Condition monitoring of semiconductor devices in a converter

Abstract

A method of monitoring condition of a modular multilevel converter, wherein the modular multilevel converter comprises submodules having a capacitor and controllable switches. The method comprising se

Description

FIELD OF THE INVENTION The invention relates to monitoring condition of semiconductor devices, and particularly to monitoring the condition of semiconductor switches in a converter and specifically in a modular multilevel converter. BACKGROUND OF THE INVENTION Multilevel converters are used in a wide variety of converter applications. When higher voltage levels are used, the voltage and / or current ratings of typical semiconductor devices are exceeded, and parallel or series connections of semiconductor devices are needed to handle voltages or currents. The parallel or series operation of semiconductor switches is not, however, straightforward as the switch devices need to be matched such that currents and voltages are divided evenly. In a multilevel converter voltage is divided inside the converter such that each semiconductor switch device operates on smaller voltage. These smaller voltage steps are used for building a higher voltage by connecting required amount of smaller voltages in series. Typical applications of multilevel converters include, for example, solar inverters, flexible AC transmission systems (FACTS), STATCOMs and high-voltage DC (HVDC) power transmission. Multilevel converters can also be operated in rectifying or in inverting mode. For example, in HVDC transmission applications multilevel converters are commonly used in both ends of the DC transmission line for rectifying AC voltage to a DC voltage and for inverting DC to an AC voltage. One type of multilevel converter is a modular multilevel converter (MMC) in which a required amount of active converting cells are stacked. Each cell or submodule contains its own storage capacitor together with controllable switches. The voltage of the capacitor can be either by-passed or connected in series with other capacitors for forming output voltage. Each submodule acts as an independent two-level converter generating a voltage having a value of zero volts or the capacitor voltage. An MMC converter may comprise hundreds of submodules in a one leg of the converter which builds one phase of converter. When the number of series connected submodules is increased, the voltage rating of the converter can be increased. The submodules may employ half-bridge or full-bridge topologies. In a half-bridge topology two controllable switch components are used to either connect the voltage of the capacitor to the circuit or to by-pass the capacitor, while in a full-bridge circuit four controllable switch devices are employed. Other topologies also exist for either connecting the voltage of the capacitor to a circuit or to bypass the capacitor. MMC converters are critical devices in the sense that a breakdown of the converter may lead to interruption of the operation. In HVDC transmission, for example, the operation of the converter should be as reliable as possible for ensuring the continuous power transmission. In the known devices, the working condition of the submodules cannot be determined during the use of the device. Maintenance of the device may be carried out by calculating approximate working hours and estimating when the submodule should be replaced by a new one. Another alternative is to remove a submodule from the circuit and measure its health offline. Further, limited measurements of the device can be carried out while submodules are connected to the converter. BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide a method and an apparatus for implementing the method so as to overcome the above problem. The object of the invention is achieved by a method and an apparatus which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on the idea of reducing a voltage of a selected module while it is online and connected to the converter. Once the voltage is reduced, tests and measurements may be carried out on the submodule. Measurements carried out may be curve tracing measurements, in which certain current or voltage curves are obtained from which the working condition of the switching devices can be determined. An advantage of the method and apparatus of the invention is that the condition of the submodules can be determined during the use of the converter. When redundant submodules are in the circuit, the capacitor voltage of the submodule can be discharged to a low level in which the curve tracing measurements can be safely made. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which Figure 1 shows an example of stacked submodules of a modular multilevel converter; Figure 2 shows an example of a submodule; Figure 3 shows an example of capacitor voltages; Figure 4 shows an example of a submodule with full-bridge circuit; Figures 5 and 6 show examples of voltage-current characteristics obtained with curve tracing; and Figures 7A and 7B show current and voltage waveforms which can be used in curve tracing. DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a basic circuit arrangement of a modular multilevel converter. Two cells or submodules 11 are shown in Figure 1. The number of these submodules is not limited to any certain number, and a high number of submodules may be stacked. Phase input or output is formed between the stacked submodules such that the output is formed from the center point between the stacked submodules. Further, each phase of the system is formed of similarly stacked submodules. DC voltage is connectable between the ends of the stacked submodules, and the DC voltage is divided between the capacitors of the submodules in a known manner. Figure 2 shows an example of a submodule or a cell. The submodule of the example is formed of a full bridge circuit and a capacitor. The full bridge circuit comprises four controllable switches with which the capacitor voltage can be connected to the series connection submodules or the capacitor can be by-passed in a known manner. In the method of the invention a submodule is selected and the voltage of the capacitor of the selected submodule is lowered. Once the voltage is lowered, measurements are performed on at least one controllable switch of the selected submodule. The selection of a submodule may be carried out in a timed manner, i.e. a higher level control system may keep track on submodules and on the time instant of the previous measurements. Another possibility is to select the submodules in a certain order. For example when measurements can be carried out, the selection of the submodules proceeds in a certain order which is tracked by a higher level controller. The voltage of the capacitor of a submodule may be lowered once there is one or more redundant submodules in the system. During the method of the invention, the converter is in operation and DC voltage is converted to AC voltage or AC voltage converted to DC voltage. When input voltage to the system allows to take one submodule out of use, the measurements of the invention can be enabled. Further, the converter may be built to have redundant submodules in each phase. In such the number of submodules used in active operation is less than the number of submodules of the system. Such a structure enables to have one or more submodules out of use even when operating at nominal voltages. When the input voltage to the system allows to take one submodule out of use, the submodule can be selected and the operation of the system can be continued. The voltage fed to the system is divided between the capacitors of the submodules. Any one of the submodules can be taken out of use by lowering the voltage of the capacitor while the voltage is divided between the other submodules which are in operation. Once it is noticed that the voltage of the system does not require all the submodules in operation or when redundant submodules are otherwise available, a submodule is selected as described above. The voltage of the capacitor of the selected submodule is lowered so that the measurements on a controllable switching component can be carried out. The lowering of the voltage is preferably carried out by transferring energy from the selected submodule temporarily to other submodules or to the network. The switches of the submodule are modulated in a known manner so that the voltage of the selected submodule is decreased. The voltage can alternatively be lowered with a resistive circuit connected in parallel with the capacitor. The resistive circuit comprises typically a controllable switch component, which closes the capacitor through a resistor. As current flows through the resistor, the voltage of the capacitor decreases. Figure 3 shows an example of capacitor voltages during the lowering of the voltage of one capacitor. It can be seen in Figure 3, that the voltages of the capacitors fluctuate around the nominal value with a frequency which corresponds to the operation frequency of the converter. The voltage 31 of the capacitor of the selected submodule is decreased to a level which enables to carry out the measurements. In the example of Figure 3, the voltage is lowered to a value of 0.05 p.u. The voltage of the capacitor is lowered to a desired value in a controlled manner. Figure 4 shows a submodule with four controllable switch components Tl, T2, T3, T4 and their antiparallel diodes DI, D2, D3, D4. Considering that the voltage of the capacitor C is discharged to a lower value, a switch component is selected for the measurement. Considering that the selected component of the submodule is T3, then current through the submodule is led through components T2 and D4. If the current direction is the opposite, the current flows trough components T4 and D2. Voltage affecting the component under measurement corresponds to the voltage of the capacitor. Thus in the embodiment, once the capacitor voltage is discharged to a suitable level, the capacitor is then by-passed such that the selected component is not within the current path. The selected component, although not in the current path initially, is switched ON and OFF as desired so that measurements can be carried out. When a submodule is discharged, it is advisable to carry out the measurements on all the switching components. Component T1 of the example of Figure 4 can be measured when the current path is through the components T2 / D2 and T4 / D4. When component T2 or T4 is to be measured, the submodule is by-passed using components Tl / Dl and T3 / D3. After the DC voltage has been reduced, the selected switch component is subjected to measurements in which the health of the component or an indication relating to the health can be determined. Typical curve tracing measurements to which switch components can be subjected to include gate-emitter (VGE) voltage versus collector current, collector-emitter (VCE) voltage versus collector current (IC) and measurement of input and output capacitances of a switch component. These test are known as such when semiconductor components are measured off-line and are briefly discussed in the following. In gate-emitter voltage versus collector current (or gate-source voltage versus drain current) test the gate to emitter voltage is gradually increased similarly as in a very slow turn-on transition while the DC voltage is fixed. Figure 5 shows an example of such measurement results. The operation of the test should be slow enough for the measurement units to acquire multiple of data points during the transition. The operation can be repeated for different values of collectoremitter (or drain - source) voltage which can be changed using the gate driver unit. Figure 6 shows another example of characteristic curves obtainable with the measurements. Figure 6 shows a measurement of collector-emitter voltage versus collector current. The measurement is carried out switching the component on and off with a higher frequency and lower DC voltage than in the normal operation. In Figure 6, the DC voltage is ranging from 0 V to 5 V while varying the gate voltage from 9 Volts to 19 Volts. In collector-emitter (VCE) voltage versus collector current (IC) test the switching device is switched on and off with a higher frequency than normal operation. Figure 7 shows an example of the current and voltage waveforms obtained during the test. The current through the component is preferably increased, and collector-emitter voltage and collector current are sampled at the time instant when the current is cut. Figure 7 shows drain current and drain-source voltages. More specifically, Figure 7A shows the complete waveforms while Figure 7B shows more detailed view of the waveform in which the sampling instants are more clearly shown. The sampled currents and voltages may be used to form a graph which represents collector-emitter voltage versus collector current. At different collector-emitter voltages, a slow turn-on transition can allow for extracting the amount of charge passing through the gate and through the collector-emitter with an acceptable sampling rate. As the largest variations in the input and output capacitances of the switch component are at low collector-emitter voltages, this study should be performed at reduced DC voltage. Alternatively, these capacitances can be identified via injecting a high frequency signal to the collector-emitter and gate of the device at different collector-emitter voltages and measuring the corresponding currents. The above measurements are able to provide data according to which the health of the controllable switch components can be determined. The data may comprise multiple of data points, interpolated values of various curves or estimated values. Multiple of data points can be gathered for example from measurements which produce a curve. These multiple data points may be used to form a curve having parameters, and these curve parameters may be used in estimating the health of the device. Further, different values can be obtained directly from the measurements and these values may be used as indicating the health. These values may, for example, include maximum or minimum values, maximum or minimum values with different measurement properties. The above measurements are examples of possible measurements which may be used for indicating the health of the controllable switch. The results obtained from the measurement are preferably stored in a memory together with a time stamp and an identification of the measured switch component and submodule. When measurements are repeated, the changes in the obtained results indicate more clearly the health or operation condition of the switching component in question. If, for example, change in a certain measurement result is detected, the properties of the component have been changed. This change may indicate that the component will soon fail. The measurement results, measured values or indicative data may be stored in a database which may comprise respective data on healthy device. The data on healthy device can be used to determine if the obtained data indicates as such that the device is nearing its end. This database may be located remotely of the converter and the data may be transmitted to the database using a communications network. Each controllable switch of a submodule has a respective gate driver circuit. A gate driver obtains switching instructions and produces a desired voltage to the gate of the switch. The switch is preferably an IGBT or a MOSFET. As in the embodiments of the invention specific control operations are needed for the measurements, specific switching instruction sequences are preferably stored in a memory and provided to the gate driver when desired. Further, in one or more of the health measurements collector-emitter or drain-source voltage of the device may be changed. The change in the voltage is obtained by charging or de-charging the voltage of the capacitor. For the implementation of the measurements at a certain DC voltage level, the circuitry may include means for measuring the capacitor voltage. According to an embodiment on the invention, the measurements are carried out when the temperature of the submodule or the selected switch component has reached a certain level or after a time period the selected switch component has been idle. The submodules of the converter are typically cooled with a circulating liquid. When a semiconductor switch component has been idle for a certain period of time, the cooling has brought the temperature of the switch component to a certain level. When the temperature of the device is brought to a certain level, the temperature variations do not have an effect on the obtained results. Thus in the embodiment, when the capacitor voltage is reduced to a lower value, an idle period is started for allowing the temperature of the switch device to sink. When all measurements are carried out in reduced and roughly at same temperatures, the effect of temperature variations is eliminated from the measurements. Accordingly, in one embodiment one of the measurements carried out during the condition monitoring is temperature measurement. In the embodiments shown in the drawings the submodules are formed of a full-bridge circuit. However, the topology of the submodules is not restricted to that shown and described. 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 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 of monitoring condition of a modular multilevel converter, wherein the modular multilevel converter comprises submodules having a capacitor and controllable switches, the method comprisingselecting a submodule,lowering the voltage of the capacitor of the selected submodule to a desired value,performing measurements on at least one controllable switch of the selected submodule.wherein the step of performing measurements comprisesselecting the at least one controllable switch,providing a current path in the submodule such that one or more of the controllable switches other than the at least one selected controllable switch are in the current path,controlling the selected at least one controllable switch to perform the measurements.

2. A method according to claim 1, wherein the selected at least one controllable switch is part of the current path during the step of controlling.

3. A method according to claim 1 or 2, wherein the lowering of the voltage of the capacitor comprisesproviding a resistive circuit in parallel with the capacitor.

4. A method according to any one of claims 1 to 3, wherein the lowering of the voltage of the capacitor comprisesmodulating the controllable switches of the selected submodule to lower the voltage of the capacitor.

5. A method according to any one of claims 1 to 4, wherein the voltage of the capacitor is changed during the measurements.

6. A method according to any one of claims 1 to 5, wherein the measurements are curve tracing measurements.

7. A method according to any one of claims 1 to 6, wherein the measurements comprise temperature measurements.

8. A method according to any one of the claims 1 to 7, wherein the results of the measurements are stored and compared to results of previous measurements.

9. A method according to claim 8, wherein the results of the05 12 24measurements comprise voltage or current curves, voltage or current slopes, values of current or voltage, values of capacitances and / or temperatures10. A method according to any one of the previous claims 1 to 9, wherein the method comprises5 keeping the selected submodule idle for a time period for lowering thetemperature of the controllable switches.

11. A method according to any one of the previous claim 1 to 10, wherein the during the method the converter is in active use either as a rectifier or as an inverter.10 12. A modular multilevel converter, wherein the modular multilevelconverter comprises submodules having a capacitor and controllable switches, the method comprisingmeans adapted to select a submodule,means adapted to lower the voltage of the capacitor of the selected sub-15 module, andmeans adapted to perform measurements on at least one controllable switch of the selected submodulewherein the means adapted to perform measurements comprisemeans for producing switching instructions to a selected controllable 20 switch,means for providing a current path in the submodule such that one or more of the controllable switches other than the at least one selected controllable switch are in the current path, andmeans for measuring currents and voltages during the measurement on 25 the selected controllable switch.

13. A modular multilevel converter according to claim 12, wherein means adapted to lower the voltage of the capacitor comprise a resistive circuit which is connectable in parallel with the capacitor.

14. A modular multilevel converter according to claim 12, wherein 30 means adapted to lower the voltage of the capacitor comprise means for modulating the controllable switches of the submodule.

15. A modular multilevel converter according to claim 12, wherein the means adapted to perform measurement comprises further means for measuring temperature of the selected controllable switch.

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