CN105182189A - Inverter disturbance type island detection method based on combination of voltage frequency and measured impedance - Google Patents

Abstract

The invention relates to the technical field of power system analysis and in particular relates to an inverter disturbance type island detection method based on combination of voltage frequency and measured impedance. The method is characterized by comprising the following steps: firstly analyzing island after-frequency reactive relation; controlling setting frequency-reactive feedback method inverters, and adding a feedback coefficient; if the sum of reactive power output by each inverter is not equal to zero, detecting whether an island operation state occurs or not according to the condition that whether the voltage frequency of a gird-connected inverter exceeds the limit of normal operation or not; otherwise, intensively injecting a voltage source signal at the point of common coupling of a system, wherein the signal is a single frequency signal, and the injection mode is three-phase injection; and detecting whether the island operation state occurs or not through the characteristic difference of system measurement impedance at the point of common coupling of the system. The island detection method provided by the invention can avoid failure caused by mutual interference of multiple disturbance signals in multi-inverter system scenes, and has the advantages of relatively high reliability, relatively high detection speed and simple and effective principle.

Description

Based on the inverter disturbance formula islanding detect method that electric voltage frequency and measurement impedance combine

Technical field

The present invention relates to Power System Analysis technical field, particularly relate to a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance.

Background technology

Worldwide, conventional fossil energy is increasingly exhausted, and problem of environmental pollution causes the concern of people day by day simultaneously, and therefore, national governments more and more pay attention to the development of regenerative resource.Photovoltaic system distributed power supply more and more accesses power distribution network, along with the progressively increase of distributed power source capacity, distribution system becomes many power supplys of power energy two-way flow and the complex network of Power System Interconnection from the radiant type network of a power uniflux.Occurred the problem of the grid-connected protection of distributed power generation thus, island protect is exactly one of them.

When the islet operation of distributed photovoltaic system refers to electrical network due to reason interruption of power supply such as electric fault, artificial origin or natural causes, photovoltaic system is failed to detect power down mode and is departed from electrical network, continue to system transmission of electric energy nearby, and the power supply isolated island of self-sufficiency that local load together formed does not control by public electric wire net.When distributed photovoltaic power generation system is in island operation state, can affect that the quality of power supply even damages electrical equipment, islet operation circuit continues the charged fault electric arc that can affect and extinguishes, make unsuccessful reclosing, serious time entail dangers to maintainer personal safety.Therefore; no matter be from the viewpoint of secure context or from reliability; unplanned island operation state is all unallowed; island operation state should be detected timely and effectively; and by photovoltaic DC-to-AC converter from system point of common coupling (PointofCommonCoupling; PCC) place disconnects, i.e. island protect.

Island detection method is divided into again distance type method, proactive and passive type method according to principle of work.Wherein, passive detection method is analyzed to exert oneself not mate with load nearby due to distributed power source after isolated island and is caused the electric parameters protect installation place change, and the realization of these class methods simply, but is exerted oneself at distributed power source and be there is check frequency with during load approximate match nearby; Active detecting method utilizes inverter to inject disturbance to system usually, response change before and after measuring system isolated island, can reduce or eliminate check frequency, but can the influential system quality of power supply.

The main direction of studying detected through inverter networking distributed generator islanding is at present active detection method, to reduce or to eliminate check frequency.But, along with system is to varying load, multi-inverter grid-connected development time various, in active method, the disturbing signal of each inverter interacts and causes islanding detect failure, therefore needs to carry out for the research of tradition based on the islanding detect method of the active detection method Problem of Failure of inverter disturbance.

Summary of the invention

In order to solve the problem, the technical scheme that the present invention takes is as follows:

Based on the inverter disturbance formula islanding detect method that electric voltage frequency and measurement impedance combine, it is characterized in that, said method comprising the steps of:

The idle relation of frequency before and after step 1, analysis isolated island;

Step 2, control arrange the inverter of frequency-idle feedback transmitter, and the reactive power disturbance of system chooses point of common coupling electric voltage frequency f _awith electrical network rated frequency f _gdifference, in order to make less frequency change obtain enough reactive disturbance to improve detection speed, in disturbance quantity, adding the feedback of frequency, add a feedback factor k, then inverter export reactive power Q _inv=k (f _a-f _g);

If the reactive power sum ∑ Q that each inverter of step 3 exports _inv≠ 0, then exceed the normal restriction run according to the electric voltage frequency of combining inverter, detect island operation state, if electric voltage frequency does not exceed the normal restriction run, system is not in island operation state; If the reactive power sum ∑ Q that each inverter exports _inv=0, then exceed according to the electric voltage frequency of combining inverter the restriction whether normally run and detect whether occur that the detection method of island operation state lost efficacy, perform step 4;

Step 4, concentrate injection voltage source signal at system point of common coupling place, this signal is single-frequency signals, injection mode is that three-phase injects, the data of the injecting voltage that acquisition system point of common coupling place records and current-responsive, after data processing, calculate to obtain the measurement impedance at system point of common coupling place; The impedance Z of usual electrical network _girdmuch smaller than the impedance Z of distributed power source _dGwith the impedance Z of load _load,

Z _Grid＜＜Z _DG，Z _Grid＜＜Z _load

$Z_{Islanding}=\frac{Z_{DG}{Z}_{load}}{Z_{DG}+Z_{load}}>>Z_{Grid}$

Before isolated island, the measurement impedance Z at system point of common coupling place _measfor:

$Z_{meas}=\frac{Z_{Grid}{Z}_{Islanding}}{Z_{Grid}+Z_{Islanding}}<Z_{Grid}$

After isolated island, the measurement impedance Z ' at system point of common coupling place _measfor:

Z′ _meas＝Z _Islanding＞＞Z _meas

Z _islandingrepresent the systematic survey impedance after isolated island generation, carry out impedance measurement based on impedance characteristic difference before and after above isolated island and carry out islanding detect.

In described step 3, exceed according to the electric voltage frequency of combining inverter the restriction whether normally run and detect whether occur that the mechanism that the detection method of island operation state lost efficacy is: the method exports corresponding reactive power according to the change of system real-time frequency, what make due to reactive power after isolated island is unbalanced, the electric voltage frequency of islanded system amplifies because of positive feedback and exceeds the normal restriction run, thus detects island operation state; But because local load is also real-time change, load change has randomness, if the idle change of local load and photovoltaic system islanding detect detect the just counteracting of the reactive power change exported, now this detection method lost efficacy; In addition, in multi-inverter or multi-computer cluster system, because of inverter One's name is legion, the reactive disturbance that each inverter exports can mutually be cut down or offset, and this detection method at this moment is also inefficacy.

Beneficial effect

(1) analyze and demonstrate active class method islanding detect lost efficacy causative mechanism, for avoid islanding detect lost efficacy specific aim reference is provided;

(2) because adopting centralized characteristic frequency disturbing signal to inject, the method does not rely on inverter, does not limit to by power supply type;

(3) under a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance proposed can avoid multiple inverter system scene, many disturbing signals disturb inefficacy mutually, have higher reliability;

(4) do not need frequency departure to be amplified to the process of action threshold value gradually, there is higher detection speed; Principle is simply effective.

Accompanying drawing explanation

Fig. 1 is the electrical block diagram of grid-connected inverter system.

Fig. 2 is the control structure figure of three-phase frequency-idle feedback islanding detect method.

Fig. 3 is the equivalent schematic diagram of anti-islanding detect dead methods of the present invention.

Fig. 4 a-4b is according to inverter exit potential frequency under specific embodiment of the invention normal operation and exports idle oscillogram.

Fig. 5 a-5b is isolated island situation lower frequency according to specific embodiment of the invention application three-phase frequency-idle feedback transmitter and idle oscillogram.

Fig. 6 a-6b is according to specific embodiment of the invention islanding detect failure conditions lower frequency and idle oscillogram.

Fig. 7 is the time domain on-line measurement impedance characteristic Dependence Results figure applying anti-inefficacy island detection method according to the specific embodiment of the invention.

The process flow diagram of a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance of Fig. 8.

Embodiment

Below in conjunction with accompanying drawing, the present invention is elaborated.The process flow diagram of a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance of Fig. 8.

Judge whether photovoltaic system is in island operation state according to the amplitude of the output voltage of combining inverter and frequency.In order to improve the benefit of photovoltaic system, general photovoltaic system only sends active power, under namely working in unity power factor.When photovoltaic system is incorporated to bulk power grid, the reference voltage of inverter output end is clamping down on by bulk power grid, is rated operational voltage.For three-phase balanced system, combining inverter output power is:

$\left\{\begin{array}{c}{P}_{inv}=\frac{3}{2}(u_d{i}_d+u_q{i}_q)\\ Q_{inv}=\frac{3}{2}(u_q{i}_d-u_d{i}_q)\end{array}\right.---\left(1\right)$

In formula, P _invfor active power, Q that inverter exports _invfor the reactive power that inverter exports, u _d, u _qand i _d, i _qbe respectively photovoltaic system inverter export three-phase voltage and current transformation to the voltage and current under two-phase rotating coordinate system, because the d axle of d-q coordinate system is oriented to line voltage vector, now have u _q=0:

$\left\{\begin{array}{c}{P}_{inv}=\frac{3}{2}{u}_d{i}_d\\ Q_{inv}=-\frac{3}{2}{u}_d{i}_q\end{array}\right.---\left(2\right)$

When photovoltaic system is incorporated into the power networks, u _d, u _qconstant, under power limitation control strategy, the active power that photovoltaic system exports and reactive power corresponding i respectively _d, i _q.Therefore, during research island protect, combining inverter is approximately local load and is the RLC load of parallel connection and the current source under operating in specific power.The schematic circuit of grid-connected inverter system as shown in Figure 1.

When K1, K2 close, inverter power supply is incorporated into the power networks, then have:

$P_{load}=\frac{3U_g^2}{R}=P_{inv}+\mathrm{\&Delta;}P---\left(3\right)$

$Q_{load}=3U_g^2(\frac{1}{2{\mathrm{\&pi;f}}_{g}L}-2{\mathrm{\&pi;f}}_{g}C)=Q_{inv}+\mathrm{\&Delta;}Q---\left(4\right)$

P in formula _loadfor active power, Q that load when system is normally run absorbs _loadfor the reactive power that load when system is normally run absorbs; P _invfor active power, Q that inverter exports _invfor the reactive power that inverter exports; Δ P is that matching degree, the local load of Δ Q and generating are not reactive power not matching degree for local load and electricity generation system active power; U _gfor output end voltage amplitude, f when grid-connected inverters runs _gfor output terminal frequency when grid-connected inverters runs.

When after K2 tripping, local load is powered separately by combining inverter.When combining inverter works in power limitation control pattern, after islet operation, the voltage U of local load end _island, local load end frequency f _island, P ' _loadload consumption active power, Q ' _loadload consumption reactive power.Now, then have:

$P_{load}^{\&prime;}=\frac{3U_{island}^2}{R}=P_{inv}---\left(5\right)$

$Q_{load}^{\&prime;}=3U_{island}^2(\frac{1}{2{\mathrm{\&pi;f}}_{island}L}-2{\mathrm{\&pi;f}}_{island}C)=Q_{inv}---\left(6\right)$

Local load terminal voltage U when calculating islet operation by formula (3) _island, that is:

$U_{island}=\sqrt{\frac{P_{inv}R}{3}}---\left(7\right)$

Combining inverter runs on specific power condition, i.e. Q _invwhen=0, after islet operation occurs, inverter and electrical network neither provide reactive power to load, the frequency f of local load end when calculating islet operation according to formula (4) _island, that is:

$f_{island}=\frac{1}{2\mathrm{\&pi;}\sqrt{LC}}---\left(8\right)$

Therefore, local load and electricity generation system active power not matching degree Δ P and U _islandpass be:

$\frac{\mathrm{\&Delta;}P}{P_{inv}}={\left(\frac{U_g}{U_{island}}\right)}^2-1---\left(9\right)$

Local load and generating are reactive power not matching degree Δ Q and f _islandpass be

$\frac{\mathrm{\&Delta;}Q}{P_{inv}}={\left(\frac{U_g}{U_{island}}\right)}^2{Q}_f(\frac{f_{island}}{f_g}-\frac{f_g}{f_{island}})---\left(10\right)$

In formula, for the quality factor of load.

When inverter and electrical network disconnect, if the active-power P that inverter exports _inv, inverter export reactive power Q _invdo not mate with the power needed for load, when namely Δ P and Δ Q is not equal to 0, release from formula (9) and (10), the voltage that inverter exports and frequency all will change.If after inverter and electrical network disconnect, the active-power P that inverter exports _inv, inverter export reactive power Q _invalmost equal with load power demand, i.e. Δ P=0, Δ Q=0, substitute into formula (9) and (10) and obtain U _island=U _g, f _island=f _g.After the reactive power disturbance of system chooses the difference islet operation of PCC electric voltage frequency and electrical network rated frequency, PCC electric voltage frequency is relevant with the reactive power that inverter exports, therefore change PCC electric voltage frequency by the output that disturbance inverter is idle, judge whether grid-connected inverter system exists islet operation with this.(not mentioning frequency and idle relation)

Active frequency drift method (ActiveFrequencyDrift, AFD) be the one of active alone island detection method, the principle of work of single phase frequency deflection method by frequency disturbance, the frequency of PCC voltage is offset, owing to clamping down on by bulk power grid before isolated island, frequency keeps 50Hz, the regulating action of bulk power grid is lost after isolated island, the scope that frequency shift (FS) goes out to allow, isolated island is judged with this, method realizes simple, isolated island discrimination is high, but when local quality factor are higher, easily cause and detect unsuccessfully.In the islanding detect of three-phase frequency shift (FS), adopt the reference current based on d-q conversion control method to be DC quantity, cannot directly be offset by the frequency and the frequency of phase place to electric current that change reference current.In island detection method now for three-phase frequency shift (FS), relatively more conventional is three-phase reactive disturbance method, by reactive power or current disturbing, and the frequency shift (FS) of PCC voltage after bulk power grid is disconnected.In three-phase reactive disturbance method, there are reactive power disturbance and reactive current disturbance, respectively for power limitation control and constant current controlling.

The reactive power disturbance of system chooses the difference of PCC electric voltage frequency and electrical network rated frequency, enough reactive disturbance are obtained to improve detection speed in order to make less frequency change, the feedback of frequency is added in disturbance quantity, add a feedback factor, i.e. three-phase frequency-idle feedback transmitter, as shown in Figure 2, its expression formula is as follows for its control structure figure:

Q _inv＝k(f _a-f _g)(11)

In formula, k is frequency feedback coefficient; f _afor the frequency of PCC voltage; f _gfor line voltage rated frequency, 50Hz.

The active power adopting power limitation control method control photovoltaic system to export and reactive power are definite value, and the grid-connected point voltage of photovoltaic system is rated voltage by clamping down on of Iarge-scale system, the voltage U at the PCC place under the two-phase rotating coordinate system formed after d-q conversion _dpcc, U _qpccalso be steady state value, draw reference current thus reference current introduce two current status feedback quantity ω Li _dwith ω Li _qrealize decoupling zero, generating reference voltage U _d, reference voltage U _q, then through dq-abc conversion, generate the signal wave of PWM, the voltage and current needed for being exported by PWM control inverter.

After islet operation occurs; local load and inverter output power matching degree higher time; the change of PCC electric voltage frequency is very little; due to the feedback of pull-in frequency; connect the idle skew that reactance or filter capacitor produce in the reactive disturbance of load and inverter and all can become large due to the effect of feedback; until frequency shift (FS) exceeds islanding detect scope, island protect action.

Three-phase frequency-idle feedback transmitter exports corresponding reactive power according to the change of system real-time frequency, and what make due to reactive power after isolated island is unbalanced, and the electric voltage frequency of islanded system exceeds the normal restriction run, thus detects island operation state.But because local load is also real-time change, load change has randomness, if the idle change of local load and photovoltaic system islanding detect detect the just counteracting of the reactive power change exported, now active detecting method lost efficacy; In multi-inverter or multi-computer cluster system, because of inverter One's name is legion, the reactive disturbance that each inverter exports can mutually be cut down or offset, be i.e. ∑ Q _inv=0, now active detecting method lost efficacy.

A kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance, equivalent constructions signal is as Fig. 3, be different from traditional inverter machine end perturbation scheme, and adopt the concentrated disturbing signal injection at PCC place, Injection Signal is voltage signal, this signal is specific single-frequency, and injection mode is that three-phase injects.

The impedance Z of usual electrical network _girdmuch smaller than the impedance Z of distributed power source _dGwith the impedance Z of load _load,

Z _Grid<<Z _DG,Z _Grid<<Z _load(12)

$Z_{Islanding}=\frac{Z_{DG}{Z}_{load}}{Z_{DG}+Z_{load}}>>Z_{Grid}---\left(13\right)$

Before isolated island, the measurement impedance Z at PCC place _measfor:

$Z_{meas}=\frac{Z_{Grid}{Z}_{Islanding}}{Z_{Grid}+Z_{Islanding}}<Z_{Grid}---\left(14\right)$

After isolated island, the measurement impedance Z ' at PCC place _measfor:

Z′ _meas＝Z _Ιslanding>>Z _meas(15)

Formula (14)-(15) show, Z ' _meas> > Z _meas, carry out impedance measurement islanding detect based on this impedance characteristic difference.The injecting voltage that before and after isolated island, the measurement impedance at PCC place is recorded by acquisition system point of common coupling and the data of current-responsive, calculate after data processing.

The technique effect of a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance of the present invention is below described by more concrete example.Use grid-connected inverter system circuit model as shown in Figure 1 in this example.Photovoltaic DC-to-AC converter have employed shown in Fig. 2 and controls.

Below control according to formula (11) and Fig. 2, the emulation carried out under matlab/simulink is implemented, simulation parameter: photovoltaic system is be incorporated to 220kV Infinite bus system through boosting after 3 photovoltaic DC-to-AC converter parallel connections, active power exports as 3*100kW, idle output is decided by difference on the frequency, Q _inv=k (f _a-50) Var, wherein feedback factor k=-380 ².RLC load is also connected on 10.5kV bus: active load P _load=300kW, Q _load.L=150kVar, Q _load.C=-152kVar.There is isolated island when 0.2s in system, emulation duration is 1.2s.Because three grid-connected photovoltaic DC-to-AC converters are equivalences completely, and PCC frequency is identical with inverter outlet place frequency, therefore only investigate the frequency of the voltage that No. 1 inverter exports.Its simulation result as shown in Figure 4.

Figure 4 shows that the waveform of the frequency of the voltage at the inverter outlet place (PCC is identical with it) when not having active detecting method and the reactive power of inverter output.Be subject to the impact of photovoltaic system control strategy, at about 0.4s, the reactive power that photovoltaic system exports just tends towards stability, and be namely from now, system cloud gray model is in steady state (SS).Because the power match degree that the power of inverter output and load consume is higher, after bulk power grid departs from, during photovoltaic system islet operation, the power that photovoltaic system exports can be dissolved by load just completely, and system does not fluctuate substantially, can normally run.Now, the islet operation energy long period continues.But be unfavorable for the personal safety of maintainer and the safety of power system device.

Figure 5 shows that the isolated island situation lower frequency of the present invention's application three-phase frequency-idle feedback transmitter and idle oscillogram.After employing is such as formula the three-phase frequency shown in (11)-idle feedback transmitter photovoltaic islanding detect strategy, other simulated conditions are constant, and simulation result as shown in Figure 5.The frequency of system just drops to below 49.5Hz in 0.2s after isolated island, and the island operation state of photovoltaic system can be detected faster.The reactive power that photovoltaic system exports rises along with the decline of frequency, and frequency is declined further.Learn thus, three-phase frequency-idle feedback transmitter is the islanding detect that can be applicable to photovoltaic system.

Three-phase frequency-idle feedback transmitter exports corresponding reactive power according to the change of system real-time frequency, and what make due to reactive power after isolated island is unbalanced, and the electric voltage frequency of islanded system exceeds the normal restriction run, thus detects island operation state.But, because local load is also real-time change, load change has randomness, if the idle change of local load and photovoltaic system islanding detect detect the just counteracting of the reactive power change exported, now this detection method is in the multi-inverter or multi-computer cluster system lost efficacy, because of inverter One's name is legion, the reactive disturbance that each inverter exports can mutually be cut down or offset, and now this detection method lost efficacy.

The emulation of failure Mechanism is carried out, simulation parameter: photovoltaic system is be incorporated to 220kV Infinite bus system through boosting after 3 photovoltaic DC-to-AC converter parallel connections, and active power exports as 3*100kW, and idle output is decided by difference on the frequency, Q under matlab/Simulink _inv=k (f _a-50) Var, wherein feedback factor k=-380 ².RLC load is: active load is also connected on 10.5kV bus: P _load=300kW, Q _load.L=150kVar, Q _load.C=-152kVar.There is isolated island when 0.2s in system, emulation duration is 1.2s.In order to simulate the real-time change of local load reactive power, the idle value of feedback of one of them photovoltaic system is set as, and two other idle value of feedback obtains negative sum, that is: Q _inv.3=-2*k (f _a-50) Var.PCC frequency is identical with inverter outlet place frequency, therefore investigates the frequency of the voltage that No. 1 inverter exports and idle.When becoming when Figure 6 shows that load there is the simulation result figure detecting dead band in three-phase frequency-idle feedback transmitter.

Learnt by Fig. 6, when the reactive power equilibrium that the photovoltaic system of varying duty at that time and feedback exports or each inverter export reactive disturbance mutual Interference Cancellation, to there is dead band in three-phase frequency-idle feedback transmitter, the frequency of voltage will fluctuate near 50Hz, can not exceed the threshold value of frequency.The idle of photovoltaic system output is also near 0 also along with the change of frequency changes by a small margin, and the idle sum of three photovoltaic systems is close to 0.

A kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance that application the present invention proposes, adopts the theory structure shown in Fig. 3, with under failure Mechanism emulates same condition and parameter above, carries out enforcements and verifies.Time domain on-line measurement impedance plot when stable state and isolated island, as shown in Figure 7.

As seen from Figure 7, the method has higher accuracy of detection, and have more significant impedance characteristic amount difference before and after isolated island, therefore the method also has higher sensitivity.Meanwhile, Fig. 7 shows, the method has higher detection speed, namely island state detected fast after isolated island in 15ms.

Therefore, by adopting a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance of the present invention, analyzing and demonstrating the causative mechanism that active class method islanding detect lost efficacy, losing efficacy for avoiding islanding detect and specific aim reference is provided; Because adopting centralized disturbing signal to inject, the method does not rely on inverter, does not limit to by power supply type, and under avoiding multiple inverter system scene, many disturbing signals disturb inefficacy mutually, has higher reliability; Do not need frequency departure to be amplified to the process of action threshold value gradually, there is higher detection speed; Principle is simply effective.

It should be noted that; above-mentioned embodiment is only the present invention's preferably embodiment; can not limiting the scope of the invention be understood as, not depart under concept thereof of the present invention, all protection scope of the present invention is belonged to modification to any subtle change that the present invention does.

Claims (2)

1., based on the inverter disturbance formula islanding detect method that electric voltage frequency and measurement impedance combine, it is characterized in that, said method comprising the steps of:

Step 1, analyze the relation of the reactive power that point of common coupling place electric voltage frequency and inverter export before and after isolated island, draw by the idle output of disturbance inverter to change the frequency of point of common coupling;

Step 2, control arrange the inverter of frequency-idle feedback transmitter, and the reactive power disturbance of system chooses point of common coupling electric voltage frequency f _awith electrical network rated frequency f _gdifference, in order to make less frequency change obtain enough reactive disturbance to improve detection speed, in disturbance quantity, adding the feedback of frequency, add a feedback factor k, then inverter export reactive power Q _inv=k (f _a-f _g);

If the reactive power sum ∑ Q that each inverter of step 3 exports _inv≠ 0, then exceed the normal restriction run according to the electric voltage frequency of combining inverter, detect island operation state, if electric voltage frequency does not exceed the normal restriction run, system is not in island operation state; If the reactive power sum ∑ Q that each inverter exports _inv=0, then whether exceed the normal restriction run according to the electric voltage frequency of combining inverter and judge whether system occurs that the detection method of island operation state lost efficacy at this moment (a little unclear and coherent), perform step 4;

Step 4, concentrate injection voltage source signal at system point of common coupling place, this signal is single-frequency signals, injection mode is that three-phase injects, the data of the injecting voltage that acquisition system point of common coupling place records and current-responsive, after data processing, calculate to obtain the measurement impedance at system point of common coupling place; The impedance Z of usual electrical network _girdmuch smaller than the impedance Z of distributed power source _dGwith the impedance Z of load _load,

Z _Grid<<Z _DG,Z _Grid<<Z _load

$Z_{Islanding}=\frac{Z_{DG}{Z}_{load}}{Z_{DG}+Z_{load}}>>Z_{Grid}$

Before isolated island, the measurement impedance Z at system point of common coupling place _measfor:

$Z_{meas}=\frac{Z_{Grid}{Z}_{Islanding}}{Z_{Grid}+Z_{Islanding}}<Z_{Grid}$

After isolated island, the measurement impedance Z ' at system point of common coupling place _measfor:

Z′ _meas＝Z _Ιslanding>>Z _meas

Z _islandingrepresent the systematic survey impedance after isolated island generation, carry out impedance measurement based on impedance characteristic difference before and after above isolated island and carry out islanding detect.

2. according to a kind of inverter disturbance formula islanding detect method combined based on electric voltage frequency and measurement impedance described in claim 1, it is characterized in that, in described step 3, exceed according to the electric voltage frequency of combining inverter the restriction whether normally run and detect whether occur that the mechanism that the detection method of island operation state lost efficacy is: the method exports corresponding reactive power according to the change of system real-time frequency, what make due to reactive power after isolated island is unbalanced, the electric voltage frequency of islanded system amplifies because of positive feedback and exceeds the normal restriction run, thus detect island operation state, but because local load is also real-time change, load change has randomness, if the idle change of local load and photovoltaic system islanding detect detect the just counteracting of the reactive power change exported, now this detection method lost efficacy, in addition, in multi-inverter or multi-computer cluster system, because of inverter One's name is legion, the reactive disturbance that each inverter exports can mutually be cut down or offset, and this detection method at this moment is also inefficacy.