DE102023201739A1 - Power converter and operating method for a power converter - Google Patents

Abstract

A power converter for supplying a connected load from a connected supply network is specified, wherein the power converter has a plurality of controllable semiconductor switching elements and a control device for controlling the semiconductor switching elements, and wherein the control device is designed to determine at least one test time, to determine at the test time whether the power provided by the power converter of the connected load is less than a threshold value, and if this is the case, to switch to a special operating mode for at most a definable intermediate load period, in which the power converter exchanges reactive power with the supply network, wherein the amount of the reactive power is at least the threshold value.

Description

The invention relates to a power converter with a plurality of controllable semiconductor switching elements and a control device for controlling the semiconductor switching elements, as well as an operating method for such a power converter.

The service life of controllable semiconductor switching elements such as IGBTs (IGBT = Insulated Gate Bipolar Transistor) or MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistor) is influenced by their temperature. The junction temperature of the switching element in particular plays a major role. The temperature is subject to constant changes during operation. In so-called semiconductor modules with several semiconductor switching elements, the temperature of the individual switching elements can also differ despite the same operating situation. In practice, the failure of a single switching element often causes the entire module to fail. The service life of individual switching elements is therefore of central importance.

Various approaches are known to increase the service life of the semiconductor switching elements used in a power converter. From "Active thermal management for a single-phase H-Bridge inverter employing switching frequency control", 2015, PCIM, it is known to vary the switching frequency between 10 kHz and 30 kHz. During periods of low load, the switching frequency and thus the temperature of the semiconductor switching elements is increased - relative to operation at a lower switching frequency. During periods of high load, on the other hand, the switching frequency and thus the temperature of the semiconductor switching elements is reduced - relative to operation at a higher switching frequency. This results in a smaller variation in temperature.

The disadvantage of this and other known methods is that they lead to a waste of power, especially during prolonged periods of low load.

It is an object of the present invention to provide a power converter and an operating method for a power converter in which the service life of the semiconductor switching elements used is increased, wherein in particular the problems mentioned at the outset are reduced.

This object is achieved by a power converter having the features specified in claim 1. A further solution consists in an operating method having the features specified in claim 13.

The power converter according to the invention is designed to supply a connected load from a connected supply network. It comprises a plurality of controllable semiconductor switching elements and a control device for controlling the semiconductor switching elements.

The control device is designed to determine at least one test time. It is further designed to determine at the test time whether the power provided by the power converter of the connected load is less than a threshold value. If this is the case, the control device switches to a special operating mode for at most a definable intermediate load period, in which the power converter exchanges reactive power with the supply network, the amount of the reactive power being at least the threshold value.

In the operating method according to the invention for a power converter, at least one test time is determined and at the test time, i.e. when the test time is reached, it is determined whether the power delivered by the power converter is less than a threshold value. If this is the case, a change is made to a special operating mode for a maximum of a definable intermediate load period in which the power converter provides reactive power, the amount of the reactive power being at least the threshold value.

In the operating method according to the invention for a power converter for supplying a connected load from a connected supply network, at least one test time is determined at which test time it is determined whether the power provided by the power converter to the connected load is less than a threshold value. If this is the case, a change is made to a special operating mode for a maximum of a definable intermediate load period in which the power converter exchanges reactive power with the supply network, the amount of the reactive power being at least the threshold value.

The above approach results in heat being generated in areas of low load. This keeps the switch materials in an elastic state, reducing the influence of switching operations on the service life and increasing the overall service life. This is a surprising effect, as it is normally assumed that an increased temperature leads to a reduced service life.

References to this effect can be found, for example, in “Power cycling results of high power IGBT modules close to 50 Hz heating process”, PCIM 2019, Chemnitz and “Impact of Thermal Cyc ling Frequency on IGBT Power Module Lifetime”, PCIM 2022, Montpellier University.

This can advantageously improve the durability of a semiconductor switching element while reducing losses compared to other known methods.

• Es ist zweckmäßig, den Sonder-Betriebsmodus zu beenden, wenn der Stromrichter wieder hohe Leistung bereitstellen muss. Die Zwischenlastdauer stellt daher einen maximalen Wert dar, der nur eingehalten wird, wenn der Stromrichter während der Zwischenlastdauer keine Leistung an angeschlossene Lasten bereitstellen muss.

Advantageous embodiments of the power converter and method 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 also with those of several subclaims. Accordingly, the following features can also be provided:

• It is advisable to end the special operating mode when the converter has to provide high power again. The intermediate load duration therefore represents a maximum value that is only maintained if the converter does not have to provide power to connected loads during the intermediate load duration.

The control device can also be designed to determine test times so that they occur periodically with a definable frequency. In other words, the test times occur at fixed intervals. Their location is therefore also known in advance. If a power is being delivered at a test time that is lower than the threshold value, the system switches to the special operating mode, otherwise not.

The control device can be designed to use a frequency of at least 5 Hz. In other words, the test times occur at least every 200 ms, and at shorter intervals at higher frequencies. This is advantageous because the effect of extending the service life occurs primarily at intermediate heating frequencies of at least 5 Hz.

In an alternative development of the invention, the control device can be designed to detect a period of a definable minimum length in which the output power remains below a threshold value and then to set the test time within the detected period after a waiting time has elapsed. In this design, the test times are not determined at fixed intervals, but are determined depending on the actual power output. To do this, an attempt is made to find a period in which the output power does not exceed the threshold value for the entire period. The test time is then determined within the period so that it is offset by the waiting time after the start of the period.

The control device can be designed to use 200 ms as the minimum length of the period, in particular 100 ms or 50 ms. Here again, the effect of extending the service life occurs primarily at intermediate heating frequencies of at least 5 Hz. Such a frequency is maintained by the periods mentioned.

The control device can be designed to use the minimum length of the period as the waiting time. If the power converter has no information on load cycles and thus the length of periods with low power is not known in advance, the time to switch to the special operating mode can only be selected after the minimum length of the period has elapsed.

If, however, the profile of the power delivered is known in advance, the control device can be designed to use a waiting time that differs from the minimum length of the period. In this case, the check to see whether such a period exists can be carried out using stored data instead of measurements. The test time can then be at any time within the period, since the period is known in advance.

The control device can be designed to use at least 20 ms, in particular at least 50 ms, as the intermediate load duration. The intermediate load duration can also be less than 100 ms. With such an intermediate load duration, a balance is created between the effect of the service life extension and a generally increased average temperature.

The control device can be designed to use at least 10%, in particular at least 25%, of the rated power of the power converter as converted reactive power. Furthermore, the control device can be designed to use at most 75% of the rated power of the power converter as converted reactive power. This increases the temperature sufficiently, but avoids unnecessary stress on the components.

The power converter can be, for example, a three-phase two-level converter. However, any other power converters can also be considered, such as multi-level power converters.

• 1 einen Umrichter mit einer Steuereinrichtung,
• 2 ein erstes Steuerverfahren,
• 3 ein zweites Steuerverfahren,
• 4 einen Temperaturverlauf bei einem Betrieb des Umrichters ohne das Steuerverfahren,
• 5 einen Temperaturverlauf bei einem Betrieb des Umrichters mit dem Steuerverfahren.

In the following, the invention is described and explained in more detail using the embodiments shown in the figures. They show:

• 1 a converter with a control device,
• 2 a first tax procedure,
• 3 a second tax procedure,
• 4 a temperature curve when the converter is operated without the control method,
• 5 a temperature curve when operating the converter with the control method.

1 shows a simply constructed three-phase converter 10 as an example of a power converter according to the invention. The converter has an active rectifier 12 with input connections 13a...c and an inverter 14 with output connections 15a...c. Rectifier 12 and inverter 14 are connected via a DC intermediate circuit 16. The DC intermediate circuit 16 comprises a capacitor 17.

The rectifier 12 and the inverter 14 each comprise three half-bridges connected in parallel, each made up of two IGBTs 18 arranged in series. The center points of the half-bridges are the input connections 13a...c for the rectifier 12 and the output connections 15a...c for the inverter 14. The converter 10 is connected to a supply network 20, for example a 400 V local network, via the input connections 13a...c. A load 29, for example an electric motor, is connected to the output connections 15a...c. The upper external connections of all half-bridges are connected to one another and to the upper connection of the DC intermediate circuit 16. The lower external connections of all half-bridges are also connected to one another and to the lower connection of the DC intermediate circuit 16.

The IGBTs 18 each comprise a control terminal 19 (gate). The control terminals 19 are connected to a control device 11. The control device 11 determines switching times for the IGBTs 18 in a known manner according to a control method, for example pulse width modulation, and outputs corresponding signals to the control terminals 19. During operation and under load, these signals have a frequency that depends on the type of converter and the type of semiconductor switches used, but in this example is 16 kHz.

The resulting conduction losses and switching losses ensure that under load all IGBTs 18 are heated to a very variable temperature, which can be in the range of 120 °C, for example. During periods of low load or no load, however, the temperature of the IGBTs 18 drops to approximately room temperature, depending on the duration of a no-load phase and other environmental influences.

This temperature curve is exemplary in 4 shown using three temperature curves, where T _IGBT represents the temperature of one of the IGBTs 18, T _FWD the temperature of the associated freewheeling diode and T _H the temperature of the heat sink. The time period shown is 1.2 s long in total. In the range from t = 0.1 s to t = 0.4 s there is a first load range in which the converter 10 provides power for the connected load 29. During this time period the temperature of the IGBTs 18 rises to approximately 110 °C to 120 °C. The fluctuation in the temperature of the IGBT 18 caused by switching operations is also visible, the temperature of which in 4 is shown.

After t = 0.4 s, the provision of power ends and the converter 10 no longer has to deliver power until t = 1 s. During this period, the temperature of the IGBTs 18 drops to approximately the ambient temperature with time constants of slightly less than 0.1 s.

In order to minimize the lifespan of the IGBTs 18 in the case of such or a similar power curve, a control method is used according to this embodiment of the invention which 2 This control method complements and extends the control method used to generate the switching times for the IGBTs 18, for example symmetrical space vector pulse width modulation. Therefore, in 2 only those steps are shown that go beyond the normal generation of the switching times. The control method is implemented together with the generation of the switching times for the IGBTs 18 in the control device 11.

The tax procedure according to 2 assumes that the periods of low power supply are not known in advance. In this case, the control device 11 must consider the current power supply of the converter 10.

In a first step 21, the control device 11 checks whether the power currently provided to the load 29 is below a threshold value. The threshold value is set in such a way that it is determined whether there is a period of low power output. In this example, 10% of the rated power of the converter is used as the threshold value. If this is 30 kW, the threshold value is therefore 3 kW. As long as this is not the case, the control device 11 remains in this first step 21.

It is clear that there are several options for testing the power supply. It is not necessary to measure the electrical power in the sense of a current-voltage product but the control device 11 can also consider current or voltage alone or just the switching pattern of the IGBTs 18. It is sufficient to state that a value representing the power supply is compared with a threshold value. In this text, power, power supply and power output are used synonymously.

If the power output falls below the threshold value, the control device switches to the second step 22. In the second step 22, the control device checks whether the power output exceeds the threshold value. If this is the case, the control device 11 returns to the first step. The control device 11 also checks whether a time interval of a definable minimum length has passed since switching to the second step. As long as this is not the case and the power output remains below the threshold value, the control device 11 also remains in the second step 22.

The check to see whether there is a period of low load is carried out continuously. For example, the control system can save a first point in time as soon as the load falls below the threshold. It then calculates how much time has passed since the first point in time. If the load exceeds the threshold again, the check is suspended.

If the time interval of the minimum length has elapsed, the control device 11 changes to a third step 23. In this third step 23, the control device 11 switches the generation of the switching signals for the IGBTs 18 so that in addition to the required power output, which can also be 0, reactive power is now exchanged with the supply network 20. The reactive power is dimensioned such that the temperature of the IGBTs 18 rises to approximately 80 °C. This process is referred to as a special operating mode. The reactive current is absorbed and emitted by suitable switching operations of the IGBTs 18 of the rectifier 12.

The control device 11 then checks in a fourth step 24 whether the required power output changes. If this is the case, the generation of the switching signals is adjusted again. If the required power exceeds the threshold value, the special operating mode is interrupted in this example and the system returns to the first step 21.

Even after the expiration of an intermediate load period, which in this embodiment is 50 ms, the control device 11 changes back to the first step 21 and thus out of the special operating mode, regardless of the current power requirement.

The 2 The method described assumes that the control device 11 has no advance information about the power output curve for the converter 10. In this case, the control device 11 is dependent on determining the correct times for switching to and from the special operating mode by considering the current power requirement. The power requirement typically results from control signals from a higher-level control system, which are in 1 is not shown. It does not usually have to be obtained from measurement signals, although this option can also be used.

In an alternative embodiment, the periods of low power output of the control device 11 can already be known in advance, since the load profile is known. In this case, the ongoing check based on the actual load is not necessary and only previously known points in time need to be compared with the current time.

In this case, a tax procedure can be used which is 3 is shown. In a first step 31, a fixed interval is defined in which it is checked whether it is possible to switch to the special operating mode. In the selected example, this interval should be 200 ms, but in other examples other intervals such as 100 ms, 150 ms or 50 ms can be used. The defined interval results in fixed points in time. In the example with an interval of 200 ms, these fixed points in time are, for example, 0.2 s, 0.4 s, 0.6 s, 0.8 s, 1 s, etc. The fixed points in time can be filtered in the first step 31 in order to keep only those points in time at which it should switch to the special operating mode. Those points in time at which the power output to the load 29 is below the threshold value for the duration of the intermediate load period from that point in time are kept, while other points in time are sorted out and discarded.

In a second step 32, it is checked whether one of the resulting points in time has just been reached. If this is the case, the control device 11 switches to a third step 33. In this third step 33, the control device 11 switches the generation of the switching signals for the IGBTs 18 so that, in addition to the required power output, reactive power is now exchanged with the supply network 20. The reactive power is again dimensioned such that the temperature of the IGBTs 18 rises to approximately 80 °C. The system therefore switches to the special operating mode.

After the intermediate load period has elapsed, the control device 11 switches back to the second step 32 and thus out of the special operating mode. A check as to whether the required power output changes is unnecessary in this embodiment, since the points in time have already been filtered accordingly in advance.

A temperature curve for the IGBTs 18 of the converter resulting from this method is shown in 5 The timing of the power supply for the connected load 29 corresponds to that shown in 4 The control device 11 remains between t = 0.1 s and t = 0.4 s in the first step 21. Then the output power falls below the threshold value up to t = 1 s.

For an interval of 200 ms, starting at t = 0 s, the candidates for switching to the special operating mode are t = 0 s, t = 0.2 s, t = 0.4 s, t = 0.6 s, etc. 5 It can be seen that the times t = 0.2 s and t = 0.4 s and t = 1 s can be discarded because there is a power output and the temperature of the IGBTs 18 is still high anyway. The times t = 0.4 s and t = 0.6 s, however, are used. At these times, the control device 11 changes to the special operating mode and remains there for the full intermediate load duration of 50 ms before returning to the second step 32.

Reference symbols

10

Inverter

11

Control device

12

active rectifier

13a...c

Input connections

14

Inverter

15a...c

Output connections

16

DC intermediate circuit

17

capacitor

18

IGBT

19

Control contact

29

load

20

Supply network

21, 31

first step

22, 32

second step

23, 33

third step

24, 34

fourth step

TIGBT

Temperature curve

Claims (13)

Power converter (10) for supplying a connected load (29) from a connected supply network (20), wherein the power converter (10) has a plurality of controllable semiconductor switching elements (18) and a control device (11) for controlling the semiconductor switching elements (18), and wherein the control device (11) is designed to - determine at least one test time, - determine at the test time whether the power provided by the power converter (10) to the connected load (29) is less than a threshold value and - if this is the case, switch to a special operating mode for at most a definable intermediate load period, in which the power converter (10) exchanges reactive power with the supply network (20), wherein the amount of the reactive power is at least the threshold value. Power converter (10) to Claim 1 , in which the control device (11) is designed to determine test times such that they occur periodically with a definable frequency. Power converter (10) to Claim 2 , in which the control device (11) is designed to use at least 5 Hz as the frequency. Power converter (10) to Claim 1 , in which the control device (11) is designed - to detect a period of a definable minimum length in which the output power remains below a threshold value, - to set the test time within the detected period after expiry of a waiting time. Power converter (10) to Claim 4 , in which the control device (11) is designed to use 200 ms as the minimum length of the period, in particular 100 ms or 50 ms. Power converter (10) to Claim 4 or 5 , in which the control device (11) is designed to use the minimum length of the period as the waiting time. Power converter (10) to Claim 4 or 5 , in which the control device (11) is designed to use a time different from the minimum length of the period as the waiting time. Power converter (10) according to one of the preceding claims, in which the control device (11) is designed to use at least 20 ms, in particular at least 50 ms, as the intermediate load duration. Power converter (10) according to one of the preceding claims, in which the control device (11) is designed to use less than 100 ms as the intermediate load duration. Power converter (10) according to one of the preceding claims, in which the control device (11) is designed to use at least 10%, in particular at least 25% of the rated power of the power converter (10) as converted reactive power. Power converter (10) according to one of the preceding claims, in which the control device (11) is designed to use at most 75% of the rated power of the power converter (10) as converted reactive power. A power converter (10) according to any preceding claim, which is a three-phase two-level converter. Operating method for a power converter (10) for supplying a connected load (29) from a connected supply network (20), in which - at least one test time is determined, - at the test time it is determined whether the power provided by the power converter (10) to the connected load (29) is less than a threshold value, - in this case a change is made to a special operating mode for at most a definable intermediate load period, in which the power converter (10) exchanges reactive power with the supply network (20), the amount of the reactive power being at least the threshold value.

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