FI122617B - Security arrangements - Google Patents

Description

PROTECTION ARRANGEMENT

TECHNICAL FIELD The present invention relates to an arrangement for protecting a power electronics device, such as a frequency converter, in the event of a fault. In particular, the invention relates to a protection arrangement of the resistors limiting the charging current of the DC capacitor filter capacitor when the DC bus or motor cable has an earth fault during charging.

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Prior art

Of the many alternatives to converting drives, PWM technology has become the most common due to its many known technical advantages.

15 In the solution, the voltage of the supply network is rectified, the DC voltage of the DC link formed is filtered by a high capacitance capacitor, and finally, by means of fast power semiconductor switches, an output voltage consisting of pulses is generated.

20 Because of the high capacitance capacitor, the drive cannot be directly connected to the supply network, but the capacitors must first be charged with a limited current to almost their final voltage to avoid a large switching current surge. According to the prior art, the charging of capacitors is most commonly arranged by means of the charging resistors 25, for example as shown in Figs. 2A and 2B. Charging resistors are also used in some applications to maintain capacitor voltage while the device is in sleep mode. The normal way is to select resistors of the same size for all stages ^ for the sake of uniform heating, simplicity of component selection, etc. ^.

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The patent publication WO 2008/031915 has become known as the so-called.

£ dynamic braking, which means short-circuiting the motor terminals

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by controlling the power semiconductor switches of the lower or upper branches of the drive output phases simultaneously. The switching of the switches to conductive m ° 35 can be arranged to be carried out at very low voltage according to F 20085922, i.e. while the DC capacitor is still charging.

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Due to mechanical knocks, installation faults, aging of the insulation, etc., it is sometimes the case that there is an earth fault in the cable between the drive and the motor or inside the motor, i.e. a low impedance connection between the phase conductor and the ground potential. In such a situation, with the output phase earth fault and the dynamic brake activated, an intermediate circuit capacitor charging circuit is formed where the charging resistor current does not charge the capacitor but passes directly through the charging resistor between the earth fault and the supply network. In such a situation, for a sufficient time 10, the resistor is destroyed, since economic dimensioning generally requires consideration only of the normal capacitor charge situation. When the charging resistor is destroyed, the drive becomes inoperable, which means a major maintenance operation and a long downtime.

It is known to use fuses or a circuit breaker in the charging circuit as an overcurrent and short-circuit protection, and it is desirable, even in the above-described grounding situation, to operate before the charging resistors are destroyed. In this case, replacing the fuses or drying the circuit breaker (in addition to removing the earth fault itself) would be sufficient, which is considerably less than repairing or replacing the drive.

The problem here, however, is that the charge current in the fault situation is not necessarily so high that a fuse or circuit breaker dimensioned according to the normal charge current surge would operate fast enough to protect the selected resistor on the same basis. As a result, the resistors may need to be oversized, which increases costs and space.

Summary of the Invention

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It is an object of the present invention to provide a novel arrangement for protecting a power electronics device, such as a frequency converter, in the event of a failure, which avoids the aforementioned drawbacks and

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it is made possible that in the event of a failure, the charging circuit protection component, e.g., a circuit breaker, will surely work before the resistors or other components on the m ° 35 current path are damaged. This object is achieved by an arrangement according to the invention, which is characterized in what is stated in the characterizing part of the independent claim. Other preferred embodiments of the invention are claimed in the dependent claims.

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According to the invention, the charging resistors are located in at least two phases, and their resistance values are dimensioned such that in one phase the resistance value is clearly lower than in the other 5 phases involved in charging.

In a normal charge, the resistors operate in series, and according to a preferred embodiment of the invention, the sum of the resistance values limiting the charge current is the same as the sum of the conventionally dimensioned resistance values (this applies when charging is performed in two steps).

In the event of a fault where the earth fault is either in the motor cable or in the drive intermediate circuit, instead of the phase with a low resistor resistance, the fault current is so high that the charging circuit protection component 15 e.g. the circuit breaker quickly cuts off the charging circuit.

The application of the invention does not require any particular dimensioning of the conductor or device protecting component, e.g., a circuit breaker or any additional components. Thus, it does not increase the cost compared to a conventional solution, but eliminates one possibility of device failure.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in more detail by way of example with reference to the accompanying drawings, in which:

Figure 1 shows the main circuit and motor of the frequency converter, Figure 2A shows a known intermediate circuit charging arrangement, Figure 2B shows a known intermediate circuit charging arrangement,

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Fig. 3 shows the charging current and the intermediate circuit voltage, o Fig. 4 shows a known embodiment of a dynamic brake, g Fig. 5 shows a circuit in the event of a failure,

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Figure 6 shows the charging circuit currents in the event of a failure,

Fig. 7 shows the charging circuit currents in the event of a failure with the charging arrangement according to the invention m ° 35.

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Detailed Description of the Invention

FIG. output voltage (steps 10 U, V, W). The frequency converter can supply, for example, a three-phase AC motor M. The load bridge INU is a full-wave bridge where the control unit CU controls pulsed-width modulation of the top and bottom branched power semiconductor switches (normally IGBT) with parallel diodes.

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In this example, the network bridge REC is a non-controllable diode full-wave bridge. The network bridge may also be controlled so that it incorporates the same three-phase controlled bridge switching as the load bridge INU.

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To limit mains current harmonics, a filter may be coupled between the mains bridge and the supply network, which in its simplest form is a three-phase AC choke, or, in the case of a diode bridge, a DC choke coupled between the mains bridge and DC link.

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Because of the considerable capacitance value of the discharge capacitor Cdc, it must normally be charged with special arrangements before the network bridge can be connected to the supply network. Figures 2A and 2B show the most common charging solutions based on the use of charging resistors.

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9 30 o In Fig. 2A, a main contactor Ki is used whose contacts ϊ remain open until the intermediate circuit capacitor Cdc is nearly charged

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to its final voltage through resistors Rn and R12. For safety reasons, e.g. for a short circuit, it is normal to protect the device with, for example, the figure

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° 35 with fuses F1 ... F3. Because the fuses are sized according to the main current, protection of the cm charging resistors may require smaller additional fuses or a circuit breaker (not shown).

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The solution of Fig. 2B uses a separate charge circuit including resistors R1 and R2 and a diode bridge REC1. The resistors may also be located between the rectifier bridge and the filter capacitor CDc (R1 'and R2'). Because the charging circuit has a rated current less than that of the main circuit, it typically uses 5 thin conductors which, for safety reasons, are protected by, for example, the circuit breaker K2 shown in the figure. When the charging situation has ended, i.e. when the capacitor Cdc has been charged to almost full voltage, the contacts of the main contactor K1 are closed. The charging circuit may remain connected to the main contactor or may be opened, for example, via auxiliary contacts of the main contactor (not shown).

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Figure 3 shows, in relation to Figure 2B, characteristic curves of the charge current iRi and DC link voltage udc as a function of time t. The peak load current Irip occurs immediately at the intermediate circuit voltage of 0 and decreases as the intermediate circuit voltage Udc increases. If the 15 Ki contacts of the main contactor are not closed immediately after charging is complete, as is possible in some applications, the charging current will remain to supply internal drive load. This level of charge current amplitude is indicated in the figure by Irh_.

20 According to WO 2008/031915, a so-called. dynamic braking can be accomplished by controlling all power semiconductor switches on the same branch of the load bridge to conduct a short circuit to the motor circuit. In the example of Figure 4, such a short circuit formed by controlling the downstream power semiconductor switches Vu2, V2, and VW2 is illustrated by drawing it with a thick line and other components irrelevant to this function and by a thin dashed line.

^ Short circuit control for dynamic brake circuit is advantageous ^ done with as low intermediate circuit voltage as possible

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9 30, for example, according to the method disclosed in Fl 20085922. If there is an earth fault in the o motor circuit in such a situation, the charging circuit £ in Fig. 2B may, during the charging of the intermediate circuit, form an example of Fig. 5.

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in the example where the earth fault current is marked by a dashed line, ί In the example, the earth fault current is in the W phase of the motor circuit, of which the earth fault current has a direct path past the intermediate circuit capacitor m ° 35 through the lower branch of the W phase power semiconductor switch Vw2, DC via diodes, resistor R1 or R2, and circuit breaker K2 for 6 to a phase with a negative current to ground potential.

Figure 6 illustrates the characteristic curves of currents as a function of time t 5 in the example case of Figure 5. The positive half-cycles of the currents h and k (above the 0-line) that occur when the instantaneous values of the voltages Li and l_2 are positive charge the intermediate circuit capacitor Cdc and decrease in amplitude from peak hp as the capacitor voltage rises to a final value. In contrast, the current pulses traveling during the negative half-cycles of the phase voltages do not charge the capacitor, but run along the paths shown in the thick line of Figure 5, so that they are limited only by resistors R1 and R2. Thus, the negative peak peak current of -hp is not affected by the charging of the capacitor.

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The problem with this situation is that since the charging current remains permanently high in the fault situation described, the heating power of the charging resistors R1 and R2 also remains high, which is not normally rated for. However, dimensioning the fuses or circuit breaker K2 to protect the resistors is difficult because the fault current may not deviate so much from the normal dimensioning that they operate fast enough.

The solution to the problem according to the invention is that the resistance value of one resistor R1 or R2 is clearly lower than that of the other. For example, if traditionally dimensioned, the resistances of both resistors are 1, according to the invention, the value of R1 can be 0.1 and the value of R2 is 1.9. When dimensioned this way, the magnitude of the normal charge current remains unchanged, but in the event of a fault, the peak values of the earth fault current through Ri increase with the resistance values,

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o 30 or 1 / 0.1 = 10 times. In this case, the current may be so high that the component protecting the conductors or the device o, e.g., a circuit breaker, will operate quickly enough to protect the resistors. It is obvious to a person skilled in the art that everyone

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to protect the phases, the protective component must be multipole, ie it must cut off all phases. No phase-specific fuse

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° 35 such protection may be provided, o

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Fig. 7 shows characteristic curves of currents when the load resistors are dimensioned according to the invention. The current in describes the current through resistor R1, which has a lower resistance value in this example, whose positive peak values fall from lup to lnL when the capacitor charges, and whose negative peak values remain at -lup. Similarly, ii2 represents the current of resistor R2, which is less in relation to the resistor values (peak values h2p, I12L, and -li2p).

The ratio of resistance values in accordance with the invention can easily determine how large a fault current is generated, which makes it easy to select a circuit breaker. In principle, the value of the lower resistor can be up to 0 (i.e., short-circuiting) if the components in the current path can withstand the stress of a high earth fault current before the conductors or component protecting the device operate.

It will be apparent to one skilled in the art that various embodiments of the invention are not limited to the above example, but may vary within the scope of the following claims. The dimensioning of the charging resistors of the invention also protects them, for example, when either terminal of the intermediate circuit capacitor is earthed or the capacitor is energized through the charging circuit when the application requires the main contactor 20 to be opened while the device is in sleep mode. If the charging circuit is three-phase, the smaller resistor according to the invention is used in only one phase.

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Claims (3)

8 An arrangement for protecting a power electronics device, such as a frequency converter (REC, INU), in the event of a failure, 5 having a high capacitance capacitor (Cdc) charged from at least two phases of the grounded supply network by phase-dependent charging resistors (R1, R2) charged while the device is in Sleep mode, and wherein the step-by-step charging currents pass through the same multi-phase conductor or device protecting component, e.g., a circuit breaker (K2); during the charge current, characterized in that the resistance value of one phase charge resistor is lower than that of the other phase charge resistors, and 20 m the single lower resistance value is dimensioned such that the earth-fault current flowing through it causes an overcurrent of said multi-phase protective component and thereby breaks the charging circuit before the pulsed energy tolerance of any charging resistor or other component in the current path is substantially exceeded. 25 An arrangement according to claim 1, characterized in that said lower resistance value is less than 50% of the other resistance values. CM CM o 30 Arrangement according to Claim 1 or 2, characterized in that the arrangement is arranged to protect the resistors limiting the charging current 1 of the DC capacitor of the DC power supply device Q_ with a DC link. tn 1 35 o CM 9