CN103941118A - Island detecting method for photovoltaic power generation grid-connected inverter trunking system - Google Patents

Abstract

An islanding detection method for grid-connected inverter cluster systems for photovoltaic power generation. When an island occurs in the inverter cluster system and the voltage frequency of the grid-connected point decreases, the phase difference between the current and voltage of the traditional sliding mode frequency drift method is corrected, and the error compensation angle is added to offset the impact of the line impedance on the blind area and detection of the sliding mode frequency drift method. The impact caused by the reaction speed improves the reliability of island detection. If the frequency of the grid-connected point voltage increases, the expression of the traditional sliding mode frequency drift method is used. At this time, the system line impedance will not have a negative impact on the effectiveness of the islanding detection.

Description

Islanding detection method for grid-connected inverter cluster system of photovoltaic power generation

technical field

The invention relates to a detection method suitable for a grid-connected inverter cluster system.

Background technique

Islanding refers to the phenomenon that in the photovoltaic grid-connected power generation system, the power grid is cut off due to failure or maintenance, but the photovoltaic power generation system does not stop supplying power to local loads, forming a power supply island phenomenon. The occurrence of isolated islands will cause huge hidden dangers to the safety of maintenance personnel and the stable operation of the power grid. Therefore, it is of great practical significance to study the islanding detection technology of microgrids. In recent years, photovoltaic power generation systems have gradually developed towards high density and multiple access points. The effectiveness of traditional island detection methods, especially active methods, has been challenged. Island detection technology suitable for photovoltaic grid-connected inverter cluster systems has become a hotspot of current research.

The sliding mode frequency drift method (SMS) is an active local island detection method with positive feedback effect. The basic principle of the sliding mode frequency drift method is that the inverter controls the output current to be at the same frequency as the grid-connected point voltage, but the leading voltage is a constant the phase of:

${\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; SMS</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>$

Among them, θ _m is the maximum phase offset, f _PCC is the voltage frequency of the grid-connected point of the inverter, f ₀ is the rated frequency of the power grid 50Hz, and f _m is the frequency when the maximum phase offset θ _m occurs. When the photovoltaic inverter is connected to the grid, θ _SMS = 0, there is no phase difference between the current and the voltage, and the frequency of the grid-connected point of the system will not change; once islanding occurs, the frequency of the inverter cluster system is no longer clamped by the grid. Due to the positive feedback effect of the sliding mode frequency method, the grid-connected point frequency will eventually exceed the set threshold, and the occurrence of the islanding phenomenon can be detected.

The distribution of inverter cluster systems with multiple access points is relatively scattered, and the line impedance between each inverter and the grid-connected point cannot be ignored. At this time, there is a phase difference between the outlet voltage collected by the inverter and the grid-connected point voltage , resulting in the deviation of the current phase output by the inverter from the theoretical value under the action of the mode frequency drift method. When the current phase difference caused by the line impedance is opposite to the direction of θ _SMS , the positive feedback effect of the mode frequency drift method (SMS) will be reduced. Affects the effectiveness of island detection. The situation with the most serious impact is that the phase difference between the grid-connected point voltage and current of the inverter cluster system is equal to the phase angle of the load, and the internal power of the island is completely matched. Taking the case of pure resistive load as an example, as shown in Figure 2, two inverters The system structure when there is line impedance in the case of parallel operation of transformers, when the load is purely resistive, the grid-connected point voltage and current waveforms when the sliding mode frequency drift method fails due to line impedance are shown in Figure 3.

Figure 4 shows the analysis of the detection blind zone of the traditional sliding mode frequency drift method in the inverter cluster system where the line impedance cannot be ignored. For the case where the system frequency f drops after islanding occurs, θ _SMS <0, and the current phase should lag behind At this time, the line impedance will cause θ _SMS to be offset, and the detection blind zone will increase; if θ _SMS > 0 after islanding occurs, the phase error caused by line impedance will be in the same direction as θ _SMS , and the detection blind zone will decrease, and the line impedance will not change. Will reduce the effectiveness of island detection. On the whole, because the system line impedance will cause the detection blind zone of the traditional SMS method to shift, and the frequency rise or fall is random after the islanding occurs, so for a specific system, it is impossible to judge whether the traditional SMS method is effective, reducing the islanding detection reliability.

Contents of the invention

The purpose of the present invention is to overcome the disadvantage that the effectiveness of the islanding detection method is reduced due to line impedance when the existing sliding mode frequency drift method is applied to the inverter cluster system, and propose a method suitable for islanding detection of the grid-connected inverter cluster system new method. The invention can effectively reduce the island detection blind area under the condition that the line impedance of the system cannot be ignored, and improve the reaction speed.

The technical solution adopted by the present invention to solve the technical problem is: to propose a novel island detection method, which improves the expression method of the phase angle θ _SMS in the traditional mode frequency drift method, so as to reduce the blind area of island detection. When an island occurs in the inverter cluster system and the voltage frequency of the grid-connected point decreases, the phase difference between the current and voltage of the traditional sliding mode frequency drift method is corrected, and the error compensation angle is added to offset the detection blind zone and reaction speed of the inverter drift method. impact and improve the reliability of island detection. The expression of phase angle θ _SMS among the present invention is:

${\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; SMS</span>}}<span\; class="notranslate">\; =</span>\left\{\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; <</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\end{array}\right.$

In the above formula: θ _m is the maximum phase offset, f _PCC is the voltage frequency of the grid-connected point of the inverter, f ₀ is the rated frequency of the power grid 50Hz, and f _m is the frequency when the maximum phase offset θ _m occurs. For the case of f _PCC >f ₀ when the frequency rises after islanding occurs, the expression of the phase angle θ _SMS is the same as that of the traditional mode frequency drift method. When the photovoltaic inverter is connected to the grid, the phase angle θ _SMS = 0, and there is no phase difference between the current and the voltage; once the islanding phenomenon occurs, the voltage frequency of the grid-connected point is no longer clamped by the grid. If the frequency of the grid-connected point produces a Small increase, then θ _SMS > 0, the current phase will be ahead of the voltage, resulting in a shorter time for the current to reach the next zero-crossing point, and the current and voltage of the system in the island state satisfy the relationship: That is, the grid-connected point voltage Current output by inverter cluster system only It is determined by the local load impedance Z, so the time for the grid-connected point voltage to reach the next zero-crossing point will also be shortened, and the frequency will increase. At this time, the phase angle θ _SMS will continue to increase due to positive feedback until the grid-connected point frequency will exceed the set threshold. , the islanding phenomenon can be detected; similarly, when the islanding occurs, the grid-connected point frequency drops, and the phase angle θ _SMS <0, the frequency will also decrease continuously through the positive feedback, and the occurrence of the islanding phenomenon can be detected.

If the frequency drops f _PCC <f ₀ after the islanding phenomenon occurs, a constant angle θ ₀ is subtracted from the phase angle θ _SMS , and θ ₀ >0, this angle is the error phase compensation angle, and its function is to cancel the line The phase lead of the inverter current caused by the impedance ensures that the angle at which the current lags behind the voltage reaches the theoretical value. The value of the error phase compensation angle θ ₀ is determined by the number of inverters in the system and the distribution distance of each inverter. In order to ensure the power quality of the system, the error phase compensation angle θ ₀ should not exceed the maximum phase offset θ _m , Pick:

θ ₀ ＝Kθ _m

Where K=K _n K _l , K _n represents the proportion of inverters distributed far away in the system, and its expression is:

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}$

Among them, n and m represent the total number of inverters and the number of inverters in the inverter cluster that are far away from the public grid-connected point, respectively. It can be seen that K _n ∈ (0,1) at this time; K _l represents the inverter cluster For the overall distribution, when the distribution of each inverter is relatively scattered and the distance from the public grid-connected point is relatively large, a larger value of _Kl can be taken, while for a closely distributed grid-connected inverter cluster system, the value of _Kl can be smaller , and its value range is K _l ∈ (0.1,1).

The control strategy of the error phase compensation angle is as follows: when the system detects that the frequency of the grid-connected point changes, it starts timing. Since GB/T15945 stipulates that the islanding protection should operate within 0.2s after the islanding phenomenon occurs, it is set when the change range reaches 0.5Hz, The duration reaches 0.1s, and θ ₀ is generated when the islanding protection is not in action, which effectively avoids the drastic change of the current phase caused by the system frequency disturbance.

The beneficial effect of the invention is that it can suppress the influence of the line impedance of the inverter cluster system on the effectiveness of the active island detection method, reduce the island detection blind area of the sliding mode frequency drift method in the presence of line impedance, and improve the reaction speed.

Description of drawings

The workflow diagram of the SMS method after Fig. 1 improvement;

Fig. 2 Inverter cluster structure when there is line impedance in the case of pure resistive load;

Figure 3 The voltage and current waveforms of the grid-connected point when the line impedance causes the island power to fully match under the pure resistive load;

Figure 4 The traditional SMS method has an island detection blind area when the line impedance cannot be ignored;

Figure 5. Comparison of islanding detection blind areas before and after the improvement of the SMS method in the case that the inverter cluster line impedance cannot be ignored.

Detailed ways

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

The present invention improves the expression of the phase angle θ _SMS in the traditional mode frequency drift method, which is:

${\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; SMS</span>}}<span\; class="notranslate">\; =</span>\left\{\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; <</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\end{array}\right.$

Where θ _m is the maximum phase offset, f _PCC is the voltage frequency of the grid-connected point of the inverter, f ₀ is the rated frequency of the power grid 50Hz, and f _m is the frequency when the maximum phase offset θ _m occurs. It can be seen that the phase angle θ _SMS expression is the same as the traditional mode frequency drift method when the frequency increases after the island occurs; if the frequency decreases after the island occurs, a constant angle θ ₀ is subtracted from the original phase angle θ _SMS , and θ ₀ >0, this angle is the error phase compensation angle, its function is to offset the phase lead of the inverter current caused by the line impedance, and ensure that the angle at which the current lags behind the voltage reaches the theoretical value.

The value of θ ₀ needs to be determined according to the structure of the photovoltaic inverter cluster. The factors considered include the number of inverters in the system and the distribution distance of each inverter. In order to ensure the power quality of the system, θ ₀ should not exceed θ _m , take:

θ ₀ ＝Kθ _m

Where K=K _n K _l , K _n represents the proportion of inverters distributed far away in the system, and its expression is:

${\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; m</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}}$

Among them, n represents the total number of inverters, and m represents the number of inverters in the inverter cluster that are far away from the public grid-connected point. It can be seen that K _n ∈ (0,1) at this time; K _l represents the overall inverter cluster For the distribution of inverters, when the distribution of inverters is relatively scattered and the distance from the public grid-connected point is relatively long, a larger value of _Kl can be taken, while for a system with a tight distribution, the value of _Kl can be smaller, and the value range is K _l ∈ (0.1,1).

The simplified θ _SMS expression is as follows:

${\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; SMS</span>}}<span\; class="notranslate">\; =</span>\left\{\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\\ {\mathrm{<span\; class="notranslate">\; \&theta;</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&CenterDot;</span>\frac{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; K</span>}<span\; class="notranslate">\; ]</span>& <span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; PCC</span>}}<span\; class="notranslate">\; <</span>{\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; )</span>\end{array}\right.$

Fig. 5 shows the comparison results of the island detection blind area between the present invention and the traditional modulus frequency drift method. As shown in Figure 5, when the inverter cluster system has line impedance, the detection blind zone of the present invention is smaller than the low-frequency part of the traditional mode frequency drift method, that is, it is less affected by the line impedance, and the high-frequency part is not affected by the line impedance. Affected by line impedance, it will not be affected after improvement.

In order to ensure that the present invention will not affect the power quality of the power grid under the grid-connected operation of the inverter cluster system, the following control strategy is adopted: when the frequency of the grid-connected point is detected to start timing, when the change lasts long enough and After the displacement is large enough, the error phase compensation angle θ ₀ is generated. For example, it can be set that when the frequency of the grid-connected point drops for 0.1s and the island protection does not operate, θ ₀ is generated, which effectively avoids the current caused by the system frequency disturbance. A case where the phase changes drastically.

Fig. 1 shows the working flow diagram of the method of the present invention. As shown in Figure 1, the inverter equipped with the method of the present invention continuously samples and analyzes the grid-connected point voltage frequency f _PCC during operation, and once the grid-connected point frequency changes, the inverter judges the grid-connected point voltage frequency f _PCC Rising or falling, if f _PCC >f ₀ , the inverter will operate according to the traditional sliding mode frequency drift method, that is, control the phase between the output current and the grid-connected point voltage

If the grid-connected point frequency f _PCC < f ₀ , the DSP of the inverter starts timing, and the inverter continues to detect the grid-connected point frequency. When f _PCC < f ₀ lasts for 0.1s, it means that islanding may occur, and the grid-connected point frequency decline, at this time, according to the method of the present invention, the phase difference between the output current and the grid-connected point voltage is controlled

If f _PCC < f ₀ and the duration fails to reach 0.1s, it is considered that the grid-connected point frequency changes due to system disturbance, and the phase error compensation angle θ ₀ will not be added. The final output current of the inverter is i=I _m cos(2πf ₀ t+θ _SMS ).

Claims (3)

1. an island detection method that is applicable to photovoltaic power generation grid-connected inverter group system, is characterized in that: when inverter group system generation isolated island, and grid-connected point voltage frequency is when reduce, to the phase differential θ between electric current and voltage in traditional sliding formwork frequency drift method _sMSadd error compensation angle θ ₀:

${\mathrm{\&theta;}}_{\mathrm{SMS}}={\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})-{\mathrm{\&theta;}}_0;$

If isolated island is grid-connected point voltage frequency rising after occurring, the phase differential θ between electric current and voltage _sMSexpression formula identical with traditional sliding formwork frequency drift method:

${\mathrm{\&theta;}}_{\mathrm{SMS}}={\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0}),$

Wherein: θ _mfor maximum phase skew, f _pCCfor grid-connected inverters point voltage frequency, f ₀for electrical network rated frequency, f _mfor producing maximum phase skew θ _mtime frequency.

2. the island detection method for photovoltaic power generation grid-connected inverter group system according to claim 1, is characterized in that: described error compensation angle θ ₀adopt following control strategy: when detecting after grid-connected dot frequency changes, start timing, when amplitude of variation reaches 0.5Hz, duration, reach 0.1s, and error compensation angle θ in the island protect situation of not moving ₀produce.

3. according to claim 1 for combining inverter group system island detection method, it is characterized in that: described error phase offset angle θ ₀value according to the structures shape of photovoltaic DC-to-AC converter cluster, the factor of consideration comprises the inverter quantity in combining inverter group system, and the distribution distance of each inverter, gets error phase offset angle θ ₀=K θ _m, K=K wherein _nk _l; K _nthe distribution distance shared ratio of inverter far away in expression system, its expression formula is wherein n represents total inverter number, and m represents the inverter number of units that inverter group system middle distance is public and site is far away, so K _n∈ (0,1); K _lrepresent inverter cluster overall distribution situation, if each inverter distributes, comparatively disperse, distance is public and site is distant, gets larger K _lvalue; For the combining inverter group system closely that distributes, K _lvalue is less, and its span is K _l∈ (0.1,1).