EP3526893A1

EP3526893A1 - Modular multilevel converter with switch frequency control using flux error hysteresis

Info

Publication number: EP3526893A1
Application number: EP16805083.9A
Authority: EP
Inventors: Rodrigo Alonso Alvarez Valenzuela; Sascha KUBERA
Original assignee: Siemens AG
Current assignee: Siemens Energy Global GmbH and Co KG
Priority date: 2016-11-30
Filing date: 2016-11-30
Publication date: 2019-08-21
Also published as: WO2018099552A1

Abstract

The invention relates to, among others, a method for operating a modular multilevel converter (10) which has at least one converter module (KM1-KM6) with sub-modules (SM) electrically connected in series. Each sub-module (SM) comprises at least two switches (S) and an energy storage unit. In the method, the voltage at the at least one converter module (KM1-KM6) is ascertained, thereby forming actual voltage values (Uk). The actual voltage values (Uk) are compared with target voltage values (Uks), and at least one of the switches (S) of the submodules (SM) is switched if a voltage deviation value (H) formed based on the differential values between the actual voltage values (Uk) and the target voltage values (Uks) deviates by a degree defined by a specified hysteresis band which is determined by an upper hysteresis band threshold (+Hmax) and a lower hysteresis band threshold (-Hmax). The upper hysteresis band threshold, the lower hysteresis band threshold, or both hysteresis band thresholds are modified in a regular or irregular manner with a control variable (K) in order to achieve a specified converter behavior, and the control variable (K) is formed using at least one measurement value. According to the invention, the current flowing through the converter module (KM1-KM6) is measured, thereby forming a current measurement value (Ik), and the control variable (K) is formed at least also using the current measurement value (Ik).

Description

description

MODULAR MULTI LEVEL CONVERTER WITH SWITCHING FREQUENCY CONTROL BY MEANS OF FLUX ERROR HYSTERESIS

The invention relates to a method for operating a modular Multilevelumrichters, a control device for a modular Multilevelumrichter and a modular Multilevelumrichter as such.

From the publication "Control of Switching Frequency for Modular Multilevel Converters by a Variable Hysteresis Band Modulation" (Sascha Kubera, Rodrigo Alvarez, Jörg Dorn, 18th European Conference on Power Electronics and Applications EPE16 ECCE Europe, 5-9 September 2016 , Karlsruhe, Germany) is a method for operating a modular

Multilevel converter, which has at least one converter module with electrically connected in series submodules known. In the previously known method, the voltage is measured on the at least one converter module to form voltage actual values. The voltage actual values are compared with voltage setpoints and the switches of the submodules are switched on or off when a voltage deviation value formed as a function of the difference values between the voltage actual values and the voltage setpoints deviates over a measure defined by a predetermined hysteresis ^band . The hysteresis is modi ^¬ fied with a control number. The control quantity is calculated using a measured value, namely the respective switching frequency of the

Converter module, formed.

The invention is based on the object, a still further improved method for operating a modular

Specify multilevel converter.

This object is achieved by a method having the features according to claim 1. Advantageous embodiments of the method according to the invention are specified in subclaims. Accordingly, the invention provides that the current through the converter module to form a current measurement value is gemes ^¬ sen and the control variable is formed at least under Heranzie- hung the measured current value.

A significant advantage of the method according to the invention is the fact that the maximum voltage difference or the maximum voltage swing between the capacitor voltages of the submodules can be reduced by the inclusion of the current through the respective converter module. This circumstance makes it possible in an advantageous manner to reduce the switching frequency for switching the submodules, which in turn reduces the switching losses of the multilevel converter and a particularly efficient operation of the multilevel converter is possible.

Especially advantageous is considered when the measured current value ^¬ a Polynombildung to form a

Is subjected to the polynomial result value and the control quantity is formed using the polynomial result value.

The polynomial formation is preferably a polynomial of at least second degree, in which the current measurement value with a predetermined first constant to form a first

Polynomial auxiliary value is multiplied, the current measured value after squaring with a predetermined second constant is multiplied to form a second polynomial auxiliary value and the first and second polynomial auxiliary value are added.

It is particularly advantageous in view of the above angespro ^¬ chene reduction of the maximum voltage swing between the capacitor voltages of the submodules when the Polynombildung a Polynombildung third or at least the third degree (ie, fourth, fifth, .... degree) is, at which the current measurement value is multiplied by a predetermined first constant to form a first polynomial auxiliary value, the current measured value after squaring with a predetermined second Multiplied constant to form a second polynomial auxiliary value, the measured current value after magnitude formation and exponentiation to the third power multiplied by a predetermined third constant to form a third polynomial auxiliary value and the first, second and third

Polynomial value added.

Also, it is considered advantageous if a first

Auxiliary control is formed using the current measurement, a second auxiliary control variable is formed, in response to a frequency deviation value indicating the deviation between the actual switching frequency of the submodules and a predetermined target switching frequency, and formed the control variable using the first and second auxiliary control variable becomes.

The frequency deviation value is preferably formed by means of an integrator which integrates a difference value indicating the difference between the actual switching frequency of the submodules and the predetermined setpoint switching frequency over a predetermined time constant.

Moreover, it is advantageous if the measured current value ei ^¬ ner polynomial formation to produce a

Is subjected to the polynomial result value and the first auxiliary control variable is formed by using the polynomial value.

The polynomial result value is preferably multiplied by the second auxiliary control variable directly or, after multiplication by a predetermined auxiliary parameter, to form the first auxiliary control variable.

The control variable is preferably formed by forming a difference between the first and second auxiliary control variables. The above-mentioned voltage deviation value is preferred by integrating the difference value between the voltage ^¬ actual values and the voltage setpoints formed over time. Incidentally, the voltage across the at least one converter module can be measured directly or calculated by adding the submode voltages of the switched submodules.

The invention also relates to a control device for controlling a Multilevelumrichters, the Minim ^¬ least comprises a converter module with electrically series-connected submodules, wherein each submodule includes at least two switches and an energy storage device and wherein the control device is designed such that the voltage at the at least one converter module determines the formation of Spannungsistwerten, the actual voltage compares with clamping ^¬ voltage setpoints, and at least one of the switches of the submodules switches when the voltage values of the voltage command values over a predetermined through a

Hysteresis, which is defined by an upper Hysteresebandschwelle and a lower Hysteresebandschwelle differ defined level, wherein the upper Hysteresebandschwelle, the un ^¬ tere Hysteresebandschwelle or both Hysteresebandschwellen to achieve a predetermined inverter behavior regularly or irregularly modified with a control number and wherein the control variable is formed using at least one measured value.

With regard to such a control device, it is provided according to the invention that the control device measures the current through the

Converter module to form a current reading measures and the control size at least by using the

Current reading forms. With regard to the advantages of the control device according to the invention, reference is made to the above statements in connection with the method according to the invention. The invention also relates to a

Multilevel converter equipped with such a control device.

The invention will be explained in more detail with reference to Ausführungsbeispie ^¬ len; thereby show by way of example

Figure 1 shows an embodiment of an inventive

Multilevelumrichter,

Figure 2 shows an embodiment of a submodule, the

Formation of converter modules in the

Multilevel inverter according to Figure 1 can be used

FIG. 3 shows a further exemplary embodiment of a submodule that can be used to form converter modules in the multilevel converter according to FIG. 1,

4 shows an exemplary embodiment of a method for operating the multilevel converter according to FIG. 1 and in this context an exemplary embodiment of an advantageous mode of operation of a control ^{device of} the multilevel converter according to FIG. 1, FIG.

FIG. 5 shows measured value profiles during operation of the multi ^¬ level converter according to FIG. 1 in the case of control of the converter modules without consideration of the current through the respective converter module,

FIG. 6 shows measured value profiles during operation of the multi ^¬ level converter according to FIG. 1 in the case of control of the converter modules taking into account the current through the respective converter module, that is to say in an operation according to FIG. 7 shows a further exemplary embodiment of a method for operating the multilevel converter according to FIG. 1 and, in this context, a further exemplary embodiment for an advantageous mode of operation of a control device of the multilevel converter according to FIG

FIG. 1

For the sake of clarity, the same reference numbers are always used in the figures for identical or comparable components.

FIG. 1 shows a multilevel converter 10 which has three AC voltage connections LI, L2 and L3, at each of which an alternating current can be fed into or removed from the multilevel converter 10. Two DC voltage connections, at which a direct current Idc can be fed into or removed from the multilevel converter 10, are identified in FIG. 1 by the reference symbols L + and L-. The DC voltage at the DC voltage terminals L + and L- carries the reference numeral Udc.

The Multilevelumrichter 10 has three series circuits Rl, R2 and R3, whose external connections are the connections Gleichspannungsan ^¬ L + and L- Multilevelumrichters of the tenth The series circuits R1, R2 and R3 each comprise two series-connected converter modules (see reference numbers KM1-KM6).

Each of the converter modules KM1-KM6 has at least two submodules SM connected in series, each comprising at least two switches and one capacitor. Embodiments of suitable sub-modules SM are exemplified below erläu ^¬ tert in connection with Figures 2 and 3. FIG. The multilevel converter 10 has a control device 20, which is suitable for driving the submodules SM and thus for controlling the converter modules KM1-KM6. The STEU ^¬ ereinrichtung 20 has for this purpose a computing device 21 and a memory 22. In the memory 22, a control program module SPM is stored, which determines the operation of the computing device 21. 2 shows an embodiment for a submodule SM, which comprises two switches S, two diodes D and a gate Kondensa ^¬ C. The components mentioned form a half-bridge circuit which, by activating the switches S-on the part of the control device 20 according to FIG. 1-permits unipolar operation of the capacitor C.

FIG. 3 shows an exemplary embodiment of a submodule SM which comprises four switches S, four diodes D and one capacitor C. The components mentioned form a full-bridge circuit which, by activating the switches S-on the part of the control device 20 according to FIG. 1-permits bipolar operation of the capacitor C.

FIG. 4 shows an exemplary embodiment for an advantageous mode of operation of the control device 20 in the context of the operation of the multilevel converter 10 according to FIG. 1.

The following explanations relate to the control of the converter module KM1 according to FIG. 1; the control of the other converter modules KM2 to KM6 takes place preference ^¬, in an identical or at least comparable form, so that in this respect, reference is made to the statements relating to the control of the converter module KML. The controller 20 measures the current I through the

Converter module KMl forming a current measurement Ik. The measured current value Ik is ^¬ fed into a Polynombildner 100 performs Polynombildung third degree to form a Polynomergebniswerts fourteenth

For this purpose, the polynomial former 100 has a first multiplier 110, which has the current measurement value Ik with a first constant kl multiplied, which makes a first

Polynomial value II is generated according to:

11 = kl * Ik

A squarer 120 and a downstream second

Multipliers 130 of the polynomial generator 100 subject the current measurement Ik to squaring and a multiplication by a second constant k2, whereby a second

Polynomial value 12 is formed according to:

12 = Ik ² * k2

In addition, the polynomial generator 100 has a magnitude and potential generator 140 and a third multiplier 150, which subject the current measurement Ik to a power of three powers and a magnitude, and subsequently perform a multiplication by a third constant k3. In this way, a third polynomial auxiliary value 13 is formed according to:

13 = I Ik I ³ * K3

A summation 160 of the Polynombildners 100 adds the three Polynomhilfswerte II, 12 and 13 to form the already mentioned be ^¬ Polynomergebniswerts 14 according to:

14 = II + 12 + 13 The polynomial result value 14 is multiplied in a multiplier 170 by an auxiliary parameter k4 to form a modified polynomial result value 15.

For the selection of the auxiliary parameters kl, k2, k3, a range between -10 and +10 is suitable. The auxiliary parameter k4 used to scale to a normalized value and corresponding preference ^¬ as the branch current maximum occurring in the stationary case. For example, if the power converter is dimensioned that at maximum active and reactive power in stationary operation, a branch current of 2 kA flows, then k4 would be k4 preferably k = 1/2.

The modified polynomial result value 15 is subsequently multiplied by a frequency deviation value F in a multiplier 215 to form a first auxiliary control quantity K1.

The frequency deviation value F is preferably as follows averages ^¬: The control means 20 detects in addition to the measured current value Ik, the respective switching frequency f with which the sub-modules SM of the converter module of Figure 1 KM1 are currently actually operated. The switching frequency f and one for the operation of the controller 20 predetermined nominal switching frequency fs in a subtractor 190 a difference formation to form a frequency difference value df unterzo ^¬ gene, which is subsequently integrated by an integrator 200 under Bil ^¬ extension of the frequency offset value F. In the ^¬ integrator 200 operates with a time constant Ti, the German lent is smaller than the line period of the electrical network to which the Multilevelumrichter 10 is connected as shown in FIG. 1

The frequency deviation value F is ^¬ fed together with the first auxiliary control variable Kl in a subtractor 220, the output side generates a control variable K. With the control quantity K, an upper hysteresis threshold + Hmax and a lower hysteresis threshold -Hmax are formed. For example, the control quantity K can directly define the upper hysteresis band threshold; in this case, the un ^¬ tere Hysteresebandschwelle -Hmax is preferably formed by an inverter 230 which inverts the sign of the control variable K. In the embodiment according to Figure 4 of the Fre quenzabweichungswert ^¬ F is advantageously directly ei ^¬ ne second auxiliary control number K2, which is fed together with the first auxiliary control variable Kl in the difference former 220th An overhead by a further processing of the Fre ^¬ quenzabweichungswerts F during the generation of the second auxiliary control variable K2 thus omitted. The upper hysteresis band threshold + Hmax and the lower one

Hysteresebandschwelle -Hmax are fed to a switching module 300, the output side control signals ST to Umschal ^¬ th of the submodules SM of the converter module KM1 generated according to FIG. 1 For this purpose, the switching module 300 compares egg NEN voltage deviation value H having a defined in the switching module 300 hysteresis curve HK and generates the Steuersig ^¬ dimensional ST for switching the submodules when the Spannungsab weichungswert ^¬ H a defined by the Hystersekurve HK hysteresis represented by the upper hysteresis band threshold + Hmax and the lower hysteresis band threshold -Hmax is limited leaves. If the voltage deviation value H lies within the hysteresis band defined by the hysteresis curve HK, it is not necessary to switch over the submodules SM. The switch module 300, the control signals ST that one of the switches of the submodules SM To ^¬ turn produce, produce, as, for example, from the above Veröffentli ^¬ chung "Control of Switching Frequency for a modular multilevel converter by a variable hysteresis band modulation" already known is.

In the embodiment according to Figure 4 of the voltage is formed ^¬ deviation value H by integrating by an integrator 400, which subjects a voltage difference dU value of an integration with a time constant At.

The polynomial value 15 changes temporally in the range <20 ms (depending on the polynomial degree: 1st order = 50 Hz => 20) ms, 2nd order = 100 Hz => 10 ms, etc.), and substantially shorter than the time constant Ti of the integrator 200, which is preferably in a range between 100 ms and several seconds before ^¬.

The voltage difference value dU is formed by forming the difference between the respective actual voltage Uk of the voltage on the converter module KML according to Figure 1 as well as a respectively specified ^differently surrounded voltage setpoint Uks; the difference formation for forming the voltage difference value dU can be effected by a difference generator 410.

5 shows an exemplary waveform of the switching frequency f, the profile of the upper Hysteresebandschwelle + Hmax, the profile of the lower Hysteresebandschwelle -Hmax, the course of the current or of the current measurement value Ik, the course of egg ^¬ nes capacitor difference value DUC each over identi ^¬ rule Timeline t. The capacitor difference value dUc indicates the difference between the capacitor voltage of the submodule SM of the converter module KM1 with the highest capacitor voltage and the capacitor voltage of the submodule SM with the lowest capacitor voltage of the converter module KM1.

FIG. 5 shows the course of the measured values F, Ik and dUc for the case in which the hysteresis thresholds + Hmax and -Hmax are set exclusively taking into account the frequency difference value df or the frequency deviation value F, as described in the abovementioned publication "Control of Switching Frequency for Modular Multilevel Converters by a Variable Hysteresis Band Modulation "is the case.

FIG. 6 shows the positive influence of the polymer former 100 of the control device 20 according to FIG. 4 on the course of the measured values, in particular on the course of the capacitor difference value dUc. It can be seen that HK to show more often at those points over time by dynamically changed depending on the current measuring values ^¬ Ik hysteresis curve where the Capacitor difference value dUc is large. In other words, the polymer formers 100 performs in the embodiment variant ge ^¬ Mäss Figure 4 to an optimized profile of the

Capacitor difference dUc, because the deviation between the largest and smallest capacitor voltage in the submodules SM of the converter module KM1 is smaller overall.

Due to the reduction of the capacitor difference values dUc, the possibility of reducing the nominal switching frequency fs according to FIG. 4 as a further positive consequence arises overall. The advantage of a reduced setpoint switching frequency fs is that, with a lower switching frequency, the switching losses of the switches in the submodules decrease. FIG. 7 shows a further exemplary embodiment of an advantageous construction or an advantageous mode of operation of the control device 20 in the context of the operation of the

Multilevel converter 10 according to FIG. 1. In conjunction with FIG. 7, reference is again made to the converter module KM1 according to FIG.

In the embodiment according to FIG. 7, the control ^device 20 exclusively evaluates the current I through the ^device

Converter module KM1 or the current measured value Ik. The measured current value Ik is a polynomial formation by the

Polynomializer 100 subjected, as already explained in connection with the embodiment of FIG 4 de ^¬ . In contrast to the embodiment of Figure 4, the control size K and thus the upper

Hysteresis band threshold + Hmax and the lower one

Hysteresebandschwelle -Hmax formed directly by means of modifi ^¬ ed polynomial result value 15. Including the respective switching frequency f or the deviation df of the respective switching frequency f from a predetermined desired switching frequency fs does not serve in the embodiment according to FIG. 7 to influence the hysteresis thresholds + Hmax or -Hmax. Incidentally, the above statements apply in hang with the figure 4 in the embodiment according to FIG 7 accordingly.

The controller 20, as shown in Figure 1, comprise a computer 21 and a memory 22, in which the function modules shown in Figures 4 and 7 are stored as soft ^¬ ware module, for example, within the control program module SPM. Although the invention in detail by preferred execution ^¬ examples has been illustrated and described in detail, the invention is not limited by the disclosed examples and other variations can be derived therefrom by the skilled artisan without departing from the scope of the invention.

Reference sign list

10 multilevel converters

20 control device

21 computing device

22 memory

100 polynomialers

110 multipliers

120 squarers

130 multipliers

140 amount and potency builders

150 multipliers

160 summator

170 multipliers

190 difference formers

200 integrator

215 multipliers

220 subtractors

230 inverters

300 switching module

400 integrator

410 difference formers

C capacitor

D diode

df frequency difference value dU voltage difference value dUc capacitor difference value f switching frequency

F Frequency deviation value fs Target frequency

H voltage deviation value

HK hysteresis curve

I electricity

II polynomial value

12 polynomial value

13 polynomial value

14 polynomial value 15 polynomial value

Idc DC

Ik current reading

K control size

kl constant

k2 constant

k3 constant

k4 auxiliary parameters

Kl auxiliary control size

K2 auxiliary control size

KM1-KM6 converter module

LI AC voltage connection

L2 AC voltage connection

L3 AC connection L + DC connection

L- DC voltage connection

Rl series connection

R2 series connection

R3 series connection

S switch

SM submodule

SPM control program module

ST control signal

t time

Ti time constant

Udc DC voltage

Uk voltage actual value

Uk voltage setpoint

At time constant

+ Hmax upper hysteresis band threshold -Hmax lower hysteresis band threshold

Claims

claims

1. Method for operating a modular

Multilevel converter (10) having at least one converter module (KM1-KM6) with electrically connected in series submodules (SM), each submodule (SM) each having at least two switches (S) and an energy store, wherein the method

the voltage at the at least one converter module (KM1-KM6) is determined to form voltage actual values (Uk),

- the voltage actual values (Uk) are compared with voltage setpoints (Uks) and

- At least one of the switches (S) of the submodule (SM) is switched, when a function of the difference ^values between the voltage actual values (Uk) and the voltage setpoints (Uks) formed voltage deviation value (H) via a through a predetermined Hystereseband formed by an upper Hysteresebandschwelle (+ Hmax) and a lower Hysteresebandschwelle (-Hmax) is set to deviate defi ned ^¬ measure,

- where the upper hysteresis band threshold (+ Hmax), the lower hysteresis band threshold (-Hmax) or both

Hysteresebandschwellen (-Hmax, + Hmax) may be modified periodically or to achieve a predetermined inverter behavior irr ^¬ connected to a control quantity (K) and

- wherein the control variable (K) is formed by using at least one measured value,

d a d u r c h e c e n c i n e s that

- The current through the converter module (KM1-KM6) is measured to form a current reading (Ik) and

- The control variable (K) is formed at least also by using the current measurement value (Ik).

2. The method according to claim 1,

d a d u r c h e c e n c i n e s that

the current measuring value (Ik) is subjected to polynomial formation to produce a polynomial value (14), and - the control quantity (K) using the

Polynomer result value (14) is formed.

3. The method according to claim 2,

d a d u r c h e c e n c i n e s that

the polynomial formation is at least a second degree polynomial formation, in which

- the current measuring value (Ik) is multiplied by a predetermined first constant (kl) to form a first polynomial auxiliary value (II),

- The current measurement (Ik) after squaring with a given second constant (k2) is multiplied to form a second polynomial auxiliary value (12) and

- the first and second polynomial auxiliary values are added.

4. The method according to claim 2 or 3,

d a d u r c h e c e n c i n e s that

the polynomial formation is a third or at least third degree polynomial formation, in which

the current measured value (Ik) is multiplied by a predetermined second constant (k2) after squaring to form a second polynomial auxiliary value (12),

- the current measuring value (Ik) by magnitude formation and Potenzie ^¬ tion to the third power with a predetermined third constant (k3) to form a third

Polynomial auxiliary value (13) is multiplied and

- the first, second and third Polynomhilfswert added ^¬ to.

5. Method according to one of the preceding claims, characterized in that

a first auxiliary control variable (K1) is formed using the current measured value (Ik),

- A second auxiliary control variable (K2) is formed, and

although depending on a frequency deviation value (F) indicating the deviation between the actual switching frequency of the sub-modules (SM) and a predetermined target switching frequency (fs), and

- The control size (K) is formed using the first and second auxiliary control variable (K1, K2).

6. The method according to claim 5,

d a d u r c h e c e n c i n e s that

the frequency deviation value (F) is formed by means of an integrator (200) which integrates a difference value (F) indicating the difference (df) between the actual switching frequency of the submodules (SM) and the predetermined nominal switching frequency (fs) over a predetermined time constant (Ti) ,

7. The method according to any one of the preceding claims 5 to 6, d a d u c h e c e n e c i n e that t

the current measuring value (Ik) is subjected to polynomial formation to produce a polynomial value (14), and

- The first auxiliary control variable (Kl) is formed using the polynomial value (14).

8. The method according to claim 7,

d a d u r c h e c e n c i n e s that

the polynomial result value is multiplied immediately or after multiplication by a predetermined auxiliary parameter (k4) by the second auxiliary control variable (K2) to form the first auxiliary contour variable (K1).

9. The method according to any one of the preceding claims 5 to 8, d a d u c c h e c e n e c e in that e

the control variable (K) is formed by forming the difference between the first and second auxiliary control variables (K1, K2).

10. The method according to any one of the preceding claims, d a d u r c h e c e n e c e s in that e

the voltage deviation value (H) by integration of the Diffe ^¬ ence value between the Spannungsistwerten (Uk) and the voltage command values (Uks) is formed over time.

11. Control device (20) for controlling a modular

Multilevel converter (10), which has at least one converter module (KM1-KM6) with electrically connected submodules (SM),

wherein each submodule (SM) comprises at least two switches (S) and an energy store, and

wherein the control device (20) is designed such that it

- compares the actual voltage values (Uk) with voltage setpoints (Uks) and

- at least one of the switches (S) of the submodules (SM) environmentally switched on if the actual voltage (Uk) over a bottom of the clamping ^¬ voltage setpoints (Uks) by a predetermined hysteresis band defined by an upper Hysteresebandschwelle (+ Hmax) and a Hysteresis band threshold (-Hmax) is set, deviate the defined dimension,

Hysteresebandschwellen (-Hmax, + Hmax) to obtain a regular or irregular predetermined inverter behavior ^¬ equipped with a control variable (K) can be modified and - wherein the control quantity (K) is formed by reference to at least one measured value,

d a d u r c h e c e n c i n e s that

- The control device (20) the current through the

Converter module (KM1-KM6) to form a current measurement (Ik) measures and

- forms the control variable (K) at least also using the measured current value (Ik).

12. multilevel converter (10),

d a d u r c h e c e n c i n e s that

this is equipped with a control device (20) according to claim 11.