A Capacitor Voltage Balancing Control Strategy for a Bridge Arm Alternating Current Converter

Abstract

The present invention discloses a capacitance voltage balance control strategy for a bridge-arm alternating converter, considering the conduction angle of the DS of the series IGBTs in the AAC α As a control quantity for maintaining the energy balance of the SS, an improved AAC energy balance criterion is determined; a three-level capacitance voltage balance control method is adopted, and the conduction angle of the DS is used to control the total energy balance of all the SS in the six arms, and the energy distribution between different SS is adjusted through the zero-sequence voltage. The present invention achieves a good balance effect of the sub-module capacitance voltage of the AAC and independent active / reactive power control.

Description

Technical Field

[0001] The present invention relates to the technical field of high-voltage direct current power transmission, and specifically to a capacitor voltage balance control strategy for an alternate arm converter. Background Art

[0002] The modular multilevel converter (MMC) is recognized as the most promising topology in high-voltage direct current power transmission systems, with advantages such as easy expansion and modularization, low loss, and flexible four-quadrant power flow control capabilities. Each phase arm of the MMC consists of many series-connected sub-modules (SM). Among them, the half-bridge sub-module (HBSM) is widely used due to its simple structure. However, since the DC fault current can flow through the anti-parallel diode of the HBSM, the HBSM does not have the ability to clear DC short-circuit faults, and an additional DC circuit breaker is required to cut off the fault current. Although improved sub-modules such as full-bridge sub-modules, clamped double sub-modules, and cross-connected sub-modules have the ability to block DC faults, they will increase the system complexity and cost.

[0003] The alternate arm converter (AAC) is a new DC power transmission topology proposed in recent years, which combines low structural complexity and fault handling capabilities. The AAC mainly consists of a two-level switch based on series IGBTs (referred to as "direction switch (DS)") and a sub-module stack based on full-bridge sub-modules (referred to as SubmoduleStack (SS)). The former DS alternately conducts the upper and lower two arm of one phase, and the latter SS is used to adjust the phase current into a sine wave with a low harmonic distortion rate. Compared with the traditional MMC based on HBSM, the AAC can save about 40% of the number of sub-modules and 50% of the sub-module capacitance value. In addition, the AAC can also provide multiple control modes, such as a blocking mode and a static synchronous compensator mode, to achieve fault ride-through during DC faults.

[0004] Since the AAC was proposed, many scholars have conducted extensive research on its mechanism analysis, circuit modeling and control design. The energy balance of all SMs is the most basic issue for the stable operation of modular converters. For traditional MMCs, the total output voltage of the upper and lower sub-module strings of each phase bridge arm is clamped by the DC bus voltage. Therefore, the energy balance of all SMs can be well achieved through the nearest level modulation (NLM) and capacitor voltage sorting algorithm. However, since the upper and lower arm bridges of each phase of AAC are complementary, the turned-on SS is in series with the DC side, which is very different from the traditional MMC. In order to achieve constant capacitor voltage of AAC sub-modules, the energy accumulation of SS in each half cycle needs to be strictly controlled to zero. Merilin et al. derived the energy balance condition of AAC, the so-called "optimal operating point". The DC bus voltage and AC voltage amplitude need to strictly meet a fixed proportional relationship to maintain the constant capacitor voltage of SS. However, in actual systems, these voltages usually change with the AC and DC power flows, and a small voltage deviation will destroy the energy balance condition and cause the SS capacitor voltage to shift. In order to expand the energy balance operation range of AAC, some scholars have proposed a "short overlap mode", that is, the upper and lower bridge arms are closed simultaneously during the overlap period to construct an energy exchange path between the SS and the DC side, and use the circulating current to control the charging and discharging of the SS. However, the overlap time in this control mode is usually less than 10°, and the discontinuous circulating current makes it difficult to achieve a satisfactory capacitor voltage balance effect. Some scholars have further extended the overlap time to 60°, which is called the "extended overlap mode". In this mode, the circulating current flows alternately through the three-phase bridge arms, and there is always a circulating current path for the energy balance of the submodule at all times. In addition, the "extended overlap mode" can suppress the sixth harmonic of the DC current, so the DC filter capacity can be reduced. However, the circulating current in the overlapping control mode will increase the conduction loss of the converter and may even cause grid-connected current distortion.

[0005] In addition to overlapping control, some scholars have used the power factor angle to correct the power balance condition of AAC, and proposed a method to dynamically adjust the power factor angle when the voltage on the DC side or AC side changes to maintain the constant voltage of the SM capacitor. Since the upper and lower bridge arms are strictly complementary, the circulating current of traditional overlapping control is eliminated. However, this method cannot achieve independent control of active power and reactive power, and its application scenarios are limited. Recently, scholars such as Heya Yang pointed out that the conduction angle of DS is another degree of freedom for controlling the energy balance condition of AAC, which can be coordinated with the power factor angle to jointly control the capacitor voltage balance of SM. This method also avoids the introduction of circulating current, but there is currently a lack of feasible control solutions. Summary of the invention

[0006] In view of the above problems, the object of the present invention is to provide a capacitor voltage balance control strategy for a bridge-arm alternating current converter, so as to achieve a good balance effect of the capacitor voltage of the AAC sub-module and independent active / reactive power control. The technical solution is as follows:

[0007] A capacitor voltage balance control strategy for a bridge-arm alternating current converter includes the following steps:

[0008] Step 1: Considering the conduction angle α of the DS of the series IGBT in the AAC as the control quantity for maintaining the energy balance of the SS, an improved AAC energy balance criterion is determined;

[0009]

[0010] wherein, V m is the amplitude of the grid voltage, is the angle by which the grid current lags behind the grid voltage; α is the conduction angle, that is, the radian by which the conduction signal of the DS lags behind the phase voltage; V dc is the amplitude of the DC voltage; K is the ratio of the AC voltage amplitude to the DC voltage;

[0011] Step 2: Adopt a three-level capacitor voltage balance control method for capacitor energy equalization control, including:

[0012] A: The first-level balance control: the balance control of the sum of the SS capacitor energies

[0013] According to the topological structure of the AAC, the average value of the capacitor voltages of 6 SSs is used as the feedback quantity, and a conduction angle is obtained through closed-loop for the use of 6 DS switches; the closed-loop control expression of the conduction angle is:

[0014]

[0015] wherein:

[0016]

[0017]

[0018] wherein: K P1 , K I1 are the coefficients of the PI controller; V * C is the reference value of the SS capacitor voltage; is the reference value of the power factor angle, s is the integral operator, I * d is the reference value of the d-axis current, is the average value of the total capacitor voltages of the upper and lower bridge-arm sub-modules, sgn(·) is the sign function, x is the input of the sign function, N is the number of SSs formed by cascading full-bridge sub-modules included in each bridge arm; v Ciis the sum of the capacitor voltages of all sub-modules;

[0019] B: The second-level balance control: Capacitor energy balance control between SSs

[0020] Step a: Implement the balance control of the capacitor voltages of 6 SSs by using the zero-sequence voltage injection method:

[0021] Divide a power frequency cycle into 6 regions, and select different feedback amounts of the SS capacitor voltages for control in different regions; The basis for selecting the feedback amounts in different regions is:

[0022] 1) Under the condition of the change of the conduction angle, the SS corresponding to the feedback amount fully input in this region;

[0023] 2) In this region, the current polarity of the SS corresponding to the feedback amount is unique;

[0024] 3) In this region, the convergence of the energy balance control reaches the preset target;

[0025] Step b: Determine the region number N T The expression of is:

[0026]

[0027] Where:

[0028]

[0029] In the formula: ceil(·) is the ceiling function; f1(·) is the angle conversion function used to convert the angle range to [0, 2π]; θ is the starting phase angle of region I;

[0030] Step c: Select a proportional controller to achieve the rapid balance of energy between modules, and the control link is expressed as:

[0031]

[0032] In the formula: K P2 is the proportional controller coefficient, is the sliding average value of the SS capacitor voltage, i = 1, 2, 3,..., 6; reflects that when the phase current polarities are different, the directions of capacitor energy regulation are different; i * d is the d-axis current reference value;

[0033] C: The third-level balance control: Capacitor voltage balance control between sub-modules inside SS

[0034] For NLM modulation, a traditional capacitor voltage sorting algorithm is used to achieve the capacitor voltage balance of each SM; for carrier phase-shifted modulation, multiple capacitor voltage regulators are used to generate the compensation voltage, which is added to the original modulation voltage of different SMs to achieve the capacitor voltage balance between SMs;

[0035] D: Overall control strategy: including traditional double closed-loop control and three-level capacitor voltage balance method.

[0036] Furthermore, the overall control strategy is specifically as follows:

[0037] The outer loop controller is selected as the active / reactive power, DC link voltage / reactive power of the control system, or the AC side voltage under the dq axis;

[0038] The inner loop controller is used to track the reference dq axis current generated by the outer loop;

[0039] The three-phase output phase voltage reference is added to the zero-sequence voltage V0 obtained from the second-level balance control to calculate the output voltage reference of all SSs

[0040] Through different modulation methods and their corresponding third-level balance control, the drive signals of all SSs are generated;

[0041] Among them, the conduction angle α of the DS is calculated through the first-level balance control and then the drive signals of all DSs are generated by the pulse generator.

[0042] The beneficial effects of the present invention are: The present invention proposes a novel capacitor voltage balance control strategy for the bridge-arm alternating converter, uses the conduction angle of the DS to control the total energy balance of all SSs in the six arms, and adjusts the energy distribution between different SSs through the zero-sequence voltage; achieves a good balance effect of the capacitor voltage of the AAC sub-module and independent active / reactive power control. Description of the Drawings

[0043] Figure 1 is the topological structure of the bridge-arm alternating converter.

[0044] Figure 2 is the schematic diagram of the current and voltage of SS1; (a) traditional mode; (b) the mode considering the conduction angle of the DS proposed by the present invention.

[0045] Figure 3 is the schematic diagram of the partition; (a) (b)

[0046] Figure 4 is the overall control block diagram of the AAC system. Specific Embodiments

[0047] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0048] 1. Improved energy balance condition of AAC

[0049] The topology of the leg - alternating converter is as Figure 1 shown. Each phase includes upper and lower legs. Each leg includes a sub - module stack (SS) composed of N full - bridge sub - modules in cascade and a direction switch composed of series - connected IGBT / diodes. v DC and i DC are the DC voltage and current. For the i - th SS (i = 1, 2, 3, …, 6), v SSi , i SSi , v Cin , v Ci are the branch voltage, branch current, capacitor voltage of sub - module n, and the sum of capacitor voltages of all sub - modules (TVASC) respectively; v x , i x (x = a, b, c) are the three - phase grid voltages and currents respectively. Let their expressions be:

[0050]

[0051] In the formula: V m , I m are the amplitudes of the grid voltage and current; ω is the grid angular frequency; is the angle by which the grid current lags behind the grid voltage, and is the initial phase of the x - th phase.

[0052] Taking SS1 as an example for analysis, for the AAC under the traditional operation mode, the DS switch conducts once in each half power - frequency cycle according to the polarity of the phase current. The waveforms of the branch voltage and current of SS1 are as Figure 2 (a) shown, and their expressions are:

[0053] v SS1 = 12v dc - v a (2)

[0054] i SS1 = i a (3)

[0055] In order to maintain the constant capacitor voltage of SS1, the integral of the product of v SS1 and i SS1 in half a power - frequency cycle needs to be 0, and we can get:

[0056]

[0057] The energy balance criterion for AAC can be derived as follows:

[0058]

[0059] It can be seen that when the AC and DC side voltages change with the system power flow, the energy balance of the SS can be maintained by controlling the power factor angle However, the active and reactive powers of the system will be limited by and independent active / reactive power control cannot be performed.

[0060] Figure 2 (b) shows the branch voltage and current waveforms of SS1 considering the conduction angle of the DS, where the conduction signal of the DS lags the phase voltage by α radians (α is called the conduction angle). Then the energy change of SS1 within half a power frequency cycle is:

[0061]

[0062] Let ΔE SS1 be equal to 0, then the improved AAC energy balance criterion can be obtained as:

[0063]

[0064] In the formula, K is the ratio of the AC voltage amplitude to the DC voltage. Comparing formula (5) and formula (7), it can be found that the conduction angle α is another control quantity independent of the power factor angle for maintaining the energy balance of the SS. By controlling the conduction angle, the AAC can operate in a wider range of voltage and power factor angles.

[0065] 2. Three-level capacitor voltage balance control method

[0066] According to the energy balance criterion of formula (7), it can be directly inferred that if the DS switches of each bridge arm obtain the corresponding conduction angles according to the closed-loop change of their respective capacitor voltages (a total of 6 conduction angles are obtained), the energy balance of all SSs can be achieved. However, the upper and lower bridge arms of the same phase need to conduct complementarily. If two different conduction angles are used to conduct for half a power frequency cycle each, it will cause the phase current to be discontinuous or abnormal direct-through of the bridge arm to generate current spikes. Therefore, each phase bridge arm can only have one conduction angle, and by alternately using the capacitor voltages of the upper and lower SSs as feedback quantities to obtain the conduction angles, the energy balance of all SSs can be achieved. However, the convergence speed of this method is slow, it cannot adapt to the scenario of fast change of the system active / reactive power, and since the feedback quantity is segmented, a continuous integral controller cannot be used, and thus the static error-free control of the capacitor voltage cannot be achieved.

[0067] In order to achieve the balance of the voltages of 6 SS capacitors, this paper proposes a "three - level" capacitor energy balance control method. The first level is the balance control of the sum of the energies of 6 SS capacitors, the second level is the balance control of the capacitor energies between SSs, and the third level is the balance control of the energies between the capacitors of all sub - modules within each SS. The three - level control method is introduced as follows:

[0068] (1) Balance control of the sum of SS capacitor energies

[0069] Due to the alternating conduction of SSs, its capacitor voltage will fluctuate with energy change within half of the power frequency cycle and remain unchanged within the other half cycle. The voltage spectrum contains a large number of low - order harmonics (mainly the first, second, third, and fourth harmonics) in addition to the DC component, but the sum of the capacitor voltages of 6 SSs only contains the DC component and the sixth - order frequency component. Therefore, in order to eliminate the influence of the irregular fluctuation of the capacitor voltage on the closed - loop control of the conduction angle (adding a filter with a low cut - off frequency will affect the phase margin of the system), this paper uses the average value of the capacitor voltages of 6 SSs as the feedback quantity, and obtains a conduction angle through closed - loop for 6 DS switches to use.

[0070] The closed - loop control expression of the conduction angle can be obtained as follows:

[0071]

[0072] Where:

[0073]

[0074]

[0075] In the formula: K P1 、K I1 are the coefficients of the PI controller; V * C is the reference value of the SS capacitor voltage. Since only one conduction angle is used, under the control of Equation (8), only the balance control of the sum of the energies of 6 SS capacitors can be achieved. Therefore, additional control is needed to adjust the energy distribution between SSs so that the energies of all SSs are finally balanced.

[0076] (2) Balance control of the capacitor energies between SSs

[0077] From Figure 2 (b), it can be seen that the energy of SS within half of the power frequency cycle is related not only to the conduction angle but also to the output voltage v SSRegarding (assuming the phase current remains unchanged). The zero-sequence voltage injection method is a typical modulation method that only changes the magnitude of the phase voltage without changing the magnitude of the phase current in a Y-connected system and has extensive applications in the capacitor voltage balance of neutral-point clamped three-level converters. A similar idea can also be adopted in the AAC system to achieve the balance control of the SS capacitor voltage.

[0078] Suppose a zero-sequence voltage V0 is added to the three-phase phase voltages output by the converter simultaneously. Then we have:

[0079] v′ x =v x +V0 (11)

[0080] Then Equation (2) will change to:

[0081] v′ SS =1 / 2v dc -v x -V0 (12)

[0082] According to the operating principle of AAC, three SSs are conducting at any given time. The sum of the currents of the upper-bridge-arm SSs is equal to the sum of the currents of the lower-bridge-arm SSs. For example, when SS1, SS4, and SS6 are conducting, we have:

[0083] i SS1 =i SS4 +i SS6 (13)

[0084] The energy changes of each SS caused by the zero-sequence voltage can be obtained as:

[0085]

[0086] ΔE SS1 =-(ΔE SS4 +ΔE SS6 ) (15)

[0087] It can be seen that the energy reduced by one SS is equal to the total energy increased by the other two SSs, indicating that the zero-sequence voltage V0 will affect the energy distribution among the three currently conducting SSs but will not change the total system energy. To achieve the balance control of the capacitor voltages of six SSs with a single zero-sequence voltage, a power frequency period needs to be divided into six regions, and different SS capacitor voltage feedback amounts are selected for control in each region. Therefore, how to divide the regions and how to select the feedback amounts are the keys to the energy balance among SSs. The basis for selecting the feedback amounts in each region in this paper is mainly: (1) When the conduction angle changes, the SS corresponding to the feedback amount is fully involved in this region; (2) In this region, the current polarity of the SS corresponding to the feedback amount is unique; (3) In this region, the energy balance control has good convergence.

[0088] Figure 3 (a) shows the schematic diagram of each SS current waveform when the power factor angle is greater than or equal to zero. The expressions of x1 to x6 in the figure are as follows:

[0089]

[0090] The following takes Figure 3 SS1 in (a) as an example for analysis. According to the first and second bases, the regions of x1 - x2 and x5 - x6 will change with the changes of α and , so they are not suitable. The region of x2 - x5 is a fixed region, which meets the above bases and is a suitable region. In addition,

[0091] In the region of x2 - x3, |i SS1 | > |i SS5 | > |i SS4 |, and the zero - sequence voltage has the largest weight on the change of ΔE SS1 .

[0092] In the region of x3 - x4, |i SS5 | or |i SS6 | > |i SS1 | > |i SS4 |, and the zero - sequence voltage has a medium weight on the change of ΔE SS1 .

[0093] In the region of x4 - x5, |i SS6 | > |i SS4 | > |i SS1 |, and the zero - sequence voltage has the smallest weight on the change of ΔE SS1 .

[0094] The greater the weight, the better the convergence of the energy balance control. In particular, at the point of x5, the control of the energy of the SS1 capacitor cannot be achieved. According to the third basis, the region of x2 - x4 is the best region for controlling the energy of the SS1 capacitor. According to the same rule, the ranges of each region are as shown by the rectangular frame regions in Figure 3 (a). The length of each region is π / 3. It is possible to choose to control the capacitor voltages of SS1, SS6, SS3, SS2, SS5, and SS4 in regions I - VI respectively. Similarly, the partitioning results when the power factor angle is less than 0 are as shown in Figure 3 (b). The coordinate range of region I is for controlling the capacitor voltage of SS1 in this region. The subsequent regions II - V are used to control the capacitor voltages of SS6, SS3, SS2, and SS5, SS4.

[0095] Based on the above analysis, the expression of the region number N T is as follows:

[0096]

[0097] Wherein:

[0098]

[0099] In the formula: ceil(x) is the upward rounding function; f1(x) is the angle conversion function (Angle Resolver) used to convert the range of angle x to [0, 2π]; θ is the starting phase angle of Region I.

[0100] Since the feedback quantity is a piecewise function and an integral controller cannot be used, but the total energy of the 6 SSs has achieved zero-static error tracking by the conduction angle control loop. Therefore, this paper selects a proportional controller to achieve the rapid balance of energy between modules, and the control link can be expressed as:

[0101]

[0102] In the formula: K P2 is the proportional controller coefficient, is the sliding average value of the SS capacitor voltage. The sliding average value filtering can eliminate the low-frequency pulsation of the SS capacitor voltage and better reflect the change trend of the capacitor energy. is used to reflect that the direction of capacitor energy regulation is different when the phase current polarities are different.

[0103] (3) Capacitor voltage balance control between internal sub-modules of SS

[0104] Since the capacitor energy of each SS has achieved balance through the first-stage and second-stage controls, it is easy to achieve the energy balance between the SMs inside each SS. For NLM modulation, the traditional capacitor voltage sorting algorithm (Sorting Algorithm, SA) can be used to achieve the capacitor voltage balance of each SM. For carrier phase-shifted modulation, multiple capacitor voltage regulators (Capacitor Voltage Regulator, CVR) can be used to generate compensation voltages and add them to the original modulation voltages of different SMs, so as to achieve good capacitor voltage balance between SMs.

[0105] (4) Overall control strategy

[0106] The overall control block diagram of the AAC system proposed in the present invention is as Figure 4 shown, mainly including the traditional double closed-loop control and the three-stage capacitor voltage balance method. The outer-loop controller can be selected as the active / reactive power of the control system, the DC link voltage / reactive power, or the AC side voltage under the dq axis. The inner-loop controller is used to track the reference dq axis current generated by the outer loop. Then, the three-phase output phase voltage reference Add it to the zero-sequence voltage V0 obtained from the second-level control to calculate the output voltage reference of all SSs. Generate the drive signals of all SSs through different modulation methods and their corresponding third-level controls. In addition, the conduction angle α of the DS is calculated through the first-level balance control and then the drive signals of all DSS are generated by the pulse generator.

Claims

1. A capacitor voltage balance control strategy for a bridge arm alternating current converter, characterized in that, It includes the following steps: Step 1: Consider the conduction angle α of the DS of the series-connected IGBTs in the AAC as the control quantity for maintaining the energy balance of the SS, and determine the improved AAC energy balance criterion; where V m is the amplitude of the grid voltage, is the angle by which the grid current lags the grid voltage; α is the conduction angle, i.e., the radian by which the conduction signal of DS lags the phase voltage; V dc is the amplitude of the DC voltage; K is the ratio of the AC voltage amplitude to the DC voltage; Step 2: Adopt a three-level capacitor voltage balance control method for capacitor energy equalization control, including: A: The first-level balance control: The balance control of the sum of the SS capacitor energies According to the topology of the AAC, use the average value of the capacitor voltages of 6 SSs as the feedback quantity, and obtain a conduction angle for 6 DS switches through closed-loop; the closed-loop control expression of the conduction angle is: Where: Where: K P1 , K I1 are the coefficients of the PI controller; V * C is the reference value of the SS capacitor voltage; is the reference value of the power factor angle, s is the integral operator, I * d is the reference value of the d-axis current, is the average value of the total voltage of the sub-module capacitors of the upper and lower bridge arms, sgn(·) is the sign function, x is the input of the sign function, N is the number of SSs formed by cascading full-bridge sub-modules included in each bridge arm; v Ci is the sum of the capacitor voltages of all sub-modules; B: The second-level balance control: The capacitor energy balance control between SSs Step a: Adopt the zero-sequence voltage injection method to achieve the balance control of the capacitor voltages of 6 SSs: Divide a power frequency cycle into 6 regions, and select different SS capacitor voltage feedback quantities for control in different regions; the basis for selecting the feedback quantities in different regions is: 1) In the case of the change of the conduction angle, the SS corresponding to the feedback quantity fully input in this region; 2) In this region, the current polarity of the SS corresponding to the feedback quantity is unique; 3) In this region, the convergence of the energy balance control reaches the preset target; Step b: Determine the area number N T The expression of Where: In the formula: ceil(·) is the ceiling function; f1(·) is the angle conversion function used to convert the angle range to [0, 2π]; θ is the starting phase angle of region I; Step c: Select a proportional controller to achieve the rapid balance of the energy between modules, and the control link is expressed as: Where: K P2 is the proportional controller coefficient, is the sliding average value of the SS capacitor voltage, i = 1, 2, 3, …, 6; reflects that the direction of capacitor energy regulation is different when the phase current polarities are different; i * d is the d-axis current reference value; C: The third-level balance control: The capacitor voltage balance control between the internal sub-modules of the SS For NLM modulation, adopt the traditional capacitor voltage sorting algorithm to achieve the balance of the capacitor voltages of each SM; for carrier phase-shifted modulation, use multiple capacitor voltage regulators to generate the compensation voltage and add it to the original modulation voltage of different SMs to achieve the balance of the capacitor voltages between SMs; D: The overall control strategy: It includes the traditional double closed-loop control and the three-level capacitor voltage balance method.

2. The capacitor voltage balance control strategy of the bridge arm alternating current converter according to claim 1, characterized in that The specific overall control strategy is: The outer-loop controller is selected as the active / reactive power, DC link voltage / reactive power of the control system, or the AC side voltage under the dq axis; The inner-loop controller is used to track the reference dq-axis current generated by the outer loop; Add the three-phase output phase voltage reference to the zero-sequence voltage V0 obtained from the second-level balance control to calculate the output voltage reference of all SSs Generate the drive signals of all SSs through different modulation methods and their corresponding third-level balance control; Among them, the conduction angle α of the DS is calculated through the first-level balance control and then generates the drive signals of all DSs through the pulse generator.

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