CN202050244U

CN202050244U - Parallel type active power filter

Info

Publication number: CN202050244U
Application number: CN2011201628804U
Authority: CN
Inventors: 董密; 杨建�; 粟梅; 孙尧; 于晶荣; 王辉; 敖杰; 肖亮
Original assignee: Jiangxi Zhongneng Electrical Science & Technology Co Ltd
Current assignee: Jiangxi Z-Energy Electric Technology Co Ltd
Priority date: 2011-05-20
Filing date: 2011-05-20
Publication date: 2011-11-23
Anticipated expiration: 2021-05-20

Abstract

The utility model discloses a parallel type active power filter applicable to a three-phase four-wire power grid system, which mainly comprises a three-phase full-bridge inverter circuit, a half-bridge chopper circuit, double capacitances and a control unit. The three-phase full-bridge inverter circuit is electrically connected with the three-phase four-wire power grid system and is connected with the half-bridge chopper circuit in parallel; the other side of the half-bridge chopper circuit is electrically connected with the middle point of the double capacitances at the direct current side through inductances, and is used for controlling the voltage of the parallel type active power filter at the direct current side; and the control unit employs a double closed-loop control structure. Compared with the prior art, the double closed-loop control structure of the parallel type active power filter can better compensate the harmonic current of a power grid and effectively inhibit unbalance of the three phases.

Description

Parallel active power filter

Technical Field

The utility model relates to a parallel type active power filter suitable for electric wire netting system especially relates to a parallel type active power filter suitable for three-phase four-wire electric wire netting system.

Background

With the continuous development of power electronic technology, more and more power electronic devices are widely applied to various fields, and simultaneously, a large amount of harmonic waves are injected into a power grid. Harmonic waves in a power grid are extremely harmful, the power supply voltage waveform of the power grid can be distorted, the power supply quality is reduced, the energy loss of a power system is increased, the service life of power supply and utilization equipment is shortened, and electromagnetic interference can be caused to other electronic equipment.

Because the Power quality problem caused by the Power grid harmonic wave is increasingly paid attention, an Active Power Filter (APF) as a novel Power electronic device for dynamically inhibiting the harmonic wave and compensating the reactive current becomes the most effective and promising compensation mode for solving the increasingly serious Power harmonic wave pollution problem.

Currently, three-phase three-wire system APF has been widely studied, and there are many research results, which have been applied in practical systems. However, since the three-phase four-wire system power supply mode is the most important power supply mode in the low-voltage power distribution system and is widely applied to power systems such as industry, office and civil buildings, urban power supply, factory power supply and the like, the problems caused by the imbalance of the harmonic waves and the three phases are more and more emphasized, and the proportion of the harmonic waves is increased year by year.

In conclusion, it is significant to compensate harmonic waves and three-phase imbalance existing in the three-phase four-wire system. How to enable the active power filter to not only reject positive sequence and negative sequence harmonic components in three-phase four-wire system APF three-phase current, but also compensate zero sequence harmonic current of a system is a very important technical problem, and becomes a current research difficulty.

SUMMERY OF THE UTILITY MODEL

In order to solve the technical problem, the utility model provides a parallelly connected type active power filter suitable for three-phase four-wire power grid system.

According to an aspect of the present invention, there is provided a parallel type active power filter. The parallel type active power filter includes:

the three-phase full-bridge inverter circuit is electrically connected with the three-phase four-wire power grid system;

the half-bridge chopper circuit is connected with the three-phase full-bridge inverter circuit in parallel and is used for controlling the direct-current side voltage of the parallel active power filter;

the half-bridge chopper circuit is electrically connected with the midpoint of the direct-current side double capacitor (120) through an inductor, and the midpoint of the double capacitors is grounded; and

the control unit adopts double closed-loop control, an inner ring is a voltage ring submodule for maintaining the voltage on the direct current side of the parallel active power filter to be stable, and an outer ring is a current ring submodule for controlling the output current of the parallel active power filter, wherein,

the half-bridge chopper circuit is composed of two power switching tubes connected in series, and each power switching tube is composed of a power tube and an anti-parallel diode.

The three-phase full-bridge inverter circuit is composed of three groups of bridge arms which are connected in parallel, each bridge arm is composed of two power switch tubes which are connected in series, each power switch tube is composed of a power tube and an anti-parallel diode, and the three-phase full-bridge inverter circuit is electrically connected with the three-phase four-wire power grid system through a three-phase inductor.

According to another aspect of the present invention, there is provided a parallel active power filter, further comprising a control unit, wherein the control unit adopts a double closed-loop control, an inner loop is a voltage loop submodule for maintaining a stable voltage at a dc side of the parallel active power filter, and an outer loop is a current loop submodule for controlling an output current of the parallel active power filter.

And the voltage ring sub-module is directly or indirectly electrically connected with two ends of the double capacitor.

Compared with the prior art, this is novel has following advantage:

compare traditional parallel type active power filter, this neotype parallel type active power filter has changed traditional parallel type active power filter's structure, for example, has newly increased half-bridge chopper circuit 112 and has adopted inductance connection neutral point clamp circuit for can resist positive sequence and negative sequence harmonic component in the three-phase four-wire system APF three-phase current according to this neotype parallel type active power filter, can compensate the zero sequence harmonic current of system again. More specifically, the three-phase full-bridge inverter circuit control 111 is used for outputting current, and the half-bridge chopper circuit 112 is used for eliminating direct-current terminal voltage fluctuation caused by zero-sequence current, so that the coupling of the direct-current terminal voltage and the current control can be well realized, and the three-phase four-wire active power filter system has practical application value.

In addition, the novel parallel active power filter applicable to the three-phase four-wire power grid system also adopts a control mode of double closed-loop control, wherein the inner loop is a voltage loop which provides energy feedforward control for stabilizing the voltage of a direct-current end and inhibiting zero-sequence current disturbance; the outer ring is a current ring, the current ring is controlled by PWM (Pulse Width Modulation), and compared with the prior art, the double-closed-loop control structure can better compensate the harmonic current of a power grid and inhibit the three-phase imbalance of the system. In addition, the method for calculating the reference current by energy feedforward adopted in the novel parallel active power filter considers the compensation of system harmonic wave and reactive power from the aspect of energy, compensates three-phase imbalance, further improves the performance of the parallel active power filter, has strong robustness to load sudden change, and ensures the stability of the system.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

Drawings

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the invention without limiting the invention. In the drawings:

fig. 1 is a schematic structural diagram of a parallel type active power filter suitable for a three-phase four-wire grid system according to a first embodiment of the present invention;

fig. 2 is a schematic diagram of an equivalent single-stage, double-pole switch for one switch leg in a parallel active power filter according to a first embodiment of the present invention;

fig. 3 is a schematic structural diagram of a parallel type active power filter suitable for a three-phase four-wire grid system according to a second embodiment of the present invention;

fig. 4 is a schematic energy relationship diagram of a parallel active power filter suitable for a three-phase four-wire grid system according to a second embodiment of the present invention;

FIG. 5 is a schematic diagram of a voltage loop energy feed forward control in accordance with a second embodiment of the present invention;

FIG. 6 is a schematic diagram of the grid phase current, phase voltage and neutral current waveforms without harmonic compensation using the novel filter;

fig. 7 is a schematic diagram of the waveforms of the grid phase current, phase voltage and neutral current when harmonic compensation is performed using the novel filter.

Detailed Description

The following detailed description will be provided with reference to the accompanying drawings and examples, so that how to apply the technical means to solve the technical problems and achieve the technical effects can be fully understood and implemented. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features of the embodiments may be combined with each other, and the technical solutions formed are all within the scope of the present invention.

First embodiment

A first embodiment of the present invention will be described with reference to fig. 1.

Fig. 1 is a schematic configuration diagram of a parallel type active power filter (hereinafter, also simply referred to as an active power filter) applied to a three-phase four-wire grid system according to a first embodiment of the present invention.

As shown in fig. 1, an active power filter, which is suitable for a three-phase four-wire grid system according to the present embodiment, includes a three-phase full-bridge inverter circuit 111 electrically connected to the three-phase four-wire grid system via a three-phase inductor to be grid-connected to the three-phase four-wire grid system. The three-phase full-bridge inverter circuit 111 is composed of three groups of parallel-connected bridge arms, each group of bridge arms is composed of two series-connected power switch tubes, each power switch tube is composed of a power tube and an anti-parallel diode, and the middle point of each group of single-phase bridge inverter circuit is electrically connected to one of three buses in an alternating current power grid.

The active power filter suitable for a three-phase four-wire grid system according to the present embodiment further includes a half-bridge chopper circuit 112 connected in parallel with the three-phase full-bridge inverter circuit 111. The half-bridge chopper circuit 112 is used to control the dc-side voltage of the active power filter. Preferably, the half-bridge chopper circuit 112 is directly connected in parallel with the three-phase full-bridge inverter circuit 111 without any other device. The half-bridge chopper circuit 112 may be formed by two power switching tubes connected in series, and the power switching tubes may be formed by power tubes and anti-parallel diodes. In addition, the active power filter according to the present embodiment further includes a dc terminal double capacitor 120. The half-bridge chopper circuit 112 is connected to the midpoint of the dc-side double capacitor 120 through an inductor to clamp and control the midpoint voltage of the capacitor, so that zero-sequence current does not flow through the double capacitors, thereby stabilizing the dc-side voltage.

For ease of understanding, the inventor further illustrates an equivalent single-stage, two-pole switch schematic diagram of one switch leg in an active power filter according to the present novel first embodiment by fig. 2.

The state of each switch can be represented as two states as shown in equation (1). More specifically, each electronic switching device is denoted by S, and referring to fig. 3, each leg may be represented as a single-stage double-pole switch, and the active power filter according to the present embodiment is controlled by switching control in the inverter. The state of each bridge arm satisfies equation (2).

S₁+S₂＝1，S₁·S₂＝0 ...............(2)

Wherein S is₁And S₂Respectively showing the upper arm on and the lower arm on.

Two capacitors C at the DC double-capacitor end₁＝C₂If C, the state space model of each bridge arm is

Wherein,

represents the current flowing on the direct current side;

represents the dc side voltage;

L_Frepresents a dc side inductance value;

wherein,

I_F＝[i_Fa i_Fb i_Fc i_Fd]^T，V_C＝[v_c1 v_c2]^T，V_S＝[V_Sa V_Sb V_Sc 0]^T，

D = [\begin{matrix} d_{a} & (1 - d_{a}) \\ d_{b} & (1 - d_{b}) \\ d_{c} & (1 - d_{c}) \\ d_{d} & (1 - d_{d}) \end{matrix}]

i_Fa、i_Fb、i_Fc、i_Fdrepresenting an actual output current value of the active power filter;

V_Sa、V_Sb、V_Sbrepresenting a three-phase network side voltage value;

v_c1、v_c2representing the voltage value of the double capacitors on the direct current side;

d represents the switching states of four power switching tubes;

where d is the conduction rate of each switch, d is greater than or equal to 0 and less than or equal to 1, and the like.

Second embodiment

This embodiment is a further modification of the first embodiment. For convenience of explanation, the same configurations as those of the foregoing embodiments will not be expanded in detail, and only differences from the foregoing embodiments will be emphasized.

Fig. 3 is a schematic diagram of an active power filter suitable for a three-phase four-wire power grid system according to a second embodiment of the present invention.

Referring to fig. 1 and 3, the inverter and chopper circuit 110 is electrically connected to the dc-side double capacitor 120, wherein the inverter and chopper circuit 110 is composed of a three-phase full-bridge inverter circuit 111 and a half-bridge chopper circuit 112 connected in parallel.

In particular, the active power filter of the present embodiment further includes a control unit. The control unit adopts a double closed-loop control strategy comprising a voltage loop submodule and a current loop submodule which are electrically connected with each other. The inner ring is a voltage ring sub-module serving as a voltage ring, and energy feedforward control is adopted to maintain the voltage stability of the direct current end of the active power filter and inhibit zero-sequence current disturbance; the outer ring is a current ring submodule used as a current ring and used for compensating harmonic current of a power grid.

The voltage loop sub-module is electrically connected to the dual capacitor (120) either directly or indirectly. Due to the existence of the three-phase four-wire zero current, the direct current voltage flows through to generate the fluctuation of the direct current voltage, and the difference value of the fluctuation and the reference value is the energy generated by the zero current. Therefore, the voltage loop submodule can obtain a reference current as an instantaneous harmonic compensation reference value of the inverter according to the change of the energy at the direct current side

The current loop submodule, which is a current loop, may then be dependent on the reference current of the active power filter

The output current of the parallel active power filter is controlled. For example, the current loop sub-module may control the inverter output current using PWM, so that the grid harmonic current may be effectively compensated.

More specifically, a preferred energy feedforward calculation reference current is detailed with reference to fig. 4 and 5

The method of (1).

Wherein,

a reference voltage value representing the direct current side capacitance setting;

v_dcrepresenting the actual voltage value of the direct current side capacitor;

Δw_dcrepresents the energy value generated by zero current;

from an energy perspective, the fluctuations in dc side energy can also be expressed as:

<math><mrow> <msub> <mi>Δw</mi> <mi>dc</mi> </msub> <mo>=</mo> <msub> <mi>w</mi> <mi>dc</mi> </msub> <mo>-</mo> <msub> <mi>w</mi> <mi>dc</mi> </msub> <mrow> <mo>(</mo> <mn>0</mn> <mo>)</mo> </mrow> <mo>=</mo> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>t</mi> </msubsup> <mrow> <mo>(</mo> <msub> <mi>v</mi> <mrow> <mi>c</mi> <mn>1</mn> </mrow> </msub> <msub> <mi>i</mi> <mrow> <mi>c</mi> <mn>1</mn> </mrow> </msub> <mo>+</mo> <msub> <mi>v</mi> <mrow> <mi>c</mi> <mn>2</mn> </mrow> </msub> <msub> <mi>i</mi> <mrow> <mi>c</mi> <mn>2</mn> </mrow> </msub> <mo>)</mo> </mrow> <mi>dt</mi> </mrow></math>

<math><mrow> <mo>=</mo> <mo>-</mo> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>t</mi> </msubsup> <mrow> <mo>(</mo> <msub> <mi>p</mi> <mi>F</mi> </msub> <mo>+</mo> <msub> <mi>L</mi> <mi>F</mi> </msub> <munder> <mi>Σ</mi> <mrow> <mi>i</mi> <mo>=</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>,</mo> <mi>c</mi> <mo>,</mo> <mi>d</mi> </mrow> </munder> <msub> <mi>i</mi> <mi>Fi</mi> </msub> <msub> <mover> <mi>i</mi> <mo>·</mo> </mover> <mi>Fi</mi> </msub> <mi>dt</mi> <mo>)</mo> </mrow> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>5</mn> <mo>)</mo> </mrow> </mrow></math>

<math><mrow> <mo>=</mo> <mo>-</mo> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>t</mi> </msubsup> <mrow> <mo>(</mo> <msub> <mi>p</mi> <mi>F</mi> </msub> <mo>+</mo> <msub> <mi>p</mi> <msub> <mi>L</mi> <mi>F</mi> </msub> </msub> <mo>)</mo> </mrow> <mi>dt</mi> </mrow></math>

wherein,

p_Frepresenting an instantaneous active power value of the active power filter;

representing the instantaneous active power on the connecting inductance.

From the equation (5), the energy variation of the DC end is only related to the instantaneous power p of the filter_FAnd instantaneous active power on half-bridge chopped current and direct current end connection inductorAnd (4) correlating. However, in practical systems, due to IGBT conduction and switching losses, the power consumption loss p needs to be considered_loss. The variation of the dc terminal energy is determined by the following equation.

<math><mrow> <msub> <mi>Δw</mi> <mi>dc</mi> </msub> <mo>=</mo> <mo>-</mo> <msubsup> <mo>&Integral;</mo> <mn>0</mn> <mi>t</mi> </msubsup> <mrow> <mo>(</mo> <msub> <mi>p</mi> <mi>F</mi> </msub> <mo>+</mo> <msub> <mi>p</mi> <mi>loss</mi> </msub> <mo>+</mo> <msub> <mi>p</mi> <msub> <mi>L</mi> <mi>F</mi> </msub> </msub> <mo>)</mo> </mrow> <mi>dt</mi> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>6</mn> <mo>)</mo> </mrow> </mrow></math>

From equation (6), there is a linear relationship between the change of the dc side energy and the active power. Thus, from an energy perspective, the APF energy relationship is shown in FIG. 4, where p is_intThe active power generated for the half-bridge chopper circuit can be represented by the following formula,

p_{int} = p_{loss} + p_{L_{F}} . . . (7)

according to the conventional APF controller, the minimum oscillation frequency of the instantaneous active power consumed by the load is 2 ω_sWherein ω is_sIs the fundamental frequency of the power grid. The conventional functions of a low-pass filter, a high-pass filter and a notch filter are thus respectively

Wherein,

ω_h＝2ω_s，ξ_h＝1。

from the instantaneous power balance it can be derived:

<math><mrow> <msub> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>s</mi> </msub> <mo>=</mo> <msub> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>L</mi> </msub> <mo>+</mo> <msub> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>int</mi> </msub> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>11</mn> <mo>)</mo> </mrow> </mrow></math>

<math><mrow> <msub> <mi>p</mi> <mi>F</mi> </msub> <mo>=</mo> <msub> <mover> <mi>p</mi> <mo>~</mo> </mover> <mi>L</mi> </msub> <mo>-</mo> <msub> <mover> <mi>p</mi> <mo>~</mo> </mover> <mi>s</mi> </msub> <mo>-</mo> <msub> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>int</mi> </msub> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>12</mn> <mo>)</mo> </mrow> </mrow></math>

<math><mrow> <msub> <mi>q</mi> <mi>F</mi> </msub> <mo>=</mo> <msub> <mover> <mi>q</mi> <mo>&OverBar;</mo> </mover> <mi>L</mi> </msub> <mo>+</mo> <msub> <mover> <mi>q</mi> <mo>~</mo> </mover> <mi>L</mi> </msub> <mo>-</mo> <msub> <mover> <mi>q</mi> <mo>~</mo> </mover> <mi>s</mi> </msub> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>13</mn> <mo>)</mo> </mrow> </mrow></math>

wherein,

representing the average value of the active energy of the power grid;

representing the fluctuation value of the active energy of the power grid;

the average value of the active energy on the load side is represented;

representing the fluctuation value of the active energy on the load side;

representing the average value of active power generated by a half-bridge chopper circuit;

p_Frepresenting an instantaneous active power value of the active power filter;

q_Frepresenting an instantaneous reactive power value of the active power filter;

representing the load side reactive energy average value;

representing a load side reactive energy fluctuation value;

representing the reactive energy fluctuation value of the power grid;

wherein

And

can be obtained from the following formulae respectively,

<math><mrow> <msubsup> <mi>i</mi> <mi>S</mi> <mo>*</mo> </msubsup> <mo>·</mo> <msub> <mi>v</mi> <mi>L</mi> </msub> <mo>=</mo> <msubsup> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>S</mi> <mo>*</mo> </msubsup> <mo>+</mo> <msubsup> <mover> <mi>p</mi> <mo>~</mo> </mover> <mi>S</mi> <mo>*</mo> </msubsup> <mo>=</mo> <msubsup> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>L</mi> <mo>*</mo> </msubsup> <mo>+</mo> <msubsup> <mover> <mi>p</mi> <mo>~</mo> </mover> <mi>S</mi> <mo>*</mo> </msubsup> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>14</mn> <mo>)</mo> </mrow> </mrow></math>

<math><mrow> <msub> <mi>i</mi> <mi>L</mi> </msub> <mo>·</mo> <msub> <mi>v</mi> <mi>L</mi> </msub> <mo>=</mo> <msub> <mover> <mi>p</mi> <mo>&OverBar;</mo> </mover> <mi>L</mi> </msub> <mo>+</mo> <msub> <mover> <mi>p</mi> <mo>~</mo> </mover> <mi>L</mi> </msub> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>15</mn> <mo>)</mo> </mrow> </mrow></math>

wherein,

representing a reference current value of the grid;

v_Lrepresenting the voltage value of the nonlinear load end;

i_Lrepresenting the current value of the nonlinear load end;

a reference value representing an average value of the grid active energy;

a reference value representing a power grid active energy fluctuation value;

a reference value representing an average value of the functional quantities at the nonlinear load end;

representing a current reference value provided by the active power filter;

representing an instantaneous active power reference value of the active power filter;

the transfer function (17) can be obtained from the equations (4), (11) to (16).

<math><mrow> <mfrac> <mrow> <msubsup> <mover> <mi>P</mi> <mo>&OverBar;</mo> </mover> <mi>L</mi> <mo>*</mo> </msubsup> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> </mrow> <mrow> <msub> <mi>ΔW</mi> <mi>dc</mi> </msub> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> </mrow> </mfrac> <mo>=</mo> <mo>-</mo> <mfrac> <mrow> <mi>H</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <mi>C</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <mo>+</mo> <mi>D</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <mi>LPF</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> </mrow> <mrow> <mn>1</mn> <mo>-</mo> <mi>LPF</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> </mrow> </mfrac> <mo>=</mo> <mo>-</mo> <mo>[</mo> <mi>H</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <mi>C</mi> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <msub> <mi>F</mi> <mn>1</mn> </msub> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <mo>+</mo> <msub> <mi>F</mi> <mn>2</mn> </msub> <mrow> <mo>(</mo> <mi>s</mi> <mo>)</mo> </mrow> <mo>]</mo> <mo>.</mo> <mo>.</mo> <mo>.</mo> <mrow> <mo>(</mo> <mn>17</mn> <mo>)</mo> </mrow> </mrow></math>

Wherein F₁(s) and F₂(s) are each defined as

F_{1} (s) = \frac{1}{1 - LPF (s)},

F_{2} (s) = D (s) \frac{LPF (s)}{1 - LPF (s)} . . . (18)

Wherein,

lpf(s) represents the transfer function value of the low-pass filter;

c(s) represents the transfer function value of a proportional controller, which is proportional to k, which can be defined as k ═ ω_f；

H(s) represents the transfer function value of the notch filter;

d(s) represents the transfer function value of the differentiator.

Therefore, the functional quantity required by the nonlinear load is provided by the power grid in whole, and the reference current value required by the power grid can be obtainedCurrent value i required by actual load_LThe difference of (a) is the reference current provided by the parallel active power filterAs shown in fig. 5. Therefore, the filtering of the harmonic wave of the power grid, the stability of the direct-current end voltage and the compensation of three-phase unbalance can be simultaneously ensured.

Simulation experiment effect

To verify the effectiveness of the proposed method, experimental studies were performed for a specific application. A single-phase full-bridge uncontrollable rectifier with one phase connected with a neutral line is adopted as a load. Insulated Gate Bipolar Transistors (IGBT) are adopted as power switching devices in the full-bridge inverter and the half-bridge chopper circuit. The control algorithm is implemented in DSP of TMS320VC33 and FPGA of XC3S 500E. The sample time is 30 mus and the actual code execution time is 19.4 mus. The instruction cycle of the DSP is 13.3 ns. The clock frequency of the PWM in the FPGA is 5 kHz. L is_FThe reference voltage of the DC end is 300V at 10 mH. Fig. 6 is a schematic diagram of the grid phase voltages, phase currents, and neutral current when harmonic compensation is not performed with a filter according to the present invention. Fig. 7 is a schematic diagram of the network phase voltage, phase current, and neutral current compensated for by the filter according to the present invention. It can be seen that after compensation, the harmonics of the currents of the phases are reduced, the current distortion rate is significantly reduced, and the grid current is substantially kept in synchronism with the grid voltage, eliminating the neutral current.

Although the embodiments of the present invention have been described above, the description is only for the convenience of understanding the present invention, and is not intended to limit the present invention. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A parallel active power filter, comprising:

a control unit adopting double closed-loop control, an inner loop being a voltage loop submodule for maintaining a voltage on a direct current side of the parallel active power filter stable, and an outer loop being a current loop submodule for controlling an output current of the parallel active power filter, wherein,

2. Parallel type active power filter according to claim 1,

the three-phase full-bridge inverter circuit is composed of three groups of mutually parallel bridge arms, each bridge arm is composed of two power switch tubes connected in series, each power switch tube is composed of a power tube and an anti-parallel diode, and

the three-phase full-bridge inverter circuit is electrically connected with the three-phase four-wire power grid system through a three-phase inductor.

3. The parallel active power filter according to claim 1, further comprising a control unit employing a double closed loop control, wherein an inner loop is a voltage loop submodule for maintaining a voltage on a direct current side of the parallel active power filter stable, and an outer loop is a current loop submodule for controlling an output current of the parallel active power filter.

4. Parallel type active power filter according to claim 3,

the voltage ring sub-module is electrically connected with two ends of the double capacitor directly or indirectly.

5. Parallel type active power filter according to claim 4,

and the voltage loop submodule obtains a reference current serving as an instantaneous harmonic compensation reference value of the inverter according to the change of the energy of the direct current side of the parallel active power filter.

6. Parallel type active power filter according to claim 5,

and the current loop submodule controls the output current of a three-phase full-bridge inverter circuit in the parallel active power filter according to the reference current.