CN103094918B - A kind of single-phase grid-connected device improving the quality of power supply - Google Patents

Abstract

The invention discloses a single-phase grid-connected device for improving power quality, which includes a single-phase voltage-type inverter based on a fully-controlled power electronic device, and the DC side of the single-phase voltage-type inverter is connected to a distributed power generation system; Including one end connected to the inverter output and the other end connected to the power system through the filter inductor C _inv , and the filter inductor L _filter that suppresses the high frequency component of the inverter output current, the reactance of the grid capacitor C _inv is greater than the filter inductance L _filter reactance. The invention can reduce the DC capacitor voltage of the grid-connected inverter, reduce the cost of switching elements and switching loss, realize the injection of active power generated by distributed power sources into the system through the grid-connected inverter, and improve the power quality at the same time.

Description

A single-phase grid-connected device for improving power quality

technical field

The invention relates to distributed power generation and power quality control in a power system, and belongs to the field of converter technology in electrical engineering.

Background technique

Traditional fossil energy is decreasing day by day, and new energy represented by wind energy and solar energy has been rapidly developed and applied. New energy power generation equipment includes both large-capacity centralized power generation systems and small-capacity distributed power generation devices. Small-capacity wind turbines and solar photovoltaic arrays are often used in conjunction with batteries and other energy storage devices to form a distributed power generation system. To connect the electric energy of the distributed generation system to the low-voltage distribution network for users, a series of AC and DC conversions are required. The last link of the conversion is often completed by the grid-connected inverter that converts DC to AC. The grid-connected inverter is connected to the low-voltage grid in parallel, and controls the energy exchange between the grid and the distributed generation system.

Grid-connected inverters can not only inject active power from distributed power sources into the system, but also have the functions of reactive power compensation and harmonic control. Traditional grid-connected devices often connect the inverter output terminal and the grid through a reactor. In order to realize the above functions, the output voltage of the inverter must meet certain requirements. For example, when the inverter is connected to the 220V grid, the output voltage of the inverter needs to reach a level higher than the grid voltage, and the corresponding DC side voltage of the inverter must be higher than the peak value of the grid voltage.

Because the output voltage of distributed power is often relatively low, some devices increase the output voltage by connecting a step-up transformer to the AC side of the grid-connected inverter, so as to connect to the grid, such as "a grid-connected inverter with composite functions and grid-connected inverter control method” (Chinese invention patent, publication date: February 15, 2012, application publication number: CN 102355151 A). If you want to avoid the use of a step-up transformer, you need to increase the output voltage of the distributed power supply through an additional link so that the inverter can be connected to the distribution network. Generally, a DC/DC converter is used to boost the voltage. For example, "Solar Photovoltaic Grid-connected Inverter" (China Invention Patent, Authorized Announcement Date September 28, 2011, Authorized Announcement No. CN 101304224 B) and "Unified Photovoltaic and Wind Power Grid-connected Device with Reactive Power Compensation and Harmonic Control Functions" "(Chinese invention patent, publication date: August 15, 2007, publication number CN101017982A). However, when the conversion ratio of the DC/DC converter is high, the structure will be more complicated, and the efficiency will also decrease during the conversion process of increasing the voltage.

Contents of the invention

The purpose of the present invention is to reduce the DC capacitor voltage of the grid-connected inverter, reduce the cost of switching elements and switching loss, and provide a method for connecting the inverter to the power distribution system through the grid-connected capacitor and filter inductance, so as to realize the The grid inverter injects the active power generated by the distributed power into the system, and can improve the power quality at the same time as a single-phase grid-connected device.

In order to achieve the above objectives, the present invention provides a single-phase grid-connected device for improving power quality, including a single-phase voltage-type inverter based on a fully-controlled power electronic device, the DC side of the single-phase voltage-type inverter and the distributed Connected to the power generation system; it also includes a grid-connected capacitor C _inv that is connected to the output of the inverter and the other end is connected to the power system through a filter inductor, and a filter inductor L _filter that suppresses high-frequency components in the inverter output current, and grid-connected capacitor C _inv The reactance is greater than the reactance of the filter inductance L _filter .

In the above device, the filter inductor and the grid-connected capacitor resonate at a certain harmonic frequency of the power distribution system. In the above device, the size of the grid-connected capacitor C _inv and the filter inductance L _filter is obtained by the following method: first, the impedance X _cf of the series branch formed by the grid-connected capacitor C _inv and the filter inductance L _filter is determined by the following formula:

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}$

Among them, V _S is the system voltage at the access point of the inverter, and Q ₀ is the design reactive power; after that, the grid-connected capacitor C _inv and the filter inductance L _filter are calculated according to the following formula group:

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; in\; v</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}$

$\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N\&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; N\&omega;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}$

Among them, ω=2πf, f is the operating frequency of the power distribution system, and N is the harmonic order of the selected filter inductor and grid-connected capacitor resonating in the system.

In the technical solution provided by the present invention, due to the use of grid-connected capacitors, when the grid-connected inverter injects active power into the system and provides reactive power compensation and harmonic compensation, the DC side voltage of the inverter can be operated at a level much lower than that of the system The level of voltage peaks. The reduction of the DC side voltage reduces the withstand voltage requirements of the selected fully-controlled power electronic devices, reduces the cost of the device, and also reduces the switching loss during the operation of the inverter. The proposed single-phase grid-connected device for improving power quality can not only inject power generated by distributed power sources such as wind turbines and solar photovoltaic arrays into the system, but also improve the power factor of the system and reduce system harmonics. Considering that distributed power generation systems often have large output power fluctuations, it is difficult for grid-connected inverters to operate at rated capacity for a long time. Grid-connected inverters provide power quality management functions, which can improve equipment utilization and bring additional benefits. Shorten the payback period of the entire grid-connected device.

Description of drawings

Fig. 1 is a schematic structural diagram of a single-phase grid-connected device for improving power quality provided by the present invention;

Fig. 2 is the equivalent circuit of the single-phase grid-connected device for improving power quality provided by the present invention;

Fig. 3 is a vector diagram during operation of the single-phase grid-connected device for improving power quality provided by the present invention;

Fig. 4 is the vector diagram when the single-phase grid-connected device for improving power quality provided by the present invention operates in design reactive power;

Figure 5 is an equivalent circuit of a unidirectional grid-connected device using a grid-connected inductor;

Figure 6 is a vector diagram of the operation of the unidirectional grid-connected device using grid-connected inductors;

Fig. 7 is the equivalent circuit of the single-phase grid-connected device for improving power quality provided by the present invention when compensating harmonics;

Fig. 8 is a block diagram of the control principle of the single-phase grid-connected device for improving power quality provided by the present invention in a simulation example;

Fig. 9 is a simulation example in which the single-phase grid-connected device for improving power quality provided by the present invention provides active and reactive power and harmonic compensation voltage and current waveforms to the system;

Fig. 10 is a waveform diagram of DC voltage, active output and reactive output when the single-phase grid-connected device for improving power quality provided by the present invention provides active and reactive power and harmonic compensation to the system in a simulation example.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

As shown in Figure 1, this embodiment provides a single-phase grid-connected device for improving power quality, which includes a single-phase voltage-type inverter based on a fully-controlled power electronic device (IGBT or MOSFET) (the dotted line in the figure Internal part), one end is connected to the inverter output and the other end is connected to the grid-connected capacitor C _inv of the power system through the filter inductor, the filter inductor L _filter that suppresses the high-frequency component of the inverter output current, and the control device. Wherein, the control device is used to control the power electronic devices in the inverter, which belongs to the known technology, so it is not shown in FIG. 1 . The DC side of the single-phase voltage type inverter is connected to the distributed power generation system, and the grid-connected capacitor is set according to the reactive power compensation capacity required by the power distribution system.

The equivalent circuit of the single-phase grid-connected device provided in this embodiment at the basic operating frequency (for example, 50Hz) of the power distribution system is shown in Figure 2, as shown in formula (1), the capacitor C in Figure 2 is equivalent to the grid-connected capacitor C _inv and the filter inductance L _filter are connected in series, the impedance of the series branch is:

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&omega;\; C</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; \&omega;\; C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&omega;L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Among them, ω=2πf, f is the operating frequency of the power distribution system. In this embodiment, the grid-connected capacitor C _inv is used to provide reactive power compensation, and the inductor L _filter is used for filtering. The reactance of the grid-connected capacitor C _inv will be greater than the reactance of the inductor L _filter , so the entire branch is equivalent to a purely capacitive branch .

The single-phase grid-connected device provided in this embodiment can provide continuous and stable reactive power compensation to the distribution network, and the capacitive impedance of the series branch can be determined by the following formula:

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

Among them, V _S is the system voltage at the access point of the grid-connected inverter, and Q ₀ is the design reactive power. When the unit of voltage is volts (V), the unit of power is volt-amperes (VA). Considering that the reactive power of the load is constantly changing, Q ₀ can be calculated by the following formula:

${\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}\underset{\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&Integral;</span>}}\mathrm{<span\; class="notranslate">\; Qdt</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Q ₀ is actually an average power, and the time T can be selected for one day or one week or other lengths of time, mainly selected according to the load fluctuation of the installation site. According to the capacitive impedance value, the corresponding capacitance can be calculated according to the following formula.

$\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; \&omega;x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

According to the positive direction of the inverter output current in Figure 2, when the inverter injects active power into the system and provides reactive current to compensate the inductive load, its output current can be expressed as:

_Icf ＝ _Icfp - _jIcfq (5)

Among them, I _cfp represents active current, and I _cfq represents reactive current. The inverter output voltage V _invc f can be calculated by formula (6),

V _invcf =V _S +V _Cf (6)

where _{VS is the system voltage at the access point of the grid-connected inverter, and Vcf is} the voltage drop across the capacitor in the equivalent circuit. The vector diagram is shown in Figure 3. It can be seen from Figure 3 that because the voltage on the capacitor in the equivalent circuit is obtained by rotating the current 90 degrees clockwise, in order to meet the required output current, the output voltage of the inverter is lower than the system voltage , the required voltage of the DC side capacitor of the inverter is correspondingly greatly reduced. When the reactive power that the system needs to compensate is the designed reactive power Q ₀ , the output voltage of the inverter is perpendicular to the system voltage, and the inverter only needs to provide voltage for the injection of active current. The vector diagram is shown in Figure 4.

In a traditional single-phase grid-connected device using grid-connected inductors, the grid-connected capacitors and filter inductors in Figure 1 will be replaced by grid-connected inductors, and the equivalent circuit is shown in Figure 5. When the single-phase grid-connected device using grid-connected inductors injects active power into the system and provides reactive current to compensate inductive loads, the vector diagram is shown in Figure 6, where V _Lf is the voltage drop on the inductance in the equivalent circuit, The inverter output voltage V _invLf can be calculated by formula (7),

V _invLf =V _S +V _Lf (7)

It can be clearly seen from Figure 5 that the inverter output voltage must be higher than the system voltage to output the required current. The voltage at the DC side of the corresponding inverter will also be much higher than the single-phase grid-connected device using grid-connected capacitors provided in this embodiment.

When the single-phase grid-connected device provided in this embodiment compensates the harmonic current generated by the load, the equivalent circuit for the fundamental frequency in Fig. 2 is no longer applicable, and a new equivalent circuit is shown in Fig. 7 . When the harmonic current flows, the impedance of the capacitance-inductance series branch is calculated by formula (8).

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; LCh</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; h\&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h\&omega;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

Where h represents the order of harmonics. In order to simultaneously inject active power into the system, compensate reactive power and harmonics, the output voltage of the inverter can be calculated by formula (9).

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; invc</span>}}<span\; class="notranslate">\; =</span>\sqrt{{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cf</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; LCh</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 9</span>}<span\; class="notranslate">\; )</span>$

Among them, I _h is the harmonic compensation current provided by the single-phase grid-connected device to the system.

In this embodiment, the filter inductor L _filter connected in series between the grid-connected capacitor and the power distribution system is used to suppress the high-order harmonics of the inverter output current, and the filter inductor can resonate with the grid-connected capacitor at a certain harmonic of the system Wave frequency, such as 3, 5 or 7 times. Assuming that the selected harmonic order is N, at this harmonic frequency, the equivalent impedance of the capacitor-inductance series branch is zero, as shown in formula (10), which can further reduce the output voltage of the inverter.

${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; LCN</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; N\&omega;C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; N\&omega;L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 10</span>}<span\; class="notranslate">\; )</span>$

According to the formulas (1), (2) and (10), the values of grid-connected capacitors and filter inductances can be obtained. Finally, the DC voltage of the inverter can be selected according to formula (11).

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; \&Greater\; Equal;</span>\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; invc</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 11</span>}<span\; class="notranslate">\; )</span>$

The schematic block diagram of the control principle of the single-phase grid-connected device provided in this embodiment is shown in Fig. 8, and the control method adopted therein includes: first, calculate the instantaneous power of the single-phase according to the system voltage and load current, and calculate the instantaneous power from the distributed generation The system obtains the signal of its active power output; then, the reference for calculating the inverter output current is calculated from the active power generated by the distributed generation system, the reactive power to be compensated, the harmonic power, and the power signal for adjusting the inverter DC voltage Afterwards, the hysteresis pulse width modulation method is used to control the inverter output current to track the reference signal, so that the grid-connected inverter can inject active power into the system and compensate reactive power and harmonics at the same time.

Below is a simulation example of the present invention:

In this simulation example, the proposed single-phase grid-connected device for improving power quality is connected in parallel in a single-phase system with a phase voltage of 220V. The grid-connected capacitor is 53uF, the filter inductance is 4mH, and the DC side capacitor voltage is 165V. The hysteresis PWM frequency is 10kHz.

The simulation results are shown in Figure 9 and Figure 10. Figure 9 contains three waveforms, the first of which is the access point voltage waveform, the second is the system side current, and the third is the load current. The difference between the two is The current injected by the grid-connected inverter; the first waveform in Figure 10 is the DC voltage waveform of the inverter; the second is the active power waveform injected by the grid-connected inverter into the system; Injected reactive power. The grid-connected device of the present invention is connected to the power grid in parallel, so that the current waveform on the system side that was originally consistent with the load current changes. Taking the simulation run to 0.8 seconds as an example, the parameters in the system change as shown in the following table after the grid-connected device is connected. Show:

Active power (W) Reactive power (Var) power factor Current harmonics (THD%) system side 710 0 0.994 6.61% load 800 793 0.707 16%

Claims (4)

1. A single-phase grid-connected device for improving power quality, including a single-phase voltage-type inverter based on a fully-controlled power electronic device, and the DC side of the single-phase voltage-type inverter is connected to a distributed power generation system; its characteristics It also includes the grid-connected capacitor C _inv that is connected to the inverter output and the other end is connected to the power system through a filter inductor, and the filter inductor L _filter that suppresses the high-frequency component of the inverter output current, and the reactance of the grid-connected capacitor C _inv Greater than the reactance of the filter inductance L _filter ; the filter inductance and the grid-connected capacitor resonate at a certain harmonic frequency of the power distribution system. 2. The single-phase grid-connected device for improving power quality according to claim 1, characterized in that: the size of the grid-connected capacitor C _inv and the filter inductance L _filter is obtained by the following method: first, determine the grid-connected by the following formula The impedance X _cf of the series branch formed by the capacitor C _inv and the filter inductance L _filter : ${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}$ Among them, V _S is the system voltage at the access point of the inverter, and Q ₀ is the design reactive power; after that, the grid-connected capacitor C _inv and the filter inductance L _filter are calculated according to the following formula group: ${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&omega;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}$ $\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; N\&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; N\&omega;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}$ Among them, ω=2πf, f is the operating frequency of the power distribution system, and N is the harmonic order of the selected filter inductor and grid-connected capacitor resonating in the system. 3. the single-phase grid-connected device improving power quality according to claim 2, is characterized in that, described Q _gets an average power, calculates by following formula: ${\mathrm{<span\; class="notranslate">\; Q</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}\underset{\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{\mathrm{<span\; class="notranslate">\; T</span>}}{<span\; class="notranslate">\; \&Integral;</span>}}\mathrm{<span\; class="notranslate">\; Qdt</span>}$ Wherein, T is a certain length of time. 4. The single-phase grid-connected device for improving power quality according to claim 2 or 3, wherein the _DC voltage V of the inverter is selected according to the following formula: ${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; \&Greater\; Equal;</span>\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; invc</span>}}$ V _invc is the output voltage of the inverter, calculated by the following formula: ${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; invc</span>}}<span\; class="notranslate">\; =</span>\sqrt{{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; Cf</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; cf</span>}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; h</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; LCh</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; h</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}$ X _LCh is the impedance of the series branch composed of the grid-connected capacitor C _inv and the filter inductance L _filter when the harmonic current flows, which is calculated by the following formula: ${\mathrm{<span\; class="notranslate">\; x</span>}}_{\mathrm{<span\; class="notranslate">\; LCh</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; h\&omega;</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; inv</span>}}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; h\&omega;</span>}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; filter</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$ Among them, h represents the number of harmonics; ω=2πf, f is the operating frequency of the power distribution system; I _cf is the output current when the inverter injects active power into the system and provides reactive current to compensate the inductive load; I _h is the harmonic compensation current provided by the single-phase grid-connected device to the system.