AU731178B2 - Protective device for power converter - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): Hitachi, Ltd.

9 9* ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION TITLE: Protective device for power converter The following statement is a full description of this invention, including the best method of performing it known to me/us:- 9 9*9* la BACKGROUND OF THE INVENTION The present invention relates to a protective device for power converter for converting a direct current into an alternating current or inverting an alternating current into a direct current by using semiconductor element modules of IGBTs or power transistors.

These days, power converter, such as inverters and converters that include modular semiconductor switching devices, including GTO thyristors and power transistors, are used extensively in electric car control equipment. Above all, in electric cars using power converter, the inverter for example, which incorporate semiconductor element modules, such as IGBTs or power transistors, the semiconductor switching devices often suffer open-circuit failure. This open-circuit failure is caused by detachment of the soldered connection parts in the module by overcurrent that results from a short-circuit of the power source, such as an inverter-phase short-circuit. A phaseshort-circuit current responsible for an open-circuit accident rises to a hundred and tens of thousands of or hundreds of thousands of ampere. When the soldered connection parts open, arc discharge occurs and breaks the surrounding devices and sometimes extends to a greater accident, such as a fire.

One of solutions to this problem has been to 2 insert a high-speed fuse in the arm of each phase of the inverter as described in JP-A-3-78432. This high-speed fuse is used to protect the module element against the outsidewall rupture caused by its breakage by a shirt-circuit of the power source, for example. The above-mentioned patent publication describes how to increase of wiring inductance by connection with a high-speed fuse and how to install a snubber for this purpose.

The above-mentioned requires a large-capacity 10 high-speed fuse and this fuse is relatively expensive. The .use of the high-speed fuse poses a problem: the internal e inductance of this fuse itself and an increase in inductance of the wire for installing the fuse require snubber reinforcement for the semiconductor switching device and lead to loss increases. The results will be cost increase and lower conversion efficiency of the power converter.

SUMMARY OF THE INVENTION To solve the above problem, the object of the present invention is, in power converter using semiconductor element modules, to realize a simple-structured protective device to protect against spread of damage by breakage of the semiconductor element module by an open-circuit failure caused by a power-source short-circuit accident for example and also prevent resulting cost increase and conversion efficiency decrease of the whole power converter.

The object of the present invention can be achieved by installing a protective device in power 3 converter that has connected in series across a directcurrent power source a semiconductor element module as upper and lower arms for one phase, and also has an alternatingcurrent output terminal provided at a series connection point of the semiconductor element module. This protective device for one phase of the power converter comprises a series circuit, including a semiconductor element and a fuse, connected in parallel with the series-connected semiconductor element module. The series circuit is also connected to both ends of the semiconductor element module, wherein the semiconductor element is capable of breaking down or breaking over at an applied voltage higher than a specified voltage and coming into a conducting state.

The object of the present invention can also be 15 achieved by installing a protective device in power •*oo converter that has a semiconductor element module as upper and lower arms for one phase connected in series across a oe direct-current power source and also has an alternatingcurrent output terminal provided at a series connection 20 point of the semiconductor element module. This protective device comprises a series circuit, including a thyristor element and a fuse, connected in parallel with the seriesconnected semiconductor element module. The protective device is also connected to both ends of the semiconductor element module. The thyristor element is controlled so as to turn on at an abnormal voltage by open-circuit failure caused by overcurrent of the semiconductor element module.

4 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a protective device for a power converter using semiconductor element modules according to a first embodiment of the present invention; Fig. 2 is a block diagram of a protective device for a power converter using semiconductor element modules according to a second embodiment of the present invention; Fig. 3 is a block diagram of a protective device for a power converter using semiconductor element modules according to a third embodiment of the present invention; .oo and 4 is a block diagram of a protective device of a power converter using semiconductor element modules according to a fourth embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS Fig. 1 shows an embodiment of the present invention, which is an example in which a protective device in power converter using semiconductor element modules is applied to a two-level inverter type electric car.

Reference numeral 1 denotes a pantograph to supply power to an electric car from an overhead conductor connected to a D.C. voltage, not shown, 2 a line breaker, 3 a charging resistance (or a current reducing resistance) to a filter circuit at the next stage, and 4 a high-speed breaker. 5 denotes a reactor of the filter circuit and forms a LC filter jointly with each of the capacitors separately provided for the respective phases, which will be 5 described later. 6, 7 and 8 denote converters of the Vand W-phases as components of a two-level three-phase power converter for converting a direct current into a three-phase alternating current of two-level potential and vice versa by switching a semiconductor element module by a PWM control circuit, not shown. Shown here as an example is a converter 6 of U-phase.

61 and 62 denote IGBTs of the semiconductor module, which form upper and lower arms of a two-level 10 inverter, and 63 denotes a filter capacitor for one phase.

This capacitor has the voltage of the overhead conductor o :charged in normal operation and this voltage is used as a power supply voltage for the U-phase converter 6. This filter capacitor voltage is similarly used as a power 15 voltage for the V-phase or the W-phase. 64 and 65 denote e e wire inductances, for example, that occur when a conveter is formed by connecting IGBTs or a filter capacitor.

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oo ~66 and 67 denote the parts related to the present invention, and 66 denotes a semiconductor element that breaks down or breaks over at a specified voltage and comes into a conducting state. 67 denotes a fuse. Those two parts are connected in series, and this series circuit is connected in parallel to both ends of IGBTs 61 and 62 connected in series.

68 denotes an alternating current output terminal of the U-phase connected to a connected point of IGBTs 61 and 62 in series. This terminal and the other alternating current output terminals of the V- and W-phases are 7 6 connected to a three-phase induction motor (omitted in Fig.

1) as an inverter load.

In an inverter type electric car configured as described, when the upper and lower arms IGBT61, IGBT62 of the U-phase converter 6 both come into a conducting state caused by a noise-induced malfunction or a breakage of one element for example, which leads to a power-source shortcircuit accident in which the filter capacitor 63 is shorted.

Meanwhile, the U-phase filter capacitor 63 is :connected through the wire inductances 64 and 65 to the S• other V- and W-phases. Generally, the wire inductance of each phase is so low as several U H or less and therefore a o oe power-source short-circuit at the U-phase filter capacitor develops directly to short-circuit accidents at the filter capacitors of the V- and W-phases. Consequently, the powersource short-circuit current flowing through the upper and lower arms IGBT61 and IGBT62 of the U-phase where the accident started surges to a hundred and tens of thousands 20 of or even hundreds of thousands of ampere, very often causing the soldered connection parts to be detached in the IGBT module, which leads to an open-circuit accident. When the soldered connection parts open, arc discharge occurs, and sometimes the surrounding devices are broken or the accident develops to a fire. As a countermeasure, it has been proposed to prevent an open-circuit failure by breakage of the element by inserting a fuse in the arms of each phase of the inverter as described before. Because a current

I,-

7 always flows through the fuse, the fuse is required to have a specified current capacity, which pushes up cost.

Furthermore, the increase in wire inductance attending on the addition of a fuse makes it necessary to reinforce the snubber for each element, which involves a problem of increased loss and so on. The present invention has been made on condition that an open-circuit failure by breakage of the element is unavoidable and has as its object to prevent breakage of the surrounding devices by arc discharge at an open-circuit failure and prevent an accident from spreading to a fire for example.

''*Description will be made of the operation of the structure related to the present invention shown in the embodiment in Fig. 1. When IGBTs 61 and 62 as the upper and r 15 lower arms for the U-phase are both brought into a *o conducting state by a noise-induced malfunction, for example, a power-source short-circuit current of a hundred and tens of thousands of to hundreds of thousands of ampere flows as mentioned above, energy by this power-source short- 20 circuit current is accumulated in the wire inductances 64, Meanwhile, in the module having IGBTs 61, 62 where the power-source short-circuit current flows, the soldered connection parts are detached by Joule heat generated by the overcurrent and those IGBT elements are in an open-circuit accident. The IGBT element being in the open-circuit state (either 61 or 62 or both) forcibly cuts off the power-source short-circuit current, causing arc discharge to flash by the energy stored in the wire inductances 64, 65, for example.

8 This arc discharge energy is huge because the power-source short-circuit current is so large as a hundred and tens of thousands of to hundreds of thousands of ampere even if the wire inductances are small, with the result that the module ruptures, breaking the surrounding devices and sometimes developing to a fire.

The embodiment of the present invention shown in Fig. 1 utilizes an abnormal voltage that occurs at the time of arc discharge and uses the semiconductor element 66 that 10 breaks down or breaks over at this abnormal voltage and comes into a conducting state to suppress arc discharge by *°*inserting the semiconductor element 66 in parallel with .i :IGBTs 61, 62 which may suffer an open-circuit failure, and this embodiment also makes the fuse 67 consume the 15 accumulated energy in the wire inductances.

More specifically, the discharge energy of the IGBT element in the state of open-circuit failure is shunted into the series circuit of the semiconductor element 66 (in a conducting state by abnormal voltage) and the fuse 67.

20 While arc discharge is suppressed by this shunting action, the open-circuit state of the broken IGBT element becomes serious, but when the arc discharge energy has all been commutated to the series circuit, the open-circuit action and the arc discharge of the broken IGBT element come to a stop. In the fuse 67 through which all the arc discharge energy has flown, the moment the consumed energy in the fuse has been added up and the total value exceeds the i 2 t endurable amount of the fuse 67, the fuse starts to melt.

i 9 By consuming the arc discharge energy in the fuse, the fuse 67 prevents arc discharge from occurring in itself. By the actions mentioned above, damage by the breakage of the IGBT element by open-circuit failure can be prevented from spreading to other damage.

Depending on the open-circuit state, in other words, depending on the distance between the open terminals of the broken part of the IGBT element, it is considered possible that arc discharge occurs by a voltage (the filter capacitor voltage in most cases) applied between the open terminals after the above-mentioned fuse blew. However, the °o*arc discharge energy in this case is in the open state of the terminals when a power-source short-circuit state did not exist in advance. Therefore, the amount of energy is 15 smaller than the arc discharge energy by the above-mentioned e*o.

power-source short-circuit current and it is not such a level as to rupture the module. In addition, when a powersource short-circuit occurs, the other protective units will be actuated to cut off a circuit or cause the filter 20 capacitor voltage to be discharged. Therefore, even if arc discharge occurs after the above-mentioned fuse blew, the power converter is automatically protected against it.

The semiconductor element 66 and the fuse 67 in the embodiment in Fig. 1 may be a small-capacity and less expensive one because a current does not flow through them in normal operation of the inverter. Those parts need not be inserted in the arm, and they do not increase the wire inductance to increase loss. As the semiconductor element 0 10 66, a so-called breakover element that breaks over at a specified voltage and comes into a conducting state or a diode that breaks down at a specified voltage and comes into a conducting state is used. The voltage capacity should be selected at a level higher than the maximum peak voltage normally applied to an element that forms an arm of the inverter.

According to the first embodiment of the present invention described above, the effects are, in power converter using semiconductor element modules, to realize a simple-structured protective device to protect against spread of damage by breakage of the semiconductor element module by an open-circuit failure caused by a power-source short-circuit accident for example and also prevent 15 resulting cost increase and conversion efficiency decrease of the whole power converter, including the protective S. device.

Fig. 2 is a schematic diagram of power converter using a semiconductor element module and its protective S 20 device according to a second embodiment of the present invention. A difference from the first embodiment in Fig. 1 is that fuses for respective phases V and W) are put together whereas a fuse for each phase V or W) is included in each inverter in the first embodiment. Note that the other numerals indicate the same components as in the first embodiment in Fig. 1. More specifically, one end of a series-connected semiconductor element module of each of the V- and W-phases is connected to one end of 11 corresponding semiconductor element 66, 76 or 86, which breaks down or breaks over at a voltage higher than a specified voltage and comes into a conducting state. The other end of each semiconductor element is connected to one end of the common fuse 9, and the other end of the 9 is connected to the series-connected semiconductor element module. In case of a short-circuit accident of the U-phase power source, for example, the actions described in detail referring to the first embodiment shown in Fig. 1 take place in the circuit connected in series with the semiconductor element 66 and the fuse 9, arc-discharge accidents are reduced in the IGBT having an open-circuit failure, thereby preventing the accident from spreading to destruction or catching a fire of the surrounding devices.

15 According to a second embodiment of the present invention, the fuses installed separately in the converters of the V- and W-phases as shown in Fig. 1 are put together. Therefore, it becomes possible to further reduce cost of the protective device in a power converter using 20 semiconductor element modules.

Fig. 3 is a block diagram showing a power converter and its protective device using semiconductor modules according to a third embodiment of the present invention. Differences from the embodiments in Figs. 1 and 2 are that the semiconductor element connected in series with the fuse is replaced with a thyristor element and that a control circuit is provided to turn on the thyristor element. More specifically, in the third embodiment, 661 12 denotes a thyristor element, 69 denotes a control circuit for the thyristor 661, and the control circuit 69 is formed by an abnormal voltage detector 691 and a gate circuit 692 of the thyristor. The other numerals indicate the same components in the first embodiment in Fig. 1. In the third embodiment, the abnormal voltage detector 691 detects an abnormal voltage which develops in an open-circuit failure of IGBT 61 or 62 in a power-source short-circuit described above, and in response to output from the abnormal voltage 10 detector 691, the thyristor's gate circuit is driven to turn on the thyristor to enable the series circuit of the S•thyristor element 661 and the fuse 67 to perform the actions described in detail in the first embodiment shown in Fig. i.

Therefore, arc-discharge accidents are reduced in the IGBT that has an open-circuit failure, thereby preventing the accident from spreading to destruction or catching a fire of the surrounding devices.

The abnormal voltage detector 691 is configured to be able to detect an abnormal voltage after a power-source 20 short-circuit, in other words, an abnormal voltage resulting from an open-circuit failure of an element after the voltage across the abnormal voltage detector has become near zero volt by a power-source short-circuit. Therefore, unlike in the first embodiment in Fig. i, in detection of an abnormal voltage, any optional abnormal voltage to detect can be selected regardless of the maximum peak voltage normally applied to the semiconductor elements forming the arms.

Thus, it becomes possible to detect abnormal voltages of 13 smaller voltage values. The effect is that when an opencircuit failure occurs, arc discharge at low energy level can be suppressed securely.

Fig. 4 is a block diagram of a power converter and its protective device using semiconductor modules according to a fourth embodiment of the present invention. A difference from the third embodiment shown in Fig. 3 is that the fuses installed separately in the converters of different phases V and W) in the third embodiment are 10 put together in the fourth embodiment. The other numerals indicate the same components as in the first embodiment.

*°°More specifically, one end of each of the thyristor elements 661, 761 and 861 of respective phases is connected to a corresponding series-connected module of semiconductor elements IGBTs of each phase, and one end of a common fuse 9 is connected to the other end of the corresponding thyristor element. The other end of the fuse 9 is connected to the other end of the series-connected semiconductor element module. When a short-circuit occurs in the power source of i 20 the U-phase, the series circuit of the thyristor 661 and the fuse9 performs the actions described in detail in the third embodiment shown in Fig. 3 to suppress arc discharge at the IGBT that has an open-circuit accident to prevent a spread of the accident to breakage and a fire of the surrounding devices. According to the fourth embodiment, the fuses installed separately for the respective phases V and W) are now put together, and a resulting effect is cost reduction of the protective device for the power converter 14 using a semiconductor element modules.

In the embodiments of the present invention, a two-level inverter circuit was shown and the arm had one element, but they are not restrictive but illustrative, and the present invention may be applied to cases where the arms are formed by a series circuit including two or more elements and also to, for example, inverter circuits for translating a direct current into an alternating current of three-level potentials.

10 The embodiments of the present invention have been 0. described showing cases where the filter capacitor was oeoo* S" installed for each phase, but the filter capacitors may be put gathered.

According to the present invention that has been o* described, in a power converter using semiconductor element modules, a simple-structured protective device against the spread of the open-circuit accident in the semiconductor element module can be realized by inserting a series circuit of a semiconductor element and a fuse, the semiconductor 20 element being capable of breaking over or breaking down at an abnormal voltage at arc discharge caused by an opencircuit accident of a semiconductor element module in a power-source short-circuit, or by inserting a series circuit of a semiconductor switching element and a fuse, the semiconductor switching element being capable of turning on by output from the abnormal voltage detector, between the terminals of the series-connected semiconductor element where a power-source short-circuit may occur. This PA\OPERUCM\22332-0rocspo=c.doo.I5/OI/1 protective device obviates the need to reinforce the snubber or the like, thus preventing cost increase of the whole power converter. An additional effect is that because a current does not flow through the protective device in normal operation of the power converter, a decrease in conversion efficiency can be prevented.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

15 The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

*oo eeo ee*

Claims (4)

1. In a power converter having a semiconductor element module as upper and lower arms for one phase connected in series across a direct-current power source and also having an alternating-current output terminal provided at a series-connection point of said semiconductor element module, a protective device for each phase of said power converter, comprising: a series circuit, including a semiconductor element and a fuse, connected in parallel with said series- e connected semiconductor element module, said series circuit being connected to both ends of said semiconductor element 0. S. module, wherein said semiconductor element is capable of breaking down or breaking over at an applied voltage higher than a specified voltage and coming into a conducting state.

2. In a power converter having a plurality of converters connected across a direct-current power source, each said converter having a semiconductor element module inserted in series as upper and lower arms for one phase and o also having an alternating-current output terminal provided at a series connection point of said semiconductor element module, a protective device for each phase of said power converter, comprising: a semiconductor element for one phase capable of breaking down or breaking over at an applied voltage higher than a specified voltage and coming into a conducting state; and a fuse connected in series with said semiconductor 17 element, wherein said semiconductor element has one end connected to one end of said series-connected semiconductor element module of one phase, and said semiconductor element has the other end connected to one end of a common fuse.

3. In a power converter having connected in series across a direct-current power source a semiconductor element module as upper and lower arms for one phase and also having an alternating-current output terminal provided at a series connection point of said semiconductor element module, a S. protective device for each phase of said power converter, S" comprising: a series circuit, including a thyristor element e. and a fuse, connected in parallel with said series-connected eo semiconductor element module, said series circuit being connected to both ends of said semiconductor element module, wherein said thyristor element is controlled so as to turn on at an abnormal voltage by open-circuit failure caused by overcurrent of said semiconductor element module. S•

4. In a power converter having a plurality of converters connected across a direct-current power source, each said converter having a semiconductor element module inserted in series as upper and lower arms for one phase, and also having an alternating-current output terminal provided at a series connection point of said semiconductor element module, a protective device for each phase of said power converter, comprising: a thyristor element controlled to turn on at an 18 abnormal voltage by open-circuit failure caused by overcurrent of said semiconductor element module, said thyristor element having one end connected to one end of said series-connected semiconductor element module and the other end connected to one end of a common fuse, said fuse having the other end connected to the other end of said series-connected semiconductor element module. SS C edge *o CS S. SC.. P:OPERUCMU2332-00cspco.do -19- A power converter substantially as hereinbefore described with reference to the drawings and/or Examples. DATED this 15 th day of January, 2001 HITACHI, LTD. By its patent attorneys Davies Collison Cave **ee *ee