WO2014102443A1

WO2014102443A1 - Semiconductor switch arrangement

Info

Publication number: WO2014102443A1
Application number: PCT/FI2012/051306
Authority: WO
Inventors: Marko TAKALA; Tero KENTALA; Harri Mattlar; Matti KÄHKIPURO; Tommi Rantanen
Original assignee: Abb Oy
Priority date: 2012-12-28
Filing date: 2012-12-28
Publication date: 2014-07-03
Also published as: CN104885363A

Abstract

A semiconductor switch arrangement comprising for each current phase a first and a second semiconductor switch (2a, 2b), a first and a second diode (3a, 3b), means for measuring voltages from input and output lines of said semiconductor switch and means for measuring current (8) arranged in series with said semiconductor switch. Additionally, the arrangement comprises a control element (14) configured to switch on the semiconductor switch arrangement at zero-voltage point and to switch off the semiconductor switches at zero-current point.

Description

SEMICONDUCTOR SWITCH ARRANGEMENT

BACKGROUND

The present invention relates to a method and an arrangement for switching electric circuits.

One of the problems associated with conventional mechanical switches is that an arc tends to develop between the contact surfaces at switch off, which causes wear of the switches.

BRIEF DESCRIPTION

An object of the present solution is thus to provide a new method and an arrangement for implementing the method. The objects of the invention are achieved by a method and an arrangement which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The solution is based on the idea that an electric circuit is switched on as off step by step utilizing the characteristics of a pair of semiconductor switches for each current phase and a diode connected in series with each semiconductor switch, wherein each diode may be an external diode or an internal diode of the other semiconductor switch.

An advantageous feature of the method and arrangement of the so- lution is that it is possible to arrange a zero-voltage connection and a zero- current disconnection of an electric circuit, which enables providing a solution that doesn't develop an arc at switch off.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the solution will be described in greater detail by means of preferred embodiments with reference to the attached [accompanying] drawings, in which

Figures 1 a and 1 b are schematic views of a semiconductor switch arrangements for a 1 -phase AC electric circuit;

Figure 2 shows schematically a method for switching current in an electric circuit;;

Figure 3 shows schematically another method for switching current in an electric circuit;

Figure 4 is a schematic graph illustrating phase voltage and phase current with respect to time during a switch on event in an electric circuit com- prising a semiconductor switch arrangement;

Figure 5 is a schematic graph illustrating phase voltage and phase current with respect to time during a switch off event in an electric circuit comprising a semiconductor switch arrangement;

Figure 6 illustrates schematically an embodiment of a semiconductor switch arrangement;

Figure 7 illustrates schematically another embodiment of a semiconductor switch arrangement;

Figure 8 illustrates schematically a semiconductor switch arrange- ment for a 3-phase AC electric circuit as a block diagram;

Figure 9 illustrates schematically switching on a 3-phase semiconductor switch arrangement with zero-voltage as a block diagram;

Figure 10 illustrates schematically switching off a 3-phase semiconductor switch arrangement with zero-current as a block diagram;

Figure 1 1 illustrates schematically as a block diagram an overcur- rent switch off logic for an embodiment comprising an active crowbar;

Figure 12 illustrates schematically an embodiment of a semiconductor switch arrangement for a 3-phase AC electric circuit comprising a serial isolation switch; and

Figure 13 illustrates schematically a method for switching off an electric circuit at an overcurrent event.

DETAILED DESCRIPTION

Parts and components of similar structure and/or function are re- ferred to with same reference numbers in the figures.

Figures 1 a and 1 b are schematic views of a semiconductor switch arrangements for a 1 -phase AC electric circuit. The embodiment illustrated in Figure 1 a comprises a semiconductor switch arrangement 1 connected to a 1 - phase alternating current (AC) electric circuit. The semiconductor switch ar- rangement of Figure 1 a comprises a first semiconductor switch 2a and a second semiconductor switch 2b connected in parallel. A first diode 3a is connected in series with the first semiconductor switch 2a and a second diode 3b is connected in series with the second semiconductor switch 2b, such that the forward-biased directions of the first and the second diode and the internal di- ode of the respective first and second semiconductor switch connected in se- ries with the diode are opposite. A 1 -phase AC source 4, an input resistance 5a, an input inductance 5b, a load current 10, a load resistance 6a and load inductance are also illustrated in the figure. Additionally, the arrangement of Figure 1 a also comprises means for measuring voltage 7 and means for measuring current 8 arranged in series with said semiconductor switch to measure current. Preferably, the semiconductor switch arrangement also comprises a overvoltage protection element 9 for absorbing the inductive energy caused by the switch off the electric circuit in an overcurrent event. The over- voltage protection element 9 can, thus, be used to limit the voltage over the semiconductor switches to prevent them from breaking. The semiconductor switch arrangement of Figure 1 a may be used to control switching the electric circuit on at a zero-voltage point and off at a zero-current point.

The switch on at a zero-voltage point may be achieved by a method according to Figure 2, wherein the polarity of the phase voltage is detected 201 in response to a command to switch on the electric circuit. The polarity of the phase voltage may be detected by the means for measuring voltage 7. The semiconductor switch that is connected in series with the diode that is reverse- biased may then be switched on 202 to a conductive state. This means that if the voltage is positive, the second semiconductor switch 2b is switched on to a conductive state, and if the voltage is negative, the first semiconductor switch 2a is switched on to a conductive state. At this point, no current is flowing in the main circuit, because the respective diode 3b or diode 3a connected in series with the semiconductor switch 2b, 2a switched on to a conductive state is reverse-biased and, thus, prevents the current flow in the circuit. When the polarity of the phase voltage is changed, the current starts to flow in the main circuit through the semiconductor switch that is in conductive state, which is the first semiconductor switch 2a or the second semiconductor switch 2b depending on the original polarity of the voltage, and the respective diode, which is the first diode 3a or the second diode 3b respectively and depending on the original polarity of the voltage, that is connected in the forward direction after the change of the polarity of the phase current. Simultaneously, the other semiconductor switch that was still switched off, that is the semiconductor switch that is connected in series with the diode that was connected in forward direction during the originally detected polarity of the voltage, is switched on 203 to a conductive state in response to the change of polarity of the phase voltage. Hence, the current flows in the main circuit during both the current polarities. Also the second polarity change of the phase voltage may be detected by means for measuring voltage 7. The main circuit was, thus, switched on at zero-voltage. This is a beneficial way to switch on an electric circuit as current peaks can be avoided and there is no voltage shock related to the switch on event and, thus, electromagnetic interference can be avoided, for example.

An example of the switching on an electric circuit with such an arrangement and according to such a method is shown in Figure 4, where the phase voltage 12 and the phase current 1 1 are shown with respect to time. In the example, the switch arrangement receives a command to switch on the electric circuit at the moment of time to and as the phase voltage is negative, the first semiconductor switch 2a is switched on to a conductive state. When the phase voltage 12 changes polarity at the moment of time ti , the phase current 1 1 starts to flow in the main circuit and also, at the same moment, the second semiconductor switch 2b is switched on to a conductive state.

The switch off at a zero-current point may be achieved by a method according to Figure 3, wherein the polarity of the phase voltage and phase current is detected 301 in response to a command to switch off the electric circuit. The polarity of the phase voltage and the change thereof may be detected by the means for measuring voltage 7 and the polarity of the phase current and the change thereof may be detected by the means for measuring current 8. The semiconductor switch 2a, 2b that is connected in series with the diode 3a, 3b that is reverse-biased is switched off 302 to a non-conductive state in response to the polarity of the phase voltage and the phase current being the same. That means that if the voltage and the current are both positive, the second semiconductor switch 2b is switched off to a non-conductive state, and if the voltage and current are both negative, the first semiconductor switch 2a is switched off to a non-conductive state. If the polarity of the phase voltage and the phase current is not the same, that is one is negative and one is positive, no action is taken until a phase voltage and phase current of the same polarity are detected. Thus, when the phase current is changing its direction, it no longer has a current path due to the diode 3a, 3b connected in series with the semiconductor switch 2a, 2b that is still in conductive state is reverse- biased and, on the other hand, the other semiconductor switch in non- conductive state also prevents the current flow. The semiconductor switch that is still in conductive state is then switched off 303 to a non-conductive state after a time substantially equal the AC half cycle has passed from switching off of the other semiconductor switch in step 302. This way it can be ensured that the polarity of the voltage and the current are opposite to semiconductor switch conductive current direction. The phase voltage should also be monitored to ensure that the current is not switched on again due to a phase difference be- tween the phase current and the phase voltage. This is a beneficial way to switch off an electric circuit, as a switch off event at zero-current point prevents development of a voltage peak that might cause electromagnetic interference to the arrangement itself or other equipment. Additionally, a fast switch off of a mechanical switch might not need to be used, which could prolong the lifetime of the mechanical switch.

An example of the switching off an electric circuit with such an arrangement and according to such a method is shown in Figure 5, where the phase voltage 12 and the phase current 1 1 are shown with respect to time. In the example, the switch arrangement receives a command to switch off the electric circuit at the moment of time to at which both the phase voltage 12 and the phase current 1 1 are negative, the first semiconductor switch 2a is switched off to a non-conductive state first. When the phase current 1 1 is changing it direction at the moment of time ti , the current no longer has a current path and the current in the main circuit is switched off. When a time sub- stantially equal to AC half cycle has passed at moment of time t₂, also the second semiconductor switch 2b is switched off to a non-conductive state to prevent the current from returning during the next half cycle. In other words, in Figure 5 substantially t2-tO=T/2. This makes the arrangement and method independent of the frequency of the electric circuit the arrangement is connected to and, thus, enables the same equipment to be used in an electric circuit of any frequency, such as 50 Hz, 60 Hz or 400 Hz.

Figure 1 b illustrates a more simple and cost-efficient semiconductor switch arrangement 1 . In this embodiment, the first semiconductor switch 2a and the second semiconductor switch 2b are connected in series and, thus, the internal diode (so called body diode) of the first semiconductor switch 2a replaces the second diode 3b and the internal diode (so called body diode) of the second semiconductor switch 2b replaces the first diode 3a. The connections may otherwise be similar and this embodiment can realize the methods shown in Figures 2 and 3 in an equal manner. This embodiment can be used when the maximum forward current specified for the internal diode of the semiconductor switch is sufficient for the application in question. Figure 6 illustrates an embodiment of a semiconductor switch arrangement, wherein it further comprises a main switch 13. The semiconductor switch arrangement 1 may be similar to those illustrated in Figures 1 a and 1 b in all other respects, for example, and it may be used to realize the method of Figures 2 and 3. In Figure 6 the semiconductor switch 2a, 2b and diode 3a, 3b part is similar to that of Figure 1 a. The main switch 13 may comprise a mechanical switch, which may be for instance an ultra-fast bi-stabile mechanical switch that is particularly beneficial in short circuit events, but different types of mechanical switches may be used in different embodiments. In such an em- bodiment, the main switch 13 may be in a non-conductive state at the moment when a command is received to switch on the electric circuit. The semiconductor switches 2a, 2b may then be switched on according to the method explained in connection with Figure 2 to switch a current on in the electric circuit. Then, when both semiconductor switches 2a, 2b are in conducting state, the main switch 13 may be switched on to a conductive state. The current path may then be commutated from the semiconductor switch 2a, 2b branches to the main switch 13 branch due to lower on-state losses. The semiconductor switches 2a, 2b may then be switched off to a non-conductive state.

When a command to switch off the electric circuit is received, the semiconductors switches 2a, 2b may first be switched on to a conductive state. Then, the main switch 13 may be switched off to a non-conductive state. After this, the main circuit may be switched off to block the current flow as is described in connection with Figure 3, for example.

Figure 7 illustrates another embodiment, which may be otherwise similar to that of Figure 6, but the semiconductor switch 2a, 2b and diode 3a, 3b part is similar to that of Figure 1 b.

Figure 8 illustrates a semiconductor switch arrangement for a 3- phase AC electric circuit as a block diagram. In a 3-phase AC application of a semiconductor switch arrangement 1 described above, a semiconductor switch arrangement according to Figure 1 a, 1 b, 6 or 7 and a method according to Figure 2 and/or 3 may be applied separately for each phase. The Figure 8 also illustrates a control element 14 and some other components and units that may be used in controlling the semiconductor switch arrangement. A same control element 14 may be configured to be used in both 1 -phase and 3-phase appli- cations.

The main switches 13 of the 3-phase switch may comprise mechan- ical switches, such as ultra-fast bi-stabile mechanical switches. The semiconductor switch arrangement 1 may also comprise semiconductor switches 2a, 2b optionally comprising diodes 3a, 3b, overvoltage protection elements 9, means for measuring voltages of the phase lines 7 and means for measuring current 8 arranged in series with semiconductor switches to measure current of the phase lines 8. In different embodiments, the semiconductor switch arrangement 1 may further comprise at least one of the following: a graphical user interface 15, a control element 14, such as a programmable integrated circuit (IC) like a field-programmable gate array (FPGA), a microcontroller (MCU) or a digital signal processor (DSP), an active crowbar 17, a current measurement circuit of an active crowbar 16, overcurrent protection circuits 18 comprising means for determining a maximum current value 19, means for determining a maximum current rate 20, driving circuits of the main switches 21 of the phase lines, driving circuits of the semiconductor switches 22 of the phase lines and a driving circuit of the active crowbar 23.

Each phase of the 3-phase semiconductor switch arrangement contains a main switch 13 with a semiconductor switch 2a, 2b and an overvoltage protection element 9 connected in parallel. This semiconductor switch may be preferably fully controllable so that it can be switched on and off at will. The semiconductor switch 2a, 2b may comprise an insulated-gate bipolar transistor (IGBT), a gate turn-off thyristor (GTO) or an integrated gate-commutated thy- ristor (IGCT).

According to an embodiment, the graphical user interface 15 may be connected to a control element 14. The graphical user interface may be configured to receive input from a user to set up settings of the 3-phase semiconductor switch arrangement, provide commands to the 3-phase semiconductor switch arrangement and have status information and measurements from the 3-phase semiconductor switch arrangement.

According to an embodiment, means for measuring voltage 7 of the phase lines may be connected to the control element 14. Voltages may be measured from each phase from input and from the output of the switch components 13, 2a, 2b. The control element 14 may control the switch on function of the 3-phase switch based on the voltage measurements by the means for measuring voltage 7. The control element 14 may also use voltage measure- ments to control a normal switch off function of the 3-phase switch.

According to an embodiment, means for measuring current 8 of the phase lines may be connected to the control element 14 and to the overcurrent protection circuits 18. The means for measuring current 8 may be connected in each phase in series with the switch components 13, 2a, 2b between the switch components and an optional active crowbar circuit. The control element 14 may control the normal switch off function of the switch based on the current measurements by the means of measuring current 8 of the phase lines. Overcurrent protection circuit 18 connected to the means for measuring current 8 of the phase lines and to the control element 14 may process the current measurements by the means for measuring current 8 of the phase lines and provide at least one of the following information types to the control element: the maximum of the absolute values of the phase currents and the maximum value of the rate of the current changes of the phase currents. The control element 14 may compare the determined maximum values to a reference value with its internal comparator. The control element 14 may then detect an over- current event if the determined maximum value is above the reference value. The reference value may be set in the graphical user interface 15, for example.

According to an embodiment, the current measurement of the active crowbar is connected to the control element 14. The current measurement of the active crowbar is connected in series with the semiconductor switch of the active crowbar to measure current through the semiconductor switch of the active crowbar. The control element 14 controls the semiconductor switch of the active crowbar based on the current measurement.

According to an embodiment, the driving circuits of the semiconductor switches 22 of the phases may be connected between a control element 14 and the semiconductor switches 2a, 2b of the phases. The control element may be configured to generate control signals to the driving circuits of the semiconductor switches 22 of the phases to switch the semiconductor switches on or off.

According to an embodiment, the driving circuits of the main switch- es 21 of the phases may be connected between a control element 14 and the main switches 13 of the phases. The control element 14 may be configured to generate control signals to the driving circuits of the main switches 21 of the phases to move the, in certain embodiments mechanical, contacts of the main switches 13 an open or closed position.

According to an embodiment, the semiconductor switch arrangement comprises a so called active crowbar 17 that may be used to give a cur- rent path for the inductive load current when switching off in overcurrent events. A driving circuit of the active crowbar 23 may be connected between a control element 14 and the active crowbar 17. The control element 14 may then be configured to generate control signals to the driving circuit of the active crowbar 23, which switch a semiconductor switch of the active crowbar on or off. A benefit of such an embodiment is that the energy stored in the load in such a switch off event can be absorbed in the resistive parts of the load instead of absorbing the load energy only in the overvoltage protection element 9. This can prolong the lifetime of the overvoltage protection element.

The semiconductor switches may thus comprise two semiconductor switches 2a, 2b in each phase line. Some examples of different types of suitable semiconductor switches have been discussed above in connection with other embodiments. For each semiconductor switch there may also be a diode 3a, 3b connected in series. These two diode-semiconductor switch pairs in one phase may be connected in reverse-parallel so that when the semiconductor switches are switched on a positive current, that is a current from source to load, flows through a diode-semiconductor pair comprising a first semiconductor switch 2a and a first diode 3a, and negative current, that is current from load to source, flows through the other diode-semiconductor pair comprising a second semiconductor switch 2b and a second diode 3b. An overvoltage protection element 9, such as described in connection with other embodiments, may be used as an overvoltage protection. These overvoltage protection elements 9 may be connected in parallel to the main switch 13 and the semiconductor switches 2a, 2b to limit the voltage across the switches and absorbing the inductive energy of the main circuit when breaking the current in overcurrent event.

According to an embodiment, the semiconductor switches 2a, 2b may all be controlled individually through semiconductor switch driver circuits 22 comprising drivers, such as optocoupler driver circuits. In different embodi- ments, the parallel main switches 13 may be controlled through main switch driver circuits 21 comprising drivers, such as optocoupler drivers. The use of optocouplers is beneficial, as they provide good voltage isolation between primary and secondary and low propagation delay. Voltages may be measured by means for measuring voltage 7 from both sides of the switch components 2a, 2b, 3a, 3b, 13: from the source side (U_nS) and from the load side (U_nl_). Current in the phase lines are measured with means for measuring current 8, such as current transducers that have good accuracy, wide frequency band- with and good galvanic isolation between primary and secondary. Closed Loop Hall Effect current transducers can be appropriate, for example. The overcur- rent protection circuits may comprise analog electronics.

According to an embodiment, switching on a 3-phase semiconductor switch arrangement may be done with zero-voltage without having electromagnetic interference according to a method substantially similar to that described in connection with Figure 2, but the switching must, naturally, be made separate for each phase. A zero-voltage switch may be done with the semi- conductor switches 2a, 2b and the main switches may be kept open as long as all of the semiconductor switches are switched on, as is described in connection with Figures 6 and 7, for example. The semiconductor switches in each phase may be switched on based on the polarity of the voltage over the switches, referred to as U_nS - U_nl_ in Figure 9, wherein S refers to source, L refers to load and n refers to each voltage phase. If the voltage over the switches is positive, the second semiconductor switch 2b (represented by S2, S4, S6 in the block diagram of Figure 9) of the phase is switched on and if the voltage over the switches is negative, the first semiconductor switch 2a (represented by S1 , S3, S5 in the block diagram of Figure 9) of the phase is switched on. After one semiconductor switch is switched on the diode 3a, 3b in series with it is then reverse-biased. Thus the current does not flow in the phase line until the polarity of the voltage over the switch components is changed. When the current flows through one semiconductor switch, the other semiconductor switch of the phase is also switched on. In 3-phase system the current in the main circuit starts to flow after one phase is switched on, if the neutral line is connected and after two phases are switched on, if the neutral line is not connected. After all of the phase lines are switched on the optional main switches 13 of all phases are switched on. Because of the bigger on-state voltage of the semiconductor switches the currents are then commutated to the branches of the main switches. These principles are illustrated in Figure 9 as a block diagram representing the normal switch on logic of the 3-phase application.

According to an embodiment, switching off the 3-phase semiconductor switch arrangement may be done with zero-current thus preventing arcing between mechanical contacts when switching off an inductive load. Zero- current switch off may be done with the semiconductor switches according to the same principles described above in connection with Figure 3, for example. To reach that effect, the phase currents need to be first commutated to the semiconductor switches by switching the semiconductor switches on and then switching the ultra-fast switches off in a similar way to that of a 1 -phase AC application explained in connection with Figures 6 and 7, for example. In the switch off event, the means for measuring current 8 and the means for measuring voltage 7 of the phase lines may be used to achieve a zero-current switch off with the semiconductor switches. The semiconductor switches are switched off based on the polarity of the phase voltages and phase currents as is explained in connection with Figure 3, for example, but separately for each phase. If the current in one phase is positive, that is current is directed from source to load, the second semiconductor switch 2b (represented by S2, S4, S6 in block diagram of Figure 10), where the negative current flows, is switched off. If the current in one phase is negative, the first semiconductor switch 2a (represented by S1 , S3, S5 in block diagram of Figure 10), where the positive current flows, is switched off. In Figure 10, the phase currents are referred to by l_n, where n refers to each phase, the phase voltages are referred to by U_n, where n refers to each phase. Now after one semiconductor switch of a phase line is switched off the current in that phase does not break until the polarity of the phase current is changed. The other semiconductor switch that was still in conductive state may then also be switched off after a delay of AC half cycle (called ½ grid period or half grid period in Figure 10). Because of possible phase difference between the current and the voltage, the switch off event has to be done when the polarity of the phase voltage and the phase current are the same. Otherwise, the current can switch on again. In a 3-phase AC application the current in the main circuit is switched off after all phases are switched off, if neutral is connected, and after two phases are switched off, if neutral is not connected. These principles are illustrated in Figure 10 as a block diagram representing the normal switch off logic of the 3-phase application.

The switch off at an overcurrent event may be achieved by a method according to Figure 13. According to an embodiment, overcurrent protection of a 3-phase AC switch may be done based on the value of the current and/or based on the rate of the current change. According to an embodiment, the control element 14 may detect 131 an overcurrent event based on the value of the current determined by means for measuring current 8. The control element may detect an overcurrent event in response to the measured value of the cur- rent in one phase being above a predetermined current limit value. In some embodiments, the current limit value may be set in a user interface, such as a graphical user interface. The user interface may be such as has been described in connection with other embodiments, for example.

According to an embodiment, the control element 14 may detect an overcurrent event based on the rate of the current change. The control element may detect 131 an overcurrent event in response to the measured value of the rate of the current change in one phase being above a current change rate limit value. In some embodiments, the current change rate limit value may be set in a user interface, such as a graphical user interface. The user interface may be such as has been described in connection with other embodiments, for example. An advantage of the overcurrent protection based on the rate of the current change is that an unusual current change rate indicating the overcurrent can be detected at lower overcurrent values, that is much before it is in the dangerous level from the system point of view, which decreases the stress of the components. The detection method is then safer than the traditional over- current protection based on the current level as the overcurrent doesn't reach an equally high value by the time of switch off. In other embodiments, both the current value and the rate of current change may be used for detecting an overcurrent event.

According to an embodiment, the control element 14 may, after detecting an overcurrent event, generate 132 a switch on signal to the semiconductor switches 2a, 2b and switch off signal to the main switches 13 to commu- tate the current to the semiconductor switches 2a, 2b. After a predefined time, when the insulation gap between the contacts of the main switch 13 is sufficient, the control element 14 may generate 133 a switch off signal to the semiconductor switches 2a, 2b to break the current in the main circuit. The sufficient insulation gap is when the breakdown voltage of the insulation gap is bigger than the protection level, that is the clamping voltage, of the overvoltage protection element 9 connected in parallel with the switch components 2a, 2b, 13. After turning off the semiconductor switches 2a, 2b the voltage over the switch components starts to increase because of the inductance in the main circuit. When the voltage reaches the protection level, the overvoltage protection element forms a low resistance shunt across the switch components. The energy stored in the inductances of the main circuit is then absorbed 134 in the overvoltage protection element 9. In different embodiments, the isolation gap of the main switch 13 may comprise air or other isolating substance or a combination thereof.

The lifetime of the overvoltage protection element 9 typically depends on the energy absorbed in the overvoltage protection element. The big- ger the transients are the shorter the overall lifetime of the overvoltage protection element will be. Thus, a significant benefit of the overcurrent protection based on the rate of the current change is that it detects the overcurrent event faster than overcurrent protection based on the value of the current and thus makes the overcurrent transients much smaller. This will then further increase the overvoltage protection element lifetime.

According to an embodiment, a so called active crowbar circuit may be used to further increase the lifetime of the overvoltage protection element 9 in systems where the load inductance is relatively big. Figure 1 1 is a block diagram illustrating overcurrent switch off logic in such an embodiment when an active crowbar is connected, as described above. Such an active crowbar circuit may then be connected to the load side. An overcurrent event may be detected 1 1 1 in a manner similar to one of those described in connection with other embodiments. The semiconductor switch arrangement 1 may be switched off in a manner similar to that described in steps 132 and 133 in con- nection with Figure 13, wherein the steps 1 12 and 1 13 correspond to step 132 and steps 1 14 and 1 15 correspond to step 133, for example. However, in an embodiment comprising such active crowbar 17, the control element 14 may then generate a switch on signal 1 16 to the semiconductor switch of the active crowbar after generating a switch off signal 1 15 to the semiconductor switches 2a, 2b of the phase lines. Thus, the inductive load current may be commutated through the active crowbar absorbing the load energy in the resistive parts of the load instead of the overvoltage protection element. In embodiments, in which the load is active, having both inductive and mechanical energy stored, the mechanical energy can sustain or even increase the current through the active crowbar, which can eventually destroy the semiconductor. In such embodiments, a current measurement circuit of an active crowbar 16, which may comprise for instance a current transducer, may be provided to prevent this by measuring the current through the semiconductor switch of the active crowbar 17. The current measurement circuit of the active crowbar 16 may then be connected to the control element 14, which may generate a switch off signal to the semiconductor switch of the active crowbar 17 in response to the current value exceeding a predefined crowbar current limit value. After this, the rest of the load energy may be absorbed in the overvoltage protection element 9.

According to another embodiment, the current value exceeding a predefined crowbar current limit value may be detected using an optocoupler driver circuit for driving the active crowbar 23 that comprises an integrated de- saturation detection of the semiconductor switch of the active crowbar 17 and which gives a fault status feedback further to the control element 14. This kind of driver may comprise an IGBT driver that detects overcurrent event when the voltage between the collector and the emitter of the IGBT is over a limit value, for example. According to an embodiment, the driver may switch the semiconductor switch of the active crowbar off automatically and give a fault status feedback to the control element 14 in response to detecting a desaturation of the semiconductor switch of the active crowbar. After receiving the fault signal the control element may then generate a reset signal to the driver activating the driver and the semiconductor switch of the active crowbar can be switched on again if wanted. A benefit of this kind of an embodiment is that by using this type of an optocoupler driver circuit the current measurement circuit of the active crowbar 16 may be left out, although both have been illustrated in Figure 8, for example.

Figure 12 illustrates schematically an embodiment of a semiconductor switch arrangement for a 3-phase AC electric circuit that may be similar to the embodiment illustrated in Figure 8, for example, but that comprises a serial isolation switch 24 for each phase. Such a switch may be used for galvanic isolation of the switch arrangement for ensuring electrical safety during maintenance work, for example. According to certain embodiments, such a serial isolation switch 24 may be provided in any of the embodiments described above.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The in- vention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1 . A semiconductor switch arrangement for switching electric circuits, wherein the switch arrangement comprises for each current phase:

a first and a second semiconductor switch that are controllable for switching on and off at will;

a first and a second diode for controlling available current paths, means for measuring voltages from input and output lines of said semiconductor switch;

means for measuring current arranged in series with said semicon- ductor switch to measure a current of the phase line; and

a control element configured to switch on the semiconductor switch arrangement at zero-voltage point in response to a command to switch on the electric circuit and to switch off the semiconductor switches at zero-current point in response to a command to switch off the electric circuit by controlling the switch on and switch off the semiconductor switches in response to the voltage and current measurements.

2. A semiconductor switch arrangement according to claim 1 , wherein the first and the second semiconductor switches of the semiconductor switch arrangement are connected in series and first and second diodes com- prise the internal diodes of the semiconductor switches and are used for controlling the available current paths.

3. A semiconductor switch arrangement according to claim 1 , wherein the first and the second semiconductor switch of the semiconductor switch arrangement are connected in parallel, the first diode is connected in series with the first semiconductor switch and the second is connected in series with the second semiconductor switch such that the forward-biased directions of the first and the second diode and the internal diode of the respective first and second semiconductor switch connected in series with the diode are opposite.

4. A semiconductor switch arrangement according to any one of claims 1 to 3, wherein the semiconductor switch arrangement further comprises a main switch connected in parallel with the semiconductor switches for achieving lower on-state losses in a conductive state.

5. A semiconductor switch arrangement according to any one of claims 1 to 4, wherein the electric circuit is a 3-phase AC circuit.

6. A semiconductor switch arrangement according to any one of claims 1 to 4, wherein the electric circuit is a 1 -phase AC circuit.

7. A semiconductor switch arrangement according to any one of claims 1 to 6, wherein the semiconductor switch arrangement further compris- es an overvoltage protection element for each phase.

8. A semiconductor switch arrangement according to any one of claims 1 to 7, wherein the semiconductor switch arrangement further comprises an active crowbar.

9. A semiconductor switch arrangement according to any one of claims 1 to 8, wherein the semiconductor switch arrangement further comprises a serial isolation switch for each current phase for galvanic isolation.

10. A semiconductor switch arrangement according to any one of claims 1 to 9, wherein the control element is configured to

detect the polarity of the phase voltage in response to a command to switch on the electric circuit,

switch on to a conductive state the semiconductor switch that is connected in series with the diode that is reverse-biased, and

switch on to a conductive state the semiconductor switch connected in series with the diode that was forward-biased during the originally detected polarity of the voltage in response to the change of polarity of the phase voltage.

1 1 . A semiconductor switch arrangement according to any one of claims 1 to 9, wherein the control element is configured to

detect the polarity of the phase voltage and phase current in re- sponse to a command to switch off the electric circuit,

switch off a semiconductor switch that is connected in series with the diode that is reverse-biased in response to the polarity of the phase voltage and the phase current being the same, and

switch off the semiconductor switch still in the conductive state to a non-conductive state after a time substantially equal to the AC half cycle has passed from switching off of the other semiconductor switch.

12. A semiconductor switch arrangement according to any one of claims 7 to 9, wherein the control element is configured to

detect an overcurrent event in response to at least one of the follow- ing: a measured value of the current in one phase being above a predetermined current limit value and a measured value of the rate of the current change in one phase being above a current change rate limit value;

generate a switch on signal to the semiconductor switches and a switch off signal to the main switches to commutate the current to the semiconductor switches;

generate, after a predefined time, a switch off signal to the semiconductor switches to break the current in the main circuit;

absorb, after the breaking of the current in the main circuit, energy stored in the inductances of the main circuit in the overvoltage protection element.

13. A method for switching current in an electric circuit, the electric circuit comprising a semiconductor switch arrangement comprising a first and a second semiconductor switch, a first and a second diode, means for measuring voltages from input and output lines of said semiconductor switch and means for measuring current, the method comprising:

detecting a polarity of a phase voltage in response to a command to switch on the electric circuit,

switching on to a conductive state a semiconductor switch that is connected in series with a diode that is reverse-biased, and

switching on to a conductive state a semiconductor switch connect- ed in series with the diode that was forward-biased during the originally detected polarity of the voltage in response to the change of polarity of the phase voltage.

14. A method according to claim 13, the method further comprising: detecting the polarity of the phase voltage and phase current in re- sponse to a command to switch off the electric circuit,

switching off semiconductor switch that is connected in series with the diode that is reverse-biased in response to the polarity of the phase voltage and the phase current being the same, and

switching off the semiconductor switch still in the conductive state to a non-conductive state after a time substantially equal to the AC half cycle has passed from switching off of the other semiconductor switch.

15. A method according to claim 13 or 14, wherein the semiconductor switch further comprises an overvoltage protection element, the method further comprising:

detecting an overcurrent event in response to at least one of the following: a measured value of the current in one phase being above a predeter- mined current limit value and a measured value of the rate of the current change in one phase being above a current change rate limit value;

generating a switch on signal to the semiconductor switches and a switch off signal to the main switches to commutate the current to the semi- conductor switches;

generating, after a predefined time, a switch off signal to the semiconductor switches to break the current in the main circuit;

absorbing, after the breaking of the current in the main circuit, energy stored in the inductances of the main circuit in the overvoltage protection element.