CN102998559B - Island detection method, device and electric system - Google Patents

Abstract

The embodiment of the present invention provides a kind of island detection method, device and electric system.Method comprises: the electric voltage frequency in the circuit form inverter, virtual reactive load and network load detects, and described virtual reactive load carries out energy exchange and not consumed energy; If the described electric voltage frequency detected exceeds setting range, then determine that described inverter, described virtual reactive load and described network load form isolated island.Realization can not have an impact to grid-connected current quality, thus ensures the normal operation of electric system.

Description

Island detection method, device and electric system

Technical field

The present invention relates to electric and electronic technical field, particularly relate to a kind of island detection method, device and electric system.

Background technology

Under electrical network normal operation, by main power supply system and the common peripherad load supplying of distributed power source, when main power supply system fault or maintenance, after associated switchgear disconnects, independently powered to the load by distributed power source.After main distribution system disconnects, distributed power source forms a little island network together with load, is called isolated island.Isolated island is a part for electrical network, still keeps a kind of state run when other spaced-apart with electrical network.Isolated island can be supplied to load due to abnormal voltage or frequency and cause consumer to damage, and the series of problems such as distribution system misoperation, also may cause personnel to get an electric shock, therefore need to detect isolated island.

Existing island detection method, by inverter to load harmonic electric current, when inverter is connected with electrical network with load, the frequency of inverter output end can not change, when inverter and load and electrical network disconnect, the frequency of inverter output end can offset, thus can detect the generation of isolated island.But this method can impact grid-connected current quality, affect the normal operation of electric system.

Summary of the invention

The embodiment of the present invention provides a kind of island detection method, inverter and electrical network.Avoid impacting grid-connected current quality to carry, ensure the normal operation of electric system.

First aspect, the embodiment of the present invention provides a kind of island detection method, comprising:

Electric voltage frequency in the circuit form inverter, virtual reactive load and network load detects, and described virtual reactive load carries out energy exchange and not consumed energy;

If the described electric voltage frequency detected exceeds setting range, then determine that described inverter, described virtual reactive load and described network load form isolated island.

In a kind of embodiment of first aspect, the lagging reactive power that described virtual reactive load absorbs increases along with the increase of detected described electric voltage frequency; Or the capacitive reactive power that described virtual reactive load absorbs increases along with the reduction of detected described electric voltage frequency.

In conjunction with first aspect and the first embodiment thereof, in the second embodiment of first aspect, if the described electric voltage frequency detected is the ratings of mains frequency, then the reactive power that described virtual reactive load absorbs is 0, and the reactive power sent is 0.

In conjunction with first aspect and the first embodiment thereof and the second embodiment, in the third embodiment of first aspect, described to inverter, virtual reactive load and network load form circuit in electric voltage frequency detect before, also comprise: produce described virtual reactive load by described inverter.

In conjunction with in first aspect and the first embodiment, the second embodiment and the third embodiment, in the 4th kind of embodiment of first aspect, the electric current that described virtual reactive load absorbs is: iq=id × (f-f _n) × k; Wherein, id is the active current that described inverter sends, and f is detected described electric voltage frequency, f _nfor the ratings of mains frequency, k is scale-up factor, and iq is the electric current that described virtual reactive load absorbs.

Second aspect, the embodiment of the present invention provides a kind of isolated island detecting device, comprising:

Detection module, detects for the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, and described virtual reactive load carries out energy exchange and not consumed energy;

Determination module, if exceed setting range for the described electric voltage frequency of described detection module detection, then determines that described inverter, described virtual reactive load and described network load form isolated island.

In a kind of embodiment of second aspect, the lagging reactive power that described virtual reactive load absorbs increases along with the increase of detected described electric voltage frequency; Or the capacitive reactive power that described virtual reactive load absorbs increases along with the reduction of detected described electric voltage frequency.

In conjunction with second aspect and the first embodiment thereof, in the second embodiment of second aspect, if the described electric voltage frequency that described detection module detects is the ratings of mains frequency, then the reactive power that described virtual reactive load absorbs is 0, and the reactive power sent is 0.

In conjunction with second aspect and the first embodiment thereof and the second embodiment, in the third embodiment of second aspect, described virtual reactive load is produced by described inverter.

In conjunction with in second aspect and the first embodiment, the second embodiment and the third embodiment, in the 4th kind of embodiment of second aspect, the electric current that described virtual reactive load absorbs is: iq=id × (f-f _n) × k; Wherein, id is the active current that described inverter sends, and f is detected described electric voltage frequency, f _nfor the ratings of mains frequency, k is scale-up factor, and iq is the electric current that described virtual reactive load absorbs.

The third aspect, the embodiment of the present invention also provides a kind of electric system, comprising: electrical network, at least one inverter, network load, at least one virtual reactive load, network load and the isolated island detecting device provided as the embodiment of the present invention.

In a kind of embodiment of the third aspect, described isolated island detecting device and described inverter integrate.

The island detection method that the embodiment of the present invention provides, device and electric system, virtual reactive load is produced in electrical network, detected by the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, the electric voltage frequency realized when detecting exceeds setting range, then determine that inverter, virtual reactive load and network load form isolated island.Because virtual reactive load is not when the circuit that electrical network and inverter, virtual reactive load and network load form disconnects, virtual reactive load is absorbing reactive power neither, does not also send reactive power, therefore, can not have an impact to grid-connected current quality, thus ensure the normal operation of electric system.

Accompanying drawing explanation

In order to be illustrated more clearly in the embodiment of the present invention or technical scheme of the prior art, introduce doing one to the accompanying drawing used required in embodiment or description of the prior art simply below, apparently, accompanying drawing in the following describes is some embodiments of the present invention, for those of ordinary skill in the art, under the prerequisite not paying creative work, other accompanying drawing can also be obtained according to these accompanying drawings.

Fig. 1 is the process flow diagram of an island detection method provided by the invention embodiment;

The model schematic of the inverter disengaging electrical network islet operation that Fig. 2 provides for the embodiment of the present invention;

Fig. 3 is the structural representation of an isolated island detecting device provided by the invention embodiment;

Fig. 4 is the structural representation of the unified embodiment of power train provided by the invention.

Embodiment

For making the object of the embodiment of the present invention, technical scheme and advantage clearly, below in conjunction with the accompanying drawing in the embodiment of the present invention, technical scheme in the embodiment of the present invention is clearly and completely described, obviously, described embodiment is the present invention's part embodiment, instead of whole embodiments.Based on the embodiment in the present invention, those of ordinary skill in the art, not making the every other embodiment obtained under creative work prerequisite, belong to the scope of protection of the invention.

Fig. 1 is the process flow diagram of an island detection method provided by the invention embodiment, and as shown in Figure 1, the method comprises:

Electric voltage frequency in S101, the circuit that forms inverter, virtual reactive load and network load detects, and virtual reactive load carries out energy exchange and not consumed energy.

If the electric voltage frequency that S102 detects exceeds setting range, then determine that inverter, virtual reactive load and network load form isolated island.

Above-mentioned steps can be performed by the pick-up unit be arranged in electrical network, or, can also be performed by the inverter in electrical network.

Wherein, network load can be the loadcarrying part in electrical network, such as: RLC(resistance, inductance, capacity; Resistance, inductance, electric capacity) load etc.Virtual reactive load can be the module that can absorb and send reactive power be arranged in electrical network; Or virtual reactive load can also be produced by the inverter in electrical network.

Virtual reactive load can be only carry out energy exchange and not catabiotic a kind of control module.When electric system normally works, electrical network is connected with inverter, virtual reactive load and network load.Under this enforcement scene, the electric voltage frequency of the circuit of inverter, virtual reactive load and network load composition is in steady state (SS), this electric voltage frequency is generally the ratings of mains frequency, therefore, the electric voltage frequency detected is in the scope of setting usually, and this setting range is the normal range of operation of mains frequency.Under this enforcement scene, due to virtual reactive load neither absorbing reactive power also do not send reactive power, that is, the reactive power that virtual reactive load absorbs is 0, and the reactive power sent is 0.Therefore, grid-connected current quality can not be affected, thus ensure the normal operation of electric system.

When electric system is broken down or overhauls, electrical network and inverter, virtual reactive load and network load disconnect.Under this enforcement scene, the electric voltage frequency of the circuit of inverter, virtual reactive load and network load composition plays pendulum, and the electric voltage frequency detected departs from the scope of setting usually, namely departs from the normal range of operation of mains frequency.Therefore, when detecting that the electric voltage frequency of the circuit that inverter, virtual reactive load and network load form exceeds setting range, then determine that inverter, virtual reactive load and network load form isolated island.And under this enforcement scene, the lagging reactive power that virtual reactive load absorbs can increase along with the increase of detected electric voltage frequency; Or the capacitive reactive power that virtual reactive load absorbs can increase along with the reduction of detected electric voltage frequency.

Optionally, embodiments provide a kind of embodiments possible of virtual reactive load, the electric current that this virtual reactive load absorbs can be: iq=id × (f-f _n) × k;

Wherein, id is the active current that inverter sends, and f is detected electric voltage frequency, f _nfor the ratings of mains frequency, k is scale-up factor, and iq is the electric current that virtual reactive load absorbs.

Concrete, when electrical network is connected with inverter, virtual reactive load and network load, the electric voltage frequency f detected can be identical with mains frequency ratings, therefore, and f-f _n=0, now reactive load neither absorb idle do not send yet idle; When electrical network and inverter, virtual reactive load and network load disconnect, when inverter, virtual reactive load and network load form isolated island, the electric voltage frequency f detected can increase or reduce, when f increases, load absorbs lagging reactive power to be increased, when f reduces, load absorbs capacitive reactive power to be increased.

The inverter that Fig. 2 embodiment of the present invention provides departs from the model schematic of electrical network islet operation, as shown in Figure 2, an inverter is comprised with system, network load comprises a resistance (R), an inductance (L) and electric capacity (C) for example and is described, produce virtual reactive load by inverter, and carry out islanding detect.

LC load resonant frequency is 50Hz, and active power reactive power is mated completely.In order to make inverter islet operation, then have:

$P=\frac{{\mathrm{Ug}}^2}{R}---\left(1\right)$

Q=0

Wherein, P is the active power that inverter exports, and Q is the reactive power that inverter exports.

Inverter produces reactive current disturbance, i.e. virtual reactive load:

Δiq=id×(f-50)×k（2）

Wherein, id is active current, and f is the electric voltage frequency of the circuit of inverter, virtual reactive load and the network load composition that inverter detects, k is scale-up factor.

So also the voltage phasor of site is:

$\stackrel{\·}{U}g=(\mathrm{id}+j\×\mathrm{iq})\×(R//\mathrm{j\ωL}//\frac{1}{\mathrm{j\ωc}})$

$=\sqrt{{\mathrm{id}}^2+{\mathrm{iq}}^2}\∠a\tan \frac{\mathrm{iq}}{\mathrm{id}}\×\frac{\mathrm{R\ωL}}{\sqrt{{\left(\mathrm{\ωL}\right)}^2+{({\mathrm{R\ω}}^2\mathrm{LC}-R)}^2}}\∠-a\tan \frac{{\mathrm{R\ω}}^2\mathrm{LC}-R}{\mathrm{\ωL}}---\left(3\right)$

$=\sqrt{{\mathrm{id}}^2+{\mathrm{iq}}^2}\×\frac{\mathrm{R\ωL}}{\sqrt{{\left(\mathrm{\ωL}\right)}^2+{({\mathrm{R\ω}}^2\mathrm{LC}-R)}^2}}\∠(a\tan \frac{\mathrm{iq}}{\mathrm{id}}-a\tan \frac{{\mathrm{R\ω}}^2\mathrm{LC}-R}{\mathrm{\ωL}})$

Wherein, ω is the voltage angle speed of the circuit of inverter, virtual reactive load and network load composition:

ω=2πf

Then the phase offset of grid-connected point voltage is:

$\mathrm{\Δ\θ}=a\tan \frac{\mathrm{iq}}{\mathrm{id}}-a\tan \frac{{\mathrm{R\ω}}^2\mathrm{LC}-R}{\mathrm{\ωL}}---\left(4\right)$

Due to except reactive current disturbance quantity, inverter does not export idle, therefore:

iq=Δiq=id×(f-50)×k（5）

In addition, the resonance frequency of LC is 50Hz, therefore:

${\mathrm{\ω}}_0=\frac{1}{\sqrt{\mathrm{LC}}}=2{\mathrm{\πf}}_N=2\mathrm{\π}\×50---\left(6\right)$

The quality factor of load are:

$Q_f=R\sqrt{C/L}=R/\left({\mathrm{\ω}}_{0}L\right)---\left(7\right)$

In conjunction with (4), (5), (6) and (7), the phase offset that can obtain grid-connected point voltage is:

$\mathrm{\Δ\θ}=a\tan [(f-50)\×k]-a\tan [Q_f\×(\frac{2\mathrm{\πf}}{{\mathrm{\ω}}_0}-\frac{{\mathrm{\ω}}_0}{2\mathrm{\πf}})]$

$=a\tan [(f-50)\×k]-a\tan [Q_f\×(\frac{f}{50}-\frac{50}{f})]$

Order:

α ₁(f)=(f-50)×k

${\mathrm{\α}}_2\left(f\right)=Q_f\×(\frac{f}{50}-\frac{50}{f})$

So:

α ₁(50)=α ₂(50)=0

α ₁(f)′=k

${\mathrm{\α}}_2{\left(f\right)}^{\′}=Q_f(\frac{1}{50}+\frac{50}{f^2})$

If the minimum frequency allowed is f _minthen:

${\mathrm{\α}}_2{\left(f\right)}^{\′}=Q_f(\frac{1}{50}+\frac{50}{f^2})\≤Q_f(\frac{1}{50}+\frac{50}{{f_{\min }}^2})$

Order:

$k>Q_f(\frac{1}{50}+\frac{50}{{f_{\min }}^2})$

So, because atan (x) is monotonically increasing function in (-∞ ,+∞), therefore:

The Δ θ >0 as f>50Hz, thus frequency f continues to increase;

The Δ θ <0 as f<50Hz, thus frequency f continues to reduce;

Therefore, system plays pendulum, and final frequency f offsets out allowed band, thus realizes non-blind area islanding detect.

The island detection method that the present embodiment provides, virtual reactive load is produced in electrical network, detected by the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, the electric voltage frequency realized when detecting exceeds setting range, then determine that inverter, virtual reactive load and network load form isolated island.Because virtual reactive load is not when the circuit that electrical network and inverter, virtual reactive load and network load form disconnects, virtual reactive load is absorbing reactive power neither, does not also send reactive power, therefore, can not have an impact to grid-connected current quality, thus ensure the normal operation of electric system.Detected by the electric voltage frequency of the time-dependent system formed inverter, virtual reactive load and network load, non-blind area islanding detect can be realized.

Further, under the enforcement scene that electric system comprises multiple inverter, each inverter can be separate, produces virtual reactive load respectively.In the embodiment of the present invention, reactive disturbance direction is determined by the electric voltage frequency detected, the lagging reactive power that virtual reactive load absorbs increases along with the increase of detected electric voltage frequency; Or the capacitive reactive power that virtual reactive load absorbs increases along with the reduction of detected electric voltage frequency.Therefore, each inverter reactive disturbance direction is synchronous, and the reactive disturbance avoiding each inverter cancels each other.

Fig. 3 is the structural representation of an isolated island detecting device provided by the invention embodiment, and as shown in Figure 3, this isolated island detecting device comprises: detection module 31 and determination module 32;

Detection module 31, detects for the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, and virtual reactive load carries out energy exchange and not consumed energy;

Determination module 32, if exceed setting range for the electric voltage frequency of detection module detection, then determines that inverter, virtual reactive load and network load form isolated island.

It should be noted that, the isolated island detecting device that the embodiment of the present invention provides, can be the pick-up unit be arranged on separately in electrical network, or its function can also be realized by the inverter in electrical network.

Optionally, the lagging reactive power that virtual reactive load absorbs can increase along with the increase of detected electric voltage frequency; Or the capacitive reactive power that virtual reactive load absorbs can increase along with the reduction of detected electric voltage frequency.

Optionally, if the electric voltage frequency that detection module 31 detects is the ratings of mains frequency, then the reactive power that virtual reactive load absorbs can be 0, and the reactive power sent can be 0.

Optionally, virtual reactive load can be produced by inverter.

Optionally, the electric current that virtual reactive load absorbs can be: iq=id × (f-f _n) × k;

Wherein, id is the active current that inverter sends, and f is detected electric voltage frequency, f _nfor the ratings of mains frequency, k is scale-up factor, and iq is the electric current that virtual reactive load absorbs.

The isolated island detecting device that the embodiment of the present invention provides is the actuating equipment of the island detection method that the embodiment of the present invention provides, and its detailed process performing island detection method see the associated description in embodiment of the method, can not repeat them here.

The isolated island detecting device that the present embodiment provides, virtual reactive load is produced in electrical network, detected by the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, the electric voltage frequency realized when detecting exceeds setting range, then determine that inverter, virtual reactive load and network load form isolated island.Because virtual reactive load is not when the circuit that electrical network and inverter, virtual reactive load and network load form disconnects, virtual reactive load is absorbing reactive power neither, does not also send reactive power, therefore, can not have an impact to grid-connected current quality, thus ensure the normal operation of electric system.

The present invention also provides a kind of electric system, comprising: electrical network, at least one inverter, network load, at least one virtual reactive load, network load and isolated island detecting device.

Wherein, isolated island detecting device can be the pick-up unit be arranged on separately in electrical network, or its function can also be realized by the inverter in electrical network, that is, isolated island detecting device can integrate with inverter.

Fig. 4 is the structural representation of the unified embodiment of power train provided by the invention, as shown in Figure 4, this embodiments provides a kind of typical structure of electric system, comprises two inverters: inverter 1 and inversion 2, and network load comprises: resistance, an inductance and an electric capacity; Inverter 1 and inverter 2 can produce a virtual reactive load respectively.The function of isolated island detecting device can be realized by inverter 1 and inverter 2.

Concrete, electric voltage frequency in the circuit of the virtual reactive load that inverter 1 can produce inverter 1, inverter 1 and network load formation detects, if the electric voltage frequency detected exceeds setting range, then can determine that the virtual reactive load that inverter 1, inverter 1 produce and network load form isolated island.Similar, electric voltage frequency in the circuit of the virtual reactive load that inverter 2 can produce inverter 2, inverter 2 and network load formation detects, if the electric voltage frequency detected exceeds setting range, then can determine that the virtual reactive load that inverter 2, inverter 2 produce and network load form isolated island.

In the present embodiment, electric system comprises 2 inverters, and these two inverters separately can produce virtual reactive load respectively.The electric voltage frequency that reactive disturbance direction is detected by inverter is determined, the lagging reactive power that virtual reactive load absorbs increases along with the increase of detected electric voltage frequency; Or the capacitive reactive power that virtual reactive load absorbs increases along with the reduction of detected electric voltage frequency.Therefore, these 2 inverter reactive disturbance directions are synchronous, and the reactive disturbance avoiding 2 inverters cancels each other.

It should be noted that, the present embodiment only comprises 2 inverters for electric system and is described, but not in this, as the restriction to the embodiment of the present invention.Electric system is comprised to the enforcement scene of more than 2 inverters, each inverter reactive disturbance direction is synchronous, and the reactive disturbance avoiding each inverter cancels each other.

The electric system that the present embodiment provides, virtual reactive load is produced in electrical network, detected by the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, the electric voltage frequency realized when detecting exceeds setting range, then determine that inverter, virtual reactive load and network load form isolated island.Because virtual reactive load is not when the circuit that electrical network and inverter, virtual reactive load and network load form disconnects, virtual reactive load is absorbing reactive power neither, does not also send reactive power, therefore, can not have an impact to grid-connected current quality, thus ensure the normal operation of electric system.

Those skilled in the art can be well understood to, for convenience and simplicity of description, only be illustrated with the division of above-mentioned each functional module, in practical application, can distribute as required and by above-mentioned functions and be completed by different functional modules, inner structure by device is divided into different functional modules, to complete all or part of function described above.The system of foregoing description, the specific works process of device and unit, with reference to the corresponding process in preceding method embodiment, can not repeat them here.

In several embodiments that the application provides, should be understood that, disclosed system, apparatus and method, can realize by another way.Such as, device embodiment described above is only schematic, such as, the division of described module or unit, be only a kind of logic function to divide, actual can have other dividing mode when realizing, such as multiple unit or assembly can in conjunction with or another system can be integrated into, or some features can be ignored, or do not perform.Another point, shown or discussed coupling each other or direct-coupling or communication connection can be by some interfaces, and the indirect coupling of device or unit or communication connection can be electrical, machinery or other form.

The described unit illustrated as separating component or can may not be and physically separates, and the parts as unit display can be or may not be physical location, namely can be positioned at a place, or also can be distributed in multiple network element.Some or all of unit wherein can be selected according to the actual needs to realize the object of the present embodiment scheme.

In addition, each functional unit in each embodiment of the application can be integrated in a processing unit, also can be that the independent physics of unit exists, also can two or more unit in a unit integrated.Above-mentioned integrated unit both can adopt the form of hardware to realize, and the form of SFU software functional unit also can be adopted to realize.

If described integrated unit using the form of SFU software functional unit realize and as independently production marketing or use time, can be stored in a computer read/write memory medium.Based on such understanding, the part that the technical scheme of the application contributes to prior art in essence in other words or all or part of of this technical scheme can embody with the form of software product, this computer software product is stored in a storage medium, comprising some instructions in order to make a computer equipment (can be personal computer, server, or the network equipment etc.) or processor (processor) perform all or part of step of method described in each embodiment of the application.And aforesaid storage medium comprises: USB flash disk, portable hard drive, ROM (read-only memory) (ROM, Read-OnlyMemory), random access memory (RAM, RandomAccessMemory), magnetic disc or CD etc. various can be program code stored medium.

The above, above embodiment only in order to the technical scheme of the application to be described, is not intended to limit; Although with reference to previous embodiment to present application has been detailed description, those of ordinary skill in the art is to be understood that: it still can be modified to the technical scheme described in foregoing embodiments, or carries out equivalent replacement to wherein portion of techniques feature; And these amendments or replacement, do not make the essence of appropriate technical solution depart from the spirit and scope of each embodiment technical scheme of the application.

Claims (11)

1. an island detection method, is characterized in that, comprising:

Electric voltage frequency in the circuit form inverter, virtual reactive load and network load detects, and described virtual reactive load carries out energy exchange and not consumed energy;

If the described electric voltage frequency detected exceeds setting range, then determine that described inverter, described virtual reactive load and described network load form isolated island;

The electric current that described virtual reactive load absorbs is: iq=id × (f-f _n) × k;

Wherein, id is the active current that described inverter sends, and f is detected described electric voltage frequency, f _nfor the ratings of mains frequency, k is scale-up factor, and iq is the electric current that described virtual reactive load absorbs.

2. method according to claim 1, is characterized in that, the lagging reactive power that described virtual reactive load absorbs increases along with the increase of detected described electric voltage frequency; Or the capacitive reactive power that described virtual reactive load absorbs increases along with the reduction of detected described electric voltage frequency.

3. method according to claim 1 and 2, is characterized in that, if the described electric voltage frequency detected is the ratings of mains frequency, then the reactive power that described virtual reactive load absorbs is 0, and the reactive power sent is 0.

4. method according to claim 3, is characterized in that, described to inverter, virtual reactive load and network load form circuit in electric voltage frequency detect before, also comprise:

Described virtual reactive load is produced by described inverter.

5. an isolated island detecting device, is characterized in that, comprising:

Detection module, detects for the electric voltage frequency in the circuit that forms inverter, virtual reactive load and network load, and described virtual reactive load carries out energy exchange and not consumed energy;

Determination module, if exceed setting range for the described electric voltage frequency of described detection module detection, then determines that described inverter, described virtual reactive load and described network load form isolated island;

The electric current that described virtual reactive load absorbs is: iq=id × (f-f _n) × k;

Wherein, id is the active current that described inverter sends, and f is detected described electric voltage frequency, f _nfor the ratings of mains frequency, k is scale-up factor, and iq is the electric current that described virtual reactive load absorbs.

6. device according to claim 5, is characterized in that, the lagging reactive power that described virtual reactive load absorbs increases along with the increase of detected described electric voltage frequency; Or the capacitive reactive power that described virtual reactive load absorbs increases along with the reduction of detected described electric voltage frequency.

7. the device according to claim 5 or 6, is characterized in that, if the described electric voltage frequency that described detection module detects is the ratings of mains frequency, then the reactive power that described virtual reactive load absorbs is 0, and the reactive power sent is 0.

8. the device according to claim 5 or 6, is characterized in that, described virtual reactive load is produced by described inverter.

9. the device according to claim 5 or 6, is characterized in that, described device is inverter.

10. an electric system, is characterized in that, comprising: electrical network, at least one inverter, network load, at least one virtual reactive load and the isolated island detecting device as described in any one of claim 5-9.

11. electric system according to claim 10, is characterized in that, described isolated island detecting device and described inverter integrate.