A slmmc topology suitable for medium voltage flexible direct current and a control method thereof

Abstract

This invention pertains to the field of medium-voltage flexible DC transmission and provides a super-level modular multilevel converter (SLMMC) topology and its control method suitable for medium-voltage flexible DC transmission. The converter consists of several high-voltage submodules and a small number of low-voltage submodules; both high-voltage and low-voltage submodules are half-bridge structures. When applied to medium-voltage flexible DC transmission systems, this topology and its control method can output more AC voltage levels while maintaining a smaller number of submodules, thus improving voltage quality. This invention, through coordinated operation of the high-voltage and low-voltage submodules and capacitor energy control of the low-voltage submodules, maintains good stability and controllability under steady-state conditions, while also exhibiting excellent dynamic performance. Compared to conventional MMCs, it significantly reduces investment costs and operating losses, demonstrating excellent economic efficiency and engineering application value.

Description

Technical Field

[0001] This invention relates to the field of medium-voltage flexible DC transmission, specifically to a super-level modular multilevel converter (SLMMC) topology suitable for medium-voltage flexible DC transmission and its control method. Background Technology

[0002] Modular multilevel converters (MMCs) have been practically applied in high-voltage direct current (HVDC) transmission due to their low harmonic content, flexible control methods, and good scalability, achieving good operational results. Unlike high-voltage MMCs, which contain hundreds of sub-modules per arm, medium-voltage DC distribution networks use MMCs with only a limited number of sub-modules. Their output voltage, under the nearest-level modulation (NLM) strategy, has a high low-order harmonic content, which may not meet the requirements of the DC distribution network, necessitating the use of filters to suppress current distortion. While carrier phase-shifted PWM (CPS-PWM) can effectively reduce low-order harmonic content, it relies on complex voltage equalization control and circulating current suppression algorithms. In summary, in the medium-voltage DC distribution field, conventional MMCs struggle to achieve a sufficient number of output levels, resulting in significant voltage waveform distortion and high harmonic content.

[0003] Innovating converter topologies is the mainstream technical approach for improving the quality of output AC voltage. Examples include Optimized Configuration MMC (MC) and Fractional-Level MMC (MC). These topologies achieve a doubling of output voltage by replacing some conventional submodules with submodules that have lower capacitor voltages and inserting them between the remaining conventional submodules. However, these methods generally suffer from problems such as difficulty in balancing the capacitor voltages of the submodules, necessitating the design of redundant submodules. Currently, research on converter topologies that can increase the number of output voltage levels and thus improve voltage quality while maintaining a relatively small number of submodules is still lacking.

[0004] To address the aforementioned issues, this invention proposes an SLMMC topology and its control method suitable for medium-voltage flexible DC transmission. Summary of the Invention

[0005] The purpose of this invention is to propose a super-level modular multilevel converter (SLMMC) topology and its control method suitable for medium-voltage flexible DC to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A super-level modular multilevel converter (SLMMC) topology suitable for medium-voltage flexible DC transmission, the converter topology comprising several high-voltage submodules (HVSMs) and several low-voltage submodules (LVSMs), specifically: Each bridge arm consists of m high-voltage submodules and n low-voltage submodules connected in series, where m≥1 and n≥1; The high-voltage submodule is a half-bridge submodule, and its capacitor voltage rating is [value missing]. u CH ; The low-voltage submodule is a half-bridge submodule, and its capacitor voltage rating is [value missing]. u CL ; The rated capacitor voltage of the high-voltage submodule is k times that of the low-voltage submodule, where k = n+1 and k≥2 and is an integer; The capacitance of the low-voltage submodule is k times that of the high-voltage submodule.

[0008] Preferably, the converter topology uses a low-voltage submodule to insert transition levels between the output levels of the high-voltage submodule to refine the output waveform, and the number of its output levels... N out for: .

[0009] This invention further protects a control method for a superlevel modular multilevel converter (SLMMC) topology suitable for medium-voltage flexible DC transmission, comprising: Modulation steps: After generating the bridge arm voltage through inner and outer loop control and circulating current suppression, determine the number of high-voltage sub-modules and low-voltage sub-modules to be put into the bridge arm, and select the corresponding high-voltage and low-voltage sub-modules to be put into operation according to the direction of the bridge arm current, so as to achieve the modulation and equalization of the converter. Low-voltage submodule energy control steps: Based on the deviation between the average capacitor energy of each phase low-voltage submodule and the reference value, a DC voltage correction amount is generated. The correction amount is added to the DC voltage to make the DC component of the submodule capacitor charging current zero, thereby maintaining the stability of the low-voltage submodule capacitor voltage.

[0010] Preferably, in the modulation step: Taking the upper arm of phase A as an example, the number of high-voltage submodules is determined according to the following formula:

[0011] The number of high-voltage submodules is determined by the following formula:

[0012] in, N aUH This represents the number of high-voltage submodules in the upper arm of phase A. N aUL This refers to the number of low-voltage submodules; u aU The bridge arm voltage of the upper bridge arm of phase A; u CH This refers to the rated capacitor voltage of the high-voltage submodule. u CL This refers to the rated capacitor voltage of the low-voltage submodule.

[0013] Preferably, in the low-voltage submodule energy control step: Taking the upper bridge arm of phase A as an example, the DC component of the submodule capacitor charging current is determined according to the following formula:

[0014] in, N The number of sub-modules on each bridge arm; U C This is the average value of the capacitor voltages of all submodules; U DC This is the DC bus voltage; I DC It is direct current; U m This refers to the amplitude of the phase AC voltage. I m The amplitude of the alternating current; φ This represents the phase angle difference between the bridge arm voltage and the bridge arm current. The capacitor voltage of the submodule is determined according to the following formula:

[0015] in, C SM This refers to the capacitance value of the high-voltage submodule. A 1 item and A The two terms represent the fundamental frequency component and the second harmonic component of the charging current of the submodule capacitor, respectively.

[0016] Preferably, the low-voltage submodule energy control step further includes: The average capacitor energy of each phase low-voltage submodule is calculated. Taking phase A as an example, its average capacitor energy is determined according to the following formula:

[0017] in, WCLa The average capacitance energy of the A-phase low-voltage submodule; u CLai The first phase of A was invested i The capacitor voltage of each low-voltage submodule; Compare the calculated average capacitance energy with the reference value. W CLref The reference value was compared. W CLref The corresponding low-voltage submodule is the rated voltage. u CL Energy at that time; A DC voltage correction is generated based on the comparison results. U LDC And apply a DC voltage to it; DC voltage correction amount U LDC Limiting is applied to keep the low-voltage submodule capacitor voltage within [0.9]. u CL 1.1 u CL Fluctuations within a certain range.

[0018] Preferably, the DC voltage correction amount U LDC Determined according to the following formula:

[0019] in, U LDC This is to add a correction amount for the DC voltage; U Lm This refers to the capacitor voltage of the low-voltage submodule.

[0020] Compared with the closest existing technology, the technical solution provided by the present invention has the following beneficial effects: (1) The superlevel modular multilevel converter topology provided by the present invention allows for free selection within a reasonable range. k The value (usually 2~4) allows for flexible selection of the output level number based on actual engineering conditions and requirements. N out Size; (2) The modulation method provided by the present invention is suitable for different k The system is a high-performance, modular multilevel converter that maintains good stability and controllability under both steady-state and dynamic conditions. (3) The energy control method for low voltage submodule capacitor voltage provided by the present invention does not require redundant design of low voltage submodule, has a simple structure, and the existing engineering control system can fully meet the requirement of one PI controller for each bridge arm when the number of submodules is small. (4) The technical solution provided by this invention achieves increased output AC voltage level using fewer sub-modules, reduces voltage harmonic content, and improves voltage quality. Under the premise of the same output AC voltage level, compared with conventional MMC and existing innovative converter topologies, this invention has fewer sub-modules, which can effectively reduce losses. At the same time, it reduces the number of power electronic devices used in medium-voltage flexible DC grids, effectively reducing costs in existing projects. Attached Figure Description

[0021] Figure 1 This is the superlevel modular multilevel converter topology in Embodiment 1 of the present invention; Figure 2 This is a block diagram of the modulation strategy in Embodiment 2 of the present invention; Figure 3 This is a block diagram of the low-voltage submodule energy control in Embodiment 3 of the present invention; Figure 4 This is a simulation result diagram under steady-state conditions in Embodiment 4 of the present invention; Figure 5 This is a simulation result diagram under dynamic working conditions in Embodiment 5 of the present invention. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0023] This invention proposes an ultra-level modular multilevel converter topology suitable for medium-voltage flexible DC, which includes several high-voltage submodules (HVSM) and several low-voltage submodules (LVSM). The high-voltage submodules are half-bridge submodules, which are more numerous and mainly undertake the power transmission task of the system. The low-voltage submodules are half-bridge submodules, which are fewer in number and mainly used to increase the number of output levels.

[0024] Each bridge arm consists of m high-voltage submodules (capacitor voltage rated at [value]). u CH ) and n low-voltage submodules (capacitor voltage rated value) u CL The high-voltage submodule is composed of multiple capacitors connected in series, and the rated capacitor voltage of the high-voltage submodule is equal to that of the low-voltage submodule. k ( k = n+1) times ( k (≥2 and are integers), the capacitance value of the low-voltage submodule is equal to that of the high-voltage submodule. k times.

[0025] Between the output levels of the high-voltage submodule, appropriate transition levels are inserted using the low-voltage submodule to refine the output waveform. This effectively reduces the level height of the generated voltage step waveform, increases the number of steps in the step waveform, and thus increases the number of output levels. N out for: (1) Furthermore: (1) When the submodule is in the active state, the bridge arm current flows through the capacitor to charge it, and the charging current causes a change in the capacitor voltage. The charging current of the submodule capacitor contains multiple components, including a DC component, a fundamental frequency component, and a second harmonic component. Through derivation, the DC component... i Cau_DC The specific expression (taking the upper arm of phase A as an example) can be expressed as: (2) In the formula, N The number of submodules on each bridge arm, U C This is the average value of the capacitor voltages of all submodules. U DC This is the DC bus voltage. I DC It is direct current. U m The phase AC voltage amplitude, I m The amplitude of the alternating current. φ This represents the phase angle difference between the bridge arm voltage and the bridge arm current.

[0026] (2) Based on the characteristics of the submodule capacitor, the voltage of the submodule capacitor... u CaU The expression can be written as: (3) In the formula, C SM This refers to the capacitance value of the high-voltage submodule. A 1 item and A The two terms represent the fundamental frequency component and the second harmonic component of the charging current of the submodule capacitor, respectively.

[0027] As can be seen from equation (3), the DC component of the charging current decreases over time. i Cau_DC This causes the submodule capacitor to continuously charge or discharge, leading to a gradual deviation of the capacitor voltage from its equilibrium value. The fundamental frequency and second harmonic current components will superimpose their corresponding fundamental frequency and second harmonic components on the submodule capacitor voltage, causing the voltage to fluctuate periodically. Therefore, ensuring that the DC current component of the charging current is zero is crucial for achieving capacitor voltage stability.

[0028] (3) The high-voltage submodule can achieve voltage equalization using the traditional bubble sorting method, just like the submodules in a conventional MMC. However, the capacitor voltage stability and equalization of the low-voltage submodule require consideration of the capacitor voltage stability condition proposed in step (2). Therefore, it is considered to generate a correction amount through the energy control of the low-voltage submodule and add it to the DC voltage to make it... i Cau_DC The voltage is set to zero, thus maintaining the stability of the low-voltage submodule capacitor voltage.

[0029] Based on the above, this invention further proposes a control method for an ultra-level modular multilevel converter topology suitable for medium-voltage flexible DC, specifically including a modulation method and a low-voltage submodule energy control method.

[0030] Modulation method: (1) In the inner and outer loop control and circulating current suppression, the bridge arm voltage is generated. u aU (Taking the upper bridge arm of phase A as an example) After that, determine the number of high-voltage submodules to be put into this bridge arm. N aUH : (4) (2) Determine the number of high-voltage submodules to be deployed in this bridge arm. N aUH Then, the number of low-voltage submodules to be deployed in the bridge arm was determined. N aUL : (5) (3) Select the appropriate high and low voltage sub-modules according to the direction of the bridge arm current to achieve the modulation and equalization of the converter.

[0031] Low-voltage submodule energy control method: (1) Low-voltage submodule energy control is used to adjust the capacitor energy of each phase low-voltage submodule to achieve the required reference value. W CLref The corresponding low-voltage submodule is the rated voltage. u CL Energy at that time. Average capacitance energy of low-voltage submodules in each phase (taking phase A as an example). W CLa for: (6) In the formula, u CLai The first phase of A was invested i A low-voltage submodule.

[0032] (2) When the bridge arm current is constant, the charging current characteristics of the submodule are mainly determined by the DC component in the bridge arm modulation voltage. UDC and fundamental frequency AC component amplitude U m The two components together determine the DC component in the submodule capacitor charging current. i Cau_DC When the value is zero, the voltage of the submodule capacitor can be stable.

[0033] make i Cau_DC To be zero, I DC and I m Treat it as a constant: (7) In the formula, U LDC To add a correction amount for the DC voltage. U Lm For the low-voltage submodule capacitor voltage, ideally it is within [0.9]. u CL 1.1 u CL Fluctuations within a certain range can be used to limit the amplitude. U LDC To balance the energy of the capacitors in the low-voltage submodule.

[0034] The following description, in conjunction with relevant accompanying drawings and specific examples, further illustrates the super-level modular multilevel converter (SLMMC) topology and its control method suitable for medium-voltage flexible DC proposed in this invention. The specific details are as follows.

[0035] Example 1: This embodiment provides an ultralevel modular multilevel converter topology, the specific process of which is as follows: like Figure 1 As shown, the Superlevel Modular Multilevel Converter (SLMMC) topology proposed in this embodiment consists of a high-voltage submodule (HVSM) and a low-voltage submodule (LVSM). The number of high-voltage submodules is m, and the number of low-voltage submodules is n. By inserting appropriate transition levels between the output levels of the high-voltage submodules, the output waveform is refined, achieving a (m+1)(n+1) level output.

[0036] Example 2: This embodiment provides a modulation strategy based on an Superlevel Modular Multilevel Converter (SLMMC) topology, the specific process of which is as follows: Figure 2 The principle of the modulation strategy block diagram is to first determine the number of high-voltage submodules to be activated. NaUH Number of low-voltage submodules deployed N aUL It is determined by the difference between the bridge arm voltage and the stepped waveform of the high-voltage submodule.

[0037] Example 3: This embodiment provides a method for energy control of low-voltage submodules, the specific process of which is as follows: To address the issue of the low-voltage submodule's inability to self-balance voltage, the low-voltage submodule energy controller introduces a small DC component into the modulation of the low-voltage submodule. U LDC To maintain the energy balance stored in the low-voltage submodules on the three-phase six-bridge arm. After adding this DC component, the DC component of the current flowing through the capacitor of the low-voltage submodule satisfies equation (4). i Cau_DC The condition of zero is used to achieve stability and balance of the capacitor voltage in the low-voltage submodule.

[0038] Example 4: This embodiment is used to simulate and test the effectiveness of the Superlevel Modular Multilevel Converter (SLMMC) topology and its control method provided by this invention under steady-state operating conditions. The specific process is as follows: The simulation was conducted on a 20kV dual-terminal flexible DC power grid with a rated power of 10 MW. Each bridge arm consists of 4 high-voltage submodules and 4 low-voltage submodules, capable of outputting 25-level AC voltage. Figure 4 The waveforms for each key quantity.

[0039] Figure 4 (a) shows the circulating current waveform of the SLMMC in the converter station. When the circulating current control is activated at 2s, the peak-to-peak value of the SLMMC circulating current is significantly reduced, and the circulating current is effectively suppressed.

[0040] Figure 4 (b), (c) and (d) are the waveforms of the arm current (taking the upper arm of phase A as an example) of the SLMMC in the converter station, the grid-side AC voltage and AC current waveforms, respectively. The system output performance is good in steady state.

[0041] Figure 4 (e) and Figure 4 (f) represents the capacitor voltage waveforms of the high-voltage and low-voltage submodules, respectively. During steady-state operation, the maximum fluctuation rate of both remains within ±10%, which meets the system operation requirements.

[0042] In summary, under steady-state conditions, no overvoltage or overcurrent phenomena were observed in any of the key quantities, thus meeting the system operation requirements.

[0043] Example 5: This embodiment is used to simulate and test the dynamic performance of the Superlevel Modular Multilevel Converter (SLMMC) topology and its control method provided by this invention. Its steady-state simulation parameters are consistent with those of Embodiment 4. Figure 5 The waveforms of key quantities throughout the entire process are shown.

[0044] Figure 5 (a) shows the waveforms of active and reactive power. Under extreme operating conditions with sudden load and reactive power changes, both active and reactive power can follow the changes of the reference value well throughout the process. At the moment of active (reactive) change, reactive (active) power fluctuates within the normal range, and the system has good transient performance.

[0045] Figure 5 (b) Figure 5 (c) and Figure 5 (d) Represents the waveforms of circulating current, AC voltage, and AC current, respectively. The circulating current suppressor can effectively suppress the circulating current and maintain a stable circulating current of the converter during load changes. During several moments of active and reactive power changes, the AC voltage and AC current can be output smoothly without significant fluctuations during the transition process.

[0046] Figure 5 (e) and Figure 5 (f) shows the capacitor voltage waveforms of the high-voltage and low-voltage submodules, respectively. The capacitor voltages of both the high-voltage and low-voltage submodules can quickly transition to the new stable operating state, and the fluctuation range is within the allowable range.

[0047] In summary, under extreme operating conditions such as load and reactive power surges, the system can successfully transition to a new stable operating state, demonstrating good dynamic recovery capabilities.

[0048] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims. The information disclosed in the background section of the present invention is merely intended to enhance the understanding of the overall background technology of the present invention and should not be construed as an admission or implication in any way that such information constitutes prior art known to those skilled in the art.

Claims

1. An SLMMC topology suitable for medium-voltage flexible DC transmission, characterized in that, The converter topology includes several high-voltage submodules and several low-voltage submodules, specifically: Each bridge arm consists of m high-voltage submodules and n low-voltage submodules connected in series, where m≥1 and n≥1; The high-voltage submodule is a half-bridge submodule, and its capacitor voltage rating is [value missing]. u CH ; The low-voltage submodule is a half-bridge submodule, and its capacitor voltage rating is [value missing]. u CL ; The rated capacitor voltage of the high-voltage submodule is k times that of the low-voltage submodule, where k = n+1 and k≥2 and is an integer; The capacitance of the low-voltage submodule is k times that of the high-voltage submodule.

2. The SLMMC topology according to claim 1, characterized in that, The converter topology uses a low-voltage submodule to insert transition levels between the output levels of the high-voltage submodule to refine the output waveform, and its output level count... N out for: 。 3. A control method for an SLMMC topology applicable to medium-voltage flexible DC as described in any one of claims 1-2, characterized in that, include: Modulation steps: After generating the bridge arm voltage through inner and outer loop control and circulating current suppression, determine the number of high-voltage sub-modules and low-voltage sub-modules to be put into the bridge arm, and select the corresponding high-voltage and low-voltage sub-modules to be put into operation according to the direction of the bridge arm current, so as to achieve the modulation and equalization of the converter. Low-voltage submodule energy control steps: Based on the deviation between the average capacitor energy of each phase low-voltage submodule and the reference value, a DC voltage correction amount is generated. The correction amount is added to the DC voltage to make the DC component of the submodule capacitor charging current zero, thereby maintaining the stability of the low-voltage submodule capacitor voltage.

4. The control method according to claim 3, characterized in that, In the modulation step: Taking the upper arm of phase A as an example, the number of high-voltage submodules is determined according to the following formula: The number of high-voltage submodules is determined by the following formula: in, N aUH This represents the number of high-voltage submodules in the upper arm of phase A. N aUL This refers to the number of low-voltage submodules; u aU The bridge arm voltage of the upper bridge arm of phase A; u CH This refers to the rated capacitor voltage of the high-voltage submodule. u CL This refers to the rated capacitor voltage of the low-voltage submodule.

5. The control method according to claim 3, characterized in that, In the energy control steps of the low-voltage submodule: Taking the upper bridge arm of phase A as an example, the DC component of the submodule capacitor charging current is determined according to the following formula: in, N The number of sub-modules on each bridge arm; U C This is the average value of the capacitor voltages of all submodules; U DC This is the DC bus voltage; I DC It is direct current; U m This refers to the amplitude of the phase AC voltage. I m The amplitude of the alternating current; φ This represents the phase angle difference between the bridge arm voltage and the bridge arm current. The capacitor voltage of the submodule is determined according to the following formula: in, C SM This refers to the capacitance value of the high-voltage submodule. A 1 item and A The two terms represent the fundamental frequency component and the second harmonic component of the charging current of the submodule capacitor, respectively.

6. The control method according to claim 3 or 5, characterized in that, The low-voltage submodule energy control steps further include: The average capacitor energy of each phase low-voltage submodule is calculated. Taking phase A as an example, its average capacitor energy is determined according to the following formula: in, W CLa The average capacitance energy of the A-phase low-voltage submodule; u CLai The first phase of A was invested i The capacitor voltage of each low-voltage submodule; Compare the calculated average capacitance energy with the reference value. W CLref The reference value was compared. W CLref The corresponding low-voltage submodule is the rated voltage. u CL Energy at that time; A DC voltage correction is generated based on the comparison results. U LDC And apply a DC voltage to it; DC voltage correction amount U LDC Limiting is applied to keep the low-voltage submodule capacitor voltage within [0.9]. u CL 1.1 u CL Fluctuations within a certain range.

7. The control method according to claim 6, characterized in that, The DC voltage correction amount U LDC Determined according to the following formula: in, U LDC This is to add a correction amount for the DC voltage; U Lm This refers to the capacitor voltage of the low-voltage submodule.

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