Condition monitoring of semiconductor devices in converters

Abstract

A method for monitoring the state of a modular multilevel converter, wherein the modular multilevel converter includes a submodule having a capacitor and a controllable switch. The method includes: selecting a submodule; controlling the controllable switch of the selected submodule to form a current path through the submodule by controlling at least one controllable switch to a conducting state and controlling at least one controllable switch to a blocking state; disconnecting the voltage of the capacitor of the selected submodule from the submodule; and performing a measurement on at least one controllable switch controlled to a blocking state.

Description

Technical Field

[0001] The present invention relates to state monitoring of semiconductor devices, in particular to state monitoring of semiconductor switches in converters, in particular to state monitoring of semiconductor switches in modular multi-level converters. Background Art

[0002] Multilevel converters are used in a wide variety of converter applications. When higher voltage levels are used, the voltage rating and / or current rating of typical semiconductor devices is exceeded and a parallel or series connection of semiconductor devices is required to handle the voltage or current. However, parallel or series operation of semiconductor switches is not simple because the switching devices need to be matched so that the current and voltage are evenly distributed. In a multilevel converter, the voltage is divided inside the converter so that each semiconductor switching device operates at a smaller voltage. These smaller voltage steps are used to build up a higher voltage by connecting the required number of smaller voltages in series.

[0003] Typical applications of multilevel converters include, for example, solar inverters, flexible AC transmission systems (FACTS), STATCOM, and high voltage DC (HVDC) power transmission. Multilevel converters can also operate in rectification mode or inversion mode. For example, in HVDC transmission applications, multilevel converters are typically used at both ends of a DC transmission line to rectify AC voltage to DC voltage and invert DC voltage to AC voltage.

[0004] One type of multilevel converter is a modular multilevel converter (MMC), in which the required number of active conversion units are stacked. Each unit or submodule contains its own storage capacitor and a controllable switch. The voltage of the capacitor can be bypassed or connected in series with other capacitors to form the output voltage. Each submodule acts as an independent converter that generates a voltage with zero volts or non-zero voltage value. An MMC converter can include hundreds of submodules in one branch of the converter, which constitutes one phase of the converter. When the number of submodules connected in series increases, the voltage rating of the converter can be increased.

[0005] The submodules can use a half-bridge or full-bridge topology. In the half-bridge topology, two controllable switch components are used to connect the voltage of the capacitor to the circuit or bypass the capacitor, while in the full-bridge circuit, four controllable switch devices are used. There are also other topologies for connecting the voltage of the capacitor to the circuit or bypassing the capacitor.

[0006] MMC converters are critical devices, since a failure of the converter may result in an interruption of operation. For example, in HVDC transmission, the operation of the converter should be as reliable as possible to ensure continuous power transmission. In known devices, the operating status of the submodules cannot be determined during the use of the device. Maintenance of the device can be performed by calculating the approximate operating time and estimating when a submodule should be replaced with a new one. Another alternative is to remove the submodule from the circuit and measure its health status offline. Further, limited measurements can be made on the device when the submodule is connected to the converter. Summary of the invention

[0007] An object of the present invention is to provide a method and an apparatus for implementing the method in order to overcome the above-mentioned problems. The object of the present invention is achieved by a method and an apparatus, which are characterized by what is stated in the independent claims. Preferred embodiments of the invention are disclosed in the dependent claims.

[0008] The invention is based on the idea of ​​disconnecting the voltage of the capacitor of the submodule and performing a measurement on the controllable switch component controlled to the blocking state. During the measurement, the output of the submodule is zero, i.e. the voltage of the capacitor is not connected to the output of the submodule, forming a current path through the submodule. The measurement performed can be a curve tracing measurement, in which certain current or voltage curves are obtained, from which the operating state of the switching device can be determined.

[0009] An advantage of the method and apparatus of the present invention is that the state of the submodule and therefore the state of the converter can be determined during the use of the converter. The method and apparatus of the present invention can be implemented using a small amount of additional circuitry. By means of the method and apparatus of the present invention, automation of the state monitoring of the device can be achieved. Further, the state monitoring can provide information that can predict faults and replace submodules before a fault situation occurs. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the following, the present invention is described in more detail by means of preferred embodiments with reference to the accompanying drawings, in which:

[0011] Figure 1 An example of stacked submodules of a modular multilevel converter is shown;

[0012] Figure 2A and Figure 2B An example of a submodule is shown;

[0013] Figure 3 An embodiment of the present invention is shown;

[0014] Figure 4 and Figure 5 shows an example of voltage-current characteristics obtained by curve tracing;

[0015] Figure 6 An example of using stacked submodules in STATCOM is shown. DETAILED DESCRIPTION

[0016] Figure 1 The basic circuit arrangement of one phase of a modular multilevel converter is shown. Two cells or submodules, cell 1 and cell N, are Figure 1 As shown. The number of these submodules is not limited to any particular number, and a large number of submodules can be stacked. Further, each phase of the system is formed by similarly stacked submodules. A DC voltage can be connected between the ends of the stacked submodules, and the DC voltage is distributed between the capacitors of the submodules in a known manner. Figure 1 The circuit of is highly simplified so that only the stacked submodules are shown, which are connected to the AC voltage via inductors.

[0017] Figure 2A An example of a submodule or unit is shown. The submodule of this example consists of a full bridge circuit and a capacitor. The full bridge circuit comprises four controllable switches, via which the capacitor voltage can be connected to the series-connected submodules, or the capacitor can be bypassed in a known manner. Figure 2B Another example of a submodule including a half-bridge circuit and a capacitor is shown. Figure 6 A STATCOM connected to a network for regulating power in the network is shown. Figure 6 In the example of FIG. 4 , the Y-connected STATCOM includes four stacked submodules in each phase.

[0018] In the method of the present invention, a submodule is selected from a modular multilevel converter, and the controllable switches of the selected submodule are controlled to form a current path through the submodule. The formed current path refers to a path without the capacitor of the submodule. Since the capacitor is not connected to the path, the output of the module is zero volts. The other controllable switches are controlled to a blocking state. Consider Figure 2A When switches S1 and S3 are controlled to be on and switches S2 and S4 are controlled to be blocked, a current path through the submodule is formed through switches S1 and S3, and the output voltage of the submodule is zero volts. Correspondingly, when switches S2 and S4 are controlled to be on and switches S1 and S3 are blocked, a current path through the submodule is formed, and the output voltage of the submodule is zero volts. When connected to a submodule having a half-bridge circuit, such as Figure 2B , a current path is formed by controlling switch S2 to be conductive and S1 to be blocked. Using the current path through the components, the output voltage of the submodule is zero volts, the capacitor is bypassed, and thus the voltage of the capacitor is not connected to the output.

[0019] In the present invention, a submodule is selected so as to perform a measurement on at least one controllable switch of the selected submodule. The selection of the submodule can be done in a timed manner, i.e. the higher level control system can keep track of the submodules and the moments of previous measurements. Another possibility is to select the submodules in a certain order. For example, when measurements can be performed, the selection of the submodules is done in a certain order, which is tracked by the higher level controller.

[0020] Further, in the present invention, the voltage of the capacitor of the selected submodule is disconnected from the submodule. With the disconnection of the voltage of the capacitor, it is referred to as an action that results in a state in which the capacitor is not electrically connected to the rest of the submodule. When disconnected, the voltage of the capacitor cannot generate a current flowing to the submodule. Further, in the present invention, when the voltage of the capacitor is disconnected from the submodule, a measurement is performed on at least one controllable switch that does not carry current.

[0021] The measurement of the present invention can be enabled when the input voltage of the system allows one submodule to be taken out of service. Further, the converter can be constructed to have redundant submodules in each phase. In this case, the number of submodules used in active operation is less than the number of submodules of the system. This structure can make one or more submodules out of service even when operating at nominal voltage.

[0022] When the input voltage of the system allows to stop using one submodule, the submodule can be selected and the operation of the system can be continued. The voltage fed into the system is divided between the capacitors of the submodules.

[0023] Once it is noted that the voltage of the system does not require all submodules to be operational or when redundant submodules are available, as described above, a submodule is selected. Further, according to an embodiment of the present invention, the measurement may be performed if the duration in time of the measurement is shorter than a half period of the fundamental voltage of the converter, as described in more detail below.

[0024] The following references Figure 3 The operation of the method and its embodiments are described. Figure 3 An embodiment of the device of the present invention is shown. Figure 3 An embodiment of a submodule with a full-bridge converter is shown. The full-bridge converter is formed by four controllable switching devices such as IGBTs s1, s2, s3, s4. The output voltage Vout is obtained from the point between switches s1 and s2 and switches s3 and s4. The switches can be used to control the voltage of the capacitor C to the output or to create a current path through the submodule that bypasses the capacitor C, whereby the output voltage is zero volts.

[0025] Figure 3A gate driver circuit Ctrl1 connected to the electrodes of each controllable switch element is shown. The gate driver circuit is a circuit capable of providing a suitable gate voltage to the controllable switch element in a known manner. The gate driver obtains control instructions from a central controller Central ctrl or another device that calculates or obtains instructions on when to control components to turn on and off. Further, Figure 3 A voltage converter 31 is shown which generates an auxiliary voltage for the gate driver.

[0026] According to the invention, the submodule comprises a disconnecting switching device s5, which is arranged to disconnect the voltage of the capacitor of the submodule from the full bridge. Figure 3 As shown, similar to the switch of the bridge circuit, the gate drive circuit Ctrl1 is connected to control the disconnect switch device. When the disconnect switch device is controlled to the blocking state, the voltage of the capacitor C does not affect the bridge circuit, that is, the current cannot flow from the capacitor or flow to the capacitor. Therefore, the disconnect switch device s5 is located in the submodule to disconnect the capacitor. The switch device s5 can be the same type as the switch device of the bridge circuit. However, the type of the disconnect switch device can also be any other controllable switch device. The disconnect switch device also obtains control from the Central ctrl device.

[0027] The measurement of the present invention can be performed when the required voltage is so low that all submodules are not needed when redundant submodules are installed, or the measurement can be performed within a time period that is less than half a cycle of the frequency of the voltage. In the first two of the above cases, any of the submodules of the converter can be controlled to be in an idle state. That is, for example, when a redundant submodule is installed, one submodule is always in a state where current flows through the submodule and the output from the module is zero volts. The module in the idle state can be freely selected. Similarly, when the required voltage is low, the freely selectable submodule is in an idle state. On the other hand, when all submodules are used, the measurement should be performed during the time period when the actively used submodule is conducting (i.e., when the output from the module is zero volts).

[0028] When a submodule is selected in the method, a current path through the submodule is formed using switches s1 and s3 or s2 and s4. Once the current path is formed, the output voltage is zero and the capacitor can be disconnected using switch s5. It should be noted that when measuring the active submodule, a current path is formed due to the modulation of the converter, and in the present invention, the time period of the current path is used for measurement.

[0029] Once switch s5 is opened to disconnect the capacitor, the controllable switch device in the blocking state when the capacitor is disconnected can be measured. That is, if the current path is formed using switches s1 and s3, switches s2 and s4 can be measured at once. During the measurement, switches s2 and s4 are controlled to be ON and OFF according to the needs of the measurement.

[0030] The measurements performed are preferably curve tracking measurements. The gate driver unit Ctrl1 is preferably constructed to perform the required measurements. For this reason, the gate driver unit is shown as being connected to each terminal of the controlled switch. Typical curve tracking measurements include collector-emitter voltage and collector current (VCE and IC), gate-emitter voltage and collector current (VGE and IC), gate-emitter capacitance and collector-emitter voltage (CGE and VCE), collector-emitter capacitance and collector-emitter voltage (CCE and VCE), collector-gate capacitance and collector-emitter voltage (CGC and VCE) and device temperature. Curve tracking measurements are known. In curve tracking, different operation-related curves can be determined. In curve tracking, different current or voltage scans are generated for the semiconductor switch terminals. For curve tracking, the gate driver unit is connected to each terminal of the controllable semiconductor switch, that is, to the collector, gate and emitter. The gate driver may be able to generate a variable voltage and / or current for each of the terminals. For example, the collector-emitter voltage can be changed during curve tracking. Further, the gate driver may include circuitry for measuring the current and / or voltage in each terminal of the controlled switch component.While curve tracing methods are well known, some measurements are briefly discussed below.

[0031] Typical curve tracing measurements that can be made on a switching element include measurements of the gate-emitter (VGE) voltage versus collector current, the collector-emitter (VCE) voltage versus collector current (IC), and the input and output capacitance of the switching component. These tests are known when semiconductor components are measured offline and are briefly discussed below.

[0032] In the gate-emitter voltage vs. collector current (or gate-source voltage vs. drain current) test, the gate-to-emitter voltage is gradually increased, similar to a very slow turn-on transition when the DC voltage is fixed. Figure 4 An example of these measurements is shown. The operation of the test should be slow enough so that the measurement unit acquires multiple data points during the transition. This operation can be repeated for different values ​​of the collector-emitter (or drain-source) voltage, which can be varied using the gate driver unit.

[0033] Figure 5 Another example of a characteristic curve that can be obtained by measurement is shown. Figure 5The measurement of collector-emitter voltage and collector current is shown. The measurement is made by switching the component on and off at a higher frequency and lower DC voltage than normal operation. Figure 5 In the example, the DC voltage ranges from 0V to 5V, while the gate voltage varies from 9V to 19V.

[0034] The slow turn-on transitions at different collector-emitter voltages can allow the amount of charge through the gate and through the collector-emitter to be extracted at an acceptable sampling rate. Since the maximum changes in the input and output capacitances of the switching element are at low collector-emitter voltages, this study should be performed with reduced DC voltage. Alternatively, these capacitances can be identified by injecting high-frequency signals into the collector-emitter and gate of the device at different collector-emitter voltages and measuring the corresponding currents.

[0035] This process can be repeated, one switching device at a time, one submodule at a time, until all submodules are covered. The process can then be repeated after specific time intervals and used to identify changes in semiconductor device parameters. These changes can then be used to identify the health status of the device.

[0036] Similar to the controllable semiconductor switches s1 . . . s4 , the open switch component s5 can be curve traced when a current path through the switches s1 and s3 or s2 and s4 is formed, ie, when the voltage of the capacitor is not connected to the output.

[0037] The above measurements can provide data from which the health state of the controllable switch component can be determined. The data may include multiple data points, interpolations of various curves, or estimates. For example, multiple data points can be collected from measurements that produce a curve. These multiple data points can be used to form a curve with parameters, and these curve parameters can be used to estimate the health state of the device. Further, different values ​​can be obtained directly from the measurements, and these values ​​can be used to indicate the health state. For example, these values ​​can include maximum or minimum values, maximum or minimum values ​​with different measurement characteristics. The above measurements are examples of possible measurements that can be used to indicate the health state of the controllable switch.

[0038] The results obtained from the measurements are preferably stored in a memory together with a timestamp and the identification of the switch component and submodule measured. When the measurements are repeated, changes in the results obtained more clearly indicate the health or operating state of the switch component in question. For example, if a change in a certain measurement result is detected, the properties of the component have changed. The change may indicate that the component will soon fail.

[0039] The measurement results, measured values ​​or indicative data may be stored in a database, which may include corresponding data about the health device. The data about the health device may be used to determine whether the data obtained indicates that the device is about to end. The database may be located remotely from the converter, and the data may be transmitted to the database using a communication network.

[0040] According to an embodiment of the present invention, a measurement is performed when the temperature of a submodule or a selected switch component has reached a certain level or after the selected switch component has been idle for a period of time. The submodules of the converter are usually cooled using a circulating liquid. After the semiconductor switch component has been idle for a period of time, the cooling causes the temperature of the switch component to reach a certain level. When the temperature of the device reaches a certain level, temperature changes will not have an impact on the results obtained. Therefore, in this embodiment, before starting the curve tracing, a certain idle time is started to allow the temperature of the switching device to drop to a steady-state level. When all measurements are performed at a reduced, known and approximately the same temperature, the effects of temperature changes are eliminated from the measurements. Therefore, in one embodiment, one of the measurements performed during condition monitoring is a temperature measurement.

[0041] In the embodiment shown in the figures, the submodule is formed by a full bridge circuit. However, the topology of the submodule is not limited to the topology shown and described.

[0042] It is obvious to a person skilled in the art that, as technology advances, the inventive concept can be implemented in various ways.The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Claims

1. A method for monitoring the state of a modular multilevel converter, wherein the modular multilevel converter comprises a submodule having a capacitor and a controllable switch, the method comprising: Select the submodule, controlling the controllable switches of the selected submodule to form a current path through the submodule by controlling at least one controllable switch to a conducting state and controlling at least one controllable switch to a blocking state, disconnecting the voltage of the capacitor of the selected submodule from the submodule, The measurement is performed on at least one controllable switch which is controlled into a blocking state.

2. The method of claim 1 , wherein disconnecting the voltage of the capacitor from the submodule comprises: The current path is blocked from the capacitor.

3. The method of claim 2, wherein blocking the current path from the capacitor comprises: The controllable semiconductor switch is controlled to a blocking state.

4. The method according to any one of claims 1 to 3, wherein the measurement is a curve tracking measurement.

5. The method according to any one of claims 1 to 3, wherein the measurements comprise temperature measurements.

6. The method according to any one of claims 1 to 3, wherein the results of the measurements are stored and compared with the results of previous measurements. 7 . The method according to claim 6 , wherein the result of the measurement comprises a voltage curve or a current curve, a voltage slope or a current slope, a current value or a voltage value, or a capacitance value.

8. The method according to any one of claims 1 to 3, wherein the method comprises: The selected submodule is kept idle for a period of time to reduce the temperature of the controllable switch. 9 . The method according to claim 1 , wherein during the method the converter is in active use as a rectifier or as an inverter.

10. A modular multi-level converter, wherein the modular multi-level converter comprises a sub-module having a capacitor and a controllable switch, the converter comprising: A component suitable for selecting a submodule; means adapted to control the controllable switches of the selected submodules to form a current path through the submodules by controlling at least one controllable switch to a conducting state and controlling at least one controllable switch to a blocking state; means adapted to disconnect the voltage of said capacitor of a selected submodule from said submodule; Means adapted to perform measurements on at least one controllable switch controlled to a blocking state.

11. A modular multi-level converter according to claim 10, wherein the means adapted to control the controllable switch comprises a gate drive circuit.

12. The modular multi-level converter of claim 11, wherein each submodule comprises a voltage converter for providing an auxiliary voltage to the gate drive circuit from the voltage of the capacitor.

13. A modular multi-level converter according to claim 11 or 12, wherein the means adapted to perform the measurements comprise: A component for generating a switching command to a selected controllable switch; as well as means for measuring current and voltage across selected controllable switches during said measuring period.

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