CN103983873B - Island detection method suitable for multi-inverter parallel operation and based on impedance measurement - Google Patents

Abstract

The invention discloses an island detection method suitable for multi-inverter parallel operation and based on impedance measurement, and belongs to the field of large-scale new energy station integrated grid connection islanding protection. The method includes the steps that firstly, the opening state and the closing state of a PCC point circuit breaker are detected in real time; secondly, if the PCC point circuit breaker is disconnected, the third step is executed, or else the first step is executed; thirdly, delay is conducted, and if a switch is kept in the disconnected state within one-second delay, the fourth step is executed, or else the first step is executed; fourthly, uncharacteristic harmonic currents are injected, timing is started, and time is set as t; fifthly, the impedance Z of uncharacteristic harmonic is measured; sixthly, if Z is larger than Zset, it is judged that an island exists, and the eighth step is executed, or else the seventh step is executed; seventhly, if t is larger than or equal to one second, the eighth step is executed, or else the fifth step is executed; eighthly, injection of the uncharacteristic harmonic currents is stopped. According to the method, harmonic is injected only when the island exists, and the quality of electric energy is not affected in the DG system grid connection operation process; an inverter is not needed for injection, mutual interference between injected harmonic is avoided, and therefore misjudgment and blind regions are avoided.

Description

It is applicable to the island detection method based on impedance measurement that multi-inverter parallel runs

Technical field

The invention belongs to extensive new forms of energy and concentrate grid-connected anti-isolated island protection field, how inverse be applicable to particularly to one Become the island detection method based on impedance measurement of device parallel running.

Background technology

Isolated island is after distributed system (DG system) disconnects with major network, and distributed generation system is still negative to this locality carried Carry power supply, thus form a uncontrollable self-centered supply network of Utilities Electric Co..Isolated island be divided into planned isolated island and Unplanned property isolated island, planned isolated island refers to predict at the beginning of Electric Power Network Planning, and transports for its safety and stability Row takes the isolated island of corresponding measure in advance, and unplanned property isolated island refers to the unexpected isolated island when Electric Power Network Planning, when non-meter When drawing isolated island generation, serve serious problem can to the safe and stable operation band of power system.The Main Function of isolated island detection It is, when distributed system departs from major network, to detect island state in time, prevent unplanned islet operation.The most conventional Island detection method can be divided on off state detection method, passive means, active method three major types.On off state detection method relies on communication party Method, when distributed system sends opening of point of common coupling (Point of Common Coupling, the PCC) that line is connected with electrical network When closing disconnection, send signal of communication to distributed power source, after distributed power source receives signal, out of service, this method Advantage is that detection efficiency is higher, but has the disadvantage that high cost；Passive means is according to DG intrasystem electric parameters during generation isolated island Change carry out isolated island detection, when the advantage of this kind of method is properly functioning for electrical network without impact, but power approximate Isolated island cannot be effectively detected the when of coupling；Active method refers to utilize inverter to inject disturbance to system, measures PCC point and is produced Raw change, when DG system grid connection is run, injects disturbance and is diluted by electrical network, and the electric parameters of PCC point is unaffected, and isolated island is transported During row, disturbance produces considerable influence at PCC point, according to this difference, carries out isolated island detection.Active method mainly has active frequency inclined Shifting method (Active Frequency Drift, AFD), sliding formwork frequency deviation method (Slip-Mode Frequency Shift, SMS), Sandia frequency deviation method (Sandia Frequency Shift, SFS), meritorious/reactive power disturbance method, this kind of side The advantage of method is that check frequency is less, the quality of power supply can be produced impact when shortcoming is properly functioning.

Island detection method based on impedance method is a kind of conventional active alone island detection method, and impedance method is according to isolated island Before and after generation, the equivalent impedance of PCC point this difference different carries out isolated island detection.It is respectively also as shown in Fig. 1 (a) and 1 (b) System equivalent figure when network operation and generation isolated island, when being incorporated into the power networks, the equivalent impedance that equivalent impedance is bulk power grid of PCC point With the parallel value of the equivalent impedance of DG system, occur after isolated island, the equivalent impedance that equivalent impedance is DG of PCC point, due to big the most electric The equivalent impedance of net is much smaller compared with the equivalent impedance of DG system, and therefore grid-connected with under isolated island two states, the equivalent of PCC point hinders Anti-difference is bigger.Impedance method utilizes inverter to send uncharacteristic harmonics, then measures the equivalent uncharacteristic harmonics impedance of PCC point, Whether exceed predetermined threshold value according to equivalent uncharacteristic harmonics impedance to judge whether isolated island.Relative to other active method, resistance Anti-method has lot of advantages: one, and no matter whether power match, and when isolated island occurs when, the equivalent impedance of PCC point is certain The amount that can become, therefore, would not be the same with AFD method as SFS method, when initial frequency is changed to zero when, blind area can be produced； Its two, SFS method and AFD method need to be controlled disturbance quantity, if disturbance quantity is the least, frequency possibly cannot be made to occur quickly partially Move, if too big, the quality of power supply can be affected again under normal operation, and impedance method need not consider to inject asking of disturbance quantity Topic, because the size of harmonic electric current is unrelated with the harmonic impedance size measured, only need to inject the harmonic wave that can accurately measure Electric current；Its three, isolated island occur after PCC point equivalent impedance can moment produce change, isolated island detect can be quickly completed.

Impedance method has good detection performance being applied to single inverter when, but to be applied to multiple stage inverse when it The when of becoming device parallel connection, it will some problems occur.First, owing to traditional impedance method controls inverter continuously to electricity Net injects uncharacteristic harmonics, can produce harmonic pollution, affect the quality of power supply, when a small amount of inverter under electrical network normal operation The when of parallel running, for bulk power grid, harmonic pollution is negligible, but when the scale of photovoltaic generation is more come The biggest, a large amount of inverter parallels, and the when that the harmonic wave of these inverters injection can being overlapped mutually, can be to bulk power grid The quality of power supply produce impact；Second, owing to uncharacteristic harmonics electric current is persistently to inject electrical network, and study display power system The time of middle generation isolated island is the shortest, thus causes waste；3rd, owing to disturbance needs to be injected by inverter, Along with the expansion of distributed power generation scale, have increasing inverter parallel, when these inverter parallels Time, the harmonic wave that they are sent can interfere with each other, and harmonic amplitude may be made to increase, in this case, it will occurs by mistake Sentence, the when of detraction mutually between harmonic wave, may make detection that blind area occurs.

Equivalent when multi-inverter parallel runs measures figure as in figure 2 it is shown, for m platform inverter, when using based on resistance The when of the isolated island detection scheme of anti-measurement, shown in its measuring principle such as formula (1):

$Z=\left|\frac{\stackrel{\·}{I}{Z}_h}{{\stackrel{\·}{I}}_m}\right|---\left(1\right)$

When multi-inverter parallel when, it is analyzed as a example by two, it is assumed that m platform and n-th inverter all use Isolated island detection scheme based on impedance measurement, the harmonic current that they send is respectivelyWithTwo harmonic currents have phase Same frequencies omega, but amplitude I_m、I_nAnd phase angleDifference, if ${\stackrel{\·}{I}}_m=I_m\sin (\mathrm{\ωt}+{\∂}_m),{\stackrel{\·}{I}}_n=I_n\sin (\mathrm{\ωt}+{\∂}_n),$ And $\left|{\stackrel{\·}{I}}_m\right|>\≤\stackrel{\·}{|I_n}|,$ Then when they utilize isolated island detection scheme based on impedance measurement simultaneously when, the harmonic current sent will enter Row superposition, as shown in formula (2):

$\stackrel{\·}{I}={\stackrel{\·}{I}}_m+{\stackrel{\·}{I}}_n=\sqrt{I_m^2+I_n^2+{2I}_m{I}_n\cos ({\∂}_m-{\∂}_n)}\sin (\mathrm{\ωt}+\∂)--\left(2\right)$

Wherein, $\∂=\arctan \frac{I_n\cos {\∂}_n+I_m\cos {\∂}_m}{I_n\sin {\∂}_n+I_m\sin {\∂}_m}.$

(1) blind area becomes big situation.

If worked as $\arccos (-\frac{|{\stackrel{\&CenterDot;}{I}}_n}{2|{\stackrel{\&CenterDot;}{I}}_m})<{\&PartialD;}_m-{\&PartialD;}_n<2\mathrm{\&pi;}-\arccos (-\frac{|{\stackrel{\&CenterDot;}{I}}_m}{2|{\stackrel{\&CenterDot;}{I}}_m}),$ Time, then the amplitude of electric current mutually detracts, The current value of synthesis will reduce by relatively single current.It is generalized to multiple stage inverter parallel, by resultant currentAs single current Synthesis is continued, it is seen then that can mutually subtract meeting above-mentioned angular relationship when when harmonic current synthesizes when with other electric current Damage.

In the case of electric current mutually detracts, resultant current will reduce, if Z_lEquivalent impedance, Z is loaded for this locality_setFor The threshold value of isolated island detection, when there is isolated island, if there isSituation, for m platform inverter, arise that Check frequency.In like manner, for other inverter, if meeting above-mentioned condition between phase angle, all it is likely to occur check frequency.

(2) situation about judging by accident.

$\left|\stackrel{\&CenterDot;}{I}\right|={\stackrel{\&CenterDot;}{I}}_m+{\stackrel{\&CenterDot;}{I}}_n=\sqrt{I_m^2+I_n^2+{2I}_m{I}_n\cos ({\&PartialD;}_m-{\&PartialD;}_n)}---\left(3\right)$

Wherein, $|{\stackrel{\&CenterDot;}{I}}_m+{\stackrel{\&CenterDot;}{I}}_n|\&le;\stackrel{\&CenterDot;}{I}|$

From formula (3), when ${\&PartialD;}_m-{\&PartialD;}_n>2\mathrm{\&pi;}-\arccos (-\frac{\left|{\stackrel{\&CenterDot;}{I}}_m\right|}{2\left|{\stackrel{\&CenterDot;}{I}}_n\right|})$ Or ${\&PartialD;}_m-{\&PartialD;}_n<-\arccos (-\frac{\left|{\stackrel{\&CenterDot;}{I}}_m\right|}{2\left|{\stackrel{\&CenterDot;}{I}}_n\right|})$ Time, electric current Between can mutually strengthen, particularly whenDuring close to 0,Close toThere is not isolated island when, If metFor m platform inverter, then there will be the situation of erroneous judgement, for the inversion of other parallel running Device, if meeting above-mentioned condition between phase angle, it is possible to the situation of erroneous judgement occurs.

By analyzing it is recognized that while in the case of separate unit or a small amount of inverter parallel, isolated island based on impedance method Detection scheme has lot of advantages, but in the case of multi-inverter parallel, its detection performance is substantially reduced, and not only can affect Quality of power supply time properly functioning, and due to the impact between harmonic, it will make its power of test significantly weaken, may Occur that blind area increases or the situation of erroneous judgement occurs.

Summary of the invention

The deficiency existed for above-mentioned prior art, the present invention propose a kind of be applicable to multi-inverter parallel run based on The island detection method of impedance measurement, it is characterised in that concretely comprising the following steps of this island detection method:

Step one: detect the circuit breaker switch state of point of common coupling PCC in real time；

Step 2: if killer switch disconnects, perform step 3；Otherwise return step one；

Step 3: carry out the time delay of 1 second, if the time delay inner breaker switch at 1 second remains off, performs step Four；Otherwise, step one is returned；

Step 4: open harmonic wave generating device, injects uncharacteristic harmonics electric current at point of common coupling PCC, starts timing, If the time is t；

Step 5: utilize inverter to measure the uncharacteristic harmonics impedance Z at now point of common coupling PCC；

Step 6: if uncharacteristic harmonics impedance Z is more than threshold value Z_set, it is determined that for isolated island, perform step 8；Otherwise perform Step 7；

Step 7: if t was more than or equal to 1 second, perform step 8；Otherwise return step 5；

Step 8: locking harmonic wave generating device, stops injecting uncharacteristic harmonics electric current.

Described Z_set=1.5Z_max, wherein, Z_maxEquivalent harmonic wave impedance maximum when being incorporated into the power networks for distributed system.

The invention have the benefit that, relative to traditional impedance method, the inventive method changes the injection of harmonic current Position, it is not necessary to inverter injects, it is to avoid the harmonic wave interactional problem injected during multi-inverter parallel, it is to avoid Erroneous judgement and the problem of blind area；Control the harmonic injection time, it is not necessary to persistently inject, it is only necessary to be likely to occur the situation of isolated island Lower injection, it is to avoid the detection method impact on quality of power supply time properly functioning；The inventive method is carried out for harmonic Optimized choice.

Accompanying drawing explanation

Fig. 1 is system equivalent figure；

System equivalent figure when Fig. 1 (a) is for being incorporated into the power networks；

For there is system equivalent figure during isolated island in Fig. 1 (b)；

Fig. 2 is equivalent measurement figure during multi-inverter parallel operation；

Fig. 3 is the island detection method flow chart that the present invention proposes；

Fig. 4 is the island detection method schematic diagram that the present invention proposes；

Fig. 5 is the equivalent circuit diagram during operation of DG system grid connection；

Fig. 6 is equivalent circuit diagram during DG system generation isolated island.

Detailed description of the invention

The invention will be further described below in conjunction with the accompanying drawings.

It is illustrated in figure 3 the island detection method flow chart that the present invention proposes, then concretely comprises the following steps:

Step one: detect the circuit breaker switch state of PCC in real time；

Step 2: if killer switch disconnects, perform step 3；Otherwise return step one；

Step 3: carry out the time delay of 1 second, if the time delay inner breaker switch at 1 second remains off, performs step Four；Otherwise, step one is returned；

Step 4: open harmonic wave generating device, injects uncharacteristic harmonics electric current at PCC, starts timing, if the time is t；

Step 5: utilize inverter to measure the uncharacteristic harmonics impedance Z at now PCC；

Step 6: if uncharacteristic harmonics impedance Z is more than threshold value Z_set, it is determined that for isolated island, perform step 8；Otherwise perform Step 7；

Step 7: if t was more than or equal to 1 second, perform step 8；Otherwise return step 5；

Step 8: locking harmonic wave generating device, stops injecting uncharacteristic harmonics electric current.

Harmonic wave generating device as shown in Figure 4, is installed on the chopper of PCC point, utilizes logic by the schematic diagram of the method Control, make harmonic wave generating device match with the chopper of PCC point, when the disconnect switch of PCC point closes, harmonic wave generating device Locking, does not injects uncharacteristic harmonics electric current to DG systemWhen the disconnect switch of PCC point disconnects, harmonic wave generating device starts, Uncharacteristic harmonics electric current is injected to DG systemInverter is now utilized to measure the uncharacteristic harmonics impedance of PCC point.When DG system When being incorporated into the power networks, as it is shown in figure 5, the disconnect switch Guan Bi of PCC point, the uncharacteristic harmonics electric current of injectionBulk power grid will be flowed to With DG system, now the measurement impedance value at decanting point is the least, when DG system generation isolated island, as shown in Fig. 6, PCC point Disconnect switch disconnects, uncharacteristic harmonics electric currentTo flow into DG system, now the measurement impedance value of PCC point is bigger, it will super Go out threshold value, isolated island can be detected.

Select the harmonic wave injected will consider 3 points: first, selection uncharacteristic harmonics, namely non-existent humorous in system Ripple, so it can be avoided that the interference of harmonic wave that originally just has of distributed system；Second, use the harmonic wave that frequency is higher, it is possible to Amplify harmonic impedance, be more prone to measure；3rd, it is impossible to select too high frequency, frequency is too high will cause harmonic voltage excessive, Affect voltage waveform.

For threshold value Z_setChoosing method and principle: because different DG system equivalent harmonic impedance is different, different orphans Under the pattern of island, equivalent harmonic impedance is the most different, therefore for different DG systems, should use different threshold values.Before and after isolated island The characteristic of equivalent harmonic wave impedance, it is considered under various load conditions, if equivalent harmonic wave impedance maximum when DG system grid connection is run For Z_max, then threshold value Z_setAdjust as Z_set=1.5Z_max。

Isolated island testing requirement completes in 2 seconds (s), i.e. disconnects at killer switch, after the time delay of 1s the most again Guan Bi, harmonic wave generating device starts, and to harmonic wave generating device locking, this process time is not to be exceeded 2s, after switching off Time delay 1s is to get rid of disturbance and the action of automatic reclosing, and therefore the harmonic injection time of harmonic injection device should not surpass Cross 1s.

The above, the only present invention preferably detailed description of the invention, but protection scope of the present invention is not limited thereto, Any those familiar with the art in the technical scope that the invention discloses, the change that can readily occur in or replacement, All should contain within protection scope of the present invention.Therefore, protection scope of the present invention should be with scope of the claims It is as the criterion.

Claims (2)

1. it is applicable to the island detection method based on impedance measurement that multi-inverter parallel runs, it is characterised in that do not affect DG Quality of power supply when system grid connection is run, and can quickly detect isolated island after isolated island occurs, eliminate tradition based on inverter The detection of impedance type isolated island send out inverter and inject influencing each other and the check frequency that produces or erroneous judgement between uncharacteristic harmonics Situation, thus there is not the situation of check frequency or erroneous judgement, concretely comprising the following steps of this island detection method:

Step one: detect the on off state of PCC chopper in real time；

Step 2: if killer switch disconnects, perform step 3；Otherwise return step one；

Step 3: carry out the time delay of 1 second, if the time delay inner breaker switch at 1 second remains off, performs step 4； Otherwise, step one is returned；

Step 4: the open uncharacteristic harmonics generating means being arranged on PCC, injects uncharacteristic harmonics electric current at PCC, and starts Timing, if the time is t；

Step 5: utilize inverter to measure the uncharacteristic harmonics impedance Z at now PCC；

Step 6: if uncharacteristic harmonics impedance Z is more than threshold value Zset, it is determined that for isolated island, perform step 8；Otherwise perform step Seven；

Step 7: if t was more than or equal to 1 second, perform step 8；Otherwise return step 5；

Step 8: locking uncharacteristic harmonics generating means, stops injecting uncharacteristic harmonics electric current.

The island detection method based on impedance measurement being applicable to multi-inverter parallel operation the most according to claim 1, It is characterized in that, described Z_set=1.5Z_max, wherein, Z_maxEquivalent harmonic wave impedance maximum when being incorporated into the power networks for distributed system.