A method for limiting the output current of an active power filter

Abstract

The present invention proposes a method for limiting the output current of an active power filter. This method achieves composite control by comprehensively limiting the instantaneous and effective values ​​of the active power filter's output current. The instantaneous value of the active power filter's output current is dynamically acquired in real time and compared with a set limit value, using a proportional coefficient of Kr. The effective value of the active power filter's output current is then compared with the limit value, using a proportional coefficient of Kp. The proportional coefficient K is set to Kr*Kp. The proportional coefficient K is then added to the command current of the next acquisition cycle to limit the current amplitude. The method proposed in the present invention simultaneously addresses the problems of poor waveform quality and slow response speed of current clipping methods for active power filters, as well as the proportional limiting method.

Description

Technical Field

[0001] The present invention relates to the technical field of power quality detection and control, and in particular to an output current limiting algorithm for an active power filter (APF). Background Art

[0002] With the rapid development of modern power electronics technology in recent years, the number of power electronics equipment and nonlinear loads such as generators, transformers, and inverters in power grids has increased dramatically, leading to a continuous increase in voltage and current harmonic levels in the grid. Ideal AC power does not exist in reality. As the power industry's requirements for power quality continue to increase, high-performance power filters are gaining increasing attention. Shunt active power filters (APFs) have been widely researched and applied in recent years due to their excellent suitability for harmonic control and reactive power compensation of current-source harmonic sources. Output current limiting is a key technology in APF control systems. Current commonly used methods include clipping or proportional limiting within a power frequency cycle. Clipping limits the portion of the output waveform that exceeds a set value, introducing harmonic components into the output waveform after clipping, impacting the output waveform quality and control system efficiency. While proportional limiting within a power frequency cycle protects the APF output waveform, its response speed lags by one cycle. Complex load conditions, especially those with impact loads, place high demands on the limiting response time. Long response times can cause equipment downtime or burnout. Therefore, it is necessary to propose a new current limiting method that can improve the response speed while maintaining the output waveform quality and further ensure the safety of the equipment. Summary of the Invention

[0003] The invention provides an active power filter output current limiting method for simultaneously solving the problems of poor waveform quality of the current active power filter output current clipping limiting method and slow response speed of the proportional limiting method.

[0004] The embodiments of the present invention are as follows:

[0005] The present invention provides a method for limiting the output current of an active power filter, comprising the following steps:

[0006] The effective value Ir and instantaneous value Ip of the output current of the active power filter are measured; the set limited effective value is compared with the effective value Ir, and the ratio is Kr, that is, Kr = limited effective value / Ir; the set limited instantaneous value is compared with the instantaneous value Ip, and the ratio is Kp, that is, Kp = limited instantaneous value / Ip; according to the actual situation of the limit requirement, the threshold value Tr of the ratio Kr and the threshold value Tp of the ratio Kp are determined; when the ratio Kr ≥ the threshold value Tr, Kr is taken as the value of the ratio Kr. =Tr; when the ratio Kr < the threshold value Tr, take Kr = Kr; when the ratio Kp ≥ the threshold value Tp, take Kp = Tp; when the ratio Kp < the threshold value Tp, take Kp = Kp; the proportional coefficient K is equal to the product of the ratio Kr and the ratio Kp, that is, K = Kr*Kp; in the next sampling period, multiply the current command value I by the proportional coefficient K to obtain the scaled-down current command value Ir; repeat the above steps in each sampling period.

[0007] Furthermore, the current is a harmonic current.

[0008] Furthermore, when the signal is a harmonic current, the effective value Ir can be determined by using a sliding film effective value calculation method or a traditional effective value formula.

[0009] Furthermore, the current limiting method is also used for harmonic voltages.

[0010] Furthermore, the instantaneous value Ip is a current amplitude sample within one cycle.

[0011] Furthermore, under normal circumstances, the threshold Tr is equal to 1, and the threshold Tr can be adjusted according to actual needs to regulate the influence of the effective value of the current on the output signal.

[0012] Furthermore, under normal circumstances, the threshold value Tp is equal to 1, and the threshold value Tp can be adjusted according to actual needs to regulate the influence of the instantaneous value of the current on the output signal.

[0013] Furthermore, there is no order requirement for solving the ratio Kr and the ratio Kp. The ratio Kr can be solved first, and then the ratio Kp; the ratio Kp can also be solved first, and then the ratio Kr; or even the two ratios can be solved at the same time.

[0014] The active power filter output current limiting method proposed in the present invention solves the problems of poor waveform quality of the current active power filter output current clipping limiting method and slow response speed of the proportional limiting method by limiting the instantaneous value and effective value of the active power filter output current. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 Flowchart of the method for limiting the output current of the active power filter;

[0017] Figure 2 The overall structure diagram of the APF system including the current limiting control link described in the present invention;

[0018] Figure 3 Waveforms of system current, load current and APF output compensation current using the traditional clipping and limiting control method;

[0019] Figure 4 Waveforms of system current, load current and APF output compensation current using the traditional proportional limiting control method;

[0020] Figure 5 Waveforms of system current, load current and APF output compensation current using the amplitude limiting control method of the present invention. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0022] According to an embodiment of the present invention, a method for limiting the output current of an active power filter is provided. Figure 1The flow chart of the method for limiting the output current of the active power filter includes the following steps: measuring the effective value Ir and the instantaneous value Ip of the output current of the active power filter; comparing the set limited effective value with the effective value Ir, and the ratio is Kr, that is, Kr = limited effective value / Ir; comparing the set limited instantaneous value with the instantaneous value Ip, and the ratio is Kp, that is, Kp = limited instantaneous value / Ip; determining the threshold value Tr of the ratio Kr and the threshold value Tp of the ratio Kp according to the actual situation of the limiting requirement; when the ratio When the value Kr ≥ the threshold value Tr, Kr = Tr; when the ratio Kr < the threshold value Tr, Kr = Kr; when the ratio Kp ≥ the threshold value Tp, Kp = Tp; when the ratio Kp < the threshold value Tp, Kp = Kp; the proportional coefficient K is equal to the product of the ratio Kr and the ratio Kp, that is, K = Kr*Kp; in the next sampling period, the current command value I is multiplied by the proportional coefficient K to obtain a scaled-down current command value Ir; the above steps are repeated in each sampling period.

[0023] In a preferred embodiment of the present invention, the current is a harmonic current.

[0024] In a preferred embodiment of the present invention, when the signal is a harmonic current, the effective value Ir can be determined by using a sliding film effective value calculation method or a traditional effective value formula.

[0025] In a preferred embodiment of the present invention, the current limiting method can also be used for harmonic voltages.

[0026] In a preferred embodiment of the present invention, the instantaneous value Ip is a current amplitude sample within one cycle.

[0027] In a preferred embodiment of the present invention, under normal circumstances, the threshold Tr is equal to 1; the threshold Tr can be adjusted according to actual needs to regulate the impact of the effective value of the current on the output signal.

[0028] In a preferred embodiment of the present invention, under normal circumstances, the threshold value Tp is equal to 1; the threshold value Tp can be adjusted according to actual needs to regulate the influence of the instantaneous value of the current on the output signal.

[0029] In a preferred embodiment of the present invention, there is no order requirement for solving the ratio Kr and the ratio Kp. The ratio Kr can be solved first, and then the ratio Kp; the ratio Kp can be solved first, and then the Kr; or even the two ratios can be solved at the same time.

[0030] The application of the present invention is described below with reference to a specific example. In this embodiment, the APF compensation system adopts a three-phase three-wire structure, and the main circuit is as follows. Figure 2As shown, the load consists of a three-phase diode rectifier with a resistive inductive load. There are two groups of loads in total, which are used to simulate load mutations. The equipment used mainly includes: an active power filter, two three-phase diode rectifier loads, an oscilloscope, and three current probes. Operation steps: (1) Use the traditional cut-off current limiting control method to limit the harmonic current extracted by the load, limit the peak value to 55A, and perform clipping and limiting at the moment of 0.5s load sudden increase, and limit the instantaneous value to within 55A; (2) Use the traditional effective value comparison control method to limit the harmonic current extracted by the load, limit the effective value to 25A, and perform effective value limitation at the moment of 0.5s load sudden increase, and respond only at 20ms, which is a slow response speed; (3) Use the limiting control method of the present invention to limit the effective value at the moment of 0.5s load sudden increase, and achieve full response at 6ms.

[0031] Figure 3 、 Figure 4 and Figure 5 The waveforms of the system current, load current, and APF output compensation current using the traditional clipping limiter, the traditional proportional limiter, and the limiter control method of the present invention are shown. The vertical axis in the figure represents the current amplitude, and the load surge begins at 0.5s. Figure 3 After a sudden load increase, the output current of the APF shows obvious clipping phenomenon. Figure 4 It responds 20ms after a sudden load increase. Figure 5 It fully responds within 6ms after a load surge, and there is no clipping.

[0032] Figure 3 and Figure 4 The command current limiting method belongs to the open-loop control method based on load current, which has the disadvantages of clipping and slow dynamic response speed, and cannot eliminate the error caused by interference. Figure 5 The amplitude limiting control method of the present invention has the advantages of fast dynamic response speed and no clipping phenomenon. The reasons for this phenomenon are: (1) the dual-parameter method proposed by the present invention realizes composite control, that is, it integrates the instantaneous current value limitation and the effective value limitation; (2) the output current of the active power filter is compared with the limit value, which is equivalent to current feedforward. The feedforward method itself can accelerate the dynamic response speed. In addition, the real-time comparison of the instantaneous current value can achieve fast dynamic response; (3) the control method proposed by the present invention belongs to closed-loop control from the control perspective, which can achieve accurate and fast tracking. In the feedback control system, no matter whether it is an external disturbance or an internal system change, as long as the controlled quantity deviates from the limit value, a corresponding control action will be generated to eliminate the deviation.

[0033] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for limiting the output current of an active power filter, characterized in that: The following steps are involved: Measuring the effective value Ir and instantaneous value Ip of the output current of the active power filter; comparing the set limited effective value with the effective value Ir, the ratio is Kr, that is, Kr = limited effective value / Ir; comparing the set limited instantaneous value with the instantaneous value Ip, the ratio is Kp, that is, Kp = limited instantaneous value / Ip; Determine a threshold Tr of the ratio Kr and a threshold Tp of the ratio Kp, wherein the threshold Tr is equal to 1 and the threshold Tp is equal to 1; When the ratio Kr≥the threshold Tr, Kr=Tr; when the ratio Kr<the threshold Tr, Kr=Kr; When the ratio Kp≥the threshold Tp, take Kp=Tp; when the ratio Kp<the threshold Tp, take Kp=Kp; The proportional coefficient K is equal to the product of the ratio Kr and the ratio Kp, that is, K=Kr*Kp; In the next sampling period, the current command value I is multiplied by the proportional coefficient K to obtain a current command value with a reduced scale; Repeat the above steps in each sampling period.

2. The method for limiting the output current of an active power filter according to claim 1, wherein: The current is a harmonic current.

3. The method for limiting the output current of an active power filter according to claim 2, wherein: The effective value Ir is determined by using a synovial effective value calculation method or a traditional effective value formula.

4. The method for limiting the output current of an active power filter according to claim 1, wherein: The current limiting method can also be used for harmonic voltages.

5. The method for limiting the output current of an active power filter according to claim 1, wherein: The instantaneous value Ip is a current amplitude sample within one cycle.

6. The method for limiting the output current of an active power filter according to claim 1, wherein: There is no order requirement for solving the ratio Kr and the ratio Kp.

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