Light load control method and system for cascaded multilevel converter

Abstract

The application provides a light load control method and system of a cascade multilevel converter. The light load control method comprises: a sampling module sampling grid-connected current of the cascade multilevel converter; a voltage equalization enabling module judging whether the cascade multilevel converter is in a light load state or a standby state according to the grid-connected current; if the cascade multilevel converter is in the light load state or the standby state, the voltage equalization enabling module cuts off each voltage equalization control loop or reduces the effect of each voltage equalization control loop, so as to avoid the situation that voltages of each independent direct current source gradually diverge until out of control in the prior art, and thus the voltage equalization and stability of each independent direct current source in the CHB light load small current case or the standby operation mode are ensured.

Description

Technical Field

[0001] This invention relates to the field of cascaded multilevel converter technology, and in particular to a light-load control method and system for cascaded multilevel converters. Background Technology

[0002] The cascaded H-bridge (CHB) multilevel rectifier converter can achieve good performance by combining carrier phase-shift modulation and voltage equalization control on the basis of voltage and current control of the main loop. When used as a rectifier, the DC side of each H-bridge is used to provide a stable DC source.

[0003] However, since the DC sides of each H-bridge are independent, differences in switching devices, switching signal delays, and loads can lead to DC side imbalances in each H-bridge stage. This imbalance affects the stable operation of the entire system. Simultaneously, the DC side voltage imbalance impacts the grid current quality, causing uneven voltage distribution across the H-bridge switching devices and potentially leading to device breakdown and damage. Therefore, a key control measure in CHB rectification applications is DC side voltage balancing control, and several different control strategies are currently available.

[0004] However, when the CHB converter is running under light load, the existing control strategies may result in the voltage of each independent DC source gradually diverging until it becomes uncontrollable. Summary of the Invention

[0005] In view of this, embodiments of the present invention provide an H-bridge light-load control method and system to ensure voltage equalization and stability of each independent DC source under light-load and low-current conditions of CHB or in standby operation mode, thereby avoiding additional costs and losses and reducing the complexity of system control.

[0006] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0007] The first aspect of this invention discloses a light-load control method for a cascaded multilevel converter, wherein the cascaded multilevel converter includes at least two power units, and the AC sides of each power unit are cascaded; the light-load control method includes:

[0008] The sampling module samples the grid-connected current of the cascaded multilevel converter;

[0009] The voltage equalization enable module determines whether the cascaded multilevel converter is in a light load state or a standby state based on the grid-connected current.

[0010] If the cascaded multilevel converter is in a light load state or a standby state, the voltage equalization enable module will cut off each voltage equalization control loop, or reduce the effect of each voltage equalization control loop.

[0011] Optionally, the voltage equalization enable module determines whether the cascaded multilevel converter is in a light-load state or a standby state based on the grid-connected current, including:

[0012] The judgment module in the voltage equalization enable module obtains the effective value extracted based on the grid-connected current.

[0013] The judgment module determines whether the valid value is less than a preset threshold.

[0014] If the effective value is less than the preset threshold, the judgment module determines that the cascaded multilevel converter is in a light-load state or a standby state; otherwise, the judgment module determines that the cascaded multilevel converter is not in a light-load state or a standby state.

[0015] Optionally, determining whether the valid value is less than a preset threshold includes:

[0016] When the effective value decreases, it is determined whether it is less than a first preset threshold; if so, the effective value is determined to be less than the preset threshold; otherwise, the effective value is determined to be greater than or equal to the preset threshold.

[0017] When the effective value increases, it is determined whether it is less than a second preset threshold; if so, the effective value is determined to be less than the preset threshold; otherwise, the effective value is determined to be greater than or equal to the preset threshold; the second preset threshold is greater than the first preset threshold.

[0018] Optionally, the first preset threshold is the lower limit of the hysteresis comparison of the light load and heavy load boundary values;

[0019] The second preset threshold is the upper limit of the hysteresis comparison of the light load and heavy load limit values.

[0020] Optionally, the voltage equalization enabling module includes: a judgment module, and a switch corresponding to each of the power units;

[0021] In the light-load control method, the voltage equalization enabling module disconnects each voltage equalization control loop, including:

[0022] The judgment module controls each of the switches, using the modulation signal output from the main control loop as the modulation signal for each of the power units.

[0023] Optionally, the voltage equalization enabling module includes: a judgment module, and a voltage equalization output adjustment module corresponding to each of the power units;

[0024] In the light-load control method, the voltage equalization enabling module reduces the effect of each voltage equalization control loop, including:

[0025] The judgment module controls each of the voltage equalization output adjustment modules, and the effect of each voltage equalization control loop changes positively correlated with the grid-connected current.

[0026] Optionally, the effect of adjusting the voltage equalization control loop changes positively with the grid-connected current, including performing the following on each of the power units:

[0027] As the grid-connected current changes, the modulation signal output by the corresponding voltage equalization control loop changes in a positive correlation.

[0028] The modulation signal corresponding to the change in the equalizing control loop is superimposed with the modulation signal output by the main control loop;

[0029] The superposition result is used as the modulation signal for the corresponding power unit.

[0030] Optionally, after the voltage equalization enable module determines whether the cascaded multilevel converter is in a light-load state or a standby state based on the grid-connected current, the method further includes:

[0031] If the cascaded multilevel converter is not in a light load state or standby state, the voltage equalization enable module superimposes the modulation signal output by each voltage equalization control loop with the modulation signal output by the main control loop, and uses each superposition result as the modulation signal of the corresponding power unit.

[0032] Optionally, the voltage equalization enabling module includes: a judgment module, and a switch or voltage equalization output adjustment module corresponding to each power unit;

[0033] In the light load control method, the voltage equalization enabling module superimposes the modulation signal output from each voltage equalization control loop with the modulation signal output from the main control loop, and uses each superposition result as the modulation signal for the corresponding power unit, including the following actions performed on each power unit:

[0034] The judgment module controls the corresponding switch or the corresponding voltage equalization output adjustment module to use the result of superimposing the modulation signal output by the voltage equalization control loop and the modulation signal output by the main control loop as the modulation signal for the corresponding power unit.

[0035] A second aspect of this invention discloses a light-load control system for a cascaded multilevel converter, comprising: a controller and a main circuit;

[0036] The main circuit includes at least two power units, and the AC sides of each power unit are cascaded, with the two ends of the cascaded circuit serving as the AC side of the main circuit.

[0037] The DC side of each power unit serves as a DC side interface of the main circuit.

[0038] Each of the power units is controlled by the controller, which is used to execute the light-load control method for the cascaded multilevel converter as described in any of the first aspects of the present invention.

[0039] Optionally, the power unit is an H-bridge.

[0040] Based on the above embodiments of the present invention, a light-load control method and system for a cascaded multilevel converter is provided. The light-load control method includes: a sampling module sampling the grid-connected current of the cascaded multilevel converter; a voltage equalization enable module determining whether the cascaded multilevel converter is in a light-load state or a standby state based on the grid-connected current; if the cascaded multilevel converter is in a light-load state or a standby state, the voltage equalization enable module cuts off each voltage equalization control loop, or reduces the effect of each voltage equalization control loop, to avoid the situation in the prior art where the voltage of each independent DC source gradually diverges until it becomes uncontrollable, thereby ensuring the voltage equalization and stability of each independent DC source under light-load and low-current conditions or in standby operation mode, without additional cost and loss, and reducing the complexity of system control. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0042] Figure 1(a) is a basic topology diagram of a single-phase cascaded H-bridge converter provided in an embodiment of the present invention;

[0043] Figure 1(b) is a basic topology diagram of a three-phase cascaded H-bridge converter provided in an embodiment of the present invention;

[0044] Figure 2 This is a schematic diagram of carrier phase-shifting PWM modulation provided in an embodiment of the present invention;

[0045] Figure 3 A schematic diagram of a typical pressure equalization control method provided in an embodiment of the present invention;

[0046] Figure 4 This is a schematic diagram of a hardware voltage equalization structure using parallel resistors provided in an embodiment of the present invention;

[0047] Figure 5 This is a schematic diagram of a hardware voltage equalization structure for an auxiliary resistor provided in an embodiment of the present invention;

[0048] Figure 6 A flowchart illustrating a light-load control method for a cascaded multilevel converter provided in an embodiment of the present invention;

[0049] Figure 7 A flowchart illustrating a method for determining whether a cascaded multilevel converter is in a light-load state or a standby state, provided for an embodiment of the present invention;

[0050] Figure 8 A schematic diagram illustrating the principle of light-load and heavy-load conversion determination provided in an embodiment of the present invention;

[0051] Figure 9 A schematic diagram illustrating the principle of light-load control of a cascaded multilevel converter provided in an embodiment of the present invention;

[0052] Figure 10 A schematic diagram illustrating the principle of light-load control of another cascaded multilevel converter provided in an embodiment of the present invention;

[0053] Figure 11(a) is a simulation diagram of a three-phase grid-connected current provided by an embodiment of the present invention;

[0054] Figure 11(b) is a simulation diagram of the average voltage of a three-phase DC bus provided in an embodiment of the present invention;

[0055] Figure 12(a) is another simulation diagram of three-phase grid-connected current provided by an embodiment of the present invention;

[0056] Figure 12(b) is a simulation diagram of the average voltage of a three-phase DC bus provided in an embodiment of the present invention;

[0057] Figure 13 This is a schematic diagram of a light-load control system for a cascaded multilevel converter provided in an embodiment of the present invention. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] In this application, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0060] As can be seen from the background technology, the CHB multilevel rectifier converter, based on the voltage and current control of the main loop, combined with carrier phase-shift modulation (CPS-PWM) and voltage equalization control, can achieve good performance. The basic topology of CHB is shown in Figure 1(a) and Figure 1(b), taking three cascaded H-bridge units (N=3) as an example.

[0061] The modulation coordination among multiple H-bridges determines the final harmonic characteristics of the overall CHB output voltage. Carrier phase-shift modulation (CPS-PWM) is the standard modulation method for CHB multilevel converters. Its main advantages in implementation are modularity and uniformity: each H-bridge unit has the same modulation wave, the same carrier amplitude, and a carrier phase difference of Π / N. The modulation process of each unit can be implemented in its own local controller, facilitating system production, maintenance, and expansion; the switching frequencies of all switches are the same and fixed, and the switching losses and fundamental power of each unit are basically consistent. The purpose of staggering the carrier phases is to ensure that the PWM pulses output by different H-bridge units are also staggered by an angle within the switching cycle. While ensuring that the fundamental voltage output by each unit is the same, the carrier phase-shifting process ensures that the switching harmonic phase angles are uniformly staggered, achieving mutual cancellation. This results in no corresponding switching harmonics in the voltage at the CHB AC port, increasing the lowest switching harmonic frequency band by N times. CPS-PWM's ability to significantly increase the equivalent switching frequency and its inherent uniformity establish its position as the standard modulation method for CHB topologies. A schematic diagram of carrier phase-shift modulation is shown below. Figure 2 As shown.

[0062] The CHB multilevel structure enables bidirectional power flow control between AC and DC. When used as a rectifier, the DC side of each power unit provides a stable DC source. However, the DC sides of each power unit are independent of each other. Due to differences in switching devices, switching signal delays, and load variations, imbalances occur on the DC sides of each level. These imbalances affect the stable operation of the entire system. Simultaneously, the DC voltage imbalance affects the grid current quality, and the uneven voltage experienced by the switching devices in each power unit can even lead to device breakdown and damage. Therefore, a key control aspect of CHB in rectification applications is DC-side voltage balancing control. For specific control strategies, please refer to [link to relevant documentation]. Figure 3 The pressure equalization control method shown.

[0063] In most cases, the CHB multilevel rectifier converter achieves good performance after employing the aforementioned carrier phase shift adjustment and voltage equalization control. However, when the converter is operating under light load, there is a possibility that the voltages of each independent DC source gradually diverge until they become uncontrollable.

[0064] There are three voltage equalization control methods. The first method is implemented through hardware voltage equalization, specifically as follows: Figure 4 As shown, Figure 4 This is a switching resistor branch connected in parallel to the DC bus. The power consumed by this resistor branch is controlled by switching switch S, thereby adjusting the equivalent loss resistance and ultimately achieving voltage balance across all units. However, the withstand voltage ratings of both the resistor and switch S should be greater than the DC voltage U. dc1 The required rated values ​​necessitate relatively large resistors and switches, resulting in high costs.

[0065] The second method involves installing a hardware voltage equalization module on the local controller that provides its own power, specifically as follows: Figure 5 As shown, the resistor connected to the bus capacitor of the main power circuit is equivalently transferred to the low-voltage board of the control, which reduces the cost of related switches and resistors to a certain extent, but also increases the capacity of the auxiliary power converter (Flgback DC / DC converter) used by the local controller.

[0066] The third method is to achieve this through software voltage equalization, particularly pulse-sequential voltage equalization control. This method only detects the polarity of the current and abandons the extraction of the current phase. Then, it adds a switching state transition in the middle of each H-bridge switching cycle to balance the charging and discharging of each unit. Although this method solves the CHB voltage equalization control problem without adding hardware, it increases the switching frequency of the switching transistors by nearly 100%, thereby increasing switching losses and heat dissipation requirements.

[0067] The voltage equalization control method described above only considers the fundamental frequency band and ignores the harmonic frequency band of the switching frequency band. However, the harmonic frequency band plays an indispensable role in some operating conditions.

[0068] In practice, the current in the same phase CHB chain is the same, but the harmonics of each H-bridge AC port voltage may differ due to various reasons. Each H-bridge has a different power angle for the same high-order harmonic current, resulting in uneven power generated by the same high-order harmonic current. This different harmonic power acts on the output DC source capacitors of different H-bridges in the same phase CHB chain, causing part of the DC bus to be charged and part to be discharged, thus causing the DC voltage to gradually diverge.

[0069] As mentioned earlier, under ideal symmetry, CPS-PWM modulation allows the low-order switching harmonics of the cascaded H-bridges to be phase-shifted by π / N, thus achieving mutual cancellation and increasing the equivalent switching frequency by a factor of N. The symmetry of CPS-PWM mainly includes three aspects: the consistency of the DC voltage of each unit, the consistency of the modulation signal of each unit, and the symmetry of the carrier phase shift angle of each unit. The first two determine the magnitude of the switching harmonics in each unit, while the last determines the symmetrical phase distribution of the switching harmonics. When the symmetry of CPS-PWM is disrupted, the condition for mutual cancellation of switching harmonics is not met, resulting in the presence of these switching harmonics that should have been eliminated in the CHB output voltage.

[0070] Voltage equalization control achieves power regulation by differentially altering the modulation waves of each unit, thus making the modulation waves of each unit inconsistent. This results in inconsistent magnitudes of the switching harmonics in each unit. Even though the uniform carrier phase shift of CPS-PWM can make these switching harmonics phase symmetrical, they still cannot be completely canceled out. The uncancelled switching subharmonics generate different harmonic powers between different H-bridges.

[0071] Theoretical analysis shows that the smaller the fundamental current of CHB, the smaller the fundamental power regulation that the voltage equalization control can generate. Even when the fundamental current approaches zero, no matter how the voltage equalization control is adjusted or how large the output duty cycle is, the fundamental power regulation it can generate will approach zero. On the other hand, the current has almost no effect on the magnitude of harmonic power. Therefore, under the condition of low fundamental output current, the harmonic power caused by the voltage equalization control may even exceed the power regulation of the voltage equalization control itself. This leads to the voltage equalization control not only failing to balance the DC voltage of each unit, but also accelerating the instability of DC voltage divergence.

[0072] Based on the above analysis, it can be seen that under light load or even standby operation conditions (small grid current), the existing control strategies may result in the voltage of each independent DC source gradually diverging until it becomes uncontrollable.

[0073] Therefore, this invention provides a light-load control method and system for a cascaded multilevel converter. In this solution, if it is determined that the cascaded multilevel converter is in a light-load state or a standby state, each voltage equalization control loop is cut off, or the effect of each voltage equalization control loop is reduced, in order to avoid the situation in the prior art where the voltage of each independent DC source gradually diverges until it becomes uncontrollable. This ensures the voltage equalization and stability of each independent DC source under light-load and low-current conditions or in standby operation mode, avoids additional costs and losses, and reduces the complexity of system control.

[0074] like Figure 6 The diagram shown is a flowchart illustrating a light-load control method for a cascaded multilevel converter provided in an embodiment of the present invention.

[0075] The cascaded multilevel converter includes at least two power units, with their AC sides cascaded. To control each power unit, the controller of this cascaded multilevel converter adds a voltage equalization enable module to the voltage equalization control loop, in addition to the existing sampling module, main control loop, and voltage equalization control loop.

[0076] The light-load control method for this cascaded multilevel converter mainly includes the following steps:

[0077] Step S601: The sampling module samples the grid-connected current of the cascaded multilevel converter.

[0078] In step S601, the grid-connected current i abc Including i a i b and i c .

[0079] Step S602: The voltage equalization enable module determines whether the cascaded multilevel converter is in a light load state or a standby state based on the grid current. If yes, proceed to step S603; otherwise, proceed to step S604.

[0080] In practical applications, the specific process by which the voltage equalization enable module in step S602 determines whether the cascaded multilevel converter is in a light-load state or a standby state based on the grid-connected current can be described as follows: Figure 7 As shown, the main steps include the following:

[0081] Step S701: The judgment module in the equalization enable module obtains the effective value extracted based on the grid-connected current.

[0082] In the specific implementation step S701, the PLL and coordinate transformation module preprocesses the sampled grid-connected current and extracts the effective value of the grid current. Then, the judgment module in the voltage equalization enable module obtains the effective value extracted by the PLL and coordinate transformation module based on the grid-connected current.

[0083] Preprocessing includes, but is not limited to, filtering the grid-connected current.

[0084] Step S702: The judgment module determines whether the valid value is less than the preset threshold. If yes, proceed to step S703; otherwise, proceed to step S704.

[0085] In the specific implementation of step S702, the judgment module compares the effective value with the preset threshold. If the effective value is found to be less than the preset threshold, it means that it is not necessary to use the voltage equalization control loop to perform voltage equalization control on the power unit. Then, step S703 is executed.

[0086] If the effective value is determined to be greater than or equal to the preset threshold, it means that the power unit needs to be controlled by the voltage equalization control loop. Then, step S704 is executed.

[0087] In practical applications, when the judgment module in step S702 determines whether the valid value is less than the preset threshold, a hysteresis judgment can be used. The specific process is as follows:

[0088] First, when the effective value decreases, it is determined whether it is less than a first preset threshold; if so, the effective value is determined to be less than the preset threshold; otherwise, the effective value is determined to be greater than or equal to the preset threshold.

[0089] Secondly, when the effective value increases, it is determined whether it is less than the second preset threshold; if so, the effective value is determined to be less than the preset threshold; otherwise, the effective value is determined to be greater than or equal to the preset threshold.

[0090] The second preset threshold is greater than the first preset threshold.

[0091] When using hysteresis judgment, the lower limit and upper limit of hysteresis comparison for light load and heavy load boundary values ​​are determined to determine the hysteresis boundary.

[0092] It should be noted that the first preset threshold is the lower limit of the hysteresis comparison of the light load and heavy load limits; the second preset threshold is the upper limit of the hysteresis comparison of the light load and heavy load limits.

[0093] For example, such as Figure 8 As shown, 2% of the full load current is used as the light load / heavy load boundary value, and ±1% is used as the hysteresis boundary. At this time, the lower limit of the hysteresis comparison of the light load / heavy load boundary value is 1%, and the upper limit of the hysteresis comparison of the light load / heavy load boundary value is 3%.

[0094] Step S703: The determination module determines whether the cascaded multilevel converter is in a light load state or a standby state;

[0095] In the specific implementation of step S703, when the effective value is less than the preset threshold, the judgment module determines that the cascaded multilevel converter is in a light load state or a standby state.

[0096] Step S704: The determination module determines that the cascaded multilevel converter is not in a light load state or a standby state.

[0097] In the specific implementation of step S704, when the effective value is greater than or equal to the preset threshold, the judgment module determines that the cascaded multilevel converter is not in a light load state or a standby state.

[0098] Step S603: The equalization enable module cuts off each equalization control loop, or reduces the function of each equalization control loop.

[0099] In the specific implementation of step S603, when it is determined that the cascaded multilevel converter is in a light load state or a standby state, it is explained that if voltage equalization control is used as in the prior art, it will cause the voltage of each independent DC source to diverge. Therefore, the voltage equalization enable module cuts off each voltage equalization control loop, or reduces the effect of each voltage equalization control loop.

[0100] In practical applications, the voltage equalization enable module includes: a judgment module, and switches corresponding to each power unit, such as... Figure 9 As shown, the judgment module is connected to the switch. The active end of the switch is connected to the main control loop, and the stationary end is connected to the voltage equalization control loop. One output of the main control loop is connected to the multiplier in the voltage equalization control loop and the adder in the subsequent stage of the voltage equalization control loop, respectively. The other output is connected to the controller.

[0101] In the light load control method, the specific process of executing step S603, where the voltage equalization enable module cuts off each voltage equalization control loop, may include:

[0102] The judgment module controls each switch, using the modulation signal output from the main control loop as the modulation signal for each power unit.

[0103] Specifically, when it is determined that the cascaded multilevel converter is in a light load state or a standby state, the judgment module controls each switch to be disconnected, so that each switch is in the disconnected state, and controls each power unit through the main control loop.

[0104] In the main control loop, a preset number of DC-side voltages are collected, such as N DC-side voltages V. dc1 ... V dcx Then, sum the N DC-side voltages to obtain the total DC-side voltage;

[0105] Calculate the average value V of the total DC-side voltage using the formula for calculating the average value. dcavg Then, calculate V. dcavg Reference value V of DC side voltage dc The difference is processed by the voltage controller to obtain the reference value i of the active current. d_ref and the reference value i of the reactive current q_ref Set to 0;

[0106] Reference value i for calculating active current d_ref With active current measurement value i d The difference is used to obtain the first value; the reference value i of the reactive current is calculated. q_ref With reactive current measurement value i q The difference is used to obtain the second value;

[0107] After the first and second values ​​are processed by the current controller, the active voltage reference signal u is obtained.d and reactive voltage reference signal u q ;

[0108] Through coordinate transformation, the active voltage reference signal u d and reactive voltage reference signal u q This is converted into a modulated signal d(t) output by the main control loop;

[0109] The modulation signal output from the main control loop is used as the modulation signal for each power unit;

[0110] Then, the modulation signal of each power unit is output to the controller, and the controller modulates the modulation signal of each power unit to obtain the drive signal of each power unit.

[0111] In practical applications, the voltage equalization enable module includes: a judgment module, and voltage equalization output adjustment modules corresponding to each power unit, such as... Figure 10 As shown, the judgment module is connected to the voltage equalization output adjustment module. One output of the main control loop is connected to the multiplier in the voltage equalization control loop, and the other output is connected to the adder in the subsequent stage of the voltage equalization control loop. The voltage equalization enable module is located between the multiplier and the adder connected to the two outputs of the main control loop.

[0112] In the light-load control method, the specific process of executing step S603, where the voltage equalization enable module reduces the effect of each voltage equalization control loop, may include:

[0113] The judgment module controls each voltage equalization output adjustment module, and the effect of each voltage equalization control loop changes positively correlated with the grid current.

[0114] Specifically, when it is determined that the cascaded multilevel converter is in a light load state or a standby state, the judgment module controls each voltage equalization output adjustment module according to the change of grid current, and adjusts the function of each voltage equalization control loop so that the function of each voltage equalization control loop changes positively with the grid current.

[0115] In other words, as the grid current decreases, the effect of each voltage equalization control loop gradually decreases. At this time, when the voltage equalization control enters the hysteresis comparison lower limit range of the set light load and heavy load limit values ​​of the light load control system, the voltage equalization effect gradually decreases, which can also improve the voltage equalization effect and stability of light load.

[0116] In each voltage equalization control loop, a preset number of DC-side voltages are collected, such as N DC-side voltages V. dc1 ... V dcx Then, sum the N DC-side voltages to obtain the total DC-side voltage. Calculate the average value V of the total DC-side voltage using the average value calculation formula. dcavg Then, calculate V. dcavgWith any DC side voltage V collected dc1 The difference;

[0117] The obtained difference is input to the voltage equalization controller, and after processing by the voltage equalization controller, the initial voltage equalization signal of each power unit is obtained.

[0118] By combining the grid voltage, grid current, and DC side total voltage, the initial voltage equalization signal of each power unit is adjusted using each voltage equalization output adjustment module to obtain the modulation signal of each power unit.

[0119] In practical applications, the specific process of adjusting the effect of each voltage equalization control loop to change positively with the grid-connected current can include performing the following on each power unit:

[0120] As the grid-connected current changes, the modulation signal output of the corresponding voltage equalization control loop changes positively correlated with the change; the modulation signal after the change of the corresponding voltage equalization control loop is superimposed with the modulation signal output of the main control loop; the superposition result is used as the modulation signal of the corresponding power unit.

[0121] Specifically, as the grid-connected current gradually decreases, the role of each voltage equalization control loop is gradually reduced. This is achieved by adjusting the initial voltage equalization signal of each power unit using each voltage equalization output adjustment module to obtain the modulation signal of each power unit. Then, the modulation signal output of the corresponding voltage equalization control loop is gradually reduced to obtain the modulation signal after the change in the corresponding voltage equalization control loop. The modulation signal after the change in the corresponding voltage equalization control loop is then superimposed with the modulation signal output by the main control loop, and the superposition result is used as the modulation signal of the corresponding power unit.

[0122] As the grid-connected current gradually increases, the role of each voltage equalization control loop gradually increases. Specifically, after adjusting the initial voltage equalization signal of each power unit using each voltage equalization output adjustment module to obtain the modulation signal of each power unit, the modulation signal output of the corresponding voltage equalization control loop is gradually increased to obtain the modulation signal after the change of the corresponding voltage equalization control loop. The modulation signal after the change of the corresponding voltage equalization control loop is superimposed with the modulation signal output of the main control loop, and the superposition result is used as the modulation signal of the corresponding power unit.

[0123] Step S604: The equalization enable module superimposes the modulation signals output by each equalization control loop with the modulation signals output by the main control loop, and uses each superposition result as the modulation signal of the corresponding power unit.

[0124] In the specific implementation of step S604, when it is determined that the cascaded multilevel converter is not in a light load state or a standby state, the voltage equalization enable module superimposes the modulation signal output by each voltage equalization control loop with the modulation signal output by the main control loop, and uses each superposition result as the modulation signal of the corresponding power unit.

[0125] In practical applications, the voltage equalization enable module includes: a judgment module, and switches corresponding to each power unit (such as...). Figure 9 (as shown) or equalizing output adjustment module (such as Figure 10 (as shown);

[0126] In the light load control method, step S604, the voltage equalization enable module superimposes the modulation signals output from each voltage equalization control loop with the modulation signals output from the main control loop, and uses each superposition result as the modulation signal for the corresponding power unit, including the following steps performed on each power unit:

[0127] The judgment module controls the corresponding switch or the corresponding voltage equalization output adjustment module, and uses the result of superimposing the modulation signal output by the corresponding voltage equalization control loop and the modulation signal output by the main control loop as the modulation signal of the corresponding power unit.

[0128] Specifically, when it is determined that the cascaded multilevel converter is not in a light load state or standby state, the judgment module controls each switch to remain closed, so that each voltage equalization control loop is connected to the main control loop. Through each voltage equalization control loop, voltage equalization control is performed on each power unit. After obtaining the modulation signal of each power unit in each voltage equalization control loop, the modulation signal output by the corresponding voltage equalization control loop is superimposed with the modulation signal output by the main control loop to obtain the superposition result, and the superposition result is used as the modulation signal of the corresponding power unit.

[0129] Specifically, when it is determined that the cascaded multilevel converter is not in a light load state or standby state, after obtaining the modulation signal of each power unit in each voltage equalization control loop, the judgment module controls the corresponding voltage equalization output adjustment module to superimpose the modulation signal output by the corresponding voltage equalization control loop with the modulation signal output by the main control loop to obtain the superposition result, and use the superposition result as the modulation signal of the corresponding power unit.

[0130] It should be noted that the modulation signals of each power unit can be obtained by referring to the execution process and principle of the voltage equalization control loop mentioned above, which will not be repeated here.

[0131] This technical solution is a software control solution, which does not add any additional hardware, meaning it will not increase any additional costs or losses. Compared with existing software methods, the system switching frequency remains unchanged, and there will be no additional switching losses. At the same time, based on existing control methods, only the control logic switching is added, without significantly increasing the complexity of system control. Therefore, this technical solution has a relative advantage.

[0132] A light-load control method for a cascaded multilevel converter (CMP) based on the above embodiments of the present invention includes: a sampling module sampling the grid-connected current of the CMP; a voltage equalization enable module determining whether the CMP is in a light-load state or a standby state based on the grid-connected current; if the CMP is in a light-load state or a standby state, the voltage equalization enable module either cuts off each voltage equalization control loop or reduces the effect of each voltage equalization control loop. In this solution, if the voltage equalization enable module determines that the CMP is in a light-load state or a standby state, the voltage equalization enable module cuts off each voltage equalization control loop or reduces the effect of each voltage equalization control loop to avoid the situation in the prior art where the voltage of each independent DC source gradually diverges until it becomes uncontrollable. This ensures the voltage equalization and stability of each independent DC source under light-load, low-current conditions or in standby operation mode, avoiding additional costs and losses, and reducing the complexity of system control.

[0133] A light-load control method for a cascaded multilevel converter provided by the above embodiments of the present invention, such as... Figure 9 The diagram shown is a schematic diagram of the principle of light-load control of a cascaded multilevel converter provided in an embodiment of the present invention. The present invention takes a three-phase CHB as an example and adds judgment logic, that is, adds a judgment module and a switch, on the basis of the existing control.

[0134] The upper part consists of a voltage-current dual-loop dq decoupling control, including dq decoupling, voltage feedforward, and coordinate transformation, which controls the grid-connected current i. abc and the total DC voltage V dc *N, used to realize the grid-connected current i abc DC side total voltage V dc Decoupling of the active and reactive current controllers of *N.

[0135] Specifically, the upper part is the main control loop, which includes three adders, one voltage controller, and two current controllers.

[0136] See Figure 9 V dc V represents the reference value for the DC-side voltage. dcavg Represents all DC-side voltages (V) dc1 ... V dcx The average value of the two is taken, and the difference is calculated by an adder. Then, the difference is used by a voltage controller to obtain the reference value i of the active current. d_ref The reference value i of reactive current. q_ref Set to 0.

[0137] Reference value of active current i d_ref The measured value of active current i dAfter the difference is calculated by the adder, the resulting difference is processed by the current controller to obtain the active voltage reference signal u. d Among them, the measured value of active current i d It is obtained by transforming the sampled grid-connected current.

[0138] Reference value i for reactive current q_ref The measured value of reactive current i q After the difference is calculated by the adder, the resulting difference is processed by the current controller to obtain the reactive voltage reference signal u. q Among them, the measured value of reactive current i q It is obtained by transforming the sampled grid-connected current.

[0139] The active voltage reference signal u d and reactive voltage reference signal u q After coordinate transformation, the modulation signal d(t) output by the main control loop is obtained.

[0140] The phase-locked loop (PLL) and coordinate transformation module on the left are used to determine the grid voltage phase angle θ and the current i required for control. dq This means converting voltage and current to the PLL coordinate system, which facilitates the separate control of active and reactive currents.

[0141] See Figure 9 V abc and i abc Representing the grid-connected voltage and grid-connected current respectively, after PLL coordinate transformation, the grid voltage phase angle θ and the current i required for control are obtained. dq The required current i dq Including the measured value of active current i d And the measured value of reactive current i q .

[0142] The superimposed part is a partial schematic diagram of the voltage equalization control loop of each H-bridge (power unit) in CHB. This voltage equalization control is based on the voltage and current dual-loop dq decoupling control output modulation signal d(t). Through the modulation signal d(t), the modulation signals d1(t), ..., dN(t) of each power unit are obtained.

[0143] Specifically, the equalization control loop includes: an adder, an equalization controller, and a multiplier.

[0144] See Figure 9 V dc1 V represents the reference value of any DC-side voltage. dcavg Represents all DC-side voltages (V) dc1 ... V dcxThe average value of the two is calculated, and the difference between them is obtained by adding them. Then, the difference is processed by the voltage equalization controller to obtain the initial voltage equalization signal of each power unit. Combined with the modulation signal d(t) output by the main control loop, the initial voltage equalization signal of each power unit is adjusted by a multiplier to obtain the modulation signal of the voltage equalization control loop corresponding to each power unit. Then, another adder is used to superimpose the modulation signal of the voltage equalization control loop corresponding to each power unit with the modulation signal output by the main control loop. The superposition result is used as the modulation signal of the corresponding power unit.

[0145] In practical applications, when the grid-connected current i abc (i a i b i c When the effective value of the signal is greater than or equal to the preset threshold, the judgment module determines that the cascaded multilevel converter is not in a light load state or a standby state, controls each switch to remain closed, and connects each voltage equalization control loop to the main control loop. The voltage equalization control loop performs voltage equalization control on each power unit, and the modulation signal output by each voltage equalization control loop is superimposed with the modulation signal output by the main control loop. The superposition results are used as the modulation signals of the corresponding power units.

[0146] Specifically, the judgment module controls the corresponding switch, superimposes the modulation signal output by the corresponding equalization control loop with the modulation signal output by the main control loop to obtain the superposition result, and uses the superposition result as the modulation signal of the corresponding power unit; the modulation signal of each power unit is output to the controller, and after being modulated by the controller, the drive signal of each power unit is output.

[0147] Preferably, the controller is a carrier phase-shifting controller (CPS-PWM).

[0148] It should be noted that the drive signal for each power unit can be a PWM signal.

[0149] When the grid current i abc When the load is less than a preset threshold, the judgment module determines that the cascaded multilevel converter is in a light load state or a standby state, controls each switch to open, and removes each voltage equalization control loop. The main control loop then controls each power unit and each switch, using the modulation signal output from the main control loop as the modulation signal for each power unit. The modulation signal of each power unit is then output to the controller, and after modulation by the controller, the drive signal for each power unit is output.

[0150] Based on the above principle, when the cascaded multilevel converter is in a light load or standby state, removing each voltage equalization control loop can ensure the voltage equalization and stability of each independent DC source under light load and low current conditions or in standby operation mode of CHB.

[0151] Based on the above embodiments of the present invention, a light-load control method for a cascaded multilevel converter, in addition to the above-mentioned method of directly disconnecting / connecting each voltage equalization control loop, can also gradually reduce the magnitude of the effect of each voltage equalization control loop as the grid current gradually decreases, such as... Figure 10 As shown, when the light-load control system enters the set light-load range, the pressure equalization effect gradually decreases. This also improves the pressure equalization effect and stability under light load. Figure 10 Taking three-phase CHB as an example, on the basis of the existing control, judgment logic is added, that is, a judgment module and a voltage equalization output adjustment module are added.

[0152] The upper part consists of voltage and current dual-loop dq decoupling control, and the left side contains a phase-locked loop (PLL) and coordinate transformation. Figure 9 The same applies, so I won't go into details here.

[0153] The superimposed part is a partial schematic diagram of the voltage equalization control loop of each H-bridge (power unit) in CHB. This voltage equalization control is based on the voltage and current dual-loop dq decoupling control output modulation signal d(t). Through the modulation signal d(t), the modulation signals d1(t), ..., dN(t) of each power unit are obtained.

[0154] Specifically, the equalization control loop includes: an adder, an equalization controller, and a multiplier.

[0155] See Figure 10 V dc1 V represents the reference value of any DC-side voltage. dcavg Represents all DC-side voltages (V) dc1 ... V dcx The average value of the two is calculated by adding them together, and then the difference is processed by the voltage equalization controller to obtain the initial voltage equalization signal. Combined with the grid voltage, grid current and DC side total voltage, the initial voltage equalization signal is adjusted by the voltage equalization output adjustment module to obtain the modulation signal.

[0156] In practical applications, when the grid-connected current i abc (i a i b i c When the effective value of the signal is greater than or equal to the preset threshold, the judgment module determines that the cascaded multilevel converter is not in a light load state or a standby state. It performs voltage equalization control on each power unit through each voltage equalization control loop, and superimposes the modulation signal output by each voltage equalization control loop with the modulation signal output by the main control loop. The superposition results are used as the modulation signals of the corresponding power units.

[0157] Specifically, in each voltage equalization control loop, after obtaining the modulation signal of each power unit, the judgment module controls the voltage equalization output adjustment module to superimpose the modulation signal output by each voltage equalization control loop with the modulation signal output by the main control loop to obtain the superposition results, and use each superposition result as the modulation signal of the corresponding power unit; output the modulation signal of each power unit to the controller, and after being modulated by the controller, output the drive signal of each power unit.

[0158] When the grid current i abc When the load is less than the preset threshold, the judgment module determines that the cascaded multilevel converter is in a light load state or a standby state, thereby reducing the effect of each voltage equalization control loop.

[0159] Specifically, in each voltage equalization control loop, after obtaining the modulation signal of each power unit, the judgment module controls each voltage equalization output adjustment module to change the modulation signal output of the corresponding voltage equalization control loop in a positive correlation with the change of the grid current, thereby obtaining the modulation signal after the change of the corresponding voltage equalization control loop. The modulation signal after the change of the corresponding voltage equalization control loop is superimposed with the modulation signal output of the main control loop to obtain the superposition result, and the superposition result is used as the modulation signal of the corresponding power unit. The modulation signal of each power unit is output to the controller, and after being modulated by the controller, the drive signal of each power unit is output.

[0160] Based on the above principle, when the cascaded multilevel converter is in a light load or standby state, reducing the effect of each voltage equalization control loop can ensure the voltage equalization and stability of each independent DC source under light load and low current conditions or in standby operation mode of CHB.

[0161] To better understand the light-load control method of the cascaded multilevel converter described above, embodiments of the present invention provide comparative simulation diagrams of simulations using and not using the light-load control method of the cascaded multilevel converter of the present invention, as shown in Figures 11(a), 11(b), 12(a), and 12(b).

[0162] The simulation was conducted under the conditions of a 3-phase 3-module circuit connected to a 10kV symmetrical power grid with a 1% load. Figures 11(a) and 11(b) are simulation diagrams of the light-load control method without the cascaded multilevel converter of the present invention, while Figures 12(a) and 12(b) are simulation diagrams of the light-load control method with the cascaded multilevel converter of the present invention. It is obvious that the light-load control method with the cascaded multilevel converter of the present invention has a better voltage equalization and stabilization effect on CHB under light load conditions.

[0163] Compared with the above embodiments of the present invention Figure 6 Corresponding to the light-load control method for a cascaded multilevel converter shown, this embodiment of the invention also provides a light-load control system for a cascaded multilevel converter, such as... Figure 13 As shown, the light-load control system of this cascaded multilevel converter includes: a controller and a main circuit;

[0164] The main circuit includes at least two power units, and the AC sides of each power unit are cascaded, with the two ends of the cascaded circuit serving as the AC side of the main circuit.

[0165] The DC side of each power unit serves as a DC side interface of the main circuit.

[0166] Each of the power units is controlled by the controller, which is used to execute the light-load control method for the cascaded multilevel converter as described in any of the above embodiments.

[0167] Preferably, the power unit is an H-bridge.

[0168] Based on the above embodiments of the present invention, a system of a cascaded multilevel converter is provided. During the execution of the light-load control method of the cascaded multilevel converter described in any of the above embodiments by the light-load control system of the cascaded multilevel converter, if the voltage equalization enable module determines that the cascaded multilevel converter is in a light-load state or a standby state, the voltage equalization enable module cuts off each voltage equalization control loop, or reduces the effect of each voltage equalization control loop, so as to avoid the situation in the prior art where the voltage of each independent DC source gradually diverges until it becomes uncontrollable. This ensures the voltage equalization and stability of each independent DC source under light-load and low-current conditions or in standby operation mode, avoids additional costs and losses, and reduces the complexity of system control.

[0169] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0170] Those skilled in the art will further recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components and steps of the various examples have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0171] In this application, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0172] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A light-load control method for a cascaded multilevel converter, characterized in that, The cascaded multilevel converter includes at least two power units, and the AC side of each power unit is cascaded. The light-load control method includes: The sampling module samples the grid-connected current of the cascaded multilevel converter; The voltage equalization enable module determines whether the cascaded multilevel converter is in a light load state or a standby state based on the grid-connected current. If the cascaded multilevel converter is in a light load state or a standby state, the voltage equalization enable module will cut off each voltage equalization control loop, or reduce the effect of each voltage equalization control loop.

2. The light-load control method for a cascaded multilevel converter according to claim 1, characterized in that, The voltage equalization enable module determines whether the cascaded multilevel converter is in a light-load state or a standby state based on the grid-connected current, including: The judgment module in the voltage equalization enable module obtains the effective value extracted based on the grid-connected current. The judgment module determines whether the valid value is less than a preset threshold. If the effective value is less than the preset threshold, the judgment module determines that the cascaded multilevel converter is in a light-load state or a standby state; otherwise, the judgment module determines that the cascaded multilevel converter is not in a light-load state or a standby state.

3. The light-load control method for a cascaded multilevel converter according to claim 2, characterized in that, Determining whether the valid value is less than a preset threshold includes: When the effective value decreases, it is determined whether it is less than a first preset threshold; if so, the effective value is determined to be less than the preset threshold; otherwise, the effective value is determined to be greater than or equal to the preset threshold. When the effective value increases, it is determined whether it is less than a second preset threshold; if so, the effective value is determined to be less than the preset threshold; otherwise, the effective value is determined to be greater than or equal to the preset threshold; the second preset threshold is greater than the first preset threshold.

4. The light-load control method for a cascaded multilevel converter according to claim 3, characterized in that, The first preset threshold is the lower limit of the hysteresis comparison of the light load and heavy load boundary values; The second preset threshold is the upper limit of the hysteresis comparison of the light load and heavy load limit values.

5. The light-load control method for a cascaded multilevel converter according to any one of claims 1 to 4, characterized in that, The voltage equalization enabling module includes: a judgment module, and switches corresponding to each of the power units; In the light-load control method, the voltage equalization enabling module disconnects each voltage equalization control loop, including: The judgment module controls each of the switches, using the modulation signal output from the main control loop as the modulation signal for each of the power units.

6. The light-load control method for a cascaded multilevel converter according to any one of claims 1 to 4, characterized in that, The voltage equalization enabling module includes: a judgment module, and a voltage equalization output adjustment module corresponding to each of the power units; In the light-load control method, the voltage equalization enabling module reduces the effect of each voltage equalization control loop, including: The judgment module controls each of the voltage equalization output adjustment modules, and the effect of each voltage equalization control loop changes positively correlated with the grid-connected current.

7. The light-load control method for a cascaded multilevel converter according to claim 6, characterized in that, The adjustment of the voltage equalization control loops varies positively with the grid-connected current, including the following adjustments performed on each power unit: As the grid-connected current changes, the modulation signal output by the corresponding voltage equalization control loop changes in a positive correlation. The modulation signal corresponding to the change in the equalizing control loop is superimposed with the modulation signal output by the main control loop; The superposition result is used as the modulation signal for the corresponding power unit.

8. The light-load control method for a cascaded multilevel converter according to any one of claims 1 to 4, characterized in that, After the voltage equalization enable module determines whether the cascaded multilevel converter is in a light-load state or a standby state based on the grid-connected current, the following steps are also included: If the cascaded multilevel converter is not in a light load state or standby state, the voltage equalization enable module superimposes the modulation signal output by each voltage equalization control loop with the modulation signal output by the main control loop, and uses each superposition result as the modulation signal of the corresponding power unit.

9. The light-load control method for a cascaded multilevel converter according to claim 8, characterized in that, The voltage equalization enabling module includes: a judgment module, and a switch or voltage equalization output adjustment module corresponding to each power unit; In the light load control method, the voltage equalization enabling module superimposes the modulation signal output from each voltage equalization control loop with the modulation signal output from the main control loop, and uses each superposition result as the modulation signal for the corresponding power unit, including the following actions performed on each power unit: The judgment module controls the corresponding switch or the corresponding voltage equalization output adjustment module to use the result of superimposing the modulation signal output by the voltage equalization control loop and the modulation signal output by the main control loop as the modulation signal for the corresponding power unit.

10. A light-load control system for a cascaded multilevel converter, characterized in that, include: Controller and main circuit; The main circuit includes at least two power units, and the AC sides of each power unit are cascaded, with the two ends of the cascaded circuit serving as the AC side of the main circuit. The DC side of each power unit serves as a DC side interface of the main circuit. Each of the power units is controlled by the controller, which is used to perform the light-load control method for the cascaded multilevel converter as described in any one of claims 1 to 9.

11. The light-load control system for the cascaded multilevel converter according to claim 10, characterized in that, The power unit is an H-bridge.

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