Power converter for connection to a DC voltage grid

Abstract

An inverter for connection to a DC supply network comprises a fuse and, in series therewith, a controllable circuit breaker, a controller being designed to switch off the circuit breaker if the current flow through the circuit breaker in the direction of the DC supply network exceeds a threshold value.

Description

[0001] Description

[0002] Power converter for connection to a DC voltage network

[0003] The invention relates to a power converter for converting between a direct voltage applied to direct voltage terminals and an alternating voltage applied to alternating voltage terminals, comprising an input circuit with one or more capacitors and an inverter circuit arranged in parallel with the input circuit with a plurality of controllable power semiconductor switches.

[0004] Multiphase electrical loads, such as electric motors in industrial environments, are often powered by converters that convert a three-phase supply voltage from a three-phase AC power grid into a modified three-phase AC voltage. These converters are therefore AC / AC converters.

[0005] Alternative industrial environments use a supply network that is a direct current network. This direct current network has a nominal voltage range between 400 V and 800 V, for example. In such environments, power converters can be used to convert the direct current to the alternating current (AC) required by the load, particularly the three-phase voltage.

[0006] Such an inverter can be protected with a fuse, which interrupts the current flow in the event of a fault. If a fault occurs in a sector of the DC network in which several such inverters are connected, and the fault occurs in one of the more powerful inverters, this inverter would also load the voltage at the input capacitors of the other connected inverters to maintain its short-circuit current. This could detrimentally trigger the fuses of the less powerful inverters. These would then have to be replaced, even though these devices were not responsible for the fault.

[0007] The object of the invention is to provide a power converter for converting between a direct voltage applied to direct voltage terminals and an alternating voltage applied to alternating voltage terminals, which avoids the problem mentioned above. A further object of the invention is to provide an operating method for such a power converter.

[0008] This object is achieved by a power converter having the features specified in claim 1. A further solution consists in the operating method having the features specified in claim 9.

[0009] The power converter according to the invention is designed to convert between a direct voltage applied to the direct voltage terminals and an alternating voltage applied to the alternating voltage terminals. The alternating voltage is a single-phase or multi-phase, in particular a three-phase alternating voltage.

[0010] The power converter comprises an input circuit with one or more input capacitors and an inverter circuit arranged in parallel with the input circuit and comprising a plurality of controllable power semiconductor switches. It is understood that the inverter circuit is connected in parallel with the input circuit with respect to its DC voltage connections. The input circuit corresponds to a DC link of an AC / AC converter, but is not preceded by a rectifier.

[0011] The inverter circuit is coupled to the AC voltage terminals on the output side, while the terminals of the input circuit are coupled to the DC voltage terminals.

[0012] Furthermore, a protective circuit with a fuse is connected between the positive pole of the input circuit and the positive DC voltage terminal. The protective circuit has a disconnect switch in series with the fuse, which includes or is formed by a controllable power semiconductor switch. The controllable power semiconductor switch is expediently a unidirectional switch with an intrinsic diode connected in parallel with the switch or implemented as a separate component.

[0013] In the operating method according to the invention for such a power converter, the circuit breaker is permanently switched on and then switched off when the current flowing from the input circuit to the DC voltage connection exceeds a threshold. For the purpose of the invention, it was recognized that this problem does not occur with AC / AC converters, since the DC link capacitors are only connected to the supply network via a rectifier, whose diodes prevent current reversal. Thus, the design prevents direct discharge of the DC link capacitors in the event of a fault in another AC / AC frequency converter connected to the supply network.

[0014] The design according to the invention can prevent the fuse from tripping if the circuit breaker is suitably connected if a fault occurs outside the power converter and therefore a connected supply voltage drops so sharply that a current flowing back from the input capacitors into the DC voltage network could cause the fuse to trip.

[0015] The term “external” refers to a fault location that is not in the power converter itself or in its load, but in the supply network or devices connected to it.

[0016] Advantageously, the power converter therefore corresponds to device class 2, according to which a device must not be damaged in the event of a short circuit in the same sector of the DC voltage network, which also includes the triggering of the fuse.

[0017] When comparing the design of an AC / AC converter with the present DC / AC converter, there is no longer a choke in the intermediate circuit required in an AC / AC converter, and the space thus freed up can be advantageously used to accommodate the fuse and the disconnector.

[0018] Advantageous embodiments of the power converter according to the invention emerge from the dependent claims. The embodiment of the independent claims can be combined with the features of one of the subclaims or, preferably, with those of several subclaims. Accordingly, the following additional features can be provided:

[0019] The negative pole of the input circuit can form the negative DC voltage terminal. It is also possible for an additional fuse to be connected between the negative pole of the input circuit and the negative DC voltage terminal. The protective circuit preferably includes a control unit for the disconnect switch. The control unit can be part of a control unit for the entire power converter, which, for example, also controls switches in the inverter circuit. For this purpose, the control unit is expediently connected to the control terminals of the controlled power semiconductor switches via driver circuits.

[0020] The controller is conveniently designed to deactivate the circuit breaker when the current flowing from the input circuit to the DC voltage terminal exceeds a threshold. In other words, the current considered here flows through the fuse and the circuit breaker, and out of the power converter toward the supply network, not toward a connected load such as an electric motor. Since the direction of the current is considered, it is understood that the comparison with the threshold value takes into account the signs, and the exceedance is considered based on the magnitude.

[0021] The threshold value is advantageously set so that the shutdown occurs before the fuse trips and interrupts the current. This control design prevents the fuse from tripping caused by a fault external to its own power converter. In other words, the fuse reacts preferentially only to a current flow into the power converter, i.e., to a fault in the power converter itself.

[0022] Again, “outside” refers to a fault location that is not in the power converter itself or in its load, but in the supply network or other devices connected to it.

[0023] If current flows in the direction corresponding to the power flow to the converter's load—i.e., from the DC network into the converter—the circuit breaker will not be switched off, even if the current exceeds the threshold. Firstly, this current direction is not taken into account for the circuit breaker's operation, since the fuse is supposed to trip here. Secondly, switching off the circuit breaker would have no effect in this case, since its reverse diode would take over the current conduction.

[0024] The controller can further be configured to leave the disconnector in the switched-on state during normal operation of the power converter. During normal operation, the current flowing through the disconnector does not exceed the threshold. Normal operation occurs when the voltage in the input circuit is approximately equal to the voltage of the DC network and the voltage in the input circuit is within a permissible voltage range for the respective power converter, for example, between 450 V and 600 V.

[0025] The advantage of permanently switching the disconnector on is that the series resistance for the supply current, i.e., the power flow to the connected load of the converter, is reduced compared to the resistance that the disconnector's reverse diode would offer alone. This reduces losses compared to operation with the disconnector off. Furthermore, a permanently switched disconnector allows excess energy from the load to be fed back into the DC voltage network, for example, during braking of a connected motor, without the need for an explicit switching action of the disconnector.

[0026] The controller may further be configured to switch off the circuit breaker during operation of the power converter when precharging of the capacitors of the input circuit takes place.

[0027] The control system can therefore be designed to switch off the isolating switch for pre-charging and in the event of a current to the DC voltage network that exceeds the threshold value, and to leave it switched on in other operating states.

[0028] The power converter expediently includes a device for determining the current through the circuit breaker, for example, a voltage measuring device, wherein the voltage measuring device is arranged such that a voltage drop across the circuit breaker is determined. The voltage measuring device thus uses the on-state resistance of the circuit breaker to measure the current. The determined voltage is essentially proportional to the magnitude of the current to be measured. Expediently, the current direction is also determined based on the sign of the resulting voltage.

[0029] The power converter can advantageously comprise further voltage measuring devices, for example for determining the voltage across the input capacitors and / or for determining the voltage in the DC voltage network.

[0030] In a preferred embodiment of the invention, the power converter has an overvoltage protection device for the disconnector. This can be, for example, a TVS diode arranged in parallel with the disconnector or a varistor arranged in parallel with the disconnector. This protects the disconnector against destruction by overvoltage. Overvoltages can occur dynamically when the disconnector is switched off, since inductively stored energy on both sides of the disconnector continues to drive the interrupted current shortly after switching off. This can cause the voltage across the disconnector to rise above the static voltage difference. The overvoltage protection device limits this voltage, for example by becoming conductive when a voltage threshold is exceeded. The overvoltage protection device can also include a capacitor for voltage limitation.

[0031] In a further preferred embodiment of the invention, the power converter comprises a discharge circuit connected in parallel with the input circuit. The discharge circuit comprises a controllable power semiconductor switch in series with a discharge resistor. A diode can be present in parallel with the discharge resistor. The discharge circuit enables the conversion of excess energy present on the load side into heat, for example, the conversion of the energy in a connected electric motor when it is braked.

[0032] In principle, the power converter allows the excess energy generated in this way to be fed back into the DC network. However, if the circuit breaker is switched off to protect the fuse, meaning there is a fault in the DC network, then the excess energy cannot be fed back into the DC network. To limit a voltage buildup in the input circuit in such a case, the controllable power semiconductor switch can be switched on or switched on in a pulsed manner, thus enabling a current flow through the discharge resistor, which converts excess energy into heat.

[0033] The inverter circuit is expediently a circuit for converting the DC voltage applied to the input circuit into a three-phase AC voltage. Known circuits can be used for this purpose. For example, the inverter circuit can comprise three parallel-connected half-bridges, each with two series-connected power semiconductor switches. The power semiconductor switches can be, for example, IGBTs or MOSFETs. Alternatively, other inverter circuits, for example, those with more than two voltage levels, can be used. Alternatively, the AC voltage can also be a single-phase AC voltage, and the inverter circuit can be a corresponding circuit, for example, with two parallel-connected half-bridges.

[0034] The inverter circuit is referred to as such, but is conveniently bidirectional and thus also capable of regenerative power, in which case it operates as a rectifier.

[0035] After the circuit breaker is switched off or the fuse blows, it is advisable to subsequently adjust the internal voltage at the input circuit to the voltage of the DC network. To do this, the input capacitors are discharged before the device is actively reconnected to the DC network.

[0036] Compensating currents caused by voltage differences between the input circuit and the DC network can destroy the circuit breaker or, in turn, inadvertently trigger the fuse. Advantageously, the power converter comprises a precharging circuit with which the current into or out of the input circuit can be limited. The precharging circuit can comprise a precharging resistor connected in series with the input capacitors. A bridging element, in particular a relay, can be connected in parallel with the precharging resistor. The controller can be configured to deactivate the relay only for the duration of the precharging, i.e., when the difference between the voltages in the input circuit and the DC network is sufficiently high, so that the current must pass through the precharging resistor.

[0037] If the voltage in the input circuit is lower than the voltage in the DC network, the input capacitors are recharged via the intrinsic diode of the power semiconductor switch in the circuit breaker. If the voltage in the input circuit is lower than the voltage in the DC network, the circuit breaker is conveniently switched on, possibly with a pre-charging resistor connected to achieve voltage equalization, or the previously described discharge circuit is used.

[0038] The invention is described and explained in more detail below with reference to the exemplary embodiments shown in the figures. They show:

[0039] Figure 1 shows a DC voltage network with two DC voltage sectors and several power converters with different connected loads, Figure 2 shows a circuit diagram of a power converter.

[0040] Figure 1 shows an example DC voltage network 10. In this example, the DC voltage network 10 is a supply network for machines in a factory hall. It uses a nominal voltage of 800 V. It further comprises two sectors 11, 12, at the interface of which a DC switch 14 is arranged. The DC switch 14 can cause the sectors 11, 12 to be isolated if a fault occurs in one of the sectors and a resulting voltage drop is detected.

[0041] In each of the sectors 11, 12, a plurality of power converters 15a...g are connected to the DC voltage network 10. The power converters 15a...g each supply a load 16a...g assigned to them. The loads 16a...g can be, for example, electrical machines, i.e., electric motors.

[0042] In this example, loads 16a...g have partially different rated powers. For example, loads 16a, d in this example have a rated power of 10 kW, loads 16b, e, g have a rated power of 5 kW, and loads 16c, f have a rated power of 50 kW.

[0043] Figure 2 shows a power converter 15a...g according to an exemplary embodiment of the invention. The power converter 15a...g comprises DC voltage terminals 21, 22 and AC voltage terminals 23, 24, 25. The DC voltage terminals 21, 22 are connected to the DC voltage network 10 (not shown in Figure 2). The AC voltage terminals 23, 24, 25 are connected to a load 16a...g, here an electric motor.

[0044] The power converter 15a...g comprises an input circuit 30, which in this example comprises a series connection of two input capacitors 31, 32 and a precharging circuit 33. The precharging circuit is a parallel connection of a relay 34 and a precharging resistor 35. The negative pole of the input circuit 30 forms the negative DC voltage terminal 22.

[0045] The power converter 15a...g has an inverter circuit 40 connected in parallel with the input circuit 30, i.e., between its negative and positive poles. The inverter circuit 40 comprises three parallel-connected half-bridges 41, 42, 43, each of which has a series connection of an upper and a lower power semiconductor switch, here IGBTs 44a...f. The nodes between each upper and lower IGBT 44a...f form the AC voltage terminals of the power converter 15a...g.

[0046] Also connected in parallel to the input circuit 30, i.e. between its negative and positive poles, the power converter 15a...g has a discharge circuit 50 which is a series of a discharge switch 51 and a parallel circuit of a diode 52 and a discharge resistor 53.

[0047] A protection circuit 60 is arranged between the positive pole of the input circuit 30 and the positive DC voltage terminal 21. The protection circuit 60 comprises a series connection of a fuse 61 and a circuit breaker, which in this example is formed by an IGBT 62.

[0048] The IGBT 62 is arranged in such a way that it can switch off and block currents flowing from the positive pole of the input circuit 30 toward the DC voltage network 10. Currents in this direction can be referred to as feedback currents, since they do not serve to supply the connected load 16a...g, but rather cause a power flow away from the load 16a...g.

[0049] Currents in the opposite direction, i.e., from the DC voltage network 10 through the protection circuit 60 into the input circuit 30, supply the load 16a...g and thus correspond to the standard operating state of the converter 15a...g. Such supply currents cannot be blocked by the IGBT 62, but are passed through by the reverse diode connected in parallel.

[0050] A bidirectional TVS diode 64 (suppressor diode) is arranged in parallel with the IGBT 62 in the circuit breaker. This provides overvoltage protection for the IGBT 62. If a voltage builds up across the IGBT 62 during operation that could destroy it, the TVS diode 64 becomes conductive, thus preventing the voltage across the IGBT 62 from increasing further.

[0051] A voltage measuring device 66 is also arranged in parallel to the IGBT 62.

[0052] Voltage measurement device 66 thus uses the on-state resistance of IGBT 62 as a measuring resistor and thereby determines the voltage drop across IGBT 62. Finally, power converter 15a...g includes a controller 70 for controlling the respective control contacts (gates) of the power semiconductor switches, i.e., in this case, IGBTs 44a...f, 62. Voltage measurement device 66 is also connected to controller 70. The controller can be in contact with higher-level control devices, which, for example, provide instructions for controlling the respective load 16a...g to controller 70 and receive measured values ​​from it.

[0053] In standard operation, i.e., with a load connected and a small difference between the voltage in the DC voltage network 10 and in the input circuit 30, the controller 70 keeps the IGBT 62 in the switched-on state. A change between the power flow directions is thus possible at any time without requiring any intervention in the IGBT 62.

[0054] In the event of a fault (short circuit) within the 15a...g converter itself, which leads to a high current flow into the 15a...g converter, the fuse interrupts the current flow. The circuit breaker, i.e., the IGBT 62, does not block such a current, regardless of its switching state.

[0055] However, if a fault occurs in one of the other power converters 15a...g, especially in one with a higher rated power than the power converter 15a...g under consideration, the voltage in the DC voltage network 10 drops, resulting in a current flow from the input circuit 30 into the DC voltage network 10, analogous to a feedback current. By measuring the voltage with the device 66, the controller 70 can determine that the voltage across the IGBT 62, and thus the proportional current through the IGBT 62, exceeds a specified threshold.

[0056] This threshold applies only to this current direction, but not to current flowing into the power converter 15a...g. In other words, the sign of the voltage is taken into account. If the threshold is exceeded, the IGBT 62 is switched off, interrupting the current flow from the power converter 15a...g. The threshold is selected so that the current flow is interrupted in such a way that the fuse does not blow. This prevents a fault external to a power converter 15a...g under consideration from blowing its fuse.

[0057] If the IGBT 62 is switched off, no excess energy can be fed back from the

[0058] Load 16a...g. If such excess energy still occurs, the discharge circuit 50 is used to dissipate the energy. For this purpose, the discharge switch 51 is turned on. This conveniently occurs in a clocked manner to control the power dissipation. This creates a controlled freewheeling circuit via the discharge resistor 53, which converts the resulting energy into heat and thus ensures its dissipation.

[0059] The IGBT 62 remains switched off as long as the voltage in the DC network 10 is lower than the voltage in the input circuit 30, i.e. UDC < Uj n . In one embodiment, switching on can occur if UDC >= Uj n and Uj n is within a permitted voltage range. The permitted voltage range depends on the design of the 15a...g converter and can, for example, be a range from 620 V to 780 V.

[0060] Another operating state in which IGBT 62 is switched off is the precharging of the capacitors 31, 32 of the input circuit 30. This occurs to bring the voltage of the capacitors 31, 32 from a lower voltage to approximately the voltage of the DC voltage network 10. For this purpose, the precharging circuit is used, and by switching off the relay 34, the precharging resistor is introduced into the current path, thereby limiting the current flowing into the capacitors 31, 32. If Uj n » UDC and Uj n even in the permitted voltage range, the IGBT 62 is switched on and normal operation begins.

[0061] Reference symbol

[0062] 10 DC network

[0063] 11, 12 sectors of the DC network

[0064] 14 DC switches

[0065] 15a... g power converter

[0066] 16a... g load

[0067] 21 , 22 DC voltage connections

[0068] 23, 24, 25 AC voltage connections

[0069] 30 input circuit

[0070] 31, 32 input capacitors

[0071] 33 Precharge circuit

[0072] 34 relays

[0073] 35 Precharge resistor

[0074] 40 Inverter circuit

[0075] 41, 42, 43 half bridges

[0076] 44a...f IGBTs

[0077] 50 discharge circuit

[0078] 51 Discharge switch

[0079] 52 diodes

[0080] 53 Discharge resistance

[0081] 60 protective circuit

[0082] 61 Fuse

[0083] 62 IGBT

[0084] 64 TVS diode

[0085] 66 Device for measuring voltage

[0086] 70 Control

Claims

Patent claims 1. Power converter (15a... g) for converting between a DC voltage applied to DC voltage terminals (21, 22) and an AC voltage applied to AC voltage terminals (23, 24, 25), comprising - an input circuit (30) with one or more input capacitors (31, 32), the negative pole of which is connected to the negative DC voltage terminal (22) of the power converter, - an inverter circuit (40) arranged in parallel with the input circuit (30) and having a plurality of controllable power semiconductor switches (44a...f), wherein the inverter circuit (40) is coupled to the AC voltage terminals (23, 24, 25), - a protective circuit (60) with a fuse (61) between the positive pole of the input circuit (30) and the positive DC voltage terminal (21), wherein the protective circuit (60) has a circuit breaker (62) in series with the fuse (61), which is a controllable power semiconductor switch.

2. Power converter (15a...g) according to claim 1, wherein the protective circuit (60) has a control (70) for the isolating switch (62), which is designed to switch off the isolating switch (62) when a current flowing from the input circuit (30) to the DC voltage terminal (21) exceeds a threshold value.

3. Power converter (15a...g) according to claim 2, wherein the controller (70) is designed to leave the isolating switch (62) in the switched-on state during operation of the power converter (15a...g) if the current does not exceed the threshold value.

4. Power converter (15a...g) according to claim 2 or 3, wherein the controller (70) is designed to switch off the isolating switch (62) during operation of the power converter (15a...g) when a pre-charging of the input capacitors (31, 32) of the input circuit (30) takes place.

5. Power converter (15a...g) according to one of the preceding claims, which has a device (66) for determining the current through the circuit breaker (62).

6. Power converter (15a...g) according to claim 5, wherein the device (66) for determining the current through the isolating switch (62) comprises a voltage measuring device (66), wherein the voltage measuring device (66) is arranged such that a voltage drop across the isolating switch (62) is determined.

7. Power converter (15a...g) according to one of the preceding claims with an overvoltage protection device (64) for the isolating switch (62), in particular a TVS diode (64) arranged in parallel with the isolating switch or a varistor arranged in parallel with the isolating switch (62).

8. Power converter (15a...g) according to one of the preceding claims, comprising a discharge circuit (50) connected in parallel to the input circuit (30), wherein the discharge circuit (50) comprises a controllable power semiconductor switch (51) in series with a discharge resistor (53).

9. Operating method for a power converter (15a...g) according to one of the preceding claims, in which the isolating switch (62) is permanently switched on and is switched off when a current flowing from the input circuit (30) to the DC voltage terminal (21) exceeds a threshold value.

10. Operating method according to claim 9, wherein the isolating switch (62) is switched off for precharging the input capacitors (31, 32) of the input circuit (30).

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