JPH02184222A - Overcurrent protective system for inverter - Google Patents

Abstract

PURPOSE:To perform overcurrent protection of an inverter through a simple structure by arranging an overcurrent detector in the DC circuit of an inverter main circuit and simultaneously interrupting all semiconductor elements constituting the negative pole side arm of the main circuit with an output signal from the overcurrent detector. CONSTITUTION:Upon short-circuit of a main circuit 10A, DC short-circuit current flows through a DC circuit and fed to the comparator in an overcurrent detector 23. When the DC current exceeds over a set level, the comparator produces an interruption signal which is held for a predetermined interval. The interruption signal is fed to all gate circuits of a negative pole side arm and signals for turning respective bipolar transistors off are provided forcibly. Consequently, the gate circuit is interrupted and thereby the short circuit of a DC power source 12 is interrupted thus eliminating short-circuit current. The short-circuit signal is also fed to a controller in order to perform protective operation against fault.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an overcurrent protection system for an inverter, and more specifically, the present invention relates to an overcurrent protection system for an inverter. The present invention relates to an overcurrent protection method for preventing damage to semiconductor elements due to short-circuit current when an inverter output is short-circuited.

(Prior art) IGBTs have the advantages of bipolar transistors, such as high breakdown voltage and easy increase in capacity, and the advantages of power MO8FETs, such as high-speed switching performance, simplicity of the drive circuit, and low power consumption. It has recently been attracting attention as a new device.

Figure 4 shows the equivalent circuit of this IGBT.
T is an N-channel MOSFET51. NPN transistor 52. PNP transistor 53 and transistor 52
It is characterized in that it consists of a base-emitter shorting resistor 54 and constitutes a parasitic thyristor circuit consisting of transistors 52 and 53.

Next, FIG. 5 shows a drive circuit for driving the induction motor 14 by a known three-phase inverter using self-extinguishing semiconductor elements such as the above-mentioned IGBT and bipolar transistor. In the figure, 10 is a main circuit using self-arc-extinguishing semiconductor elements 11a to llf in the positive and negative arms, 12 is a DC power supply, and 13 is a current transformer CT installed in each phase on the AC side. , , CT, and 15 is a control device for the main circuit IO. In addition, each element 11a-l
For convenience, illustration of the gate circuit of lf is omitted.

This type of inverter consists of elements 1 connected in series to each other by on/off signals output from a control device 15.
By turning 1a=11f on and off alternately,
An AC voltage of arbitrary magnitude and frequency can be obtained as an output, and it is widely used as a variable speed drive device for AC motors such as the induction motor 14, and as an uninterruptible power supply device.

By the way, in such an inverter, it is important to protect the elements 11a=11f without destroying them if an overcurrent failure occurs for some reason on the output side of the inverter. As a conventional overcurrent protection system of this type, first, as described above, a current transformer CT is provided for each phase in the current detector 13.

~CT, is input to the control device 15, and the control device 15
Now, it is determined whether each phase current has reached the failure level. If it is determined that the failure level has been reached, an operation stop command is output from the control device 15 to the main circuit IO.

Generally, among overcurrent failures, an output short-circuit failure can be cited as a serious failure that can lead to damage to the elements 11a to 11f.

As shown in FIG. 6, if the output lines of the main circuit IO are short-circuited for some reason, a short-circuit 16 of the DC power supply 12 is formed via the elements 11a and lie. In this case, a large current flows through the elements 11a and lie due to the short circuit 16, increasing the possibility of element destruction.

Conventionally, however, as an overcurrent protection method in this case, the current is detected by the current detector 13 as described above, the detected value is waveform-shaped by a filter, and then the control device 13
The current is input to the arithmetic circuit No. 5 to detect an overcurrent, and outputs an operation stop command to turn off the elements 11a and 11e. In this case, it usually takes about 20 to 30) Isec until an output short circuit actually occurs and the device is turned off.

When bipolar transistors are used as the elements 11a to 11f, bipolar transistors have the ability to protect against power supply short circuits by turning off the elements within 40 to 50 II seconds after an output short circuit occurs. There is no particular problem by applying the overcurrent protection method in the event of an output short circuit.

(Problems to be Solved by the Invention) However, in the case of IGBTs, the allowable overcurrent time is currently shorter than that of bipolar transistors, which is about 10 to 15 Ilsec. For this reason, the above-mentioned overcurrent protection method cannot provide overcurrent protection when the output of an inverter using IGBTs is short-circuited, and in the worst case, there is a problem that the device may be destroyed.

In addition, when using the above-mentioned overcurrent protection method, a high-speed and highly accurate current detector 13 is required, but such a current detector 13 is extremely expensive and reduces the overall cost of the inverter. This has the disadvantage that it leads to an increase in costs. Furthermore, in the past, there was a problem that overcurrent protection was impossible if an output short circuit occurred inside the current detector 13 (on the main circuit 10 side).

The present invention has been made to solve the above problems,
The purpose of this is to quickly and reliably protect semiconductors and elements to prevent element destruction in the event of an overcurrent failure due to an inverter output short circuit, etc., and to reduce costs by allowing the use of inexpensive current detectors. An object of the present invention is to provide an overcurrent protection method for an inverter that is simple in structure and reduces overcurrent.

(Means for Solving the Problem) In order to achieve the above object, the present invention provides an overcurrent detector in the DC circuit of the inverter main circuit to detect overcurrent flowing to semiconductor elements due to an output short circuit, etc. The present invention is characterized in that all semiconductor elements constituting the negative arm of the main circuit are simultaneously cut off by the output signal of the current detector.

(Function) According to the present invention, when an output short circuit occurs in the inverter, for example, a short circuit is formed between the positive and negative pole side arms and the DC power supply, and a large current flows through the DC circuit of the inverter main circuit, so that this short circuit failure can be prevented. It can be detected by an overcurrent detector. When an overcurrent is detected, the output signal (cutoff signal) from the overcurrent detector is used to instantaneously and forcibly shut off all semiconductor elements on the negative arm, thereby eliminating the short circuit in the DC power supply and reducing the short circuit current. It acts to eliminate element damage and prevent element destruction.

Here, since all the semiconductor elements on the negative side arm have a common potential, the cutoff signal from the overcurrent detector can be input directly to these gate circuits without signal isolation.
All semiconductor elements on the negative side arm can be shut off simultaneously at high speed without using a control device.

(Example) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a main circuit 10A of a three-phase inverter to which the overcurrent protection system according to this embodiment is applied. In the same figure, 20a to 2Of are I as self-extinguishing semiconductor elements forming the positive and negative side arms.
GBTs 21a to 21f are diodes connected in parallel to IGBTs 20a to 20f, respectively, and 22 is the main circuit 10.
A current detection resistor connected to the negative bus of the DC circuit of A,
23 has an input side connected to one end of the current detection resistor 22,
The output side includes a control device (not shown), an IGBT of the negative side arm, and gate circuits 24d to 2Of of 20d to 2Of.
These are the overcurrent detection amounts connected to 4f. In addition,
The gate circuits of the IGBTs 20a to 20c on the positive side arm are not illustrated. Further, since the constituent elements other than those described above are the same as those shown in FIG. 5, they are given the same reference numerals and detailed description thereof will be omitted.

Furthermore, FIG. 2 shows an example of the overcurrent detector 23.
This overcurrent detector 23 detects the direct current Ioc of the main circuit 10A.
The comparator 23A outputs a cutoff signal by comparing the overcurrent set value Is'' with the overcurrent setting value Is, and the comparator 23A has a function of holding the cutoff signal for a certain period of time.

Next, the operation of this embodiment when the inverter output is short-circuited will be explained with reference to the timing chart of FIG.

Now, if the output side of the main circuit 10A is short-circuited, a short-circuit DC current flows through the negative bus of the DC circuit of the main circuit 10A. This current is detected as a direct current IDc by the current detection resistor 22.
is detected by the comparator 23 of the overcurrent detector 23.
It is input to A. When this DC current Ioc exceeds the overcurrent set value 2 at time t1 in FIG. 3, the comparator 23A outputs a cutoff signal and holds this cutoff signal for a certain period of time.

The cutoff signal is transmitted to all IGBTs 20d to 20f on the negative side arm without going through the arithmetic circuit of the control device (not shown).
These gate circuits 24d to 24f forcefully output an off signal to each IGBT 20d to 20f in response to the cutoff signal, thereby blocking them all at once. As a result, the short circuit of the DC power supply 12 is cut off, and the short circuit current disappears because there is no path for it to flow. Further, since the cutoff signal is also input to the control device, the control device also performs a protection operation in parallel at the time of a normal failure.

Note that since the IGBTs 20d to 20f on the negative side arm have a common potential, the gate circuits 24d to 24f have a common potential. Therefore, the potential of the overcurrent detector 23 is
By making them the same as 20d to 20f, the cutoff signal can be input directly to the gate circuits 24d to 24f without going through the control device, and there is no conventional time delay between failure detection and protective operation. No, IGBT20d~
20f can be shut off instantly. For this reason, I.G.
The element can be turned off within a relatively short time allowed by the BT's performance, and element destruction can be reliably prevented.

In the above embodiment, IG is used as a self-extinguishing semiconductor element.
Although the inverter using BT has been explained, other self-extinguishing devices such as bipolar transistors and MOSFEs are also available.
The present invention is also applicable to an inverter using T or the like.

Further, the present invention is effective in various cases of protecting semiconductor elements from overcurrent failures caused by causes other than output short circuits, and can also be applied to single-phase inverters.

(Effects of the Invention) As described above, according to the present invention, all semiconductor elements of the negative side arm are forcibly cut off without going through the main circuit control device using a cutoff signal that detects an overcurrent due to an output short circuit or the like. Therefore, even in inverters that use semiconductor devices such as IGBTs, which have a relatively short overcurrent tolerance, it is possible to quickly eliminate overcurrent and prevent device destruction. can.

Furthermore, the present invention can be realized by simply adding a simple overcurrent detector and wiring to a conventional inverter;
This can contribute to simplifying the circuit configuration and reducing costs.

In addition, it is more economical because there is no need to use an expensive current detector on the AC output side of the main circuit, and it is also possible to detect overcurrent failures inside this current detector (on the main circuit side). It has the effect of

[Brief explanation of the drawing]

1 to 3 show an embodiment of the present invention,
Fig. 1 is a block diagram of the main parts of a three-phase inverter to which this embodiment is applied, Fig. 2 is an explanatory diagram of an overcurrent detector, Fig. 3 is a timing chart showing the operation of this embodiment, and Figs. Fig. 6 is for explaining the prior art, and Fig. 4 is for explaining the prior art.
An equivalent circuit diagram of a GBT, and FIG. 5 is a configuration diagram of the main parts of a three-phase inverter. FIG. 6 is an explanatory diagram when the output of the three-phase inverter is short-circuited. 10A...Main circuit 13...Current detector 20a to 20f-IGBT 23...Overcurrent detector 24d to 24f...Gate circuit 12...DC power supply 14...Induction motor 22 ...Current detection resistor 23A...Comparator

Claims (1)

[Claims] By detecting an overcurrent flowing through a semiconductor element constituting the main circuit of an inverter and turning off the semiconductor element,
In the overcurrent protection method for an inverter that protects the semiconductor elements, an overcurrent detector for detecting the overcurrent is provided in the DC circuit of the main circuit, and an output signal of the overcurrent detector is used to detect the negative arm of the main circuit. An overcurrent protection method for an inverter that is characterized by simultaneously cutting off all semiconductor elements that make up the inverter.