CN114268233A

CN114268233A - Novel power equalization modulation strategy for cascaded H-bridge

Info

Publication number: CN114268233A
Application number: CN202111391283.3A
Authority: CN
Inventors: 顾军; 张维国; 许青春; 宋飞; 张明; 杜治斌
Original assignee: Anhui University of Science and Technology
Current assignee: Anhui University of Science and Technology
Priority date: 2021-11-23
Filing date: 2021-11-23
Publication date: 2022-04-01

Abstract

The invention discloses a novel power equalization modulation strategy suitable for a cascaded H-bridge multi-level converter, and belongs to the technical field of multi-level PWM converters. Based on the ideas of carrier reconstruction and carrier freedom degree recombination, the method firstly reconstructs three triangular carriers to ensure that the three triangular carriers are uniformly distributed in different interlayer periods for circulation; secondly, adjusting the vertical degree of freedom of the sine modulation wave; and then comparing the modulation wave with the carrier wave to obtain an optimized PWM pulse signal. The method of the invention not only can realize power balance among all output units, so that the direct current voltage discharge is kept consistent, and the quality of the output voltage of the converter is improved; the time required by equalization is shortened to the maximum extent; while also having uniformity of average switching frequency. In addition, the method can be expanded to n cascade units, so that the practicability of the converter is improved.

Description

Novel power equalization modulation strategy for cascaded H-bridge

Technical Field

The invention belongs to the technical field of multi-carrier PWM (pulse width modulation) of a multi-level converter, and particularly relates to a novel power balance modulation method suitable for a symmetrical cascade H-bridge multi-level inverter.

Background

In recent years, multi-level inverters gradually replace traditional two-level inverters in the high-voltage and high-power field due to the advantages of high output voltage level, low switching stress, small electromagnetic interference, small voltage change rate (dv/dt), and the like, and become research hotspots. The multi-level inverter can be classified into a clamp type and a cascade type. The clamped multi-level inverter can be further classified into a flying capacitor clamped (FC), a diode clamped (NPC) and a hybrid clamped (hybrid clamped). The cascade H-bridge (CHB) inverter does not need to consider the voltage-sharing problem of a flying capacitor at the direct current side of an FC inverter, and the problems that NPC (neutral point clamped) type switching tubes are not uniformly distributed, the optimal design of a heat dissipation system is complex and the like are solved. And because the structure is simple and the modularized design is easy, the device can be widely applied to the occasions of new energy power generation, high-voltage high-power direct-current transmission, comprehensive control of electric energy quality and the like.

For the CHB type inverter, selecting an appropriate modulation strategy helps to improve Total Distortion (THD) of the output voltage, increase output direct current component, achieve power balance among units, and the like. The conventional Pulse Width Modulation (PWM) strategy has Level-Shifted PWM (LS-PWM) modulation and Carrier Phase Shifted PWM (CPS-PWM) modulation. Under the traditional CPS-PWM modulation strategy, the output average power among the units of the cascaded H bridge is naturally balanced, but the output voltage harmonic distribution is not optimal. Under the LS-PWM strategy, the harmonic content is less, but the power self-balance among the units cannot be realized. The unbalanced output power can lead the loss of the direct-current power supply to be uneven, the loss distribution of the power device is uneven, the switching tube generates heat unevenly, the optimal design of a heat dissipation system is difficult, and the stability of the system is influenced. For the problem of unbalanced output power, the current solutions are: 1. the carrier rotation method, such as 1/2 modulation wave period rotation, 1/4 modulation wave period rotation, and the like, is power equalization that is realized by changing in the modulation wave layer, and as the number of cascade units increases to n units, the equalization time also increases to n/2 modulation wave periods to realize power equalization. 2. The pulse allocation strategy based on the linear congruence method can realize power equalization, but the random function is introduced, so that the control difficulty and the uncertainty of the equalization time are increased. Therefore, the power equalization problem is solved, and the equalization duration, the harmonic content of the output voltage and the control complexity are comprehensively considered, so that the practicability of the modulation strategy is improved.

Disclosure of Invention

Object of the Invention

The invention provides a novel modulation strategy aiming at the problem of unbalanced output power of each unit of a cascaded H-bridge multi-level inverter under the traditional LS-PWM modulation strategy, solves the contradiction between unbalanced output power and optimized output phase voltage of a symmetrical cascaded H-bridge under the traditional modulation strategy, and simultaneously shortens the power balancing time.

Technical scheme

The technical scheme of the invention is as follows:

1) the cascade H-bridge multi-level inverter is formed by cascading n H-bridges, and the direct current sides of the cascade H-bridge multi-level inverter are all isobaric direct current voltage sources which are not in common with the ground, namely U_dc1＝U_dc2＝…＝U_dcn＝E。

2) For n cascade units, the method of the invention needs n triangular carriers with equal amplitude and same frequency, which are arranged in a transverse alternate stacking way. Taking the first layer carrier of three-unit cascade as an example to construct a novel carrier. First, the second half period of the first carrier (triangular wave) is shifted up by 1 unit. Next, the second carrier front half cycle is shifted up by 2 units, and the second half cycle is also shifted up by 2 units. The first half cycle of the third carrier is then shifted up by 1 unit. With half a carrier period as the frequency shift target, the first layer carrier frequency shift rule is: 0-1-2-2-1-0. The second layer carrier translation rule is as follows: 0-1-0- (-1) - (-1) -0. The third layer of carrier translation rules are: 0- (-1) - (-2) - (-2) - (-1) -0. The same layer is connected with each other after translation. The carrier period after the three-unit cascade reconstruction is 3 times of the triangular carrier period.

3) The novel carrier wave and the modulation wave are intersected to generate an optimized PWM control signal. When the modulation wave is positioned in the first half period and the modulation wave is larger than the corresponding carrier wave, the PWM control signal enables the output of the H bridge to be positive, and otherwise the output of the H bridge is 0. When the modulation wave is positioned in the second half period and the modulation wave is smaller than the corresponding carrier wave, the PWM control signal enables the output of the H bridge to be negative, otherwise, the output of the H bridge is 0.

3) The theoretical total number of levels of the inverter output voltage is 2n + 1.

Advantageous effects

The method of the invention not only can balance the output power of each cascade unit of the cascade H-bridge multi-level inverter, but also can shorten the power balance time, and can ensure the phase voltage harmonic elimination performance to be optimal, thereby improving the output characteristic of the inverter. Meanwhile, the method can also be applied to a cascaded H bridge multi-level inverter with n units, so that the practicability of the inverter topology is improved.

Drawings

Fig. 1 is a circuit diagram of a CHB type three-cell inverter topology.

Fig. 2 is a diagram of a conventional PD and carrier degree of freedom reconfiguration modulation strategy. Wherein, (a) the legacy PD modulation strategy and (b) the carrier degree of freedom modulation strategy.

Fig. 3 is an output state transition diagram.

Fig. 4 is a conventional 1/4 cycle-by-cycle modulation scheme.

Fig. 5 is a diagram of a novel power equalization modulation scheme.

FIG. 6 is a diagram illustrating corresponding outputs at different reference voltages.

Fig. 7 is a schematic diagram of a conventional PD modulation method.

Fig. 8 is a diagram of a novel power equalization modulation scheme.

Fig. 9 is a simulation waveform diagram of the output of the conventional modulation strategy, the novel modulation strategy and the optimized modulation strategy.

Fig. 10 is a graph of the novel modulation output voltage waveform.

Fig. 11 is a graph of the output power of each unit of the novel modulation.

Fig. 12 is a graph of inverter output voltage spectrum, wherein: (a) and (b) outputting a voltage frequency spectrum diagram under the traditional PD modulation strategy, and modulating the output voltage frequency spectrum diagram.

Detailed Description

The technical solution of the present invention will be clearly and completely described below by taking a symmetric three-unit cascaded H-bridge as an example and combining with the accompanying drawings. The described three-unit modulation method is only an integral part of the embodiments of the present invention, not all embodiments. The invention can be extended to n units, and obviously all other embodiments obtained by those skilled in the art without creative efforts will fall within the protection scope of the invention based on the specific method of the invention.

Fig. 1 shows a topology circuit diagram of a CHB type three-cell inverter. The main topology is formed by mutually cascading three H-bridge units. Wherein H₁Right half-bridge midpoint of bridge and H₂The midpoints of the left half-bridges of the bridge are connected to each other. In turn, H₂Right half-bridge midpoint of bridge and H₃The left half-bridge of the bridge is connected with the midpoint, and the alternating voltage is from H₁Left arm midpoint of bridge and H₃And outputting the middle point of the right bridge arm of the bridge. The unit half-bridges are connected in parallel and then connected in series with a direct current voltage source. Wherein the DC voltage sources are independent of each other and u_dc1＝u_dc2＝u_dc3＝E。Q_ijIs the parallel connection of an IGBT and a freewheeling diode. The output voltage expression is shown in formula (1):

u_o＝u_o1+u_o2+u_o3 (1)

the relation expression between the output voltage of the ith (i equals to 1,2,3) unit and the on-off of the IGBT is as shown in formula (2): wherein u is_oFor the inverter output of the total voltage u_oiThe voltage is output for the ith cell. i.e. i_oThe output current (positive direction shown in fig. 1) can be expressed by equation (3). Wherein I is the output current amplitude, omega_mIn order to output the angular frequency of the voltage,

is the output impedance angle.

Fig. 2 is a diagram of a conventional PD and carrier degree of freedom reconfiguration modulation strategy. As shown in fig. 2, for any cascaded unit, the carrier period before reconstruction is T₁After reconstruction, the carrier period becomes T₀＝3T₁. When the carrier frequency is 6000Hz and the modulation wave frequency is 50 Hz. At T₁In the time period, the voltage increment of the sine modulation wave is as follows:

wherein M is a modulation ratio, and M < 1.

Thus, at T₁The time period output voltage increment is less than 3E and approximately 0. So that each unit can be in any carrier period T₀The internal output power is:

wherein i is 1,2, 3.

Because the circuits are connected in series, the currents of all points are equal, and the voltage value is approximately unchanged. The output power of each unit in the novel modulation strategy is in direct proportion to the turn-on time of the unit. As shown in equation (6).

As shown in fig. 2(b), the degree of freedom adjustment satisfies that the output times of the respective cells are equal, as shown in equation (7). Therefore, each cell is at T₀The power balance can be satisfied in time.

t₁+t₂+t₃+t₄＝t₅+t₆+t₇＝t₈+t₉+t₁₀ (7)

The cell outputs have three level states, namely E, -E and 0. When the output voltage is + -E, the output state is recorded as + -1, and when the output voltage is 0When so, the output state is noted as 0. The output states of the three H bridges are combined to obtain an output state function S ═ S₁S₂S₃. The process of converting the output state of the conventional PD modulation method can be represented as fig. 3. As can be seen from fig. 3, there are only two output states that are switched to each other at each layer of carriers. For example, in the case where the modulated wave is located within E-2E, it is larger than the layer carrier v_cr2+The output level state is 011 at time, and 001 at time less. In the process of level conversion, the third unit is always in an on state, the first unit is always in an off state, and the second unit is always in a high-frequency on-off state. As can be seen from the output states corresponding to the levels in table 1, when the outputs are ± E, ± 2E, 0, there are many output redundant switch states. Of these redundant state switching states, rejecting the state opposite to the output polarity still has a rich switching margin. Therefore, in the modulation process, the output power balance of each unit can be finally realized on the premise of not changing the total output voltage by rearranging and combining the redundant states theoretically.

TABLE 1 output states corresponding to the levels

The degree of freedom in the modulation method includes two blocks: carrier wave degree of freedom and modulated wave degree of freedom. The degrees of freedom include amplitude, frequency, phase, vertical offset, etc. of the carrier. In the modulation method, the degrees of freedom of the modulated wave are commonly found in: magnitude, phase, and vertical offset. The vertical offset of the carrier wave is controlled to be regularly recombined, so that the distribution of the output voltage of each unit can be changed, and an ideal output effect is achieved. After the degree of freedom is adjusted, on the one hand: the method has the advantages of low harmonic content output by the traditional PD modulation strategy and has the characteristic of power balance of the traditional CPS modulation strategy. On the other hand: the time required for power equalization can be shortened.

The method is realized by performing vertical freedom degree adjustment on the conventional PD modulated wave to make the carrier wave uniformDistributed between layers as shown in fig. 2(b), and the modulated wave before modulation is shown in fig. 2 (a). In conventional PD modulation schemes, an arbitrary carrier, e.g., v_cr2+And is distributed only in a certain layer. V after modulation by carrier wave degree of freedom_cr2+The output power is uniformly distributed between 0E and 3E, and the diversity of the output condition is increased. In the PD modulation strategy, there are 2 corresponding output states: 110 and 001, the output state after the carrier adjustment is changed into 6 types: 110. 001, 101, 011, 100, 010. Compared with two output states before adjustment, 4 output states, namely 101, 011, 100 and 010, are added after adjustment, and redundant switch states among output levels are fully utilized, so that the output states are not single.

Fig. 4 is a conventional 1/4 cycle carrier rotation modulation scheme. On the basis of traditional PD modulation, 1/4 periods of carrier waves are recombined to realize power balance among output units. The first cycle of the carrier wave is realized within 3 modulated wave periods. In this modulation method, the redundant switch states shown in table 1 are used for each cell when the output voltages are ± E and ± 2E.

The output power expression in the modulation method of fig. 4 is (8):

in the first 3/2 modulated wave period, the output power of each unit is as follows:

from the formula (9): p_uo1＝P_uo2＝P_uo3＝P₁₁+P₁₂+P₂₁+P₂₂+P₃₁+P₃₂。

The output is balanced in power within 3/2 modulation wave periods. But this equalization time will increase as the number of output cells increases. When the output is N stages connected in series, the required equalization time will be (N/2) T_o。

As shown in fig. 5. Modulated wave (v) in the figure_ref) Is a sine wave, v_cr1+、v_cr2+、v_cr3+、v_cr1-、v_cr2-、v_cr3-Is a carrier wave. At 3T_cWithin a time period (T)_cA triangular wave period), the carrier wave is uniformly distributed in the interval of 0-3E and 3T_cIs one carrier cycle period. Therefore, the shortest power balancing time of the novel modulation method is the carrier period, so that compared with the conventional 1/4 modulation wave period alternation method, the method greatly shortens the time required by output power balancing among the CHB units.

The output conditions at different reference voltages during a large carrier period are shown in fig. 6.

When modulation ratio M>2/3, setting a carrier cycle period as T₁，T₁＝3T_c，v_refIs a constant value, v_cr1+、v_cr2+、v_cr3+Is a carrier wave, t₁、t₂、 t₃… is the on time.

At different reference voltages, the on-time is calculated as follows:

1)0<v_refwhen the content is less than or equal to 1/3, the opening time of each unit is shown as the formula (10). As can be seen from the calculation, the output time of each unit is equal, and the output time is shown in the formula (11). By substituting this into equation (6), it can be calculated that the average output power of each cell in one carrier period is Po1 ═ Po2 ═ Po 3. Therefore, when the modulation wave is less than 1/3, the output of each unit can reach power balance.

2)1/3<v_refWhen the output time length is less than or equal to 2/3, the output time length of each unit can be calculated according to the figure 6 and is equal after the sum is accumulated in one carrier periodAs in equation (12), the output power is equal.

3)2/3<v_refWhen the output duration is less than or equal to 1, the cumulative sum of the output durations of the high levels in one carrier period can be obtained according to fig. 6 in the same way, as shown in equation (13). Therefore, the output power of each unit is also equal.

In the above three cases, the effective output time of each unit is equal when the unit is output under different reference voltages. The following conclusions can be drawn: the novel modulation strategy can ensure that the output power of each unit is balanced in the full modulation range. The duration it takes to achieve power equalization is one carrier period. And the action times of the three units are approximately the same, so that the average switching frequency of the switching tube tends to be consistent.

Fig. 8 shows a schematic diagram of the novel power equalization modulation, in which the modulation wave is obtained by shifting the half period of the sine wave by 3E units upward, as shown in equation (14), and k is a real number.

The modulation being applied only to v_cr1+、v_cr2+、v_cr3+Three carriers. This carrier consists of 3 periodic triangular carriers (as shown in fig. 7) of equal amplitude, identical frequency, and different positions. Taking the first layer carrier of three-unit cascade as an example to construct a novel carrier. First, the second half period of the first carrier (triangular wave) is shifted up by 1 unit. Next, the second carrier front half cycle is shifted up by 2 units, and the second carrier front half cycle is also shifted up by 2 units. The first half cycle of the third carrier is then shifted up by 1 unit. Taking half carrier period as frequency shift object, the first layer carrier frequency shift ruleComprises the following steps: 0-1-2-2-1-0. The second layer carrier translation rule is as follows: 0-1-0- (-1) - (-1) -0. The third layer carrier frequency translation rule is as follows: 0- (-1) - (-2) - (-2) - (-1) -0. The same layer is connected with each other after translation.

When the modulation wave is in the first half period and is larger than the corresponding carrier wave, the PWM control signal enables the output of the H bridge to be positive, and otherwise, the output of the H bridge is 0. When the modulation wave is in the second half period and is smaller than the corresponding carrier wave, the PWM control signal enables the output of the H bridge to be negative, otherwise, the output is 0.

Taking a three-unit CHB type inverter as an example, a simulation platform is built by utilizing Matlab2018a/Simulink, and the characteristics and performance of the novel power balance modulation method are further explained. The specific parameters are set as follows: the DC side power supply voltage E is 1000V, the load resistance R is 50 omega, the filter inductance L is 10mH, and the fundamental frequency f_mIs 50Hz, a triangular carrier frequency f_cIs 7.2 kHz.

Fig. 9 shows waveforms of output voltage and current under the conventional modulation and the novel modulation strategy. It is measured that under the conventional PD modulation, when the modulation ratio M is 0.9, the ratio of the average powers is:

the output power of each unit is greatly unbalanced. After improvement, under the same modulation degree, the output average power ratio of each unit is as follows: 12.10 kW: 12.10 kW: the 12.10kW output satisfied power balance, and the output voltage waveform is shown in fig. 10. The output of each cell is shown in fig. 11. The output conditions of each unit under different modulation ratios of the conventional modulation and the novel modulation are shown in table 2. The novel modulation realizes the output power balance among all units in the full modulation range.

Fig. 12 is a graph of CHB output voltage spectrum. According to an output spectrogram, the amplitude of the fundamental wave of the output voltage is 2695V and the harmonic content is 22.47% under the traditional PD modulation strategy, and the amplitude of the fundamental wave of the output voltage is 2695V and the harmonic content is 22.47% under the modulation strategy based on the degree of freedom recombination. Therefore, the novel modulation strategy can enable the harmonic content of the output phase voltage to be consistent with the traditional PD modulation strategy and the output harmonic is mainly concentrated near the frequency of the modulation wave and the sideband harmonic.

TABLE 2 average power output of each unit under different modulation ratios

It will be understood that modifications and variations can be resorted to by those skilled in the art, and that all such modifications and variations are intended to be included within the scope of the invention as defined by the appended claims.

Claims

1. A novel power equalization modulation strategy suitable for a cascaded H-bridge multi-level inverter is characterized by comprising the following steps:

(1) the method is suitable for an n-unit cascade H-bridge multi-level inverter, and the direct current sides of the inverters of the topology are all independent and isobaric power supplies.

(2) When the method is applied to an n-unit cascade H-bridge multi-level inverter, n novel carriers with equal amplitude, same frequency and different phases are required to be transversely and alternately arranged. And when the modulation wave is in the positive half cycle, the modulation wave and a PWM wave generated by comparing the carrier wave control the left bridge arm of the H-bridge unit. And when the modulation wave is in the negative half cycle, the modulation wave and the carrier wave are compared to generate a PWM wave to control the right bridge arm of the H-bridge unit. The carrier wave and the modulation wave are in periodic circulation, and the modulation wave is compared with the novel carrier wave to generate a PWM control signal, so that the output power balance of the cascade unit is realized, and the optimal harmonic elimination performance of the output voltage is kept.