CN105808901B - Method for determining on-state loss of modular multilevel converter - Google Patents
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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
Technical Field
The invention relates to a calculation method of a power electronic technology, in particular to a method for determining on-state loss of a modular multilevel converter.
Background
Since 2002, R.Marquart and A. L esnicar of Federal national defense university in Munich, Germany propose a Modular Multilevel Converter (MMC), which has been widely developed and applied in the field of voltage source type high-voltage converters due to its topological structure advantages, and its topological structure is shown in FIG. 1. in the flexible direct-current transmission system based on the voltage source type Converter, the Siemens 2010 adopts MMC converters in both the Transbay power grid interconnection project of the United states and the flexible direct-current transmission demonstration project of the south China in 2011, and in the high-voltage direct-current transformer for the direct-current power grid, the high-voltage DC/DC Converter (direct-current transformer) based on the MMC has become a research hotspot both at home and abroad, and in the field of motor application, a great number of researchers have researched the variable frequency control technology of asynchronous motors based on the MMC.
The MMC current converter mainly uses an Insulated Gate Bipolar Transistor (IGBT) as a power switching device. Limited by the voltage withstanding level of a single IGBT, the number of the IGBTs used is large to meet the requirement of high voltage application, so the loss of the MMC converter is mainly caused by the power switching devices, and the operation stability of the IGBTs is also one of the key factors affecting the reliable operation of the whole system. The main reason for the operational failure of the IGBT is that its junction temperature is too high, so good cooling design and system optimization are the prerequisite for reliable operation of the system. In a flexible direct-current transmission system based on MMC, the system works at 50Hz, and the loss research of a power device provides theoretical support for system cooling design, parameter model selection and the like; in a high-voltage DC/DC converter based on MMC, a system works under a medium-frequency working condition (500 Hz-1 kHz), and along with the increase of working frequency, the volume of passive devices such as a capacitor and an inductor in the converter is reduced, and the loss of power devices such as IGBT is increased, so that the loss research of MMC is indispensable in the compromise optimization design of system loss and volume; in the MMC-based motor dragging technology, a system works under a variable frequency working condition according to a speed regulation requirement, and the relation between the loss and the frequency of an MMC converter needs to be researched and loss calculation under different frequencies needs to be carried out.
The evaluation method for the loss of the power device can be divided into three types of test detection, physical modeling and mathematical analysis. The test detection method is only suitable for low-voltage and low-power occasions, and the physical modeling method is based on a large number of device manufacturing parameters and is difficult to obtain. At present, a mathematical analysis method is adopted in MMC converter loss research, a characteristic function of a power device is fitted according to some device characteristic parameters provided by a manufacturer, and then loss estimation or online loss calculation based on average current and effective current of the power device is carried out. Loss estimation based on average current and effective current cannot give an analytical expression of MMC loss, and the quantitative relation between the MMC loss and the modulation degree, power factor, active transmission power and the like of a current converter cannot be simply obtained; the online loss calculation needs to obtain the voltage, current, driving signals and the like of the MMC converter at each moment for operation, and the practicability is not strong in the system optimization design stage, so that the MMC converter is difficult to be seamlessly connected with an optimization design program.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a method for determining the on-state loss of the modular multilevel converter, and the method is based on the instantaneous current of an IGBT and a diode, effectively solves the analytic expression of the on-state loss of the modular multilevel converter, is favorable for the quantitative analysis of the on-state loss of the MMC and is convenient for realizing the optimized design program of a system.
The purpose of the invention is realized by adopting the following technical scheme:
the invention provides a method for determining on-state loss of a modular multilevel converter, 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;
the improvement is that the method comprises the steps of:
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: and determining the three-phase on-state loss of the modular multilevel converter.
Further, 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):
Vce_125＝U125+R125·iT(1)；
Vce_25＝U25+R25·iT(2)；
wherein: i.e. iTIs the on-current of the IGBT; vce_125And Vce_25Representing 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 isjThe operating junction temperature; vce_TjRepresenting the IGBT junction temperature as TjA lower collector-emitter voltage; u shapeTjRepresenting the IGBT junction temperature as TjA lower threshold voltage; rTjRepresenting the IGBT junction temperature as TjA 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):
iT1＝|iarm|·GT1·ST1；(arm＝up/down) (6)；
wherein: i.e. iupIs the upper bridge arm current; i.e. idownIs the lower bridge arm current; i.e. iarmIs the bridge arm current; i isdThe current is the current of the direct current side of the MMC current converter; i ismThe 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;
the phase current of the MMC alternating side lags behind the phase voltage; i.e. iT1Current is turned on for T1; gT1A drive signal of IGBTT 1; sT1The 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; pT1conFor a junction temperature of TjOn-state loss of the lower IGBT T1; vce_TjFor a junction temperature of TjCollector-emitter voltage of the lower IGBT T1; ts is the period of the output alternating voltage of 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 shapedIs a direct current side voltage; u shapemOutputting the peak value of the single-phase alternating-current 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;
driving signals of T2 in the n upper bridge arm submodules respectively;
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;
step 1.5: and (3) calculating the on-state loss of all IGBTs in the upper bridge arm:
adding the conduction 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, Pup_IGBTonThe 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: pdown_IGBTonThe 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'2Respectively corresponding angles 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):
further, the step 2 comprises: the on-state losses of all diodes in the upper and lower arms are shown in the following formula (17):
wherein: pDiodeonThe on-state losses of all diodes in the upper bridge arm and the lower bridge arm are obtained; u shapefThe junction temperature of the diode is the threshold voltage under Tj; rfThe on-resistance of the diode at a junction temperature Tj.
Further, the step 3 comprises the following steps:
step 3.1: calculating on-state loss of single-phase power device of MMC (modular multilevel converter)
Adding the on-state loss of the IGBT obtained in the step 1 and the step 2 and the same-platform loss of the diode to obtain the on-state loss of all single-phase power devices of the MMC current converter, wherein the on-state loss is shown as the following formula (18):
wherein: pon_phaseThe 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):
Pon_total＝3Pon_phase(19)；
wherein: pon_totalThe on-state loss of the three-phase power device of the MMC converter is achieved.
Compared with the closest prior art, the technical scheme provided by the invention has the following excellent effects:
1. the MMC converter on-state loss calculation method provided by the invention is obtained based on the inversion of the instant current conducted by each power device, and the physical significance is clear;
2. the MMC converter on-state loss calculation method provided by the invention can calculate and obtain the on-state loss analytical expression of each part of power devices in the MMC converter, and the on-state loss analytical expression comprises the upper bridge arm submodule, the lower bridge arm submodule and the on-state loss of T1, T2, diode D1 and diode D2 in all the submodules, thereby being beneficial to realizing the comparative analysis of the loss of each part;
3. the MMC converter on-state loss calculation expression provided by the invention can obtain the quantitative relation between each part of on-state loss and the converter modulation degree, power factor and active transmission power, and is convenient for the research of loss inhibition measures;
4. the MMC converter on-state loss calculation method provided by the invention has a wide application range, and can be suitable for loss analysis of flexible direct-current power transmission systems based on the MMC converter, isolated DC/DC converters based on the MMC, motor dragging based on the MMC and the like.
Drawings
Fig. 1 is a circuit topology diagram of a modular multilevel converter provided by the present invention;
fig. 2 is a topological diagram of a submodule in the modular multilevel converter provided by the invention;
FIG. 3 is a graph showing the relationship between the collector-emitter voltage and collector current of the IGBT provided by the present invention;
FIG. 4 is a graph of the relationship between diode turn-on voltage and forward current provided by the manufacturer of the present invention;
FIG. 5 is a current waveform diagram of an MMC converter provided by the present invention; wherein, (a) is a schematic diagram of a current waveform of an upper bridge arm of the MMC converter, and (b) is a schematic diagram of a current waveform of a lower bridge arm of the MMC converter.
Detailed Description
The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.
The invention provides a method for determining on-state loss of a modular multilevel converter, wherein a circuit topological diagram of the modular multilevel voltage source type converter is shown in figure 1 and consists of three phases, and each phase consists 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; the method comprises the following steps:
step 1: calculating 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: calculating the on-state loss of diodes in single-phase upper and lower bridge arms of the modular multilevel converter;
and step 3: and calculating the three-phase on-state loss of the modular multilevel converter.
The step 1 comprises the following substeps:
step 1.1: calculating the on-state voltage drop of the IGBT under the normal working junction temperature through device relation curve fitting and an interpolation method;
the insulated gate bipolar transistor IGBT collector emitter voltage-collector current curves provided by manufacturers are used for fitting, and curves at two test junction temperatures of 25 ° and 125 ° are generally given, as shown in fig. 3. The relational expressions of the IGBT collector-emitter voltage and the conduction current under 25 degrees and 125 degrees can be obtained by fitting the curve, and the relational expressions are shown as 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 at the working junction temperature, as shown in formula (3).
Vce_125＝U125+R125·iT(1)；
Vce_25＝U25+R25·iT(2)；
Wherein: i.e. iTIs the on-current of the IGBT; vce_125And Vce_25Representing 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 isjThe operating junction temperature; vce_TjRepresenting the IGBT junction temperature as TjA lower collector-emitter voltage; u shapeTjRepresenting the IGBT junction temperature as TjA lower threshold voltage; rTjRepresenting the IGBT junction temperature as TjLower forward on-resistance.
Step 1.2: the on-state loss of T1 in a single half-bridge submodule is calculated:
taking phase a as an example, according to the MMC operation mechanism, the upper and lower arm currents can be represented by equations (4) and (5), whose approximate waveforms are shown as (a) and (b) in fig. 5. The half-bridge submodule structure shown in fig. 2 is obtained, the conditions that T1 is turned on in the submodule are that the trigger signal of T1 is positive and the bridge arm current is negative, so the on current of T1 is expressed as formula (6); the on-state loss of the IGBT can be obtained by averaging the product of the collector-emitter voltage and the conducting current at the moment of the IGBT conducting in an alternating current fundamental wave period. Thus, the on-state loss of T1 can be expressed as equation (7):
iT1＝|iarm|·GT1·ST1；(arm＝up/down) (6)；
wherein: i.e. iupIs the upper bridge arm current; i.e. idownIs the lower bridge arm current; iarm is bridge arm current; i isdThe current is the current of the direct current side of the MMC current converter; i ismThe 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;
the phase current of the MMC alternating side lags behind the phase voltage; i.e. iT1Current is turned on for T1; gT1A drive signal of T1; sT1As a conditional function of T1, when the bridge arm current direction is negative, the value is 1, when the bridge arm current direction is positive, the value is 0, and the positive direction of the bridge arm current is as shown in fig. 1; pT1conFor a junction temperature of TjOn-state loss for lower T1; vce_TjRepresenting the IGBT junction temperature as TjA lower collector-emitter voltage; ts is the period of the MMC inverter outputting the alternating voltage.
Step 1.3: calculating the on-state loss of T1 in all submodules of upper bridge arm
The number of the upper bridge arm sub-modules is assumed to be n, and the junction temperatures of the IGBTs in the sub-modules are equal. The on-state losses of all the upper IGBTs in the upper bridge arm are added, and the on-state losses of T1 in all the submodules of the upper bridge arm are obtained as an expression (8). And the reaction is simplified by the formula (6) to obtain the formula (9). Because the number of the sub-modules used by the modular multilevel converter in a high-voltage application occasion is large, according to an MMC operation mechanism, the sum of T1 driving signals in each sub-module of an upper bridge arm can be represented as an expression (10).
Wherein:
the on-state loss of T1 in all submodules of the upper bridge arm;
respectively is the conduction current of T1 in the n upper bridge arm submodules;
driving signals of T1 in the n upper bridge arm submodules respectively; u shapedIs a direct current side voltage; u shapemAnd outputting the peak value of the single-phase alternating-current voltage for the MMC converter.
Step 1.4: calculating the on-state loss of T2 in all submodules of upper bridge arm
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 formula (11). When the dead zone is not considered, the driving signal of T2 in the sub-module is complementary to the driving 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 the sub-modules T2 in the upper arm can be expressed as equation (13).
Wherein:
the on-state loss of T2 in all submodules of the upper bridge arm;
driving signals of T2 in the n upper bridge arm submodules respectively;
as a conditional function of T2, the bridge arm current direction is 0 when negative and 1 when positive.
Step 1.5: and (3) calculating the on-state loss of all IGBTs in the upper bridge arm:
and adding the conduction losses of all the T1 and T2 in the upper bridge arm to obtain the on-state losses of all the IGBTs in the upper bridge arm. And (3) simplifying the on-state loss of all IGBTs in the upper bridge arm into three integral expressions according to the current zero crossing point of the upper bridge arm, wherein the integral expressions are shown as a formula (14).
Wherein, Pup_IGBTonThe on-state loss of all IGBTs in the upper bridge arm is obtained; theta1、θ2The angles corresponding to the zero-crossing of the upper arm current are shown in fig. 5 (a).
Step 1.6: and (3) calculating the on-state loss of all IGBTs of the lower bridge arm:
and (3) calculating the on-state losses of all the IGBTs of the lower bridge arm by adopting the same method as the on-state losses of all the IGBTs of the upper bridge arm, wherein the on-state losses are as shown in a formula (15).
Wherein: pdown_IGBTonThe 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'2The angles corresponding to the lower arm current flowing through zero are shown in fig. 5 (b).
Step 1.7: calculating the on-state loss of all IGBTs in the upper and lower bridge arms
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 (16)
Step 2: calculating the on-state losses of all diodes in upper and lower bridge arms
The on-state losses of all diodes in the upper and lower bridge arms can be obtained by the same calculation method as in step 1, as shown in formula (17).
Wherein: pDiodeonThe on-state losses of all diodes in the upper bridge arm and the lower bridge arm are obtained; u shapefFitting relation curves of 125-degree and 25-degree forward conduction voltages-conduction currents provided by manufacturers for the threshold voltage of the diode with the junction temperature of Tj and solving the relation curves by an interpolation method to obtain the relation curves of the diode forward conduction voltages-conduction currents as shown in FIG. 4; rfIs twoThe junction temperature of the diode is the on-resistance under Tj, and is obtained by fitting a relation curve of the diode forward conduction voltage-on current of 125 degrees and 25 degrees provided by a manufacturer and performing interpolation operation.
And step 3: calculating the on-state loss of the MMC current converter three-phase power device:
step 3.1: calculating the on-state loss of the single-phase power device of the MMC current converter:
and (3) adding the losses obtained in the step (1) and the step (2) to obtain the on-state losses of all single-phase power devices of the MMC converter, as shown in a formula (18).
Wherein: pon_phaseThe on-state loss of the single-phase power device of the MMC converter is achieved.
Step 3.2: calculating on-state loss of three-phase power device of MMC (modular multilevel converter)
Since the MMC inverter operates symmetrically in three phases and the losses of the three-phase switching devices are approximately the same, the on-state loss of the three-phase power device of the MMC inverter can be expressed as formula (19):
Pon_total＝3Pon_phase(19)；
wherein: pon_totalThe on-state loss of the three-phase power device of the MMC converter is achieved.
Finally, it should be noted that: although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can make modifications and equivalents to the embodiments of the present invention without departing from the spirit and scope of the present invention, which is set forth in the claims of the present application.
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):
Vce_125＝U125+R125·iT(1)；
Vce_25＝U25+R25·iT(2)；
wherein: i.e. iTIs the on-current of the IGBT; vce_125And Vce_25Representing 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 isjThe operating junction temperature; vce_TjRepresenting the IGBT junction temperature as TjA lower collector-emitter voltage; u shapeTjRepresenting the IGBT junction temperature as TjA lower threshold voltage; rTjRepresenting the IGBT junction temperature as TjA 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. iupIs the upper bridge arm current; i.e. idownIs the lower bridge arm current; i.e. iarmIs the bridge arm current; i isdThe current is the current of the direct current side of the MMC current converter; i ismThe 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. iT1Current is turned on for T1; gT1Is 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; pT1conFor a junction temperature of TjOn-state loss of the lower IGBT T1; vce_Tj1For a junction temperature of TjCollector-emitter voltage of the lower IGBT T1; t issA 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 shapedIs a direct current side voltage; u shapemOutputting 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, Pup_IGBTonThe 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: pdown_IGBTonThe 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: pIGBTonThe 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: pDiodeonThe on-state losses of all diodes in the upper bridge arm and the lower bridge arm are obtained; u shapefThe junction temperature of the diode is the threshold voltage under Tj; rfThe on-resistance of the diode with the junction temperature Tj; n is the number of upper bridge arm sub-modules, ImThe peak value of the phase current at the alternating current side of the MMC converter is obtained; i isdThe 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 shaped，UmRespectively, 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: pon_phaseThe 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):
Pon_total＝3Pon_phase(19)；
wherein: pon_totalThe 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, ImThe peak value of the phase current at the alternating current side of the MMC converter is obtained; i isdThe 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 shaped，UmRespectively 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 shapeTjRepresenting the IGBT junction temperature as TjA lower threshold voltage; pIGBTonThe on-state losses of all IGBTs in the upper bridge arm and the lower bridge arm are obtained; pDiodeonThe on-state losses of all diodes in the upper bridge arm and the lower bridge arm are obtained; u shapefIs the threshold voltage at diode junction temperature Tj.
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