Converter off-grid disordered sequence droop control method and system adaptive to unbalanced load

Abstract

The application discloses a method and system for off-grid sequence separation droop control of a converter adapted to unbalanced load, which comprises the following steps: performing positive sequence droop control on active power and reactive power under a positive sequence fundamental frequency to obtain a positive sequence voltage vector and a positive sequence fundamental angle; performing negative sequence droop control on active power and reactive power under a negative sequence fundamental frequency to obtain a negative sequence voltage vector and a negative sequence fundamental angle; obtaining a negative sequence voltage dq-axis component reference value according to the positive sequence fundamental angle and the negative sequence fundamental angle; simultaneously, taking the positive sequence voltage vector and 0 as a positive sequence voltage dq-axis component reference value; and obtaining a modulation voltage of a power unit of the converter according to the positive sequence voltage dq-axis component reference value and the negative sequence voltage dq-axis component reference value, the positive sequence dq-axis component and the negative sequence dq-axis component of the converter serving as feedback values, and voltage and current double-loop regulation control being based. The application realizes active power and reactive power sequence separation extraction control, improves load voltage unbalance degree, realizes load negative sequence current sharing, enhances system stability, and has stronger unbalanced load capacity.

Description

Technical Field

[0001] This invention belongs to the field of energy storage converter control technology, specifically, it relates to an off-grid sequential droop control method and system for converters adapted to unbalanced loads. Background Technology

[0002] With the development of the new energy industry, the power demand for power electronic converters is gradually increasing. One approach is to increase the converter voltage level, mainly limited by the voltage withstand ratings of electrical components such as IGBTs, capacitors, and inductors. Looking at the changes in energy storage converter voltage levels, AC voltage has gradually shifted from 315V and 380V to the current 630V and 690V, while the corresponding DC voltage level has gradually shifted from 1000V to 1500V. The increase in voltage leads to a corresponding increase in converter capacity and efficiency. However, due to the voltage equalization characteristics of series electrochemical cells, the reliability of the system must be considered when further increasing the voltage level. Another approach is to increase the converter current level, currently commonly achieved by hard parallel connection of IGBTs. However, this method leads to some capacity loss due to current sharing issues. Therefore, parallel connection at the converter level has become a current research hotspot.

[0003] In the existing technology, "An Optimized Control Method for Parallel Operation of Energy Storage Converters in a Three-Phase Unbalanced System" (CN103986174B) provides an optimized control method for parallel operation of energy storage converters in a three-phase unbalanced system. When the converters are connected to the grid, the parallel converters typically operate in PQ mode. Their control characteristics determine that only high-frequency circulating currents, such as those at the switching frequency, exist between the converters. At the power frequency and low harmonic frequencies, the circulating current is relatively small due to the control of the current regulator. However, when the converters are off-grid, how to achieve equal current or power distribution among the parallel converters is a pressing issue. If the current or power distribution is uneven, individual converters are prone to overload, affecting the stability of the system. In the "Off-grid Control Method for Multi-Machine Parallel Energy Storage Power Converters under Complex Loads" (CN113472006A), each energy storage converter calculates its own fundamental positive-sequence output power. Through virtual synchronous machine control and droop control, as well as secondary frequency and voltage regulation, frequency and amplitude reference values ​​of the AC side output voltage are generated. Droop control, with its characteristic of not requiring high-speed communication assistance, has become the mainstream method for solving the power sharing problem. However, droop control is greatly affected by the line impedance characteristics, and there is active and reactive power coupling, which affects the control of voltage and frequency, thus worsening the power distribution characteristics of the parallel system. In particular, when there is an unbalanced load in the load, the negative sequence current further exacerbates the difficulty of power sharing. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a converter off-grid droop control method and system adapted to unbalanced loads, which solves the power sharing problem caused by negative sequence current under unbalanced loads and is easy to apply in engineering.

[0005] The present invention adopts the following technical solution.

[0006] This invention proposes a converter off-grid droop control method adapted to unbalanced loads, comprising:

[0007] Step 1: Obtain the active and reactive power of the converter at the positive sequence fundamental frequency and at the negative sequence fundamental frequency.

[0008] Step 2: Perform positive-sequence droop control on the active power and reactive power at the positive-sequence fundamental frequency to obtain the positive-sequence voltage vector and positive-sequence fundamental angle; perform negative-sequence droop control on the active power and reactive power at the negative-sequence fundamental frequency to obtain the negative-sequence voltage vector and negative-sequence fundamental angle.

[0009] Step 3: Based on the positive-sequence fundamental wave angle and the negative-sequence fundamental wave angle, the negative-sequence voltage vector is reoriented to the rotating coordinate system to obtain the reference value of the negative-sequence voltage dq-axis component; at the same time, the positive-sequence voltage vector and 0 are used as the reference value of the positive-sequence voltage dq-axis component.

[0010] Step 4: Based on the reference values ​​of the positive and negative sequence voltage dq axis components, and using the positive and negative sequence dq axis components of the converter as feedback values, the modulation voltage of the converter power unit is obtained based on the voltage and current dual closed-loop regulation control.

[0011] Step 1 includes:

[0012] Step 1.1: Collect the grid-side voltage and current of the converter, and use the positive-sequence fundamental wave angle to perform positive and negative sequence decomposition and rotation transformation to obtain the positive and negative sequence dq axis components of the grid-side voltage and current in the rotating coordinate system;

[0013] Step 1.2: Collect the AC filter capacitor voltage and the AC side current of the power unit of the converter, and use the positive sequence fundamental wave angle to perform positive and negative sequence decomposition and rotation transformation to obtain the positive and negative sequence dq axis components of the AC filter capacitor voltage and the AC side current of the power unit in the rotating coordinate system.

[0014] Step 1.3: Calculate the positive-sequence active power, positive-sequence reactive power, negative-sequence active power, and negative-sequence reactive power using the positive and negative sequence dq axis components of the grid-side voltage and current.

[0015] In step 1.3, the positive-sequence active power, positive-sequence reactive power, negative-sequence active power, and negative-sequence reactive power satisfy the following relationships:

[0016]

[0017] In the formula,

[0018] P p Q p These are positive-sequence active power and positive-sequence reactive power, respectively.

[0019] P n Q n These are negative-sequence active power and negative-sequence reactive power, respectively.

[0020] U 2pd I 2pd These represent the positive-sequence d-axis components of voltage and current on the grid side of the converter, respectively.

[0021] U 2pq I 2pq These are the positive-sequence q-axis components of voltage and current on the grid side of the converter, respectively.

[0022] U 2nd I 2nd These represent the negative-sequence d-axis components of voltage and current on the grid side of the converter, respectively.

[0023] U 2nq I 2nq These are the negative-sequence q-axis components of voltage and current on the grid side of the converter, respectively.

[0024] Step 2 includes:

[0025] Step 2.1: Based on the positive sequence active power and positive sequence reactive power, and using positive sequence fundamental wave droop control, obtain the positive sequence voltage amplitude and positive sequence fundamental wave frequency, and obtain the positive sequence fundamental wave angle from the positive sequence fundamental wave frequency through an integration element.

[0026] Step 2.2: Based on the negative sequence active power and negative sequence reactive power, and using negative sequence fundamental wave droop control, obtain the negative sequence voltage amplitude and the difference between the positive and negative sequence fundamental wave frequencies. Then, sum the negative sequence fundamental wave frequency by taking the inverse of the positive sequence fundamental wave frequency. Finally, obtain the negative sequence fundamental wave angle through an integration process.

[0027] In step 2.1, the positive sequence voltage amplitude and the positive sequence fundamental frequency satisfy the following relationship:

[0028]

[0029] In the formula,

[0030] ω p It is the positive sequence fundamental frequency.

[0031] ω0 is the rated frequency.

[0032] k ppThe droop factor is the positive-sequence active power.

[0033] U p This is the positive sequence voltage amplitude.

[0034] U0 is the rated voltage amplitude.

[0035] k pq This is the droop factor for positive-sequence reactive power.

[0036] In step 2.2, the negative sequence voltage amplitude and the difference between the positive and negative sequence fundamental frequencies satisfy the following relationship:

[0037]

[0038] In the formula,

[0039] Δω is the difference between the positive and negative sequence fundamental frequencies.

[0040] k np The droop factor for negative sequence active power.

[0041] U n The magnitude of the negative sequence voltage.

[0042] k nq This is the droop factor for negative sequence reactive power.

[0043] The negative sequence fundamental frequency satisfies the following relationship:

[0044] ω n =-ω p -Δω

[0045] In the formula, ω n It is the negative sequence fundamental frequency.

[0046] Step 3 includes:

[0047] Step 3.1: Based on the negative sequence voltage amplitude and the negative sequence fundamental angle, the reference value of the negative sequence voltage dq axis component in the rotating coordinate system is obtained by reorienting it by taking the inverse value of the positive sequence fundamental angle.

[0048] Step 3.2: Use the positive sequence voltage amplitude and 0 as reference values ​​for the positive sequence voltage dq axis components in the rotating coordinate system.

[0049] In step 3.1, the reference values ​​of the negative sequence voltage dq-axis components in the rotating coordinate system satisfy the following relationship:

[0050]

[0051] In the formula,

[0052] These are the reference values ​​for the dq-axis components of the negative sequence voltage, respectively.

[0053] θ n The angle of the negative sequence fundamental wave.

[0054] θ p The angle is the positive sequence fundamental wave angle.

[0055] Step 4 includes:

[0056] Step 4.1: The reference values ​​of the positive sequence voltage dq-axis component and the negative sequence voltage dq-axis component are used together as the reference value input;

[0057] Step 4.2: Use the positive and negative sequence dq axis components of the AC filter capacitor voltage and the AC side current of the power unit as feedback values;

[0058] Step 4.3: Based on the voltage and current dual closed-loop regulation control, the modulation voltage in the rotating coordinate system is obtained; wherein, the voltage and current dual closed-loop regulation control includes: voltage outer loop control and current inner loop control.

[0059] Step 4.4: The modulation voltage in the rotating coordinate system is transformed by a rotational transformation with the positive sequence fundamental wave angle as input to obtain the modulation voltage in the three-phase stationary coordinate system.

[0060] Step 4.5: After the modulated voltage is output in the three-phase stationary coordinate system, the power unit of the drive converter is activated.

[0061] In another aspect, the present invention proposes an off-grid sequence droop control system for converters adapted to unbalanced loads, comprising: a data acquisition module, a positive and negative sequence decomposition module, a positive sequence power calculation module, a negative sequence power calculation module, a positive sequence fundamental droop control module, a negative sequence fundamental droop control module, an integral element, a negative sequence redirection module, and a voltage and current dual closed-loop regulation control module.

[0062] The acquisition module is used to acquire the grid-side voltage and current of the converter, as well as the AC filter capacitor voltage and the AC-side current of the power unit.

[0063] The positive and negative sequence decomposition module is used to perform positive and negative sequence decomposition and rotation transformation on the grid-side voltage and current of the converter using the positive sequence fundamental wave angle to obtain the positive and negative sequence dq axis components of the grid-side voltage and current in the rotating coordinate system; and to perform positive and negative sequence decomposition and rotation transformation on the AC filter capacitor voltage and power unit AC side current of the converter using the positive sequence fundamental wave angle to obtain the positive and negative sequence dq axis components of the AC filter capacitor voltage and power unit AC side current in the rotating coordinate system.

[0064] The positive sequence power calculation module is used to calculate positive sequence active power and positive sequence reactive power using the positive and negative sequence dq axis components of grid-side voltage and current.

[0065] The negative sequence power calculation module is used to calculate negative sequence active power and negative sequence reactive power using the positive and negative sequence dq axis components of grid-side voltage and current.

[0066] The positive-sequence fundamental frequency droop control module is used to obtain the positive-sequence voltage amplitude and positive-sequence fundamental frequency based on the positive-sequence active power and positive-sequence reactive power and the positive-sequence fundamental frequency droop control.

[0067] The negative sequence fundamental frequency droop control module is used to obtain the negative sequence voltage amplitude and the difference between the positive and negative sequence fundamental frequencies based on the negative sequence active power and negative sequence reactive power, and then obtain the negative sequence fundamental frequency by inverting and summing the difference with the positive sequence fundamental frequency.

[0068] The integral element is used to calculate the positive-sequence fundamental frequency by integrating the positive-sequence fundamental frequency to obtain the positive-sequence fundamental angle, and to calculate the negative-sequence fundamental frequency by integrating the negative-sequence fundamental frequency to obtain the negative-sequence fundamental angle.

[0069] The negative sequence redirection module is used to obtain the reference value of the negative sequence voltage dq axis component in the rotating coordinate system by reorienting it based on the negative sequence voltage amplitude and the negative sequence fundamental angle by taking the inverse value of the positive sequence fundamental angle.

[0070] The voltage and current dual closed-loop regulation control module includes: a voltage outer loop control unit, a current inner loop control unit, a rotation transformation unit, and a modulation output unit. This module uses the positive-sequence voltage amplitude and zero as reference values ​​for both the positive and negative-sequence voltage dq-axis components in the rotating coordinate system, and the positive and negative-sequence dq-axis components of the AC filter capacitor voltage and the AC side current of the power unit as feedback. Through the voltage outer loop control unit and the current inner loop control unit, the modulated voltage in the rotating coordinate system is obtained. The rotation transformation unit then performs a rotation transformation with the positive-sequence fundamental angle as input, ultimately obtaining the modulated voltage in the three-phase stationary coordinate system. The modulated voltage is output by the modulation output unit to drive the power unit.

[0071] The beneficial effects of this invention are that, compared with the prior art, this invention realizes the sequential extraction and control of active and reactive power, improves the load voltage imbalance, realizes the equal distribution of negative sequence current in the load, enhances system stability, and has a stronger ability to cope with unbalanced loads compared with the virtual impedance method. Attached Figure Description

[0072] Figure 1 This is a flowchart of a converter off-grid sequential droop control method for adapting to unbalanced loads proposed in this invention;

[0073] Figure 2 This is a circuit diagram of a converter connected in parallel with an off-grid load in an embodiment of the present invention;

[0074] Figure 3This is a schematic diagram of the power sampling location in an embodiment of the present invention;

[0075] Figure 4 This is a schematic diagram of positive and negative sequence power calculation in an embodiment of the present invention;

[0076] Figure 5 This is a schematic diagram of positive and negative sequence power calculation in an embodiment of the present invention;

[0077] Figure 6 This is a schematic diagram of the sequential droop control in an embodiment of the present invention;

[0078] Figure 7 This is a schematic diagram of voltage and current dual closed-loop control in an embodiment of the present invention.

[0079] Figure 8 This is a waveform diagram of the three-phase voltage of the power grid in an embodiment of the present invention;

[0080] Figure 9 This is a waveform diagram of the three-phase current of the power grid in an embodiment of the present invention;

[0081] Figure 10 This is an improved negative sequence d-axis current waveform diagram of the first two modules in an embodiment of the present invention;

[0082] Figure 11 This is a diagram of the negative sequence d-axis current waveform of the two modules after the improvement in this embodiment of the invention;

[0083] Figure 12 This is an improved negative-sequence active power waveform diagram of the first two modules in an embodiment of the present invention;

[0084] Figure 13 This is an improved negative-sequence reactive power waveform diagram of the first two modules in this embodiment of the invention;

[0085] Figure 14 This is a diagram of the negative-sequence active power waveform of the two modules after the improvement in this embodiment of the invention;

[0086] Figure 15 This is a diagram of the negative-sequence reactive power waveform of the two modules after the improvement in this embodiment of the invention. Detailed Implementation

[0087] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0088] Example 1.

[0089] This invention proposes an off-grid sequential droop control method for converters adapted to unbalanced loads. The power values ​​at the positive-sequence fundamental frequency and the negative-sequence fundamental frequency are extracted separately for sequential droop control. Then, the voltage vector obtained by the negative-sequence droop control is redirected to the negative-sequence coordinate system symmetrical to the positive-sequence coordinate system. After dual closed-loop regulation of positive and negative sequence voltage and current, the negative-sequence current is evenly distributed.

[0090] like Figure 1 As shown, the sequence droop control method includes:

[0091] Step 1: Obtain the active and reactive power of the converter at the positive sequence fundamental frequency and the active and reactive power at the negative sequence fundamental frequency.

[0092] Specifically, step 1 includes:

[0093] Step 1.1: Collect the grid-side voltage and current of the converter, and use the positive-sequence fundamental wave angle to perform positive and negative sequence decomposition and rotation transformation to obtain the positive and negative sequence dq axis components of the grid-side voltage and current in the rotating coordinate system.

[0094] Step 1.2: Collect the AC filter capacitor voltage and the AC side current of the power unit of the converter, and use the positive sequence fundamental wave angle to perform positive and negative sequence decomposition and rotation transformation to obtain the positive and negative sequence dq axis components of the AC filter capacitor voltage and the AC side current of the power unit in the rotating coordinate system.

[0095] Specifically, the positive and negative order decomposition and rotation transformation in steps 1.1 and 1.2 are conventional algorithms, with the T / 4 delay method being preferred for positive and negative order decomposition.

[0096] Figure 2 This is a typical schematic diagram of a converter connected in parallel off-grid with a load. The DC side of the converter's power unit is connected to the battery, and the AC side is connected to the load through an LCL filter circuit. When the converters are connected in parallel, the load end is the common connection point. Figure 3 This is a schematic diagram of the power sampling location, illustrating the voltage and current data that need to be collected by the sequential droop control method proposed in this invention.

[0097] Among them, I 1a I 1b and I 1c These are the three-phase currents on the AC side of the power unit, U 1a U 1b and U 1c These are the three-phase voltages of the AC filter capacitor in the LCL filter circuit, I. 2a I 2b and I 2c These are the grid-side currents, U and U, respectively. 2a U 2b and U 2c These are the grid-side voltages, respectively.

[0098] Step 1.3: Calculate the positive-sequence active power, positive-sequence reactive power, negative-sequence active power, and negative-sequence reactive power using the positive and negative sequence dq axis components of the grid-side voltage and current.

[0099] Specifically, the positive-sequence active power, positive-sequence reactive power, negative-sequence active power, and negative-sequence reactive power satisfy the following relationships:

[0100]

[0101] In the formula,

[0102] P p Q p These are positive-sequence active power and positive-sequence reactive power, respectively.

[0103] P n Q n These are negative-sequence active power and negative-sequence reactive power, respectively.

[0104] U 2pd I 2pd These represent the positive-sequence d-axis components of voltage and current on the grid side of the converter, respectively.

[0105] U 2pq I 2pq These are the positive-sequence q-axis components of voltage and current on the grid side of the converter, respectively.

[0106] U 2nd I 2nd These represent the negative-sequence d-axis components of voltage and current on the grid side of the converter, respectively.

[0107] U 2nq I 2nq These are the negative-sequence q-axis components of voltage and current on the grid side of the converter, respectively.

[0108] Step 2: Perform positive-sequence droop control on the active power and reactive power at the positive-sequence fundamental frequency to obtain the positive-sequence voltage vector and positive-sequence fundamental angle; perform negative-sequence droop control on the active power and reactive power at the negative-sequence fundamental frequency to obtain the negative-sequence voltage vector and negative-sequence fundamental angle.

[0109] Specifically, step 2 includes:

[0110] Step 2.1: Based on the positive sequence active power and positive sequence reactive power, and using positive sequence fundamental wave droop control, obtain the positive sequence voltage amplitude and positive sequence fundamental wave frequency, and obtain the positive sequence fundamental wave angle from the positive sequence fundamental wave frequency through an integration process.

[0111] In step 2.1, the positive sequence voltage amplitude and the positive sequence fundamental frequency satisfy the following relationship:

[0112]

[0113] In the formula,

[0114] ω p It is the positive sequence fundamental frequency.

[0115] ω0 is the rated frequency.

[0116] k pp The droop factor is the positive-sequence active power.

[0117] U p This is the positive sequence voltage amplitude.

[0118] U0 is the rated voltage amplitude.

[0119] k pq This is the droop factor for positive-sequence reactive power.

[0120] Step 2.2: Based on the negative sequence active power and negative sequence reactive power, and using negative sequence fundamental wave droop control, obtain the negative sequence voltage amplitude and the difference between the positive and negative sequence fundamental wave frequencies. Then, sum the negative sequence fundamental wave frequency by taking the inverse of the positive sequence fundamental wave frequency. Finally, obtain the negative sequence fundamental wave angle through an integration process.

[0121] In step 2.2, the negative sequence voltage amplitude and the difference between the positive and negative sequence fundamental frequencies satisfy the following relationship:

[0122]

[0123] In the formula,

[0124] Δω is the difference between the positive and negative sequence fundamental frequencies.

[0125] k np The droop factor for negative sequence active power.

[0126] U n The magnitude of the negative sequence voltage.

[0127] k nq This is the droop factor for negative sequence reactive power.

[0128] 12. The converter off-grid droop control method for adapting to unbalanced loads according to claim 6, characterized in that,

[0129] The negative sequence fundamental frequency satisfies the following relationship:

[0130] ω n =-ω p -Δω

[0131] In the formula, ω n It is the negative sequence fundamental frequency.

[0132] Step 3: Based on the positive-sequence fundamental wave angle and the negative-sequence fundamental wave angle, the negative-sequence voltage vector is reoriented to the rotating coordinate system to obtain the reference value of the negative-sequence voltage dq-axis component; at the same time, the positive-sequence voltage vector and 0 are used as the reference value of the positive-sequence voltage dq-axis component.

[0133] Specifically, step 3 includes:

[0134] Step 3.1: Based on the negative sequence voltage amplitude and the negative sequence fundamental angle, the reference value of the negative sequence voltage dq axis component in the rotating coordinate system is obtained by reorienting it by taking the inverse value of the positive sequence fundamental angle.

[0135] In step 3.1, the reference values ​​of the negative sequence voltage dq-axis components in the rotating coordinate system satisfy the following relationship:

[0136]

[0137] In the formula,

[0138] These are the reference values ​​for the dq-axis components of the negative sequence voltage, respectively.

[0139] θ n The angle of the negative sequence fundamental wave.

[0140] θ p The angle is the positive sequence fundamental wave angle.

[0141] Step 3.2: Use the positive sequence voltage amplitude and 0 as reference values ​​for the positive sequence voltage dq axis components in the rotating coordinate system.

[0142] Step 4: Based on the reference values ​​of the positive and negative sequence voltage dq axis components, and using the positive and negative sequence dq axis components of the converter as feedback values, the modulation voltage of the converter power unit is obtained based on the voltage and current dual closed-loop regulation control.

[0143] Specifically, step 4 includes:

[0144] Step 4.1: The reference values ​​of the positive sequence voltage dq-axis component and the negative sequence voltage dq-axis component are used together as the reference value input;

[0145] Step 4.2: Use the positive and negative sequence dq axis components of the AC filter capacitor voltage and the AC side current of the power unit as feedback values;

[0146] Step 4.3: Based on the voltage and current dual closed-loop regulation control, the modulation voltage in the rotating coordinate system is obtained; wherein, the voltage and current dual closed-loop regulation control includes: voltage outer loop control and current inner loop control.

[0147] Step 4.4: The modulation voltage in the rotating coordinate system is transformed by a rotational transformation with the positive sequence fundamental wave angle as input to obtain the modulation voltage in the three-phase stationary coordinate system.

[0148] Step 4.5: After the modulated voltage is output in the three-phase stationary coordinate system, the power unit of the drive converter is activated.

[0149] In another aspect, this invention proposes an off-grid droop control system for converters that adapts to unbalanced loads, comprising:

[0150] The module includes: acquisition module, positive and negative sequence decomposition module 1, positive sequence power calculation module 2, negative sequence power calculation module 3, positive sequence fundamental frequency droop control module 4, negative sequence fundamental frequency droop control module 5, integral element 6, negative sequence redirection module 7, and voltage and current dual closed-loop regulation control module.

[0151] The acquisition module is used to acquire the grid-side voltage and current of the converter, as well as the AC filter capacitor voltage and the AC-side current of the power unit.

[0152] Positive and negative order decomposition module, such as Figure 4 As shown, the positive-sequence fundamental angle is used to decompose and rotate the grid-side voltage and current of the converter into positive and negative sequence components, obtaining the positive and negative sequence dq-axis components of the grid-side voltage and current in the rotating coordinate system; as shown... Figure 5 As shown, it is also used to perform positive and negative sequence decomposition and rotation transformation on the AC filter capacitor voltage and AC side current of the power unit of the converter using the positive sequence fundamental wave angle, so as to obtain the positive and negative sequence dq axis components of the AC filter capacitor voltage and AC side current of the power unit in the rotating coordinate system.

[0153] Positive sequence power calculation module, such as Figure 4 As shown, positive-sequence active power and positive-sequence reactive power are calculated using the positive and negative sequence dq-axis components of grid-side voltage and current.

[0154] Negative sequence power calculation module, such as Figure 4 As shown, it is used to calculate negative sequence active power and negative sequence reactive power using the positive and negative sequence dq axis components of grid-side voltage and current;

[0155] Positive sequence fundamental droop control module, such as Figure 6 As shown, it is used to obtain the positive sequence voltage amplitude and positive sequence fundamental frequency based on the positive sequence active power and positive sequence reactive power, and on the basis of positive sequence fundamental frequency droop control.

[0156] Negative sequence fundamental droop control module, such as Figure 6 As shown, it is used to obtain the negative sequence voltage amplitude and the difference between the positive and negative sequence fundamental frequencies based on the negative sequence active power and negative sequence reactive power, and then obtain the negative sequence fundamental frequency by inverting and summing the difference with the positive sequence fundamental frequency.

[0157] In the points system, such as Figure 6As shown, the positive-sequence fundamental frequency is calculated by integrating the positive-sequence fundamental frequency to obtain the positive-sequence fundamental angle, and the negative-sequence fundamental frequency is calculated by integrating the negative-sequence fundamental frequency to obtain the negative-sequence fundamental angle.

[0158] Negative order redirection modules, such as Figure 6 As shown, the method is used to obtain the reference value of the dq axis component of the negative sequence voltage in the rotating coordinate system by reorienting it by taking the inverse value of the positive sequence fundamental wave angle based on the negative sequence voltage amplitude and the negative sequence fundamental wave angle.

[0159] Voltage and current dual closed-loop regulation control module, such as Figure 7 As shown, it includes: a voltage outer loop control unit 8, a current inner loop control unit 9, a rotation transformation unit 10, and a modulation output unit 11; a voltage and current dual closed-loop regulation control module, which uses the positive sequence voltage amplitude and 0 as reference values ​​for the positive sequence voltage dq-axis components and the negative sequence voltage dq-axis components in the rotating coordinate system as reference inputs, and uses the positive and negative sequence dq-axis components of the AC filter capacitor voltage and the AC side current of the power unit as feedbacks. Through the voltage outer loop control unit and the current inner loop control unit, the modulation voltage in the rotating coordinate system is obtained; then, the rotation transformation unit performs rotation transformation with the positive sequence fundamental wave angle as input, and finally obtains the modulation voltage in the three-phase stationary coordinate system; after the modulation voltage is output by the modulation output unit, it drives the power unit to operate.

[0160] Example 2.

[0161] The output voltage changes after a three-phase unbalanced load (resistance values ​​of 2 ohms, 2 ohms, and 0.9 ohms, star connection) is connected to the converter output side as follows: Figure 8 As shown, the normal phase voltage amplitude is 310V. Due to the separate positive and negative sequence control, the converter output voltage maintains good balance under unbalanced loads, resolving the voltage imbalance problem under unbalanced loads. However, the presence of an unbalanced load while the voltage remains balanced leads to three-phase current imbalance, as shown... Figure 9 As shown, negative-order components appear.

[0162] Due to the difference in the impedance of the lines connected to parallel module 1 and parallel module 2, the negative sequence current distribution is unbalanced. Without using the off-grid droop control method for the converter, the negative sequence active current of the two modules is as follows: Figure 10 As shown, after using the off-grid droop control method for converters, the distribution of negative sequence active current in the modules was improved, making the negative sequence active current of the two converters more balanced, such as... Figure 11 As shown.

[0163] Due to the use of a droop control strategy, the negative-sequence active power (calculated with negative-sequence q-axis voltage orientation) also maintains good balance. The negative-sequence active and reactive power of the two modules before and after the improvement are as follows: Figures 12 to 15 As shown.

[0164] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0165] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0166] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0167] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0168] Various aspects of this disclosure are described herein with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this disclosure. It should be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer-readable program instructions.

[0169] These computer-readable program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, or other programmable data processing apparatus to produce a machine such that, when executed by the processor of the computer or other programmable data processing apparatus, they create means for implementing the functions / actions specified in one or more blocks of the flowchart and / or block diagram. These computer-readable program instructions can also be stored in a computer-readable storage medium that causes a computer, programmable data processing apparatus, and / or other device to operate in a particular manner; thus, the computer-readable medium storing the instructions comprises an article of manufacture that includes instructions for implementing aspects of the functions / actions specified in one or more blocks of the flowchart and / or block diagram.

[0170] Computer-readable program instructions may also be loaded onto a computer, other programmable data processing apparatus, or other device to cause a series of operational steps to be performed on the computer, other programmable data processing apparatus, or other device to produce a computer-implemented process, thereby causing the instructions executed on the computer, other programmable data processing apparatus, or other device to perform the functions / actions specified in one or more boxes of a flowchart and / or block diagram.

[0171] The flowcharts and block diagrams in the accompanying drawings illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer program products according to various embodiments of the present disclosure. In this regard, each block in a flowchart or block diagram may represent a module, segment, or portion of an instruction containing one or more executable instructions for implementing a specified logical function. In some alternative implementations, the functions marked in the blocks may occur in a different order than those shown in the drawings. For example, two consecutive blocks may actually be executed substantially in parallel, and they may sometimes be executed in reverse order, depending on the functions involved. It should also be noted that each block in the block diagrams and / or flowcharts, and combinations of blocks in the block diagrams and / or flowcharts, may be implemented using a dedicated hardware-based system that performs the specified function or action, or using a combination of dedicated hardware and computer instructions.

[0172] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A method for off-grid droop control of a converter adapted to unbalanced loads, characterized in that, The method comprises: Step 1, obtaining the active power and reactive power of the converter at the positive sequence fundamental frequency, and the active power and reactive power at the negative sequence fundamental frequency; Step 2, performing positive sequence droop control on the active power and reactive power at the positive sequence fundamental frequency to obtain a positive sequence voltage vector and a positive sequence fundamental angle, and performing negative sequence droop control on the active power and reactive power at the negative sequence fundamental frequency to obtain a negative sequence voltage vector and a negative sequence fundamental angle; Step 3, according to the positive sequence fundamental angle and the negative sequence fundamental angle, the negative sequence voltage vector is reoriented to the rotating coordinate system to obtain a negative sequence voltage dq-axis component reference value; at the same time, the positive sequence voltage vector and 0 are taken as the positive sequence voltage dq-axis component reference value; Step 4, according to the positive and negative sequence voltage dq-axis component reference values, taking the positive and negative sequence dq-axis components of the converter as feedback values, and based on voltage and current double-loop regulation control, a modulation voltage of the converter power unit is obtained.

2. The off-grid separate sequence droop control method for the converter adapting to unbalanced load according to claim 1, wherein step 1 comprises: Step 1.1, collecting the grid-side voltage and current of the converter, performing positive and negative sequence decomposition and rotating transformation using the positive sequence fundamental angle to obtain the positive and negative sequence dq-axis components of the grid-side voltage and current in the rotating coordinate system; Step 1.2, collecting the AC filter capacitor voltage and power unit AC side current of the converter, performing positive and negative sequence decomposition and rotating transformation using the positive sequence fundamental angle to obtain the positive and negative sequence dq-axis components of the AC filter capacitor voltage and power unit AC side current in the rotating coordinate system; Step 1.3, using the positive and negative sequence dq-axis components of the grid-side voltage and current to calculate the positive sequence active power, the positive sequence reactive power, the negative sequence active power and the negative sequence reactive power.

3. The off-grid separate sequence droop control method for the converter adapting to unbalanced load according to claim 2, wherein in step 1.3, the positive sequence active power, the positive sequence reactive power, the negative sequence active power and the negative sequence reactive power respectively satisfy the following relationship: In the formula, 4. The off-grid separate sequence droop control method for the converter adapting to unbalanced load according to claim 3, wherein step 2 comprises: Step 2.1, based on the positive sequence fundamental droop control, according to the positive sequence active power and the positive sequence reactive power, obtaining the positive sequence voltage amplitude and the positive sequence fundamental frequency, and obtaining the positive sequence fundamental angle through the integral link from the positive sequence fundamental frequency; P p , Q p are the positive sequence active power and the positive sequence reactive power, respectively, P n , Q n are the negative sequence active power and the negative sequence reactive power, respectively, U 2pd , I 2pd Vd and Id are the voltage and current positive sequence d-axis components of the grid side of the converter, respectively, U 2pq , I 2pq Vq and Iq are the voltage and current positive sequence q-axis components of the grid side of the converter, respectively, U 2nd , I 2nd Vd, Id are the voltage and current negative sequence d-axis components of the converter grid side respectively, U 2nq 、I 2nq are the voltage and current negative sequence q-axis components of the grid side of the converter, respectively. Step 2.2, based on the negative sequence fundamental droop control, according to the negative sequence active power and the negative sequence reactive power, obtaining the negative sequence voltage amplitude and the positive and negative sequence fundamental frequency difference, and obtaining the negative sequence fundamental frequency by taking the negative of the positive sequence fundamental frequency and summing, and obtaining the negative sequence fundamental angle through the integral link.

5. The off-grid separate sequence droop control method for the converter adapting to unbalanced load according to claim 4, wherein in step 2.1, the positive sequence voltage amplitude and the positive sequence fundamental frequency satisfy the following relationship: In the formula, ω0 is the rated frequency, U0 is the rated voltage amplitude, 6. The off-grid separate sequence droop control method for the converter adapting to unbalanced load according to claim 5, wherein ​ ω p is the positive sequence fundamental frequency, ​ k pp k is the droop coefficient for positive sequence active power, U p is the positive sequence voltage magnitude, ​ k pq is the droop coefficient for positive sequence reactive power. ​ In step 2.2, the negative sequence voltage amplitude and the positive and negative sequence fundamental frequency difference satisfy the following relationship: In the formula, Δω is the positive and negative sequence fundamental frequency difference, k np k is the droop coefficient for negative sequence active power, U n is the negative sequence voltage magnitude, k nq is the droop coefficient for negative sequence reactive power.

7. The off-grid sequence separation droop control method of the unbalanced load adaptive converter according to claim 6, characterized in that, The negative sequence fundamental frequency satisfies the following relationship: ω n = -ω p -Δω ω n is the negative sequence fundamental frequency.

8. The off-grid sequence separation droop control method of the unbalanced load adaptive converter according to claim 7, characterized in that, Step 3 includes: Step 3.1, according to the negative sequence voltage amplitude and the negative sequence fundamental angle, using the method of reorientation with the opposite value of the positive sequence fundamental angle, the negative sequence voltage dq axis component reference value in the rotating coordinate system is obtained; Step 3.2, using the positive sequence voltage amplitude and 0 as the positive sequence voltage dq axis component reference value in the rotating coordinate system.

9. The off-grid sequence separation droop control method of the unbalanced load adaptive converter according to claim 8, characterized in that, In step 3.1, the negative sequence voltage dq axis component reference value in the rotating coordinate system satisfies the following relationship: In the formula, respectively negative sequence voltage dq-axis component reference values, θ n is the negative sequence fundamental angle, θ p is the positive sequence fundamental angle.

10. The off-grid sequence separation droop control method of the unbalanced load adaptive converter according to claim 9, characterized in that, Step 4 includes: Step 4.1, the positive sequence voltage dq axis component reference value and the negative sequence voltage dq axis component reference value are used as the reference value input together; Step 4.2, using the positive and negative sequence dq axis components of the alternating current filter capacitor voltage and the power unit alternating current side current as the feedback value; Step 4.3, based on the voltage and current double closed loop regulation control, the modulated voltage in the rotating coordinate system is obtained; wherein the voltage and current double closed loop regulation control includes: voltage outer loop control, current inner loop control; Step 4.4, the modulated voltage in the rotating coordinate system, after the rotation transformation with the positive sequence fundamental angle as the input, the modulated voltage in the three-phase stationary coordinate system is obtained; Step 4.5, after the modulated voltage in the three-phase stationary coordinate system is output, the power unit of the converter is driven to act.

11. A unbalanced load adaptive converter off-grid sequence separation droop control system using the method of any one of claims 1-10, characterized in that, The system includes: acquisition module, positive and negative sequence decomposition module, positive sequence power calculation module, negative sequence power calculation module, positive sequence fundamental droop control module, negative sequence fundamental droop control module, integral element, negative sequence reorientation module, voltage and current double closed loop regulation control module; The acquisition module is used to acquire the grid side voltage and current of the converter, and acquire the alternating current filter capacitor voltage and the power unit alternating current side current of the converter; The positive and negative sequence decomposition module is used to perform positive and negative sequence decomposition and rotation transformation on the grid side voltage and current of the converter using the positive sequence fundamental angle, to obtain the positive and negative sequence dq axis components of the grid side voltage and current in the rotating coordinate system; the positive and negative sequence decomposition and rotation transformation are performed on the alternating current filter capacitor voltage and the power unit alternating current side current of the converter using the positive sequence fundamental angle, to obtain the positive and negative sequence dq axis components of the alternating current filter capacitor voltage and the power unit alternating current side current in the rotating coordinate system; The positive sequence power calculation module is used to calculate the positive sequence active power and the positive sequence reactive power using the positive and negative sequence dq axis components of the grid side voltage and current; a negative sequence power calculation module configured to calculate negative sequence active power and negative sequence reactive power using positive and negative sequence dq-axis components of grid-side voltage and current; a positive sequence fundamental droop control module configured to obtain positive sequence voltage amplitude and positive sequence fundamental frequency based on positive sequence active power and positive sequence reactive power according to positive sequence fundamental droop control; a negative sequence fundamental droop control module configured to obtain negative sequence voltage amplitude and positive-negative sequence fundamental frequency difference based on negative sequence active power and negative sequence reactive power according to negative sequence fundamental droop control, and obtain negative sequence fundamental frequency by taking the inverse of the sum of the positive sequence fundamental frequency and the positive-negative sequence fundamental frequency difference; an integral element configured to obtain positive sequence fundamental angle by integral calculation of the positive sequence fundamental frequency, and obtain negative sequence fundamental angle by integral calculation of the negative sequence fundamental frequency; a negative sequence reorientation module configured to obtain negative sequence voltage dq-axis component reference value in a rotating coordinate system by using a method of reorientation with the inverse value of the positive sequence fundamental angle according to negative sequence voltage amplitude and negative sequence fundamental angle; a voltage and current double closed loop regulation control module including a voltage outer loop control unit, a current inner loop control unit, a rotation conversion unit and a modulation output unit; the voltage and current double closed loop regulation control module is configured to use the positive sequence voltage amplitude and 0 as the reference value input of the positive sequence voltage dq-axis component reference value and the negative sequence voltage dq-axis component reference value in the rotating coordinate system, use the positive and negative sequence dq-axis components of the AC filter capacitor voltage and the power unit AC side current as feedback, obtain the modulation voltage in the rotating coordinate system through the voltage outer loop control unit and the current inner loop control unit, and finally obtain the modulation voltage in the three-phase stationary coordinate system by using the positive sequence fundamental angle as the input of the rotation conversion unit; the modulation voltage is output through the modulation output unit to drive the power unit to act.

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