A modular multilevel converter with harmonic control capability and control method thereof

Abstract

The present invention discloses a modular multilevel converter with harmonic control capability and a control method thereof, wherein the modular multilevel converter comprises a bridge arm series submodule for normal operation and an additional redundant submodule; during operation, the redundant submodule is switched on and off to control the desired AC side harmonic voltage, and the base frequency active power and reactive power transmission characteristics of the converter are not affected while providing the AC side harmonic control function. The present invention is based on a functional composite modular multilevel converter, and can avoid the need to separately set an active filter, which is beneficial to reducing engineering investment costs.

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, the vast majority of the published literature has basically only considered the independent device configuration scheme when studying the design method of active filters, and rarely considered how to embed the filtering ability into the existing grid converter devices mentioned above. Considering the influence of harmonic voltage / current on the converter parameters and control system design, designing a functional composite converter scheme is an effective way to improve the engineering economy, so it is necessary to conduct further research on this. Summary of the Invention

[0006] Aiming at the existing problems of power grid harmonic governance, the present invention provides a modular multilevel converter with harmonic governance ability and its control method, enabling the functional reuse modular multilevel converter to have harmonic governance ability, so as to avoid setting up an active filter separately, which is beneficial to reducing the engineering investment cost.

[0007] For this reason, the present invention adopts the following technical solutions.

[0008] In the first aspect, the present invention provides a modular multilevel converter with harmonic governance ability, including series sub-modules of bridge arms for normal operation and additional redundant sub-modules;

[0009] The number of the redundant sub-modules is calculated according to the following method:

[0010] Step 1, according to the short-circuit impedance percentage of the converter transformer U k % and the inductance value of the bridge arm reactor L a calculate the equivalent inductance on the valve side of the converter L eq , and the converter transformer is used to connect the modular multilevel converter and the AC bus of the converter station:

[0011]

[0012] Among them, f N represents the rated operating frequency of the AC power grid; U vN represents the rated voltage of the valve side winding of the converter transformer, S N represents the rated capacity of the converter transformer; y represents the conversion coefficient of the converter bridge arm inductance converted to the equivalent inductance on the valve side;

[0013]

[0014]

[0015] U Among them, U vh,maxrepresents the maximum amplitude of the harmonic voltage output by the converter bridge arm; the subscript j represents the harmonic order; j max represents the maximum value of the harmonic order; U shj,max represents the connection point of the converter j the maximum value of the harmonic voltage amplitude of the K represents the voltage transformation ratio of the network / valve side windings of the converter transformer; I vhj,max represents the maximum amplitude of the harmonic current output by the converter valve side;

[0016] Step 3: Combine the requirements for the modulation degree of the bridge arm harmonic voltage and calculate the number of redundant sub-modules required for the bridge arm N r :

[0017]

[0018] wherein, M h,max represents the maximum allowable modulation degree of the harmonic voltage output by the converter bridge arm; U cN represents the rated value of the capacitor voltage of the series sub-module of the bridge arm.

[0019] Further, in Step 1, when the modular multilevel converter is a modular multilevel AC / DC converter y is equal to 1 / 2, and when the modular multilevel converter is a modular multilevel matrix-type AC / AC converter y is equal to 1 / 3.

[0020] Further, according to different connection methods of the converter bridge arms, the applicable topological forms of the modular multilevel converter include star, triangle, double-star, and matrix types.

[0021] Further, according to whether there is a polarity change in the output voltage of the bridge arm, the series sub-module of the bridge arm of the modular multilevel converter adopts a bipolar sub-module topology or a unipolar sub-module topology.

[0022] In a second aspect, the present invention provides a control method for the above-mentioned modular multilevel converter, which outputs the desired harmonic voltage on the AC side through redundant sub-module switching control during operation, and does not affect the fundamental frequency active power and reactive power transmission characteristics of the converter while providing the AC side harmonic governance function.

[0023] Further, the converter control system will use the harmonic current measurement values of other incoming lines at the connection point of the converter obtained through real-time detection I hjAs the reference value of the harmonic current output on the AC side of the converter, the harmonic voltage output of the converter arm is regulated through a closed-loop control system, so that the harmonic current output on the AC side of the converter I shj is equal in magnitude and opposite in phase to the harmonic currents of other incoming lines at the point of common coupling.

[0024] Furthermore, the harmonic current on the AC side is a harmonic current of a single frequency or a harmonic current containing multiple frequency components.

[0025] Furthermore, when a series sub-module of the converter arm fails and exits the operation, to meet the normal power transmission requirements of the power transmission system, the switching control of the redundant sub-modules converts some redundant sub-modules into series sub-modules of the arm, which are used to synthesize the fundamental frequency component of the arm output voltage, and preferentially meet the requirements of the fundamental frequency component in the arm output voltage. At this time, the harmonic output capacity on the AC side of the converter decreases, but it still has the ability to control harmonics.

[0026] Based on the above technical solutions, the present invention has the following beneficial effects: The present invention enables the function-reuse modular multilevel converter to have the ability to control harmonics, can avoid setting up an active filter separately, is conducive to reducing the engineering investment cost, and improving the engineering economy. Description of the Drawings

[0027] Figure 1 is a schematic diagram of the topology structure of the modular multilevel converter;

[0028] Figure 2 is a schematic diagram of the topology structure of the series sub-module of the arm;

[0029] Figure 3 is a schematic diagram of the principle of the control method of the modular multilevel converter of the present invention;

[0030] Figure 4 is a schematic diagram of the simulation model of the present invention. Detailed Embodiments

[0031] To describe the present invention more specifically, the technical solutions of the present invention will be described in detail below with reference to the drawings and specific embodiments.

[0032] Figure 1 is a schematic diagram of the topology structure of the modular multilevel converter. The modular multilevel converter takes the arm as the basic structural unit, and each converter arm includes an arm reactor and a series sub-module. According to different connection methods of the converter arms, the specific topology forms applicable to the modular multilevel converter include but are not limited to star (see (a) of Figure 1 ), delta (see (b) of Figure 1 ), double-star (see (c) of Figure 1 ), matrix (see (d) of Figure 1such as (d)). Among them, the star connection method and the delta connection method are more common in active filters and dynamic reactive power compensation devices that adopt cascaded multilevel topologies; the double-star connection method is the standard topology of the modular multilevel AC / DC converter in flexible DC transmission projects; the representative of the matrix topology is the modular multilevel AC / AC matrix converter that has great development potential in current flexible low-frequency power transmission systems.

[0033] Figure 2 It is a schematic diagram of the topology structure of the series-connected sub-module of the bridge arm. According to whether there is a polarity change in the output voltage of the bridge arm, the series-connected sub-module of the bridge arm of the modular multilevel converter can adopt a bipolar sub-module topology and a unipolar sub-module topology. The bipolar sub-module topology is represented by the full-bridge sub-module, and its characteristic lies in the three-level voltage output ability: positive U c , 0, negative U c , as shown in Figure 2 (a). The unipolar sub-module topology is represented by the half-bridge sub-module. The output voltage of the sub-module only includes positive U c and 0 levels, as shown in Figure 2 (b). The bridge arm sub-modules of cascaded multilevel active filters and modular multilevel AC / AC converters usually adopt the bipolar sub-module topology; the bridge arm sub-modules of modular multilevel AC / DC converters generally adopt the unipolar sub-module topology, and according to actual needs, the unipolar sub-module and the bipolar sub-module can also be used simultaneously to construct a hybrid converter scheme.

[0034] Figure 3 It is the schematic diagram of the control method of the modular multilevel converter. The converter control system will use the harmonic current measurement value I hj of other incoming lines at the converter connection point obtained by real-time detection as the reference value of the harmonic current output on the AC side of the converter, and adjust the harmonic voltage output by the converter bridge arm through a closed-loop control system, so that the harmonic current I shj output on the AC side of the converter is equal in magnitude and opposite in phase to the harmonic current of other incoming lines at the connection point. According to design requirements, the harmonic current on the AC side for detection and control 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 redundant sub-modules, and the power frequency component is mainly borne by the series-connected sub-modules of the bridge arm. Combining with Figure 3 the equivalent circuit in, the method for configuring the number of redundant sub-modules is further described.

[0035] Step 1: According to the percentage of the short-circuit impedance of the converter transformer U k % and the inductance value of the bridge arm reactorL 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:

[0036] (1)

[0037] 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.

[0038] Step 2: Determine the maximum value of the harmonic voltage output by the converter bridge arm based on the converter harmonic current control capacity.

[0039] (2)

[0040] 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.

[0041] Step 3: Calculate the number of redundant submodules required for the bridge arm based on the bridge arm harmonic voltage modulation requirements N r :

[0042] (3)

[0043] 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.

[0044] 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.

[0045] The following are specific applications of the present invention.

[0046] 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%.

[0047] Consider using modular multi-level matrix converter to filter the third and fifth harmonic currents.

[0048] 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.

[0049]

[0050] Then, the requirement for the output harmonic voltage of the converter bridge arm is determined according to formula (2).

[0051]

[0052] Finally, calculate the number of redundant submodules required for the bridge arm N r, where the maximum allowable modulation degree of the harmonic voltage output by the converter bridge arm is taken M h,max is 0.95, and the rated value of the capacitor voltage of the series sub-module in the bridge arm U cN is 2.15 kV. The calculation results according to Equation (3) show that at this time N r it can be taken:

[0053]

[0054] Therefore, the number of sub-modules used for harmonic suppression is set to 6. At this time, the total number of full-bridge sub-modules in the bridge arm is set to 62, including 56 effective modules and 6 harmonic suppression and redundant modules. The harmonic suppression modules account for 9.4% of the number of bridge arm modules, which has a relatively significant economic advantage compared with setting up an active filter alone.

[0055] Based on the above parameters, modeling and simulation are carried out using PSCAD / EMTDC. The simulation model is as Figure 4 shown. 5th and 7th harmonics are injected at other incoming lines, and the active filtering function of the modular multilevel converter is unlocked at the 3rd second. The simulation results show that there are large 5th and 7th harmonics in the system before the 3rd second, and the harmonics are filtered out after the 3rd second, and it is basically close to a sine wave.

[0056] Those skilled in the art can obviously make various modifications to the above embodiments easily and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art to the present invention according to 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 percentage of the short-circuit impedance of the converter transformer U k %, and the inductance value of the bridge arm reactor L a calculate the equivalent inductance on the valve side of the converter L eq , and the converter transformer is used to connect the modular multilevel converter and the AC bus of the converter station: Among them, f N represents the rated operating frequency of the AC power grid; U vN represents the rated voltage of the valve side winding of the converter transformer, S N represents the rated capacity of the converter transformer; y represents the conversion coefficient of the converter bridge arm inductance converted to the equivalent inductance on the valve side; 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: Among them, U vh,max represents the maximum amplitude of the harmonic voltage output by the converter bridge arm; the subscript j represents the harmonic order; j max represents the maximum value of the harmonic order; U shj,max represents the maximum value of the j th harmonic voltage amplitude at the connection point of the converter; K represents the voltage transformation ratio of the grid / valve side windings of the converter transformer; I vhj,max represents the maximum amplitude of the harmonic current output on the valve side of the converter; Step 3: Calculate the required number of redundant sub-modules for the bridge arm in combination with the requirements for the modulation index of the bridge arm harmonic voltage N r : Among them, M h,max represents the maximum allowable modulation degree of the harmonic voltage output by the converter bridge arm; U cN represents the rated value of the capacitor voltage of the series sub-module of the bridge arm.

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 is equal to 1 / 2, and when the modular multilevel converter is a modular multilevel matrix AC / AC converter y is equal to 1 / 3.

3. A modular multilevel converter with harmonic control capability according to claim 1, characterized in that: According to different converter arm connection modes, the modular multilevel converter is applicable to topological forms including 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 1-4, 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 use the measured harmonic current values of other incoming lines at the connection point of the converter obtained through real-time detection I hj as the reference value of the harmonic current output on the AC side of the converter. By adjusting the harmonic voltage output of the converter arm through a closed-loop control system, the harmonic current output on the AC side of the converter I shj is equal in magnitude and opposite in phase to the harmonic current of other incoming lines at the 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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