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

Abstract

The invention discloses an island detecting method for a photovoltaic power generation grid-connected inverter trunking system. When the island phenomenon happens in the inverter trunking system and the grid-connected point voltage frequency is reduced, the phase difference between the current and the voltage in a traditional sliding formwork frequency drift method is corrected, the error compensation angle is added, the effect of line impedance on the sliding formwork frequency drift method for detecting the blind area and the response speed is offset, and the reliability of island detecting is improved. If the grid-connected point voltage frequency is increased, the expression of the traditional sliding formwork frequency drift method is adopted, and the system line impedance cannot bring the adverse effect to the validity of island detecting.

Description

Island detection method for photovoltaic power generation grid-connected inverter group system

Technical field

The present invention relates to a kind of detection method that is applicable to combining inverter group system.

Background technology

Isolated island refers in grid-connected photovoltaic system, and because fault or maintenance cause grid cut-off, and photovoltaic generating system does not stop to local load supplying, forms the phenomenon of a power supply isolated island.The generation meeting of isolated island causes huge hidden danger to the stable operation of maintainer's safety and electrical network, and the isolated island detection technique of therefore studying microgrid has important practical significance.In recent years, photovoltaic generating system is gradually to high density, multi-access point future development, tradition island detection method, the validity of especially active method is subject to new challenge, and the isolated island detection technique that is applicable to photovoltaic combining inverter group system becomes the focus of current research.

Sliding formwork frequency drift method (SMS) is a kind of active local island detection method with positive feedback effect, and the ultimate principle of sliding formwork frequency drift method is inverter control output current and grid-connected point voltage same frequency, but constant phase place of leading voltage:

${\mathrm{\θ}}_{\mathrm{SMS}}={\mathrm{\θ}}_m\sin (\frac{\mathrm{\π}}{2}\·\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})$

Wherein, θ _mfor maximum phase skew, f _pCCfor grid-connected inverters point voltage frequency, f ₀for electrical network rated frequency 50Hz, f _mfor producing maximum phase skew θ _mtime frequency.When photovoltaic inverter grid-connected moves, θ _sMS=0, between electric current and voltage, there is no phase differential, the grid-connected dot frequency of system can not change yet; Once generation isolated island, inverter group system frequency is clamped down on by electrical network no longer, and due to the positive feedback effect of sliding formwork frequency method, grid-connected dot frequency finally can exceed setting threshold, the generation of isolated island phenomenon can be detected.

The inverter group system of multi-access point distributes and comparatively disperses, each inverter and the also line impedance between site can not be ignored, now, between the outlet voltage that inverter collects and grid-connected point voltage, there is phase differential, the current phase deviation theory value that causes inverter to be exported under the effect of mould frequency drift method, the current and phase difference and the θ that cause when line impedance _sMSduring opposite direction, can cause the positive feedback effect of mould frequency drift method (SMS) to reduce, affect the validity that isolated island detects.Wherein the most serious situation of impact is the phase angle that between the grid-connected point voltage of inverter group system and electric current, phase differential equals load, isolated island internal power mates completely, the pure resistive load situation of take is example, the system architecture while there is line impedance in two inverter parallel situations as shown in Figure 2, when load is pure when resistive, grid-connected point voltage when line impedance causes sliding formwork frequency drift method to lose efficacy and current waveform are as shown in Figure 3.

Figure 4 shows that the analysis of the detection blind area in the inverter group system that traditional sliding formwork frequency drift method can not ignore in line impedance, for the situation that system frequency f declines after isolated island, θ occur _sMS<0, current phase should lag behind grid-connected point voltage, and now line impedance can cause θ _sMSoffset, detect blind area and strengthen; θ after if isolated island occurs _sMS>0, the phase error that line impedance causes and θ _sMSequidirectional, detect blind area and reduce, line impedance can not reduce the validity that isolated island detects.In general, because system line impedance meeting causes the detection blind area of traditional SMS method, be offset, and there is rear frequency rising in isolated island or decline is random, therefore for a certain particular system, cannot judge that whether traditional SMS method is effective, reduce the reliability that isolated island detects.

Summary of the invention

When the object of the invention is to overcome existing sliding formwork frequency drift method and being applied in inverter group system, the shortcoming that line impedance causes island detection method validity to reduce, proposes a kind of new method that combining inverter group system isolated island detects that is applicable to.The present invention can effectively reduce system line impedance can not ignore the isolated island detection blind area in situation, improves reaction velocity.

The present invention for the technical scheme that solves its technical matters and adopt is: propose a kind of Novel island detection method, the method has been improved phasing degree θ in traditional mould frequency drift method _sMSthe method of expression formula, to reduce isolated island, detect blind area.When inverter group system generation isolated island, and when grid-connected point voltage frequency reduces, traditional sliding formwork frequency drift method electric current and voltage phase difference are revised, added error compensation angle, thereby offset the impact that inversion drift method detection blind area and reaction velocity cause, improve the reliability that isolated island detects.Phasing degree θ in the present invention _sMSexpression formula be:

${\mathrm{\&theta;}}_{\mathrm{SMS}}=\left\{\begin{array}{cc}{\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})& (f_{\mathrm{PCC}}>f_0)\\ {\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})-{\mathrm{\&theta;}}_0& (f_{\mathrm{PCC}}<f_0)\end{array}\right.$

In above formula: θ _mfor maximum phase skew, f _pCCfor grid-connected inverters point voltage frequency, f ₀for electrical network rated frequency 50Hz, f _mfor producing maximum phase skew θ _mtime frequency.Frequency rising situation f after occurring for isolated island _pCC> f ₀, phasing degree θ _sMSexpression formula identical with traditional mould frequency drift method.When photovoltaic inverter grid-connected moves, phasing degree θ _sMS=0, between electric current and voltage, there is no phase differential; Once isolated island phenomenon occurs, and grid-connected point voltage frequency is no longer subject to electrical network and clamps down on, if now grid-connected dot frequency has produced little increase, a θ _sMS> 0, and current phase will be ahead of voltage, and the time that causes electric current to arrive next zero crossing shortens, and system current/voltage under island state meets relation: it is grid-connected point voltage the electric current of only being exported by inverter group system determine with local loaded impedance Z, the time that therefore grid-connected point voltage arrives next zero crossing also will shorten, frequency rising, now phasing degree θ _sMSbecause positive feedback effect continues to increase, until grid-connected dot frequency is by exceeding the threshold value of setting, isolated island phenomenon can be detected; In like manner, when constantly grid-connected dot frequency occurs isolated island, occur declining, phasing degree θ _sMS< 0, also, by by the effect of positive feedback, frequency constantly declined, and the generation of isolated island phenomenon can be detected.

Frequency decline f after if isolated island phenomenon occurs _pCC< f ₀, at phasing degree θ _sMSbasis on deducted a constant angle θ ₀, and θ ₀>0, this angle is error phase offset angle, its effect is that to offset the inverter current phase place that line impedance causes leading, guarantees that the angle that electric current lags behind voltage reaches theoretical value.Error phase offset angle θ ₀value determined by inverter quantity in system and the distribution distance of Ge Tai inverter, in order to guarantee system power quality, error phase offset angle θ ₀should not surpass maximum phase skew θ _m, get:

θ ₀＝Kθ _m

K=K wherein _nk _l, K _nthe distribution distance shared ratio of inverter far away in expression system, its expression formula is:

$K_n=\frac{m}{n}$

Wherein n and m represent respectively total inverter number and the inverter number of units that inverter cluster middle distance is public and site is distant, known now K _n∈ (0,1); K _lrepresent inverter cluster overall distribution situation, when each inverter distributes, disperse, distance is public and site is distant, desirable larger K _lvalue, and for distributing combining inverter group system closely, K _lvalue can be less, and its span is K _l∈ (0.1,1).

The control strategy of error phase offset angle is as follows: when system detects after grid-connected dot frequency changes, start timing; because GB/T15945 regulation island protect should move in 0.2s after isolated island phenomenon occurs; therefore set when amplitude of variation reaches 0.5Hz, duration and reach 0.1s, and θ in the island protect situation of not moving ₀produce, effectively avoided system frequency disturbance to cause the situation of current phase acute variation.

The invention has the beneficial effects as follows, can suppress the impact of inverter group system line impedance on active alone island detection method validity, reduce sliding formwork frequency drift method existing the isolated island under line impedance cases to detect blind area, improve reaction velocity.

Accompanying drawing explanation

Fig. 1 improves the workflow diagram of rear SMS method;

Inverter cluster topology while there is line impedance in the pure resistive load situation of Fig. 2;

Grid-connected point voltage and current waveform when in the pure resistive load situation of Fig. 3, line impedance causes isolated island power to mate completely;

Fig. 4 tradition SMS method can not be ignored isolated island in situation in line impedance and detect blind area;

The isolated island that before and after Fig. 5 improves, SMS method be can not ignore in situation in inverter cluster line impedance detects blind area comparison.

Embodiment

Below in conjunction with the drawings and specific embodiments, further illustrate the present invention.

The present invention has improved phasing degree θ in traditional mould frequency drift method _sMSexpression formula, for:

${\mathrm{\&theta;}}_{\mathrm{SMS}}=\left\{\begin{array}{cc}{\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})& (f_{\mathrm{PCC}}>f_0)\\ {\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})-{\mathrm{\&theta;}}_0& (f_{\mathrm{PCC}}<f_0)\end{array}\right.$

θ wherein _mfor maximum phase skew, f _pCCfor grid-connected inverters point voltage frequency, f ₀for electrical network rated frequency 50Hz, f _mfor producing maximum phase skew θ _mtime frequency.Visible, phasing degree θ in frequency rising situation after isolated island occurs _sMSexpression formula is identical with traditional mould frequency drift method; If after isolated island occurs, frequency declines, at former phasing degree θ _sMSbasis on deduct a constant angle θ ₀, and θ ₀>0, this angle is error phase offset angle, its effect is that to offset the inverter current phase place that line impedance causes leading, guarantees that the angle that electric current lags behind voltage reaches theoretical value.

θ ₀value need be according to the structures shape of photovoltaic DC-to-AC converter cluster, the factor of consideration comprises the inverter quantity in system, and the distribution distance of each inverter, in order to guarantee system power quality, θ ₀should not surpass θ _m, get:

θ ₀＝Kθ _m

K=K wherein _nk _l, K _nthe distribution distance shared ratio of inverter far away in expression system, its expression formula is:

$K_n=\frac{m}{n}$

Wherein n represents total inverter number, and m represents the inverter number of units that inverter cluster middle distance is public and site is distant, known now K _n∈ (0,1); K _lrepresent inverter cluster overall distribution situation, when each inverter distributes, disperse, distance is public and site is distant, desirable larger K _lvalue, and for distributing system closely, K _lvalue can be less, and its span is K _l∈ (0.1,1).

Abbreviation θ _sMSexpression formula is as follows:

${\mathrm{\&theta;}}_{\mathrm{SMS}}=\left\{\begin{array}{cc}{\mathrm{\&theta;}}_m\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})& (f_{\mathrm{PCC}}>f_0)\\ {\mathrm{\&theta;}}_m[\sin (\frac{\mathrm{\&pi;}}{2}\&CenterDot;\frac{f_{\mathrm{PCC}}-f_0}{f_m-f_0})-K]& (f_{\mathrm{PCC}}<f_0)\end{array}\right.$

The isolated island that Figure 5 shows that the present invention and traditional mould frequency drift method detects blind area comparative result.As shown in Figure 5, in the situation that inverter group system exists line impedance, detection of the present invention blind area more traditional mould frequency drift method low frequency part reduces to some extent, is subject to line impedance to affect reduction, and HFS is owing to not affected by line impedance, can be not influenced after improvement yet.

In order to guarantee that the present invention can not affect the quality of power supply of electrical network under inverter group system is incorporated into the power networks situation, adopt following control strategy: when detecting after grid-connected dot frequency changes, start timing, when changing lasting time long enough, and the enough error phase offset angle θ greatly afterwards of side-play amount ₀produce, for example, can set when the grid-connected dot frequency decline situation duration and reach 0.1s, and in the island protect situation of not moving, produce θ ₀, effectively avoided system frequency disturbance to cause the situation of current phase acute variation.

Figure 1 shows that the workflow diagram of the inventive method.As shown in Figure 1, be furnished with the inverter of the inventive method in operational process, continue grid-connected point voltage frequency f _pCCsample and analyze, once grid-connected dot frequency changes, inverter judges grid-connected point voltage frequency f _pCCrise or decline, if f _pCC> f ₀, inverter will, according to traditional sliding formwork frequency drift method action, be controlled output current and grid-connected point voltage phase place

If grid-connected dot frequency f _pCC< f ₀, the DSP of inverter starts timing, and inverter continues to detect grid-connected dot frequency simultaneously, works as f _pCC< f ₀duration reaches 0.1s, and the generation isolated island that expresses possibility, and the decline of grid-connected dot frequency, now, control output current and grid-connected point voltage phase differential according to the inventive method

If f _pCC< f ₀duration fails to reach 0.1s, thinks that system disturbance causes grid-connected dot frequency to change, and can not add phase error compensation angle θ ₀.The final output current i=I of inverter _mcos (2 π f ₀t+ 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).