CN105811771B - A kind of determination method based on the loss of MMC isolated form DC/DC converter switches - Google Patents

Abstract

The invention relates to a method for determining the switching loss of a DC/DC converter based on MMC isolation. The method includes: performing parameter fitting on the switching loss characteristic curve of the power device of the MMC neutron module; using an interpolation method to calculate the operating junction temperature of the sub-module Switching loss energy of power devices; determine the average switching loss of a single power device; determine the distribution of switching loss types in the AC cycle of the MMC-DC/AC/DC converter; determine the single-phase upper and lower bridge arms of the MMC-DC converter at the input end Average switching loss; determine the switching loss of all power devices of the MMC-DC/AC/DC converter, the technical solution provided by the invention adopts the method of equivalent replacement of switching loss, and effectively solves the isolation based on the modular multilevel converter The analytical expression of the switching loss of the MMC-DC/DC converter is disclosed, and a numerical calculation method of the switching loss of the MMC-DC/DC converter is disclosed. It is helpful for the quantitative analysis of the switching loss of the MMC-DC/DC converter, and facilitates the realization and integration of the system parameter optimization design program.

Description

A Method for Determining Switching Loss of Isolated DC/DC Converter Based on MMC

technical field

The invention relates to a method for determining switching loss, in particular to a method for determining switching loss of an MMC-based isolated DC/DC converter.

Background technique

The shortage of traditional energy and the deteriorating environment have greatly promoted the development and utilization of clean energy such as green renewable energy. However, limited by the consumption capacity of the local power system, most renewable energy has not been effectively utilized, and even the phenomenon of "abandoning wind" and "abandoning light" has occurred. It is urgent to carry out large-scale, high-efficiency, and safe delivery of wind and photovoltaic power Research. The DC grid technology based on conventional DC and flexible DC is one of the effective technical means to solve this situation. One of the main factors hindering the formation of the DC power grid is the lack of high-voltage and large-capacity DC/DC converters, so that DC transmission lines with different voltage levels cannot be directly connected to form a large-scale DC transmission system. At present, the research on DC/DC converter technology is mainly focused on low and medium power voltage levels. With the continuous construction of DC lines and the increasingly urgent demand for DC power grids, the technology of high-voltage and large-capacity DC/DC converters needs to be solved urgently.

The isolated high-voltage large-capacity DC/DC converter based on Modular Multilevel Converter (MMC) has become a research hotspot due to its many advantages. This topology consists of two MMC converters connected through an isolation transformer. , as shown in Figure 1. The bridge arms of the MMC converter at the input and output ends are all connected in series with sub-modules, as shown in Figure 2, which effectively alleviates the hard requirements such as drive consistency and voltage equalization faced by a large number of power devices directly connected in series in high-voltage applications. The DC/DC converter is flexible in control, and can realize bidirectional flow of energy without changing hardware circuits and control strategies. The input and output sides are electrically isolated through an isolation transformer, effectively preventing the spread of system faults. The isolation transformer can work at 500Hz to 1kHz. With the increase of the operating frequency, the volume of passive components such as capacitors and inductors in the converter decreases, but the loss of power devices such as IGBT increases. Therefore, in the optimization design of system loss and volume compromise In-depth research on the loss of MMC-DC/AC/DC converter is essential.

The loss of power devices is composed of on-state loss and switching loss. The evaluation methods of the loss can be divided into three categories: 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, which is difficult to obtain. At present, the research on the loss of MMC converters adopts 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 quantitative relationship between MMC switching loss and voltage modulation degree, the number of bridge arm sub-modules, transformer operating frequency and active transmission power cannot be easily obtained, which is not convenient for quantitative analysis of switching loss and system optimization design.

Contents of the invention

In order to solve the deficiencies in the above-mentioned prior art, the purpose of the present invention is to provide a method for determining switching losses based on MMC isolated DC/DC converters. The analytical expression of the switching loss of the isolated DC/DC converter of the flat converter discloses a numerical calculation method of the switching loss of the MMC-DC/DC converter. It is helpful for the quantitative analysis of the switching loss of the MMC-DC/DC converter, and facilitates the realization and integration of the system parameter optimization design program.

The object of the present invention is to adopt following technical scheme to realize:

The present invention provides a method for determining the switching loss of an MMC-based isolated DC/DC converter. The isolated DC/DC converter includes an isolation transformer and MMCs connected at both ends thereof; both MMCs are connected to a DC system; The modular multilevel converter MMC is composed of three phases, and each phase is composed of upper and lower bridge arms with the same structure in series; the midpoint of the upper and lower bridge arms is connected to the AC terminal;

Each of the upper and lower bridge arms includes a reactor and N submodules with the same structure; after the submodules of each bridge arm are cascaded, one end passes through the reactor and the modular multilevel converter The sub-modules of each bridge arm are cascaded and 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 bus bars of the DC end of the modular multi-level voltage source converter. ; The sub-module is composed of a half-bridge and a capacitor branch connected in parallel with it, 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 an insulated gate bipolar transistor IGBT and its Composed of parallel freewheeling diodes FWD;

Its improvement is that described method comprises the following steps:

Step 1: Carry out parameter fitting to the switching loss characteristic curve of the power device of the MMC neutron module;

Step 2: Use the interpolation method to calculate the switching loss energy of the sub-module power device at the working junction temperature;

Step 3: Determine the average switching loss of a single power device;

Step 4: Determine the distribution of switching loss types in the AC cycle of the MMC-DC/AC/DC converter;

Step 5: Determine the average switching loss of the single-phase upper and lower bridge arms of the MMC converter at the input end;

Step 6: Determine the switching losses of all power devices of the MMC-DC/AC/DC converter.

Further, in the step 1, for the switching loss characteristic curve of the power device in the sub-module, the quadratic polynomial of the following formula (1) is used to fit and extract the switching loss characteristic parameters, and to obtain the Energy loss for one switching action:

Among them: E _{sw_k} represents the turn-on, turn-off or diode reverse recovery loss (E _on , E _off or E _rec ) of the IGBT at a junction temperature of k°C; _idev represents the current flowing through the power device, which is the collector for the IGBT The current i _C is the forward current i _F for the diode; a _{sw_k} , b _{sw_k} , and c _{sw_k} are parameters determined by fitting the energy consumption curves at 25°C and 125°C respectively, and the IGBT turn-on loss curves are fitted to obtain a _{on_k} , b _{on_k} , c _{on_k} , a _{off_k} , b _{off_k} , c _{off_k} are obtained by fitting the IGBT turn-off loss curve, and a _{rec_k} , b _{rec_k} , _{crec_k} are obtained by fitting the diode reverse recovery loss curve.

Further, in the step 2, according to the fitting results of the energy consumption curves of the power devices in the sub-modules at 25°C and 125°C, the interpolation method is used to calculate the switching loss energy of the power devices in the sub-modules at the working junction temperature T _j °C, as shown in the following formula Shown in (2); Bring formula (1) into formula (2) and simplify, obtain formula (3)-(6):

Among them: E _swTj represents the switching loss energy of the power device in the sub-module at the junction temperature Tj°C; a _swTj , b _swTj , and c _swTj are the parameters in the switching loss expression of the power device in the sub-module at the junction temperature Tj°C; E _{sw_125} represents the turn-on, turn-off or diode reverse recovery loss of the IGBT at a junction temperature of 125°C; E _{sw_25} represents the turn-on, turn-off or diode reverse recovery loss of the IGBT at a junction temperature of 25°C; a _{sw_125} , b _{sw_125} , c _{sw_125} represent the parameters determined by fitting the energy consumption curve at 125°C, a _{on_125,} b _{on_125} , c _{on_125} are obtained by fitting the IGBT turn-on loss curve, and a _{off_125} , b _{off_125} , b off_125 are obtained by fitting the IGBT turn-off loss curve c _{off_125} , a _{rec_125} , b _{rec_125 , and crec_125 are obtained by fitting the diode reverse recovery loss curve; a sw_25 , b sw_25 , and c sw_25 represent the parameters} determined by fitting the energy consumption curve at 25°C, and the IGBT turn-on loss curve is calculated Fitting a _{on_25} , b _{pn_25} , c _{on_25} , and fitting the IGBT turn-off loss curve to get a _{off_25} , b _{off_2} 5 , c _{off_25} , and fitting the diode reverse recovery loss curve to get a _{rec_25} , b _{rec_25} , c _{rec_25} .

Further, in the step 3, the corresponding switching loss energy is accumulated according to the times of turning on and off of the power devices in the sub-module, and time-averaged to obtain the average switching loss of each part; in the sub-module The calculation formulas of turn-on loss P _T1on and turn-off loss P _T1off of IGBTT1 and reverse blocking loss P _D1rec of diode D1 are as follows:

Among them: u _dc is the capacitor voltage of the sub-module; v _ceref is the reference voltage reference value provided by the manufacturer to calculate the switching loss; E _onTju is the energy loss generated by the uth turn-on of IGBT T1 at the working junction temperature T _j ℃; E _offTju is the energy loss generated by the u-th turn-off of IGBT T1 at the operating junction temperature T _j ℃; E _recTju is the energy loss generated by the u-th reverse blocking of D1 at the operating junction temperature T _j ℃; T _s is the MMC - DC/AC/DC converter AC side duty cycle; w is the number of times of switching in T _s cycle time.

Further, said step 4 includes the following steps:

Step 4.1: Determine the conditions under which a single half-bridge submodule produces different types of switching losses:

For the half-bridge sub-module structure in Figure 2, the direction of the bridge arm current and the switching sequence of the sub-modules affect the switching loss type; when the bridge arm current direction is positive, the IGBT will be generated during the transition from the input state to the cut-off state of the sub-module Turn-on loss of T2 and reverse blocking loss of diode D1;

Step 4.2: Determine the type of switching loss in the upper and lower bridge arms of the MMC converter:

According to the operating mechanism of the MMC-DC/AC/DC converter, the expressions of the voltage and current of the upper and lower bridge arms are shown in equations (10)-(13); when the output voltage of the bridge arm rises, part of the bridge arm sub-modules are The cut-off state is converted to the active state, and the corresponding switching loss is generated according to the current direction of the bridge arm; when the output voltage of the bridge arm drops, some bridge arm sub-modules are converted from the cut-off state to the active state:

Among them: U _up is the output voltage of the upper bridge arm; U _down is the output voltage of the lower bridge arm; U _d is the DC side voltage; U _m is the peak output voltage of the MMC-DC/AC/DC converter AC side; θ is the AC voltage Phase angle; m is the voltage modulation degree; i _up is the current of the upper bridge arm, I _d is the current of the DC side of the MMC converter; I _m is the peak value of the current of the AC side; is the phase angle difference between the AC side voltage and current; k is the current modulation degree.

Further, said step 5 includes the following steps:

Step 5.1: Perform equivalent replacement of the switching loss, so that the switching types generated by the upper and lower bridge arms at the same time are consistent;

The analysis of step 4.2 shows that the types of switching losses generated by the upper and lower bridge arms at the same time are not necessarily the same, which makes the calculation and solution of the switching loss of the system complicated. Under the premise of ensuring that the total switching loss of the system remains unchanged, through the equivalent replacement of switching loss, the switching types generated by the upper and lower bridge arms at the same time are consistent, which greatly simplifies the calculation.

Step 5.2: Calculate the average switching loss of the single-phase upper and lower bridge arms of the MMC converter at the input end:

Solve the switching loss in the fundamental wave period, as shown in the following formula (14):

a _onrecTj = a _onTj + a _recTj (15);

b _onrecTj = b _onTj + b _recTj (16);

c _onrecTj = c _onrecTj + c _recTj (17);

Among them: P _{sw_1} is the switching loss of the power device in the single-phase upper and lower bridge arms of the MMC converter; N1 indicates the number of switching times of the inner bridge arm sub-module in [0, π/2); N2 indicates [π/2, π) The number of switching conversions of the sub-module of the inner bridge arm; N3 represents the number of switching conversions of the sub-module of the inner bridge arm in [π, 3π/2); N4 represents the switching conversion of the sub-module of the inner bridge arm in [3π/2, 2π) The number of times; i _uph and i _downh are [0, π/2) the corresponding upper bridge arm current and lower bridge arm current when the inner bridge arm is switching for the hth time; i _upi and i _downi are [π/2, π ) the corresponding upper bridge arm current and lower bridge arm current during the i-th switching conversion of the inner bridge arm; i _upj and i _downj are [π, 3π/2) the corresponding upper bridge arm current during the j-th switching conversion of the inner bridge arm arm current and lower arm current; i _upk and i _downk are [3π/2, 2π) the corresponding upper arm current and lower arm current when the inner bridge arm is switching for the kth time; U _cap is the rated capacitance of the sub-module Voltage; a _onrecTj , b _onrecTj , c _onrecTj represent the sum of the fitting parameters of the IGBT turn-on loss curve and the diode reverse cut-off loss curve at Tj junction temperature respectively; Ts is the duty cycle of the AC side of the MMC-DC/AC/DC converter;

Step 5.3: Put the upper and lower bridge arm current expressions (12) and (13) into the formula (14) to simplify:

Step 5.4: Simplify the average switching loss using the quantitative relationship corresponding to the modulation strategy:

Assuming that the number of sub-modules in the upper and lower arms of the half-bridge sub-module is n, based on the nearest level approximation modulation strategy, the reference voltage of the upper and lower bridge arms is compared with the threshold voltage value to generate a corresponding modulation signal; the threshold voltages are respectively For: 0, A total of n+1, θ _1h represents the angle corresponding to the hth intersection of the bridge arm voltage modulation wave with the threshold voltage in (0, π/2]; θ _2i represents the bridge arm voltage modulation wave in (π/2, π ] in the angle corresponding to the i-th intersection with the threshold voltage; θ _3j represents the angle corresponding to the j-th intersection of the bridge arm voltage modulation wave in [π, 3π/2) and the threshold voltage; θ _4k represents the bridge arm voltage modulation The angle corresponding to the kth intersection of the wave with the threshold voltage in [3π/2, 2π); get the analytical expression (19)-(22), and put the formula (19)-(22) into the formula (18) Get the relational formula (24), and get the formula (25) by summing the series:

a _onoffrecTj = a _onTj + a _offTj + a _recTj (26);

b _onoffrecTj = b _onoffTj + b _offTj + b _recTj (27);

c _onoffecTj = c _onTj + c _offTj + c _recTj (28);

Among them: [] is the rounding algorithm, Q represents the rounding function, which reflects the influence of different voltage modulation degrees on the loss; h represents the number of intersections between the bridge arm voltage modulation wave and the threshold voltage within (0,π/2]; a _onoffrecTj , b _onoffrecTj , c _onoffrecTj respectively represent the sum of curve fitting parameters of IGBT turn-on loss, turn-off loss and diode reverse cut-off loss when the junction temperature is Tj.

Further, said step 6 includes the following steps:

Step 6.1: Calculate the switching loss of the three-phase power device of the MMC converter at the input end;

The MMC converter operates symmetrically. According to step 5, the switching loss of the three-phase power device of the input MMC converter is solved, as shown in the following formula (29):

P _{sw_3} = 3*P _{sw_1} (29);

Step 6.2: Calculate the switching loss of the three-phase power device of the MMC converter at the output end: as shown in the following formula (30):

P' _{sw_3} = 3*P' _{sw_1} (30);

Step 6.2: Calculate the switching losses of all power devices in the MMC-DC/AC/DC converter

Add the switching losses of the input and output MMCs in steps 6.1 and 6.2 to obtain the switching losses of all power devices in the MMC-DC/AC/DC converter, as shown in the following equation (31):

P _total = P _{sw_3} + P' _{sw_3} (31);

Among them: P _total is the switching loss of all power devices in the MMC-DC/AC/DC converter.

Compared with the closest prior art, the excellent effect that the technical solution provided by the present invention has is:

1. The MMC-DC/AC/DC converter switching loss calculation method provided by the present invention is based on each power device being turned on and off at each turn-on and turn-off moment, and the method of loss equivalent replacement is deduced, and the physical meaning is clear;

2. The MMC-DC/AC/DC converter switching loss calculation method provided by the present invention can calculate the analytical expression of each power device switching loss in the converter, including the turn-on and turn-off on-state loss of the IGBT and the diode reverse Block loss, which can realize comparative analysis between various losses;

3. The MMC-DC/AC/DC converter switching loss calculation expression provided by the present invention can obtain the quantitative relationship between the converter switching loss and the voltage modulation degree, the number of bridge arm sub-modules, the transformer operating frequency, and the power transmission power, which is convenient System parameter optimization design and loss suppression measures research.

Description of drawings

Figure 1 is a topology diagram of an isolated DC/DC converter based on MMC;

Fig. 2 is a circuit topology diagram of a modular multilevel converter provided by the present invention;

Fig. 3 is the characteristic curve diagram of IGBT turn-on and turn-off losses provided by the present invention;

Fig. 4 is a characteristic curve diagram of diode reverse blocking provided by the present invention;

Fig. 5 is the current waveform diagram of the bridge arm of the MMC converter provided by the present invention, wherein: the dotted line is the current waveform diagram of the upper bridge arm, and the solid line is the current waveform diagram of the lower bridge arm;

Fig. 6 is the bridge arm voltage waveform diagram of the MMC converter provided by the present invention; wherein: the dotted line is the voltage waveform diagram of the upper bridge arm, and the solid line is the voltage waveform diagram of the lower bridge arm;

Fig. 7 is the MMC converter loss distribution diagram after equivalent replacement provided by the present invention; Wherein: the dotted line is the upper bridge arm loss diagram, and the solid line is the lower bridge arm loss diagram;

Fig. 8 is a quantitative relationship diagram corresponding to the closest level approach modulation strategy provided by the present invention;

FIG. 9 is a flowchart of a method for determining switching loss of an MMC-based isolated DC/DC converter provided by the present invention.

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings.

The present invention provides a method for calculating the switching loss of an isolated DC/DC converter based on a modular multilevel converter MMC, wherein: the isolated DC/DC converter includes an isolation transformer and MMCs connected at both ends thereof; Each MMC is connected to the DC system; the modular multilevel converter MMC is composed of three phases, and each phase is composed of upper and lower bridge arms with the same structure in series; the midpoint of the upper and lower bridge arms is connected The AC side of the modular multilevel converter;

Each of the upper and lower bridge arms includes a reactor and N submodules with the same structure; after the submodules of each bridge arm are cascaded, one end passes through the reactor and the modular multilevel converter The sub-modules of each bridge arm are cascaded and 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 bus bars of the DC end of the modular multi-level voltage source converter. ; The sub-module is composed of a half-bridge and a capacitor branch connected in parallel with it, 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 an insulated gate bipolar transistor IGBT and its The parallel freewheeling diode FWD is composed; the flow chart of the calculation method is shown in Figure 9, including the following steps:

Step 1: Perform parameter fitting according to the switching loss characteristic curve of the power device:

Aiming at the switching loss characteristic curves of power devices (IGBT and diode) provided by the manufacturer, the quadratic polynomial as in formula (1) is used to fit and extract the switching loss characteristic parameters, so as to obtain the switching loss characteristic curve under a given on-state current energy loss. Refer to Figure 3 and Figure 4 for switching loss characteristic curves.

Where: E _{sw_k} represents the turn-on, turn-off or diode reverse recovery loss energy (E _on , E _off or E _rec ) of the IGBT at a junction temperature of k°C; _idev represents the current flowing through the power device, and for the IGBT Electrode current i _C , for a diode it is forward current i _F ). a _{sw_k} , b _{sw_k} , c _{sw_k} are the parameters determined by fitting the energy consumption curves at 25°C and 125°C, a _{on_25} , b _{on_25} , c _{on_25} are obtained by fitting the IGBT turn-on loss curve, and the IGBT turn-off loss curve is calculated A _{off_25} , b _{off_25} , c _{off_25} are obtained by fitting, and a _{rec_25} , b _{rec_25} , and _{crec_25} are obtained by fitting the diode reverse recovery loss curve.

Step 2: Calculate switching loss energy at operating junction temperature using interpolation

According to the fitting results of energy consumption curves at 25°C and 125°C, the interpolation method is used to calculate the switching loss energy at the working junction temperature Tj°C, as shown in formula (2). Substitute formula (1) into formula (2) for simplification to obtain formulas (3)-(6).

Among them: E _swTj represents the switching loss energy of the power device at the junction temperature of Tj°C; a _swTj , b _swTj , c _swTj are the parameters in the switching loss expression of the power device at the junction temperature of Tj°C; E _{sw_125} represents the junction temperature of IGBT turn-on, turn-off or diode reverse recovery loss at 125°C; E _{sw_25} means turn-on, turn-off or diode reverse recovery loss of IGBT at a junction temperature of 25°C; a _{sw_125} , b _{sw_125} , c _{sw_125} mean through 125°C energy consumption curve fitting determined parameters, fitting the IGBT turn-on loss curve to get a _{on_125} , b _{on_125} , c _{on_125} , fitting the IGBT turn-off loss curve to get a _{off_125} , b _{off_125} , c _{off_125} , for the diode The reverse recovery loss curve is fitted to get a _{rec_125} , b _{rec_125} , _{crec_125} ; a _{sw_25} , b _{sw_25} , c _{sw_25} represent the parameters determined by fitting the energy consumption curve at 25°C, and the IGBT turn-on loss curve is fitted to get a _{on_25} , b _{on_25} , c _{on_25} , a _{off_25} , b _{off_25} , c _{off_25} are obtained by fitting the IGBT turn-off loss curve, and a _{rec_25} , b _{rec_25} , _{crec_25} are obtained by fitting the diode reverse recovery loss curve.

Step 3: Calculate the average switching losses of a single power device:

According to the number of switching actions, the corresponding switching loss energy is accumulated, and then averaged over time to obtain the average switching loss of each part. For example, the calculation formulas for the turn-on loss and turn-off loss of T1 and the reverse blocking loss of D1 in the sub-module shown in Figure 2 are as follows:

Among them: u _dc is the capacitor voltage of the sub-module; v _ceref is the reference voltage reference value provided by the manufacturer to calculate the switching loss; E _onTju is the loss energy generated by the u-th turn-on of T1 at the working junction temperature T _j ℃; E _offTju E recTju is the energy loss generated by the u-th turn-off of D1 at the working junction temperature T _j ℃; E _recTju is the loss energy generated by the u-th reverse blocking of D1 at the working junction temperature T _j ℃; T _s is the MMC-DC / AC/DC converter AC side duty cycle; w is the number of times of switching in T _s cycle time.

Step 4: Determine the distribution of switching loss types in the AC cycle of the MMC-DC/AC/DC converter

Step 4.1: Determine the conditions under which a single half-bridge submodule produces different types of switching losses:

The structure of the half-bridge sub-module is shown in Figure 2. The direction of the bridge arm current and the switching sequence of the sub-modules all affect the switching loss. For example, when the current direction of the bridge arm is positive, the turn-on loss of T2 and the reverse blocking loss of D1 will be generated during the transition from the switch-on state to the cut-off state of the sub-module. All cases are listed in Table 1.

Table 1 Switching loss of a single half-bridge sub-module

Step 4.2: Determine the switching loss type of the upper and lower bridge arms of the MMC converter:

According to the operating mechanism of the MMC-DC/AC/DC converter, the expressions of the voltage and current of the upper and lower bridge arms are shown in equations (10)-(13), considering the reactive power transferred in the DC/AC/DC converter The power is very small, and the power factor of the input and output MMC can be very high, that is Figures 5 and 6 are waveform diagrams of the current and voltage of the upper and lower bridge arms. When the output voltage of the bridge arm rises, some of the sub-modules of the bridge arm switch from the cut-off state to the input state, and corresponding switching losses are generated according to the direction of the bridge arm current; when the output voltage of the bridge arm drops, some of the bridge arm sub-modules switch from the cut-off state to put into state. Combining Table 1, Figure 5 and Figure 6, the distribution of switching losses of the upper and lower bridge arms can be obtained, as shown in Table 2 and Figure 6. Since T1 and T2, D1 and D2 have the same type, the resulting switching loss is only related to the bridge arm current, and the switching loss generated by which switching device will not be subdivided here.

Table 2 Distribution of switching losses of upper and lower bridge arms

Among them: U _up is the output voltage of the upper bridge arm; U _down is the output voltage of the lower bridge arm; U _d is the DC side voltage; U _m is the peak output voltage of the MMC-DC/AC/DC converter AC side; θ is the AC voltage Phase angle; m is the voltage modulation degree; i _up is the current of the upper arm; i _down is the current of the lower arm; I _d is the current of the DC side of the MMC converter; I _m is the peak value of the current of the AC side; is the phase angle difference between the AC side voltage and current; as shown by the dotted line in Figure 5, where θ ₁ and θ ₂ are the zero-crossing points of the upper arm current; i _down is the lower arm current, as shown by the solid line in Figure 5, where θ′ ₁ and θ′ ₂ are the zero-crossing points of the lower bridge arm current;

Step 5: Calculate the average switching loss of the single-phase upper and lower arms of the input MMC converter

Step 5.1: Equivalent replacement of switching losses

Take phase A of the MMC converter at the input end as an example. _In Figure ₆ , the lower bridge arm in curve A1A2 produces turn _{- on} loss and reverse blocking loss, and the lower bridge arm in curve A5A6 produces turn-off loss. Since curves A ₁ A ₂ and A ₅ A ₆ correspond to the same magnitude of the bridge arm current and the same output voltage of the bridge arm, under the condition that the sum of switching losses in the fundamental wave period remains unchanged, it can be considered that the curve A ₁ A ₂ corresponds to the lower bridge The turn-off loss generated in the time period of arm A ₅ A ₆ , the curve A ₅ A ₆ corresponds to the turn-on loss and reverse blocking loss generated in the time period of the lower bridge arm A ₁ A ₂ . In the same way, replace the losses in curves A ₂ A ₃ and A ₄ A ₅ , replace the losses in curves B ₁ A ₂ and A ₅ B ₆ , and replace the losses in curves A ₂ B ₃ and B ₄ A ₅ Make a replacement. The loss distribution after equivalent replacement is shown in Fig. 7.

Step 5.2: Calculate the average switching loss of the single-phase upper and lower arms of the input MMC converter

According to Fig. 7, the switching loss in the fundamental period can be solved, as shown in formula (14).

a _onrecTj = a _onTj + a _recTj (15);

b _onrecTj = b _onTj + b _recTj (16);

c _onrecTj = c _onrecTj + c _recTj (17);

Among them: P _{sw_1} is the switching loss of the power device in the single-phase upper and lower bridge arms of the MMC converter; N1 indicates the number of switching times of the inner bridge arm sub-module in [0, π/2); N2 indicates [π/2, π) The number of switching conversions of the sub-module of the inner bridge arm; N3 represents the number of switching conversions of the sub-module of the inner bridge arm in [π, 3π/2); N4 represents the switching conversion of the sub-module of the inner bridge arm in [3π/2, 2π) The number of times; i _uph and i _downh are [0, π/2) the corresponding upper bridge arm current and lower bridge arm current when the inner bridge arm is switching for the hth time; i _upi and i _downi are [π/2, π ) the corresponding upper bridge arm current and lower bridge arm current during the i-th switching conversion of the inner bridge arm; i _upj and i _downj are [π, 3π/2) the corresponding upper bridge arm current during the j-th switching conversion of the inner bridge arm arm current and lower arm current; i _upk and i _downk are [3π/2, 2π) the corresponding upper arm current and lower arm current when the inner bridge arm is switching for the kth time; U _cap is the rated capacitance of the sub-module Voltage.

Step 5.3: Bring the upper and lower arm current expressions into the simplification

Bring the upper and lower arm current expressions (12) and (13) into simplified form:

Step 5.4: Simplify the analytical expression for the average switching loss using the quantitative relationship corresponding to the modulation strategy

Assuming that the number of sub-modules in the upper and lower bridge arms is large, and the number is n, based on the nearest level approximation modulation strategy, the reference voltages of the upper and lower bridge arms are compared with a series of threshold voltage values to generate corresponding modulation signals. The threshold voltages are: 0, A total of n+1. Each θ _1h , θ _2i , θ _3j , θ _4k is the angle corresponding to the intersection of the bridge arm voltage modulation wave and the threshold voltage, and the analytical expressions can be obtained as (19)-(22), as shown in Figure 8. And put the formula (19)-(22) into the formula (18) to get the relational formula (24), and get the formula (25) through the summation of the sequence.

a _onoffrecTj = a _onTj + a _offTj + a _recTj (26);

b _onoffrecTj = b _onoffTj + b _offTj + b _recTj (27);

c _onoffrecTj = c _onoffTj + _coffTj + c _recTj (28);

Among them: [] is the rounding algorithm, Q represents the rounding function, which reflects the influence of different voltage modulation degrees on the loss; h represents the number of intersections between the bridge arm voltage modulation wave and the threshold voltage within (0,π/2]; a _onoffrecTj , b _onoffrecTj , c _onoffrecTj respectively represent the sum of curve fitting parameters of IGBT turn-on loss, turn-off loss and diode reverse cut-off loss when the junction temperature is Tj.

Step 6: Calculate the switching losses of all power devices of the MMC-DC/AC/DC converter

Step 6.1: Calculate the switching losses of the three-phase power devices of the MMC converter at the input

Since the MMC converter operates symmetrically, the switching loss of the three-phase power device of the input MMC converter can be solved according to step 5, as shown in equation (29).

P _{sw_3} = 3*P _{sw_1} (29);

Step 6.2: Calculate the switching losses of the three-phase power devices of the MMC converter at the output

Using the same method as step 5 and 6.1, the switching loss of the three-phase power device of the MMC converter at the output end can be obtained, as shown in formula (30).

P' _{sw_3} = 3*P' _{sw_1} (30);

Step 6.2: Calculate the switching losses of all power devices in the MMC-DC/AC/DC converter

Add the switching losses of the input and output MMCs in steps 6.1 and 6.2 to obtain the switching losses of all power devices in the MMC-DC/AC/DC converter, as shown in equation (31).

P _total = P _{sw_3} + P' _{sw_3} (31);

Among them: P _total is the switching loss of all power devices in the MMC-DC/AC/DC 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 Any modification or equivalent replacement that does not deviate from the spirit and scope of the present invention is within the protection scope of the claims of the pending application of the present invention.

Claims (6)

1. a kind of determination method based on the loss of MMC isolated form DC/DC converter switches, the isolated form DC/DC converter packets Include isolating transformer and its MMC of both ends connection；Two MMC are accessed in straight-flow system；Modularization multi-level converter MMC by Three-phase is constituted, and is made of identical upper and lower two bridge arm of concatenated structure per phase；The midpoint link block of upper and lower two bridge arm The exchange end of multilevel converter；

Each bridge arm includes 1 reactor and the identical submodule of N number of structure in upper and lower two bridge arm；The son of each bridge arm One end is connected by reactor with the end that exchanges of modularization multi-level converter after module-cascade；The sub-module cascade of each bridge arm The other end is connect with cascade submodule one end of another two-phase bridge arm afterwards, forms modular multilevel voltage source converter direct current The positive and negative anodes busbar at end；The submodule is made of half-bridge capacitor branches connected in parallel, and the half-bridge is by upper bridge arm under Bridge arm is constituted, and the upper bridge arm and lower bridge arm are by insulated gate bipolar transistor IGBT and fly-wheel diode connected in parallel FWD is formed；

It is characterized in that, the method includes following step：

Step 1：Parameter fitting is carried out to the switching loss characteristics of MMC Neutron module power devices；

Step 2：Utilize the switching loss energy of submodule power device under interpolation calculation working junction temperature；

Step 3：Determine the average switch loss of single power device；

Step 4：Determine the distribution of switching loss type in MMC-DC/AC/DC converter ac cycles；

Step 5：Determine the average switch loss of the single-phase upper and lower bridge arm of input terminal MMC transverters；

Step 6：Determine the switching loss of all power devices of MMC-DC/AC/DC converters；

The step 4 includes the following steps：

Step 4.1：Determine that single half-bridge submodule generates the condition of different switching loss types：

For half-bridge sub-modular structure, the switching change over order of bridge arm current direction and submodule influences switching loss class Type；When bridge arm current direction is that just, submodule has input state to become cutting off the turn-on consumption for generating IGBT T2 in state procedure It is lost with the reverse blocking of diode D1；

Step 4.2：Determine switching loss type in the upper and lower bridge arm of MMC transverters：

According to MMC-DC/AC/DC converter operation mechanisms, expression formula such as formula (10)-(13) institute of upper and lower bridge arm voltage and electric current Show；When bridge arm output voltage rises, part bridge arm submodule is converted to input state by excision state, according to bridge arm current side To the corresponding switching loss of generation；When bridge arm output voltage declines, part bridge arm submodule is converted to input by excision state State：

Wherein：U_upFor upper bridge arm output voltage；U_downFor lower bridge arm output voltage；U_dFor DC voltage；U_mFor MMC-DC/AC/ DC converter exchange side output voltage peak values；θ is the phase angle of alternating voltage；M is voltage modulated degree；i_upFor upper bridge arm current, I_d For MMC Converter DC-side electric currents；I_mFor ac-side current peak value；For the phase angle difference of exchange side voltage and electric current；K is electric current Modulation degree.

2. determining method as described in claim 1, which is characterized in that in the step 1, for submodule power device in the block Switching loss characteristics are fitted and extract switching loss characterisitic parameter using the quadratic polynomial of such as following formula (1), seek The energy loss of given on state current switch motion next time：

Wherein：E_{sw_k}Indicate that junction temperature is the opening of IGBT at k DEG C, turns off or the reverse recovery loss (E of diode_on, E_offOr E_rec)；i_devThe electric current of power device is flowed through in expression, is collector current i for IGBT_C, it is forward current i for diode_F； a_{sw_k}、b_{sw_k}、c_{sw_k}Be respectively by the determining parameter of 25 DEG C and the fitting of 125 DEG C of energy consumption curves, to IGBT turn-on consumptions curve into Row fitting obtains a_{on_k}、b_{on_k}、c_{on_k}, IGBT turn-off power loss curves are fitted to obtain a_{off_k}、b_{off_k}、c_{off_k}, to two poles Pipe reverse recovery loss curve is fitted to obtain a_{rec_k}、b_{rec_k}、c_{rec_k}。

3. determining method as claimed in claim 2, which is characterized in that in the step 2, according to 25 DEG C and 125 DEG C of submodules The energy consumption curve fitting result of middle power device utilizes interpolation calculation working junction temperature T_jPower device is opened in submodule at DEG C Loss of energy is closed, as shown in following formula (2)；It brings formula (1) into formula (2) and carries out abbreviation, obtain formula (3)-(6)：

Wherein：E_swTjIndicate that junction temperature is the switching loss energy of power device in submodule at Tj DEG C；a_swTj、b_swTj、c_swTjRespectively Parameter at being Tj DEG C for junction temperature in submodule in the switching loss expression formula of power device；E_{sw_125}Indicate that junction temperature is at 125 DEG C The opening of IGBT turns off or the reverse recovery loss of diode；E_{sw_25}Indicate that junction temperature is the opening of IGBT at 25 DEG C, turns off or two The reverse recovery loss of pole pipe；a_{sw_125}、b_{sw_125}、c_{sw_125}It indicates through the determining parameter of 125 DEG C of energy consumption curve fittings, it is right IGBT turn-on consumption curves are fitted to obtain a_{on_125}、b_{on_125}、c_{on_125}, IGBT turn-off power loss curves are fitted to obtain a_{off_125}、b_{off_125}、c_{off_125}, diode reverse recovery losses curve is fitted to obtain a_{rec_125}、b_{rec_125}、c_{rec_125}； a_{sw_25}、b_{sw_25}、c_{sw_25}It indicates, by the determining parameter of 25 DEG C of energy consumption curve fittings, to be fitted IGBT turn-on consumption curves Obtain a_{on_25}、b_{on_25}、c_{on_25}, IGBT turn-off power loss curves are fitted to obtain a_{off_25}、b_{off_25}、c_{off_25}, to diode Reverse recovery loss curve is fitted to obtain b_{rec_25}、b_{rec_25}、c_{rec_25}。

4. determining method as described in claim 1, which is characterized in that in the step 3, according to power device in submodule The number turned on and off adds up corresponding switching loss energy, and it is average that the time is carried out to it, you can obtains each section Average switch loss；The turn-on consumption P of IGBT T1 in submodule_T1onWith turn-off power loss P_T1offAnd diode D1's is reversed Block loss P_D1recCalculation formula it is as follows：

Wherein：u_dcFor submodule capacitor voltage；v_cerefFor producer provide calculating the reference voltage base value of switching loss； E_onTjuIt is IGBT T1 in working junction temperature T_jThe u times loss of energy for opening generation at DEG C；E_offTjuIt is IGBT T1 in working junction temperature T_jThe loss of energy that the u times shutdown generates at DEG C；E_recTjuIt is D1 in working junction temperature T_jThe loss that the u times reverse blocking generates at DEG C Energy；T_sFor the MMC-DC/AC/DC converter exchange side work periods；W is T_sThe number switched in cycle time.

5. determining method as described in claim 1, which is characterized in that the step 5 includes the following steps：

Step 5.1：Switching loss is subjected to equivalent replacement so that the switchtype that upper and lower bridge arm is generated in synchronization is consistent；

Step 5.2：Calculate the average switch loss of the single-phase upper and lower bridge arm of input terminal MMC transverters：

The switching loss in the primitive period is solved, as shown in following formula (14)：

a_onrecTj=a_onTj+a_recTj(15)；

b_onrecTj=b_onTj+b_recTj(16)；

c_onrecTj=c_onTj+c_recTj(17)；

Wherein：P_{sw_1}For the switching loss of power device in the single-phase upper and lower bridge arm of MMC transverters；N1 indicate [0, pi/2) interior bridge arm The number of submodule switching conversion；N2 indicate [pi/2, π) conversion of interior bridge arm submodule switching number；N3 expressions [π, 3 pi/2s) in The number of bridge arm submodule switching conversion；N4 indicate [3 pi/2s, 2 π) conversion of interior bridge arm submodule switching number；i_uphAnd i_downh For [0, pi/2) the h times switching conversion of interior bridge arm when corresponding upper bridge arm current and lower bridge arm electric current；i_upiAnd i_downiFor [pi/2, π) Corresponding upper bridge arm current and lower bridge arm electric current when interior bridge arm ith switching conversion；i_upjAnd i_downjFor [π, 3 pi/2s) interior bridge arm Corresponding upper bridge arm current and lower bridge arm electric current when j switching conversion；i_upkAnd i_downkFor [3 pi/2s, 2 π) interior bridge arm kth time switching Corresponding upper bridge arm current and lower bridge arm electric current when conversion；U_capFor submodule capacitance rated voltage；a_onrecTj、b_onrecTj、 c_onrecTjThe sum of IGBT turn-on consumptions curve and the reversed cut-off loss curve fitting parameter of diode under Tj junction temperatures is indicated respectively；T_s For the MMC-DC/AC/DC converter exchange side work periods；

Step 5.3：It is simplified to bring upper and lower bridge arm current expression formula (12) and (13) into formula (14)：

Step 5.4：Simplify average switch using the corresponding quantitative relationship of modulation strategy to be lost：

Assuming that upper and lower bridge arm Neutron module number is n in half-bridge submodule, modulation strategy is approached by nearest level and is obtained, upper and lower bridge Arm reference voltage generates corresponding modulated signal compared with threshold voltage value；Threshold voltage is respectively：0、 Total n+1, θ_1hIndicate bridge arm voltage modulating wave (0, pi/2] in the h times angle corresponding with threshold voltage intersection；θ_2iTable Show bridge arm voltage modulating wave (pi/2, π] interior ith angle corresponding with threshold voltage intersection；θ_3jIndicate bridge arm voltage tune

Wave processed [π, 3 pi/2s) interior jth time angle corresponding with threshold voltage intersection；θ_4kIndicate bridge arm voltage modulating wave [3 Pi/2,2 π) interior kth time angle corresponding with threshold voltage intersection；Obtain analytical expression (19)-(22), and by formula (19)- (22) it brings formula (18) into and obtains relational expression (24), sum to obtain formula (25) through ordered series of numbers：

a_onoffrecTj=a_onTj+a_offTj+a_recTj(26)；

b_onoffrecTj=b_onTj+b_offTj+b_recTj(27)；

c_onoffrecTj=c_onTj+c_offTj+c_recTj(28)；

Wherein：[] is rounding algorithm, and Q indicates bracket function, influence of the reaction different voltages modulation degree to loss；H indicate (0, Pi/2] in the number that intersects with threshold voltage of bridge arm voltage modulating wave；a_onoffrecTj、b_onoffrecTj、c_onoffrecTjIt indicates respectively The sum of IGBT turn-on consumptions, turn-off power loss and the reversed cut-off loss curve fitting parameter of diode when junction temperature is Tj.

6. determining method as described in claim 1, which is characterized in that the step 6 includes the following steps：

Step 6.1：Calculate the switching loss of input terminal MMC transverter three phase power devices；

MMC transverter symmetrical operations solve the switching loss of input terminal MMC transverter three phase power devices according to step 5, as follows Shown in formula (29)：

P_{sw_3}=3*P_{sw_1}(29)；

Step 6.2：Calculate the switching loss of output end MMC transverter three phase power devices：As shown in following formula (30)：

P′_{sw_3}=3*P '_{sw_1}(30)；

Step 6.2：Calculate the switching loss of all power devices in MMC-DC/AC/DC converters

Input terminal in step 6.1 and 6.2 is added with the switching loss of output end MMC to own in MMC-DC/AC/DC converters The switching loss of power device, as shown in following formula (31)：

P_total=P_{sw_3}+P′_{sw_3}(31)；

Wherein：Psw_1, P`sw_1 indicate the switch damage of input side and the single-phase upper and lower bridge arm power device of outlet side MMC respectively Consumption；P_totalFor the switching loss of all power devices in MMC-DC/AC/DC converters.