APF parallel operation confluence point control method and system

Abstract

The invention relates to the technical field of harmonic suppression, in particular to an APF parallel operation confluence point control method and system, and the method comprises the steps: enabling all parallel APFs to be connected into a power grid through a confluence point; collecting a current signal and a voltage signal at a grid side, a load side and a confluence point side; a first protection strategy based on reactive overcompensation and a second protection strategy based on excessive circulating current are configured at the APF end or the centralized control end; and controlling the on-off of the confluence point contactor according to the first protection strategy or the second protection strategy. When a fault occurs, the contactor or the circuit breaker is automatically disconnected in time so as to protect the APF main circuit from being damaged.

Description

Technical Field

[0001] The present invention relates to the technical field of harmonic control, and in particular to an APF parallel confluence point control method and system. Background Art

[0002] Parallel active power filters (APFs) usually detect harmonic currents from the load side as the command current of the APF. In a system where multiple APFs operate in parallel, ideally, each APF operates synchronously, and the current size and phase emitted by each device are synchronized. However, in reality, due to the large rate of change of harmonic currents and the fact that the APF itself will also generate additional harmonics, the APF generates reactive (inductive and capacitive) currents. When a certain number of parallel machines are exceeded, the current at the junction point suddenly increases and the circuit breaker emits an abnormal sound. On the other hand, the current size or phase emitted by each inverter is not synchronized. When there is a signal delay in the current emitted by multiple APFs operating in parallel, a phase difference will be generated, resulting in circulating currents between the APFs. When the circulating current is too large, it will damage the power switching devices and cause the power system to collapse. Summary of the invention

[0003] In order to solve the above technical problems, the present invention proposes an APF parallel bus point control method, which can automatically disconnect the contactor or circuit breaker in time when a fault occurs to protect the APF main circuit from damage.

[0004] The steps of the control method include:

[0005] All parallel APFs are connected to the grid through the confluence point;

[0006] Collect current and voltage signals on the grid side, load side, and confluence point side;

[0007] A first protection strategy based on reactive power overcompensation and a second protection strategy based on excessive circulating current are configured at the APF end or the centralized control end;

[0008] The opening and closing of the bus contactor is controlled according to the first or second protection strategy.

[0009] In another aspect of the present invention, an APF parallel bus point control system is provided, comprising a processor and a memory, wherein the memory stores instructions, and when the instructions are executed by at least one processor, the steps of any of the above control methods are implemented.

[0010] The technical solution of the present invention can compensate for inductive reactive power, prevent capacitive reactive power caused by overcompensation, and effectively weaken the circulation current at the confluence point. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 : Schematic diagram of APF multi-machine parallel topology structure; TA: mutual inductor, KM: AC contactor, QF: circuit breaker;

[0012] Figure 2 : Schematic diagram of APF parallel connection generating circulating current;

[0013] Figure 3 : A flow chart of control method steps. DETAILED DESCRIPTION

[0014] The present invention relates to Figure 1 The busbar shown in FIG. 1 includes a contactor KM and a circuit breaker QF connecting multiple parallel APF machines to the output end of the power grid.

[0015] The active power filter of the present invention refers to a parallel type active power filter.

[0016] In related terms, PCC is the common connection point, which is usually the first point of access to the network after the grid connection point.

[0017] Circulating current is the reactive current that flows from one APF to another, and does not flow into the load. Circulating current will bring additional losses, resulting in uneven distribution of compensation currents among APFs and causing grid voltage and current distortion.

[0018] Two APFs in parallel can be equivalent Figure 2 , where Uout1 and Uout2, Z1 and Z2, Ic1 and Ic2 are the output voltage, output impedance and output compensation current of the two parallel APFs respectively; Zs and Ic are the system equivalent impedance and the total output current of the APF.

[0019] The output currents of the two APFs are not equal. At this time, a circulating current will appear in the compensation device. At this time, the circulating current I is calculated according to formula I C :

[0020]

[0021] Through the following embodiments, various aspects of the present disclosure are explained with reference to the attached drawings. Some aspects of the present disclosure can be implemented without these details. In other cases, well-known circuits, structures and technologies are not shown in detail to avoid obscuring the understanding of the description.

[0022] Some solutions, by Figure 1 The main controller in the Figure 3 Steps:

[0023] All parallel APFs are connected to the grid through the confluence point;

[0024] Collect current and voltage signals on the grid side, load side, and confluence point side;

[0025] A first protection strategy based on reactive power overcompensation and a second protection strategy based on excessive circulating current are configured at the APF end or the centralized control end;

[0026] The opening and closing of the bus contactor is controlled according to the first or second protection strategy.

[0027] Before the nth APF is connected to the grid through the confluence point, the following steps are performed:

[0028] Calculate the reactive power at the junction point based on the junction point current and the junction point voltage;

[0029] Calculate the reactive power on the load side according to the load side current and the load side point voltage;

[0030] If the reactive power on the confluence point side is greater than the reactive power on the load side, the first protection strategy is executed and the contactor connected to the nth APF is disconnected;

[0031] If the reactive power on the bus-point side is less than the reactive power on the load side, the nth APF is allowed to be connected to the grid.

[0032] Some scenarios also include the following steps performed by APF:

[0033] Use fast Fourier transform to perform harmonic decomposition of the output voltage on the bus side and the load side;

[0034] Calculate the proportion of high-order harmonics on the confluence point side to the harmonics on the APF compensation side.

[0035] Calculate the proportion of higher-order harmonics on the load side to the harmonics on the APF compensation side.

[0036] If the proportion of high-order harmonics exceeds 30%, a delayed grid connection request is sent to the main controller.

[0037] When the nth APF is connected to the grid, the second protection strategy is executed, which specifically includes the following steps: calculating the circulating current of each APF, modulating the circulating current compensation signal of each APF and the harmonic compensation signal of the APF together into an APF output signal; performing delay compensation on the control delay of the reference current of each APF;

[0038] For the nth APF connected to the grid, the following steps are used to output the circulating current compensation signal:

[0039] S1 calculates the A phase circulating current Ina, B phase circulating current Inb, and C phase circulating current Inc.

[0040] S2 decomposes the circulating current of each phase into positive-sequence circulating current IP, negative-sequence circulating current IN and zero-sequence circulating current IZ.

[0041] S3 performs independent PI control on the positive-sequence circulating current, negative-sequence circulating current and zero-sequence circulating current to perform circulating current compensation.

[0042] Compensating the control delay of the reference current of each APF includes a method based on Smith predictor compensation, and the transfer function of the Smith predictor is shown as follows:

[0043]

[0044] Among them, Irefi is the APF circulating current reference current at the i-th sampling time, Irefi+1 is the APF estimated circulating current reference current at the i+1-th sampling time, Gc(s) is the PID algorithm transfer function, Gp(s) is the transfer function of the first-order system, s is the frequency, and τ is the control delay.

[0045] The Smith predictor compensation method is a technology in existing public literature, such as the literature: Dong Min, Dong Guangshan; AGC control system using Smith predictor low-pass filter compensation, DOI: 10.13228 / j.boyuan.issn1001-0963.20130452

[0046] In some schemes, the PI control in step S3 specifically includes: decomposing the positive-sequence circulating current IP and the negative-sequence circulating current IN into reactive component and active component respectively by using dq transformation, setting the reference values ​​of the reactive components of the positive-sequence circulating current, the negative-sequence circulating current and the zero-sequence circulating current to 0, and independently controlling the reactive components of the negative-sequence circulating current, the zero-sequence circulating current and the positive-sequence circulating current by using PI controllers respectively.

[0047] The signals of the positive-sequence circulating current and the negative-sequence circulating current after PI control are subjected to dq inverse transformation and added to the zero-sequence circulating current suppression signal to obtain the circulating current suppression signal of the nth APF.

[0048] In some other schemes, the method for compensating the control delay of the reference current of each APF also includes directly using the APF circulating current reference current at the i-th sampling time as the APF estimated circulating current reference current at the i+1-th sampling time.

[0049] Some specific solutions, for the nth APF connected to the grid, in the S1 step for implementing the output circulating current compensation signal, when the calculated circulating current value of any phase exceeds the warning value, in order to prevent the circulating current overload from causing damage to the inverter, the contactor of the busbar is immediately cut off.

[0050] In some solutions, the APF paralleling and converging device includes multiple APFs connected in parallel, multiple processors and memories implemented by DSPs, switches electrically connecting the output end of each APF to the converging point, and circuit breakers arranged at the output end of the converging point to the power grid. Instructions are stored in the memory, and when the instructions are executed by at least one processor, all steps of the above solution are implemented.

[0051] The APF parallel confluence device includes 2-6 APFs in parallel. The incorporation of too many APFs will result in a slower response speed of the circulating current control at the confluence point.

[0052] Some solutions propose an APF parallel bus point control system, including a processor and a memory, wherein the memory stores instructions, and when the instructions are executed by at least one processor, the steps of any of the above control methods are implemented. Preferably, the processor is a plurality of parallel computing processors.

[0053] The embodiments and functional operations of the subject matter described in this specification may be implemented in: digital electronic circuitry, tangibly implemented computer software or firmware, computer hardware, including the structures disclosed in this specification and their structural equivalents, or a combination of more than one of the above. The embodiments of the subject matter described in this specification may be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on one or more tangible non-transitory program carriers, for execution by a data processing device or to control the operation of a data processing device.

[0054] Alternatively or additionally, the program instructions may be encoded on an artificially generated propagation signal, such as a machine-generated electrical, optical, or electromagnetic signal, which is generated to encode information for transmission to an appropriate receiver device for execution by a data processing device. The computer storage medium may be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination of one or more of the above devices.

[0055] The processes and logic flows described in this specification may be performed by one or more programmable computers that execute one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows may also be performed by special purpose logic circuits, such as FPGAs (field programmable gate arrays) or ASICs (application specific integrated circuits), and the apparatus may also be implemented as special purpose logic circuits.

[0056] Although this specification contains many specific implementation details, these should not be interpreted as limitations on the scope of any invention or the scope of what can be claimed, but rather as an illustration of features that can concretize a particular embodiment of a particular invention. The specific features described in this specification in the context of independent embodiments can also be implemented in combination with a single embodiment. Conversely, the various features described in the context of a single embodiment can also be implemented independently in multiple embodiments, or in any suitable sub-combination. In addition, although the features can be described above as a combination effect and even initially required, one or more features from the required combination can be removed from the combination in some cases, and the required combination can be turned to a sub-combination or a variation of the sub-combination.

Claims

1. A method for controlling APF parallel bus points, characterized in that: The steps of the control method include: All parallel APFs are connected to the grid through the confluence point; Collect current and voltage signals on the grid side, load side, and confluence point side; A first protection strategy based on reactive power overcompensation and a second protection strategy based on excessive circulating current are configured at the APF end or the centralized control end; The opening and closing of the bus contactor is controlled according to the first or second protection strategy.

2. The control method according to claim 1, characterized in that: Before the nth APF is connected to the grid through the confluence point, the following steps are performed: Calculate the reactive power at the junction point based on the junction point current and the junction point voltage; Calculate the reactive power on the load side according to the load side current and the load side point voltage; If the reactive power on the confluence point side is greater than the reactive power on the load side, executing the first protection strategy to disconnect the contactor connected to the nth APF; If the reactive power on the confluence point side is less than the reactive power on the load side, the nth APF is allowed to be connected to the grid.

3. The control method according to claim 2, characterized in that: The following steps are also included: The reactive current at the bus point is judged to be inductive reactive current or capacitive reactive current by the phase of the bus point output relative to the grid-side current or the positive or negative of the relative grid-side voltage; If it is an inductive reactive current, the nth APF is allowed to be connected to the grid; If it is a capacitive reactive current, the grid connection of the nth APF is delayed.

4. The control method according to claim 1, characterized in that: The following steps are performed by APF: Use fast Fourier transform to perform harmonic decomposition of the output voltage on the bus side and the load side; Calculate the proportion of higher-order harmonics on the confluence point side to the harmonics on the APF compensation side. Calculate the proportion of high-order harmonics on the load side to the harmonics on the APF compensation side. If the proportion of high-order harmonics exceeds 30%, a delayed grid connection request is sent to the main controller.

5. The control method according to claim 1, characterized in that: When the nth APF is connected to the grid, the second protection strategy is executed, which specifically includes the following steps: calculating the circulating current of each APF, modulating the circulating current compensation signal of each APF and the harmonic compensation signal of the APF together into an APF output signal; performing delay compensation on the control delay of the reference current of each APF; For the nth APF connected to the grid, the following steps are used to output the circulating current compensation signal: S1 calculates the A phase circulating current Ina, B phase circulating current Inb, and C phase circulating current Inc. S2 decomposes the circulating flow of each phase into positive sequence circulating flow I P 、Negative sequence circulation I N and zero sequence circulating current I Z S3 performs independent PI control on the positive-sequence circulating current, negative-sequence circulating current and zero-sequence circulating current to perform circulating current compensation.

6. The control method according to claim 5, characterized in that: Compensating the control delay of the reference current of each APF includes a method based on Smith predictor compensation, and the transfer function of the Smith predictor is shown as follows: Among them, I refi is the APF circulating reference current at the i-th sampling time, I refi+1 is the APF estimated circulating reference current at the i+1th sampling time, G c (s) is the PID algorithm transfer function, G p (s) is the transfer function of the first-order system, s is the frequency, and τ is the control delay.

7. The control method according to claim 5, characterized in that: The PI control in step S3 specifically includes: using dq transformation to convert the positive sequence circulating current I P and negative sequence circulation I N Decompose it into reactive component and active component, set the reference values ​​of the reactive components of positive-sequence circulating current, negative-sequence circulating current and zero-sequence circulating current to 0, and independently control the reactive components of negative-sequence circulating current, zero-sequence circulating current and positive-sequence circulating current by PI controllers respectively.

8. The control method according to claim 6, characterized in that: Step S3 also includes: performing dq inverse transformation on the positive-sequence circulating current and negative-sequence circulating current signals after PI control, and adding the signals to the zero-sequence circulating current suppression signal to obtain the circulating current suppression signal of the nth APF.

9. The control method according to claim 5, characterized in that: The method for compensating the control delay of the reference current of each APF also includes directly using the APF circulating current reference current at the i-th sampling time as the APF estimated circulating current reference current at the i+1-th sampling time.

10. An APF parallel bus point control system, characterized in that: The invention comprises a processor and a memory, wherein the memory stores instructions, and when the instructions are executed by at least one processor, the steps of the method according to any one of claims 1 to 9 are implemented.

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