CN103743976A - Side-frequency harmonic current disturbance island detection method - Google Patents

Abstract

The invention relates to a side-frequency harmonic current disturbance island detection method. An island detection technology is one of key technologies in a grid -connected inverter. A side-frequency harmonic current disturbance method is an active island detection method. The method is characterized in that micro side-frequency components are filled in grid-connected current and whether an island state happens or not is judged by detecting the side-frequency components in grid-side voltage through a detection network. The side-frequency harmonic current disturbance method not only is suitable for resistive loads, but also is suitable for high-quality-factor harmonic loads; the side-frequency harmonic current disturbance method not only is suitable for single-phase grid-connected inverters, but also is suitable for three-phase grid-connected inverters and does not influence the waveform quality of the grid-connected current.

Description

Side frequency harmonic current disturbance island detection method

Technical field

The present invention relates to a kind of active alone island detection method of combining inverter, relate in particular to a kind of side frequency harmonic current disturbance island detection method.

Background technology

In photovoltaic generation and wind-power electricity generation distributed electricity generation system, combining inverter is connected with public electric wire net, jointly to local load supplying, as shown in Figure 1.When so-called " isolated island " refers to electrical network decompression, distributed generation system still keeps a certain part circuit in decompression electrical network to continue the state of power supply.Due to fault or the reason such as artificial, combining inverter may all constitute a threat to local consumer, combining inverter and personal safety etc. in islet operation state, and therefore, combining inverter must possess reliable isolated island and detect and defencive function.

Island detection method comprises passive type and active two large classes.Passive type island detection method is the most basic, is also the basis of current active alone island detection method, mainly comprises overvoltage/undervoltage and crosses two kinds of methods of under-frequency.Active alone island detection method is by introducing disturbance to the output of combining inverter, when isolated island is occurred, voltage or the frequency of point of common coupling (PCC) exceed setting range, thereby detect island state, mainly comprise two kinds of methods of frequency deviation method and variation method.

When the validity of existing island detection method and isolated island, the characteristic relation of load is very large.When load is resistive load, existing detection method can detect island state substantially; When RLC parallel resonance load that load is high quality factor, the validity of existing detection method can reduce greatly, and even inspection does not measure island state.

Figure 2 shows that the load of RLC parallel resonance.The relation of its input impedance and input signal angular frequency as the formula (1).

$\left|Z\left(\mathrm{\&omega;}\right)\right|=\left|\frac{\stackrel{\&CenterDot;}{U}}{\stackrel{\&CenterDot;}{I}}\right|=\frac{\mathrm{<figure-callout\; id="1"\; label="R"\; filenames="HDA0000450297730000013.png"\; state="\{\{state\}\}">R</figure-callout>}}{\sqrt{1+{Q_f}^2\&CenterDot;{(\frac{\mathrm{\&omega;}}{{\mathrm{\&omega;}}_0}-\frac{{\mathrm{\&omega;}}_0}{\mathrm{\&omega;}})}^2}}---\left(1\right)$

$Q_f=R\&CenterDot;\sqrt{\frac{C}{L}}$

Wherein, quality factor

Resonance frequency ${\mathrm{\&omega;}}_0=\frac{1}{\sqrt{\mathrm{LC}}}$

According to formula (1), can draw the input impedance of RLC parallel resonance load and the relation of input signal angular frequency, as shown in Figure 3.

When island state occurs, if load is the load of RLC parallel resonance, the output voltage angular frequency of combining inverter is easily stabilized in resonance frequency omega ₀place, and quality factor q _fhigher, like this.Work as resonance frequency omega ₀while just equaling the angular frequency of line voltage, the frequency of locating by detection point of common coupling (PCC) just cannot judge whether island state occurs.The present invention puts forward for this situation.

Summary of the invention

In order to overcome the deficiency of existing island detection method, the invention provides a kind of active alone island detection method that does not rely on load characteristic.

Technical solution of the present invention is:

A side frequency harmonic current disturbance island detection method, its special character is: the island state of single-phase grid-connected inverter is detected and comprised the following steps:

1] in the grid-connected current of combining inverter, inject first-harmonic side frequency composition; Described first-harmonic side frequency composition electric current is suc as formula (2) second:

${i_0}^{*}={I_m}^{*}\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;{I_m}^{*}\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(2\right)$

Wherein first is the first-harmonic composition of the grid-connected current of setting, I _m ^*for the current peak of the first-harmonic set, ω _sfor electrical network angular frequency, (ω _s+ ω ₁), (ω _s-ω ₁) be side frequency frequency, ω ₁be the deviation value of current perturbation component frequency and electrical network angular frequency, M is a very little value, ω ₁<< ω _s,

that frequency is (ω _s+ ω ₁), (ω _s-ω ₁) the ratio of current perturbation component amplitude and fundamental current component amplitude,

2] detect point of common coupling place voltage, if voltage exists second of formula (4), island state occurs:

$u_o=U_m\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;U_m\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(4\right)$

Wherein first is the first-harmonic composition of point of common coupling place voltage, and second is the side frequency composition of point of common coupling place voltage.

As the optimization of the island state detection method to single-phase grid-connected inverter, the 0.5-2% that the amplitude of first-harmonic side frequency composition of the present invention is fundamental voltage amplitude; Frequency departure value ω ₁be not more than 2% of fundamental frequency.The amplitude of first-harmonic side frequency composition is preferably 1% of fundamental voltage amplitude; Frequency departure value ω ₁be preferably 1% of fundamental frequency.

Adopt side frequency harmonic current disturbance island detection method, for the island state of three-phase grid-connected inverter, detect and comprise the following steps:

1] in the grid-connected current of combining inverter, inject first-harmonic side frequency composition, grid-connected current instruction is provided by following formula (7):

$\left\{\begin{array}{c}{i_{\mathrm{oA}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t\\ {i_{\mathrm{oB}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos ({\mathrm{\&omega;}}_{s}t-\frac{2\mathrm{\&pi;}}{3})\\ {i_{\mathrm{oC}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos ({\mathrm{\&omega;}}_{s}t+\frac{2\mathrm{\&pi;}}{3})\end{array}\right.----\left(7\right)$

2] detecting point of common coupling place voltage, is ω if the voltage magnitude at point of common coupling place is subject to frequency ₁the modulation of low frequency signal, island state occurs.

As the optimization of the island state detection method to three-phase grid-connected inverter, the 0.5-2% that the amplitude of first-harmonic side frequency composition of the present invention is fundamental voltage amplitude; Frequency departure value ω ₁be not more than 2% of fundamental frequency.The amplitude of first-harmonic side frequency composition is preferably 1% of fundamental voltage amplitude; Frequency departure value ω ₁be preferably 1% of fundamental frequency.

Compared with prior art, advantage is in the present invention:

Side frequency harmonic current method of perturbation of the present invention does not rely on load characteristic, has both been applicable to resistive load situation, is applicable to again the resonant load situation of high quality factor; Both be applicable to single-phase grid-connected inverter, be applicable to again three-phase grid-connected inverter, do not affected the waveform quality of grid-connected current simultaneously.

Accompanying drawing explanation

Fig. 1 is distributed generation system schematic diagram;

Fig. 2 is the load of RLC parallel resonance;

Fig. 3 is the input impedance characteristic of RLC parallel resonance load.

Embodiment

In the grid-connected current of combining inverter, inject small first-harmonic side frequency composition, the side frequency of locating voltage by detecting point of common coupling (PCC) becomes to assign to judge whether island state occurs.This method can be called side frequency harmonic current method of perturbation.

Suppose grid-connected current instruction i _o ^*as the formula (2),

${{\mathrm{<figure-callout\; id="0"\; label="i"\; filenames="HDA0000450297730000013.png"\; state="\{\{state\}\}">i</figure-callout>}}_0}^{*}={I_m}^{*}\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;{I_m}^{*}\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(2\right)$ In formula, ω _sfor electrical network angular frequency, M is a very little value, ω ₁<< ω _s

Grid-connected current i _oby the form presenting as the formula (3).

$i_o=I_m\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;I_m\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(3\right)$

In the grid-connected current shown in formula (3), first first-harmonic composition that is electric current, second be electric current side frequency composition, side frequency frequency (ω _s+ ω ₁), (ω _s-ω ₁) and ω _svery approaching.

When island state occurs, if load is the load of RLC parallel resonance, resonance frequency omega ₀just equal the angular frequency of line voltage _s, the voltage that point of common coupling (PCC) is located is by the form presenting as the formula (4).

$u_o=U_m\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;U_m\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(4\right)$

When line voltage is normal, the voltage that point of common coupling (PCC) is located is determined by external power grid, can not have side frequency composition as the formula (4).According to above characteristic, just can judge whether island state occurs.

Formula (2) deformable is the form shown in formula (5).

$\begin{array}{c}{i_o}^{*}={I_m}^{*}\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;{I_m}^{*}\&CenterDot;{I_m}^{*}\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]\\ ={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t\end{array}---\left(5\right)$

Formula (4) deformable is the form shown in formula (6).

$\begin{array}{c}{u}_o=U_m\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;U_m\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]\\ =U_m\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t\end{array}---\left(6\right)$

By formula (6), can be found out, adopt side frequency harmonic current method of perturbation, when island state occurs, the voltage waveform that point of common coupling (PCC) is located is amplitude modulation waveform, and sinusoidal wave amplitude is subject to the modulation of low frequency signal, and the frequency of this low frequency signal is ω ₁.In the specific implementation, by analyzing point of common coupling (PCC), locate voltage magnitude whether to have frequency be ω ₁low-frequency fluctuation just can judge whether island state occurs.

Formula (5) is depicted as the concrete form of single-phase grid-connected inverter grid-connected current instruction, and for three-phase grid-connected inverter, grid-connected current instruction is provided by formula (7).

$\left\{\begin{array}{c}{i_{\mathrm{oA}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t\\ {i_{\mathrm{oB}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos ({\mathrm{\&omega;}}_{s}t-\frac{2\mathrm{\&pi;}}{3})\\ {i_{\mathrm{oC}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos ({\mathrm{\&omega;}}_{s}t+\frac{2\mathrm{\&pi;}}{3})\end{array}\right.----\left(7\right)$

While injecting the side frequency composition of first-harmonic in giving grid-connected current, the amplitude of side frequency composition and frequency departure value ω ₁can not too greatly can not be too small than the amplitude of first-harmonic and frequency, if cross conference, first-harmonic grid-connected current waveform is impacted, easily produce harmonic wave, if too little, be not easy to detect, therefore the amplitude of first-harmonic side frequency composition can be the 0.5%-2% of fundamental voltage amplitude, is preferably 1% of fundamental voltage amplitude; The frequency departure value ω of first-harmonic side frequency composition ₁for below 2% of fundamental frequency, be preferably fundamental frequency 1%.

Claims (6)

1. a side frequency harmonic current disturbance island detection method, is characterized in that: the island state of single-phase grid-connected inverter is detected, comprise the following steps:

1] in the grid-connected current of combining inverter, inject first-harmonic side frequency composition; Described first-harmonic side frequency composition electric current is suc as formula (2) second:

${i_0}^{*}={I_m}^{*}\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;{I_m}^{*}\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(2\right)$

I _o ^*for grid-connected current instruction, first first-harmonic composition for the grid-connected current of setting in formula (2), I _m ^*for the current peak of the first-harmonic set, ω _sfor electrical network angular frequency, (ω _s+ ω ₁), (ω _s-ω ₁) be side frequency frequency, ω ₁the deviation value of current perturbation component frequency and electrical network angular frequency, ω ₁<< ω _s,

that frequency is (ω _s+ ω ₁), (ω _s-ω ₁) the ratio of current perturbation component amplitude and fundamental current component amplitude,

2] detect the voltage u of point of common coupling place _oif voltage exists second of formula (4), island state occurs:

$u_o=U_m\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t+\frac{M}{2}\&CenterDot;U_m\&CenterDot;[\cos ({\mathrm{\&omega;}}_s+{\mathrm{\&omega;}}_1)t+\cos ({\mathrm{\&omega;}}_s-{\mathrm{\&omega;}}_1)t]---\left(4\right)$

Wherein first is the first-harmonic composition of point of common coupling place voltage, and second is the side frequency composition of point of common coupling place voltage.

2. side frequency harmonic current disturbance island detection method according to claim 1, is characterized in that:

The 0.5-2% that the amplitude of described first-harmonic side frequency composition is fundamental voltage amplitude;

Described frequency departure value ω ₁be not more than 2% of fundamental frequency.

3. side frequency harmonic current disturbance island detection method according to claim 2, is characterized in that:

The amplitude of described first-harmonic side frequency composition is fundamental voltage amplitude 1%;

Described frequency departure value ω ₁for 1% of fundamental frequency.

4. a side frequency harmonic current disturbance island detection method, is characterized in that: the island state for three-phase grid-connected inverter detects, and comprises the following steps:

1] in the grid-connected current of combining inverter, inject first-harmonic side frequency composition, grid-connected current instruction is provided by following formula (7):

$\left\{\begin{array}{c}{i_{\mathrm{oA}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos {\mathrm{\&omega;}}_{s}t\\ {i_{\mathrm{oB}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos ({\mathrm{\&omega;}}_{s}t-\frac{2\mathrm{\&pi;}}{3})\\ {i_{\mathrm{oC}}}^{*}={I_m}^{*}\&CenterDot;(1+M\&CenterDot;\cos {\mathrm{\&omega;}}_{1}t)\&CenterDot;\cos ({\mathrm{\&omega;}}_{s}t+\frac{2\mathrm{\&pi;}}{3})\end{array}\right.----\left(7\right)$

2] detecting point of common coupling place voltage, is ω if the voltage magnitude at point of common coupling place is subject to frequency ₁the modulation of low frequency signal, island state occurs.

5. side frequency harmonic current disturbance island detection method according to claim 4, is characterized in that:

The 0.5-2% that the amplitude of described first-harmonic side frequency composition is fundamental voltage amplitude;

Described frequency departure value ω ₁be not more than 2% of fundamental frequency.

6. side frequency harmonic current disturbance island detection method according to claim 5, is characterized in that:

The amplitude of described first-harmonic side frequency composition is fundamental voltage amplitude 1%;

Described frequency departure value ω ₁for 1% of fundamental frequency.

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