A three-phase four-bridge-arm networking inverter strategy for three-phase unbalanced load

Abstract

The application provides a three-phase four-bridge-arm networking inverter strategy for a three-phase unbalanced load, which provides a path for the midpoint current by adding a fourth bridge arm, and realizes the technical requirements of high-quality power supply and reliable expansion under an unbalanced load through voltage and current information sampling, an ABC bridge arm modulation wave generation method and an N bridge arm modulation wave generation method. Since the application provides a path for the midpoint current by adding a fourth bridge arm, the problems of neutral point drift, voltage distortion, excessive neutral line current, parallel networking circulating current and uneven power of a traditional inverter under an unbalanced load are solved, the application is suitable for a three-phase four-wire inverter system with an unbalanced load, such as a micro-grid, energy storage, an uninterruptible power supply, distributed power grid connection and the like, can effectively suppress voltage distortion caused by an unbalanced load, and improves power supply quality. The application is suitable for being applied as a three-phase four-bridge-arm networking inverter strategy for a three-phase unbalanced load.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a three-phase four-bridge arm grid inverter strategy for three-phase unbalanced loads. Background Technology

[0002] With the rapid development of distributed generation and microgrid technologies, the performance requirements for inverters are becoming increasingly stringent. In off-grid mode, inverters need to provide stable voltage support for local loads. In practical applications, mixed single-phase / three-phase load connections and load switching often lead to a three-phase imbalance in the system.

[0003] When traditional inverters are networked, the lack of a neutral line circuit in the presence of a three-phase unbalanced load leads to severe voltage asymmetry in the three-phase output, affecting power quality. Therefore, it is necessary to propose a three-phase four-arm networked inverter strategy for three-phase unbalanced loads to meet the requirements for high-quality power supply and reliable capacity expansion under unbalanced load conditions. Summary of the Invention

[0004] To effectively suppress voltage distortion caused by unbalanced loads, this invention provides a three-phase four-arm grid inverter strategy for three-phase unbalanced loads. This strategy provides a path for the neutral current by adding a fourth arm, and achieves the technical requirements of high-quality power supply and reliable capacity expansion under unbalanced loads through voltage and current information sampling, ABC arm modulation wave generation method and N arm modulation wave generation method.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A strategy for a three-phase four-arm grid-connected inverter for three-phase unbalanced loads is implemented through the following steps: Step 1: Collect real-time values ​​of three-phase line voltage U AB , U BC , U CA Collect real-time DC bus voltage values U dc Collect real-time values ​​of three-phase line current I A , I B , I C The sampling frequency is entirely equivalent to the switching frequency of the power electronic system. f s Calculate the N-arm current. I N .

[0006] Step 2: The six switching transistors of the three bridge arms ABC are based on... UAB , U BC , U CA The effective value is used for feedback control. The modulation method can be SPWM or SVPWM to generate the modulation wave signal of the ABC bridge arm, output the control signal of the ABC bridge arm switching transistor, and output a three-phase symmetrical sinusoidal voltage with constant voltage frequency.

[0007] Step 3: Generate the N-arm modulated wave, which contains a DC component. m dc and communication weight m ac Generate DC component m dc Steps 4 and 5 need to be performed to generate the AC component. m ac Step 6 needs to be performed.

[0008] Step 4: Collect the voltage between the load center point and the DC side negative terminal, and calculate... U O The sampling frequency is equivalent to the switching frequency of the power electronic system. f s Power grid frequency metering f n Calculated within each power frequency cycle U O The average value is calculated as u O .

[0009] Step 5: with U dc / 2 is the control target, with u O For feedback, a PI or PID controller is used to generate the DC component of the N-arm modulated wave. m dc The calculation period is equivalent to the power frequency period.

[0010] Step 6: According to I A , I B , I C Offline adjustment ratio coefficient K Generates the AC component of the N-arm modulated wave. m ac .

[0011] Step 7: Add the DC and AC components of the N-arm modulation wave generated in Steps 5 and 6 in real time to generate the N-arm switching control signal. m N ,Right now:m N = m dc + m ac This completes the control of the three-phase four-bridge inverter.

[0012] The technical effects and advantages of this invention are as follows: Because this invention provides a path for the neutral point current by adding a fourth bridge arm, it solves the problems of neutral point drift, voltage distortion, excessive neutral current, and uneven power distribution in parallel grids caused by unbalanced loads in traditional inverters. It is suitable for three-phase four-wire inverter systems that require unbalanced loads, such as microgrids, energy storage, uninterruptible power supplies, and distributed generation grid connection. It effectively suppresses voltage distortion caused by unbalanced loads and improves power quality. It is suitable as a strategy for three-phase four-bridge-arm grid inverters designed for three-phase unbalanced loads. Attached Figure Description

[0013] Figure 1 This is a topology diagram of a three-phase four-bridge arm grid inverter for three-phase unbalanced loads according to the present invention. Detailed Implementation

[0014] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0015] A strategy for a three-phase four-arm grid-connected inverter for three-phase unbalanced loads is implemented through the following steps: Step 1: Collect real-time values ​​of three-phase line voltage U AB , U BC , U CA Collect real-time DC bus voltage values U dc Collect real-time values ​​of three-phase line current I A , I B , I C The sampling frequency is entirely equivalent to the switching frequency of the power electronic system. f s Calculate the N-arm current. I N .

[0016] Furthermore, step 1 calculates... I N The specific calculation method is as follows:

[0017] Step 2: The six switching transistors of the three bridge arms ABC are based on... U AB , U BC , U CA The effective value is used for feedback control. The modulation method can be SPWM or SVPWM to generate the modulation wave signal of the ABC bridge arm, output the control signal of the ABC bridge arm switching transistor, and output a three-phase symmetrical sinusoidal voltage with constant voltage frequency.

[0018] Step 3: Generate the N-arm modulated wave. The N-arm modulated wave contains a DC component. m dc and communication weight m ac Generate DC component m dc Steps 4 and 5 need to be performed to generate the AC component. m ac Step 6 needs to be performed.

[0019] Step 4: Collect the voltage between the load center point and the DC side negative terminal, and calculate... U O The sampling frequency is equivalent to the switching frequency of the power electronic system. f s Power grid frequency metering f n Calculated within each power frequency cycle U O The average value is calculated as u O .

[0020] Furthermore, step 4 calculates... u O The specific calculation method is as follows: First, calculate the number of sampling points for each power frequency cycle. N , N = f s / f n .

[0021] Then calculate according to the following method. u O :

[0022] in, Indicates N consecutive times U O The real-time sampling results are all sums of the values.

[0023] Step 5: withU dc / 2 is the control target, with u O For feedback, a PI or PID controller is used to generate the DC component of the N-arm modulated wave. m dc The calculation period is equivalent to the power frequency period.

[0024] Step 6: According to I A , I B , I C Offline adjustment ratio coefficient K Generates the AC component of the N-arm modulated wave. m ac .

[0025] Furthermore, the specific process of step 6 is as follows: inductance L N voltage at both ends U L It can be represented as:

[0026] d I L and d t respectively I L and t The minute components.

[0027] because I L Since it is a sinusoidal quantity, I L The differential component is equivalent to I L The negative of the integral, i.e.:

[0028] In a discrete sampling system, the integral of the sum of the three-phase line currents is defined as follows: Sum ,Right now: ,but: .

[0029] Ideally, and after normalizing the carrier's positive and negative values, the AC component of the N-arm modulated wave... m ac It can be represented as: .

[0030] In practice, considering factors such as inductor parameter calibration error and carrier wave value, a proportional coefficient is defined.K ,make Offline adjustment ratio coefficient K ,make U O Equal to U dc / 2, then it can be confirmed K The value completes the output of the AC component of the modulated wave of the N-arm bridge.

[0031] Step 7: Add the DC and AC components of the N-arm modulation wave generated in Steps 5 and 6 in real time to generate the N-arm switching control signal. m N ,Right now: m N = m dc + m ac Complete the control of the three-phase four-bridge inverter.

[0032] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A strategy for a three-phase four-arm grid-connected inverter for three-phase unbalanced loads, implemented through the following steps: Step 1: Collect real-time values ​​of three-phase line voltage U AB , U BC , U CA Collect real-time DC bus voltage values U dc Collect real-time values ​​of three-phase line current I A , I B , I C The sampling frequency is entirely equivalent to the switching frequency of the power electronic system. f s Calculate the N-arm current. I N ; Step 2: The six switching transistors of the three bridge arms ABC are based on... U AB , U BC , U CA The effective value is used for feedback control. The modulation method can be SPWM or SVPWM to generate the modulation wave signal of the ABC bridge arm, output the control signal of the ABC bridge arm switching transistor, and output a three-phase symmetrical sinusoidal voltage with constant voltage frequency. Step 3: Generate the N-arm modulated wave, which contains a DC component. m dc and communication weight m ac ; Generate DC component m dc Steps 4 and 5 need to be performed to generate the AC component. m ac Step 6 needs to be performed; Step 4: Collect the voltage between the load center point and the DC side negative terminal, and calculate... U O The sampling frequency is equivalent to the switching frequency of the power electronic system. f s Power grid frequency metering f n Calculated within each power frequency cycle U O The average value is calculated as u O ; Step 5: with U dc / 2 is the control target, with u O For feedback, a PI or PID controller is used to generate the DC component of the N-arm modulated wave. m dc The calculation period is equivalent to the power frequency period; Step 6: According to I A , I B , I C Offline adjustment ratio coefficient K Generates the AC component of the N-arm modulated wave. m ac ; Step 7: Add the DC and AC components of the N-arm modulation wave generated in Steps 5 and 6 in real time to generate the N-arm switching control signal. m N ,Right now: m N = m dc + m ac This completes the control of the three-phase four-bridge inverter.

2. The three-phase four-arm grid inverter strategy for three-phase unbalanced loads according to claim 1, characterized in that: The calculation in step 1 I N The specific calculation method is as follows: 。 3. The three-phase four-arm grid inverter strategy for three-phase unbalanced loads according to claim 2, characterized in that: The calculation in step 4 u O The specific calculation method is as follows: First, calculate the number of sampling points for each power frequency cycle. N , N = f s / f n , Then calculate u O : ； in, Indicates N consecutive times U O The real-time sampling results are all sums of the values.

4. The three-phase four-arm grid inverter strategy for three-phase unbalanced loads according to claim 1, characterized in that: The specific process of step 6 is as follows: inductance L N voltage at both ends U L It can be represented as: ； Where, d I L and d t respectively I L and t The trace components; because I L Since it is a sinusoidal quantity, I L The differential component is equivalent to I L The negative of the integral, i.e.: ； In a discrete sampling system, the integral of the sum of the three-phase line currents is defined as follows: Sum ,Right now: ,but: ; Ideally, and after normalizing the carrier's positive and negative values, the AC component of the N-arm modulated wave... m ac It can be represented as: ； In practice, considering factors such as inductor parameter calibration error and carrier wave value, a proportional coefficient is defined. K ,make Offline adjustment ratio coefficient K ,make U O Equal to U dc / 2, then it can be confirmed K The value is used to complete the output of the AC component of the modulated wave of the N-arm bridge.

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