CN105808901B - A method for determining the conduction loss of a modular multilevel converter - Google Patents

Abstract

The invention relates to a method for determining on-state loss of a modular multilevel converter, which comprises the following steps: determining the on-state loss of the IGBTs in the single-phase upper bridge arm and the single-phase lower bridge arm of the modular multilevel converter; determining on-state loss of diodes in single-phase upper and lower bridge arms of the modular multilevel converter; and determining the three-phase on-state loss of the modular multilevel converter. The on-state loss analytical expression of the modular multilevel converter is effectively solved based on the instantaneous current of the IGBT and the diode, the quantitative analysis of the on-state loss of the MMC is facilitated, and the optimal design of a system is facilitated.

Description

A method for determining the conduction loss of a modular multilevel converter

technical field

The invention relates to a calculation method of power electronic technology, in particular to a method for determining the on-state loss of a modular multilevel converter.

Background technique

Since 2002, R. Marquart and A. Lesnicar of the Federal University of Defense in Munich, Germany jointly proposed the Modular Multilevel Converter (MMC). The field of flow device has been widely developed and applied, and its topology is shown in Figure 1. In terms of flexible DC transmission systems based on voltage source converters, MMC converters were used in the Transbay grid interconnection project put into operation by Siemens in the United States in 2010 and in the Nanhui flexible DC transmission demonstration project in China in 2011; In terms of high-voltage DC transformers, MMC-based high-voltage DC/DC converters (DC transformers) have become a research hotspot at home and abroad; in the field of motor drive applications, a large number of scholars have studied the frequency conversion control technology of asynchronous motors based on MMC converters.

The main power switching device used in the MMC converter is an insulated gate bipolar transistor IGBT (InsulatedGate Bipolar Transistor, IGBT). Limited by the withstand voltage level of a single IGBT, in order to meet the needs of high-voltage applications, a large number of them are used, so the loss of the MMC converter is mainly caused by the power switching device, and the operation stability of the IGBT also affects the reliable operation of the entire system. one of the key factors. The main reason for the failure of IGBT operation is that its junction temperature is too high, so good cooling design and system optimization are the prerequisites for the reliable operation of the system. In the flexible DC transmission system based on MMC, the system works at 50Hz, and the loss research of power devices provides theoretical support for system cooling design, parameter selection, etc. 500Hz～1kHz), as the operating frequency increases, the volume of passive devices such as capacitors and inductors in the converter decreases while the loss of power devices such as IGBTs increases. Therefore, the research on the loss of MMC in the optimal design of system loss and volume compromise must be carried out. Indispensable; in the motor driving technology based on MMC, the system works in variable frequency conditions according to the speed regulation requirements, and it is necessary to study the relationship between MMC converter loss and frequency and carry out loss calculation at different frequencies.

The evaluation methods of power device loss can be divided into three categories: test detection, physical modeling and mathematical analysis. The experimental detection method is only suitable for low-voltage and low-power applications, and the physical modeling method is difficult to obtain based on a large number of device manufacturing parameters. At present, the loss research of MMC converters adopts the mathematical analysis method. According to some device characteristic parameters provided by the manufacturer, the characteristic function of the power device is fitted, and then the loss estimation or online loss calculation based on the average current and effective current of the power device is carried out. The loss estimation based on average current and effective current fails to give the analytical expression of MMC loss, and the quantitative relationship between MMC loss and converter modulation degree, power factor, active power transmission power, etc. cannot be simply obtained; online loss calculation needs to obtain MMC conversion It is not practical to perform calculations on the voltage, current and driving signal of the current transformer at each moment, which is not practical in the system optimization design stage, and it is difficult to seamlessly connect with the optimization design program.

SUMMARY OF THE INVENTION

In view of the deficiencies of the prior art, the purpose of the present invention is to provide a method for determining the on-state loss of a modularized multilevel converter. The present invention effectively solves the problem of modularized multilevel commutation based on the instantaneous currents of IGBTs and diodes. The analytical expression of the on-state loss of the MMC is helpful for the quantitative analysis of the on-state loss of the MMC, and it is convenient for the realization of the system optimization design program.

The purpose of this invention is to adopt following technical scheme to realize:

The invention provides a method for determining the on-state loss of a modularized multilevel converter. The modularized multilevel voltage source converter is composed of three phases, and each phase is composed of upper and lower two connected in series with the same structure. The bridge arm is formed; the midpoint of the upper and lower bridge arms is connected to the AC end of the modular multilevel converter;

Each bridge arm of the upper and lower bridge arms includes a reactor and N sub-modules with the same structure; after the sub-modules of each bridge arm are cascaded, one end passes through the reactor and the modular multi-level converter. After the sub-modules of each bridge arm are cascaded, the other end is connected to one end of the cascaded sub-modules of the other two-phase bridge arms to form the positive and negative busbars of the DC end of the modular multi-level voltage source converter. ; The sub-module is composed of a half bridge and its parallel capacitor branch, the half bridge is composed of an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are both composed of insulated gate bipolar transistors IGBT and its It is composed of freewheeling diode FWD connected in parallel;

The improvement lies in that the method comprises the following steps:

Step 1: Determine the IGBT on-state losses in the single-phase upper and lower arms of the modular multilevel converter;

Step 2: Determine the diode conduction loss in the single-phase upper and lower arms of the modular multilevel converter;

Step 3: Determine the three-phase on-state losses of the modular multilevel converter.

Further, the step 1 includes the following steps:

Step 1.1: Calculate the IGBT on-state voltage drop at normal operating junction temperature by device relationship curve fitting and interpolation:

Using the IGBT collector-emitter voltage-collector current curve to fit the junction temperature at 25° and 125°, the expressions of the IGBT collector-emitter voltage and on-current are obtained, as shown in the following equations (1) and ( 2), and then use the interpolation method to calculate the relationship expression between the IGBT collector-emitter voltage and the on-current at the working junction temperature, as shown in the following formula (3):

V _{ce_125} =U ₁₂₅ +R ₁₂₅ ·i _T (1);

V _{ce_25} =U ₂₅ +R ₂₅ ·i _T (2);

Among them: i _T is the on-current of the IGBT; V _{ce_125} and V _{ce_25} represent the collector-emitter voltage when the IGBT junction temperature is 125° and 25°, and U ₁₂₅ and U ₂₅ represent the IGBT junction temperature when the junction temperature is 125° and 25°. The threshold voltage is fitted by the relationship curve between the collector-emitter voltage and the collector current provided by the manufacturer; R ₁₂₅ and R ₂₅ are the forward on-resistance at the junction temperature of 125° and 25° of the IGBT, and the collector-emitter provided by the manufacturer _Tj is the working junction temperature; V _{ce_Tj} represents the collector-emitter voltage when the IGBT junction temperature is T _j ; U _Tj represents the threshold voltage when the IGBT junction temperature is T _j ; R _Tj represents the forward conduction resistance when the junction temperature of the IGBT is T _j ;

Step 1.2: Calculate the on-state loss in a single half-bridge submodule;

The upper and lower bridge arm currents are expressed as equations (4) and (5); the condition for the conduction of IGBT T1 in the sub-module is that the IGBT T1 trigger signal is positive and the bridge arm current is negative, so the conduction current of IGBT T1 is expressed as Eq. (6); The on-state loss of IGBT can be obtained by taking the average value of the product of the collector-emitter voltage and on-current at the time of IGBT turn-on, and the on-state loss of IGBT T1 is expressed as (7) )Mode:

i _T1 =|i _arm | · G _T1 · S _T1 ; (arm=up/down) (6);

为MMC交流侧相电流滞后于相电压的角度；i_T1为T1导通电流；G_T1为IGBTT1的驱动信号；S_T1为IGBTT1的条件函数，当桥臂电流方向为负时取值为1，桥臂电流方向为正时取值为0；P_T1con为结温为T_j下IGBT T1的通态损耗；V_{ce_Tj}为结温为T_j下IGBT T1的集射极电压；Ts为MMC换流器输出交流电压的周期；Where: i _up is the current of the upper bridge arm; i _down is the current of the lower bridge arm; i _arm is the current of the bridge arm; I _d is the current of the DC side of the MMC converter; I _m is the peak value of the phase current of the AC side of the MMC converter; θ is the phase angle of the AC side phase voltage of the MMC converter;

is the angle at which the phase current of the MMC AC side lags the phase voltage; i _T1 is the conduction current of T1; G _T1 is the driving signal of IGBTT1; S _T1 is the condition function of IGBTT1, when the current direction of the bridge arm is negative, the value is 1, When the current direction of the bridge arm is positive, the value is 0; P _T1con is the on-state loss of IGBT T1 when the junction temperature is T _j ; V _{ce_Tj} is the collector-emitter voltage of IGBT T1 when the junction temperature is T _j ; Ts is the MMC commutation The cycle of the output AC voltage of the device;

Step 1.3: Calculate the on-state loss of T1 in all submodules of the upper arm:

Suppose the number of sub-modules in the upper arm is n, and the junction temperature of the IGBTs in each sub-module is the same; by summing the on-state losses of all IGBTs in the upper arm, the conduction loss of IGBT T1 in all sub-modules of the upper arm is obtained as Equation (8), and using Equation (6) to simplify to obtain Equation (9); according to the MMC operation mechanism, the sum of the IGBT T1 drive signals in each sub-module of the upper bridge arm is expressed as Equation (10):

为上桥臂的所有子模块中IGBT T1的通态损耗；

分别为n个上桥臂子模块中IGBT T1的导通电流；

分别为n个上桥臂子模块中IGBT T1的驱动信号；U_d为直流侧电压；U_m为MMC换流器输出单相交流电压的峰值；in:

is the on-state loss of IGBT T1 in all sub-modules of the upper bridge arm;

are the on-currents of the IGBT T1 in the n upper-arm sub-modules respectively;

are the driving signals of the IGBT T1 in the n upper-arm sub-modules respectively; U _d is the DC side voltage; U _m is the peak value of the single-phase AC voltage output by the MMC converter;

Step 1.4: Calculate the on-state losses of IGBT T2 in all submodules of the upper arm:

Referring to the calculation method of the on-state loss of T1 in all sub-modules in the upper bridge arm in steps 1.2 and 1.3, the on-state loss of T2 in all sub-modules in the upper bridge arm can be obtained, as shown in the following formula (11); In the sub-module, the driving signal of IGBT T2 is complementary to the driving signal of IGBT T1, as shown in the following formula (12); the sum of the driving signals of IGBT T2 of each sub-module of the upper bridge arm is expressed as the following formula (13):

为上桥臂的所有子模块中T2的通态损耗；

分别为n个上桥臂子模块中T2的驱动信号；

为T2的条件函数，当桥臂电流方向为负时为0，桥臂电流方向为正时为1；in:

is the on-state loss of T2 in all sub-modules of the upper bridge arm;

are the drive signals of T2 in the n upper bridge arm sub-modules respectively;

is the condition function of T2, when the current direction of the bridge arm is negative, it is 0, and it is 1 when the current direction of the bridge arm is positive;

Step 1.5: Calculate the on-state losses of all IGBTs in the upper arm:

Add the conduction losses of all IGBT T1 and IGBT T2 in the upper arm to obtain the conduction losses of all IGBTs in the upper arm; according to the zero-crossing point of the upper arm current, convert the conduction losses of all IGBTs in the upper arm into It is simplified to three integral formulas, as shown in formula (14):

Among them, P _{up_IGBTon} is the on-state loss of all IGBTs in the upper bridge arm; θ ₁ and θ ₂ are the corresponding angles when the current of the upper bridge arm crosses zero;

Step 1.6: Calculate the on-state losses of all IGBTs in the lower arm:

Calculate the on-state loss of all IGBTs in the lower arm as shown in Equation (15):

为下桥臂子模块中T1的通态损耗之和；

为下桥臂子模块中T2的通态损耗之和；θ'₁、θ'₂分别为下桥臂电流过零时对应的角度；Among them: P _{down_IGBTon} is the on-state loss of all IGBTs in the lower arm;

is the sum of the on-state losses of T1 in the lower arm sub-module;

is the sum of the on-state losses of T2 in the sub-module of the lower bridge arm; θ' ₁ and θ' ₂ are the corresponding angles when the current of the lower bridge arm crosses zero;

Step 1.7: Calculate the on-state losses of all IGBTs in the upper and lower legs:

Add the on-state losses obtained in steps 1.5 and 1.6 to obtain the on-state losses of all IGBTs in the upper and lower arms, and bring the current expressions (4) and (5) of the upper and lower arms into the simplified formula (16):

Further, the step 2 includes: the on-state losses of all diodes in the upper and lower bridge arms are shown in the following formula (17):

Among them: P _Diodeon is the on-state loss of all diodes in the upper and lower arms; U _f is the threshold voltage at the diode junction temperature of Tj; R _f is the on-resistance of the diode at the junction temperature of Tj.

Further, described step 3 comprises the following steps:

Step 3.1: Calculate the on-state loss of the single-phase power device of the MMC converter

The on-state loss of the IGBT and the same-stage loss of the diode obtained in steps 1 and 2 are added to obtain the on-state loss of all power devices of the MMC converter single-phase, as shown in the following formula (18):

Among them: P _{on_phase} is the on-state loss of the single-phase power device of the MMC converter;

Step 3.2: Calculate the on-state losses of the three-phase power devices of the MMC converter:

The on-state loss of the three-phase power device of the MMC converter is shown in the following formula (19):

P _{on_total} = 3P _{on_phase} (19);

Among them: P _{on_total} is the on-state loss of the three-phase power device of the MMC converter.

Compared with the closest prior art, the technical solution provided by the present invention has the following excellent effects:

1. The calculation method of the on-state loss of the MMC converter provided by the present invention is based on the instantaneous current of the conduction of each power device, and the physical meaning is clear;

2. The method for calculating the on-state loss of the MMC converter provided by the present invention can calculate the analytical expression of the on-state loss of each part of the power device in the MMC converter, including the upper bridge arm sub-module, the lower bridge arm sub-module and all The on-state losses of T1, T2, diode D1 and diode D2 in the sub-module are helpful for the comparative analysis of the losses of each part;

3. The calculation expression of the on-state loss of the MMC converter provided by the present invention can obtain the quantitative relationship between the on-state loss of each part and the converter modulation degree, power factor, and active power transmission power, which is convenient for the study of loss suppression measures;

4. The MMC converter on-state loss calculation method provided by the present invention has a wide range of applications, and can be applied to the flexible DC transmission system based on the MMC converter, the isolated DC/DC converter based on the MMC and the motor drag based on the MMC. Motion loss analysis.

Description of drawings

1 is a circuit topology diagram of a modular multilevel converter provided by the present invention;

Fig. 2 is the topology diagram of the modularized multilevel converter neutron module provided by the present invention;

3 is a graph showing the relationship between the collector-emitter voltage-collector current of the IGBT provided by the present invention for manufacturers;

Fig. 4 is the relationship curve diagram of diode conduction voltage-forward current provided by the manufacturer provided by the present invention;

5 is a current waveform diagram of the MMC converter provided by the present invention; wherein (a) is a schematic diagram of the current waveform of the upper arm of the MMC converter, and (b) is a schematic diagram of the current waveform of the lower arm of the MMC converter.

Detailed ways

The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

The present invention provides a method for determining the on-state loss of a modularized multilevel converter, wherein the circuit topology diagram of the modularized multilevel voltage source converter is shown in FIG. The upper and lower bridge arms have the same structure; the midpoint of the upper and lower bridge arms is connected to the AC end of the modular multilevel converter;

Each bridge arm of the upper and lower bridge arms includes a reactor and N sub-modules with the same structure; after the sub-modules of each bridge arm are cascaded, one end passes through the reactor and the modular multi-level converter. After the sub-modules of each bridge arm are cascaded, the other end is connected to one end of the cascaded sub-modules of the other two-phase bridge arms to form the positive and negative busbars of the DC end of the modular multi-level voltage source converter. ; The sub-module is composed of a half bridge and its parallel capacitor branch, the half bridge is composed of an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are both composed of insulated gate bipolar transistors IGBT and its The freewheeling diode FWD connected in parallel consists of the following steps:

Step 1: Calculate the IGBT on-state losses in the single-phase upper and lower arms of the modular multilevel converter;

Step 2: Calculate the diode conduction loss in the single-phase upper and lower arms of the modular multilevel converter;

Step 3: Calculate the three-phase on-state losses of the modular multilevel converter.

The step 1 includes the following sub-steps:

Step 1.1: Calculate the IGBT on-state voltage drop under normal operating junction temperature through device relationship curve fitting and interpolation;

Use the IGBT collector-emitter voltage-collector current curve provided by the manufacturer for fitting, and usually give the curves under two test junction temperatures of 25° and 125°, as shown in Figure 3. By fitting the curve, the relationship expression between the IGBT collector-emitter voltage and the on-current at 25° and 125° can be obtained, as shown in equations (1) and (2). Then use the interpolation method to calculate the relationship expression between the IGBT collector-emitter voltage and the on-current at the working junction temperature, as shown in formula (3).

V _{ce_125} =U ₁₂₅ +R ₁₂₅ ·i _T (1);

V _{ce_25} =U ₂₅ +R ₂₅ ·i _T (2);

Among them: i _T is the on-current of the IGBT; V _{ce_125} and V _{ce_25} represent the collector-emitter voltage when the IGBT junction temperature is 125° and 25°, and U ₁₂₅ and U ₂₅ represent the IGBT junction temperature when the junction temperature is 125° and 25°. The threshold voltage is fitted by the relationship curve between the collector-emitter voltage and the collector current provided by the manufacturer; R ₁₂₅ and R ₂₅ are the forward on-resistance at the junction temperature of 125° and 25° of the IGBT, and the collector-emitter provided by the manufacturer _Tj is the working junction temperature; V _{ce_Tj} represents the collector-emitter voltage when the IGBT junction temperature is T _j ; U _Tj represents the threshold voltage when the IGBT junction temperature is T _j ; R _Tj represents the forward on-resistance at the junction temperature _Tj of the IGBT.

Step 1.2: Calculate the on-state loss of T1 in a single half-bridge submodule:

Taking phase A as an example, according to the MMC operation mechanism, the upper and lower arm currents can be expressed as equations (4) and (5), and their approximate waveforms are shown in (a) and (b) in Figure 5 . From the half-bridge sub-module structure shown in Figure 2, the condition for T1 to be turned on in the sub-module is that the T1 trigger signal is positive and the bridge arm current is negative, so the on-current of T1 is expressed as formula (6); IGBT conduction The on-state loss of the IGBT can be obtained by taking the average value of the product of the collector-emitter voltage at the on-time and the on-current in an AC fundamental cycle. Therefore, the on-state loss of T1 can be expressed as equation (7):

i _T1 =|i _arm | · G _T1 · S _T1 ; (arm=up/down) (6);

为MMC交流侧相电流滞后于相电压的角度；i_T1为T1导通电流；G_T1为T1的驱动信号；S_T1为T1的条件函数，当桥臂电流方向为负时取值为1，桥臂电流方向为正时取值为0，桥臂电流的正方向如图1所示；P_T1con为结温为T_j下T1的通态损耗；V_{ce_Tj}表示IGBT结温为T_j下的集射极电压；Ts为MMC换流器输出交流电压的周期。Among them: i _up is the current of the upper bridge arm; i _down is the current of the lower bridge arm; _iarm is the current of the bridge arm; I _d is the current of the DC side of the MMC converter; is the phase angle of the AC side phase voltage of the MMC converter;

is the angle at which the phase current of the MMC AC side lags the phase voltage; i _T1 is the conduction current of T1; G _T1 is the driving signal of T1; S _T1 is the condition function of T1, when the current direction of the bridge arm is negative, the value is 1, When the current direction of the bridge arm is positive, the value is 0, and the positive direction of the bridge arm current is shown in Figure 1; P _T1con is the on-state loss of T1 _when the junction temperature is T _{j ;} Collector-emitter voltage; Ts is the period of the output AC voltage of the MMC converter.

Step 1.3: Calculate the on-state loss of T1 in all submodules of the upper arm

It is assumed that the number of sub-modules on the upper bridge arm is n, and the junction temperatures of the IGBTs in each sub-module are equal. Adding the on-state losses of all the upper IGBTs in the upper bridge arm, the on-state loss of T1 in all sub-modules of the upper bridge arm can be obtained as formula (8). And use formula (6) to simplify to obtain formula (9). Due to the large number of sub-modules used in the modular multi-level converter in high-voltage applications, according to the MMC operation mechanism, the sum of the T1 drive signals in each sub-module of the upper bridge arm can be expressed as equation (10).

为上桥臂的所有子模块中T1的通态损耗；

分别为n个上桥臂子模块中T1的导通电流；

分别为n个上桥臂子模块中T1的驱动信号；U_d为直流侧电压；U_m为MMC换流器输出单相交流电压的峰值。in:

is the on-state loss of T1 in all sub-modules of the upper bridge arm;

are the on-currents of T1 in the n upper-arm sub-modules, respectively;

are the driving signals of T1 in the n upper-arm sub-modules respectively; U _d is the DC side voltage; U _m is the peak value of the single-phase AC voltage output by the MMC converter.

Step 1.4: Calculate the on-state loss of T2 in all submodules of the upper arm

Referring to the calculation method of the on-state loss of T1 in all sub-modules in the upper bridge arm in steps 1.2 and 1.3, the on-state loss of T2 in all sub-modules in the upper bridge arm can be obtained, as shown in equation (11). When the dead zone is not considered, the drive signal of T2 in the sub-module is complementary to the drive signal of T1, which can be expressed as Equation (12). When the number of sub-modules is large, the sum of the driving signals of each sub-module T2 of the upper bridge arm can be expressed as formula (13).

为上桥臂的所有子模块中T2的通态损耗；

分别为n个上桥臂子模块中T2的驱动信号；

为T2的条件函数，当桥臂电流方向为负时为0，桥臂电流方向为正时为1。in:

is the on-state loss of T2 in all sub-modules of the upper bridge arm;

are the drive signals of T2 in the n upper bridge arm sub-modules respectively;

It is the condition function of T2, when the current direction of the bridge arm is negative, it is 0, and it is 1 when the current direction of the bridge arm is positive.

Step 1.5: Calculate the on-state losses of all IGBTs in the upper arm:

Summing up the conduction losses of all T1 and T2 in the upper arm gives the conduction losses of all IGBTs in the upper arm. According to the zero-crossing point of the upper arm current, the on-state losses of all IGBTs in the upper arm are simplified into three integral equations, as shown in equation (14).

Among them, P _{up_IGBTon} is the on-state loss of all IGBTs in the upper bridge arm; θ ₁ and θ ₂ are the corresponding angles when the current of the upper bridge arm crosses zero, as shown in (a) in FIG. 5 .

Step 1.6: Calculate the on-state losses of all IGBTs in the lower arm:

Using the same method as the calculation of the on-state losses of all IGBTs in the upper bridge arm, the on-state losses of all IGBTs in the lower bridge arm are calculated as shown in Equation (15).

为下桥臂子模块中T1的通态损耗之和；

为下桥臂子模块中T2的通态损耗之和；θ'₁、θ'₂分别为下桥臂电流过零时对应的角度，如图5中的(b)所示。Among them: P _{down_IGBTon} is the on-state loss of all IGBTs in the lower arm;

is the sum of the on-state losses of T1 in the lower arm sub-module;

is the sum of the on-state losses of T2 in the sub _- module of the lower arm _;

Step 1.7: Calculate the on-state losses of all IGBTs in the upper and lower legs

Add the on-state losses obtained in steps 1.5 and 1.6 to obtain the on-state losses of all IGBTs in the upper and lower arms, and bring the current expressions (4) and (5) of the upper and lower arms into the simplified formula (16)

Step 2: Calculate the on-state losses of all diodes in the upper and lower legs

Using the same calculation method as in step 1, the on-state losses of all diodes in the upper and lower arms can be obtained, as shown in equation (17).

Among them: P _Diodeon is the on-state loss of all diodes in the upper and lower bridge arms; U _f is the threshold voltage of the diode when the junction temperature is Tj, the relationship between the forward conduction voltage and the conduction current of 125° and 25° provided by the manufacturer The curve is fitted and obtained by interpolation method. The relationship between the forward voltage and the current of the diode is shown in Figure 4; R _f is the on resistance of the diode when the junction temperature is Tj. And 25 ° forward conduction voltage - conduction current relationship curve fitting and obtained after interpolation.

Step 3: Calculate the on-state losses of the three-phase power devices of the MMC converter:

Step 3.1: Calculate the on-state loss of the single-phase power device of the MMC converter:

By adding up the losses obtained in steps 1 and 2, the on-state losses of all the power devices of the single-phase MMC converter can be obtained, as shown in equation (18).

Among them: P _{on_phase} is the on-state loss of the single-phase power device of the MMC converter.

Step 3.2: Calculate the on-state losses of the three-phase power devices of the MMC converter

Due to the three-phase symmetrical operation of the MMC converter, the losses of the three-phase switching devices are approximately the same, so the on-state loss of the three-phase power devices of the MMC converter can be expressed as equation (19):

P _{on_total} = 3P _{on_phase} (19);

Among them: P _{on_total} is the on-state loss of the three-phase power device of the MMC converter.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art can still implement the present invention. Modifications or equivalent replacements are made in any manner, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention are all within the protection scope of the claims of the present invention for which the application is pending.

Claims (3)

1. A method for determining on-state loss of a modular multilevel converter is disclosed, wherein the modular multilevel converter is composed of three phases, and each phase is composed of an upper bridge arm and a lower bridge arm which are connected in series and have the same structure; the middle points of the upper bridge arm and the lower bridge arm are connected with an alternating current end of the modular multilevel converter;

each of the upper bridge arm and the lower bridge arm comprises 1 reactor and N submodules with the same structure; after the sub-modules of each bridge arm are cascaded, one end of each bridge arm is connected with the alternating current end of the modular multilevel converter through the reactor; after the sub-modules of each bridge arm are cascaded, the other end of each bridge arm is connected with one end of the cascaded sub-modules of the other two bridge arms to form a positive and negative bus of the direct current end of the modular multilevel voltage source type converter; the submodule is composed of a half bridge and a capacitor branch circuit connected in parallel with the half bridge, the half bridge is composed of an upper bridge arm and a lower bridge arm, and the upper bridge arm and the lower bridge arm are composed of an Insulated Gate Bipolar Transistor (IGBT) and a freewheeling diode (FWD) connected in parallel with the IGBT;

characterized in that the method comprises the following steps:

step 1: determining the on-state loss of the IGBTs in the single-phase upper bridge arm and the single-phase lower bridge arm of the modular multilevel converter;

step 2: determining on-state loss of diodes in single-phase upper and lower bridge arms of the modular multilevel converter;

and step 3: determining the three-phase on-state loss of the modular multilevel converter;

the step 1 comprises the following steps:

step 1.1: and (3) calculating the on-state voltage drop of the IGBT under the normal working junction temperature by a device relation curve fitting and interpolation method:

carrying out junction temperature fitting at 25 degrees and 125 degrees by using an IGBT collector-emitter voltage-collector current curve to obtain expressions of IGBT collector-emitter voltage and breakover current, wherein the expressions are shown in the following formulas (1) and (2); and then calculating by using an interpolation method to obtain a relational expression of the IGBT collector-emitter voltage and the conduction current under the working junction temperature, as shown in the following formula (3):

V_{ce_125}＝U₁₂₅+R₁₂₅·i_T(1)；

V_{ce_25}＝U₂₅+R₂₅·i_T(2)；

wherein: i.e. i_TIs the on-current of the IGBT; v_{ce_125}And V_{ce_25}Representing collector-emitter voltages, U, at 125 deg. and 25 deg. of IGBT junction temperature₁₂₅And U₂₅Threshold voltages representing junction temperatures of the IGBT under 125 degrees and 25 degrees are obtained by fitting a relation curve of collector-emitter voltage-collector current provided by a manufacturer; r₁₂₅And R₂₅Fitting forward on-resistance for IGBT junction temperature under 125 DEG and 25 DEG by relation curve of collector-emitter current provided by manufacturers; t is_jThe operating junction temperature; v_{ce_Tj}Representing the IGBT junction temperature as T_jA lower collector-emitter voltage; u shape_TjRepresenting the IGBT junction temperature as T_jA lower threshold voltage; r_TjRepresenting the IGBT junction temperature as T_jA lower forward on-resistance;

step 1.2: calculating the on-state loss in a single half-bridge submodule;

the upper and lower bridge arm currents are represented by equations (4) and (5); the condition that the IGBT T1 in the sub-module is turned on is that the IGBT T1 trigger signal is positive and the bridge arm current is negative, so the on current of the IGBT T1 is expressed by equation (6); the on-state loss of the IGBT can be obtained by averaging the product of the collector-emitter voltage and the on-current at the turn-on time of the IGBT in one ac fundamental wave period, and the on-state loss of the IGBT T1 is expressed by the following equation (7):

wherein: i.e. i_upIs the upper bridge arm current; i.e. i_downIs the lower bridge arm current; i.e. i_armIs the bridge arm current; i is_dThe current is the current of the direct current side of the MMC current converter; i is_mThe peak value of the phase current at the alternating current side of the MMC converter is obtained; theta is the phase angle of the alternating-current side phase voltage of the MMC converter;

lagging the phase current of the alternating current side of the MMC converter to the angle of the phase voltage; i.e. i_T1Current is turned on for T1; g_T1Is the drive signal of IGBT T1;

the value is 1 when the direction of the bridge arm current is negative and 0 when the direction of the bridge arm current is positive, which is a condition function of IGBTT 1; p_T1conFor a junction temperature of T_jOn-state loss of the lower IGBT T1; v_{ce_Tj1}For a junction temperature of T_jCollector-emitter voltage of the lower IGBT T1; t is_sA period of outputting an alternating voltage for the MMC converter;

step 1.3: the on-state loss of T1 in all submodules of the upper bridge arm is calculated as:

setting the number of the upper bridge arm sub-modules as n, and enabling the junction temperatures of the IGBTs in the sub-modules to be equal; adding the on-state losses of all IGBTs in the upper bridge arm to obtain the on-state loss of the IGBT T1 in all sub-modules of the upper bridge arm as a formula (8), and simplifying by using a formula (6) to obtain a formula (9); according to the MMC operation mechanism, the sum of the IGBT T1 driving signals in each submodule of the upper bridge arm is expressed as an expression (10):

wherein:

the on-state loss of the IGBT T1 in all the submodules of the upper bridge arm;

the conduction currents of the IGBT T1 in the n upper bridge arm submodules are respectively;

driving signals of IGBT T1 in the n upper bridge arm submodules respectively; u shape_dIs a direct current side voltage; u shape_mOutputting the peak value of the single-phase alternating-current phase voltage for the MMC current converter;

step 1.4: the on-state losses of the IGBT T2 in all submodules of the upper bridge arm are calculated as:

referring to the calculation methods of the on-state losses of T1 in all the submodules in the upper bridge arm in steps 1.2 and 1.3, the on-state losses of T2 in all the submodules in the upper bridge arm can be obtained, as shown in the following formula (11); when the dead zone is not considered, the driving signal of the IGBT T2 in the submodule is complementary to the driving signal of the IGBT T1, as shown in the following formula (12); the sum of the driving signals of the sub-modules IGBT T2 of the upper arm is expressed as the following formula (13):

wherein:

the on-state loss of T2 in all submodules of the upper bridge arm;

respectively being T2 in n upper bridge arm submodulesThe drive signal of (1);

as a conditional function of T2, when the bridge arm current direction is negative, it is 0, and when the bridge arm current direction is positive, it is 1;

a drive signal of IGBTT 2;

step 1.5: and (3) calculating the on-state loss of all IGBTs in the upper bridge arm:

adding the on-state losses of all IGBTs T1 and IGBT T2 in the upper bridge arm to obtain the on-state losses of all IGBTs in the upper bridge arm; according to the current zero crossing point of the upper bridge arm, reducing the on-state loss of all IGBTs in the upper bridge arm into three integral expressions, wherein the integral expressions are shown as the following formula (14):

wherein, P_{up_IGBTon}The on-state loss of all IGBTs in the upper bridge arm is obtained; theta₁、θ₂Respectively corresponding angles when the current of the upper bridge arm crosses zero;

step 1.6: and (3) calculating the on-state loss of all IGBTs of the lower bridge arm:

the on-state losses of all the IGBTs of the lower bridge arm are calculated as shown in the following formula (15):

wherein: p_{down_IGBTon}The on-state losses of all IGBTs of a lower bridge arm are obtained;

the sum of the on-state losses of T1 in the lower bridge arm submodule;

the sum of the on-state losses of T2 in the lower bridge arm submodule; theta'₁、θ'₂Are respectively asThe corresponding angle when the current of the lower bridge arm crosses zero;

step 1.7: and (3) calculating the on-state losses of all IGBTs in the upper bridge arm and the lower bridge arm:

adding the on-state losses obtained in the steps 1.5 and 1.6 to obtain the on-state losses of all IGBTs in the upper bridge arm and the lower bridge arm, and substituting the upper bridge arm current expression (4) and the lower bridge arm current expression (5) into a simplified expression to obtain an expression (16):

wherein: p_IGBTonThe on-state losses of all the IGBTs in the upper and lower bridge arms.

2. The determination method according to claim 1, wherein the step 2 includes: the on-state losses of all diodes in the upper and lower arms are shown in the following formula (17):

wherein: p_DiodeonThe on-state losses of all diodes in the upper bridge arm and the lower bridge arm are obtained; u shape_fThe junction temperature of the diode is the threshold voltage under Tj; r_fThe on-resistance of the diode with the junction temperature Tj; n is the number of upper bridge arm sub-modules, I_mThe peak value of the phase current at the alternating current side of the MMC converter is obtained; i is_dThe current is the current of the direct current side of the MMC current converter;

lagging the phase current of the alternating current side of the MMC converter to the angle of the phase voltage; u shape_d，U_mRespectively, the voltages at the dc side of the MMC converter, which outputs the peak value of the single-phase ac phase voltage.

3. The determination method according to claim 1, wherein said step 3 comprises the steps of:

step 3.1: calculating the on-state loss of the single-phase power device of the MMC current converter:

adding the on-state losses of the IGBT and the diode obtained in the steps 1 and 2 to obtain the on-state losses of all single-phase power devices of the MMC converter, wherein the on-state losses are shown as the following formula (18):

wherein: p_{on_phase}The method comprises the following steps of (1) realizing on-state loss of a single-phase power device of the MMC converter;

step 3.2: calculating the on-state loss of the MMC current converter three-phase power device:

the on-state loss of the three-phase power device of the MMC converter is shown as the following formula (19):

P_{on_total}＝3P_{on_phase}(19)；

wherein: p_{on_total}The method comprises the steps of (1) realizing on-state loss of a three-phase power device of the MMC converter; n is the number of upper bridge arm sub-modules, I_mThe peak value of the phase current at the alternating current side of the MMC converter is obtained; i is_dThe current is the current of the direct current side of the MMC current converter;

lagging the phase current of the alternating current side of the MMC converter to the angle of the phase voltage; u shape_d，U_mRespectively representing the voltages of the direct current sides of the MMC converters, and outputting the peak value of the single-phase alternating-current phase voltage by the MMC converters; u shape_TjRepresenting the IGBT junction temperature as T_jA lower threshold voltage; p_IGBTonThe on-state losses of all IGBTs in the upper bridge arm and the lower bridge arm are obtained; p_DiodeonThe on-state losses of all diodes in the upper bridge arm and the lower bridge arm are obtained; u shape_fIs the threshold voltage at diode junction temperature Tj.

Images