Harmonic current compensation method based on improved filter and active power filter

Abstract

The present application belongs to the technical field of power quality compensation of distribution network, and particularly relates to a harmonic current compensation method under three-phase unbalanced conditions based on an improved filter and an active power filter. In view of the low accuracy of the existing fundamental positive sequence component detection method under three-phase unbalanced conditions, the present application adopts the following technical scheme: the harmonic current compensation method under three-phase unbalanced conditions based on an improved filter, comprising: collecting three-phase load actual current, performing Clark transformation to obtain alpha and beta components; using an improved filter to obtain two groups of orthogonal signals of the alpha and beta components of the three-phase load actual current to obtain the estimated values of the fundamental positive sequence current alpha and beta components; performing difference between the estimated values and actual values to obtain the alpha and beta components of the current to be compensated; and converting the alpha and beta components of the current to be compensated into three-phase current values to be compensated through inverse Clark transformation. The present application has the beneficial effect of improving the detection accuracy of the fundamental positive sequence current, thereby improving the harmonic compensation effect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power quality compensation of power distribution network, and particularly relates to a harmonic current compensation method under three-phase imbalance condition based on an improved filter and an active power filter. BACKGROUND

[0002] With the input of power electronic devices and the grid connection of distributed energy in the power distribution network, the harmonic pollution problem of the power distribution network cannot be ignored. In addition, affected by the asymmetric access of load and the asymmetric short-circuit fault of the power distribution network, the three-phase imbalance problem of the power distribution network has become one of the most common power quality problems. When the three-phase voltage of the network side is unbalanced, the operating state of the rectifier load type nonlinear load will change greatly, and negative sequence and low-order harmonic currents will be injected into the power grid. Influenced by this, the frequency sequence of the harmonic current of the nonlinear load of the power distribution network changes, causing the harmonic detection error of the active power filter to increase and reducing the harmonic compensation performance of the active power filter.

[0003] The harmonic detection of active power can be divided into three categories: current detection method of difference from fundamental wave, single harmonic current extraction, and current detection method based on sequence extraction. The latter two methods need multiple filter units, although they can realize the monitoring of multiple harmonic quantities, but the calculation amount is large, and they are only suitable for specific harmonic compensation. In the current detection method of difference from fundamental wave, the accuracy mainly depends on the detection accuracy of the fundamental positive sequence component. Among them, the ip-iq current detection method is the most widely used because of its simple structure and good harmonic detection effect. However, when the three-phase is unbalanced, the ip-iq current detection method is affected by the phase error of the phase-locked loop, and the detection accuracy decreases.

[0004] In view of this, some scholars have proposed a double second-order generalized integrator (DSOGI) based on a second-order generalized integrator (SOGI) to realize the extraction of the fundamental positive sequence component. However, under the condition of three-phase imbalance, the nonlinear load will generate negative sequence and 3rd harmonic currents, reducing the detection accuracy. It can be seen that how to ensure the detection accuracy of the fundamental positive sequence component of the active power filter under the condition of three-phase imbalance is the key to improving the quality of harmonic compensation.

[0005] Therefore, a positive sequence fundamental current detection method based on an improved filter is established in this paper, and on this basis, a harmonic current compensation method under three-phase imbalance condition and an active power filter model are proposed. SUMMARY

[0006] The present application aims at the low precision of the existing active power filter fundamental positive sequence component detection method under the condition of three-phase imbalance and the poor harmonic current compensation effect, and provides a harmonic current compensation method under the condition of three-phase imbalance based on an improved filter, which improves the detection precision of the fundamental positive sequence component under the condition of three-phase imbalance, thereby improving the harmonic compensation quality.

[0007] To achieve the above object, the present application adopts the following technical solution: the harmonic current compensation method under the condition of three-phase imbalance based on an improved filter, comprising:

[0008] collecting three-phase load actual currents of a nonlinear load;

[0009] performing Clark transformation on the collected three-phase load actual currents to obtain α and β components of the three-phase load actual currents;

[0010] obtaining two groups of orthogonal signals of the α and β components of the three-phase load actual currents by using a TOSSI-SOGI filter, wherein the TOSSI-SOGI filter comprises a SOGI-3 filter and a TOSSI filter, and the outputs of the SOGI-3 filter and the TOSSI filter are used as the negative feedback signals of each other, the SOGI-3 filter is a second-order generalized integrator with an angular frequency of 3 times the harmonic angular frequency, and the TOSSI filter is a third-order sine integrator;

[0011] obtaining the α and β component estimates of the fundamental positive sequence current according to the two groups of orthogonal signals of the α and β components of the three-phase load actual currents;

[0012] obtaining the α and β components of the negative sequence and harmonic currents to be compensated by subtracting the α and β component estimates of the fundamental positive sequence current from the α and β components of the actual current;

[0013] converting the α and β components of the negative sequence and harmonic currents to be compensated into three-phase current values to be compensated by performing inverse Clark transformation. The harmonic current compensation method under the condition of three-phase imbalance based on an improved filter has the advantages that the two groups of orthogonal signals of the α and β components of the three-phase load actual currents are obtained by using the TOSSI-SOGI filter, the α and β components of the negative sequence and harmonic currents to be compensated are obtained by subtracting the α and β component estimates of the fundamental positive sequence current from the α and β components of the actual current, the harmonic detection error caused by the 3 times harmonic and negative sequence current is reduced, the detection precision of the positive sequence fundamental current of the nonlinear load is improved, and the harmonic compensation effect of the active power filter is improved.

[0014] As an improvement, the three-phase load actual currents of the nonlinear load are collected by using a current sensor;

[0015] The specific expression of the collected three-phase load actual currents is:

[0016]

[0017]

[0018]

[0019] Among them, i a i b and i c These are the three-phase currents (a, b, and c) of the load, respectively. and These represent the positive-sequence and negative-sequence amplitudes of the h-th harmonic current, respectively. and ω and t represent the positive-sequence phase and negative-sequence phase of the h-th harmonic current, respectively, where ω is the fundamental angular frequency and t is time.

[0020] As an improvement, the specific expressions for the α and β components of the actual three-phase load current obtained through the Clark transformation are as follows:

[0021]

[0022]

[0023] Among them, i α i β These are the α and β components of the actual current of the three-phase load.

[0024] As an improvement, two parallel TOSSI-SOGI filters are used to obtain two sets of orthogonal signals of the α and β components of the actual three-phase load current.

[0025] As an improvement, the transfer function of the SOGI-3 filter is:

[0026]

[0027]

[0028] Where v(s) is the SOGI-3 filter input signal, and s is the integral operator, which is the SOGI-3 filter input signal i. α or i β The complex frequency domain representation, and The two orthogonal signals output from the SOGI-3 filter, ω r The third harmonic angular frequency, ω r = 2 × π × 150 rad / s, where k is a constant, take

[0029] The transfer function of the TOSSI filter is:

[0030]

[0031]

[0032] where v m (s) is the input signal of the TOSSI filter, and are two quadrature signals of the TOSSI filter output, ω is the fundamental angular frequency, and k1 and k2 are constants, which are respectively 2.33 and 2.8.

[0033] As an improvement, a set of quadrature signals output by the TOSSI-SOGI filter is:

[0034]

[0035]

[0036] Two sets of quadrature components of the current α and β components obtained by using two TOSSI-SOGI filters are respectively: and

[0037]

[0038] As an improvement, the obtained i α and i β are the estimated values of the fundamental positive sequence components.

[0039]

[0040]

[0041] wherein, and are the estimated values of the fundamental positive sequence current α and β components.

[0042] As an improvement, the α and β components of the negative sequence and harmonic currents to be compensated are obtained by subtracting the estimated values of the fundamental positive sequence current α and β components from the actual current α and β components i α and i β , and the specific expressions are:

[0043]

[0044]

[0045] wherein, i fα and i fβ are the α and β components of the negative sequence and harmonic currents to be compensated.

[0046] As an improvement, DC voltage stabilization is achieved through equivalent active current injection. The active current injection method involves generating a certain reference current value from the DC voltage via PI regulation. The specific expression is as follows:

[0047]

[0048]

[0049]

[0050] in, and These are the α and β components of the injected equivalent active current, respectively. and These are the estimated values ​​of the α and β components of the fundamental positive sequence of the grid-side voltage, respectively; ρ is the equivalent active current adjustment factor; and k... p and k i These are the proportional and integral coefficients of the voltage loop, respectively. and u dc These are the reference and actual values ​​of the DC voltage, respectively.

[0051] The value of the current to be compensated after injecting active current and for:

[0052]

[0053]

[0054] The current value to be compensated is obtained through inverse Clark transform. and Convert to the three-phase current value to be compensated The specific expression is:

[0055]

[0056] As an improvement, it also includes: generating the PWM input reference current value, i.e. the three-phase current value to be compensated, through hysteresis control, and generating the switching signals S1-S6 of the active power filter through PWM modulation, thereby realizing nonlinear load harmonic current compensation.

[0057] Active power filters based on improved filters include:

[0058] The current acquisition module is used to acquire the actual current of a three-phase load with a nonlinear load.

[0059] The Clark transform module is used to perform Clark transform on the actual three-phase load current to obtain the α and β components of the actual three-phase load current;

[0060] The TOSSI-SOGI filter module is used to obtain two sets of orthogonal signals of alpha and beta components of actual three-phase load currents, and the TOSSI-SOGI filter comprises a SOGI-3 filter and a TOSSI filter, and the outputs of the SOGI-3 filter and the TOSSI filter are used as negative feedback signals of each other, the SOGI-3 filter is a second-order generalized integrator with an angular frequency of 3 times the harmonic angular frequency, and the TOSSI filter is a third-order sine integrator;

[0061] The estimation value obtaining module is used to obtain the fundamental positive sequence current alpha and beta component estimation values according to the two sets of orthogonal signals of alpha and beta components of actual three-phase load currents.

[0062] The difference making module is used to make a difference between the fundamental positive sequence current alpha and beta component estimation values and the actual current alpha and beta components to obtain the alpha and beta components of the negative sequence and harmonic currents to be compensated.

[0063] The inverse Clark transformation module is used to convert the alpha and beta components of the negative sequence and harmonic currents to be compensated into the three-phase current values to be compensated through inverse Clark transformation.

[0064] The hysteresis control module is used to generate the PWM input reference current values, i.e., the three-phase current values to be compensated, through hysteresis control.

[0065] The switching signal module is used to generate the switching signals of the active power filter through PWM modulation. BRIEF DESCRIPTION OF DRAWINGS

[0066] Figure 1 FIG. 1 is a structural schematic diagram of an active power filter using the harmonic current compensation method of the embodiment of the present application.

[0067] Figure 2 FIG. 2 is a structural diagram of a TOSSI-SOGI filter used in the harmonic current compensation method of the embodiment of the present application.

[0068] Figure 3 FIG. 3 is a principle diagram of the harmonic current compensation method of the embodiment of the present application.

[0069] Figure 4 FIG. 4 is a comparison diagram of effects before and after compensation of the harmonic current compensation method of the embodiment of the present application. DETAILED DESCRIPTION

[0070] The technical solutions of the embodiments of the present application are explained and described below, but the following embodiments are only preferred embodiments of the present application, not all. Based on the embodiments in the embodiments, other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the present application.

[0071] Reference is made to Figures 1 to 4The application adopts the following technical scheme: a harmonic current compensation method under three-phase unbalanced conditions based on an improved filter, comprising:

[0072] collecting three-phase load actual currents of a nonlinear load;

[0073] performing Clark transformation on the collected three-phase load actual currents to obtain alpha and beta components of the three-phase load actual currents;

[0074] obtaining two groups of orthogonal signals of the alpha and beta components of the three-phase load actual currents by using a TOSSI-SOGI filter, the TOSSI-SOGI filter comprising a SOGI-3 filter and a TOSSI filter, and the outputs of the SOGI-3 filter and the TOSSI filter being used as negative feedback signals of each other, the SOGI-3 filter being a second-order generalized integrator with an angular frequency of 3 times a harmonic angular frequency, and the TOSSI filter being a third-order sine integrator;

[0075] obtaining alpha and beta component estimates of a fundamental positive sequence current according to the two groups of orthogonal signals of the alpha and beta components of the three-phase load actual currents;

[0076] obtaining alpha and beta components of a negative sequence and harmonic current to be compensated by performing difference between the alpha and beta component estimates of the fundamental positive sequence current and the actual current alpha and beta components;

[0077] converting the alpha and beta components of the negative sequence and harmonic current to be compensated into three-phase current values to be compensated by performing inverse Clark transformation.

[0078] In the prior art, the use of SOGI cascading has a great influence on bandwidth, and the use of SOGI parallel connection requires a large number of SOGI numbers. In the embodiment, a unique TOSSI-SOGI filter comprising a SOGI-3 filter and a TOSSI filter is used. Similar to the parallel form of the SOGI-3 filter and the TOSSI filter, the TOSSI filter is equivalent to a two-stage cascaded TOSSI filter in terms of filtering effect after 5 times the harmonic, and has a faster response speed. In addition, the SOGI-3 filter can realize almost lossless detection of the third harmonic of the three-phase unbalanced harmonic load, and is regarded as a notch function (i.e., suppressing specific frequencies, which can weaken or completely eliminate certain frequency components in the input signal to achieve the purpose of removing noise and interference). Since only the transfer characteristic near the notch frequency is affected, and the transfer characteristic of other frequency bands is not affected, the system bandwidth is almost not affected.

[0079] In the embodiment, the three-phase load actual currents of the nonlinear load are collected by a current sensor;

[0080] The specific expression of the collected three-phase load actual currents is:

[0081]

[0082]

[0083]

[0084] wherein, i a , i b and i c are a, b, c three-phase phase currents of the non-linear load, and are the positive sequence component amplitude and the negative sequence component amplitude of the hth harmonic current, and are the positive sequence component phase and the negative sequence component phase of the hth harmonic current, and ω is the fundamental angular frequency, and t is time.

[0085] In this embodiment, the specific expressions of the α, β components of the three-phase load actual current obtained by the Clark transformation are as follows:

[0086]

[0087]

[0088] wherein, i α , i β are the α, β components of the three-phase load actual current.

[0089] The relationship between the a, b, c three-phase phase currents i a , i b and ic of the non-linear load and the α, β components i α , i β of the three-phase load actual current is as follows:

[0090]

[0091] In this embodiment, two parallel TOSSI-SOGI filters are used to obtain two sets of orthogonal signals of the α, β components of the three-phase load actual current.

[0092] In this embodiment, the transfer function of the SOGI-3 filter is as follows:

[0093]

[0094]

[0095] wherein, v(s) is the input signal of the SOGI-3 filter (i.e. v in Figure 2 ), s is an integral operator, and is the input signal i α or i βThe complex frequency domain representation, (Right now Figure 2 In )and (Right now Figure 2 In ) represents the two quadrature signals output from the SOGI-3 filter, ω r The third harmonic angular frequency, ω r = 2 × π × 150 rad / s, where k is a constant, take

[0096] The transfer function of the TOSSI filter is:

[0097]

[0098]

[0099] Among them, v m (s) is the input signal of the TOSSI filter (i.e. Figure 2 v in m ), (Right now Figure 2 In )and (Right now Figure 2 In ) represents the two orthogonal signals output by the TOSSI filter, ω is the fundamental angular frequency, and k1 and k2 are constants, which are 2.33 and 2.8 respectively.

[0100] In this embodiment, the set of orthogonal signals output by the TOSSI-SOGI filter is:

[0101]

[0102]

[0103] Two sets of orthogonal components of the current α and β components are obtained by using two TOSSI-SOGI filters respectively. and

[0104]

[0105] In this embodiment, in a TOSSI-SOGI filter, the same v(s) (as i) α In another TOSSI-SOGI filter, the input i β Input SOGI-3 filter and TOSSI filter, SOGI-3 filter output and TOSSI filter output and Then the output of the SOGI-3 filter When input into a TOSSI filter, the TOSSI filter updates its output once, and the output of the TOSSI filter... When input into the SOGI-3 filter, the SOGI-3 filter updates its output once.

[0106] In this embodiment, The fundamental component of v(s) has the same amplitude and phase. The amplitude of the fundamental component of v(s) is the same, while The phase lag is v(s) by 90 degrees. On the other hand, i β The positive-order component lags i α The positive-order component is 90 degrees, while i β Negative order components lead i α The negative order component is 90 degrees. Therefore, i can be extracted. α and i β The estimated value of the fundamental positive sequence component is expressed as:

[0107]

[0108]

[0109] in, and These are the estimated values ​​of the α and β components of the fundamental positive sequence current, respectively. correspond Figure 3 i in α1 , correspond Figure 3 qi in β1 , correspond Figure 3 qi in α1 , correspond Figure 3 i in β1 .

[0110] In this embodiment, the α and β components of the fundamental positive sequence current are estimated. With respect to the actual current α and β components i α i β The difference is used to obtain the α and β components of the negative sequence and harmonic currents to be compensated, and the specific expression is as follows:

[0111]

[0112]

[0113] Among them, i fα i fβ These are the α and β components of the negative sequence and harmonic currents to be compensated, respectively.

[0114] In this embodiment, the DC voltage is stabilized by equivalent active current injection. The active current injection method is that the DC voltage generates a certain reference current value through PI regulation. The specific expression is:

[0115]

[0116]

[0117]

[0118] wherein, and are the α and β components of the injected equivalent active current, and are the estimated values of the α and β components of the positive sequence fundamental component of the grid-side voltage (i.e. Figure 3 and in , ρ is the equivalent active current regulation factor, k p and k i are the proportional coefficient and integral coefficient of the voltage loop, and u dc are the reference value and actual value of the DC voltage;

[0119] The compensated current value after injecting the active current is and

[0120]

[0121]

[0122] The compensated current value and is converted into the compensated three-phase current value by inverse Clark transformation. The specific expression is:

[0123]

[0124] In this embodiment, it also includes: generating the PWM input reference current value, i.e. the compensated three-phase current value, by hysteresis control, and generating the switching signal S1-S6 of the active power filter by PWM modulation, so as to realize the harmonic current compensation of the nonlinear load.

[0125] ​To verify the effect of the harmonic current compensation method under three-phase unbalanced conditions based on the improved filter proposed in the embodiment of the application, a model is established in matlab for simulation. In the simulation, the basic parameters of the active power filter are set as follows: rated line voltage 400V, filter inductance 10mH, filter resistance 1mΩ, DC capacitor 500μF, DC voltage rating 800V, and switching frequency 10kHz.

[0126] The compensation effect is shown in Figure 4 , wherein ε i and THD mi (m=a, b, c) are the negative sequence current unbalance degree and the harmonic current distortion rate of each phase, respectively. Figure 4 (a) and Figure 4 (c) are the same, and all represent the three-phase current collected, Figure 4 (b) represents the results before compensation and each index, Figure 4 (d) represents the results after compensation and each index. It can be seen that after the harmonic current compensation method under three-phase unbalanced conditions based on the improved filter in the embodiment of the application is used, the negative sequence current unbalance degree and the harmonic current distortion rate of each phase are obviously inhibited.

[0127] The harmonic current compensation method under three-phase unbalanced conditions based on the improved filter in the embodiment of the application uses the TOSSI-SOGI filter to obtain two sets of orthogonal signals of the α and β components of the three-phase load actual current, subtracts the fundamental positive sequence current α and β component estimation value from the actual current α and β component to obtain the α and β components of the negative sequence and harmonic current to be compensated, reduces the harmonic detection error caused by the 3rd harmonic and negative sequence current, improves the detection accuracy of the positive sequence fundamental current of the nonlinear load, and further improves the harmonic compensation effect of the active power filter. The TOSSI-SOGI filter has a notch function. Two sets of parallel TOSSI-SOGI filters are used to improve the speed.

[0128] The embodiment of the application also provides an active power filter based on an improved filter, which comprises:

[0129] A current collection module is configured to collect three-phase load actual current of a nonlinear load.

[0130] A Clark conversion module is configured to perform Clark conversion on the collected three-phase load actual current to obtain α and β components of the three-phase load actual current.

[0131] The TOSSI-SOGI filter module is used to obtain two sets of orthogonal signals of alpha and beta components of actual three-phase load currents, and the TOSSI-SOGI filter comprises a SOGI-3 filter and a TOSSI filter, and the outputs of the SOGI-3 filter and the TOSSI filter are used as negative feedback signals of each other, the SOGI-3 filter is a second-order generalized integrator with an angular frequency of 3 times the harmonic angular frequency, and the TOSSI filter is a third-order sine integrator;

[0132] The estimation value obtaining module is used to obtain the fundamental positive sequence current alpha and beta component estimation values according to the two sets of orthogonal signals of alpha and beta components of actual three-phase load currents.

[0133] The difference making module is used to make a difference between the fundamental positive sequence current alpha and beta component estimation values and the actual current alpha and beta components to obtain alpha and beta components of negative sequence and harmonic currents to be compensated.

[0134] The inverse Clark transformation module is used to convert the alpha and beta components of the negative sequence and harmonic currents to be compensated into three-phase current values to be compensated through inverse Clark transformation.

[0135] The hysteresis control module is used to generate PWM input reference current values, i.e. the three-phase current values to be compensated, through hysteresis control.

[0136] The switching signal module is used to generate switching signals of the active power filter through PWM modulation.

[0137] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited to this, and those skilled in the art should understand that the present application includes but is not limited to the content described in the above specific embodiment. Any modification without deviating from the functional and structural principles of the present application will be included in the scope of the claims.

Claims

1. A method for harmonic current compensation under unbalanced conditions in three-phase based on an improved filter, characterized by: The method comprises the following steps: collecting three-phase load actual current of a nonlinear load; performing Clark transformation on the collected three-phase load actual current to obtain α and β components of the three-phase load actual current; obtaining two sets of orthogonal signals of the α and β components of the three-phase load actual current by using a TOSSI-SOGI filter, the TOSSI-SOGI filter comprising a SOGI-3 filter and a TOSSI filter, and the outputs of the SOGI-3 filter and the TOSSI filter being used as negative feedback signals of each other, the SOGI-3 filter being a second-order generalized integrator with an angular frequency of 3 times a harmonic angular frequency, and the TOSSI filter being a third-order sine integrator; obtaining estimated values of α and β components of a fundamental positive sequence current according to the two sets of orthogonal signals of the α and β components of the three-phase load actual current; obtaining α and β components of a negative sequence and harmonic current to be compensated by performing subtraction on the estimated values of the α and β components of the fundamental positive sequence current and the α and β components of the actual current; converting the α and β components of the negative sequence and harmonic current to be compensated into three-phase current values to be compensated by performing inverse Clark transformation.

2. The method for improved filter based compensation of harmonic currents under unbalanced conditions in three phase as claimed in claim 1 wherein: The three-phase load actual current of the nonlinear load is collected by a current sensor. A specific expression of the collected three-phase load actual current is as follows: wherein i a , i b and i c are the a, b, c three-phase phase currents of the load, and are the positive sequence component amplitude and the negative sequence component amplitude of the hth harmonic current, and are the positive sequence component phase and the negative sequence component phase of the hth harmonic current, ω is the fundamental angular frequency, and t is time. A specific expression of the α and β components of the three-phase load actual current obtained by Clark transformation is as follows: where i α , i β are the α, β components of the actual current of the three-phase load.

3. The method for improved filter based compensation of harmonic currents under unbalanced conditions in three phase as claimed in claim 2 wherein: Two sets of orthogonal signals of the α and β components of the three-phase load actual current are obtained by using two parallel TOSSI-SOGI filters.

4. The method for improved filter based harmonic current compensation under unbalanced three phase conditions as claimed in claim 3 wherein: A transfer function of the SOGI-3 filter is as follows: where v(s) is the SOGI-3 filter input signal, s is the integration operator, and i α or i β is the complex frequency domain representation of the SOGI-3 filter input signal, and are the two quadrature signals of the SOGI-3 filter output, ω r is the 3rd harmonic angular frequency, and ω r = 2 x π x 150 rad / s, and k is a constant. A transfer function of the TOSSI filter is as follows: where v m (s) is the input signal of the TOSSI filter, and are two quadrature signals of the output of the TOSSI filter, ω is the fundamental angular frequency, and k1 and k2 are constants.

5. The method for improved filter based compensation of harmonic currents under unbalanced conditions in three phase as claimed in claim 4 wherein: One set of orthogonal signals output by the TOSSI-SOGI filter is as follows: Two sets of quadrature components of current α, β components are obtained by using two TOSSI-SOGI filters respectively and 6. The method for improved filter based compensation of harmonic currents under unbalanced conditions in three phase as claimed in claim 5 wherein: The estimated value of the fundamental positive sequence component of i α and i β is expressed as: wherein and are the estimated values of the fundamental positive-sequence current α, β components, respectively.

7. The method for improved filter based harmonic current compensation under unbalanced three phase conditions as claimed in claim 6 wherein: By fundamental positive sequence current α, β component estimation value With the actual current α, β component i α , β Subtract the α, β components of the negative sequence and harmonic currents to be compensated, and the specific expression is: where i fα , i fβ are the α, β components of the negative sequence and harmonic currents to be compensated, respectively.

8. The method for improved filter based harmonic current compensation under unbalanced three phase conditions as claimed in claim 7 wherein: The DC voltage is stabilized by injecting an equivalent active current, and a specific expression of a reference current value of the active current injection mode is as follows: wherein, and are the α, β components of the injected equivalent active current, respectively, and are the estimated values of the α, β components of the fundamental positive sequence component of the grid-side voltage, respectively, ρ is the equivalent active current regulation factor, k p and k i are the proportional and integral coefficients of the voltage loop, respectively, and u dc are the reference and actual values of the DC voltage, respectively; the current value to be compensated after injecting the active current and is: The current value to be compensated is obtained through inverse Clark transform. and Convert to the three-phase current value to be compensated The specific expression is:

9. The method for improved filter based harmonic current compensation under unbalanced conditions in three phase as claimed in claim 1 wherein: The method further comprises the following steps: A PWM input reference current value, i.e., the three-phase current value to be compensated, is generated by hysteresis control, and switching signals S1-S6 of the active power filter are generated by PWM modulation, so that the harmonic current compensation of the nonlinear load is realized.

10. An improved filter based active power filter employing the method of harmonic current compensation under three phase unbalanced conditions as claimed in any one of claims 1 to 9, characterized in that: The method comprises the following steps: A current collection module is configured to collect three-phase load actual current of a nonlinear load; A Clark transformation module is configured to perform Clark transformation on the collected three-phase load actual current to obtain α and β components of the three-phase load actual current; A TOSSI-SOGI filter module is configured to obtain two sets of orthogonal signals of the α and β components of the three-phase load actual current, the TOSSI-SOGI filter comprising a SOGI-3 filter and a TOSSI filter, and the outputs of the SOGI-3 filter and the TOSSI filter being used as negative feedback signals of each other, the SOGI-3 filter being a second-order generalized integrator with an angular frequency of 3 times a harmonic angular frequency, and the TOSSI filter being a third-order sine integrator; An estimated value obtaining module is configured to obtain estimated values of α and β components of a fundamental positive sequence current according to the two sets of orthogonal signals of the α and β components of the three-phase load actual current; A subtraction module is configured to obtain α and β components of a negative sequence and harmonic current to be compensated by performing subtraction on the estimated values of the α and β components of the fundamental positive sequence current and the α and β components of the actual current. An inverse Clark transformation module is configured to convert the α and β components of the negative sequence and harmonic current to be compensated into three-phase current values to be compensated through inverse Clark transformation; A hysteresis control module is configured to generate PWM input reference current values, i.e. the three-phase current values to be compensated, through hysteresis control; A switching signal module is configured to generate switching signals of the active power filter through PWM modulation.

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