KR20040035526A - Method and Apparatus for Protecting against Load Short-circuit of a High Voltage Multi-level Converter - Google Patents

Abstract

PURPOSE: A method and an apparatus for protecting a load element of a high-voltage DC power supply unit using a multi-level converter are provided to interrupt the power from the load by minimizing the amount of short current in the short circuit due to a spark-down phenomenon. CONSTITUTION: An apparatus for protecting a load element of a high-voltage DC power supply unit using a multi-level converter includes a power line switch group, a switch, an inductor, a couple of diodes, a couple of resistors, a couple of switches, and a couple of capacitors. The power line switch group is serially connected to multi-phase input lines. The switch is serially connected to a positive output terminal of a PWM converter. The inductor is connected between the switch and a load. The diodes and the resistors are connected in parallel to the inductor. The switches and the capacitors are serially connected to a connection point between the switch and the inductor.

Description

TECHNICAL AND APPARATUS for Protecting against Load Short-circuit of a High Voltage Multi-level Converter}

The present invention relates to an apparatus and method for protecting a load short-circuit current of a multilevel converter, and more particularly, to a DC power supply for supplying DC power to a load in which an arc frequently occurs due to a spark, such as an ion source. Limits the magnitude of short circuit current in case of load short circuit due to spark down, and cuts off the power from short circuited load quickly and protects the power supply and load from short circuit current without any damage. In addition, the present invention relates to a method and apparatus for enabling normal operation by quickly reapplying DC power when the load is restored to normal.

AC / DC power converters that supply special loads such as ion sources require precise output voltage stabilization control performance and excellent protection against load short-circuits. In the event of frequent load short circuits inside the ion source, it is necessary to cut off the power supply quickly from the load to suppress and eliminate the rise of the short circuit current. However, it is important to turn off the power again as soon as possible after the short-circuit factor disappears inside the ion source. This is to make the overall operating time of the system as long as possible.

In order to satisfy these characteristics, vacuum tubes were used in the early days. However, as the capacity of GTO thyristors became larger in the 1980s, a method of replacing an existing vacuum tube with a GTO thyristor was studied. GTO thyristors have improved performance over conventional methods due to their high speed on / off characteristics, low loss characteristics and semi-permanent lifespan that are incomparable with vacuum tubes. However, in order to apply to high voltage of several tens of kV or more, due to the limitation of the voltage rating of the GTO thyristor, it is inevitable to use several GTO thyristors in series. This not only increases the size and weight of the stack of GTO thyristors, but also creates a demanding condition for turning on and off multiple GTO thyristors simultaneously. Therefore, to solve this problem, the inverter method has been researched and developed. Inverter type basically consists of rectifier, inverter, boost transformer and diode full-wave rectifier. The inverter is responsible for the blocking of the output DC power supply in this manner. However, the disadvantage of the inverter method is that the system configuration is complicated and requires a boost transformer separately.

Recently, researches on multi-level converters have been actively conducted for the purpose of applying to high-voltage large-capacity systems. A new type of power conversion method has been proposed that uses the concept of multi-level converters to quickly cut off and reload DC power. However, one of the methods has a problem in that a large AC input current, which is several times the rated value in a load short circuit, is energized for half a cycle. The other drawback is that the current continues to flow through the load until the DC power is interrupted in the event of a load short-circuit until the energy stored in the dc-side inductor is completely discharged.

In order to solve the above problems, the present invention basically uses a multi-level converter to simplify the structure of the power supply device so that there is no increase in AC input current in the event of a load short-circuit accident, and at the same time when the DC output power is cut off, the load current The aim is to provide a new type of converter for a direct current power supply which is zero.

According to an object of the present invention, in a PWM (PulseWidth Modulation) converter for converting an AC power source into a DC power source, a power line switch group installed in series with each of a multiphase input line, and the amount of the converter ( A switch connected in series to the output terminal of +), an inductor connected in series between the switch and the load, a pair of series connected diodes and resistors connected in parallel with the inductor, a pair of connection points between the switch and the inductor Including a structure in which a switch connected in series and a capacitor are connected and the same pair is repeatedly connected to the negative output terminal of the converter continuously, the DC output power is cut off quickly in the event of a load short circuit to prevent excessive short-circuit current from flowing. As well as safeguarding the load device, even when the load is restored to normal Provides a converter with output short-circuit protection, which supplies power to the load and recovers to its normal state within the shortest time.

According to another object of the present invention, in the PWM converter that minimizes the short-circuit current in the event of a short circuit and quickly reapplies the DC power when the load recovers from the short-circuit state, the inductor connected in series with the load To prevent a sudden rise of the short-circuit current, and when the short-circuit current exceeds a predetermined value, the PWM converter is electrically disconnected from the AC power supply, and the energy accumulated in the inductor is discharged through a resistor connected in parallel with the inductor. The output short circuit protection method of the converter having an output short circuit protection function is characterized in that the amount of the inductor energy flowing into the load is zero.

1 is a block diagram showing a multilevel converter according to the present invention;

2 is a diagram showing each switch operation state and voltage and current waveforms before and after a load short circuit accident and recovery to a normal load;

3 is a view showing waveforms of the AC input current i _a i _b , i _c and the DC output current i _o when a load short circuit accident has occurred several times;

4 is an enlarged view of a load axis before and after the time axis;

5 is an enlarged view of the time axis before and after the time of reapplying the output power after the load is restored to normal.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

First of all, in adding reference numerals to the components of each drawing, it should be noted that the same reference numerals are used as much as possible even if displayed on different drawings. In addition, in describing the present invention, when it is determined that the detailed description of the related well-known configuration or function may obscure the gist of the present invention, the detailed description thereof will be omitted.

1 is a diagram illustrating a multilevel converter according to the present invention. The basic circuit configuration is similar to a conventional multilevel converter, in which only a few auxiliary switches are added. The switches S _a , S _{b and} S _c are inserted on each side of the AC input, the switch S _dc is inserted on the positive output side of the converter, and the output stage filter capacitor C ₁ -C of the conventional multilevel converter. _n has a respective switch (S _o1 -S _on) one by one in series and inserted in is replaced by series-connected pair of capacitors and a switch, the short circuit when the load short-circuit the inductor (L) to prevent the rapid increase of current, connected in series in parallel Diode D and resistor R _dc are connected. All of these auxiliary switches remain on in normal operation and are turned off in the event of a load short circuit. The switching losses of these added auxiliary switches are therefore extremely low compared to the main switches S ₁ -S _(n-1) of the multilevel converter.

The number of auxiliary switches S _a, S _b, S _c and S _dc is constant regardless of the level number of the converter. Each of the auxiliary switches S _a, S _{b and} S _c consists of two SCR thyristors connected in anti-parallel, and acts as a simple switch rather than an AC thyristor controller. The switches S _o1 -S _on prevent the output capacitors C ₁ -C _n from discharging during the short circuit of the load. If the load returns to normal after the short-circuit is eliminated and the voltage of the output capacitor is reapplied to the load, the load voltage needs to rise as quickly as possible to the rated voltage. Therefore, to satisfy this requirement, it is important to keep the floating state in which the capacitor is not discharged.

Hereinafter, a load protection method according to a short circuit current according to the present invention will be described based on the circuit described above.

In normal operation mode, all auxiliary switches S _a , S _b , S _c , S _dc and S _o1 -S _on are turned on / off by the main switch of S ₁ -S _(n-1) according to the PWM signal generated from the controller. It remains on while repeating the off operation. In this mode, the operating states of the converter of the present invention and the conventional multilevel converter are the same.

Fig. 2 shows the switching state of each switching element which is changed back and forth when a short-circuit occurrence of the multilevel converter output occurs and when the steady state is recovered. The waveforms of the switching state, load voltage and output direct current of each switching element in each time interval are described as follows.

1) t ₀ ≤ t <t ₁

During this mode, the output capacitor voltage V _c is applied to the inductor L, so that the output current i _o increases linearly as in [Equation 1].

V_C: Sum of voltages applied to output filter capacitors C ₁ -C _n

Until this time, all switching elements remain in a steady state.

2) t ₁ ≤ t <t ₂

At time t ₁ , the DC current i _o reaches the previously set output short-circuit current detection level i _os , which causes the output short-circuit signal OS to go high. As soon as the OS signal goes high, the gate signals for thyristors S _a , S _b and S _c are turned off and the power flow direction of the multilevel converter is reversed. In other words, the converter operates in the regeneration mode. Therefore, it is possible to reduce the AC input line current to zero as soon as possible. The magnitude of the AC input line currents i _a , i _b and i _c in the regenerative mode is reduced as follows.

i_a {(t)} =-(i_b {(t)} + i_c {(t))} ~~~~~~~~~~~~

Also, the load current continues to increase linearly as shown in [Equation 5].

This mode lasts until all AC input current decreases to zero, and when the current decreases to zero, the auxiliary switch S _dc , S _o1 -S _on and the main switch S ₁ -S _{(n-1 ) Are} all turned off.

3) t ₂ ≤ t <t ₃

In this mode, the three-phase ac line currents i _a , i _{b and} i _c are all zero and the load current i _{o is} also zero. However, the inductor current is discharged through the resistor R _dc as shown in [Equation 6].

Here, τ ₁ = L / R _dc and the voltage drop of the diode in series with R _dc is ignored.After the discharge of the inductor is terminated, there is no more current flow in the circuit and all output capacitor voltages are discharged. Is maintained.

4) t ₃ ≤ t <t ₄

The short-circuit condition disappears before time t ₄ , and the load is in a steady state. The output DC power of the multilevel converter must then be reapplied to the load.

5) t ₄ ≤ t <t ₅

At time t ₄ all auxiliary switches S _a , S _b , S _c , S _dc , _{SO 1} -S _on are turned on and main switches S ₁ -S _(n-1) start normal PWM operation. When _SO1 -S _on is turned on, the DC voltage of the capacitor is applied to the load again, and the output DC current after application increases as follows.

Where τ ₂ = L / R _L and R _L is the load resistance.

[Simulation Result]

In order to confirm the effectiveness of the converter according to the invention, the multilevel converter shown in FIG. 1 was tested. The parameters used in the simulation are as follows. AC input line voltage is 220 V, L _s is 1.6 mH, R _dc is 100 Ω, C is 2200 μF, L is 2 mH, V _o is 400 V and load resistance is 40 Ω.

FIG. 3 shows waveforms of phase currents i _a , i _b , i _c and output DC currents i _o of the AC input side when the load short circuit is set to occur frequently and switching as shown in FIG. 2 occurs. A load short circuit was to occur every 30 msec and the load short duration was 10 msec. It can be seen that when the load is shorted, the magnitude of the AC input current decreases directly to zero without increasing.

FIG. 4 is an enlarged view of a load short circuit portion in FIG. 3 on a time axis. It can be seen that the output current i _o decreases to zero as soon as the AC input current i _a , i _b , i _c decreases to zero.

FIG. 5 is an enlarged view of a part of reapplying power to a load in FIG. 3. When the output capacitor is connected to the load by turning on the switches S _o- S _on , the load current increases exponentially and the increase time interval is around 200 μs.

The present invention provides a new type of DC power supply that has the capability to cut off the power from the shorted load while minimizing the short-circuit current at the time of load short-circuit, and to quickly apply the rated voltage to the load when the load is restored to the normal state. Provided is a high voltage DC power supply using a multilevel converter. The main features and advantages of the present invention are summarized as follows.

1) The converter according to the present invention has the advantages of very high efficiency, long life, and easy maintenance, compared to the conventional tube type, and its structure is simpler and lighter than the conventional GTO or inverter type power supply. It is possible.

2) The speed at which the output voltage of the DC power supply is disconnected from the shorted load is fast enough not to damage the converter and the load.

3) Unlike the existing multi-level converters, the converter output capacitor can maintain the rated output voltage without any discharge even if the load is shorted, so that the rated voltage can be quickly reapplied to the load when the load is restored to normal.

4) Compared with the conventional multi-level converter method, the magnitude of AC input current does not increase when the load is short-circuited, and when the power is cut off from the shorted load, the stored energy of the inductor connected in series on the load side flows into the load. This prevents the blocking characteristic.

Claims (7)

In a PWM (Pulse Width Modulation) converter for converting an AC power source into a DC power source, a power line switch group installed in series with each of a multiphase input line, and connected in series with a positive output terminal of the converter. Is connected to the switch, the inductor connected in series between the switch and the load, a pair of series connected diodes and resistors connected in parallel with the inductor, a pair of series connected switches and capacitors at the connection point of the switch and the inductor It is connected and the same pair is repeated and connected continuously to the negative output terminal of the converter.In the event of a load short circuit accident, the DC output power is cut off quickly to prevent excessive short-circuit current, thereby protecting the converter and the load device. Of course, even when the load is restored to normal, the DC power supply to the load Converter with an output short-circuit protection, characterized in that the recovery to the normal state in a short time. The method of claim 1, If the short circuit current reaches a predetermined amount after a short time after the short circuit accident occurs, the AC line current is turned off by turning off the power line switch group and setting the converter operation mode to a regeneration mode. Converter with output short-circuit protection characterized by rapid reduction. The method of claim 2, And the voltage of the capacitor remains in a floating state even after the load is shorted and the polyphase input line current is zero. The method of claim 3, And when the load returns to normal, the voltage of the capacitor, which remains in the floating state, is reapplied to the load, thereby rapidly increasing the load voltage to the rated voltage. The method of claim 4, wherein And each of the switches of the switch group of the polyphase input line has two thyristors connected in anti-parallel. The method of claim 5, And all switches of the converter have an IGBT or one on-off controllable element and one diode connected in parallel in parallel. In a PWM converter that minimizes the short-circuit current in the event of a short-circuit accident and quickly reapplies the DC power when the load recovers from the short-circuit state. Use an inductor in series with the load to prevent abrupt rises in short circuit current, The PWM converter is electrically disconnected from the AC power supply when the short-circuit current reaches a predetermined value or more, Discharge the energy accumulated in the inductor through a resistor connected in parallel with the inductor output short circuit protection method of the converter with an output short circuit protection, characterized in that the amount of inductor energy is introduced into the load (Zero) .