuit. Preferably, while monitoring the transient current immediately after closing the contactor or closing the main circuit circuit breaker with the auxiliary circuit breaker open, if the current value exceeds a certain value, these closing operations are performed. By repeating the process again and adjusting the timing to turn on the main circuit, it is possible to turn on the main circuit with an even smaller inrush current value. To explain the present invention together with the drawings, FIG. 4 is a circuit diagram showing the first embodiment of the present invention, in which 1 is an electric furnace electrode as a load, 2 is a main transformer, 3 is a power supply main circuit, and 4 is a circuit diagram showing a first embodiment of the present invention. 5 is a circuit breaker for closing and opening the main circuit, and 5 is a disconnector. During normal electric furnace operation, the power supply current from the main circuit 3 flows to the primary winding of the main transformer 2 via the closed circuit breaker 4 and the disconnector 5, for example, 22 [KV].
A large current is supplied to the electrode 1 by stepping down the primary voltage of 460 [V] to a secondary voltage of 460 [V]. Here, the secondary winding of the main transformer 2 is connected to a closing auxiliary circuit 6, which is a main part of the present invention, so that the secondary winding of the main transformer 2 can be individually excited from the low voltage power supply circuit 7. be. The input auxiliary circuit 6 includes a high impedance auxiliary transformer 8 for phase adjustment connected to the low voltage power supply circuit 7, and a contactor 9 that selectively connects the secondary side of the auxiliary transformer 8 to the load electrode 1. ing. As an example, if the voltage of main circuit 3 is 22
[KV] The voltage of the low voltage power supply circuit 7 is 440 [V], the main transformer 2 has a capacity of 40 [MKV] (primary rated current 1050A), and the no-load excitation current characteristics as shown in Table 1 below and as shown in Fig. 5 are obtained. A specific example of the input auxiliary circuit 6 will be described below in the case where it has the voltage power characteristics as shown in FIG. 6 and the tap power characteristics as shown in FIG.

[Table] First, considering the strength of electrode 1, etc., and setting the starting conditions for main transformer 2 to tap 6 or less, the no-load excitation capacity is 67 [KVA] as shown in Table 1. The capacity of the transformer 8 is for primary excitation and secondary excitation.
Considering the air gap difference for the next excitation, approximately 80 [KVA], which is approximately 120%, and a time rating of approximately 15 seconds are sufficient. In addition, the impedance Z _T of the auxiliary transformer 8 needs to be high in order to suppress the excitation inrush current of the main transformer 2, and if the impedance of the main transformer 2 is assumed to be zero as a worst case condition, the maximum current of the auxiliary transformer If it is set to 250%, the impedance Z _T becomes 40%. Therefore, as the auxiliary transformer 8, the output secondary voltage
460 [V], no-load secondary voltage 644 [V], rated current 100
[A], Capacity 80 [KVA], Impedance Z _T 40%
(≒1.8Ω) specifications. In FIG. 4, when the main transformer 2 is connected to the main circuit 3 by the circuit breaker 4, the circuit breaker 4 is opened first, and the contactor 9 of the connection auxiliary circuit 6 is closed when the disconnector 5 is closed. . The rush current at this time is
It flows from the low voltage circuit 7 of 440 [V] through an auxiliary transformer circuit of impedance Z _T and is suppressed by impedance Z _T . In this case, it goes without saying that a reactor may be additionally inserted on the secondary side of the auxiliary transformer 8 to further reduce the rush current. In this way, the peak of the rush current is suppressed, and when the current drops below a certain value, usually after about 5 seconds, the circuit breaker 4 can be closed and the main circuit can be turned on smoothly. In this case, the value of the current flowing through the closing auxiliary circuit 6 immediately after the contactor 9 is closed is measured, and if the value exceeds a certain value, the contactor 9 is opened and then closed again. It is also possible to further reduce the inrush current value, which increases or decreases depending on the voltage phase at the time. The embodiment shown in FIG. 4 also has the advantage that the input auxiliary circuit 6 can be constructed only from low-voltage equipment. As described above, according to the present invention, by applying initial excitation to the secondary side with a limited current before turning on the primary side of the main circuit to the main transformer, transient conditions can be prevented. Even in such a case, the main transformer will not be saturated and generation of an excessive excitation inrush current can be prevented. Furthermore, by providing the equipment for initial excitation on the secondary side of the main transformer, the cost becomes cheaper compared to the high voltage equipment on the primary side. Therefore, it can be applied to transformers that supply power to loads such as steelmaking electric furnaces that are frequently turned on and off, to stabilize power supply systems, prevent accidents and extend the life of electrical equipment such as transformers, and stabilize operations. can be achieved.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing changes in the magnetic flux in the transformer core and voltage and current in a transient state to explain the excitation inrush current (rush current), Fig. 2 is a waveform diagram of the transient excitation current and magnetic flux, and Fig. 3 is a diagram showing the relationship between magnetizing inrush current and transformer capacity, FIG. 4 is a circuit diagram showing the first embodiment of the present invention, and FIG. 5 is a diagram showing the capacity as an example.
Voltage power characteristic diagram at tap 9 of a 40MVA main transformer, Figure 6 is also a tap power characteristic diagram, 1...load (electric furnace electrode), 2...main transformer,
3...Main power circuit, 4...Turn on the main circuit. Opening circuit breaker, 5... Disconnector, 6... Closing auxiliary circuit, 7...
...Low voltage power supply circuit, 8...Auxiliary transformer, 9...Contactor, 10...Reactor, 11...Auxiliary circuit breaker.

Claims (1)

[Claims] 1. A contactor that is connected to the secondary winding of the main transformer to which the load is connected and is turned on before the main transformer is connected to the main circuit, and connected to the secondary winding through this contactor, A transformer excitation inrush current suppression device characterized by comprising a closing auxiliary circuit consisting of a high impedance auxiliary transformer that performs auxiliary excitation by passing a limited current in the same phase as the main circuit through the secondary winding. .