Modularized multi-level current converter with harmonic suppression capability and control method of modularized multi-level current converter

Abstract

The invention discloses a modular multilevel converter with harmonic suppression capability and a control method of the modular multilevel converter. The modular multilevel converter comprises a bridge arm series submodule used for normal operation and an additional redundant submodule. In the operation process, the expected alternating-current side harmonic voltage is output through switching control of the redundant submodules, and the fundamental frequency active power and reactive power transmission characteristics of the converter are not affected while the alternating-current side harmonic suppression function is provided. Based on the functional composite modular multilevel converter, an active filter can be prevented from being independently arranged, and the engineering investment cost can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of power transmission and distribution of electric power systems, and specifically relates to a modular multi-level converter with harmonic control capability and a control method thereof. Background Art

[0002] The harmonic control problem is an important factor affecting the operation performance and operating cost of the power grid. In the traditional power system based on synchronous power supply and industrial frequency AC large power grid, the harmonic current of the power grid is mainly introduced by nonlinear loads, and the main means of controlling harmonic current is to configure passive resonant filters. In modern power systems, the proportion of renewable energy power generation continues to increase, and new transmission methods such as DC transmission and low-frequency transmission play an important role in scenarios such as large-capacity and long-distance power transmission. The operation mode of the power system is flexible and gradually shows a high degree of power electronics. This means that the harmonic content of modern power systems is higher and the harmonic frequency components are more complex. At this time, the filtering effect of passive resonant filters will be greatly reduced.

[0003] Compared with passive filters, active filters based on power electronic devices have the ability to filter out multiple harmonics at the same time, are highly controllable, and have broader application prospects in modern power systems. Among them, active filters using cascaded multilevel converter topology have technical advantages such as high modularity, good output harmonic characteristics, and low device switching frequency, and have greater development potential in high-voltage and large-capacity scenarios. However, cascaded multilevel active filters are composed of a large number of power electronic switching devices and sub-module capacitors, and the investment cost of setting them up separately is relatively high. In actual projects, they are usually used in conjunction with passive filters.

[0004] It should be noted that the cascaded multilevel active filter has a very similar topology to the modular multilevel AC / DC converter commonly used in flexible DC transmission projects and the modular multilevel matrix AC / AC converter used in flexible low-frequency transmission projects. The cascaded multilevel converter topology is also used to construct dynamic reactive power compensation equipment for AC power grids. It can be considered that the above converter scheme is the specific functional performance of the modular multilevel converter topology in different application scenarios. The cascaded multilevel active filter and the dynamic reactive power compensation equipment provide harmonic control functions and dynamic reactive power support for the AC power grid respectively. The modular multilevel AC / DC converter and the modular multilevel matrix AC / AC converter mainly play the role of transmitting active power between the power frequency main grid and the DC / low-frequency transmission system. In fact, the cascaded multilevel dynamic reactive power compensation equipment, the modular multilevel AC / DC converter and the modular multilevel matrix AC / AC converter also have the ability to provide harmonic control functions for the AC power grid, and this ability can avoid the need to set up active filters separately, saving equipment investment.

[0005] So far, most of the published literature has basically only considered the independent device configuration scheme when studying the active filter design method, and rarely considered how to embed the filtering capability into the above-mentioned existing grid converter equipment. Considering the impact of harmonic voltage / current on converter parameters and control system design, designing a functional composite converter scheme is an effective way to improve engineering economy, so it is necessary to conduct further research on this. Summary of the invention

[0006] In view of the problem of harmonic control in the existing power grid, the present invention provides a modular multilevel converter with harmonic control capability and a control method thereof, so that the functional multiplexing modular multilevel converter has harmonic control capability to avoid the need for a separate active filter, which is beneficial to reducing project investment costs.

[0007] To this end, the present invention adopts the following technical solution.

[0008] In a first aspect, the present invention provides a modular multilevel converter with harmonic control capability, comprising a bridge arm series connection submodule for normal operation and an additional redundant submodule; The number of redundant submodules is calculated according to the following method: Step 1: According to the short-circuit impedance percentage of the converter transformer U k % and bridge arm reactor inductance L a Calculate the equivalent inductance of the converter valve side L eq , the converter transformer is used to connect the modular multi-level converter and the converter station AC bus:

[0009] in, f N Indicates the rated operating frequency of the AC power grid; U vN Indicates the rated voltage of the valve-side winding of the converter transformer. S N Indicates the rated capacity of the converter transformer; y Indicates the conversion coefficient of the converter bridge arm inductance to the valve side equivalent inductance; Step 2: Determine the maximum value of the harmonic voltage output by the converter bridge arm in combination with the converter harmonic current control capacity:

[0010] in, U vh,max Indicates the maximum value of the harmonic voltage output by the converter bridge arm; j Indicates the harmonic order; j maxIndicates the maximum value of the harmonic order; U shj,max Indicates the inverter grid connection point j The maximum value of subharmonic voltage amplitude, K Indicates the voltage ratio of the converter transformer grid / valve side winding; I vhj,max Indicates the maximum value of the harmonic current amplitude output on the converter valve side; Step 3: Calculate the number of redundant submodules required for the bridge arm based on the bridge arm harmonic voltage modulation requirements. N r :

[0011] in, M h,max Indicates the maximum permissible modulation degree of the harmonic voltage output by the converter bridge arm; U cN Indicates the rated voltage of the bridge arm series submodule capacitor.

[0012] Furthermore, in step 1, when the modular multilevel converter is a modular multilevel AC / DC converter y Equal to 1 / 2, when the modular multilevel converter is a modular multilevel matrix AC / AC converter y Equal to 1 / 3.

[0013] Furthermore, according to different converter arm connection modes, applicable topological forms of the modular multilevel converter include star, triangle, double star and matrix.

[0014] Furthermore, according to whether there is a polarity change in the bridge arm output voltage, the bridge arm series submodules of the modular multi-level converter adopt a bipolar submodule topology or a unipolar submodule topology.

[0015] In a second aspect, the present invention provides a control method for the above-mentioned modular multilevel converter, which outputs the desired AC side harmonic voltage through redundant sub-module switching control during operation, while providing the AC side harmonic control function without affecting the converter's fundamental frequency active power and reactive power transmission characteristics.

[0016] Furthermore, the converter control system will obtain the harmonic current measurement values ​​of other incoming lines of the converter grid connection point through real-time detection. I hj As the reference value of the harmonic current output on the AC side of the converter, the closed-loop control system adjusts the harmonic voltage output by the converter bridge arm to make the output harmonic current on the AC side of the converter I shj The harmonic current is equal in magnitude and opposite in phase to the harmonic current of other incoming lines at the grid connection point.

[0017] Furthermore, the harmonic current on the AC side is a harmonic current of a single frequency or a harmonic current containing multiple frequency components.

[0018] Furthermore, when the inverter bridge arm series submodule fails and exits operation, in order to meet the normal power transmission requirements of the power transmission system, the switching control of the redundant submodule converts some redundant submodules into bridge arm series submodules for synthesizing the fundamental frequency components of the bridge arm output voltage, giving priority to meeting the requirements of the fundamental frequency components in the bridge arm output voltage. At this time, the harmonic output capacity of the converter AC side decreases, but it still has the harmonic control capability.

[0019] Based on the above technical solution, the present invention has the following beneficial effects: the present invention enables the functional multiplexing modular multi-level converter to have harmonic control capability, which can avoid the need to separately set up an active filter, thereby helping to reduce project investment costs and improve project economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the topology structure of a modular multi-level converter; Figure 2 It is a schematic diagram of the topological structure of the bridge arm series submodules; Figure 3 It is a schematic diagram of a modular multi-level converter control method of the present invention; Figure 4 Schematic diagram of the simulation model of the present invention. DETAILED DESCRIPTION

[0021] In order to describe the present invention more specifically, the technical solution of the present invention is described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0022] Figure 1 Schematic diagram of modular multilevel converter topology. The modular multilevel converter uses bridge arms as basic structural units, and each converter bridge arm includes bridge arm reactors and series submodules. According to different converter bridge arm connection methods, the specific topological forms applicable to the modular multilevel converter include but are not limited to star type (see Figure 1 (a)), triangle (see Figure 1 (b)), binary star type (see Figure 1 (c)), matrix type (see Figure 1 (d) etc. Among them, star connection and triangle connection are often seen in active filters and dynamic reactive compensation equipment using cascaded multilevel topology; double star connection is the standard topology of modular multilevel AC / DC converters in flexible DC transmission projects; the representative of matrix topology is the modular multilevel AC / AC matrix converter with great development potential in the current flexible low-frequency transmission system.

[0023] Figure 2Figure 1 is a schematic diagram of the topological structure of the bridge arm series submodule. Depending on whether the bridge arm output voltage has a polarity change, the bridge arm series submodule of the modular multilevel converter can adopt a bipolar submodule topology or a unipolar submodule topology. The bipolar submodule topology is represented by the full-bridge submodule, which is characterized by a three-level voltage output capability: positive U c , 0, negative U c ,See Figure 2 (a). The unipolar submodule topology is represented by the half-bridge submodule, and the submodule output voltage only contains the positive U c and 0 levels, see Figure 2 (b). The bridge arm submodules of cascaded multilevel active filters and modular multilevel AC / AC converters usually adopt bipolar submodule topology; the bridge arm submodules of modular multilevel AC / DC converters generally adopt unipolar submodule topology, and according to actual needs, unipolar submodules and bipolar submodules can also be used at the same time to build a hybrid converter solution.

[0024] Figure 3 The schematic diagram of the modular multilevel converter control method is shown in FIG. The converter control system uses the harmonic current measurement values ​​of other incoming lines of the converter grid connection point obtained through real-time detection. I hj As the reference value of the harmonic current output on the AC side of the converter, the closed-loop control system adjusts the harmonic voltage output by the converter bridge arm so that the harmonic current output on the AC side of the converter I shj The harmonic currents of other incoming lines at the grid connection point are equal in magnitude and opposite in phase. According to design requirements, the harmonic current on the AC side to be detected and controlled can be a single-frequency harmonic current or a harmonic current containing multiple frequency components. The harmonic voltage output by the converter bridge arm is mainly synthesized by the redundant submodule, while the power frequency component is mainly borne by the bridge arm series submodule. Figure 3 The equivalent circuit in further illustrates the method of configuring the number of redundant sub-modules.

[0025] Step 1: According to the converter transformer short-circuit impedance percentage U k % and bridge arm reactor inductance L a Calculate the equivalent inductance of the converter valve side L eq , the converter transformer is used to connect the modular multi-level converter and the converter station AC bus: (1) in, f N Indicates the rated operating frequency of the AC power grid;U vN Indicates the rated voltage of the valve-side winding of the converter transformer. S N Indicates the rated capacity of the converter transformer; y Indicates the conversion coefficient of the converter bridge arm inductance to the valve side equivalent inductance, y The value of depends on the specific converter topology. For modular multi-level AC / DC converters, y Equal to 1 / 2, for modular multi-level matrix AC / AC converter y Equal to 1 / 3.

[0026] Step 2: Determine the maximum value of the harmonic voltage output by the converter bridge arm based on the converter harmonic current control capacity.

[0027] (2) in, U vh,max Indicates the maximum value of the harmonic voltage output by the converter bridge arm; j Indicates the harmonic order; j max Indicates the maximum value of the harmonic order; U shj,max Indicates the inverter grid connection point j The maximum value of subharmonic voltage amplitude, K Indicates the voltage ratio of the converter transformer grid / valve side winding; I vhj,max Indicates the maximum value of the harmonic current amplitude output on the converter valve side.

[0028] Step 3: Calculate the number of redundant submodules required for the bridge arm based on the bridge arm harmonic voltage modulation requirements N r : (3) in, M h,max Indicates the maximum permissible modulation degree of the harmonic voltage output by the converter bridge arm; U cN Indicates the rated voltage of the bridge arm series submodule capacitor.

[0029] When the bridge arm series submodule fails and exits operation, in order to meet the normal power transmission requirements of the transmission system, some redundant submodules will be converted into bridge arm series submodules to synthesize the fundamental frequency component of the bridge arm output voltage. Therefore, in this case, the number of redundant submodules that can be used to synthesize the bridge arm output harmonic voltage is reduced, the amplitude variation range of the bridge arm output harmonic voltage is reduced, and the harmonic output capacity of the converter AC side is reduced. However, it is worth mentioning that as long as there are still redundant submodules in the converter bridge arm, the converter theoretically retains a certain harmonic control capability.

[0030] The following are specific applications of the present invention.

[0031] Consider the fifth harmonic voltage U on the grid side of a certain system sh5 The content is 0.9%, the 7th harmonic voltage U sh7 The content is 0.35%, the 5th current harmonic I sh5 is 75A, 7th harmonic current I sh7 The current is 35A; the low-frequency converter adopts a modular multi-level matrix converter with a converter voltage level of 64kV. The number of full-bridge sub-modules in each bridge arm is 56 (excluding redundancy) according to conventional design. The voltage levels of the power frequency side and the low-frequency side grid are both 220kV. The rated capacity of the AC converter is set to 300MW, and the inductance value of the bridge arm reactor is 15mH; the rated capacity of the converter transformer is set to 330MVA, the rated voltage on the power frequency valve side is set to 64kV, and the short-circuit impedance percentage is 15%.

[0032] Consider using modular multi-level matrix converter to filter the third and fifth harmonic currents.

[0033] First, calculate the equivalent inductance of the converter valve side according to formula (1): L eq , combined with the inductance of the bridge arm reactor, we can calculate L eq About 0.0109H.

[0034]

[0035] Then, the requirement for the output harmonic voltage of the converter bridge arm is determined according to formula (2).

[0036]

[0037] Finally, calculate the number of redundant submodules required for the bridge arm N r Here, the maximum allowable modulation index of the harmonic voltage output by the converter bridge arm is taken as M h,max 0.95, the bridge arm series submodule capacitor voltage rating UcN is 2.15kV. The calculation results according to formula (3) show that at this time N r Desirable:

[0038] Therefore, the number of sub-modules used for harmonic control is set to 6. At this time, the total number of sub-modules in the entire bridge arm is set to 62, including 56 effective modules and 6 harmonic control and redundant modules. The harmonic control modules account for 9.4% of the number of bridge arm modules, which has a more significant economic advantage compared to setting up an active filter alone.

[0039] According to the above parameters, modeling and simulation are carried out based on PSCAD / EMTDC. The simulation model is as follows Figure 4 As shown in the figure, the 5th and 7th harmonics are injected at other incoming lines, and the active filtering function of the modular multilevel converter is unlocked at 3 seconds. The simulation results show that there are large 5th and 7th harmonics in the system 3 seconds ago, and the harmonics are filtered out after 3s, which is basically close to a sine wave.

[0040] It is obvious that those skilled in the art can easily make various modifications to the above embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art to the present invention based on the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A modular multilevel converter with harmonic control capability, characterized in that: It includes bridge arm series submodules for normal operation and additional redundant submodules; The number of redundant submodules is calculated according to the following method: Step 1: According to the short-circuit impedance percentage of the converter transformer U k % and bridge arm reactor inductance L a Calculate the equivalent inductance of the converter valve side L eq , the converter transformer is used to connect the modular multi-level converter and the converter station AC bus: ， in, f N Indicates the rated operating frequency of the AC power grid; U vN Indicates the rated voltage of the valve-side winding of the converter transformer. S N Indicates the rated capacity of the converter transformer; y Indicates the conversion coefficient of the converter bridge arm inductance to the valve side equivalent inductance; Step 2: Determine the maximum value of the harmonic voltage output by the converter bridge arm in combination with the converter harmonic current control capacity: ， in, U vh,max Indicates the maximum value of the harmonic voltage output by the converter bridge arm; j Indicates the harmonic order; j max Indicates the maximum value of the harmonic order; U shj,max Indicates the inverter grid connection point j The maximum value of subharmonic voltage amplitude, K Indicates the voltage ratio of the converter transformer grid / valve side winding; I vhj,max Indicates the maximum value of the harmonic current amplitude output on the converter valve side; Step 3: Calculate the number of redundant submodules required for the bridge arm based on the bridge arm harmonic voltage modulation requirements. N r : ， in, M h,max Indicates the maximum permissible modulation degree of the harmonic voltage output by the converter bridge arm; U cN Indicates the rated voltage of the bridge arm series submodule capacitor.

2. A modular multilevel converter with harmonic control capability according to claim 1, characterized in that: In step 1, when the modular multilevel converter is a modular multilevel AC / DC converter y Equal to 1 / 2, when the modular multilevel converter is a modular multilevel matrix AC / AC converter y Equal to 1 / 3.

3. The modular multilevel converter with harmonic control capability according to claim 1, characterized in that: According to different converter bridge arm connection modes, applicable topological forms of the modular multilevel converter include star, triangle, double star and matrix.

4. The modular multilevel converter with harmonic control capability according to claim 1, characterized in that: According to whether the bridge arm output voltage has a polarity change, the bridge arm series submodules of the modular multi-level converter adopt a bipolar submodule topology or a unipolar submodule topology.

5. The control method of the modular multilevel converter according to any one of claims 1 to 4, characterized in that: During operation, the redundant submodules are switched on and off to output the desired AC side harmonic voltage, which provides the AC side harmonic control function without affecting the converter base frequency active power and reactive power transmission characteristics.

6. The control method according to claim 5, characterized in that: The converter control system will obtain the harmonic current measurement values ​​of other incoming lines at the converter grid connection point through real-time detection. I hj As the reference value of the harmonic current output on the AC side of the converter, the closed-loop control system adjusts the harmonic voltage output by the converter bridge arm to make the output harmonic current on the AC side of the converter I shj The harmonic current is equal in magnitude and opposite in phase to the harmonic current of other incoming lines at the grid connection point.

7. The control method according to claim 6, characterized in that: The harmonic current on the AC side is a harmonic current of a single frequency or a harmonic current containing multiple frequency components.

8. The control method according to claim 5, characterized in that: When the inverter bridge arm series submodule fails and exits operation, in order to meet the normal power transmission requirements of the power transmission system, the switching control of the redundant submodule converts some redundant submodules into bridge arm series submodules to synthesize the fundamental frequency components of the bridge arm output voltage, giving priority to meeting the requirements of the fundamental frequency components in the bridge arm output voltage. At this time, the harmonic output capacity of the inverter AC side decreases, but it still has the harmonic control capability.

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