JPH0417029B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a power conversion device in which a smoothing reactor is interposed in a DC intermediate circuit between a thyristor rectifier on an AC power source side and a thyristor inverter on an AC load side.

[Conventional technology]

FIG. 2 is a circuit diagram showing a conventional example of an induction furnace power source, which is one of the main uses of this type of power converter. A thyristor inverter 4 is connected to the DC output side of a thyristor rectifier 2 connected to an AC power source 1 via a smoothing reactor 3, and this inverter has the following characteristics:
Power is supplied to the heating coil 6 of the induction furnace 5. A capacitor 7 for power factor improvement is connected in parallel to the heating coil 6.
The load circuit thereby forms a parallel resonant circuit. A control circuit 31 that supplies firing pulses to the thyristors of the inverter 4 synchronizes with the load voltage (resonance voltage) detected by the voltage transformer 10 and performs control to keep the control advance angle or commutation margin angle constant. conduct. Generally, inverter 4 is operated at a high output frequency. On the other hand, the control circuit 21 that supplies the ignition pulse to the thyristor of the rectifier 2 controls the voltage regulator 19
Under the action of , the firing phase of the thyristor is controlled so that the magnitude of the inverter output voltage is kept constant. For this purpose, the input section of the voltage regulator 19 receives the voltage setting signal from the voltage setting device 11,
The voltage detection signals detected by the voltage transformer 10 and the rectifier circuit 18 connected thereto are compared, and the voltage regulator 19 functions to eliminate voltage control deviations obtained from the comparison results. Furthermore, for overcurrent protection, the current detected by the current transformer 9 provided on the AC power source side of the rectifier 2 is converted into a DC signal by the rectifier circuit 15.
It is input to the current limiter 16. This current limiter 16 has the function of forcing the voltage regulator 19 to lower the voltage to keep the current near the limit value when the current detection signal exceeds a preset limit value. do. Furthermore, for overvoltage protection, an overvoltage protection thyristor 52 is connected between the input terminals of the inverter. This thyristor 52 is ignited by an overvoltage detection circuit 53 that receives a voltage detection signal obtained from the voltage transformer 10 via the rectifier circuit 18 . That is, the overvoltage detection circuit 53 fires the thyristor 52 in response when the voltage detection signal exceeds a preset limit value, thereby short-circuiting the DC input terminals of the inverter and reducing the current in the smoothing reactor 3. is bypassed, and energy injection into the resonant circuits 6 and 7 via the inverter 4 is blocked. As a result, in the resonant circuits 6 and 7, the voltage attenuates in accordance with the energy consumption due to the loss.
The overcurrent of the smoothing reactor 3 during the short circuit operation by the thyristor 52 is reduced by the current limiter 16.
This can be prevented by the action of As another conventional technique for overvoltage protection, the overvoltage detection signal from the overvoltage detection circuit 53 is interrupted by the inverter control circuit 31, and the thyristor pairs of the upper and lower arms, which are connected in series with each other, are fired at the same time. Methods of forming short circuits are also known.

[Problem to be solved by the invention]

However, in applications such as an induction furnace power supply that supplies power to a load that fluctuates rapidly, the above-mentioned overvoltage protection means cannot withstand overvoltages with a steep rise due to the unavoidable response delay of the overvoltage detection circuit 53. Reliable protection may not always be possible, and in the case of the prior art using the overvoltage protection thyristor 52, there is a problem in that the thyristor 52 must have a large capacity. In addition, in the case of the conventional technology in which the series arms in the inverter are simultaneously activated instead of the overvoltage protection thyristor 52,
Due to the resonant voltage remaining in the load, the thyristors that are fired at the same time are extinguished and the short circuit operation cannot be continued. As a result of various investigations and examinations into the cause of the occurrence of overvoltage with a steep rise, it was found that it occurs when the DC intermediate circuit continues to operate in an intermittent current state. The current intermittent state also occurs at the beginning of the current rise when the power conversion device is started and at the end of the current cutoff when the power conversion device is stopped, but this does not persist to the extent that it becomes a problem. During operation, the voltage control loop by the voltage regulator 19 adjusts the current injected into the parallel resonant circuit via the inverter in such a way that the resonant circuit voltage remains constant. In other words,
The lighter the load, the smaller the current injected into the parallel resonant circuit. Since the voltages appearing at the input sides of the rectifier 2 and the inverter 4 pulsate, the current in the DC intermediate circuit is also affected by the pulsations. Although current pulsations are suppressed by the smoothing reactor 3, they cannot be completely eliminated. Therefore, when the current required to maintain the voltage becomes small due to a light load, even if the average value of the DC intermediate circuit current is not zero, the instantaneous value becomes an intermittent waveform that repeatedly reaches zero. In the expected normal load range, a smoothing reactor with an inductance value that does not cause such current interruption is used. However, in reality, the reactor is usually selected from among standard products, and the reactor with the optimum value for the capacity and load of the inverter is not necessarily used. Furthermore, in induction melting furnaces, addition of materials,
Sudden load fluctuations occur when hot water is melted down and the water is turned on, and this cannot be avoided. In addition, voltage control systems are prone to hunting due to such sudden load fluctuations, and the more responsive voltage control is attempted, the greater this tendency becomes. Mutual interference that occurs during operation also causes hunting in the voltage control system. In this way, when hunting in the voltage control system occurs in combination with a sudden change in load, the DC intermediate circuit current becomes intermittent. When the current falls below the holding current of the thyristor in the intermittent state, the thyristor, which should be conducting, is extinguished and the current path is cut off, resulting in a high voltage being generated by the energy remaining in the smoothing reactor. The purpose of the present invention is to focus on the fact that the cause of the generation of such high voltage is due to intermittent current conditions, and to prevent the thyristor of the power converter from being destroyed before such dangerous high voltage occurs. The aim is to provide protective measures that can prevent

[Means to solve the problem]

A power conversion device according to the present invention that achieves the above object has the following features. That is, a thyristor rectifier 2 that is connected to an AC power source and converts AC to DC, a thyristor inverter 4 that converts DC to AC and supplies power to an AC load, and a DC intermediate circuit between the rectifier and the inverter. a smoothing reactor 3; a rectifier control means 21 for supplying a firing pulse to the thyristor of the rectifier; an inverter control means 31 for supplying a firing pulse to the thyristor of the inverter; and detecting the magnitude of the output voltage of the inverter. voltage detection means 10, 18; voltage setting means 11 for setting the output voltage of the inverter; and voltage detection means 11 for setting the output voltage of the inverter; voltage regulating means 19 that acts on the rectifier control means so as to eliminate the deviation; current intermittent detection means 9, 15, 24 that detects the current intermittent state of the DC intermediate circuit; and current from the current intermittent detection means. a shift signal generating means 25 for supplying a shift signal to the rectifier control means so as to forcibly operate the rectifier in an inverse conversion region in response to the intermittent detection signal; and a current generated by the current intermittent detection means at startup. It is characterized by comprising means 14 and 26 for preventing the shift signal generating means from responding to the intermittent detection signal.

[Effect]

According to the present invention, when the current intermittent detection means detects current interruption, this is immediately transmitted to the shift signal generation means, and the shift signal generation means provides a shift signal to the rectifier control means to cause the rectifier to operate in the inverse conversion region. let As a result, the DC intermediate circuit current is quickly and completely cut off from the intermittent state. Therefore, the intermittent state does not occur intermittently, and overvoltage is prevented from occurring. If the shift signal generating means responds to the current intermittent detection signal from the current intermittent detection means at the time of startup, the shift signal generating means cannot be started, so the response of the shift signal generating means is blocked for a while immediately after the startup command is given.

【Example】

FIG. 1 is a circuit diagram showing an embodiment of a power conversion device according to the present invention. In this figure, elements with the same reference numbers as in FIG. 2 indicate the same elements as in FIG. According to the present invention, the current intermittent detection circuit 24 is connected to the output side of the rectifier circuit 15 connected to the current transformer 9. This current intermittent detection circuit 24 can be configured with a comparator, for example. When the current corresponding to the DC intermediate circuit current detected from the current transformer 9 via the rectifier circuit 15 reaches an instantaneous value close to zero, this comparator responds,
Generates a current intermittent detection signal. A shift signal generating circuit 25 is connected downstream of the current intermittent detection circuit 24, and the shift signal generating circuit 25 provides a shift command signal to the rectifier control circuit 21 in response to the current intermittent detection signal. When the rectifier control circuit 21 receives the shift command signal, the voltage regulator 1
Regardless of the control signal from 9, the rectifier 2 is forced to operate in the inverse transformation region (particularly at a predetermined minimum control advance angle). As a result, smooth reactor 3
The energy is regenerated to the AC power supply side, and the DC intermediate circuit current quickly reaches complete zero. Therefore, hunting occurs in the voltage control system along with rapid load fluctuations due to addition of materials in the induction furnace 5, melting through, and tapping of hot water while the current is on.
As a result, when the current interruption starts, the current interruption is immediately stopped and a transition to a complete current zero state is performed. In this case, the output signal of the current intermittent detection circuit 24 can also be applied to the alarm 28 to activate the alarm 28. If the shift signal generation circuit 25 responds to the current discontinuity that occurs at the initial stage of current rise at startup, it will not be possible to start, so in order to avoid this, there is a means for blocking the transmission of the current discontinuity detection signal to the shift signal generation circuit 25 at the time of startup. is necessary. A timer circuit 26 is added as this blocking means, and this timer circuit, in response to the operation command switch 14, controls the current intermittent detector 24 until a predetermined period of time elapses from the time the switch is turned on. It functions to prevent the generation of intermittent detection signals. Note that the current limiter 16 shown in FIG.
Although it is preferable to provide this in the embodiment of the present invention, it is omitted in FIG.

【Effect of the invention】

As described above, according to the present invention, there are provided means for detecting intermittent smoothing reactor current, means for generating a shift signal for operating a rectifier in an inverse conversion region in response to an output signal of the smoothing reactor current, and means for detecting intermittent current at startup. By means of a simple solution consisting of means for preventing the shift signal generating means from responding to the detection signal, a shift signal is immediately applied to the rectifier control means upon detection of a current interruption to cause the rectifier to operate in the inverse conversion region. As a result, the DC intermediate circuit current is quickly released from the intermittent state and is completely cut off. Therefore, the intermittent state does not continue, and overvoltage is prevented from occurring. If the shift signal generating means responds to the current intermittent detection signal from the current intermittent detecting means at the time of startup, the shift signal generating means will not be able to start, so the response of the shift signal generating means will be blocked for a while immediately after the startup command is given.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a power converter according to the present invention, and FIG. 2 is a circuit diagram showing an embodiment of a conventional power converter. 1... AC power supply, 2... Rectifier, 3... Smoothing reactor, 4... Inverter, 5... Induction furnace, 6
... Heating coil, 7 ... Power factor correction capacitor, 9
...Current transformer, 10...Voltage transformer, 11...
Voltage setting device, 14... Operation command switch, 15...
... Rectifier circuit, 16 ... Current limiter, 18 ... Rectifier circuit, 19 ... Voltage regulator, 21 ... Rectifier control circuit, 24 ... Current intermittent detector, 25 ... Shift signal generator, 26 ... Timer circuit, 28... Alarm, 31... Inverter control circuit.

Claims (1)

[Claims] 1. A thyristor rectifier 2 that is connected to an AC power source and converts AC to DC; a thyristor inverter 4 that converts DC to AC and supplies power to an AC load; and DC between the rectifier and the inverter. a smoothing reactor 3 interposed in the intermediate circuit; a rectifier control means 21 for supplying a firing pulse to the thyristor of the rectifier; an inverter control means 31 for supplying a firing pulse to the thyristor of the inverter; Voltage detecting means 10, 18 for detecting the magnitude; Voltage setting means 11 for setting the magnitude of the output voltage of the inverter; A setting signal from the voltage setting means and a voltage detection signal from the voltage detecting means. voltage adjusting means 19 that acts on the rectifier control means so as to eliminate the deviation according to the deviation between them; current intermittent detection means 9, 15, 24 that detects the current intermittent state of the DC intermediate circuit; a shift signal generating means 25 for supplying a shift signal to the rectifier control means so as to forcibly operate the rectifier in an inverse conversion region in response to a current intermittent detection signal from the detection means; A power conversion device comprising means 14 and 26 for preventing the shift signal generating means from responding to a current intermittent detection signal generated by the means.