Neutral-point voltage balance control method for three-level back-to-back frequency converter based on zero-sequence voltage injection

Abstract

The invention discloses a neutral-point voltage balance control method for a three-level back-to-back frequency converter based on zero-sequence voltage injection, and belongs to the technical field of power electronics. The neutral-point voltage balance control method comprises the following steps: acquiring input three-phase voltage and current of a rectifier, output three-phase voltage and current of an inverter and voltage instantaneous values of two capacitors of a direct-current bus through voltage and current Hall components; the signals are input into the DSP after being subjected to conditioning circuit and A / D conversion; the DSP processor obtains a midpoint average current needing to be compensated by calculating a positive and negative bus capacitor voltage difference, and determines a rectification side midpoint average current reference value by combining the inversion side midpoint average current; based on the characteristic that zero-sequence voltage of a three-phase three-wire system does not affect line voltage, key zero-sequence voltage is selected within a limited range by establishing the relation between midpoint average current and the zero-sequence voltage on the rectification side, and PWM modulation waves are adjusted to achieve midpoint voltage balance. The method is simple in calculation, high in real-time performance and suitable for all modulation degree scenes.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics, and particularly relates to a neutral-point voltage balance control method for a three-level back-to-back frequency converter based on zero-sequence voltage injection. BACKGROUND

[0002] In recent years, high-voltage and large-capacity frequency converters based on multi-level technology are increasingly widely applied in industrial systems in China, and have achieved remarkable energy-saving effects, improved the efficiency of industrial production, and brought significant economic benefits. Generally, multi-level converters can be divided into two categories according to the voltage level: one is a high-voltage multi-level converter with a cascaded H-bridge (CHB) structure, mainly applied to 6kV and 10kV-grade high-voltage and large-capacity AC motors; the other is a medium-voltage three-level converter with a neutral-point-clamped (NPC) structure, mainly applied to 3kV and below-grade motors.

[0003] At present, three-level back-to-back frequency converters based on diode-clamped three-level converters are widely applied in medium-voltage and high-power fields such as steel rolling, railways, and mines due to their good control effect, mature control method, and simple implementation. However, for three-level back-to-back frequency converter systems, the balance control of the DC bus midpoint voltage is a key problem, and the unbalance of the midpoint voltage will lead to an increase in the harmonic distortion rate of the frequency converter output voltage, uneven voltage stress of the switching device, and serious influence on the operation stability and reliability of the system. The existing midpoint voltage balance control methods have defects such as complex calculation, limited range of applicable modulation degree, and are difficult to meet the requirements of control accuracy and real-time performance in industrial applications. SUMMARY

[0004] The purpose of the present application is to provide a three-level back-to-back frequency converter midpoint voltage balance control method based on zero-sequence voltage injection, which solves the balance control problem of the DC bus midpoint voltage in a three-level back-to-back frequency converter system, and the method is simple to calculate and applicable in various modulation degree conditions.

[0005] In order to achieve the above purpose, the technical solution adopted by the present application is as follows:

[0006] A three-level back-to-back frequency converter midpoint voltage balance control method based on zero-sequence voltage injection is implemented according to the following steps:

[0007] Step 1: Collect the three-phase voltage and current instantaneous values of the rectifier input, the three-phase voltage and current instantaneous values of the inverter output, and the voltage instantaneous values of the two DC capacitors of the DC bus in the three-level back-to-back frequency converter system through voltage Hall components and current Hall components, respectively;

[0008] Step 2, the three-phase voltage and current instantaneous values of the rectifier input, the three-phase voltage and current instantaneous values of the inverter output and the voltage instantaneous values of the two DC capacitors of the DC bus after being processed by the conditioning circuit are converted into digital signals by the A / D module and input into the DSP processor;

[0009] Step 3, the DSP processor processes the digital three-phase voltage and current instantaneous values of the rectifier input, the three-phase voltage and current instantaneous values of the inverter output and the voltage instantaneous values of the two DC capacitors of the DC bus to obtain the zero sequence voltage component to be injected;

[0010] Step 4, the DSP processor adjusts the PWM modulation wave according to the zero sequence voltage component obtained in Step 3, thereby adjusting the midpoint voltage of the three-level back-to-back frequency converter.

[0011] Preferably, the zero sequence voltage component obtained in Step 3 is implemented according to the following steps:

[0012] Step 3.1, for the three-level back-to-back frequency converter, assuming that the bus capacitor is C, the capacitor voltage between the positive bus and the intermediate bus is u H , the capacitor voltage between the intermediate bus and the negative bus is u L , and the capacitor voltage difference between the positive bus and the negative bus is:

[0013] Δu=u L -u H (1)

[0014] Therefore, the midpoint average current to be compensated for maintaining the balance of the midpoint voltage is i o :

[0015]

[0016] Step 3.2, the midpoint average current of the rectifier side of the three-level back-to-back frequency converter is defined as i o1 , the bus midpoint average current generated by the rectifier side control is i o1 , which is affected by i o and i o2 , that is, it satisfies

[0017] i o1 =i o -i o2 (3)

[0018] Step 3.3, the three-phase output reference voltage of the inverter side is set as

[0019]

[0020] wherein: V m is the phase voltage amplitude when the maximum linear modulation is satisfied m is the modulation ratio, satisfying 0 ≤ m ≤ 1; θ is the voltage phase angle. Choose half the bus voltage, i.e., V. dc / 2 represents the voltage base value, thus obtaining the per-unit value V of the three-phase reference voltage without zero-sequence voltage. u0 V v0 and V w0 :

[0021]

[0022] For a three-phase symmetrical system, assuming the load current is a continuous sinusoidal quantity across three phases, its three-phase current can be written as:

[0023]

[0024] Among them I m The phase current amplitude, The power factor angle.

[0025] Step 3.4, let the average current at the midpoint of the inverter side be i. o2 Define the DC bus midpoint voltage level as zero. For a midpoint-clamped three-level converter, the current of a phase only contributes to the bus midpoint current when the output voltage level of that phase is zero. Since voltage level changes occur within a switching cycle, the bus midpoint current also changes accordingly. We take the average value of the bus midpoint current within one switching cycle as the research object. Therefore, for the inverter side, the average bus midpoint current i within one switching cycle is... o2 With three-phase output voltage v u0 v v0 v w0 and three-phase output current i u i v i w The relationship is:

[0026]

[0027] Step 3.5, combining equations (2), (3), and (7), we obtain the average midpoint current i of the rectifier side of the three-level back-to-back inverter. o1 :

[0028]

[0029] When the control rectifier bus midpoint current satisfies equation (8), the theoretical difference between the upper and lower bus capacitor voltages is zero. That is, the above equation is the reference value of the average current at the rectifier bus midpoint in the bus midpoint voltage balance control algorithm.

[0030] Step 3.6, assuming the three-phase input reference voltage on the rectifier side is:

[0031]

[0032] Select half of the bus voltage, i.e., V. dc / 2 represents the voltage base value, thus obtaining the per-unit value V of the three-phase reference voltage without zero-sequence voltage. a0 V b0 and V c0 .

[0033]

[0034] For a three-phase three-wire system, adding a zero-sequence voltage to the three-phase reference voltage does not affect its output line voltage. If the zero-sequence voltage is v... z Then the three-phase reference voltage can be written as

[0035]

[0036] For a three-phase symmetrical system, the input current is a continuous sinusoidal quantity across three phases, and its three-phase current can be written as...

[0037]

[0038] Therefore, for the rectifier side, after applying zero-sequence voltage, the relationship between the average current at the bus midpoint and the three-phase current within one switching cycle is as follows:

[0039]

[0040] Step 3.7, therefore, by combining equations (8) and (13), the relationship between the instantaneous values ​​of the three-phase voltage and current at the rectifier input, the instantaneous values ​​of the three-phase voltage and current at the inverter output, and the instantaneous values ​​of the voltages of the two DC capacitors on the DC bus and the zero-sequence voltage components in the three-level back-to-back inverter system is as follows:

[0041]

[0042] By selecting an appropriate zero-sequence voltage v z This makes Δu approach 0, u L -u H Setting the value to 0 can improve midpoint voltage fluctuations and control midpoint potential balance.

[0043] The beneficial effects of this invention are as follows:

[0044] 1. Based on the structural characteristics of a three-level back-to-back frequency converter, this invention establishes a mathematical relationship between the neutral point voltage balance and the neutral point average current. By injecting zero-sequence voltage to adjust the neutral point average current on the rectifier side, neutral point voltage balance control is achieved. The principle is clear, the calculation process is simple, no complex control algorithm is required, the computational burden on the processor is reduced, and the real-time performance of the control is guaranteed.

[0045] 2. This invention limits the range of zero-sequence voltage, effectively avoiding overmodulation, and is applicable to all modulation scenarios of 0≤m≤1, solving the problem of limited applicability of existing methods;

[0046] 3. This invention selects the critical zero-sequence voltage at the inflection point of the piecewise function for calculation, which not only reduces the amount of calculation, but also ensures that one of the corresponding three-phase reference voltages is an integer. The bridge arm switch of that phase does not operate in the current control cycle, which significantly reduces the switching loss of the system and improves the operating efficiency of the frequency converter.

[0047] 4. This invention accurately acquires the instantaneous values ​​of various electrical quantities through voltage and current Hall effect sensors. After A / D conversion, the values ​​are processed and controlled in real time by a DSP processor. The control accuracy is high, which can quickly suppress midpoint voltage fluctuations, ensure the sinusoidal nature of the inverter output voltage, reduce harmonic distortion rate, extend the service life of switching devices, and improve the system's operational stability and reliability. Attached Figure Description

[0048] Figure 1 This is a circuit block diagram of a three-level back-to-back frequency converter, which mainly consists of an LCL filter, a diode-clamped three-level rectifier, two DC bus capacitors, a diode-clamped three-level inverter, and a three-phase AC asynchronous motor.

[0049] Figure 2 This is the voltage waveform of the two DC bus capacitors when the three-phase AC380V input three-level back-to-back frequency converter outputs 20A. Among them, channel 3 (pink) is the output current waveform, channel 2 (blue) is the capacitor voltage waveform between the positive bus and the middle bus, and channel 4 (green) is the capacitor voltage waveform between the middle bus and the negative bus.

[0050] Figure 3 This is the voltage waveform of the two DC bus capacitors when the three-phase AC380V input three-level back-to-back frequency converter outputs 38A. Channel 3 (pink) is the output current waveform, channel 2 (blue) is the capacitor voltage waveform between the positive bus and the middle bus, and channel 4 (green) is the capacitor voltage waveform between the middle bus and the negative bus.

[0051] Figure 4 This is the voltage waveform of the two DC bus capacitors when the three-phase AC380V input three-level back-to-back frequency converter outputs 48A. Channel 3 (pink) is the output current waveform, channel 2 (blue) is the capacitor voltage waveform between the positive bus and the middle bus, and channel 4 (green) is the capacitor voltage waveform between the middle bus and the negative bus. Detailed Implementation

[0052] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0053] The present invention relates to a method for controlling the midpoint voltage balance of a three-level back-to-back frequency converter based on zero-sequence voltage injection, which is applied to a diode-clamped three-level back-to-back frequency converter system. The circuit principle of this system is as follows: Figure 1 As shown, it mainly consists of an LCL filter, a diode-clamped three-level rectifier, two DC bus capacitors, a diode-clamped three-level inverter, and a three-phase AC asynchronous motor.

[0054] The specific implementation steps are as follows:

[0055] Signal acquisition: The voltage Hall effect sensor model HV200-050P and the current Hall effect sensor model CSM025M are used to acquire the instantaneous values ​​of the three-phase voltage and corresponding current at the input of the rectifier, the three-phase voltage and corresponding current at the output of the inverter, and the voltage and instantaneous voltage of the DC bus capacitor, respectively.

[0056] Signal conditioning and conversion: The acquired analog signal is sent to the conditioning circuit (model INA128) for filtering and amplification to remove interference signals and adjust the signal amplitude to the input range of the A / D module; the ADS8364 A / D conversion module from TI is used to convert the conditioned analog signal into a 16-bit digital signal and input it to the TMS320F28335 DSP processor;

[0057] Finally, a PWM modulation wave is generated through calculation and output to the drive circuit to control the switching of the rectifier bridge arm switching devices, adjust the midpoint voltage, and achieve voltage balance control.

[0058] The specific steps are as follows:

[0059] Step 1: Collect the instantaneous values ​​of the three-phase voltage and current input of the rectifier, the instantaneous values ​​of the three-phase voltage and current output of the inverter, and the instantaneous voltage values ​​of the two DC capacitors on the DC bus in the three-level back-to-back inverter system using voltage Hall effect devices and current Hall effect devices, respectively.

[0060] Step 2: The instantaneous values ​​of the three-phase voltage and current input to the rectifier, the instantaneous values ​​of the three-phase voltage and current output to the inverter, and the instantaneous voltage values ​​of the two DC capacitors on the DC bus, after being processed by the conditioning circuit, are converted into digital signals by the A / D module and input into the DSP processor.

[0061] Step 3: The DSP processor processes the instantaneous values ​​of the three-phase voltage and current input to the rectifier, the instantaneous values ​​of the three-phase voltage and current output to the inverter, and the instantaneous values ​​of the voltages of the two DC capacitors on the DC bus to obtain the zero-sequence voltage component to be injected.

[0062] Step 4: The DSP processor adjusts the PWM modulation wave according to the zero-sequence voltage component obtained in step 3, thereby adjusting the midpoint voltage of the three-level back-to-back frequency converter.

[0063] Preferably, obtaining the injected zero-sequence voltage component in step 3 is specifically implemented according to the following steps:

[0064] Step 3.1, for a three-level back-to-back frequency converter, assume the bus capacitance is C, and the voltage across the capacitor between the positive bus and the intermediate bus is u. H The capacitor voltage between the intermediate bus and the negative bus is u. L The voltage difference between the positive and negative bus capacitors is:

[0065] Δu=u L -u H (1)

[0066] Therefore, the average midpoint current that needs to be compensated to maintain midpoint voltage balance is i. o for:

[0067]

[0068] Step 3.2, define the average midpoint current on the rectifier side of the three-level back-to-back inverter as i. o1 The average current i at the midpoint of the bus generated by the rectifier-side control o1 Receive i o and i o2 The influence, i.e., satisfying

[0069] i o1 =i o -i o2 (3)

[0070] Step 3.3, assuming the three-phase output reference voltage on the inverter side is...

[0071]

[0072] Where: V m The phase voltage amplitude during maximum linear modulation satisfies m is the modulation ratio, satisfying 0 ≤ m ≤ 1; θ is the voltage phase angle. Choose half the bus voltage, i.e., V. dc / 2 represents the voltage base value, thus obtaining the per-unit value V of the three-phase reference voltage without zero-sequence voltage. u0 V v0 and V w0 :

[0073]

[0074] For a three-phase symmetrical system, assuming the load current is a continuous sinusoidal quantity across three phases, its three-phase current can be written as:

[0075]

[0076] Among them Im The phase current amplitude, The power factor angle.

[0077] Step 3.4, let the average current at the midpoint of the inverter side be i. o2 Define the DC bus midpoint voltage level as zero. For a midpoint-clamped three-level converter, the current of a phase only contributes to the bus midpoint current when the output voltage level of that phase is zero. Since voltage level changes occur within a switching cycle, the bus midpoint current also changes accordingly. We take the average value of the bus midpoint current within one switching cycle as the research object. Therefore, for the inverter side, the average bus midpoint current i within one switching cycle is... o2 With three-phase output voltage v u0 v v0 v w0 and three-phase output current i u i v i w The relationship is:

[0078]

[0079] Step 3.5, combining equations (2), (3), and (7), we obtain the average midpoint current i of the rectifier side of the three-level back-to-back inverter. o1 :

[0080]

[0081] When the control rectifier bus midpoint current satisfies equation (8), the theoretical difference between the upper and lower bus capacitor voltages is zero. That is, the above equation is the reference value of the average current at the rectifier bus midpoint in the bus midpoint voltage balance control algorithm.

[0082] Step 3.6, assuming the three-phase input reference voltage on the rectifier side is:

[0083]

[0084] Select half of the bus voltage, i.e., V. dc / 2 represents the voltage base value, thus obtaining the per-unit value V of the three-phase reference voltage without zero-sequence voltage. a0 V b0 and V c0 .

[0085]

[0086] For a three-phase three-wire system, adding a zero-sequence voltage to the three-phase reference voltage does not affect its output line voltage. If the zero-sequence voltage is v... z Then the three-phase reference voltage can be written as

[0087]

[0088] For a three-phase symmetrical system, the input current is a continuous sinusoidal quantity across three phases, and its three-phase current can be written as...

[0089]

[0090] Therefore, for the rectifier side, after applying zero-sequence voltage, the relationship between the average current at the bus midpoint and the three-phase current within one switching cycle is as follows:

[0091]

[0092] Step 3.7, therefore, by combining equations (8) and (13), the relationship between the instantaneous values ​​of the three-phase voltage and current at the rectifier input, the instantaneous values ​​of the three-phase voltage and current at the inverter output, and the instantaneous values ​​of the voltages of the two DC capacitors on the DC bus and the zero-sequence voltage components in the three-level back-to-back inverter system is as follows:

[0093]

[0094] By selecting an appropriate zero-sequence voltage v z This makes Δu approach 0, u L -u H Setting the value to 0 can improve midpoint voltage fluctuations and control midpoint potential balance.

[0095] Experimental verification: Under a three-phase AC 380V input voltage, tests were conducted at output currents of 20A, 38A, and 48A respectively. The voltage waveforms of the two DC bus capacitors were obtained as follows: Figure 2 , Figure 3 , Figure 4 As shown in the waveform diagram, after adopting the control method of this invention, the difference between the capacitor voltage between the positive bus and the intermediate bus and the capacitor voltage between the intermediate bus and the negative bus is controlled within a small range, and the voltage fluctuation is smooth. This proves that this invention can effectively achieve midpoint voltage balance control and has good control effect under different output current conditions.

Claims

1. A method for neutral point voltage balance control of a three-level back-to-back frequency converter based on zero-sequence voltage injection, characterized in that, The specific steps are as follows: Step 1: Collect the instantaneous values ​​of the three-phase voltage and current input of the rectifier, the instantaneous values ​​of the three-phase voltage and current output of the inverter, and the instantaneous voltage values ​​of the two DC capacitors on the DC bus in the three-level back-to-back inverter system using voltage Hall effect devices and current Hall effect devices, respectively. Step 2: The instantaneous values ​​of the three-phase voltage and current input to the rectifier, the instantaneous values ​​of the three-phase voltage and current output to the inverter, and the instantaneous voltage values ​​of the two DC capacitors on the DC bus, after being processed by the conditioning circuit, are converted into digital signals by the A / D module and input into the DSP processor. Step 3: The DSP processor processes the instantaneous values ​​of the three-phase voltage and current input to the rectifier, the instantaneous values ​​of the three-phase voltage and current output to the inverter, and the instantaneous values ​​of the voltages of the two DC capacitors on the DC bus to obtain the zero-sequence voltage component to be injected. Step 4: The DSP processor adjusts the PWM modulation wave according to the zero-sequence voltage component obtained in step 3, thereby adjusting the midpoint voltage of the three-level back-to-back frequency converter.

2. The method for neutral point voltage balance control of a three-level back-to-back frequency converter based on zero-sequence voltage injection according to claim 1, characterized in that, The injection of the zero-sequence voltage component in step 3 is specifically implemented according to the following steps: Step 3.1, for a three-level back-to-back frequency converter, assume the bus capacitance is C, and the voltage across the capacitor between the positive bus and the intermediate bus is u. H The capacitor voltage between the intermediate bus and the negative bus is u. L The voltage difference between the positive and negative bus capacitors is: Δu=u L -u H (1) Therefore, the average midpoint current that needs to be compensated to maintain midpoint voltage balance is i. o for: Step 3.2, define the average midpoint current on the rectifier side of the three-level back-to-back inverter as i. o1 The average current i at the midpoint of the bus generated by the rectifier-side control o1 Receive i o and i o2 The influence, i.e., satisfying i o1 =i o -i o2 (3) Step 3.3, assuming the three-phase output reference voltage on the inverter side is... Where: V m The phase voltage amplitude during maximum linear modulation satisfies m is the modulation ratio, satisfying 0 ≤ m ≤ 1; θ is the voltage phase angle. Choose half the bus voltage, i.e., V. dc / 2 represents the voltage base value, thus obtaining the per-unit value V of the three-phase reference voltage without zero-sequence voltage. u0 V v0 and V w0 : For a three-phase symmetrical system, assuming the load current is a continuous sinusoidal quantity across three phases, its three-phase current can be written as: Among them I m The phase current amplitude, The power factor angle. Step 3.4, let the average current at the midpoint of the inverter side be i. o2 Define the DC bus midpoint voltage level as zero. For a midpoint-clamped three-level converter, the current of a phase only contributes to the bus midpoint current when the output voltage level of that phase is zero. Since voltage level changes occur within a switching cycle, the bus midpoint current also changes accordingly. We take the average value of the bus midpoint current within one switching cycle as the research object. Therefore, for the inverter side, the average bus midpoint current i within one switching cycle is... o2 With three-phase output voltage v u0 v v0 v w0 and three-phase output current i u i v i w The relationship is: Step 3.5, combining equations (2), (3), and (7), we obtain the average midpoint current i of the rectifier side of the three-level back-to-back inverter. o1 : When the control rectifier bus midpoint current satisfies equation (8), the theoretical difference between the upper and lower bus capacitor voltages is zero. That is, the above equation is the reference value of the average current at the rectifier bus midpoint in the bus midpoint voltage balance control algorithm. Step 3.6, assuming the three-phase input reference voltage on the rectifier side is: Select half of the bus voltage, i.e., V. dc / 2 represents the voltage base value, thus obtaining the per-unit value V of the three-phase reference voltage without zero-sequence voltage. a0 V b0 and V c0 . For a three-phase three-wire system, adding a zero-sequence voltage to the three-phase reference voltage does not affect its output line voltage. If the zero-sequence voltage is v... z Then the three-phase reference voltage can be written as For a three-phase symmetrical system, the input current is a continuous sinusoidal quantity across three phases, and its three-phase current can be written as... Therefore, for the rectifier side, after applying zero-sequence voltage, the relationship between the average current at the bus midpoint and the three-phase current within one switching cycle is as follows: Step 3.7, therefore, by combining equations (8) and (13), the relationship between the instantaneous values ​​of the three-phase voltage and current at the rectifier input, the instantaneous values ​​of the three-phase voltage and current at the inverter output, and the instantaneous values ​​of the voltages of the two DC capacitors on the DC bus and the zero-sequence voltage components in the three-level back-to-back inverter system is as follows: By selecting an appropriate zero-sequence voltage v z This makes Δu approach 0, u L -u H Setting the value to 0 can improve midpoint voltage fluctuations and control midpoint potential balance.

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