Methods for harmonic current compensation and feeder resonance damping in microgrid loads

Abstract

The present invention discloses a method for harmonic current compensation and feeder resonance damping of a microgrid load, the method comprising: performing power control through a DG unit to obtain an instantaneous base current reference value of a connection point POC; adopting a selective harmonic detection strategy to suppress harmonic current components to obtain a harmonic current reference value for local load current compensation; calculating a reference current reference value for feeder reference active damping according to the harmonic components of the POC voltage and the active damping resistor; realizing closed-loop current tracking, and adding the instantaneous base current reference value, the harmonic current reference value for local load current compensation, and the reference current reference value for active damping to obtain a current reference value of the DG unit. The method realizes compensation for local load harmonic current and damping of feeder resonance propagation, and reduces grid current and node voltage distortion.

Description

Technical Field

[0001] The present invention relates to the technical field of distributed microgrids, and in particular to a method for microgrid load harmonic current compensation and feeder resonance damping. Background Art

[0002] With the increase of power electronic devices and nonlinear loads in the system, the structure of the distribution network is changing. These nonlinear loads and power electronic devices will bring serious harmonic voltage and current distortion to the system, thereby increasing the noise problem of sensitive loads in the distribution network and reducing the efficiency of the system. In order to solve these harmonic problems, people have developed active power filters and passive power filters. On the one hand, for passive filters, it is difficult to solve time-varying harmonic problems due to the fixed parameters of the filter circuit, so it is inconvenient to use. On the other hand, active power filters based on power converters are considered to be a better solution due to their compact structure, strong robustness to system disturbances, and adaptability to changes in system parameters.

[0003] However, for the distributed generation (DG) units connected to the end of long feeders, which are common in weak power grids such as rural distribution networks, firstly, the DG units are usually strongly coupled with nearby nonlinear loads, which may reduce the effectiveness of the system control strategy; secondly, the main grid voltage may also be distorted, which will cause trouble for power quality control; thirdly, the long feeder resonance may be caused by both the sending and receiving ends of the grid voltage distortion, but how to use the DG unit to solve the resonance propagation problem under such unfavorable conditions has not been discussed. Therefore, this paper proposes a flexible DG unit control method that simultaneously compensates for local load harmonic current and feeder resonance damping. It is realized by controlling the equivalent circuit of the DG unit by combining controllable harmonic current source and resonance damping, and the correctness of the method is verified by simulation results.

[0004] Although the currently available literature provides harmonic suppression methods for active power filters and passive power filters, there is little mention of long feeder systems and no specific processing methods are provided. Summary of the invention

[0005] The object of the present invention is to provide a method for microgrid load harmonic current compensation and feeder resonance damping, aiming to solve the above problems.

[0006] The present invention provides a method for microgrid load harmonic current compensation and feeder resonance damping, comprising:

[0007] S1. Power control is performed through the DG unit to obtain the instantaneous base current reference value of the connection point POC;

[0008] S2. Use a selective harmonic detection strategy to suppress harmonic current components and obtain a harmonic current reference value for local load current compensation;

[0009] S3. Calculate the reference current reference value of the feeder reference active damping according to the harmonic components of the POC voltage and the active damping resistance;

[0010] S4. Implement closed-loop current tracking, and obtain the current reference value of the DG unit by adding the instantaneous base current reference value, the harmonic current reference value of the local load current compensation and the base current reference value of the active damping.

[0011] By adopting the embodiment of the present invention, a multifunctional DG unit is provided, which realizes compensation for local load harmonic current and damping of feeder resonance propagation, provides active compensation for low-voltage distribution network, and reduces grid current and node voltage distortion.

[0012] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented according to the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0014] Figure 1 is a flow chart of a method for microgrid load harmonic current compensation and feeder resonance damping according to an embodiment of the present invention;

[0015] Figure 2 is a schematic diagram of a grid-connected multifunctional DG unit with a local nonlinear load according to an embodiment of the present invention;

[0016] Figure 3 is the DG unit and local load performance of the embodiment of the present invention;

[0017] Figure 4 It is the node voltage and grid current performance of the embodiment of the present invention.

[0018] Explanation of reference numerals: 1: main power grid; 2: long feeder; 3: multifunctional DG unit; 31: LCL filter; 4: local non-linear load. DETAILED DESCRIPTION

[0019] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0020] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0021] In addition, the terms "first" and "second" are used for descriptive purposes only and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "multiple" is two or more, unless otherwise clearly and specifically defined. In addition, the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0022] Method Embodiment

[0023] According to an embodiment of the present invention, a method for microgrid load harmonic current compensation and feeder resonance damping is provided. Figure 1 is a flow chart of a method for microgrid load harmonic current compensation and feeder resonance damping according to an embodiment of the present invention, such as Figure 1 As shown, the method for microgrid load harmonic current compensation and feeder resonance damping according to an embodiment of the present invention specifically includes:

[0024] S1. Perform power control through the DG unit to obtain the instantaneous base current reference value of the connection point POC.

[0025] Figure 2 Schematic diagram of a grid-connected multifunctional DG unit with a local nonlinear load according to an embodiment of the present invention. Figure 2As shown, the DG unit includes the main grid 1, the long feeder 2, the multifunctional DG unit 3 and the local nonlinear load 4. The DC source is connected to the POC through the LCL filter 31. L1 and L2 are filter inductors, and C f is the filter capacitor, V POC is the POC point voltage, I DG is the current output from the multifunctional DG unit 3 to the POC point, I out is the output current of the DG unit, the local nonlinear load 4 is connected to the POC terminal, and its current is determined by I load express.

[0026] Specifically, the solution for power control through the multifunctional DG unit mainly consists of two parts:

[0027] First, the voltage phase angle θ at the connection point POC is obtained using a phase-locked loop;

[0028] Then, according to the instructions of the energy control management system, the active and reactive current references are obtained, and further according to the voltage phase angle θ and the magnitude of the active and reactive current components, the instantaneous base current reference value of the connection point POC is calculated by formula 1:

[0029] I ref,pq =cos(θ)·g * p +sin(θ)·g * q Formula 1;

[0030] Among them, g * p is the magnitude of the actual power current component, g * q is the magnitude of the reactive current component, I ref,pq is the instantaneous base current component.

[0031] S2. A selective harmonic detection strategy is used to suppress harmonic current components and obtain a harmonic current reference value for local load current compensation.

[0032] Specifically, the DG unit current reference value should also include harmonic components to achieve appropriate harmonic compensation. The selective harmonic detection strategy used to suppress the harmonic current components is shown in Formula 2:

[0033] I ref,load =G har (s)·I load Formula 2;

[0034] Among them, I ref,load is the harmonic current reference for local load current compensation, G har(s) is the transfer function of the harmonic detector, which is expressed as shown in Formula 21:

[0035]

[0036] Among them, ω f is the fundamental frequency, h is the harmonic order, ω bp is the passband angular frequency for harmonic detection.

[0037] S3. Calculate the reference current value of the feeder reference active damping according to the harmonic components of the POC voltage and the active damping resistance.

[0038] Specifically, the multifunctional DG unit controller achieves the suppression of resonance propagation along the feeder through the following two steps:

[0039] First, since the frequency of resonance actually occurs in a wide frequency range, the harmonic components of the POC voltage are obtained by eliminating the fundamental component of the POC voltage, as shown in Formula 3:

[0040]

[0041] Where h is the harmonic order, V POC,h is the harmonic component of the POC voltage, V POC is the voltage at the POC point, ω f is the fundamental frequency, ω bp is the passband angular frequency for harmonic detection;

[0042] Secondly, the reference current reference value of the feeder reference active damping is calculated based on the harmonic components of the POC voltage and the active damping resistance, as shown in Formula 4:

[0043]

[0044] Among them, I ref,ad is the reference current value of active damping, r v is the active damping resistor.

[0045] S4. Implement closed-loop current tracking, and obtain the current reference value of the DG unit by adding the instantaneous base current reference value, the harmonic current reference value of the local load current compensation and the baseline current reference value of the active damping.

[0046] Specifically, the current reference value I of the DG unit is calculated by formula 5: ref :

[0047] I ref =I ref,pq +I ref,load +I ref,ad Formula 5.

[0048] In order to achieve fast tracking of closed-loop current without causing resonance of the LCL filter of the DG unit, a voltage-current dual-loop control scheme is used to achieve fast tracking of fundamental current and harmonic current. Specifically:

[0049] The outer current control loop uses a proportional resonant controller PR to obtain the output reference current value, as shown in the following figure.

[0050] As shown in formula 6:

[0051]

[0052] Among them, G outer (s) is the outer loop current controller, k ih is the gain of the semi-resonant controller, ω cut is the cut-off frequency of the resonant controller;

[0053] In the inner voltage control loop, the inverter output current I is measured inner as feedback value and use simple proportional control k inner Provide sufficient damping for the LCL filter to obtain the inverter output reference voltage value, as shown in Formula 7:

[0054]

[0055] Among them, G inner (s) is the transfer function of the inner loop controller, V ref,out The output reference voltage of the inverter for PWM pulse width modulation signal processing.

[0056] To verify the effectiveness of the power allocation strategy proposed in the invention, this embodiment is simulated in the Matlab / Simulink environment. Figure 2 When the DG unit performs local load harmonic current compensation and active feeder resonance damping at the same time, the system performance is as follows: Figure 3 and Figure 4 As shown, Figure 3 For DG units and local load performance, Figure 4 In this case, the DG unit provides active compensation for the system, which reduces the distortion of the grid current and node voltage, proving the effectiveness of the method of the present invention.

[0057] By adopting the embodiment of the present invention, a multifunctional DG unit is provided, which, in addition to being able to provide active power and reactive power, also realizes compensation for local load harmonic current and damping of feeder resonance propagation, and provides active compensation for the low-voltage distribution network, thereby reducing grid current and node voltage distortion.

[0058] Obviously, those skilled in the art should understand that the above modules or steps of the present invention can be implemented by a general computing device, they can be concentrated on a single computing device, or distributed on a network composed of multiple computing devices, and optionally, they can be implemented by a program code executable by a computing device, so that they can be stored in a storage device and executed by the computing device, and in some cases, the steps shown or described can be executed in a different order than here, or they can be made into individual integrated circuit modules, or multiple modules or steps therein can be made into a single integrated circuit module for implementation. Thus, the present invention is not limited to any specific combination of hardware and software.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for microgrid load harmonic current compensation and feeder resonance damping, characterized in that: include: S1. Power control is performed through the DG unit to obtain the instantaneous base current reference value of the connection point POC; The DG unit includes the main grid, long feeder, multifunctional DG unit and local nonlinear load. The DC source is connected to the POC through an LCL filter. L1 and L2 are filter inductors. C f is the filter capacitor, V POC is the POC point voltage, I DG is the current output from the multifunctional DG unit to the POC point, I out is the output current of the DG unit, the local nonlinear load is connected to the POC terminal, and its current is determined by I load express; The method described in step S1 specifically includes: The voltage phase angle θ of the connection point POC is obtained by using a phase-locked loop; According to the voltage phase angle θ and the magnitude of the active and reactive current components, the instantaneous base current reference value of the connection point POC is calculated by formula 1: I ref,pq = cos(θ)·g * p + sin(θ)·g * q Equation 1; Among them, g * p is the magnitude of the actual power current component, g * q is the magnitude of the reactive current component, I ref,pq is the instantaneous base current component; S2. Use a selective harmonic detection strategy to suppress harmonic current components and obtain a harmonic current reference value for local load current compensation; The selective harmonic detection strategy in step S2 is shown in Formula 2: I ref,load =G har (s)·I load Formula 2; Among them, I ref,load is the harmonic current reference for local load current compensation, G har (s) is the transfer function of the harmonic detector; S3. Calculate the reference current reference value of the feeder reference active damping according to the harmonic components of the POC voltage and the active damping resistance; The harmonic components of the POC voltage are obtained by eliminating the fundamental component of the POC voltage, as shown in Formula 3: Where h is the harmonic order, V POC,h is the harmonic component of the POC voltage, V POC is the voltage at the POC point, ω f is the fundamental frequency, ω bp is the passband angular frequency for harmonic detection; S4. Implement closed-loop current tracking, and obtain the current reference value of the DG unit by adding the instantaneous base current reference value, the harmonic current reference value of the local load current compensation and the base current reference value of the active damping.

2. The method according to claim 1, characterized in that The reference current reference value of the feeder reference active damping is calculated according to the harmonic component of the POC voltage and the active damping resistance in step S3, as shown in Formula 4: Among them, I ref,ad is the reference current value of active damping, r v is the active damping resistor.

3. The method according to claim 2, characterized in that The closed-loop current tracking described in step S4 specifically includes: using a voltage-current dual-loop control scheme to achieve fast tracking of fundamental current and harmonic current to avoid resonance of the LCL filter of the DG unit; The current reference value I of the DG unit is obtained by formula 5 ref : I ref =I ref,pq +I ref,load +I ref,ad Formula 5.

4. The method according to claim 3, characterized in that The voltage and current dual-loop control scheme specifically includes: In the outer current control loop, the proportional resonant controller PR is used to obtain the output reference current value, as shown in Formula 6: Among them, G outer (s) is the outer loop current controller, k ih is the gain of the semi-resonant controller, ω cut is the cut-off frequency of the resonant controller; In the inner voltage control loop, the inverter output current I is measured inner as feedback value and use simple proportional control k inner Provide sufficient damping for the LCL filter to obtain the inverter output reference voltage value, as shown in Formula 7: Among them, G inner (s) is the transfer function of the inner loop controller, V ref,out The output reference voltage of the inverter for PWM pulse width modulation signal processing.

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