CN105515003A - Active power filter for detecting harmonic wave and wattless current and detecting method thereof - Google Patents

Abstract

The invention discloses an active power filter for detecting a harmonic wave and a wattless current and a detecting method thereof. The active power filter comprises a harmonic wave and wattless current detecting circuit and a compensating current generating circuit, wherein the harmonic wave and wattless current detecting circuit and the compensating current generating circuit are successively connected in series at the current side of a power grid. The compensating current generating circuit comprises a current tracking control module, a driving circuit module and a main circuit module, wherein the current tracking control module, the driving circuit module and the main circuit module are successively connected in series. The topological structure of the main circuit module is a three-phase four-leg PWM converter. Compared with prior art, a novel harmonic wave detecting algorithm according to the invention does not require a phase-locked loop or matrix transform, thereby effectively prevent a zero-sequence harmonic leakage error caused by coordinate conversion, greatly improving detecting precision and greatly improving compensation effect.

Description

A kind of Active Power Filter-APF of harmonics and reactive current detection and detection method

Technical field

The present invention relates to field of power, exactly refer to a kind of harmonic current detecting method avoiding producing because of coordinate transform Zero-pharse harmonic lobe error be applied in active power filter for three-phase four-wire system.

Background technology

Over nearly 30 years, along with power electronic equipment extensive use in the industry, the harmonic pollution problems of electrical network is on the rise, and has a strong impact on the quality of power supply.Harmonic wave not only affects the normal work of electric equipment, and the safety and economic operation of returning electrical network brings hidden danger.

Since Active Power Filter-APF principle is suggested, the detection of harmonic current just causes to study widely and has developed multiple method so far, wave superposition method generally has: analog filter detection method, based on the detection method of FFT, based on the detection method of instantaneous reactive power theory, based on the adaptive detective method etc. of neural net, instantaneous space vector method wherein based on instantaneous reactive power theory is a kind of detection method most widely used in current Active Power Filter-APF, proposed in 1984 by Japanese scholars Akagi.H the earliest, through updating, now comprise p-q method and i _p-i _qmethod.Wherein, p-q method is applicable to the symmetrical and detection that is harmonic current in undistorted situation of line voltage; i _p-i _qmethod is when line voltage distorts and effective when line voltage is asymmetric.But all zero-sequence current be have ignored for more than three-phase imbalance load 2 kind of a method, thus produce Zero-pharse harmonic lobe error.

Summary of the invention

The present invention is directed to traditional p-q method and i _p-i _qthe problem of the zero sequence lobe error that method produces owing to carrying out coordinate transform, propose a kind of Harmonic currents detection new departure being applied to the improvement of active power filter for three-phase four-wire system, the method does not need phase-locked loop, do not need to carry out matrixing, effectively avoid Zero-pharse harmonic lobe error.

Technical scheme of the present invention is:

An Active Power Filter-APF for harmonics and reactive current detection, is included in the wave superposition circuit and offset current circuit for generating of connecting successively power network current side; Described offset current circuit for generating comprises current follow-up control module, drive circuit module and the main circuit module of connecting successively, and the topological structure of described main circuit module is the pwm converter of a three-phase four-arm; Described wave superposition circuit comprises Hall current sensor, Hall voltage transducer, signal conditioning circuit, AD7656 module, dsp controller; Described Hall current sensor, Hall voltage transducer are connected with dsp controller respectively by signal conditioning circuit, AD7656 module; Dsp controller is used for performing the harmonics and reactive current detection algorithm improved, and according to detection signal, produces drive singal, the signal controlling high-power IGBT finally exported.

Further, described main circuit module is a four bridge legs structure, is made up of 8 IGBT switching tubes, and every phase brachium pontis has upper and lower two IGBT switching tubes, and DC side is made up of a DC capacitor.

Further, between power network current and described main circuit module, be also provided with the high pass filter of RCL composition.

The technical scheme of method of the present invention is:

A detection method for the Active Power Filter-APF of harmonics and reactive current detection, comprises the following steps:

Step 1, first obtains the voltage e of electrical network respectively by Hall voltage transducer, Hall current sensor _a, e _b, e _cwith load current i _a, i _b, i _cvalue;

Step 2, according to surveyed e _a, e _b, e _cand i _a, i _b, i _cask for active power p=e _ai _a+ e _bi _b+ e _ci _c;

The active-power P that step 3 is tried to achieve according to step 2, the harmonic component in low-pass filtering elimination P, only remaining steady-state value wherein I ₁₊cos φ is exactly the amplitude of the fundamental positive sequence active component of current;

Step 4, according to the voltage of the electrical network that step 1 is measured, obtains expression formula e _a ²+ e _b ²+ e _c ²numerical value;

Step 5, the steady-state value of the active power that the line voltage recorded according to step 1 and step 3 are tried to achieve and the e that step 4 is obtained _a ²+ e _b ²+ e _c ²numerical value, and then obtain the real component i of fundamental positive sequence electric current _a1p, i _b1p, i _c1p;

Step 6, according to the load current i that step 1 is surveyed _a, i _b, i _c, therefrom deduct the real component i of the fundamental positive sequence electric current of step 5 _a1p, i _b1p, i _c1p, all deleterious currents comprising fundamental reactive current, zero-sequence current, negative-sequence current and harmonic current can be obtained, and using after its reversed polarity as offset current instruction i _ca ^*, i _cb ^*, i _cc ^*;

Step 7, selects suitable control algolithm to control according to the offset current instruction that step 6 is obtained, and generation control signal driving three-phase four-arm circuit generation PWM ripple carrys out the harmonic wave in compensation network; Thus reach the object making mains by harmonics meet power quality index.

The present invention has following technique effect:

1) describedly being thisly made up of two large divisions of the active power filter for three-phase four-wire system of real time sampling idle is involved based on the humorous of innovatory algorithm, i.e. wave superposition circuit and offset current circuit for generating, wherein offset current circuit for generating is divided into again current follow-up control module, drive circuit module and main circuit module, and wherein the topological structure of main circuit is the pwm converter of a three-phase four-arm.In addition, the high pass filter of RCL composition is mainly used to the switching harmonics electric current near filtering switching noise.A kind of algorithm of novel harmonic detecting is proposed hered in wave superposition circuit, the method does not need phase-locked loop, do not need to carry out matrixing, effectively avoid and produce Zero-pharse harmonic lobe error because of coordinate transform, make surveyed fundamental active phase current more accurate, better effects if.

2) topological structure of the main circuit of offset current circuit for generating is the pwm converter of a three-phase four-arm, as a kind of three-phase and four-line converter topology form, three-phase four-arm converter provides zero-sequence current path by additional four bridge legs, can tackle the needs of unbalanced load or unbalanced power grid.With traditional three-phase and four-line converter, the three-phase and four-line converter of especially Industrial Frequency Transformer isolation is compared, and three-phase four-arm converter has the outstanding advantages such as structure is simple, volume is little, cost is low.

3) great majority are all three-phase unbalance systems in actual applications, and traditional p-q, ip-iq, d-q detection method have ignored zero-sequence current, thus produce Zero-pharse harmonic lobe error.Herein introduce the humorous of a kind of improvement based on the general principle of instantaneous reactive power theory and involve real time sampling idle method, propose a kind of algorithm of novel harmonic detecting, the method does not need phase-locked loop, do not need to carry out matrixing, effectively avoid and produce Zero-pharse harmonic lobe error because of coordinate transform.

4) owing to adding a brachium pontis, and the load exported may be asymmetric, no longer meet Xa+Xb+Xc=0, be not inconsistent with conventional two-dimensional space vector condition, space vector is not in a plane, need be that three-dimensional space vectors just can be dealt with problems by two-dimensional space vector extension, the track of reference voltage vector is increased to three dimensions by two-dimensional space, and three-dimensional space vectors PWM controls, this control method easily realizes converter and controls in real time, linear modulationra is high, this control thought is also convenient to Digital Realization simultaneously, and by harmonic content can be reduced the conservative control of zero vector or reduce switching loss.

5) based on improving on the basis of detection algorithm, adopt the three-phase four-leg inverter of three-dimensional space vectors control and compensation current occuring circuit, the method not only detects more accurate, and compensation effect is good, but also is beneficial to Digital Realization.

Adopt MATLAB7.0 in Simulink environment, built the analogue system of active power filter for three-phase four-wire system, and this detection method is applied to the model of whole active power filter system, can finds out that the detection method of this improvement can achieve good effect by simulation result.

Accompanying drawing explanation

The structured flowchart of the active power filter for three-phase four-wire system that Fig. 1 the present invention proposes;

Fig. 2 is traditional i _p-i _qthe theory diagram of detection algorithm;

The wave superposition circuit diagram that Fig. 3 the present invention proposes;

The pwm converter topological structure of the three-phase four-arm that Fig. 4 the present invention proposes;

The Cleaning Principle figure of the innovatory algorithm that Fig. 5 the present invention proposes;

Electrical network phase voltage under the innovatory algorithm that Fig. 6 the present invention proposes and first-harmonic phase current figure;

A, b, c phase harmonic current figure of the detection under the innovatory algorithm that Fig. 7 the present invention proposes;

Comparison diagram before and after offset current under the innovatory algorithm that Fig. 8 the present invention proposes; Wherein (a) is the offset current before improving; B () is the offset current after improvement.

Embodiment

Below in conjunction with accompanying drawing, technical scheme of the present invention is described in detail:

Traditional i _p-i _qdetection side's ratio juris as shown in Figure 2.In figure $C=\left[\begin{array}{cc}sin\mathrm{\ω}t& -cos\mathrm{\ω}t\\ -cos\mathrm{\ω}t& -sin\mathrm{\ω}t\end{array}\right].$ In the method, need to use and a phase line voltage e _asynchronous sinusoidal signal sin ω t and corresponding cosine signal cos ω t, they by a phase-locked loop (PLL) and one just, cosine signal circuit for generating obtains.I can be calculated according to definition _p, i _q, draw i through LPF filtering _p, i _qdC component by i _a1, i _b1, i _c1produce, therefore by i can be calculated _a1, i _b1, i _c1, and then calculate i _ah, i _bh, i _ch.

But in three-phase four-wire system, three-phase imbalance will cause three-phase current sum non-vanishing.By symmetrical component method, threephase load electric current can be analyzed to positive sequence, negative phase-sequence and zero sequence three kinds of symmetrical component sums, namely

i＝i ₊+i _-+i ₀(1)

Positive sequence component three-phase amplitude is equal, and phase place presses A, B, C delayed 120 ° successively; Negative sequence component three-phase amplitude is equal, and phase place presses A, B, C advanced 120 ° successively; Zero-sequence component is all identical in amplitude with phase place.

i _a0＝i _b0＝i _c0(2)

Three-phase/two phase inversion can be expressed as

$\left[\begin{array}{c}{i}_{\mathrm{\α}}\\ i_{\mathrm{\β}}\end{array}\right]=C_{32}\left[\begin{array}{c}{i}_a\\ i_b\\ i_c\end{array}\right]=C_{32}\left[\begin{array}{c}{i}_{a+}\\ i_{b+}\\ i_{c+}\end{array}\right]+C_{32}\left[\begin{array}{c}{i}_{a-}\\ i_{b-}\\ i_{c-}\end{array}\right]+C_{32}\left[\begin{array}{c}{i}_{a0}\\ i_{b0}\\ i_{c0}\end{array}\right]$

Wherein

$C_{32}\left[\begin{array}{c}{i}_{a0}\\ i_{b0}\\ i_{c0}\end{array}\right]=\sqrt{2/3}\left[\begin{array}{ccc}1& -1/2& -1/2\\ 0& \sqrt{3}/2& -\sqrt{3}/2\end{array}\right]\left[\begin{array}{c}{i}_{a0}\\ i_{b0}\\ i_{c0}\end{array}\right]=\left[\begin{array}{c}0\\ 0\end{array}\right]---\left(3\right)$

Visible, when adopting instantaneous reactive power detection method, zero-sequence component is not reflected in orthogonal alpha-beta coordinate, thereby produces Zero-pharse harmonic lobe error.

China's low voltage electric network adopts three-phase four-wire system, and because the imbalance of single-phase load is distributed, zero-sequence current certainly exists, and low voltage electric network low-order harmonic content is large in addition, will produce zero sequence lobe error with the harmonic current in the method detection of grid.This just needs to do necessary improvement to detection algorithm.

As shown in figures 1 and 3, the active power filter for three-phase four-wire system of a kind of harmonics and reactive current detection of the present invention, is included in the wave superposition circuit and offset current circuit for generating of connecting successively power network current side; Described offset current circuit for generating comprises current follow-up control module, drive circuit module and the main circuit module of connecting successively, and the topological structure of described main circuit module is the pwm converter of a three-phase four-arm; Described wave superposition circuit comprises Hall current sensor, Hall voltage transducer, signal conditioning circuit, AD7656 module, dsp controller; Described Hall current sensor, Hall voltage transducer are connected with dsp controller respectively by signal conditioning circuit, AD7656 module; Dsp controller is used for performing the harmonics and reactive current detection algorithm improved, and according to detection signal, produces drive singal, the signal controlling high-power IGBT finally exported.Described main circuit module is a four bridge legs structure, is made up of 8 IGBT switching tubes, and every phase brachium pontis has upper and lower two IGBT switching tubes, and DC side is made up of a DC capacitor.The high pass filter of RCL composition is also provided with between power network current and described main circuit module.

To the detection method of the active power filter for three-phase four-wire system of the harmonics and reactive current detection of following improvement be completed in above-mentioned dsp controller, comprises the following steps:

Step 1, first obtains the voltage e of electrical network respectively by Hall voltage transducer, Hall current sensor _a, e _b, e _cwith load current i _a, i _b, i _cvalue;

Step 2, according to surveyed e _a, e _b, e _cand i _a, i _b, i _cask for active power p=e _ai _a+ e _bi _b+ e _ci _c;

The active-power P that step 3 is tried to achieve according to step 2, the harmonic component in low-pass filtering elimination P, only remaining steady-state value wherein I ₁₊cos φ is exactly the amplitude of the fundamental positive sequence active component of current;

Step 4, according to the voltage of the electrical network that step 1 is measured, obtains expression formula e _a ²+ e _b ²+ e _c ²numerical value;

Step 5, the steady-state value of the active power that the line voltage recorded according to step 1 and step 3 are tried to achieve and the e that step 4 is obtained _a ²+ e _b ²+ e _c ²numerical value, and then obtain the real component i of fundamental positive sequence electric current _a1p, i _b1p, i _c1p;

Step 6, according to the load current i that step 1 is surveyed _a, i _b, i _c, therefrom deduct the real component i of the fundamental positive sequence electric current of step 5 _a1p, i _b1p, i _c1p, all deleterious currents comprising fundamental reactive current, zero-sequence current, negative-sequence current and harmonic current can be obtained, and using after its reversed polarity as offset current instruction i _ca ^*, i _cb ^*, i _cc ^*;

Step 7, selects suitable control algolithm to control according to the offset current instruction that step 6 is obtained, and generation control signal driving three-phase four-arm circuit generation PWM ripple carrys out the harmonic wave in compensation network; Thus reach the object making mains by harmonics meet power quality index.

Fig. 4 is the main topological structure of active power filter for three-phase four-wire system offset current circuit for generating, and it is a four bridge legs structure, is made up of 8 IGBT switching tubes, and every phase brachium pontis has upper and lower two IGBT switching tubes, and DC side is made up of a DC capacitor.Compared with three-phase three brachium pontis electric capacity midsplit type APF device, although the electronic power switch device count that three-phase four-arm uses is more more, but in order to ensure that the mid point point position between DC side two electric capacity is zero in three-phase three brachium pontis electric capacity midsplit type topological structure, CONTROLLER DESIGN is needed to control two capacitance voltages, make both voltages equal, do not allow to occur DC side two unbalanced situations of capacitance voltage, cause control more complicated thus, the unreliable factor of system is increased.By above comparative analysis, several from the viewpoint of APF center line offset current fan-out capability, three-phase brachium pontis current output capability and reliability, the first-selected main circuit structure of three-phase four-wire system APF is four bridge legs topology.

The present invention improves the schematic diagram of detection algorithm as shown in Figure 5, and its cardinal principle is as follows:

General supposition line voltage e _a, e _b, e _cthree-phase symmetrical, undistorted, then:

e _a＝E _msinωt

e _b＝E _msin(ωt-120°)(4)

e _c＝E _msin(ωt+120°)

Load current i _a, i _b, i _cfirst-harmonic and harmonic wave sum can be expressed as:

$i_a=i_{a1}+\underset{i=1}{\overset{k}{\Σ}}{i}_{ak}$

$i_b=i_{b1}+\underset{i=1}{\overset{k}{\Σ}}{i}_{bk}---\left(5\right)$

$i_c=i_{c1}+\underset{i=1}{\overset{k}{\Σ}}{i}_{ck}$

Consider three-phase imbalance, electric current is divided into positive sequence, negative phase-sequence, zero sequence, then fundamental current is expressed as follows:

Wherein I ₁₊, I _1-, I ₁₀the amplitude of fundamental positive sequence, negative phase-sequence, zero-sequence component respectively, be power factor angle, θ 1-is the initial phase of first-harmonic negative phase-sequence.Harmonic current is also divided into positive sequence, negative phase-sequence, zero sequence, and k subharmonic current can be expressed as:

i _ak＝I _k+sin(kωt+θ _k+)+I _k-sin(kωt+θ _k-)+I _k0

i _bk＝I _k+sin(kωt+θ _k+-120°)+I _k-sin(kωt+θ _k-+120°)+I _k0(7)

i _ck＝I _k+sin(kωt+θ _k++120°)+I _k-sin(kωt+θ _k--120°)+I _k0

Wherein I _k+, I _k-, I _k0the amplitude of k fundamental positive sequence, negative phase-sequence, zero-sequence component respectively, θ _k+, θ _k-it is the initial phase of harmonic wave positive sequence, negative phase-sequence.Three-phase instantaneous power p can be obtained by following formula:

$\begin{array}{c}p=e_a{i}_a+e_b{i}_b+e_c{i}_c=e_a(i_{a1}+\underset{k}{\Σ}{i}_{ak})+e_b(i_{b1}+\underset{k}{\Σ}{i}_{bk})+\\ e_c+(i_{c1}+\underset{k}{\Σ}{i}_{ck})=(e_a{i}_{a1}+e_b{i}_{b1}+e_c{i}_{c1})+\underset{k}{\Σ}(e_a{i}_{ak}+e_b{i}_{bk}+e_c{i}_{ck})\\ =p_1+\underset{k}{\Σ}{p}_k\end{array}---\left(8\right)$

Formula (4), (6), (7) are substituted into p ₁, p _kabbreviation can obtain:

$\begin{array}{c}{p}_k=e_a{i}_{ak}+e_b{i}_{bk}+e_c{i}_{ck}\\ =\frac{3}{2}{E}_m{I}_{k+}\cos \left(\right(k-1)\mathrm{\ω}t+{\mathrm{\θ}}_{k+})-\frac{3}{2}{E}_m{I}_{k-}\cos \left(\right(k+1)\mathrm{\ω}t+{\mathrm{\θ}}_{k-})\end{array}---\left(10\right)$

Formula (9), formula (10) are substituted into formula (8) and obtain:

Above formula comprises direct current and series of harmonic component.Harmonic frequency is minimum reaches 100Hz, and through low-pass filtering (LFP), the harmonic component in p can elimination, only remaining steady-state value wherein it is exactly the amplitude of the fundamental positive sequence active component of current.For A phase, the fundamental positive sequence active component of current can be obtained by following formula:

In like manner can obtain the real component of other two-phase fundamental positive sequence electric currents:

From actual loading current i _a, i _b, i _cin deduct the real component i of fundamental positive sequence electric current _a1p, i _b1p, i _c1p, all deleterious currents comprising fundamental reactive current, zero-sequence current, negative-sequence current and harmonic current can be obtained, and using after its reversed polarity as offset current instruction i _ca ^*, i _cb ^*, i _cc ^*:

i _ahq＝i _a-i _a1p＝-i _ca ^*

i _bhq＝i _b-i _b1p＝-i _cb ^*(15)

i _chq＝i _c-i _c1p＝-i _cc ^*

The schematic diagram of this innovatory algorithm as shown in Figure 5.

Fig. 6 is the oscillogram applying electrical network phase voltage that MATLAB environment records and fundamental active phase current under this paper innovatory algorithm, abscissa is time parameter, ordinate represents amplitude, As time goes on the fundamental positive sequence detected as seen from the figure gains merit phase current all the time with the complete same-phase of electrical network phase voltage, for standard sine, and three-phase symmetrical, amplitude is equal, and this illustrates that the harmonic and reactive currents detected is entirely accurate.

Fig. 7 is a, b, c phase harmonic current figure of the detection under the innovatory algorithm that proposes of the present invention, and abscissa represents the time, and ordinate represents a, the amplitude of b, c three-phase current.

Fig. 8 is comparison diagram before and after the offset current under the innovatory algorithm that proposes of the present invention, and abscissa represents the time, and ordinate represents the amplitude of electric current.The electric current under the unbalanced load of compensation three-phase can be found out in the drawings, non-linear due to laod unbalance, so measured current waveform is saddle ripple, and three-phase amplitude is not etc., has height to have low, and when system drop into there is the Active Power Filter-APF of innovatory algorithm time, three-phase current is standard sine wave, and three-phase symmetrical, amplitude is equal, and known electric current obtains good compensation.

Active Power Filter-APF is primarily of wave superposition circuit, current follow-up control module; the protection of drive circuit, main circuit and necessity and auxiliary circuit composition; wherein current follow-up control part, drive circuit and main circuit form offset current circuit for generating.The effect of Harmonic currents detection part draws according to the compensation object of Active Power Filter-APF the current command signal needing to compensate, and its core real-time detects harmonic and reactive currents component in target compensation or active current accurately.Current follow-up control part is first link in offset current circuit for generating, its effect is according to the correlation between instruction current signal and actual offset current, draw the pwm signal controlling each device break-make of main circuit, (tracking control algorithm of current follow-up control module generally adopts pi to control can to adopt conventional current follow-up control module, Hysteresis control, three-dimensional space vector modulation algorithm control etc.), inside drive circuit, driving chip generally adopts chip m579621, his effect is that the positive and negative 5v signal of DSP is converted to+15 and-10, drive the signal of IGBT.The result controlled is the change ensureing active electric power filter to output current trace command signal.The effect of main circuit directly exports actual offset current.

In the description of this specification, specific features, structure, material or feature that the description of reference term " embodiment ", " some embodiments ", " illustrative examples ", " example ", " concrete example " or " some examples " etc. means to describe in conjunction with this embodiment or example are contained at least one embodiment of the present invention or example.In this manual, identical embodiment or example are not necessarily referred to the schematic representation of above-mentioned term.And the specific features of description, structure, material or feature can combine in an appropriate manner in any one or more embodiment or example.

Although illustrate and describe embodiments of the invention, those having ordinary skill in the art will appreciate that: can carry out multiple change, amendment, replacement and modification to these embodiments when not departing from principle of the present invention and aim, scope of the present invention is by claim and equivalents thereof.

Claims (4)

1. an Active Power Filter-APF for harmonics and reactive current detection, is characterized in that, is included in the wave superposition circuit and offset current circuit for generating of connecting successively power network current side;

Described offset current circuit for generating comprises current follow-up control module, drive circuit module and the main circuit module of connecting successively, and the topological structure of described main circuit module is the pwm converter of a three-phase four-arm;

Described wave superposition circuit comprises Hall current sensor, Hall voltage transducer, signal conditioning circuit, AD7656 module, dsp controller; Described Hall current sensor, Hall voltage transducer are connected with dsp controller respectively by signal conditioning circuit, AD7656 module; Dsp controller is used for performing the harmonics and reactive current detection algorithm improved, and according to detection signal, produces drive singal, the signal controlling high-power IGBT finally exported.

2. the Active Power Filter-APF of a kind of harmonics and reactive current detection as claimed in claim 1, it is characterized in that: described main circuit module is a four bridge legs structure, be made up of 8 IGBT switching tubes, every phase brachium pontis has upper and lower two IGBT switching tubes, and DC side is made up of a DC capacitor.

3. the Active Power Filter-APF of a kind of harmonics and reactive current detection as claimed in claim 1, is characterized in that: the high pass filter being also provided with RCL composition between power network current and described main circuit module.

4. a detection method for the Active Power Filter-APF of harmonics and reactive current detection, is characterized in that, comprises the following steps:

Step 1, first obtains the voltage e of electrical network respectively by Hall voltage transducer, Hall current sensor _a, e _b, e _cwith load current i _a, i _b, i _cvalue;

Step 2, according to surveyed e _a, e _b, e _cand i _a, i _b, i _cask for active power p=e _ai _a+ e _bi _b+ e _ci _c;

The active-power P that step 3 is tried to achieve according to step 2, the harmonic component in low-pass filtering elimination P, only remaining steady-state value wherein I ₁₊cos φ is exactly the amplitude of the fundamental positive sequence active component of current;

Step 4, according to the voltage of the electrical network that step 1 is measured, obtains expression formula e _a ²+ e _b ²+ e _c ²numerical value;

Step 5, the steady-state value of the active power that the line voltage recorded according to step 1 and step 3 are tried to achieve and the e that step 4 is obtained _a ²+ e _b ²+ e _c ²numerical value, and then obtain the real component i of fundamental positive sequence electric current _alp, i _blp, i _clp;

Step 6, according to the load current i that step 1 is surveyed _a, i _b, i _c, therefrom deduct the real component i of the fundamental positive sequence electric current of step 5 _alp, i _blp, i _clp, all deleterious currents comprising fundamental reactive current, zero-sequence current, negative-sequence current and harmonic current can be obtained, and using after its reversed polarity as offset current instruction i _ca ^*, i _cb ^*, i _cc ^*;

Step 7, selects suitable control algolithm to control according to the offset current instruction that step 6 is obtained, and generation control signal driving three-phase four-arm circuit generation PWM ripple carrys out the harmonic wave in compensation network; Thus reach the object making mains by harmonics meet power quality index.