JPH028533B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to an overvoltage protection device for an inverter for absorbing surge voltage entering from an inverter output wiring made of semiconductors, particularly power transistors.

[Prior art and its problems]

Overvoltage protection devices for inverters include an overvoltage protection device called a snubber circuit that absorbs internal surge voltages that occur during switching operations in the inverter, as well as an overvoltage protection device that protects against surge voltages that enter from outside, especially when the inverter An overvoltage protection device is provided to absorb surge voltage entering from the AC power input line of the rectifier, which is a DC input source. As the latter overvoltage protection device, it is known to connect a non-linear resistor, such as a ZnO element, between each of the three-phase input lines of the rectifier.

However, as the performance of semiconductors used in inverters, especially power transistors, has improved, the applications of inverters have expanded, and as a result, it has become necessary to take measures against surge voltages that enter from the inverter's output wiring. The above-mentioned known overvoltage protection devices are insufficient to protect the inverter from surge voltages entering from the output wiring.

Therefore, in order to protect the inverter from overvoltage against surge voltages entering from the output wiring, it is preferable to connect a nonlinear resistor to the inverter output wiring as well.

FIG. 1 shows an embodiment of an inverter device in which an overvoltage protection device is connected to the output wiring. 1 is a three-phase AC power supply, and one wire is generally grounded as shown at 2. 4 is an inverter device, as is well known, for example, an inverter consisting of a diode bridge rectifier, a bridge connection of power transistors each having diodes connected in antiparallel, and a DC intermediate circuit between the rectifier and the inverter. and a smoothing capacitor interposed between the two. Connected to the three-phase output wiring 5 of the inverter device is an overvoltage protection device 6 consisting of three non-linear resistors connected in a star shape and whose neutral point is grounded.

Such an overvoltage protection device can effectively absorb the surge voltage that enters the output wiring of the inverter and protect the semiconductors in the inverter from the overvoltage.

However, such conventional devices have the disadvantage of requiring large capacitance non-linear resistors, as will be explained below.

FIG. 2 shows an example of the time course of the ground voltage of the inverter output line. In this way, the ground potential of the inverter output line has a waveform that changes rapidly in accordance with the switching operation within the inverter. When the effective value of the line voltage on the AC power supply side is Ea [V], the maximum value of the ground potential of the inverter output line is √2Ea [V].

A ZnO element that is generally used as a nonlinear resistor can be equivalently represented by a nonlinear resistance element 61 for its original purpose and an interelectrode capacitance 62 in parallel with this. Therefore, the equivalent circuit viewed microscopically for the sudden change part of the potential waveform shown in FIG. 2 is as shown in FIG. , and the above elements 61 and 62. Consider the case where switch 11 is turned on with the terminal voltages of elements 61 and 62 at zero.

The transient phenomenon in this case is illustrated by the time course shown in FIG. That is, due to the LC oscillation action, the voltage v of the capacitor 62 exceeds the power supply voltage √2Ea [V] and has an oscillating waveform having a maximum value of 2×√2Ea [V].

On the other hand, the inverter output voltage limit level determined by the conduction voltage E _Z of the nonlinear resistor is set to the normal level of the inverter output voltage √2Ea [V] in order to effectively utilize the withstand voltage of the power transistors that make up the inverter. It should be selected so that it is not too large. In this case, a non-linear resistor will be used which has a conduction voltage greater than its normal level, but less than twice its normal level.

Therefore, as shown in FIG.
is clamped to its conduction voltage E _Z by a nonlinear resistor.

In this way, the nonlinear resistor frequently repeats the clamping action of absorbing overvoltage caused by its own capacitance during normal operation of the inverter, resulting in power loss. , it must have a correspondingly large capacity and be expensive.

〔Effect of the invention〕

SUMMARY OF THE INVENTION An object of the present invention is to provide an overvoltage protection device that can constantly prevent a nonlinear resistor from performing a clamping operation under normal operating conditions of an inverter.

〔Effect of the invention〕

According to the present invention, a diode bridge is connected to the AC output side of the inverter, and a parallel circuit of a non-linear resistor for overvoltage absorption and a capacitor is connected between each DC terminal of this diode bridge and the earth. This is achieved by connecting the .

According to such a configuration, the voltage of the nonlinear resistor does not suddenly change with the switching operation of the inverter, and is kept at a substantially constant voltage value, so that no vibration phenomenon occurs. Therefore, the nonlinear resistor only clamps when a surge voltage enters the inverter's output wiring.
It only needs to be a small capacity and inexpensive one.

[Embodiments of the invention]

FIG. 5 shows an embodiment of an inverter overvoltage protection device according to the present invention.

In this figure, numeral 4 is the same as the inverter device 4 in the conventional example of FIG. 1, and one of the input lines is grounded as shown by 2 as in FIG. 1. An overvoltage protection device according to the present invention is connected to the output line 5 of the inverter device 4. This overvoltage protection device consists of a three-phase diode bridge 71 connected to the output line 5 of the inverter device.
and a non-linear resistor 72 connected between each DC terminal of this diode bridge and the ground, respectively.
73, and capacitors 74 and 75 connected in parallel to each of these nonlinear resistors.

The ground voltage of the inverter output line frequently changes suddenly as the power transistors in the inverter perform switching operations (see FIG. 2). In the case of the overvoltage protection device according to the invention the capacitor 7
Due to the smoothing effect of 4, 75, the non-linear resistor 72,
Since the voltage across terminals 73 remains substantially constant regardless of switching operations, no oscillatory effects occur. Therefore, even if the conduction voltage value of the non-linear resistor is selected so as to make the inverter's output voltage limit level sufficiently low, the non-linear resistor will frequently repeat the clamping operation under normal operating conditions as in conventional equipment. Since there is no such thing as a return, a nonlinear resistor will not be destroyed even if its capacitance is reduced.

The overvoltage protection device shown in FIG. 5 can be modified in various ways without departing from the technical idea of the present invention, such as by inserting a current limiting resistor in series with each capacitor 74, 75.

〔Effect of the invention〕

As explained above, according to the present invention, by connecting parallel circuits of a nonlinear resistor and a capacitor between each DC terminal of the diode bridge connected to the inverter output line and the ground,
Since the voltage of the non-linear resistor remains almost constant regardless of the switching operation of the inverter and no oscillating operation occurs, under normal conditions where surge voltage does not enter the inverter output line, it is necessary to set the limit level sufficiently low. Even if a linear resistor is used, it is possible to avoid the inconvenience of performing a clamping operation each time an inverter switching operation is performed, thereby causing a loss. In other words, while allowing economical design of the inverter and easing the constraints on the routing of output wiring, the nonlinear resistor itself is free from damage even if its capacitance is small.

[Brief explanation of drawings]

Figure 1 is a connection diagram showing an example of a conventional device;
The figure is a waveform diagram showing an example of the ground voltage of the inverter output line over time, Figure 3 is an equivalent circuit diagram to explain the occurrence of vibration due to inverter switching operation, and Figure 4 is the operation of the equivalent circuit in Figure 3. FIG. 5 is a connection diagram showing one embodiment of the device of the present invention. Code explanation 1...AC power supply, 2...Grounding point, 4
...Inverter, 71...Diode bridge,
72, 73...Nonlinear resistor, 74, 75...Capacitor.

Claims (1)

[Claims] 1. In order to suppress surge voltage that enters from the output wiring of an inverter made of semiconductors, a diode bridge is connected to the AC output side of the inverter, and an overvoltage absorbing shield is connected between each DC terminal of this diode bridge and the ground. An inverter protection device characterized by connecting parallel circuits of a linear resistor and a capacitor.