CN105490275A - Parallel hybrid active filter and quality detection method thereof - Google Patents

Abstract

The embodiment of the invention discloses a parallel hybrid active filter, which includes an active apparatus and a passive apparatus connected between a three-phase power grid and a non-linear load in a parallel. The passive apparatus includes a passive filter group and a compensation capacitor, which are connected between the three-phase power grid and the non-linear load in a parallel. The active apparatus includes two DC-side capacitors and an inverter, the DC-side capacitors are identical in capacitance, and the two DC-side capacitors are connected in series and then connected to the inverter in parallel. The inverter includes two bridge arms that are connected in parallel, and each bridge arm includes two IGBTs connected in series. A midpoint of series connection of the two DC-side capacitors and a midpoint of the bridge arms are in parallel connection between the three-phase power grid and the non-linear load. The novel parallel hybrid active filter has the great harmonic wave inhibition and reactive compensation properties.

Description

Parallel hybrid active filter and quality detection method thereof

Technical Field

The invention relates to the technical field of active power filtering, in particular to a parallel hybrid active filter and a quality detection method thereof.

Background

The harmonics reduce the efficiency of production, transmission and utilization of electrical energy, cause overheating of electrical equipment, vibration and noise generation, and cause insulation aging, shortened service life, and even failure or burnout. The harmonic wave can cause the local parallel resonance or series resonance of the power system, so that the harmonic wave content is amplified, and the equipment such as a capacitor and the like is burnt. Harmonic waves can also cause relay protection and automatic device malfunction, which makes electric energy metering chaotic. Outside of the power system, harmonics can cause severe interference to communication equipment and electronic equipment. With the use of a large number of nonlinear loads, harmonic pollution in a power supply system is more and more serious, harmonic loss is increased day by day, and the influence on energy conservation and loss reduction in China is also more and more serious.

Many researchers and research institutions have studied harmonic suppression, and a common harmonic suppression means is to install a passive filter (PPF), an active filter (APF) or a hybrid active filter (HAPF) in which the passive filter and the active filter are combined in a power grid. However, in the medium-high voltage system, a single passive filter can only filter specific subharmonics, and a single active filter has the problems of small compensation capacity and high price. Therefore, the hybrid active filter becomes a hot spot in practical application at present.

In the prior art, a hybrid active filter is suitable for a medium-high voltage power grid and can perform large-capacity reactive compensation, but the inverter of the hybrid active filter has large loss and the whole device has high cost in the operation process.

Disclosure of Invention

The embodiment of the invention provides a parallel hybrid active filter and a quality detection method thereof, and aims to solve the problems that in the prior art, the hybrid active filter has high inverter loss and the whole device has high cost in the operation process.

In order to solve the technical problem, the embodiment of the invention discloses the following technical scheme:

the embodiment of the invention provides a parallel hybrid active filter, which comprises an active device and a passive device which are connected between a three-phase power grid and a nonlinear load in parallel, wherein,

the passive device comprises a passive filter bank and a compensation capacitor, and the passive filter bank and the compensation capacitor are respectively connected between a three-phase power grid and a nonlinear load in parallel;

the active device comprises two direct current side capacitors and an inverter, the capacitance values of the direct current side capacitors are equal, the two direct current side capacitors are connected in series and then connected in parallel with the inverter, the inverter comprises two groups of bridge arms which are connected in parallel, and the bridge arms comprise two series-connected IGBTs; the midpoint of the series connection of the two direct current side capacitors and the midpoint of the bridge arm are connected in parallel between the three-phase power grid and the nonlinear load.

Preferably, the active device further comprises a coupling transformer and a set of passive filters connected in series, both the coupling transformer and the passive filters being connected in series with the active device.

Preferably, the active device further comprises an injection capacitor for compensating a larger capacity of reactive power.

Preferably, the output end of the active device is connected in series with a three-phase inductor for filtering high-frequency burrs caused by the on-off of a switching device.

A quality detection method of a parallel hybrid active filter for detecting the parallel hybrid active filter of any one of claims 1 to 4, comprising:

calculating a power grid reference current;

calculating an error value of the power grid current according to the power grid reference current;

and determining the accuracy grade of the parallel hybrid active filter according to the error value of the power grid current.

Preferably, the calculating the grid reference current comprises:

acquiring single-phase power grid voltage and single-phase power grid current;

calculating a parameter G of susceptance properties according to the single-phase power grid voltage and the single-phase power grid current;

and calculating the reference current of the power grid according to the parameter G and the single-phase power grid voltage.

Preferably, the error value of the grid current is a difference between the single-phase grid current and the grid reference current.

Preferably, the determining the accuracy level of the parallel hybrid active filter according to the error value of the grid current comprises:

pre-storing an error threshold range corresponding to the quality grade;

searching an error threshold range corresponding to the error value according to the error value;

determining a quality level corresponding to the error value according to an error threshold range.

As can be seen from the above technical solutions, the parallel hybrid active filter provided in the embodiments of the present invention includes an active device and a passive device connected in parallel between a three-phase power grid and a nonlinear load, where the passive device includes a passive filter bank and a compensation capacitor, and the passive filter bank and the compensation capacitor are respectively connected in parallel between the three-phase power grid and the nonlinear load; the active device comprises two direct current side capacitors and an inverter, the capacitance values of the direct current side capacitors are equal, the two direct current side capacitors are connected in series and then connected in parallel with the inverter, the inverter comprises two groups of bridge arms which are connected in parallel, and the bridge arms comprise two series-connected IGBTs; and the serially connected midpoint of the two direct current side capacitors and the midpoint of the bridge arm are connected between the three-phase power grid and the three-phase load in parallel. The invention provides a novel structure of a parallel hybrid active filter, which can further reduce the cost of the whole set of device and has the main principle that: the active inverter adopts a three-phase four-switch structure, so that the number of switching devices is reduced, and the loss of the inverter is reduced; in addition, a harmonic detection method of single-phase current is adopted, the use of corresponding detection devices and control execution devices is reduced, and the detection calculation time is obviously reduced. The effectiveness of the structure is verified by simulation experiments and engineering application.

Drawings

In order to more clearly illustrate the embodiments or technical solutions in the prior art of the present invention, the drawings used in the description of the embodiments or prior art will be briefly described below, and it is obvious for those skilled in the art that other drawings can be obtained based on these drawings without creative efforts.

Fig. 1 is a schematic diagram of a topology structure of a parallel hybrid active filter according to an embodiment of the present invention;

fig. 2 is a schematic structural diagram of a three-phase four-switch inverter according to an embodiment of the present invention;

FIG. 3 is a schematic diagram of a single-phase electrical model according to an embodiment of the present invention;

fig. 4 is an equivalent circuit diagram of a parallel hybrid active filter system according to an embodiment of the present invention;

fig. 5 is a schematic flow chart of a method for detecting quality of a parallel hybrid active filter according to an embodiment of the present invention;

fig. 6 is a schematic flowchart of a process for calculating a grid reference current according to an embodiment of the present invention;

fig. 7 is a schematic diagram illustrating a process of determining an accuracy level of a parallel hybrid active filter according to an embodiment of the present invention;

fig. 8(a) is a waveform diagram of a grid current before compensation according to an embodiment of the present invention;

fig. 8(b) is a diagram of a compensated grid current waveform according to an embodiment of the present invention;

fig. 1 to 8(b), the symbols represent:

1-three-phase power grid, 2-nonlinear load, 3-passive device, 4-injection capacitor, 5-coupling transformer, 6-three-phase inductor, 7-inverter, 71-direct current side capacitor, 72-IGBT, 8-impedance of output filter, and 9-equivalent voltage source of coupling transformer valve side.

Detailed Description

In order to make those skilled in the art better understand the technical solution of the present invention, the technical solution in the embodiment of the present invention will be clearly and completely described below with reference to the drawings in the embodiment of the present invention, and it is obvious that the described embodiment is only a part of the embodiment of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

First aspect

Referring to fig. 1, a schematic diagram of a topology structure of a parallel hybrid active filter provided for an embodiment of the present invention includes an active device and a passive device 3 connected in parallel between a three-phase power grid 1 and a nonlinear load 2, where the passive device 3 includes a passive filter bank and a compensation capacitor, and the passive filter bank and the compensation capacitor are respectively connected in parallel between the three-phase power grid 1 and the nonlinear load 2; the active device comprises two direct current side capacitors 71 and an inverter 7, the capacitance values of the direct current side capacitors 71 are equal, the two direct current side capacitors 71 are connected in series and then connected with the inverter 7 in parallel, and the inverter 7 is a three-phase four-switch voltage type inverter and is a pulse width modulation inverter based on a self-turn-off device.

The active device further comprises a coupling transformer 5 and a set of passive filters connected in series, the coupling transformer 5 and the passive filters are both connected in series with the active device.

The active device further comprises an injection capacitor 4 for compensating a larger capacity of reactive power.

And the output end of the active device is connected in series with a three-phase inductor 6 for filtering high-frequency burrs caused by the on-off of a switching element.

The active device is composed of a direct current side capacitor 71 and a three-phase four-switch voltage type inverter (VSC), the VSC is a Pulse Width Modulation (PWM) inverter based on a self-turn-off device, and the active device is connected in series with a set of passive filters through a coupling transformer 5 and connected in parallel to a power grid.

The passive device 3 is subjected to most of the fundamental voltage and the active device injects the harmonic current to be compensated. The whole system is simple in structure, can well control harmonic waves in a high-power grid, and can compensate large-capacity reactive power. The biggest advantage is that the fundamental wave voltage borne by the active device is low, and the capacity of the needed inverter 7 is small. The parallel hybrid active filter carries out static compensation of reactive power by a compensation capacitor, the parallel passive filter can compensate the reactive power with larger capacity and filter harmonic current with specific times, and the active device and the passive device 3 jointly inhibit harmonic waves. Fundamental wave reactive current basically cannot flow into the coupling transformer 5 and the inverter 7, so that the filtering device has the reactive compensation capability with larger capacity, the harmonic suppression capability with large capacity and the smaller inverter capacity, and is more suitable for the harmonic treatment requirements of large industrial and mining enterprises.

Fig. 2 is a schematic structural diagram of a three-phase four-switch inverter according to an embodiment of the present invention. The inverter 7 comprises two groups of bridge arms which are connected in parallel, and each bridge arm comprises two IGBTs 72 which are connected in series; the middle point of the two series-connected direct-current side capacitors 71 and the middle point of the bridge arm are connected between the three-phase power grid 1 and the nonlinear load 2 in parallel.

Compared with a conventional three-phase six-inverter bridge, the inverter 7 provided by the embodiment of the invention reduces two switching devices, and the capacitance on the direct current side is replaced by the capacitance with the capacitance value twice as large as that of the original capacitance value. One phase of the inverter 7 is led out from the midpoint of two capacitors on the direct current side, and a three-phase inverter circuit is constructed by an IPM (intelligent power module) module with 4 IGBT72(insulated gate bipolar transistor) switching tubes and two capacitors with equal capacitance values, which is equivalent to replacing one phase of three-phase bridge arms (6 IGBT switching tubes) with a capacitor bridge arm.

The IGBT72 is a composite fully-controlled voltage-driven power Semiconductor device composed of BJT (bipolar junction transistor) and MOS (Metal-oxide-Semiconductor insulated gate field effect transistor), and the IGBT72 has the advantages of small driving power and low saturation voltage drop, and is very suitable for application in the fields of converter systems with a dc voltage of 600V or more, such as ac motors, frequency converters, switching power supplies, lighting circuits, traction drives, and the like.

The ac side voltage of a three-phase four-switch inverter obtained by kirchhoff's voltage theorem is:

$\left\{\begin{array}{c}{u}_{aN}=u_{an}+u_{nN}\\ u_{bN}=u_{bn}+u_{nN}\\ u_{cN}=u_{nN}\end{array}\right.---\left(1\right)$

for a balanced three-phase system, there are

u_aN+u_bN+u_cN＝0(2)

The formula (2) can be substituted for the formula (1):

$u_{nN}=-\frac{u_{an}+u_{bn}}{3}---\left(3\right)$

therefore, the voltage equation of the hybrid parallel active filter is:

$\left\{\begin{array}{c}{u}_{aN}=\frac{2u_{an}-u_{bn}}{3}\\ u_{bN}=\frac{2u_{bn}-u_{an}}{3}\\ u_{cN}=-\frac{u_{an}+u_{bn}}{3}\end{array}\right.---\left(4\right)$

wherein,

in FIG. 2, N is the valve side neutral point of the coupling transformer, Z_a、Z_b、Z_cN is the midpoint of the dc side capacitance, which is the impedance of the output filter. Corresponding to FIG. 2, u in the formulae (1) to (5)_anIs the voltage between point a and point n, u_bnIs the voltage between point b and point n, u_cnIs the voltage between the point c and the point n, u_aNIs the voltage between point a and point N, u_bNIs the voltage between point b and point N, u_cNIs the voltage between point c and point N, u_nNRefers to the voltage between N and N points, u_c1Is referred to as a capacitor C₁Voltage of u_c2Is referred to as a capacitor C₂Voltage of (T)_a1、T_a2、T_b1、T_b2Refers to the voltage, T, across 4 IGBT elements_a1∧u_C1Represents T_a1And u_C1Value of voltage between, T_a2∧u_C2Represents T_a2And u_C2Value of voltage between, T_b1∧u_C1Represents T_b1And u_C1Value of voltage between, T_b2∧u_C2Represents T_b2And u_C2The voltage value in between.

As can be seen from the voltage equation (4), the C-phase voltage is indirectly controlled as long as A, B two-phase voltages are controlled, so that the inverter can completely adopt a three-phase four-switch structure. In practical engineering application, the inverter structure only needs to control the on-off of four switching elements, so that partial actuating elements of a detection system and a control system can be effectively reduced, and the power consumption, the failure rate and the cost of a switching device of an active device can be reduced.

A single-phase equivalent circuit of the hybrid active filter according to the embodiment of the present invention is shown in fig. 3. The nonlinear load is regarded as a harmonic current source I_L，U_SFor the system supply voltage, the active device is controlled to an ideal controlled voltage source U_CAnd n is the coupling transformer transformation ratio. The definition and direction of the other quantities of electricity in the circuit are also shown in FIG. 3, where I_S、I_L、I_P、I_C、I_i、I_fRespectively being power grid branch current, load branch current, parallel passive branch current, active device output current, injection branch current, fundamental wave series resonance branch current, Z_S、Z_P、Z_C0、Z_f、Z_CThe impedance of the power grid, the impedance of the passive branch, the impedance of the injection capacitor, the impedance of the fundamental resonance branch and the impedance of the output filter are respectively.

According to kirchhoff's current and voltage theorem, there are:

$\left\{\begin{array}{c}{U}_S=I_S{Z}_S+I_P{Z}_P\\ I_S=I_L+I_P+I_i\\ I_P{Z}_P=I_f{Z}_f+I_i{Z}_{C0}\\ I_f=I_i+I_C\\ U_C=I_C{Z}_C+I_f{Z}_f\end{array}\right.---\left(6\right)$

reducing equation set (6) to obtain:

I_S(K₁Z_S+K₂Z_S+K₁Z_P)-I_LK₁+U_CZ_f-(K₁+K₂)U_S＝0(7)

in the formula, $\left\{\begin{array}{c}{K}_1=Z_C{Z}_f+Z_{C0}{Z}_L+Z_{C0}{Z}_C\\ K_2=Z_P(Z_f+Z_C)\end{array}\right.---\left(8\right)$

as can be seen from equation (7), the inverter is controlled to output the harmonic voltage U only by reasonable control_CCan effectively reduce the harmonic current I of the power grid_SThe content of (a). This document uses a control strategy based on the harmonic current of the grid, i.e. controlling U_C＝K·I_SAnd K is the control magnification. Equation (7) can be simplified as:

$I_S=\frac{K_1/K_1+K_2}{Z_S+\frac{K_1{Z}_P+{\mathrm{KZ}}_f}{K_1+K_2}}{I}_L+\frac{U_S}{Z_S+\frac{K_1{Z}_P+{\mathrm{KZ}}_f}{K_1+K_2}}---\left(9\right)$

the equation (9) can reversely deduce a single-phase equivalent circuit of the whole system when the control strategy of controlling the output voltage of the inverter according to the harmonic current of the power grid is adopted, as shown in fig. 4.

Wherein the equivalent impedance $Z^{\′}=\frac{K_1(Z_P-1)+{\mathrm{KZ}}_f}{K_1+K_2}---\left(9\right)$

As can be seen from fig. 4, this control strategy is essentially equivalent to equivalently increasing the harmonic impedance of the grid by controlling the output impedance of the active device. The larger the harmonic impedance of the power grid is, the more load harmonic waves are forced to flow into the passive filter, so that the filtering effect of the whole system is improved. Meanwhile, the harmonic compensation characteristic of the passive filter is greatly influenced by the impedance of the power grid, and the larger the impedance of the power grid is, the better the filtering effect of the passive filter is. Therefore, the control strategy improves the filtering effect of the whole system and simultaneously improves the grid-connected performance of the passive filter.

On the other hand, the invention also provides a quality detection method of the parallel hybrid active filter, which corresponds to the embodiment of the hybrid parallel active filter provided by the invention. The embodiment of the invention adopts a direct current control method based on the current error of the power grid for quality detection of the parallel hybrid filter. Referring to fig. 5, the specific steps include:

step S101: and calculating the reference current of the power grid.

Specifically, referring to fig. 6, step S101 includes:

step S201: and acquiring single-phase power grid voltage and single-phase power grid current.

Step S202: and calculating the parameter G of the susceptance property according to the single-phase power grid voltage and the single-phase power grid current.

Step S203: and calculating the reference current of the power grid according to the parameter G and the single-phase power grid voltage.

Step S102: and calculating an error value of the power grid current according to the power grid reference current.

The specific derivation process is as follows:

defining a single-phase grid current i_s：

i_s＝i_f+i_h＝Gu_s+i_h(10)

In the formula i_hIs the sum of harmonic and reactive currents, i_fIs a fundamental current, and G is a parameter representing a susceptance property. i.e. i_hWhen the compensated current is equal to 0, the reference current of the power grid existsAnd the sum i of the harmonic and reactive currents_hFormula (11) is satisfied, wherein T is the power grid frequency.

$1/T{\∫}_0^T{u}_s{i}_{h}dt=0---\left(11\right)$

i_h＝i_s-i_f＝i_s-Gu_s(12)

Treat formula (12) to formula (11), having:

$1/T{\∫}_0^T{u}_s(i_s-{\mathrm{Gu}}_s)dt=0---\left(13\right)$

thus, the parameter G is obtained as:

$G=\frac{1/T{\∫}_0^T{u}_s{i}_{s}dt}{1/T{\∫}_0^T{u}_s^{2}dt}=\frac{{\∫}_0^T{u}_s{i}_{s}dt}{{\∫}_0^T{u}_s^{2}dt}---\left(14\right)$

the parameter G determines the grid reference current.

The inverter adopted by the embodiment of the invention is three-phase four-switch, and only two-phase current needs to be detected. From the above analysis, the power grid phase a current error and phase B current error are respectively:

${\mathrm{\Δi}}_{sa}=i_{sa}-i_{sa}^{*}---\left(15\right)$

${\mathrm{\Δi}}_{sb}=i_{sb}-i_{sb}^{*}---\left(16\right)$

step S103: and determining the accuracy grade of the parallel hybrid active filter according to the error value of the power grid current.

Specifically, the error value of the grid current is a difference value between the single-phase grid current and the grid reference current. Referring to fig. 7, the specific implementation steps include:

step S301: error threshold ranges corresponding to the quality levels are stored in advance.

Step S302: and searching an error threshold range corresponding to the error value according to the error value.

Step S303: determining a quality level corresponding to the error value according to an error threshold range.

The algorithm is based on single-phase current harmonic detection, and therefore can be directly applied to single-phase systems, three-phase three-wire systems and three-phase four-wire systems. The embodiment of the invention adopts the high-power parallel mixed type active filter of a three-phase four-switch inverter structure, and performs test operation in a certain large casting enterprise aiming at the problems of harmonic wave treatment and reactive compensation of nonlinear smelting equipment of the enterprise.

The capacity of a rectifying device of 10kV intermediate frequency furnace smelting equipment of a large casting enterprise is 9.54MVA, 24-pulse rectification is formed by alternate phases of secondary triangles, the reactive power requirement when the load of the intermediate frequency furnace is 6MW is 4.032MVar, and the current contents of 23 th harmonic, 25 th harmonic, 47 th harmonic and 49 th harmonic are 14.57A, 12.75A, 3.94A and 3.44A respectively. The novel parallel hybrid active filter designed by the method has the following specific parameters: the capacity of an active device of the system is 300 kVA; the inverter adopts power modules F4-150R12KS4 with rated voltage of 1200V and rated current of 150A, and the C phase of the inverter consists of two capacitors with the voltage of 10000uF/1000V on the direct current side; the control part takes the DSP2812 as a core; the output filter parameter is 1 mH; the voltage transformation ratio of the transformer phase coupled to the passive filter by the active filter is 2: 1; the passive device is composed of 23 times and 25 times of filtering branches, the capacitance of the 23 times passive branch is 10uF, the inductance is 1.92mH, the capacitance of the 25 times passive branch is 10uF, and the inductance is 1.62 mH.

The grid current waveforms before and after the input for 2s are shown in fig. 8(a) and 8(b), respectively. It can be seen from the figure that after the system is put into operation, the waveform of the power grid current is improved from a distorted waveform to a nearly sine wave, and harmonic components in the power grid current are greatly reduced.

It is noted that, in this document, relational terms such as "first" and "second," and the like, may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.

The foregoing are merely exemplary embodiments of the present invention, which enable those skilled in the art to understand or practice the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (8)

1. Parallel hybrid active filter, comprising active and passive devices (3) connected in parallel between a three-phase grid (1) and a non-linear load (2), wherein,

the passive device (3) comprises a passive filter bank and a compensation capacitor which are respectively connected between the three-phase power grid (1) and the nonlinear load (2) in parallel;

the active device comprises two direct current side capacitors (71) and an inverter (7), the capacitance values of the direct current side capacitors (71) are equal, the two direct current side capacitors (71) are connected in series and then are connected with the inverter (7) in parallel, the inverter (7) comprises two groups of bridge arms which are connected in parallel, and the bridge arms comprise two IGBTs (72) which are connected in series; the middle point of the series connection of the two direct current side capacitors (71) and the middle point of the bridge arm are connected between the three-phase power grid (1) and the nonlinear load (2) in parallel.

2. The parallel hybrid active filter according to claim 1, characterized in that the active device further comprises a series connection of a coupling transformer (5) and a set of passive filters, both the coupling transformer (5) and the passive filters being connected in series with the active device.

3. Parallel hybrid active filter according to claim 1, characterized in that the active device further comprises an injection capacitor (4) for compensating a larger capacity of reactive power.

4. The parallel hybrid active filter according to claim 1, characterized in that the output end of the active device is connected in series with a three-phase inductor (6) for filtering high-frequency glitches caused by switching on and off of a switching device.

5. A quality detection method for a parallel hybrid active filter, which is used for detecting the parallel hybrid active filter according to any one of claims 1 to 4, and comprises the following steps:

calculating a power grid reference current;

calculating an error value of the power grid current according to the power grid reference current;

and determining the accuracy grade of the parallel hybrid active filter according to the error value of the power grid current.

6. The quality detection method of the parallel hybrid active filter according to claim 5, wherein the calculating the grid reference current comprises:

acquiring single-phase power grid voltage and single-phase power grid current;

calculating a parameter G of susceptance properties according to the single-phase power grid voltage and the single-phase power grid current;

and calculating the reference current of the power grid according to the parameter G and the single-phase power grid voltage.

7. The method as claimed in claim 6, wherein the error value of the grid current is a difference between the single-phase grid current and the grid reference current.

8. The method for detecting the quality of the parallel hybrid active filter as claimed in claim 5, wherein the determining the accuracy level of the parallel hybrid active filter according to the error value of the grid current comprises:

pre-storing an error threshold range corresponding to the quality grade;

searching an error threshold range corresponding to the error value according to the error value;

determining a quality level corresponding to the error value according to an error threshold range.