An asymmetric phase-shifted control method suitable for half-wave shaping MMC

Abstract

The application discloses an asymmetric phase-shifting control method suitable for a half-wave shaping MMC, and can realize wide-range operation energy balance of a converter. The method is characterized in that, in a 2pi period, a commutation unit switching function and a multi-level unit modulation voltage are not switched in a half-wave period pi, but in a positive half-wave mode, [0, pi] is taken as a reference to expand and shift an angle theta to the left and right, that is, the interval is [-theta, pi+theta], and in a negative half-wave mode, [pi, 2pi] is taken as a reference to shrink and shift an angle theta to the left and right, that is, the interval is [pi+theta, 2pi-theta]. The application adopts an asymmetric phase-shifting modulation method, generates and reverses polarities of two asymmetric partial sine waves through a commutation switch cooperating with a multi-level unit, and obtains a complete multi-level sine wave on an alternating current side. The application can eliminate the limitation of a power factor angle on a phase-shifting angle while realizing wide-range energy balance of the converter, is only affected by a modulation ratio, and compared with symmetric phase-shifting modulation, is beneficial to simplifying commutation control logic when the system is in a wide-range reactive power regulation, and has better engineering application value.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of multi-level converters, in particular to an asymmetric phase-shift control method suitable for a half-wave shaping MMC (HWS-MMC). BACKGROUND

[0002] The modular multilevel converter (MMC) has the advantages of high output power quality and easy expansion of modules, and is the core equipment in the fields of flexible DC power transmission, new energy grid-connected transmission, and flexible interconnection of distribution networks.

[0003] In existing flexible DC power transmission projects, the MMC uses a large number of sub-modules and a large amount of capacitors, which highlights the disadvantages of large size and weight, high investment cost, etc. The lightweight research of MMC is an important topic.

[0004] In recent years, some studies have proposed HWS-MMC based on the ideas of half-wave multi-level shaping and bridge arm time division multiplexing. The multi-level unit (bridge arm) of the HWS-MMC only needs to generate a sinusoidal half-wave, which can effectively reduce the number of sub-modules, improve the utilization rate of sub-modules, and realize the lightweight of the MMC. However, the commutation switch of the HWS-MMC changes the current path of the converter within one fundamental frequency period, so that the multi-level unit can only be balanced at a specific operating point. Some researchers have proposed a symmetric phase-shift modulation method, which introduces a phase-shift angle as a control degree of freedom, i.e. the positive and negative half-wave modes are shifted by an angle of θ, and the energy balance of the converter is achieved in a wide range. However, the phase-shift angle in the energy balance constraint condition of this method is affected by the modulation ratio and the power factor angle, and the control logic of the converter is complex when the active / reactive power changes in a wide range. Therefore, the present application proposes a control method based on asymmetric phase-shift modulation for HWS-MMC, which can solve the above problems. SUMMARY

[0005] The purpose of the present application is to provide an asymmetric phase-shift control method suitable for a half-wave shaping MMC, which can achieve wide-range operation energy balance of the converter, eliminate the influence of the power factor angle on the phase-shift angle, and simplify the calculation of the phase-shift angle and the commutation control logic of the converter.

[0006] To achieve the above purpose, the technical solution adopted by the present application is as follows:

[0007] An asymmetric phase-shift control method suitable for a half-wave shaping MMC, characterized in that it comprises the following steps:

[0008] Step 1: According to the half-wave shaping MMC working principle and the asymmetric phase-shift modulation method, the expression of the commutation unit switch function is determined, the voltage and current of the multi-level unit (bridge arm) are represented by the switch function, the energy accumulation expression of the multi-level unit module capacitor is derived by integral operation, and the energy balance constraint condition of the commutation switch phase-shift angle θ is obtained according to the energy balance equation

[0009] Step 2: The three-phase sinusoidal modulation voltage and its dq-axis components output by the DC voltage (active power) / reactive power-network side current double closed loop control part of the converter are calculated to obtain the modulation ratio m; according to the energy balance constraint condition described in step 1, the phase-shift angle θ0 for realizing steady-state energy balance control is calculated

[0010] Step 3: The phase-shift angle increment for realizing dynamic energy balance is obtained from the multi-level unit module capacitor voltage average value control and inter-phase balance control part, and the three-phase commutation unit phase-shift angle θ is obtained by combining the steady-state energy balance phase-shift angle θ0 u 、θ v 、θ w ;

[0011] Step 4: The commutation comparison value d of each phase commutation unit is calculated according to the three-phase commutation unit phase-shift angle θ k (k=u,v,w) and the modulation ratio obtained in step 3 k :

[0012] d k =-msinθ k ,k=u,v,w (1)

[0013] Step 5: The three-phase sinusoidal modulation voltage u k0ref described in step 2 is compared with the commutation comparison value d k in step 4 to determine the three-phase commutation unit switch function S k , as shown in equation (2); the on-off of the switch of each phase commutation unit is controlled according to S k ; when S k =1, it is the positive half-wave mode, and the commutation switches S 21 and S 12 are turned on, and S 11 and S 22 are turned off; when S k =0, it is the negative half-wave mode, and the commutation switches S 11 and S 22 are turned on, and S 21 and S 12 are turned off

[0014]

[0015] Step 6: According to the three-phase sinusoidal modulation voltage uk0ref , and the three-phase commutation cell switching function S determined in step 5 k , a reference voltage u of the multi-level bridge arm modulation is generated mckref , as follows:

[0016]

[0017] In the formula, k u = (U dc / 2) / U C , Udc is the steady-state value of the capacitor voltage on the DC side of the converter, and U C is the steady-state value of the total capacitor voltage of the multi-level bridge arm.

[0018] Step 7: According to the multi-level bridge arm reference voltage u mckref in step 6, in combination with the nearest level approximation or carrier phase-shifted multi-level modulation strategy and the phase internal submodule voltage sharing method, the three-phase multi-level unit (bridge arm) is multi-level modulated.

[0019] In the step 1, the basic principle of the asymmetric phase-shifted modulation method is that, in a 2π period, the commutation cell switching function and the multi-level unit modulation voltage do not switch in a half-wave period π, but the positive half-wave mode is expanded by θ angles to the left and right based on [0, π], i.e., the interval is [-θ, π+θ], and the negative half-wave mode is contracted by θ angles to the left and right based on [π, 2π], i.e., the interval is [π+θ, 2π-θ]; in a 2π period, two asymmetric partial sine waves with DC bias are generated by the multi-level unit, and the waveform bias elimination and polarity reversal are realized through the cooperation of the commutation switch, and the complete multi-level sine wave on the AC side is obtained.

[0020] According to the asymmetric phase-shifted modulation principle, the basic switching function of the three-phase commutation cell defined by the phase-shift angle θ is:

[0021]

[0022] According to the switching function formula (4) and the principle of the half-wave shaping MMC circuit, the expressions of the voltage u mc across the multi-level unit and the bridge arm current i mc in a 2π period are:

[0023]

[0024] In the formula, m = U m / (U dc / 2) is the modulation ratio of the converter, U m and I m are the amplitude of the AC voltage and current of the converter, is the power factor angle of the AC current of the converter lagging behind the AC voltage, and ω is the angular frequency of the AC power grid.

[0025] According to formula (5), (6), the cumulative energy DE of the multi-level unit in a 2π period is derived mc :

[0026]

[0027] According to the multi-level unit energy balance equation, i.e., DE mc = 0, the energy balance constraint condition of the commutation switch phase shift angle θ can be derived as formula (8), and it can be seen that the phase shift angle is only affected by the modulation ratio and is irrelevant to the power factor angle.

[0028]

[0029] In the step 2, the modulation ratio m is calculated according to the d, q axis components u dref , u qref of the three-phase sinusoidal modulation voltage output by the converter double closed loop control part in the synchronous rotating coordinate system.

[0030]

[0031] The modulation ratio m is substituted into formula (8) to obtain the steady-state energy balance phase shift angle θ0.

[0032] In the step 3, the phase shift angle increment Δθ0for realizing the overall dynamic energy balance is obtained by the multi-level unit module capacitor voltage average value controller, and the average phase shift angle θ1of the three-phase commutation unit is obtained by superimposing θ0; the phase shift angle increments Δθ u , Δθ v of the u phase and the v phase are obtained by the module capacitor voltage interphase balance controller, and then the phase shift angles θ u , θ v , θ w of the three-phase commutation unit are calculated, and the calculation formula is as follows:

[0033]

[0034] The present application has the following beneficial effects:

[0035] The present application adopts the asymmetric phase shift modulation method, generates and reverses the polarity of two asymmetric multi-level sine half waves through the commutation switch and the multi-level unit, and obtains the complete multi-level sine wave on the AC side. The present application can realize the wide-range operation energy balance control of the half-wave shaping MMC converter, decouples the phase shift angle and the power factor angle, makes the phase shift angle only affected by the modulation ratio, simplifies the commutation control logic when the system is in the wide-range reactive power regulation compared to the symmetric phase shift modulation, and has better engineering application value. The present application can also be applied to other hybrid MMC topologies based on half-wave alternation. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The topology of a half-wave reshaping MMC (HWS-MMC) in the application;

[0037] Figure 2 An asymmetric phase-shifting control strategy diagram suitable for the HWS-MMC provided by the application;

[0038] Figure 3 A HWS-MMC asymmetric phase-shifting modulation method principle diagram provided by the application;

[0039] Figure 4 A HWS-MMC existing symmetric phase-shifting modulation method principle diagram;

[0040] Figure 5 The steady-state waveforms of the u-phase multi-level unit bridge arm voltage, the converter AC side voltage and the multi-level unit bridge arm current provided by the embodiment of the application;

[0041] Figure 6 The dynamic waveforms of the active power, the reactive power, the DC side voltage and the phase-shifting angle with power disturbance in the system running process provided by the embodiment of the application;

[0042] Figure 7 The three-phase multi-level unit sub-module capacitor voltage steady-state waveforms provided by the embodiment of the application; DETAILED DESCRIPTION

[0043] The application will be described in further detail below with reference to the drawings and embodiments.

[0044] It should be pointed out that the specific implementation methods described herein are only used to explain the related content and are not a limitation on the application. It should be noted that the implementation methods in the application and the features in the implementation methods can be combined with each other without conflict.

[0045] Embodiment:

[0046] The half-wave reshaping MMC (HWS-MMC) circuit topology suitable for the embodiment is as shown in Figure 1 , which includes three phase circuits with the same structure, each phase circuit adopts the connection mode of multi-level units embedded in commutation units; the multi-level units can adopt a mixed configuration of half-bridge modules and full-bridge modules, which are connected in series to form a bridge arm; the commutation unit contains four commutation switches, among which the AC side commutation switches S 11 and S 12 adopt the form of multiple IGBTs in series in both directions, the DC side commutation switches S 21 and S 22 adopt the form of multiple IGBTs in single direction in series, and C dc is a DC side capacitor.

[0047] The working principle of the HWS-MMC provided by the embodiment when asymmetric phase-shifting control is adopted is as follows: in a 2π period, two asymmetric partial sine waveforms with DC bias are generated by the multi-level unit; the commutation unit is connected to the AC side and the DC side, so as to realize waveform bias elimination and polarity reversal, and obtain the complete multi-level sine wave at the AC side.

[0048] The embodiment of the application provides an asymmetric phase-shifting control method suitable for half-wave shaping MMC, and the overall control block diagram is as shown in Figure 2 The method comprises the following steps:

[0049] Step 1: according to the working principle of the half-wave shaping MMC and the asymmetric phase-shifting modulation method, the switching function of the commutation unit is determined, the voltage and current of the multi-level unit (bridge arm) are represented by the switching function, the energy accumulation expression of the multi-level unit module capacitor is derived through integral operation, and the energy balance constraint condition of the commutation switching phase-shifting angle θ is obtained according to the energy balance equation.

[0050] Specifically, the basic principle of the asymmetric phase-shifting modulation method provided by the application is as follows: as shown in Figure 3 in a 2π period, the switching function of the commutation unit and the modulation voltage of the multi-level unit are not switched in the half-wave period π, but the positive half-wave mode is outwardly expanded by θ angles on the left and right sides based on [0, π], that is, the interval is [-θ, π+θ], and the negative half-wave mode is inwardly contracted by θ angles on the left and right sides based on [π, 2π], that is, the interval is [π+θ, 2π-θ]. Figure 4 The principle diagram of the existing symmetric phase-shifting modulation method is given in

[0051] According to the asymmetric phase-shifting modulation principle, the basic switching function of the three-phase commutation unit defined by the phase-shifting angle θ is as follows:

[0052]

[0053] According to the switching function formula (1) and the HWS-MMC circuit principle, the cumulative energy ΔE of the multi-level unit in a 2π period can be derived as follows: mc

[0054]

[0055] In the formula, u mc and i mc are the terminal voltage and current of the multi-level unit, m is the modulation ratio of the converter, I m is the amplitude of the AC current of the converter, is the power factor angle of the AC current of the converter lagging behind the AC voltage, and ω is the angular frequency of the AC power grid. ​

[0056] According to the multi-level cell energy balance equation, ΔE mc = 0, the energy balance constraint condition of the commutation switch phase shift angle θ can be derived as formula (3), and it can be seen that the phase shift angle is only affected by the modulation ratio, and is irrelevant to the power factor angle.

[0057]

[0058] And in the existing symmetrical phase shift modulation method, the energy balance constraint condition that the phase shift angle θ satisfies is formula (4), and it can be seen that the phase shift angle is affected by both the modulation ratio and the power factor angle.

[0059]

[0060] Therefore, the asymmetric phase shift modulation method provided by the present application can decouple the phase shift angle and the power factor angle, and compared with the symmetrical phase shift modulation, it is beneficial to simplify the commutation control logic when the system is in a wide range of reactive power regulation.

[0061] Step 2: The three-phase sinusoidal modulation voltage and its dq axis components output by the DC voltage (active power) / reactive power - grid side current double closed loop control part of the converter are used to calculate the modulation ratio m; and the phase shift angle θ0 for realizing the steady-state energy balance control is calculated according to the energy balance constraint condition described in step 1.

[0062] Specifically, the d, q axis components u dref , u qref of the three-phase sinusoidal modulation voltage output by the double closed loop control part of the converter in the synchronous rotating coordinate system are used to calculate the modulation ratio m:

[0063]

[0064] The modulation ratio m is substituted into the energy balance constraint condition of formula (3) to obtain the steady-state energy balance phase shift angle θ0.

[0065] Step 3: The phase shift angle increment for realizing dynamic energy balance is obtained from the multi-level cell module capacitor voltage average value control and inter-phase balance control part, and the steady-state energy balance phase shift angle θ0 is combined to obtain the three-phase commutation unit phase shift angle θ u , θ v , θ w .

[0066] Specifically, the reference value u Cref of the three-phase multi-level cell module capacitor voltage average value is subtracted from the measured value , and the phase shift angle increment Δθ0 for realizing the overall dynamic energy balance is output through the PI controller, and the steady-state energy balance phase shift angle θ0 is superimposed to obtain the average phase shift angle θ1 of the three-phase commutation unit.

[0067] In the module capacitor voltage inter-phase balance control, the average value of the three-phase multi-level unit module capacitor voltage is respectively subtracted from the average value of the u-phase and v-phase module capacitor voltage and The phase shift angle increment Δθ of the u-phase and v-phase is outputted by the PI controller u , Δθ v , combined with the three-phase average phase shift angle θ1, the phase shift angle θ of the three-phase commutation unit is calculated u , θ v , θ w The calculation formula is as follows:

[0068]

[0069] Step 4: the phase shift angle θ of the commutation unit of each phase is calculated by the phase shift angle θ of the commutation unit of each phase obtained in step 3 k (k=u, v, w) and the modulation ratio k :

[0070] d k =-msinθ k , k=u, v, w (7)

[0071] Step 5: the three-phase sinusoidal modulation voltage u k0ref in step 2 is compared with the commutation comparison value d k in step 4 to determine the switching function S k of the three-phase commutation unit, as formula (8); the on-off of the switching of each phase commutation unit is controlled according to S k ; when S k =1, it is the positive half-wave mode, the commutation switches S 21 and S 12 are turned on, and S 11 and S 22 are turned off; when S k =0, it is the negative half-wave mode, the commutation switches S 11 and S 22 are turned on, and S 21 and S 12 are turned off.

[0072]

[0073] Step 6: according to the three-phase sinusoidal modulation voltage u k0ref in step 2 and the switching function S k of the three-phase commutation unit determined in step 5, the reference voltage u mckref of the multi-level bridge arm modulation is generated, as follows:

[0074]

[0075] In the formula, k u = (U dc / 2) / U C , Udc is the steady value of the capacitor voltage on the DC side of the converter, U C is the steady value of the total capacitor voltage of the multi-level bridge arm.

[0076] Step 7: according to the multi-level bridge arm reference voltage u mckref , a carrier phase-shifted multi-level modulation strategy and an intra-phase submodule voltage balancing method for correcting the modulation ratio of each module are used to modulate the three-phase multi-level unit (bridge arm).

[0077] The half-wave shaping MMC shown in Fig. Figure 1 is simulated by using the control strategy (shown in Fig. Figure 2 ) of the application. Figure 5 The steady-state waveforms of the multi-level unit bridge arm voltage, the converter AC side voltage and the multi-level unit bridge arm current are given, which are consistent with the working principle of the application: in a 2π period, the multi-level unit generates two asymmetric partial sine waveforms containing DC bias, and the complete multi-level sine wave on the AC side is obtained through the cooperation of the commutating switch; Figure 6 The dynamic waveforms of the active power, the reactive power, the DC side voltage and the phase shift angle θ0 during the system operation are given, the disturbance is that the active power changes from 16MW to 8MW at 1s, and returns to 16MW at 1.5s, the reactive power changes from 0 to 8Mvar at 2s, and returns to 0 at 2.5s, the reactive power changes from 0 to -8Mvar at 3.5s, and returns to 0 at 4.5s; Figure 7 The fluctuation waveforms of the capacitor voltage of the three-phase multi-level unit submodule are given, which show the balance of the inter-phase and intra-phase submodule capacitor voltage; the simulation results verify the effectiveness of the application.

Claims

1. An asymmetric phase-shifted control method suitable for half-wave shaped MMC, characterized in that, The method comprises the following steps: Step 1: According to the half-wave shaping MMC working principle and the asymmetric phase-shift modulation method, the expression of the commutation unit switch function is determined, the voltage and current of the multi-level unit are represented by the switch function, the energy accumulation expression of the multi-level unit module capacitor is derived by integral operation, and the energy balance constraint condition of the commutation switch phase shift angle θ is obtained according to the energy balance equation Step 2: The three-phase sinusoidal modulation voltage and the dq-axis component thereof are output from the DC voltage or active power, reactive power-network side current double closed-loop control part of the converter, the modulation ratio m is calculated, and the phase shift angle θ0 for realizing the steady-state energy balance control is calculated according to the energy balance constraint condition in step 1; Step 3: The phase-shifting angle increment for dynamic energy balance is obtained from the multi-level cell module capacitor voltage average value control and inter-phase balance control part, combined with the steady-state energy balance phase-shifting angle θ0, to obtain the three-phase commutation cell phase-shifting angle θ u , θ v , θ w ; Step 4: The phase-shifting angle θ is shifted for each phase commutating unit obtained from Step 3 k (k = u, v, w) and the modulation ratio, the commutation comparison value d of each phase commutating unit is calculated k : d k = -msin θ k k = u, v, w (1) Step 5: Convert the three-phase sinusoidal modulation voltage u from Step 2... k0ref Compare with the commutation value d in step 4 k By comparison, the switching function S of the three-phase commutation unit is determined. k As shown in equation (2); according to S k Controls the on / off state of the switches of each phase commutation unit. When S k =1, indicating positive half-wave mode, commutator switch S 21 and S 12 On, S 11 and S 22 Off, when S k =0, indicating negative half-wave mode, commutator switch S 11 and S 22 On, S 21 and S 12 Turn off; Step 6: The three-phase sinusoidal modulated voltage u k0ref is generated from the three-phase sinusoidal modulation voltage u k determined in step 5, and the three-phase commutation cell switching function S mckref as follows: where k u = (U dc / 2) / U C is the ratio of the steady-state value of the 1 / 2 DC-side capacitor voltage to the total voltage of the multilevel bridge-arm capacitors. Step 7: Reference voltage u modulated by the multi-level bridge arm according to step 6 mckref The three-phase multi-level unit is modulated by multi-level according to the recent level approximation or carrier phase-shift multi-level modulation strategy and the in-phase submodule voltage equalization method.

2. The asymmetric phase-shifted control method for half-wave shaped MMC according to claim 1, wherein: In the step 1, the basic principle of the asymmetric phase shift modulation method is that, in a 2π period, the commutation unit switching function and the multi-level unit modulation voltage are not switched in a half-wave period π, but are outwardly expanded by θ angles on the left and right of the [0, π] as the reference in the positive half-wave mode, that is, the interval is [-θ, π+θ], and are inwardly contracted by θ angles on the left and right of the [π, 2π] as the reference in the negative half-wave mode, that is, the interval is [π+θ, 2π-θ]; in a 2π period, two asymmetric partial sinusoidal waves with DC bias are generated from the multi-level unit, the waveform bias elimination and polarity reversal are realized through the cooperation of the commutation switch, and the complete multi-level sinusoidal wave on the AC side is obtained; According to the asymmetric phase shift modulation principle, the three-phase commutation unit basic switching function is determined as follows: The derived multi-level cell accumulates energy ΔE over a 2π period mc is: wherein u mc and i mc are the terminal voltage and current of the multi-level cell, I m is the amplitude of the AC current of the converter, m is the modulation ratio, is the power factor angle of the AC current of the converter lagging the AC voltage, and ω is the angular frequency of the AC grid. According to the energy balance equation of the multilevel cell, i.e., ΔE mc = 0, the energy balance constraint condition of the phase-shift angle θ of the commutation switch can be derived:

3. The asymmetric phase-shifted control method for half-wave shaped MMC of claim 1, wherein: In steps 2 and 3, the modulation ratio m is substituted into equation (6) to obtain the steady-state energy balance phase shift angle θ0; the phase shift angle increment Δθ0 for achieving overall dynamic energy balance is obtained from the multi-level unit module capacitor voltage average value controller, and superimposed with θ0 to obtain the average phase shift angle θ1 of the three-phase commutation unit; the phase shift angle increment Δθ1 of phase u and phase v is obtained from the module capacitor voltage phase equalization controller. u , Δθ v Then, the phase shift angle θ of the three-phase commutator unit can be calculated. u θ v θ w The calculation formula is as follows: .

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