A series h-bridge medium voltage motor drive system capacitance value reduction control method

Abstract

The application discloses a series H-bridge type medium-voltage motor driving system capacitor capacity reduction control method. The sub-module capacitor capacity reduction control method is to redesign the input side injected current command value of the front-end three-phase rectifier circuit of the sub-module under the condition of considering the asymmetric voltage condition of a power grid, that is, to inject negative sequence currents with two frequencies of 2ω o ‑ω s and 2ω o ‑3ω s or positive sequence currents with two frequencies of 2ω o +ω s and 2ω o +3ω s ; the instantaneous power at the input side of the sub-module is controlled by controlling the input side current of the sub-module, the difference between the instantaneous powers at the input and output sides of the capacitor, that is, the power required to be buffered by the sub-module capacitor, is reduced, and then the capacitor capacity is reduced. The application can offset the fluctuating power introduced by the existing injected current method under the asymmetric voltage condition of the power grid, further reduce the fluctuating voltage on the sub-module capacitor, and realize the reduction of the sub-module capacitor.

Description

Technical Field

[0001] This invention relates to the field of frequency converter technology, and specifically to a method for reducing the capacitance value of a series H-bridge type medium-voltage motor drive system. Background Technology

[0002] Medium-voltage motor drives are widely used in high-speed railways, mines, water conveyance, and other fields to save energy and improve motor performance. Series H-bridge multilevel converters have become one of the most mainstream medium-voltage motor drive solutions due to their advantages such as high output voltage waveform quality, modular structure, and strong fault tolerance. Because the series H-bridge medium-voltage motor drive system connects the three-phase stator windings of the motor through three single-phase H-bridge chains, the instantaneous output power of each sub-module in the H-bridge chain pulsates at twice the fundamental frequency, resulting in double-frequency ripple power on the DC capacitor of each sub-module. To absorb this double-frequency ripple power and achieve a stable DC voltage, a large electrolytic capacitor is typically installed on the DC side of each sub-module, which accounts for a significant proportion of the overall system volume and cost. More importantly, electrolytic capacitors typically have a limited lifespan under ripple power, which is one of the main obstacles to achieving a highly reliable motor drive system. Therefore, reducing the capacitor size of the sub-modules in the series H-bridge medium-voltage motor drive system is currently a research hotspot.

[0003] Active power decoupling technology is an effective method to solve this problem. Existing active power decoupling technologies typically employ active filters composed of switching devices and energy storage elements (inductors or capacitors). Additional active filters increase topology complexity, failure rate, and cost. Therefore, some studies have proposed a software-controlled current injection power decoupling technique. This technique injects current into the input side of the three-phase rectifier circuit at the front end of a submodule of a series H-bridge medium-voltage motor drive system, reducing the difference in instantaneous power between the input and output sides of the submodule capacitor—that is, the buffer power of the submodule capacitor—thus reducing the submodule capacitor's capacity without adding additional hardware. However, the injected current in these studies is derived based on normal grid operation conditions. Under normal grid operation, second-harmonic power fluctuations can be completely canceled out, and the submodule capacitor is designed to be very small. If an asymmetrical grid voltage fault occurs, existing current injection methods will generate new power fluctuations on the submodule capacitor, especially when the output frequency is close to the rated frequency. The capacitor value designed under normal grid operation will not meet the requirements, the voltage fluctuations on the submodule capacitor will still be large, and the capacitor required to stabilize the voltage will also be large. Summary of the Invention

[0004] To overcome the shortcomings of the existing technology, this invention proposes a control method for reducing the capacitance value of a series H-bridge medium-voltage motor drive system. This method involves redesigning the current injected into the input side of the submodule, injecting two frequencies of 2ω. o -ω s and 2ωo -3ω s Negative sequence current or two frequencies of 2ω o +ω s and 2ω o +3ω s The positive sequence current can offset the fluctuating power introduced by the existing injection current method under the condition of grid voltage asymmetry, further reduce the fluctuating voltage on the submodule capacitor, and realize the reduction of the submodule capacitor.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for reducing the capacitance value of a series H-bridge medium-voltage motor drive system, wherein the series H-bridge medium-voltage motor drive system includes a multi-winding transformer, a series H-bridge frequency converter, and a medium-voltage motor;

[0007] The primary winding of the multi-winding transformer is connected to the medium-voltage power grid, and the secondary winding consists of 3n windings. The series H-bridge inverter is composed of 3n sub-modules. Each sub-module is connected to one secondary winding of the multi-winding transformer on the power grid side (front end). Every n sub-modules are connected in series on the motor side (rear end) to form an H-bridge chain, forming a total of 3 H-bridge chains. The 3 H-bridge chains are connected to the three-phase stator winding of the medium-voltage motor in a Y-type structure.

[0008] The submodule consists of an input filter inductor, a front-end three-phase rectifier circuit, a DC-side capacitor, and a rear-end full-bridge inverter circuit.

[0009] The method for reducing the capacitance value of the submodule is based on the consideration of grid voltage asymmetry. It involves redesigning the input side of the three-phase rectifier circuit at the front end of the submodule to inject current commands, specifically injecting two frequencies of 2ω... o -ω s and 2ω o -3ω s Negative sequence current or two frequencies of 2ω o +ω s and 2ω o +3ω s Positive sequence current; ω o It is the output angular frequency of the back-end full-bridge inverter circuit, ω. s It is the angular frequency of the power grid;

[0010] By controlling the input current of the submodule, the instantaneous power of the submodule capacitor input side is controlled, thereby reducing the difference between the instantaneous power of the capacitor input and output sides, i.e. the power that the submodule capacitor needs to buffer, and thus reducing the capacitance value.

[0011] The method for controlling the reduction of the capacitance value of the submodule specifically includes the following steps;

[0012] Step 1: The front-end three-phase rectifier circuit of the submodule adopts an unbalanced control system structure for suppressing negative sequence current on the AC side in the dq coordinate system. The outer loop of the DC voltage on the d-axis uses a PI regulator, and the output of the PI regulator is the same as the DC current command. Correspondingly, it is compared with the DC voltage command value. Multiplying them gives the commanded value p of the average active power. * To suppress negative sequence current on the AC side and considering only the average active power p * and average reactive power q * The control utilizes instantaneous power theory to calculate the positive sequence current command value. and

[0013]

[0014] Input voltage u to the submodule sx (x = a, b, c represent the three phases A, B, and C of the power grid) positive and negative sequence separation is performed to obtain u. dp u qp u dn u qn ;

[0015] Step 2: Calculate the injection current command value considering the grid voltage asymmetry condition. and Inject current command value and Superimposed on the positive sequence current command value and In this process, a dq decoupling control strategy is adopted to control the d-axis and q-axis currents respectively, and finally generate a PWM signal to act on the switching transistors of the front-end three-phase rectifier circuit.

[0016] Step 2 specifically involves:

[0017] Step 2.1: Calculate the injected current i of this submodule. 1x and i 2x Instantaneous power p on the input side of the capacitor i ;

[0018] Under the condition of unbalanced grid voltage, the three-phase input voltage u of the submodule sx for:

[0019] u sx =U sp sin(ω s t+θ sp +θ x )+U sn sin(ω s t+θ sn -θ x )+Usz sin(ω s t+θ s z) (2)

[0021] In the formula, U sp U sn U sz Input the positive-sequence, negative-sequence, and zero-sequence voltage amplitudes to this submodule, ω s Let θ be the angular frequency of the power grid. sp θ sn θ sz Input the initial phase angles of the positive-sequence, negative-sequence, and zero-sequence components of the voltage into this submodule. Corresponding to x = a, b, c;

[0022] Inject current i into the input side of the submodule 1x and i 2x Then the three-phase input current i of the submodule x for:

[0023] i x =i sx +i 1x +i 2x

[0024] =I s sin(ω s t+θ sp +θ x )+I1sin(ω1t+θ 1x )+I2sin(ω2t+θ 2x (3)

[0025] In the formula, i sx I represents the original input current of this submodule. s I1 and I2 are the original input current amplitude of this submodule, and I1 and I2 are the injected current i of this submodule. 1x and i 2x Amplitude, ω1, ω2 are the injected current i of this submodule 1x and i 2x angular frequency, θ 1x θ 2x Inject current i into this submodule 1x and i 2x Initial phase angle;

[0026] The injected current i of this submodule can be derived from formulas (2) and (3). 1x and i 2x Instantaneous power p on the input side of the capacitor i :

[0027]

[0028] In the formula, p s Inject current i into this submodule 1x and i 2x The input instantaneous power after, p L Inject current i into this submodule 1x and i 2x The instantaneous power of the input filter inductor and the leakage inductance on the secondary side of the transformer. For input power DC, For the negative sequence voltage of the power grid and i sx The resulting fluctuations For grid voltage and injected current i 1x and i 2x The resulting fluctuation, L, is the sum of the input filter inductance of the module and the leakage inductance on the secondary side of the transformer;

[0029] in, Buffered by the submodule capacitor, It contains (ω1+ω s ), (ω1-ω s ), (ω2+ω s ), (ω2-ω s The six frequency components (ω1+ω2), (ω1-ω2), are used to cancel out the power fluctuations on the submodule output side. The specific expressions are as follows:

[0030]

[0031] in,

[0032]

[0033] Step 2.2: Calculate the instantaneous power p on the capacitor output side of this submodule. oy ;

[0034] Submodule output voltage and current fundamental instantaneous value u oy and i oy They are respectively:

[0035]

[0036] In the formula, y = u,v,w represents the three phases of the motor, U, V, W, and U o and I o These represent the fundamental amplitudes of the submodule's output voltage and current, respectively, ω. o δ represents the fundamental angular frequency of the submodule's output voltage and current, and δ is the initial phase difference between the input and output voltages. The phase difference between the output voltage and current is the power factor angle of the motor. Corresponding to y = u, v, w;

[0037] The instantaneous power p on the capacitor output side of this submodule can be obtained from formula (6). oy :

[0038]

[0039] In the formula, For output power DC flow, This refers to the output power fluctuation. Corresponding to y = u, v, w;

[0040] Step 2.3: Utilize the instantaneous power p on the capacitor input side i DC flow With the instantaneous power p on the capacitor output side oy DC flow If they are equal, find the current i used to transmit power. sx Amplitude I s :

[0041]

[0042] In the formula, the amplitude I s To find the unknown quantity, we solve equation (7) to obtain:

[0043]

[0044] Step 2.4: Utilize the instantaneous power p on the capacitor input side i Fluctuation in The instantaneous power p on the capacitor output side oy Medium volatility Cancel out, and calculate the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and injection current i 2x Amplitude I2, frequency ω2 and phase θ 2x ;

[0045] Step 2.5: Calculate the injected current i in the abc coordinate system. 1x and i 2x After performing abc / dq transformations separately, sum them to obtain the injection current command value in the dq coordinate system. and

[0046]

[0047] In the formula, T abc / dq This is the abc / dq transformation matrix.

[0048] In step 2.4:

[0049] Based on the different cancellation methods, the methods for determining the injected current can be divided into the following two types:

[0050] Method 1: The median value is (ω1+ω) s ) Components and Given that the frequency, amplitude, and phase are all equal, calculate the injected current i. 1x Then The amplitude in (ω2+ω) s ) Components and The median value is (ω1-ω) s If the component frequencies and phases are equal but the amplitudes are opposite, the injected current i can be calculated. 2x ,at this time Other (ω1+ω) s ), (ω1-ω s The components cancel each other out to zero among the three phases A, B, and C. The specific equations are as follows:

[0051]

[0052] in,

[0053]

[0054] In the formula, the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and the injected current i 2x Amplitude I2, frequency ω2 and phase θ 2x For unknown quantities to be determined;

[0055] Solving equation (10) yields:

[0056]

[0057] Right now:

[0058]

[0059] Method 2: The median amplitude is (ω1-ω) s ) Components and Since the frequency, amplitude, and phase are equal, the injected current i can be calculated. 1x Then The amplitude in (ω2-ω) s ) Components and The median amplitude is (ω1+ω) sGiven that the component frequencies and phases are equal but the amplitudes are opposite, determine the injected current i. 2x ,at this time Other (ω1+ω) s ), (ω1-ω s The components cancel each other out to zero among the three phases A, B, and C. The specific equations are as follows:

[0060]

[0061] in,

[0062]

[0063] In the formula, the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and the injected current i 2x Amplitude I2, frequency ω2 and phase θ 2x For unknown quantities to be determined;

[0064] Solving equation (13) yields:

[0065]

[0066] Right now:

[0067]

[0068] The injected current command value in step 2.5 and Further optimization is needed, and the specific steps are as follows:

[0069] (1) Utilizing the submodule output voltage u oy and output current i oy and u oy and i oy u′ obtained by 90° phase shift oy and i′ oy Find:

[0070]

[0071] in,

[0072]

[0073] (2) Using the positive and negative sequence components u of the submodule input voltage in the dq coordinate system dp u qp u dn u qn And the input voltage u without positive and negative sequence separation sd and u sq ,get:

[0074]

[0075] in,

[0076]

[0077] (3) Substitute equations (17)-(18) into equation (12) or equation (15) to obtain the optimized injection current i 1x and i 2x Substituting this into equation (16) yields the optimized injection current command value. and

[0078] 1) When using the cancellation method in step 2.4, inject the current command value. and as follows:

[0079]

[0080] in,

[0081]

[0082] 2) When using the second cancellation method in step 2.4, the injected current command value and as follows:

[0083]

[0084] in,

[0085]

[0086] u oy and u′ oy i is obtained directly from the modulation wave 2r / 3s in the motor control loop. oy and i′ oy The positive and negative sequence components of the submodule input voltage u in the dq coordinate system are obtained directly from the stator current 2r / 3s in the motor control loop. dp u qp u dn u qn And the input voltage u without positive and negative sequence separation sd and u sq They are all part of the unbalanced control system of the three-phase rectifier circuit that suppresses the negative sequence current on the AC side. Therefore, the amplitude and phase parameters required for the injection current calculation are obtained from the motor control loop and the three-phase rectifier circuit control loop of the sub-module. There is no need to add additional sensors or use complex algorithms to extract the amplitude and phase information separately before calculation.

[0087] The multi-winding transformer is either a conventional transformer or a phase-shifting transformer.

[0088] The injection current command value and This is a general expression considering the unbalanced voltage conditions of the power grid, let u sd =U sp ,u sq =0, obtaining the injected current command value when the power grid is operating normally. and

[0089] The beneficial effects of this invention are:

[0090] Because the submodule capacitor reduction control method of the series H-bridge medium-voltage motor drive system proposed in this invention redesigns the injected current command value on the input side of the three-phase rectifier circuit at the front end of the submodule under the condition of grid voltage asymmetry, the injected current i in the existing method is reduced. 1x On top of that, an additional set of currents i of other frequencies is injected. 2x This method offsets the power fluctuations introduced by existing methods under asymmetrical grid voltage conditions, further reducing the difference in instantaneous power between the input and output sides of the submodule capacitor, i.e., the buffer power of the submodule capacitor. Therefore, it overcomes the defect of large capacitor voltage fluctuations under asymmetrical grid voltage conditions in existing current injection methods, further reduces the submodule capacitor, shrinks the overall equipment size, lowers equipment costs, and improves equipment lifespan.

[0091] Because this invention redesigns and optimizes the current injected into the submodule input side under the condition of unbalanced grid voltage, the amplitude and phase parameters required for the injection current calculation can be obtained from the motor control loop and the submodule three-phase rectifier circuit control loop. There is no need to add additional sensors or use complex algorithms to extract the amplitude and phase information separately before calculation. Therefore, the method proposed in this invention is simple and reliable to implement. Attached Figure Description

[0092] Figure 1 This is a structural diagram of a series H-bridge type medium-voltage motor drive system.

[0093] Figure 2 This is a detailed circuit diagram of a series H-bridge inverter submodule.

[0094] Figure 3 This is a control block diagram of the front-end three-phase rectifier circuit of a series H-bridge inverter submodule.

[0095] Figure 4 This is a flowchart of the submodule capacitor value reduction control method proposed in this invention.

[0096] Figure 5This is a structural diagram of the series H-bridge type medium-voltage motor drive system in Embodiment 1 of the present invention.

[0097] Figure 6 The waveform diagram shows the simulation results of the traditional control method in Embodiment 1 of the present invention.

[0098] Figure 7 The waveform diagram shows the simulation results of the newly proposed control method in Embodiment 1 of the present invention.

[0099] Figure 8 The waveform diagram shows the simulation results of the traditional control method in Embodiment 2 of the present invention.

[0100] Figure 9 The waveform diagram shows the simulation results of the newly proposed control method in Embodiment 2 of the present invention. Detailed Implementation

[0101] The present invention will now be described in further detail with reference to the accompanying drawings.

[0102] To explain the working principle of the present invention, Figure 1 The following description uses a series H-bridge medium-voltage motor drive system as an example. This system consists of a multi-winding transformer, a series H-bridge frequency converter, and a medium-voltage motor. The primary winding of the multi-winding transformer is connected to the medium-voltage power grid, and the secondary winding has 3n windings. The series H-bridge frequency converter consists of 3n sub-modules. Each sub-module is connected at its front end to one secondary winding of the multi-winding transformer. Every n sub-modules are connected in series at their rear ends to form an H-bridge chain, for a total of three H-bridge chains. These three H-bridge chains are connected in a Y-shape to the three-phase stator windings of the medium-voltage motor. The specific circuit structure of the sub-modules is as follows... Figure 2 As shown, it consists of an input filter inductor, a front-end three-phase rectifier circuit, a DC-side capacitor, and a rear-end full-bridge inverter circuit.

[0103] The method for reducing the capacitance value of submodules in a series H-bridge medium-voltage motor drive system proposed in this invention is implemented as follows:

[0104] like Figure 3 As shown:

[0105] Step 1: The front-end three-phase rectifier circuit of the submodule adopts an unbalanced control system structure for suppressing negative sequence current on the AC side in the dq coordinate system. The outer loop of the d-axis DC voltage uses a PI regulator, whose output is related to the DC current command. Correspondingly, it is compared with the DC voltage command value. Multiplying them gives the commanded value p of the average active power. * To suppress negative sequence current on the AC side and considering only the average active power p * and average reactive power q * The control of this system utilizes instantaneous power theory to calculate the positive sequence current command value. and

[0106]

[0107] Input voltage u to the submodule sx (x = a, b, c represent the three phases A, B, and C of the power grid) By separating the positive and negative sequences, we can obtain u. dp u qp u dn u qn .

[0108] Step 2: Calculate the injection current command value considering the grid voltage asymmetry condition. and Injected current and Superimposed on the positive sequence current command value and In this process, a dq decoupling control strategy is adopted to control the d-axis and q-axis currents respectively, and finally generate a PWM signal to act on the switching transistors of the front-end three-phase rectifier circuit.

[0109] In step 2, under the condition of unbalanced grid voltage, the three-phase input voltage u of the submodule sx for:

[0110] u sx =U sp sin(ω s t+θ sp +θ x )+U sn sin(ω s t+θ sn +θ x )+U sz sin(ω s t+θ sz ) (2)

[0112] In the formula, U sp U sn U sz Input the positive-sequence, negative-sequence, and zero-sequence voltage amplitudes to this submodule, ω s Let θ be the angular frequency of the power grid. sp θ sn θ sz Input the initial phase angles of the positive-sequence, negative-sequence, and zero-sequence components of the voltage into this submodule. The corresponding values ​​are x = a, b, c.

[0113] Inject current i into the input side of the submodule 1x and i 2x Then the three-phase input current i of the submodulex for:

[0114] i x =i sx +i 1x +i 2x

[0115] =I s sin(ω s t+θ sp +θ x )+I1sin(ω1t+θ 1x )+I2sin(ω2t+θ 2x (3)

[0116] In the formula, i sx I represents the original input current of this submodule. s I1 and I2 are the original input current amplitude of this submodule, and I1 and I2 are the injected current i of this submodule. 1x and i 2x Amplitude, ω1, ω2 are the injected current i of this submodule 1x and i 2x angular frequency, θ 1x θ 2x Inject current i into this submodule 1x and i 2x Initial phase angle.

[0117] Submodule output voltage and current fundamental instantaneous value u oy and i oy They are respectively:

[0118]

[0119] In the formula, y = u,v,w represents the three phases of the motor, U, V, W, and U o and I o These represent the fundamental amplitudes of the submodule's output voltage and current, respectively, ω. o δ represents the fundamental angular frequency of the submodule's output voltage and current, and δ is the initial phase difference between the input and output voltages. The phase difference between the output voltage and current is the power factor angle of the motor. The corresponding values ​​are y = u, v, w.

[0120] According to such Figure 4 The procedure shown can be used to determine the injected current command value. and The calculation formula and specific steps are as follows:

[0121] (1) The injection current i can be calculated by using the fact that some terms in the instantaneous power expression on the input and output sides of the capacitor are equal. 1x and i 2xTherefore, to find the injected current i, we need to consider the system of equations. 1x and i 2x The injected current i of this submodule needs to be calculated first. 1x and i 2x Instantaneous power p on the input side of the capacitor i The specific expression:

[0122]

[0123] In the formula, p s Inject current i into this submodule 1x and i 2x The input instantaneous power after, p L Inject current i into this submodule 1x and i 2x The instantaneous power of the input filter inductor and the leakage inductance on the secondary side of the transformer. For input power DC, For the negative sequence voltage of the power grid and i sx The resulting fluctuations For grid voltage and injected current i 1x and i 2x The resulting fluctuation, L, is the sum of the input filter inductance of the module and the leakage inductance on the secondary side of the transformer.

[0124] in, Buffered by the submodule capacitor, It contains (ω1+ω s ), (ω1-ω s ), (ω2+ω s ), (ω2-ω s The six frequency components (ω1+ω2), (ω1-ω2), etc., are used to cancel out the power fluctuations on the output side of the submodule. The specific expressions are as follows:

[0125]

[0126] in,

[0127]

[0128] (2) Then calculate the instantaneous power p on the capacitor output side of this submodule. oy The specific expression:

[0129]

[0130] In the formula, For output power DC flow, This refers to the output power fluctuation. The corresponding values ​​are y = u, v, w.

[0131] The above two steps yield the specific expressions for the instantaneous power on the capacitor's input and output sides. Then, by ensuring that the specific terms in the two expressions are equal, the injected current i can be calculated. 1x and i 2x The system of equations.

[0132] (3) Utilizing the instantaneous power p on the capacitor input side i DC flow With the instantaneous power p on the capacitor output side oy DC flow If they are equal, find the current i used to transmit power. sx Amplitude I s :

[0133]

[0134] In the formula, the amplitude I s Let be the unknown quantity to be solved. Solving equation (7) yields:

[0135]

[0136] The obtained amplitude I s It can be used to subsequently optimize the expression for the injected current command value.

[0137] (4) Utilizing the instantaneous power p on the capacitor input side i Fluctuation in The instantaneous power p on the capacitor output side oy Medium volatility Cancel out, and calculate the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and injection current i 2x Amplitude I2, frequency ω2 and phase θ 2x Based on the different methods of cancellation, the methods for determining the injected current can be divided into the following two categories:

[0138] 1) Method 1: [The rest of the text appears to be a list of methods and instructions, possibly related to a specific method or approach.] The median amplitude is (ω1+ω) s ) Components and Given that the frequency, amplitude, and phase are all equal, calculate the injected current i. 1x Then The amplitude in (ω2+ω) s ) Components and The median amplitude is (ω1-ω) s If the component frequencies and phases are equal but the amplitudes are opposite, the injected current i can be calculated. 2x ,at this time Other (ω1+ω)s ), (ω1-ω s The components cancel each other out to zero among the three phases A, B, and C. The specific equations are as follows:

[0139]

[0140] in,

[0141]

[0142] In the formula, the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and the injected current i 2x Amplitude I2, frequency ω2 and phase θ 2x For unknown quantities to be determined;

[0143] Solving equation (10) yields:

[0144]

[0145] Right now:

[0146]

[0147] 2) The median amplitude is (ω1-ω) s ) Components and Since the frequency, amplitude, and phase are equal, the injected current i can be calculated. 1x Then The amplitude in (ω2-ω) s ) Components and The median amplitude is (ω1+ω) s Given that the component frequencies and phases are equal but the amplitudes are opposite, determine the injected current i. 2x ,at this time Other (ω1+ω) s ), (ω1-ω s The components cancel each other out to zero among the three phases A, B, and C. The specific equations are as follows:

[0148]

[0149] in,

[0150]

[0151] In the formula, the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and the injected current i 2x Amplitude I2, frequency ω2 and phase θ2x For unknown quantities to be determined;

[0152] Solving equation (13) yields:

[0153]

[0154] Right now:

[0155]

[0156] The two cancellation methods described above are respectively designed for frequencies of 2ω in existing methods. o -ω s The injection current and frequency are 2ω o +ω s The injected current, and additional currents i of different frequencies are injected. 2x To counteract the fluctuating power introduced by existing methods, two frequencies of 2ω are injected. o -ω s and 2ω o -3ω s Negative sequence current or two frequencies of 2ω o +ω s and 2ω o +3ω s The positive sequence current. The main difference between the injected current obtained by these two cancellation methods lies in the frequency. Since the injected current flows through the filter inductor and transformer, the higher the frequency of the current, the higher the voltage drop across the inductor. The rectifier needs to provide a higher modulation ratio to balance the voltage drop. Therefore, compared with method one, method two requires increasing the DC voltage of the submodule or using a special modulation method to extend the linear modulation region to avoid overmodulation.

[0157] (5) The injected current i in the abc coordinate system 1x and i 2x After performing abc / dq transformations separately, sum them to obtain the injection current command value in the dq coordinate system. and

[0158]

[0159] In the formula, T abc / dq This is the abc / dq transformation matrix.

[0160] It can be seen from equations (12) and (15) that the injection current i obtained through the above steps 1x and i 2x The expression is quite complex, and its injected current command value after abc / dq transformation is... and Additional algorithms are needed to extract specific phase information; therefore, the injected current command value... and The calculation formula can be further optimized, and the specific steps are as follows:

[0161] (1) Utilizing the submodule output voltage u oy and output current i oy and u oy and i oy u′ obtained by 90° phase shift oy and i′ oy We can obtain:

[0162]

[0163] in,

[0164]

[0165] (2) Using the positive and negative sequence components u of the submodule input voltage in the dq coordinate system dp u qp u dn u qn And the input voltage u without positive and negative sequence separation sd and u sq We can obtain:

[0166]

[0167] in,

[0168]

[0169] (3) Substituting equations (17)-(18) into equation (12) or equation (15), the optimized injection current i can be obtained. 1x and i 2x Substituting this into equation (16) yields the optimized injection current command value. and

[0170] 1) Using the cancellation method to inject current command value and as follows:

[0171]

[0172] in,

[0173]

[0174] 2) Injection current command value when using cancellation method two and as follows:

[0175]

[0176] in,

[0177]

[0178] u oy and u′ oy It can be directly obtained from the modulation wave 2r / 3s in the motor control loop, i oy and i′ oy It can be directly obtained from the stator current 2r / 3s in the motor control loop, and the positive and negative sequence components u of the submodule input voltage in the dq coordinate system. dp u qp u dn u qn And the input voltage u without positive and negative sequence separation sd and u sq They are all part of the unbalanced control system of the three-phase rectifier circuit that suppresses the negative sequence current on the AC side. Therefore, the amplitude and phase parameters required for the injection current calculation can be obtained from the motor control loop and the three-phase rectifier circuit control loop of the sub-module. There is no need to add additional sensors or use complex algorithms to extract the amplitude and phase information separately and then perform the calculation.

[0179] The above describes the proposed method for reducing the capacitance value of submodules in a series H-bridge medium-voltage motor drive system. The principle of this invention is as follows:

[0180] Based on the existing current injection method, the current injection command value on the input side of the submodule is redesigned, and an additional set of currents of other frequencies is injected, that is, two frequencies of 2ω are injected. o -ω s and 2ω o -3ω s Negative sequence current or two frequencies of 2ω o +ω s and 2ω o +3ω s The positive sequence current can offset the fluctuating power introduced by existing methods under the condition of unbalanced grid voltage, further reduce the difference in instantaneous power between the input and output sides of the submodule capacitor, i.e., the buffer power of the submodule capacitor, thereby further reducing the submodule capacitor, reducing the overall size of the equipment, reducing the equipment cost, and improving the equipment life.

[0181] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings.

[0182] Example 1

[0183] This embodiment employs the submodule capacitor value reduction control method proposed in this invention—injecting two frequencies of 2ω. o -ωs and 2ω o -3ω s The negative sequence current, with Figure 5 The proposed method is illustrated using a series H-bridge medium-voltage motor drive system as an example. This system consists of a multi-winding transformer, a series H-bridge frequency converter, and a medium-voltage motor. The primary winding of the multi-winding transformer is connected to the medium-voltage power grid, while the secondary winding has 15 windings. The series H-bridge frequency converter consists of 15 sub-modules. Each sub-module is connected at its front end to one secondary winding of the multi-winding transformer. Every five sub-modules are connected in series at their rear ends to form an H-bridge chain, resulting in a total of three H-bridge chains connected to the three-phase stator windings of the medium-voltage motor. The specific circuit structure of each sub-module is as follows... Figure 2 As shown, it consists of an input filter inductor, a front-end three-phase rectifier circuit, a DC-side capacitor, and a rear-end full-bridge inverter circuit.

[0184] In this embodiment, the specific parameters of the transformer and frequency converter are shown in Table 1, and the specific parameters of the motor are shown in Table 2. The circuit is simulated using MATLAB / Simulink.

[0185] Table 1 Specific parameters of transformers and frequency converters

[0186]

[0187]

[0188] Table 2 Specific parameters of the motor

[0189]

[0190] To verify that the proposed submodule capacitor reduction control method can reduce the capacitor value compared to traditional methods when considering grid voltage asymmetry, simulations were conducted with an output frequency of 40Hz and a motor speed of 1200 r / min. Figure 6 , Figure 7 The figures show the simulation waveforms of the traditional control method and the newly proposed control method, respectively. Subfigure (a) shows the grid voltage waveform u. ga u gb u gc Sub-figure (b) shows the grid current waveform i ga i gb i gc Sub-diagram (c) shows the capacitor voltage waveform u of the sub-module. dc Subgraph (d) shows the speed waveform n of the medium-voltage motor.

[0191] At the start of the simulation, the motor started under no-load and reached a speed of 1200 r / min. At t = 0.8 s, the load stepped to the rated load of 710 kW. At t = 1.2 s, a single-phase ground fault occurred in phase C of the power grid. Throughout the simulation, under normal power grid operation, both control methods, with a capacitor value of 950 μF, could control the voltage fluctuation amplitude of the submodule capacitor within 5%, and the power grid current waveform quality was good, exhibiting a three-phase symmetrical sine wave. However, under asymmetrical power grid voltage conditions, the traditional control method resulted in poor power grid current waveform quality, a sharp increase in submodule capacitor voltage fluctuation, and the 950 μF capacitor value could not meet the requirement of voltage fluctuation within 5%, necessitating a larger capacitor value. The newly proposed control method can control the voltage fluctuation amplitude of the submodule capacitor to within 62V, which is about 5% of the voltage set value (1225V), even under the worst condition of a single-phase ground fault in phase C. The grid current waveform can still maintain a three-phase symmetrical sinusoidal waveform. Compared with the traditional control method, the newly proposed control method can further reduce the capacitor value under the condition of grid voltage asymmetry.

[0192] Example 2

[0193] This embodiment employs the submodule capacitor value reduction control method proposed in this invention—injecting two frequencies of 2ω. o +ω s and 2ω o +3ω s The positive sequence current, with Figure 5 The proposed method will be explained using the series H-bridge type medium-voltage motor drive system shown as an example.

[0194] In this embodiment, the specific parameters of the transformer and frequency converter are shown in Table 3, and the specific parameters of the motor are the same as those in Table 2. The circuit is simulated using MATLAB / Simulink.

[0195] Table 3 Specific parameters of transformers and frequency converters

[0196]

[0197] To verify that the proposed submodule capacitor reduction control method can reduce the capacitor value compared to traditional methods when considering grid voltage asymmetry, the simulation was conducted with an output frequency of 50Hz and a motor rated speed of 1470r / min. Figure 8 , Figure 9 The figures show the simulation waveforms of the traditional control method and the newly proposed control method, respectively. Subfigure (a) shows the grid voltage waveform u. ga u gb u gc Sub-figure (b) shows the grid current waveform i ga i gbi gc Sub-diagram (c) shows the capacitor voltage waveform u of the sub-module. dc Subgraph (d) shows the speed waveform n of the medium-voltage motor.

[0198] In the initial stage of the simulation, the motor started under no-load and reached a speed of 1470 r / min; at t = 0.8 s, the load stepped to the rated load of 710 kW; at t = 1.2 s, a single-phase ground fault occurred in phase C of the power grid. Throughout the simulation, under normal power grid operation, both control methods, with a capacitor value of 350 μF, could control the voltage fluctuation amplitude of the submodule capacitor within 5%, and the power grid current waveform quality was good, exhibiting a three-phase symmetrical sine wave. Similarly, under asymmetrical power grid voltage conditions, the traditional control method resulted in poor power grid current waveform quality and increased voltage fluctuation in the submodule capacitor, requiring a further increase in capacitor size to meet the requirement of voltage fluctuation within 5%. The newly proposed control method, even under the worst-case scenario of a single-phase ground fault in phase C, could control the voltage fluctuation amplitude of the submodule capacitor within 85 V, approximately 5% of the voltage setpoint (1700 V). Compared to the traditional control method, the newly proposed control method can further reduce the capacitor value under asymmetrical power grid voltage conditions while maintaining good power grid current waveform quality.

[0199] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reducing the capacitance value of a series H-bridge medium-voltage motor drive system, wherein the series H-bridge medium-voltage motor drive system includes a multi-winding transformer, a series H-bridge frequency converter, and a medium-voltage motor; in, The primary winding of the multi-winding transformer is connected to the medium-voltage power grid, and the secondary winding consists of 3n windings. The series H-bridge inverter is composed of 3n sub-modules. The front end of each sub-module is connected to one secondary winding of the multi-winding transformer, and the rear ends of every n sub-modules are connected in series to form an H-bridge chain, forming a total of 3 H-bridge chains. The 3 H-bridge chains are connected to the three-phase stator winding of the medium-voltage motor in a Y-type structure. The submodule consists of an input filter inductor, a front-end three-phase rectifier circuit, a DC-side capacitor, and a rear-end full-bridge inverter circuit. The characteristic feature is that the method for reducing the capacitance value of the submodule is to redesign the injection current command value on the input side of the three-phase rectifier circuit at the front end of the module, taking into account the asymmetrical voltage condition of the power grid, that is, to inject two frequencies of 2ω. o -ω s and 2ω o -3ω s Negative sequence current or two frequencies of 2ω o +ω s and 2ω o +3ω s Positive sequence current; ω o It is the output angular frequency of the back-end full-bridge inverter circuit, ω. s It is the angular frequency of the power grid; By controlling the input current of the submodule, the instantaneous power of the submodule capacitor input side is controlled, thereby reducing the difference between the instantaneous power of the capacitor input and output sides, i.e. the power that the submodule capacitor needs to buffer, and thus reducing the capacitance value. The method for controlling the reduction of the capacitance value of the submodule specifically includes the following steps; Step 1: The front-end three-phase rectifier circuit of the submodule adopts an unbalanced control system structure for suppressing negative sequence current on the AC side in the dq coordinate system. The outer loop of the DC voltage on the d-axis uses a PI regulator, and the output of the PI regulator is the same as the DC current command. Correspondingly, it is compared with the DC voltage command value. Multiplying them together gives the commanded value p of the average active power. * To suppress negative sequence current on the AC side and considering only the average active power p * and average reactive power q * The control utilizes instantaneous power theory to calculate the positive sequence current command value. and Input voltage u to the submodule sx Separating positive and negative orders yields u dp u qp u dn u qn x = a, b, c represent the three phases A, B, and C of the power grid; Step 2: Calculate the injection current command value considering the grid voltage asymmetry condition. and Inject current command value and Superimposed on the positive sequence current command value and In this process, a dq decoupling control strategy is adopted to control the d-axis and q-axis currents respectively, and finally generate a PWM signal to act on the switching transistors of the front-end three-phase rectifier circuit. Among them, the injection current command value and The calculation formula and specific steps are as follows: Under the condition of unbalanced grid voltage, determine the injected current i of this submodule. 1x and i 2x Instantaneous power p on the input side of the capacitor i ; Calculate the instantaneous power p on the capacitor output side of this submodule. oy ; Utilizing the instantaneous power p on the capacitor input side i DC flow With the instantaneous power p on the capacitor output side oy DC flow If they are equal, find the current i used to transmit power. sx Amplitude I s ; Utilizing the instantaneous power p on the capacitor input side i Fluctuation in The instantaneous power p on the capacitor output side oy Medium volatility Cancel out, and calculate the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and injection current i 2x Amplitude I2, frequency ω2 and phase θ 2x ; The injected current i in the abc coordinate system 1x and i 2x After performing abc / dq transformations separately, sum them to obtain the injection current command value in the dq coordinate system. and 2. The method for reducing the capacitance value of a series H-bridge medium-voltage motor drive system according to claim 1, characterized in that, Step 2 specifically involves: Step 2.1: Calculate the injected current i of this submodule. 1x and i 2x Instantaneous power p on the input side of the capacitor i ; Under the condition of unbalanced grid voltage, the three-phase input voltage u of the submodule sx for: you sx =U sp sin(ω s t+θ sp +θ x )+U sn sin(ω s t+θ sn -θ x )+U sz sin(ω s t+θ sz ) (2) In the formula, U sp U sn U sz Input the positive-sequence, negative-sequence, and zero-sequence voltage amplitudes to this submodule, ω s Let θ be the angular frequency of the power grid. sp θ sn θ sz Input the initial phase angles of the positive-sequence, negative-sequence, and zero-sequence components of the voltage into this submodule. Corresponding to x = a, b, c; Inject current i into the input side of the submodule 1x and i 2x Then the three-phase input current i of the submodule x for: i x =i sx +i 1x +i 2x =I s sin(ω s t+θ sp +θ x )+I1sin(ω1t+θ 1x )+I2sin(ω2t+θ 2x ) (3) In the formula, i sx I represents the original input current of this submodule. s I1 and I2 are the original input current amplitude of this submodule, and I1 and I2 are the injected current i of this submodule. 1x and i 2x Amplitude, ω1, ω2 are the injected current i of this submodule 1x and i 2x angular frequency, θ 1x θ 2x Inject current i into this submodule 1x and i 2x Initial phase angle; The injected current i of this submodule can be obtained from formulas (2) and (3). 1x and i 2x Instantaneous power p on the input side of the capacitor i : In the formula, p s Inject current i into this submodule 1x and i 2x The input instantaneous power after, p L Inject current i into this submodule 1x and i 2x The instantaneous power of the input filter inductor and the leakage inductance on the secondary side of the transformer. For input power DC, For the negative sequence voltage of the power grid and i sx The resulting fluctuations For grid voltage and injected current i 1x and i 2x The resulting fluctuation, L, is the sum of the input filter inductance of the module and the leakage inductance on the secondary side of the transformer; in, Buffered by the submodule capacitor, It contains (ω1+ω s ), (ω1-ω s ), (ω2+ω s ), (ω2-ω s The six frequency components (ω1+ω2), (ω1-ω2), are used to cancel out the power fluctuations on the submodule output side. The specific expressions are as follows: in, Step 2.2: Calculate the instantaneous power p on the capacitor output side of this submodule. oy ; Submodule output voltage and current fundamental instantaneous value u oy and i oy They are respectively: In the formula, y = u,v,w represents the three phases of the motor, U, V, W, and U o and I o These represent the fundamental amplitudes of the submodule's output voltage and current, respectively, ω. o δ represents the fundamental angular frequency of the submodule's output voltage and current, and δ is the initial phase difference between the input and output voltages. The phase difference between the output voltage and current is the power factor angle of the motor. Corresponding to y = u, v, w; The instantaneous power p on the capacitor output side of this submodule can be obtained from formula (6). oy : In the formula, For output power DC flow, This refers to the output power fluctuation. Corresponding to y = u, v, w; Step 2.3: Utilize the instantaneous power p on the capacitor input side i DC flow With the instantaneous power p on the capacitor output side oy DC flow If they are equal, find the current i used to transmit power. sx Amplitude I s : In the formula, the amplitude I s To find the unknown quantity, we solve equation (7) to obtain: Step 2.4: Utilize the instantaneous power p on the capacitor input side i Fluctuation in The instantaneous power p on the capacitor output side oy Medium volatility Cancel out, and calculate the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and injection current i 2x Amplitude I2, frequency ω2 and phase θ 2x ; Step 2.5: Calculate the injected current i in the abc coordinate system. 1x and i 2x After performing abc / dq transformations separately, sum them to obtain the injection current command value in the dq coordinate system. and In the formula, T abc / dq This is the abc / dq transformation matrix.

3. The method for reducing the capacitance value of a series H-bridge type medium-voltage motor drive system according to claim 2, characterized in that, In step 2.4: Based on the different cancellation methods, the methods for determining the injected current can be divided into the following two types: Method 1: The median amplitude is (ω1+ω) s ) Components and Given that the frequency, amplitude, and phase are all equal, calculate the injected current i. 1x Then The amplitude in (ω2+ω) s ) Components and The median amplitude is (ω1-ω) s If the component frequencies and phases are equal but the amplitudes are opposite, the injected current i can be calculated. 2x ,at this time Other (ω1+ω) s ), (ω1-ω s The components cancel each other out to zero among the three phases A, B, and C. The specific equations are as follows: in, In the formula, the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and the injected current i 2x Amplitude I2, frequency ω2 and phase θ 2x For unknown quantities to be determined; Solving equation (10) yields: Right now: Method 2: The median amplitude is (ω1-ω) s ) Components and Since the frequency, amplitude, and phase are equal, the injected current i can be calculated. 1x Then The amplitude in (ω2-ω) s ) Components and The median amplitude is (ω1+ω) s Given that the component frequencies and phases are equal but the amplitudes are opposite, determine the injected current i. 2x ,at this time Other (ω1+ω) s ), (ω1-ω s The components cancel each other out to zero among the three phases A, B, and C. The specific equations are as follows: in, In the formula, the injected current i 1x Amplitude I1, frequency ω1 and phase θ 1x and the injected current i 2x Amplitude I2, frequency ω2 and phase θ 2x For unknown quantities to be determined; Solving equation (13) yields: Right now:

4. The method for reducing the capacitance value of a series H-bridge type medium-voltage motor drive system according to claim 3, characterized in that, The injected current command value in step 2.5 and Further optimization is needed, and the specific steps are as follows: (1) Utilizing the submodule output voltage u oy and output current i oy and u oy and i oy u′ obtained by 90° phase shift oy and i′ oy Find: in, (2) Using the positive and negative sequence components u of the submodule input voltage in the dq coordinate system dp u qp u dn u qn And the input voltage u without positive and negative sequence separation sd and u sq ,get: in, (3) Substitute equations (17)-(18) into equation (12) or equation (15) to obtain the optimized injection current i 1x and i 2x Substituting this into equation (16) yields the optimized injection current command value. and 5. The method for reducing the capacitance value of a series H-bridge type medium-voltage motor drive system according to claim 3, characterized in that, 1) When using the cancellation method in step 2.4, inject the current command value. and as follows: in, 2) When using the second cancellation method in step 2.4, the injected current command value and as follows: in, u oy and u′ oy i is obtained directly from the modulation wave 2r / 3s in the motor control loop. oy and i′ oy The positive and negative sequence components of the submodule input voltage u in the dq coordinate system are obtained directly from the stator current 2r / 3s in the motor control loop. dp u qp u dn u qn And the input voltage u without positive and negative sequence separation sd and u sq They are all part of the unbalanced control system of the three-phase rectifier circuit that suppresses the negative sequence current on the AC side. Therefore, the amplitude and phase parameters required for the injection current calculation are obtained from the motor control loop and the three-phase rectifier circuit control loop of the sub-module. There is no need to add additional sensors or use complex algorithms to extract the amplitude and phase information separately before calculation.

6. The method for reducing the capacitance value of a series H-bridge type medium-voltage motor drive system according to claim 5, characterized in that, The injection current command value and This is a general expression considering the unbalanced voltage conditions of the power grid, let u sd =U sp ,u sq =0, obtaining the injected current command value when the power grid is operating normally. and 7. The method for reducing the capacitance value of a series H-bridge medium-voltage motor drive system according to claim 1, characterized in that, The multi-winding transformer is either a conventional transformer or a phase-shifting transformer.

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