Cascade structure current generator, converter submodule test circuit and control system

Abstract

The present invention provides a cascade structure current generator, a converter submodule test circuit and a control system, wherein the test circuit includes a current generator and a measured object, wherein the current generator is based on a cascade structure and is used to control the load current of the measured object, and adopts carrier phase shift or carrier stacking modulation, which can effectively increase the power level of the current generator and reduce the output voltage and current harmonic content. At the same time, a corresponding control system for the test circuit is provided, which can make the load current, voltage and thermal stress of the tested cascade converter submodule consistent with the actual complete cascade converter system working conditions. Compared with the existing method, the present invention can increase the equivalent switching frequency of the current generator by reasonably increasing the number of current generator cascades, thereby reducing the switching frequency and power supply requirements of the devices in the current generator. At the same time, the number of current generator cascades can be flexibly adjusted to adapt to the test requirements of different numbers of tested cascade converter submodules.

Description

Technical Field

[0001] The present invention relates to the technical field of power electronics, and in particular to a current generator and converter submodule test circuit using a cascade structure, and a control system corresponding to the test circuit. Background Art

[0002] Due to their modularity, ease of expansion, and low harmonic content, cascade converters are widely used in medium- and high-voltage applications such as motor drives, HVDC transmission, and static VAR compensators. Because cascade converters contain numerous submodules, and their stable and reliable operation depends largely on their reliability, comprehensive reliability testing of these submodules is essential. In recent years, building simplified test circuits to simulate the operating conditions of the submodule under test in an actual cascade converter and performing reliability assessments based on the submodule's behavior under simulated conditions has become a preferred reliability testing solution for cascade converter submodules.

[0003] Currently, existing test circuits for simulating cascaded converter operating conditions mostly generate test current through a current generator and inject it into the sub-module under test. However, the current generator in these existing test circuits typically consists of a constant-voltage DC source and a half-bridge or full-bridge circuit.

[0004] For example, after searching, it was found that: the Chinese invention patent with patent number ZL202010942352.4 discloses a "current control method for a cascaded sub-module operating condition simulation system suitable for NLC", in which the current generator adopts a constant voltage DC source and a full-bridge structure; the Chinese invention patent with patent number ZL201910976491.6 discloses a "working condition simulation test circuit and method for a cascaded converter sub-module", in which the current generator adopts a constant voltage DC source and a half-bridge structure.

[0005] When a current generator uses a half-bridge structure, it can output positive polarity and zero polarity, for a total of two levels. When a current generator uses a full-bridge structure, it can output positive polarity, negative polarity, and zero polarity, for a total of three levels. However, due to the large number of cascaded modules, an actual cascaded converter may output dozens or even hundreds of levels. Increasing the number of levels significantly reduces current harmonics, specifically by reducing current ripple. Therefore, current ripple in actual cascaded converters is often extremely low. However, as mentioned above, the number of levels that current generators can output in existing test circuits is relatively small, limiting the reduction of current ripple in the test current or increasing the test circuit's requirements for filters and switching frequencies.

[0006] Therefore, there is a need in the art for a new cascaded converter submodule test circuit and corresponding control thereof to reduce the current ripple of the test current or reduce the requirements of the test circuit on filters and switching frequencies. Summary of the Invention

[0007] In response to the above-mentioned problems in the prior art, the present invention proposes a cascade structure current generator and converter submodule test circuit and control system to reduce the current ripple of the test current or reduce the test circuit's requirements for filters and switching frequencies.

[0008] A first aspect of the present invention provides a current generator with a bridge cascade structure, the current generator comprising:

[0009] The bridge inverter module is formed by sequentially connecting n bridge inverters in a forward direction, where n is a natural number greater than or equal to 2;

[0010] a filter, connected in series with the bridge inverter module, for reducing the harmonic content of the output current of the current generator;

[0011] Each bridge inverter uses an independently controllable power supply device, and the DC voltage output by the power supply device generates the required AC voltage under the action of the bridge inverter.

[0012] Optionally, the bridge inverter is a full-bridge or half-bridge inverter. When the current generator includes n half-bridge inverters, the current generator can output n+1 levels. When the current generator includes n full-bridge inverters, the current generator can output 2n+1 levels. Furthermore, the switching pulses of each full-bridge or half-bridge inverter are generated using carrier phase shifting, carrier stacking, or nearest-level approximation modulation to reduce harmonics in the output voltage and current.

[0013] A second aspect of the present invention provides a cascade converter submodule test circuit, comprising:

[0014] A current generator, using the above-mentioned bridge cascade structure current generator, is used to generate the load current required by the cascade converter submodule under test;

[0015] The object under test includes an inverter submodule group and a rectifier submodule group, each submodule group contains one or more forward-cascaded submodules under test, the negative pole of the last submodule under test in the rectifier submodule group is connected to the negative pole of the last submodule under test in the inverter submodule group, the output port of the first submodule under test in the inverter submodule group and the output port of the first submodule under test in the rectifier submodule group are respectively connected to the positive and negative output ports of the current generator, so that the inverter submodule group, the rectifier submodule group and the current generator form a loop.

[0016] Optionally, when the object under test has positive polarity, negative polarity and zero polarity outputs at the same time, a full-bridge inverter is used in the bridge inverter module of the current generator; when the object under test has only positive polarity and zero polarity outputs, a half-bridge inverter is used in the bridge inverter module of the current generator.

[0017] A third aspect of the present invention provides a control system for the above-mentioned cascade converter submodule test circuit, comprising:

[0018] The sampling module samples the test current signal and the voltage signal of the sub-module under test from the test circuit;

[0019] a current controller, configured to control a current generator in the test circuit;

[0020] A voltage controller, configured to control the submodule under test in the test circuit and generate a feedforward voltage signal;

[0021] A cascade converter system parameter model generates a test current reference value signal, a capacitor voltage DC component reference signal, and a modulation voltage reference signal based on the target cascade converter system, and then tracks the test current reference value signal, the capacitor voltage DC component reference signal, and the modulation voltage reference signal through the test current and capacitor voltage in the test circuit to ensure that the current and voltage operating conditions in the test circuit are similar to those of the target cascade converter system, thereby achieving testing of the submodules of the target cascade converter system;

[0022] The current controller receives the current reference value signal generated by the cascade converter system parameter model, the test current signal sampled by the sampling module, and the feedforward voltage signal calculated and generated by the voltage controller, and outputs the control pulse signal required by the bridge inverter module in the current generator;

[0023] The voltage controller receives the modulation voltage reference signal and the capacitor voltage DC component reference signal generated by the cascade converter system parameter model, the test current signal and the voltage signal of the sub-module under test sampled by the sampling module, and outputs the control pulse signal required by the sub-module under test in the object under test.

[0024] Optionally, the current controller generates the control pulse signal required by the bridge inverter module in the current generator based on proportional-integral resonant control or proportional resonant control.

[0025] Optionally, the voltage controller generates the feedforward voltage signal based on the control pulse signal of the measured submodule in the measured object and the capacitor voltage of a single measured submodule, so as to suppress interference of the output voltage of the measured object on the system.

[0026] Furthermore, the current controller generates the control pulse signal required by the bridge inverter module in the current generator in the following manner: the difference between the test current reference value signal generated by the cascade converter system parameter model and the test current sampling signal is input into a proportional, or proportional-integral, or proportional-integral resonant controller, and the feedforward voltage signal is compensated at the output port of the controller, thereby generating the modulation voltage required by the current generator; according to the modulation voltage required by the current generator, carrier phase-shift modulation, carrier stacking modulation, or nearest level approximation modulation is used to generate control pulses for all bridge inverters in the current generator, so as to ultimately enable the current generator to output the output voltage required to generate the test current.

[0027] Furthermore, the voltage controller generates the control pulse signal and the feedforward voltage signal required by the submodule under test in the object under test in the following manner:

[0028] The voltage controller superimposes the output voltage of the capacitor voltage balance control on the modulation voltage reference value, and generates the control pulse of the object under test through the same modulation method as the actual complete cascade converter system; at the same time, the voltage controller generates the feedforward voltage signal based on the control pulse signal of the measured sub-module in the measured object and the capacitor voltage calculation of the single measured sub-module.

[0029] The feedforward voltage signal has the same waveform as the pulse voltage at the port of the measured object. The generation time of the feedforward voltage pulse is calculated by the control pulse of the measured sub-module in the voltage controller, and the amplitude of the feedforward voltage pulse is calculated by the capacitor voltage of the measured sub-module sampled by the voltage controller.

[0030] Optionally, the control pulses of the current generator are generated based on the following modulation methods, wherein:

[0031] Carrier phase shift modulation requires n carrier waves ranging from -1 to 1, with the phases of each carrier wave shifted by 2π / n. By comparing the modulated wave with the n carrier waves, control pulses for n bridge inverters are generated.

[0032] Carrier stacking modulation requires n carriers with the same phase. The range of each carrier is 2 / n. The carriers are stacked from -1 to 1. By comparing the modulated wave with the n carriers, the control pulses of n bridge inverters are generated respectively.

[0033] The nearest level approximation modulation obtains the number of bridge inverter modules that need to be put into operation by rounding the modulation wave, thereby controlling the corresponding number of bridge inverters to be put into operation in the current generator.

[0034] The control system of the test circuit of the present invention can reduce the switching frequency required by the current generator and increase the cutoff frequency of the current generator filter while keeping the test current ripple range unchanged by increasing the number of bridge inverters in the current generator; or reduce the current ripple in the test circuit while keeping the switching frequency of the current generator and the cutoff frequency of the filter unchanged.

[0035] According to a fourth aspect of the present invention, a method for determining parameters of the cascaded converter submodule test circuit is provided, specifically comprising: determining the DC supply voltage in the current generator of the test circuit and the number of bridge inverter modules according to the following method:

[0036] Determining a DC supply voltage required by a bridge inverter in the current generator based on the simulated operating conditions, the number of submodules under test in the object under test, and the inductance of the filter;

[0037] Based on the current ripple calculation formula, determine the relationship between the current ripple range and the required DC supply voltage, the filter inductance, the switching frequency of the current generator, and the number of bridge inverter modules in the current generator;

[0038] The number of bridge inverter modules in the current generator is selected according to the determined relationship to limit the test current ripple to within the required range.

[0039] Compared with the prior art, the embodiments of the present invention have at least one of the following beneficial effects:

[0040] The current generator provided by the present invention adopts a bridge cascade structure and can output more levels, thereby reducing the demand for filters and switching frequencies in the test circuit while keeping the test current ripple size unchanged; or reducing the current ripple of the test current while keeping the demand for filters and switching frequencies unchanged.

[0041] The current generator provided by the present invention adopts a bridge cascade structure, which can reduce the supply voltage on the DC side of a single bridge inverter module when the maximum output voltage of the current generator is constant, thereby reducing the withstand voltage level of the switching device in the bridge inverter module.

[0042] The current generator provided by the present invention adopts a bridge cascade structure with an adjustable number, so that the current generator has good scalability. When the output voltage range required by the current generator changes, the number of bridge inverter modules in the current generator can be flexibly adjusted to meet different voltage output requirements.

[0043] The present invention provides a test circuit in which a current generator based on a cascade structure is used to control the load current of the object under test. Using carrier phase shifting or carrier stacking modulation, this effectively increases the current generator's power level and reduces the output voltage and current harmonic content. The present invention also provides a corresponding control system for the test circuit, ensuring that the load current, voltage, and thermal stress of the tested cascaded converter submodule are consistent with the operating conditions of an actual complete cascaded converter system.

[0044] The test circuit and control system provided by the present invention can increase the equivalent switching frequency of the current generator by rationally increasing the number of cascaded bridge inverters in the current generator, thereby reducing the switching frequency and power supply requirements of the components in the current generator. Furthermore, the number of cascaded bridge inverters in the current generator can be flexibly adjusted to accommodate the testing requirements of different numbers of cascaded converter submodules under test. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Other features, objects and advantages of the present invention will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0046] Figure 1 This is a circuit diagram of a current generator using a bridge cascade structure in a preferred embodiment of the present invention;

[0047] Figure 2 This is a circuit diagram for testing a cascaded converter submodule in a preferred embodiment of the present invention;

[0048] Figure 3 A control system block diagram of a cascaded converter submodule test circuit in a preferred embodiment of the present invention;

[0049] Figure 4 Schematic diagram of the topological structure of the cascade converter submodule test circuit when the submodule under test is a full-bridge submodule in a preferred embodiment of the present invention;

[0050] Figure 5 A control block diagram of a current controller of a cascade converter submodule test circuit using a bridge cascade structure current generator in a preferred embodiment of the present invention;

[0051] Figure 6 A control block diagram of a voltage controller of a cascade converter submodule test circuit using a bridge cascade structure current generator in a preferred embodiment of the present invention;

[0052] In the figure: 1-current generator; 2-measured object; 3-actual complete cascade converter system parameter model; 4-current controller; 5-voltage controller; 6-sampling module. DETAILED DESCRIPTION

[0053] The following is a detailed description of an embodiment of the present invention. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process. It should be noted that those skilled in the art may make various modifications and improvements without departing from the scope of the present invention, and these modifications and improvements fall within the scope of protection of the present invention.

[0054] Figure 1 This is a circuit diagram of a current generator using a bridge cascade structure provided in a preferred embodiment of the present invention. Figure 1 As shown, in this embodiment, a current generator 1 adopting a bridge cascade structure includes a bridge inverter module and a filter, wherein the bridge inverter module is formed by forward cascading n bridge inverters in sequence, where n is a natural number greater than or equal to 2; the filter 13 is connected in series with the bridge inverter module to reduce the harmonic content of the output current of the current generator; each bridge inverter adopts an independently controllable power supply device, and the DC voltage output by the power supply device generates the required AC voltage under the action of the bridge inverter.

[0055] In the above embodiment of the present invention, the current generator 1 adopts a bridge cascade structure, which can output more levels, thereby reducing the demand for filters and switching frequencies in the test circuit while keeping the test current ripple size unchanged; or reducing the current ripple of the test current while keeping the demand for filters and switching frequencies unchanged.

[0056] In some embodiments, filter 13 may employ a filter inductor. Furthermore, the filter inductor's value may be designed based on the test current ripple range, the DC supply voltage, the current generator switching frequency, and the number of bridge inverter modules in the current generator. Of course, in other embodiments, other types of filters may be employed as long as they can reduce the harmonic content of the current generator's output current.

[0057] In some embodiments, the bridge inverter can be a full-bridge or half-bridge inverter. Specifically, each bridge inverter includes a full-bridge or half-bridge inverter 11 and a constant-voltage DC source or DC power supply 12. The DC side of the full-bridge or half-bridge inverter 11 is connected to the DC constant-voltage source or DC power supply 12, and the AC side, under the action of a control pulse, outputs the AC output voltage required to generate the test current. Therefore, when the current generator includes n half-bridge inverters, the current generator can output n+1 levels; when the current generator includes n full-bridge inverters, the current generator can output 2n+1 levels.

[0058] Specifically, the n bridge inverters of the current generator can be powered by, but are not limited to, n independent constant-voltage DC sources or by generating n DC voltages using a transformer and rectifier circuit. More preferably, a preferred solution for generating DC voltage using a transformer and rectifier circuit is to generate multiple phase-shifted AC voltages on the secondary side of the transformer, which are then converted into multiple DC voltages using three-phase or single-phase diode rectifier circuits.

[0059] In the above embodiment of the present invention, the current generator 1 adopts an independently controllable power supply device, wherein the voltage of a single constant voltage DC source is reduced, thereby reducing the withstand voltage requirement of semiconductor devices in the bridge inverter.

[0060] Figure 2 This is a test circuit diagram of a cascade converter submodule using a bridge cascade structure current generator in a preferred embodiment of the present invention; Figure 2 As shown, in this embodiment, the main circuit of the cascade converter submodule test circuit includes a current generator 1 using a bridge cascade structure and a test object 2 composed of submodules.

[0061] The current generator 1 includes a bridge inverter module and a filter, wherein the bridge inverter module is formed by sequentially forward-connecting n bridge inverters, each bridge inverter is connected to a DC power supply 12, and the bridge inverter module is connected in series with a filter 13 composed of an inductor. The specific structure can be seen in the above Figure 1 The illustrated embodiment is illustrative.

[0062] The test object 2 includes a group of inverter submodules 21 and a group of rectifier submodules 22. The inverter submodules and rectifier submodules in these two groups are all test submodules. Each submodule group includes one or more test submodules in a forward cascade connection. The cathode of the last test submodule in the rectifier submodule group is connected to the cathode of the last test submodule in the inverter submodule group. The output port of the first test submodule in the inverter submodule group and the output port of the first test submodule in the rectifier submodule group are respectively connected to the positive and negative output ports of the current generator 1, thereby forming a loop between the inverter submodule group, the rectifier submodule group, and the current generator 1.

[0063] In the above embodiment, when the measured object 2 has positive, negative, and zero polarity outputs, a full-bridge inverter is preferably used in the bridge inverter module. When the measured object 2 has only positive and zero polarity outputs, a half-bridge inverter is preferably used in the bridge inverter module.

[0064] The cascaded converter submodule test circuit in the above embodiment of the present invention uses a cascaded current generator, which reduces the requirements for filters and switching frequencies while keeping the test current ripple size unchanged; or reduces the current ripple of the test current while keeping the requirements for filters and switching frequencies unchanged.

[0065] Figure 3 FIG1 is a control system block diagram of a cascade converter submodule test circuit in a preferred embodiment of the present invention. Figure 3 As shown, the control system in this embodiment is for Figure 2 The control system for a cascade converter submodule test circuit using a bridge-type cascade current generator is shown. The system includes a cascade converter system parameter model 3, a current controller 4, a voltage controller 5, and a sampling module 6. Here, the cascade converter system parameter model 3 is a complete cascade converter system parameter model, and the sampling module 6 is a capacitor voltage sampler for the submodule under test. The sampling module 6 samples the test circuit to obtain a test current signal and a submodule voltage signal under test. The current controller 4 controls the current generator in the test circuit, and the voltage controller 5 controls the submodule under test in the test circuit to generate a feedforward voltage signal. The cascade converter system parameter model 3 is used to generate a test current reference signal, a capacitor voltage DC component reference signal, and a modulation voltage reference signal.

[0066] Specifically, in the control system of the test circuit, refer to Figure 3 As shown, the cascade converter system parameter model 3 generates the current reference value i of the target cascade converter system. test_ref , modulation voltage reference value v _ref And the voltage DC component reference value V sm_ref The reference value is input into the current controller 4 and the voltage controller 5; the current controller 4 receives the current reference value i generated by the cascade converter system parameter model 3 test_ref , the sampled test current signal i test And the voltage controller 5 calculates the generated feedforward voltage v com , the control pulse signal required by the bridge inverter module in the output current generator 4; the voltage controller 5 receives the modulation voltage reference value v generated by the complete cascade converter system parameter model 3 _ref , voltage DC component reference value V sm_ref , the submodule voltage v sampled by the submodule capacitor voltage sampler 6 sm And the sampled test current signal i test , output the control pulse signal required by the sub-module under test in the object under test.

[0067] The control system of the above embodiment of the present invention can ensure that the load current, voltage and thermal stress of the tested cascade converter submodule are consistent with the actual working conditions of the complete cascade converter system.

[0068] In the above embodiment, the current generator 1 adopting the bridge cascade structure is controlled by the current controller 4 , and the sub-module under test is controlled by the voltage controller 5 .

[0069] Current controller 4 receives the test current reference value signal generated by the actual complete cascade converter system parameter model 3, the sampled test current signal, and the feedforward voltage signal generated by voltage controller 5. It then generates control pulses for current generator 1, which utilizes a bridge cascade structure, based on proportional-integral resonant control or proportional resonant control. The voltage feedforward signal is used to suppress interference from the output voltage of the measured object on the system controller. It can be generated in voltage controller 5 based on the control pulses of the measured submodules within the measured object and the capacitor voltage of each measured submodule. Compared to generating the feedforward voltage signal through sampling, this method reduces the delay between the feedforward voltage and the interference voltage and improves the system's control bandwidth.

[0070] The voltage controller 5 receives the modulation voltage reference value of the measured submodule and the capacitor voltage DC component reference value calculated by the complete cascade converter system parameter model, the test current signal sampled from the test circuit, and the submodule capacitor voltage sampled from the test object of the test circuit, and then superimposes the output voltage of the capacitor voltage balance control on the modulation voltage reference value, and generates the switching pulse of the test object through the same modulation method as the actual complete cascade converter system. Further, the capacitor voltage balance control can be: inputting the difference between the capacitor voltage DC component reference signal and the measured submodule voltage signal into a proportional integral controller or a proportional controller, and multiplying the output of the proportional integral controller or the proportional controller with the direction function of the test current signal, and using the product as the output of the capacitor voltage balance control. The direction function of the test current signal is 1 when the test current signal flows into the submodule and is -1 when the test current signal flows out of the submodule. At the same time, the voltage controller 5 calculates the bridge arm voltage based on the test current signal sampled from the test circuit and the submodule capacitor voltage sampled from the test object of the test circuit, and then calculates the feedforward voltage required for generating the current controller.

[0071] In some embodiments, when the current generator includes n bridge inverters, the switching pulses of each full-bridge or half-bridge inverter can be generated by carrier phase shifting, carrier stacking, or nearest level approximation modulation to reduce the harmonics of the output voltage and current. Specifically, carrier phase shift modulation requires n carriers ranging from -1 to 1, with the phase of each carrier staggered by 2π / n. The control pulses of the n bridge inverters are generated by comparing the modulation wave with the n carriers; carrier stacking modulation requires n carriers with the same phase, each carrier ranging from 2 / n, and the carriers are stacked from -1 to 1. The control pulses of the n bridge inverter modules are generated by comparing the modulation wave with the n carriers; the nearest level approximation modulation obtains the number of bridge inverter modules required by rounding the modulation wave, thereby controlling the corresponding number of bridge inverter modules to be put into the current generator.

[0072] Based on the bridge cascade structure of the above-mentioned current generator 1, in some embodiments, by reasonably increasing the number of bridge inverter modules in the current generator 1, the switching frequency required by the current generator 1 can be reduced and the filter cutoff frequency of the current generator 1 can be increased while maintaining the test current ripple range in the test circuit; or the current ripple in the cascade converter submodule test circuit can be reduced while maintaining the switching frequency of the current generator 1 and the cutoff frequency of the filter 13. At the same time, the number of bridge inverter modules in the current generator 1 can be flexibly adjusted according to the test requirements to determine the number of bridge inverters required in the current generator 1. Of course, the above two methods can also be used simultaneously. For example, the number of bridge inverter modules can be determined in the following way: first, the DC supply voltage required by the bridge inverter module in the current generator 1 can be determined based on the simulated working conditions, the number of cascade converter sub-modules under test, and the inductance of the filter inductor. Then, based on the current ripple calculation formula, the relationship between the current ripple range and the DC supply voltage, filter inductor, current generator switching frequency, and the number of bridge inverters in the current generator can be determined. Finally, a reasonable number of bridge inverters in the current generator can be selected to limit the test current ripple to a reasonable range.

[0073] Based on the above test circuit structure, another embodiment of the present invention further provides a method for determining parameters of a cascaded converter submodule test circuit.

[0074] Specifically, the DC supply voltage V of a single bridge inverter in the current generator 1 is gen It can be determined by the following method: when the current generator 1 has n bridge inverters, the rectifier submodule group and the inverter submodule group in the measured object 2 both have n smWhen the sub-module is tested, since the output voltage of the cascaded bridge inverter in the current generator 1 should be at least greater than the output voltage of the filter 13 and the output of the measured object 2, when the sub-module is running under the working condition of the static VAR compensator, the DC supply voltage V gen The following relationships must be met:

[0075]

[0076] Among them, L t is the inductance of the filter, I test is the amplitude of the test current, m is the modulation index of the submodule under test, C is the capacitance of the submodule under test, and ω is the fundamental angular frequency. At the same time, if the switching frequency of the submodule under test is low, it is necessary to appropriately increase V gen To meet the increased supply voltage requirement caused by the capacitor voltage ripple of the sub-module under test.

[0077] As a preferred embodiment, after determining V gen After the value range of , the number n of bridge inverters in the current generator 1 can be determined based on the following method. The relevant factors to be considered in determining the number n of bridge inverters in the current generator include: the maximum value of the test current ripple i ripple_max , the DC supply voltage V of a single bridge inverter in the current generator gen , filter inductor L t , carrier frequency f s Under carrier phase shift modulation, the relationship satisfied by the above variables can be expressed by the formula:

[0078]

[0079] Then in V gen When the value meets the above range, the number n of bridge inverters in the current generator must meet:

[0080]

[0081] Under carrier stacking modulation, the relationship satisfied by the above variables can be expressed by the formula:

[0082]

[0083] Then in V gen When the value meets the above range, the number n of bridge inverter modules in the current generator must meet:

[0084]

[0085] The method of the above embodiment can flexibly adjust the number of bridge inverters according to test requirements.

[0086] As a preferred embodiment, the quilt module in the object under test may be a full-bridge submodule or a half-bridge submodule. Figure 4 This is a schematic diagram of the topology of a cascaded converter submodule test circuit using a bridge-type cascade current generator, provided in a preferred embodiment of the present invention, when the quilt module is a full-bridge submodule. The submodule groups under test are divided into an inverter submodule group 21 and a rectifier submodule group 22. The submodules under test in the inverter and rectifier submodule groups are sequentially connected in forward cascade. The negative electrode of the capacitor of the last submodule 1.n in the inverter submodule group is connected to the negative electrode of the capacitor of the last submodule 2.n in the rectifier submodule group 22 via a wire. The output port of the first submodule under test 2.1 in the inverter submodule group 21 and the output port of the first submodule under test 2.2 in the rectifier submodule group 22 are connected to the positive and negative output ports of the current generator, respectively, thereby forming a loop between the two submodule groups under test and the current generator 1. When the object under test has the above structure, the test current generated by the current generator 1 with multi-level output capability can simultaneously test multiple submodules under test.

[0087] The target cascade converter system in which the submodule under test in the above-mentioned embodiments resides includes, but is not limited to, modular multilevel converters, cascaded H-bridge converters, cascaded static VAR compensators, and cascaded motor drive converters. The submodule under test primarily comprises power semiconductor devices and capacitors. The power semiconductor devices are combined into a full-bridge or half-bridge circuit, with the DC side of the bridge circuit connected to a capacitor. During operation, the submodule under test determines its switching state by controlling the on / off state of the bridge circuit, thereby controlling the submodule's output voltage level.

[0088] In a preferred embodiment, the target cascade converter system parameter model is obtained by performing theoretical calculations, simulation analysis, or experimental recording on the target cascade converter system to obtain the target cascade converter system's voltage and current operating conditions, which serve as reference values ​​for the test circuit. Therefore, as long as the current and voltage of the test circuit stabilize at their final reference values ​​under the control of current controller 4 and voltage controller 5, the operating conditions of the submodule under test in the target cascade converter system can be replicated.

[0089] In some embodiments, the modulation signal for current generator 1 is generated by inputting a test current reference signal generated by an actual complete cascade converter system parameter model 3, a test current signal sampled from a test circuit, and a feedforward voltage signal calculated and generated by a voltage controller 5 into a current controller 4. In current controller 4, the difference between the current reference value and the current sampling signal is input into a proportional, proportional-integral, or proportional-integral resonant controller, and the feedforward voltage signal is compensated at the controller's output port, thereby generating the modulation voltage required by current generator 1. A modulation method is then used to generate control pulses for all bridge inverters in current generator 1.

[0090] As a preferred embodiment, Figure 3 The current controller 4 shown in FIG can be used as Figure 5 In the current controller 4 of this structure, the current controller 4 receives the current reference value i generated by the complete cascade converter system parameter model 3. test_ref , the sampled test current signal i test And the voltage controller 5 calculates and generates the measured object port voltage v com After that, i test_ref with i test The difference is input to the Proportional Integral Resonance controller (PIR), and then the output of the PIR controller is compensated for v com , thereby offsetting the interference of the measured object on the current control loop, and then the output of the PIR controller is combined with the compensation voltage v com The sum of the values ​​is used as the modulation voltage. After the modulation voltage required by the current generator is determined, control pulses for all bridge inverter modules in the current generator are generated based on carrier phase shift modulation, carrier stacking modulation, or nearest level approximation modulation, so that the current generator outputs the output voltage required to generate the test current.

[0091] As a preferred embodiment, Figure 3 The voltage controller 5 shown in FIG. 5 can be used as Figure 6 In the voltage controller 5 of this structure, the voltage controller 5 receives the modulation voltage reference value v generated by the target cascade converter system parameter model 3. _ref , voltage DC component reference value V sm_ref , the submodule voltage v sampled by the submodule capacitor voltage sampler 6 sm And the sampled test current signal i test After that, V sm_ref With v sm The difference is input to the proportional integral controller (PI), and then the output of the proportional integral controller is multiplied by the output of the current direction function to obtain the capacitor voltage balance signal required by the voltage controller 5. Finally, the output of the balance signal is compared with v _ref The modulation voltages of the submodules are obtained by summing them up, and control pulses of the submodules are generated based on the modulation method of the target cascade converter system. Figure 6 Specifically, it is shown that when the test circuit adopts Figure 4 The structure shown is the process of generating control pulses of submodule 1.i and submodule 2.i when the target cascade converter system adopts carrier phase shift modulation.

[0092] As a preferred embodiment, Figure 3 The voltage controller 5 shown in the figure can generate a feed-forward voltage signal by the following method. The function of the feed-forward voltage signal is to offset the interference of the pulse voltage of the measured object port on the system control. Therefore, the feed-forward voltage signal should have the same waveform as the pulse voltage of the measured object port. To this end, the generation time of the feed-forward voltage pulse is calculated by the control pulse of the measured submodule in the voltage controller 5, and the amplitude of the feed-forward voltage pulse is calculated by the capacitor voltage of the measured submodule sampled by the voltage controller 5. Generating the feed-forward voltage by this method can make the delay between the feed-forward voltage and the pulse voltage of the object port smaller than that generated by sampling, thereby achieving higher control accuracy and bandwidth.

[0093] The above embodiment of the present invention, by adopting a bridge cascade structure current generator and the cascade converter submodule test circuit and control system formed thereby, can reduce the test circuit's requirements for the current generator switching frequency and the filter in the current generator while keeping the test current ripple range unchanged in the cascade converter submodule test circuit; or reduce the test current ripple range of the cascade converter submodule test circuit while keeping the current generator switching frequency and the filter in the current generator unchanged in the cascade converter submodule test circuit. At the same time, if the DC power supply voltage required when using a full bridge or half bridge is V t , then after using n full-bridge or half-bridge cascades to form a current generator, the single DC power supply voltage requirement is V t Because the target cascade converter often has many submodules and low current ripple, the embodiments of the present invention can reduce the current ripple of the test current by increasing the number of levels of the current generator in the test circuit, thereby making the test current closer to the bridge arm current of the actual system. Alternatively, within the same current ripple range, the test circuit's requirements for switching frequency and filters can be reduced, thus having great application value.

[0094] The above describes specific embodiments of the present invention. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various variations or modifications within the scope of the claims, without affecting the essence of the present invention. The technical features of the above preferred embodiments may be used in any combination, provided that they do not conflict with each other.

Claims

1. A control system for a cascaded converter submodule test circuit, characterized in that: The cascade converter submodule test circuit includes: A current generator for generating the load current required by the cascaded converter submodule under test; the current generator comprises: a bridge inverter module composed of n bridge inverters cascaded in a forward direction, where n is a natural number greater than or equal to 2; a filter connected in series with the bridge inverter module for reducing the harmonic content of the output current of the current generator; each bridge inverter employs an independently controllable power supply device, the DC voltage output by the power supply device generating the required AC voltage under the action of the bridge inverter; The measured object includes an inverter submodule group and a rectifier submodule group, each submodule group includes one or more forward-connected measured submodules, the cathode of the last measured submodule in the rectifier submodule group is connected to the cathode of the last measured submodule in the inverter submodule group, the output port of the first measured submodule in the inverter submodule group and the output port of the first measured submodule in the rectifier submodule group are respectively connected to the positive and negative output ports of the current generator, so that the inverter submodule group, the rectifier submodule group and the current generator form a loop; the measured submodule includes a full-bridge submodule or a half-bridge submodule; The control system comprises: The sampling module samples the test current signal and the voltage signal of the sub-module under test from the test circuit; a current controller, configured to control a current generator in the test circuit; A voltage controller, configured to control the submodule under test in the test circuit and generate a feedforward voltage signal; A cascade converter system parameter model generates a test current reference value signal, a capacitor voltage DC component reference signal, and a modulation voltage reference signal based on the target cascade converter system, and then tracks the test current reference value signal, the capacitor voltage DC component reference signal, and the modulation voltage reference signal through the test current and capacitor voltage in the test circuit to ensure that the current and voltage operating conditions in the test circuit are similar to those of the target cascade converter system, thereby achieving testing of the submodules of the target cascade converter system; The current controller receives the current reference value signal generated by the cascade converter system parameter model, the test current signal sampled by the sampling module, and the feedforward voltage signal calculated and generated by the voltage controller, and outputs the control pulse signal required by the bridge inverter module in the current generator; The voltage controller receives the modulation voltage reference signal and the capacitor voltage DC component reference signal generated by the cascade converter system parameter model, the test current signal and the voltage signal of the sub-module under test sampled by the sampling module, and outputs the control pulse signal and the feedforward voltage signal required by the sub-module under test in the object under test.

2. The control system for the cascaded converter submodule test circuit according to claim 1, characterized in that: The bridge inverter is a full-bridge or half-bridge inverter; When the current generator includes n half-bridge inverters, the current generator can output n+1 levels; When the current generator includes n full-bridge inverters, the current generator can output 2n+1 levels.

3. The control system for the cascaded converter submodule test circuit according to claim 2, characterized in that: For each bridge inverter, the switching pulses are generated by carrier phase shifting, carrier stacking, or nearest level approximation modulation.

4. The control system for the cascaded converter submodule test circuit according to claim 1, characterized in that: When the object under test has positive polarity, negative polarity and zero polarity output at the same time, a full-bridge inverter is used in the bridge inverter module of the current generator; When the measured object has only positive polarity and zero polarity outputs, a half-bridge inverter is used in the bridge inverter module of the current generator.

5. The control system for the cascade converter submodule test circuit according to claim 1, characterized in that: Also includes at least one of the following options: - the current controller generates the control pulse signal required by the bridge inverter module in the current generator through proportional-integral resonant control or proportional resonant control based on the test current signal reference value signal output by the cascade converter system parameter model; -The voltage controller generates a control pulse signal for the sub-module of the object under test through proportional-integral control based on the capacitor voltage DC component reference signal and the modulation voltage reference signal output by the cascade converter system parameter model, and calculates and generates the feedforward voltage signal based on the control pulse signal of the sub-module under test in the object under test and the capacitor voltage signal of a single sub-module under test, so as to suppress the interference of the output voltage of the object under test on the system.

6. The control system for the cascade converter submodule test circuit according to claim 5, characterized in that: The current controller generates the control pulse signal required by the bridge inverter module in the current generator in the following manner: Inputting the difference between the current reference value signal generated by the cascade converter system parameter model and the test current signal into a proportional, proportional-integral, or proportional-integral resonant controller, and compensating the feedforward voltage signal at the output port of the controller to generate the modulation voltage required by the current generator; According to the modulation voltage required by the current generator, carrier phase shift modulation, carrier stacking modulation or nearest level approximation modulation is used to generate control pulses for all bridge inverters in the current generator, so as to ultimately make the current generator output the output voltage required to generate the test current.

7. The control system for the cascaded converter submodule test circuit according to claim 5, characterized in that: The voltage controller generates the control pulse signal and the feedforward voltage signal required by the submodule under test in the object under test in the following manner: The voltage controller superimposes the output voltage of the capacitor voltage balance control on the modulation voltage reference value, and generates a control pulse of the object under test through the same modulation method as the actual complete cascade converter system; at the same time, the voltage controller calculates and generates the feedforward voltage signal based on the control pulse signal of the measured submodule in the measured object and the capacitor voltage signal of the single measured submodule, wherein: The capacitor voltage balance control is specifically as follows: inputting the difference between the capacitor voltage DC component reference signal and the voltage signal of the submodule under test of the submodule into a proportional-integral controller or a proportional controller, and multiplying the output of the proportional-integral controller or the proportional controller by the direction function of the test current signal, and using the product as the output of the capacitor voltage balance control, wherein the direction function of the test current signal is 1 when the test current signal flows into the submodule and is -1 when the test current signal flows out of the submodule; The feedforward voltage signal has the same waveform as the pulse voltage at the port of the measured object. The generation time of the feedforward voltage pulse is calculated by the control pulse of the measured sub-module in the voltage controller, and the amplitude of the feedforward voltage pulse is calculated by the capacitor voltage signal of the measured sub-module sampled by the voltage controller.

8. The control system for the cascade converter submodule test circuit according to claim 6, characterized in that: The control pulses of the current generator are generated based on the following modulation methods, wherein: Carrier phase shift modulation requires n carrier waves ranging from -1 to 1, with the phases of each carrier wave shifted by 2π / n. By comparing the modulated wave with the n carrier waves, control pulses for n bridge inverters are generated. Carrier stacking modulation requires n carriers with the same phase. The range of each carrier is 2 / n. The carriers are stacked from -1 to 1. By comparing the modulated wave with the n carriers, the control pulses of n bridge inverters are generated respectively. The nearest level approximation modulation obtains the number of bridge inverter modules that need to be put into operation by rounding the modulation wave, thereby controlling the corresponding number of bridge inverters to be put into operation in the current generator.

9. The control system for the cascaded converter submodule test circuit according to claim 1, characterized in that: By increasing the number of bridge inverters in the current generator, the switching frequency required for the current generator can be reduced and the cutoff frequency of the current generator filter can be increased while the test current ripple range of the test circuit remains unchanged; and / or, the current ripple in the test circuit can be reduced while the switching frequency of the current generator and the cutoff frequency of the filter remain unchanged.

10. A method for determining parameters of a control system of a cascaded converter submodule test circuit according to claim 1, characterized in that: The cascade converter submodule test circuit, the DC supply voltage in the current generator, and the number of bridge inverter modules are determined according to the following method: Determining a DC supply voltage required by a bridge inverter in the current generator based on the simulated operating conditions, the number of submodules under test in the object under test, and the inductance of the filter; Based on the current ripple calculation formula, determine the relationship between the current ripple range and the required DC supply voltage, the filter inductance, the switching frequency of the current generator, and the number of bridge inverter modules in the current generator; The number of bridge inverter modules in the current generator is selected according to the determined relationship to limit the test current ripple to within the required range.

11. The method for determining parameters of a control system of a cascaded converter submodule test circuit according to claim 10, characterized in that: When the submodule under test operates in the working condition of the static VAR compensator, the DC supply voltage V gen The following relationship is satisfied: Among them, L t is the inductance of the filter inductor, I test is the amplitude of the test current, m is the modulation index of the submodule under test, C is the capacitance of the submodule under test, and ω is the fundamental angular frequency. At the same time, if the switching frequency of the submodule under test is low, V gen To meet the increased supply voltage requirement caused by the capacitor voltage ripple of the sub-module under test.

12. The method for determining parameters of a control system of a cascaded converter submodule test circuit according to claim 10, characterized in that: The number n of bridge inverter modules in the current generator is determined based on the following factors: Test current ripple maximum value i ripple_max , the DC supply voltage V of a single bridge inverter module in the current generator gen , filter inductor L t , carrier frequency f s ; -Under carrier phase shift modulation, the relationship satisfied by the above variables is expressed by the following formula: Then in V gen When the value satisfies the selection range of the full-bridge inverter or the half-bridge inverter in the current generator, the number n of bridge inverters in the current generator satisfies: - Under carrier stacking modulation, the relationship satisfied by the above variables is expressed by the formula: Then in V gen When the value satisfies the selection range of the full-bridge inverter or the half-bridge inverter in the current generator, the number n of bridge inverters in the current generator must meet the following requirements:

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