JPH11289775A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an excitation rush current from being generated in a converter transformer at the time of starting by boosting the AC voltage such that a DC magnetization does not take place in the transformer based on the flux of the transformer measured while starting with a reduced AC voltage. SOLUTION: In a power converter 1, a control signal (c) for a converter 11 is multiplied by 6 (voltage reduction rate is about 0.3) through a multiplier 82 before being inputted to a PWM modulator 75. Output voltage of the converter 11 is thereby reduced by a factor of 6 and arush current due to a DC magnetization does not flow regardless of the remanent magnetic flux of a converter transformer 2. An excitation detector 73 determines mean value of exciting current over 1 cycle and then determines the remanent magnetic flux of the converter transformer 2. Furthermore, the magnetic flux is used for determining the time when that flux is obtained through excitation without a DC magnetization. Subsequently, the control signal is inputted, as it is, to the PWM modulator 75 to eliminate a DC magnetization from the flux in the converter transformer. The transformer 2 has AC side voltage equal to the system voltage and it can be interlinked with the system by throwing in a circuit breaker 3 (AC voltage rise).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter, and more particularly, to a voltage-type self-excited converter having a transformer for the converter.

[0002]

2. Description of the Related Art A voltage-type self-excited converter is used as an AC power supply because it operates as an AC voltage source. Therefore, various measures for suppressing the magnetization are taken.

However, even in such a case, there is a possibility that the transformer may be saturated depending on the voltage application start phase at the time of starting the power converter, and an inrush overcurrent may be generated. For this reason, a countermeasure such as providing a gap in the transformer for the converter to reduce the residual magnetic flux, and using a voltage pattern that does not cause the magnetic polarization has been taken. An example of the countermeasure is JP-A-10-14247.
No. power converter.

[0004]

However, in the countermeasure example shown here, it is assumed that the residual magnetic flux is 0, and the problem cannot be applied when there is a residual magnetic flux such as a gapless transformer. There is. Countermeasures can be taken by detecting the residual magnetic flux itself and adjusting the voltage application phase so that no demagnetization occurs. However, in a converter in a stopped state, the residual magnetic flux of the converter transformer is statically and highly reliable. Difficult to detect in degrees.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to firstly eliminate the gap of a transformer for a converter so that an ordinary transformer can be used. The second object is to make it possible to increase the magnetic flux density used by the transformer for the converter. Third, a special device for detecting the magnetic flux of the transformer is not required. Fourth, it is to prevent an inrush current to the transformer when the power converter is started. Fifth, the starting time of the power converter is reduced. As a means for this, the AC voltage at the start of the converter is reduced. The reduction amount is 50% or less. The magnetic flux of the transformer for the converter is measured from the exciting current of the transformer for the converter operating at the reduced voltage. The magnetic flux is returned to the residual magnetic flux at the start of the operation by setting the operation period to an integer cycle. The voltage is reduced to zero to maintain the magnetic flux. When the magnetic flux when normally excited becomes equal to the measured magnetic flux, operation is performed at a normal AC voltage.

The function of the present invention is to prevent the inrush current of the transformer for the converter at the start of the start by reducing the AC voltage at the start of the converter. By setting the reduction amount to 50% or less, the transformer for the converter is prevented from reaching saturation. The magnetic flux of the converter transformer can be measured from the exciting current of the converter transformer operating at the reduced voltage. The magnetic flux is returned to the residual magnetic flux at the start of the operation by setting the operation period to an integer cycle. By reducing the voltage to zero, the magnetic flux is kept at a constant value. By operating at a normal AC voltage when the magnetic flux when normally excited becomes equal to the measured magnetic flux, the converter transformer is excited without being demagnetized.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation of the present invention will be described in detail.
For simplicity, a single-phase case will be described.

A transformer having a residual magnetic flux Φ (the magnetic flux is represented by a linkage flux in consideration of the number of windings of the transformer) has an effective value U,
When an alternating voltage u of phase θ = √2 × U × sin (ω × t + θ) (1) is applied from t = 0, the magnetic flux φ of the transformer becomes φ = − (√2 × U / ω) {cos (ω × t + θ) −cos θ} + Φ (2)

When the voltage is represented by% with respect to the rated voltage and the magnetic flux is represented by% with respect to the magnetic flux amplitude when excited at the rated voltage, the above equation can be expressed as: φ = −U {cos (ω × t + θ) −cos θ} + Φ (3)

As a result, the residual magnetic flux .PHI. And the magnetic flux Ucos .theta. Due to the phase at the start of voltage application are added as a transient term. For example, when .theta. Is -.pi./2 to .pi. / 2 radian and .PHI. The magnetic flux is U (1 + cos θ) + Φ (4). In particular, when θ = 0, the maximum magnetic flux is 2U + Φ, and when excited at the rated voltage, the magnetic flux of the transformer greatly exceeds the saturation level, and an excessive inrush current flows. This relationship is shown in FIG.

For this reason, conventionally, in a transformer for a converter, a gap has been provided in an iron core so that the residual magnetic flux is substantially zero. However, even if it does so, the power converter is 100%
It can be seen that when started with voltage, the transformer for the converter is excited to 200% magnetic flux. For this reason, in order to prevent the inrush current of the excitation, it is necessary to manufacture the transformer for the converter such that the excessive excitation current does not flow even with the magnetic flux of 200%. Since a normal transformer is manufactured so as to start to be saturated at about 120%, an exciting current of about 150% results in an excessively large exciting current. For this reason, the conventional transformer for the conversion device had to be a large iron core transformer.

In the case of a normal transformer without a gap, residual magnetic flux remains. The residual magnetic flux is about 80% at the maximum. Assuming that the magnetic flux is set to 150% or less so that an excessive inrush current does not flow in the transformer for the converter, 2U + Φ = 2U + 80 ≦ 150 (5) According to (5), the AC voltage at startup must be 35% or less. No. This number also depends on the transformer design. If the iron core is made large, even 50% does not need to be demagnetized. From this, if the AC voltage U is started at 50% or less, an excessive inrush current is not generated in the transformer.

As described above, if the power converter is started with the AC voltage reduced, even if the transformer for the converter does not have a gap, no exciting inrush current is generated. Further, the iron core of the transformer for the converter can be made as large as a normal transformer or slightly larger.

Next, when the transformer is excited, the magnetic flux of the transformer can be measured without using a special magnetic flux measuring device by measuring the exciting current. It can also be measured by estimating from the relationship between the applied voltage and the instantaneous value of the exciting current. A simple and reliable method is to calculate the average value of the exciting current in one cycle, that is, the DC component. Even if the excitation characteristics of the iron core are nonlinear, the average magnetic flux Φ + U × cos θ can be estimated and measured by referring to a table from the average value of the excitation current. From this result, U × cos θ which is known as the output voltage of the converter is subtracted to obtain only the residual magnetic flux Φ.

It should also be noted that the magnetic flux of the transformer returns to almost the initial value of the residual magnetic flux when it is excited for one cycle. Not only can the residual magnetic flux Φ be measured by exciting at a low voltage for one cycle,
You can almost return to the original state. This is the same for the excitation of integer cycles.

Therefore, after measuring the residual magnetic flux, the AC voltage is temporarily reduced to zero. The transformer is maintained in a state of residual magnetic flux Φ.

If the transformer is excited so as not to be demagnetized without generating transient conditions such as residual flux, the magnetic flux ψ
Is ψ = −Ucos (ω × t + θ) (6) Assuming that the AC voltage U is a 100% voltage, the residual magnetic flux Φ is about 80% at the maximum, and there is a time point T at which the value of ψ becomes equal to the residual magnetic flux Φ. 100% from T
Is applied, the actual magnetic flux φ becomes φ = −U {cos (ω × t + θ) −cos (ω × T + θ)} + Φ = −Ucos (ω × t + θ) (7) ∵−Ucos (ω × T + θ) = Φ (8), which is the same as ψ, and can be excited without any magnetization. The above operation is shown in FIG.

The magnetic flux of the transformer may be provided with another detecting means. By applying a voltage to the transformer and performing detection, highly reliable detection is possible. The excitation period does not necessarily have to be one cycle. Since a voltage of an arbitrary waveform can be generated by the converter, it is also possible to control the voltage waveform after the magnetic flux can be estimated so as to bring the state to a state where there is no magnetization. The re-applied voltage need not be 100%,
The required voltage may be set.

FIG. 1 shows an embodiment of the present invention.

The configuration and operation of this embodiment will be described. In power converter 1, control signal generator 72 outputs control signal c of converter 11. The control signal is generated by various control operations (not shown) for operating the converter. This control signal is input to a pulse width modulator (PWM) 75 as it is after being corrected for suppressing magnetic declination as described later. The pulse width modulator generates a switching signal s for the converter 11 according to the signal c. The converter 11 operates according to the signal s. As a result, converter 11 outputs a voltage (fundamental wave component) substantially proportional to control signal c. If the conversion of the proportional coefficient is performed, the control signal c becomes the output voltage (fundamental wave component) of the converter.
This is a sine wave that is almost equal to the instantaneous value of the fundamental wave component of the output voltage. DC power supply 4 is a DC voltage source for the converter. The converter transformer 2 is excited by the output voltage of the converter. The circuit breaker 3 is a switch for connecting the converter to the power system 45, and is open when the converter is started. When starting the converter,
First, the switch 8 is set to the y position. As a result, the control signal c is multiplied by b by the multiplier 82 and input to the pulse width modulator. b is the initial excitation voltage reduction factor of the converter transformer, which is about 0.3. Since the output voltage of the converter is reduced by a factor of b, the inrush current does not flow in the converter transformer regardless of the residual magnetic flux. In this state, the converter is operated for one cycle. The current transformer 51 detects the exciting current for the converter. By obtaining the average value of the excitation current in one cycle by the excitation detector 73, the value of the residual magnetic flux of the transformer for the converter can be obtained. Further, from this value, a time T at which the magnetic flux reaches that value when excited without magnetization is obtained.

From one cycle, the switch 8 is set to the position z. Since the output voltage of the converter 11 becomes zero, the magnetic flux of the converter transformer is maintained at a constant value. This value is equal to the value of the residual magnetic flux. Waiting until time T, the switch 8 is set to the position of x. After time T, the control signal c is directly input to the pulse width modulator 75, and the magnetic flux of the transformer for the conversion device is in an excited state without magnetization. The switching time T of the switch is set by the switching time setting device 74 based on the phase signal from the phase detector 71 for detecting the phase of the AC voltage and the signal of the excitation detector.

Since the AC side voltage of the converter transformer is equal to the system voltage, the circuit breaker 3 can be turned on to interconnect the system. Thereafter, the converter may be arbitrarily controlled by the control signal generator 72. As a result, the power converter
The operation can be started without demagnetizing the converter transformer in less than one cycle.

The excitation detector 73 also inputs the current on the AC side of the converter transformer detected by the current transformer 52. This is because the magnetizing transformer always detects the exciting current from the difference between the DC side current and the AC side current to perform the magnetizing suppression control so that the converter transformer does not become magnetized. Is also a correction signal to the control signal generator 72.

Although the above description has been made on the assumption that the power converter 1 operates in connection with the power system, the same operation can be performed when an AC voltage is applied to the load. In this case, the circuit breaker 3 may be turned on from the beginning. In the first one-cycle operation, the control signal c is multiplied by b. However, during this period, the converter operates independently with being disconnected from the system, and may be used as an arbitrary reference signal. Accurate excitation can be performed irrespective of the magnitude of the control signal. For example, the frequency can be increased. In this case, it is possible to not reduce the voltage, and it is possible to shorten the magnetic flux detection time.

FIG. 4 shows another embodiment of the present invention, in which the power converter is constructed by multiple connection using a plurality of converters. In the case of multiple connections, the switching frequency of the converter is lowered. In the above description, the converter has been described as capable of generating an ideal sine waveform, but the actual voltage waveform is a pulse train of a rectangular wave. As the switching frequency decreases, the number of pulses decreases, and the magnetic flux waveform also deviates from an ideal waveform. Therefore, the switching time can be adjusted for each converter.

FIG. 5 shows still another embodiment of the present invention.
This is the case for three phases. In this case, the same operation can be performed by converting into two phases such as αβ coordinates orthogonal to each other and treating them as two sets of single phases.

[0027]

According to the present invention, there is provided a power converter which employs a transformer for a converter similar to a general transformer and can start the converter without generating an inrush current at the time of starting. Can be.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining one embodiment of the present invention.

FIG. 2 is a diagram illustrating a magnetic flux at the start of excitation of a transformer for a converter.

FIG. 3 is a diagram illustrating magnetic flux at the start of excitation of a transformer for a converter according to the present invention.

FIG. 4 is a diagram for explaining another embodiment of the present invention.

FIG. 5 is a diagram for explaining still another embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Power conversion device, 2 ... Transformer for a conversion device, 3 ... Breaker, 4 ... DC power supply, 6 ... Transformer, 8 ... Switch, 11 ...
Converter, 45 ... power system, 51, 52 ... current transformer, 71 ...
Phase detector, 72: control signal generator, 73: excitation detector, 74: switching time setting device, 75: pulse width modulator, 8
2. Multiplier.

Claims (6)

[Claims] 1. A self-excited voltage converter having a transformer, wherein the transformer is started by reducing the AC voltage and the magnetic flux of the transformer is measured, and based on the result, no bias occurs in the transformer. A power converter characterized in that the AC voltage is increased as described above. 2. The power converter according to claim 1, wherein the AC voltage at the time of starting is reduced to 50% or less. 3. The power converter according to claim 1, wherein the operation period in which the AC voltage is reduced is one cycle or an integer cycle. 4. The power converter according to claim 1, wherein a magnetic flux of the transformer is measured from an exciting current of the transformer during an AC voltage reduction operation period. 5. The power converter according to claim 1, wherein after starting the AC voltage reduction and measuring the magnetic flux of the transformer for the converter,
Once the AC voltage is dropped to zero, the magnetic flux of the converter transformer is held at a constant value, and the normal voltage is applied from the time when the magnetic flux when the converter transformer is excited without being demagnetized becomes equal to the measured magnetic flux. A power conversion device characterized in that the power conversion device is operated on 6. A self-excited voltage converter having a transformer, wherein the transformer is started by increasing the frequency of the AC voltage and measures the magnetic flux of the transformer. A power converter characterized by raising an AC voltage so as not to generate the power.