CN104793148B  Distributed generator islanding detection method based on grid entry point characteristic harmonics voltage measurement  Google Patents
Distributed generator islanding detection method based on grid entry point characteristic harmonics voltage measurement Download PDFInfo
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 CN104793148B CN104793148B CN201510219972.4A CN201510219972A CN104793148B CN 104793148 B CN104793148 B CN 104793148B CN 201510219972 A CN201510219972 A CN 201510219972A CN 104793148 B CN104793148 B CN 104793148B
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Abstract
The invention discloses a kind of distributed generator islanding detection method based on grid entry point characteristic harmonics voltage measurement, the distributed power source is connected with inverter, the inverter is passed sequentially through into port filter and transformer access power network composition Gridconnected Distributed Generation Power System, and this method includes：Choose at least two groups characteristic harmonics number of times as reference subharmonic to carry out isolated island detection according to inverter IGBT switching frequencies；The voltage of grid entry point to inverter output end and between transformer and power network synchronizes sampling, and by conversion with after decomposition, obtaining the selected corresponding reference subharmonic voltage value of at least two groups characteristic harmonics number of times；Judge whether each group exceeds this group of harmonic voltage detection threshold value scope set in advance with reference to subharmonic voltage value size；If so, and the duration exceed setting time limit value, then judge occur island effect.This method disclosed by the invention is reliable and easy to realize, strong to grid adaptability, is adapted to engineering actual use.
Description
Technical field
The present invention relates to distributed generator islanding detection technique field, more particularly to one kind is based on grid entry point characteristic harmonics electricity
Press the distributed generator islanding detection method of measurement.
Background technology
In recent years, becoming increasingly conspicuous with energy shortage and environmental problem, new energy power generation technology is in the big of national policy
Power has obtained swift and violent development under supporting, substantial amounts of generation of electricity by new energy device power supply (Distributed in a distributed manner
Generation, DG) form be linked into power network, alleviate power network pressure while also to power network bring the quality of power supply and
The problem of runing planning aspect, wherein island effect is exactly a subject matter.
In photovoltaic generation, windpower electricity generation, fuel cell power generation distributed electricity generation gridconnecting electricity generation system, when power network is powered
During because of failure accident or have a power failure maintenance and during escape, fail to detect that power down mode itself will cut off supply network immediately,
Form the isolated island for a selfenergizing being made up of the load of distribution electric station grid connection electricity generation system and surrounding, i.e. island effect.
Potential safety hazard can be brought to user equipment during islet operation, while the person of the unplanned islet operation to maintainer
Safety can also constitute potential threat, it is therefore desirable to take the isolated island control measure of correlation.Relevant control after isolated island will also be protected
Card is after whole system disturbance excision, and each isolated island can be carried out and net operation faster, makes whole power system restoration to normally
Safe operation state.
Island detection method mainly has passive type and active.Existing passive type isolated island detection method, passes through detection point
The data of cloth power grid inverter output voltage, frequency or phase are contrasted with the level threshold value set up, and judgement is
No generation island effect.The defect of this method is due to that fluctuation when power network is normally run may also cause erroneous judgement.If in addition,
The power output of distributed power source matches with bearing power, then when power network power failure, and grid entry point voltage and frequency change are very
It is small to be not enough to detect whether occur island effect, i.e., in the presence of very big check frequency.Existing active alone island detection method
It is, to certain disturbance, the frequency or phase of output voltage is occurred accordingly to the amplitude, frequency, phase of inverter output current
Skew, to determine whether to occur island effect, but the detection method will influence the electric energy matter of power network due to the addition of disturbance quantity
Amount.
Therefore, how isolated island is accurately detected on the premise of the power network quality of power supply is not influenceed, being that distributed power source is gridconnected grinds
The emphasis studied carefully.
The content of the invention
It is an object of the invention to provide a kind of distributed generator islanding detection based on grid entry point characteristic harmonics voltage measurement
Method, this method is reliable and easy to realize, strong to grid adaptability, is adapted to engineering actual use.
The purpose of the present invention is achieved through the following technical solutions：
A kind of distributed generator islanding detection method based on grid entry point characteristic harmonics voltage measurement, the distributed power source with
Inverter is connected, and the inverter is passed sequentially through into port filter and transformer access power network composition Gridconnected Distributed Generation Power System,
This method includes：
Choose at least two groups characteristic harmonics number of times as reference subharmonic to carry out orphan according to inverter IGBT switching frequencies
Detect on island；
The voltage of grid entry point to inverter output end and between transformer and power network synchronizes sampling, and passes through
Conversion is with after decomposition, obtaining the selected corresponding reference subharmonic voltage value of at least two groups characteristic harmonics number of times；
Judge whether each group exceeds this group of harmonic voltage detection threshold value set in advance with reference to subharmonic voltage value size
Scope；
If so, and the duration exceed setting time limit value, then judge occur island effect.
Further, include before synchronizing sampling：
Sampling analysis cutoff frequency f is determined according to inverter IGBT switching frequencies_{h}With sample frequency f_{s}, wherein, f_{s}≥2f_{h}；
Sampling interval T is set_{s}, sampled point isAnd N is equal to 2 integral number power, f_{0}For fundamental frequency.
Further, this method also includes determining harmonic voltage detection threshold value scope, and its step is：
System impedance model is set up, wherein, the parameter in the system impedance model is converted to same voltage class, power network
End is equivalent to voltage source and connected with system impedance, and inverter is equivalent to harmonic voltage source；
Set up according to the system impedance model and isolated island does not occur with occurring system harmonic impedance model during isolated island, and in terms of this
Calculate at least two groups selected characteristic harmonics number of times corresponding grid entry point when isolated island does not occur with occurring isolated island and refer to subharmonic
Voltage；
According to each group of characteristic harmonics number of times do not occur isolated island with occur isolated island when grid entry point with reference to subharmonic voltage come
Determine this group of harmonic voltage detection threshold value scope.
Further, this method also includes：Parameter in system impedance model is converted to same voltage class；
Wherein, voltage class is designated as U_{N}, system short circuit capacity is S_{d}, then system impedance X_{S}For：
The inductance value and capacitance for entering port filter are designated as L and C respectively, and system frequency is designated as f_{0}, then port filter sense is entered
Anti X_{L}With capacitive reactance X_{C}Respectively：
X_{L}=2 π f_{0}L；
Transformer rated capacity is designated as S_{N}, shortcircuit impedance is designated as U_{k}, then transformer impedance X_{T}For：
System load capacity is designated as S_{L}, phasor power factor is designated asThen equivalent load resistance R_{load}And equivalent reactance
X_{load}For：
Further, this method also includes at least two groups characteristic harmonics number of times selected by calculating when isolated island does not occur pair
The grid entry point answered refers to subharmonic voltage, and it includes：
When isolated island not occurring in system harmonic impedance model system load be equivalent to open circuit, parameter therein with it is selected
Characteristic harmonics number of times h is related：System impedance X_{S,h}=jhX_{S}；Transformer impedance X_{T,h}=jhX_{T}；Enter the capacitive reactance X of port filter_{C,h}=
j(X_{C}/h)；Enter port filter induction reactance X_{L,h}=jhX_{L}；Load impedanceFor inverter output harmonic wave voltage；
Calculate gridconnected inverters point a harmonic voltage
By gridconnected inverters point a harmonic voltageBe converted to grid entry point and refer to subharmonic voltage
Wherein, n is transformer primary side to secondary side noload voltage ratio.
Further, this method also includes：At least two groups selected characteristic harmonics number of times are calculated when occurring isolated island pair
The grid entry point answered refers to subharmonic voltage, and it includes：
System impedance is separated with system in system harmonic impedance model during generation isolated island, and distributed power source passes through inverter pair
Load is directly powered, and parameter therein is related to selected characteristic harmonics number of times h：System impedance X_{S,h}=jhX_{S}；Transformer hinders
Anti X_{T,h}=jhX_{T}；Enter the capacitive reactance X of port filter_{C,h}=j (X_{C}/h)；Enter port filter induction reactance X_{L,h}=jhX_{L}；Load minimum impedance
Calculate gridconnected inverters point a harmonic voltage
By gridconnected inverters point a harmonic voltageBe converted to grid entry point and refer to subharmonic voltage
Wherein, n is transformer primary side to secondary side noload voltage ratio.
It is further, described that according to each group of characteristic harmonics number of times, when isolated island does not occur with occurring isolated island, grid entry point is referred to
Subharmonic voltage determines that this group of harmonic voltage detection threshold value scope includes：
When isolated island not occurring according to selected characteristic harmonics number of times h subharmonic voltage is referred to occurring grid entry point during isolated island
Virtual value U_{pcc,h}With U'_{pcc,h}It is U to determine corresponding harmonic voltage detection threshold value scope_{pcc,h}~U'_{pcc,h}。
As seen from the above technical solution provided by the invention, this method is applied to the distribution by gridconnected inverters
Power supply electricity generation system, this method existing passive detection method when both gridconnected inverters power output can be avoided to be matched with bearing power
The check frequency of presence, also solves the problem that the electrical energy power quality disturbance problem that active detection method is brought；Meanwhile, this method is reliable
And be easily achieved, it is strong to grid adaptability, it is adapted to engineering actual use.
Brief description of the drawings
In order to illustrate the technical solution of the embodiments of the present invention more clearly, being used required in being described below to embodiment
Accompanying drawing be briefly described, it should be apparent that, drawings in the following description are only some embodiments of the present invention, for this
For the those of ordinary skill in field, on the premise of not paying creative work, other can also be obtained according to these accompanying drawings
Accompanying drawing.
Fig. 1 is the isolated island detection principle diagram of Gridconnected Distributed Generation Power System provided in an embodiment of the present invention；
Fig. 2 is a kind of distributed generator islanding based on grid entry point characteristic harmonics voltage measurement provided in an embodiment of the present invention
The flow chart of detection method；
Fig. 3 is the schematic diagram of system impedance model provided in an embodiment of the present invention；
To be provided in an embodiment of the present invention the schematic diagram of system harmonic impedance model during isolated island does not occur for Fig. 4；
The schematic diagram of system harmonic impedance model when Fig. 5 is generation isolated island provided in an embodiment of the present invention；
Fig. 6 is the distributed generator islanding detection provided in an embodiment of the present invention based on grid entry point characteristic harmonics voltage measurement
Step flow chart.
Embodiment
With reference to the accompanying drawing in the embodiment of the present invention, the technical scheme in the embodiment of the present invention is carried out clear, complete
Ground is described, it is clear that described embodiment is only a part of embodiment of the invention, rather than whole embodiments.Based on this
The embodiment of invention, the every other implementation that those of ordinary skill in the art are obtained under the premise of creative work is not made
Example, belongs to protection scope of the present invention.
In Gridconnected Distributed Generation Power System, distributed power source is connected with inverter, and the inverter passes sequentially through entrance filtering
Device accesses power network with transformer.When distributed power source is by gridconnected inverters, because load impedance is far longer than system impedance,
Do not occur to be equivalent to open circuit during isolated island, inverter produces particular harmonic injected system, and corresponding harmonic voltage is produced in grid entry point.Hair
During raw isolated island, distributed power source is directly powered by inverter to load, and system impedance separation, grid entry point harmonic voltage will be rapid
Increase, the present invention is by detecting that grid entry point characteristic harmonics voltage change determines whether isolated island.Therefore, for containing gridconnected inverters
Isolated island detection principle diagram such as Fig. 1 of Gridconnected Distributed Generation Power System is, it is necessary to measure inverter output end and grid entry point with reference to secondary humorous
Wave voltage size, and calculate do not occur isolated island and occur two kinds of isolated island in the case of grid entry point refer to subharmonic voltage size, really
Characteristic harmonics voltage ratio is determined compared with threshold range, so as to determine whether occur island effect.Come below by specific embodiment to this
Detailed introduction is done in invention.
Fig. 2 is a kind of distributed generator islanding based on grid entry point characteristic harmonics voltage measurement provided in an embodiment of the present invention
The flow chart of detection method.As shown in Fig. 2 this method mainly comprises the following steps：
Step 21, according to inverter IGBT switching frequencies choose at least two groups characteristic harmonics number of times as with reference to subharmonic come
Carry out isolated island detection.
In the embodiment of the present invention, to ensure reliability of islanding detection, at least choosing two groups of inverter IGBT, (insulated gate is double
Bipolar transistor) nearby characteristic harmonics voltage carries out isolated island detection to switching frequency as reference subharmonic.
Step 22, the voltage of grid entry point to inverter output end and between transformer and power network are synchronized and adopted
Sample, and by conversion with after decomposition, obtaining the selected corresponding reference subharmonic voltage value of at least two groups characteristic harmonics number of times.
As shown in figure 1, can be entered by PT (voltage transformer) to inverter output end and grid entry point (PCC grid entry points) voltage
Row synchronized sampling, then changed by A/D, after Fourier decomposition, each group is can obtain with reference to subharmonic voltage size.
Exemplary, synchronized sampling obtains the finite length sequence u (n) that a length is S, is changed by A/D, carries out in Fu
Leaf is decomposed, and obtains each group with reference to subharmonic voltage size：
Wherein,It is as follows for empty unit；H is selected characteristic harmonics number of times.
In addition, needing to carry out the setting of relevant parameter before sampling is synchronized, specifically include：According to inverter IGBT
Switching frequency determines sampling analysis cutoff frequency f_{h}With sample frequency f_{s}, wherein, f_{s}≥2f_{h}；Sampling interval T is set_{s}, sampled point isWherein f_{0}For fundamental frequency, to reduce Fourier decomposition amount of calculation and complexity, increase isolated island detection rings
Speed is answered, sampled point N is further defined as the integral number power equal to 2.
Step 23, judge each group with reference to subharmonic voltage value size whether beyond this group of harmonic voltage set in advance examine
Survey threshold range；If so, being then transferred to step 24；Otherwise, it is transferred to and repeats step 22.
In the embodiment of the present invention, each group of harmonic voltage detection threshold value scope can be determined by calculating accordingly, specifically
The step of it is as follows：
1st, system impedance model is set up, wherein, the parameter in the system impedance model is converted to same voltage class, electricity
Net end is equivalent to voltage source and connected with system impedance, and inverter is equivalent to harmonic voltage source.
The schematic diagram of system impedance model is illustrated in figure 3, wherein：X_{S}For system impedance, X_{T}For transformer impedance, X_{C}For
Enter port filter capacitive reactance, X_{L}To enter port filter induction reactance, R_{load}And X_{load}For load equivalent resistance and reactance, U_{S}For line voltage
Source,For inverter equivalent voltage source, a points are gridconnected inverters point.
Abovementioned parameter is converted to same voltage class, specifically：
Voltage class is designated as U_{N}(lowvoltagegrade), system short circuit capacity is S_{d}, then system impedance X_{S}For：
The inductance value and capacitance for entering port filter are designated as L and C (being singlephase) respectively, and system frequency is designated as f_{0}, then enter
Port filter induction reactance X_{L}With capacitive reactance X_{C}Respectively：
X_{L}=2 π f_{0}L；
Transformer rated capacity is designated as S_{N}, shortcircuit impedance is designated as U_{k}(%), then transformer impedance X_{T}For：
System load capacity is designated as S_{L}, phasor power factor is designated asThen load equivalent resistance R_{load}And equivalent reactance
X_{load}For：
2nd, set up according to the system impedance model and isolated island does not occur with occurring system harmonic impedance model during isolated island, and with this
Calculate at least two groups selected characteristic harmonics number of times corresponding grid entry point reference time when isolated island does not occur with occurring isolated island humorous
Wave voltage.
In this step, using characteristic harmonics voltage near two groups of inverter IGBT switching frequencies selected by abovementioned steps 11
The reference subharmonic voltage compared as detection.
1) load impedance is far longer than system impedance in system harmonic impedance model (as shown in Figure 4) when isolated island not occurring,
Open circuit is equivalent to, is not considered during calculating, Model Parameter is related to selected characteristic harmonics number of times h：System impedance X_{S,h}=
jhX_{S}；Transformer impedance X_{T,h}=jhX_{T}；Enter the capacitive reactance X of port filter_{C,h}=j (X_{C}/h)；Enter port filter induction reactance X_{L,h}=
jhX_{L}；For inverter output harmonic wave voltage；
Calculate gridconnected inverters point a harmonic voltage
Because practical test points are located at the grid entry point of transformer highvoltage side bus, it is therefore desirable to by harmonic voltage conversion to height
Grade is pressed, i.e., by gridconnected inverters point a harmonic voltageBe converted to grid entry point and refer to subharmonic voltage
Wherein, n be transformer primary side to secondary side noload voltage ratio, i.e. lowvoltage to voltage levels noload voltage ratio.
2) system impedance is separated with system in system harmonic impedance model (as shown in Figure 5) when occurring isolated island, distributed electrical
Source is directly powered by inverter to load, and parameter therein is related to selected characteristic harmonics number of times h：System impedance X_{S,h}
=jhX_{S}；Transformer impedance X_{T,h}=jhX_{T}；Enter the capacitive reactance X of port filter_{C,h}=j (X_{C}/h)；Enter port filter induction reactance X_{L,h}=
jhX_{L}；Load minimum impedance
Calculate gridconnected inverters point a harmonic voltage
Similarly, by gridconnected inverters point a harmonic voltageBe converted to grid entry point and refer to subharmonic voltage
3rd, according to each group of characteristic harmonics number of times, when isolated island does not occur with occurring isolated island, grid entry point refers to subharmonic voltage
To determine this group of harmonic voltage detection threshold value scope.
When can not occur isolated island according to selected characteristic harmonics number of times h subharmonic is referred to occurring grid entry point during isolated island
Voltage effective value U_{pcc,h}With U'_{pcc,h}It is U to determine corresponding harmonic voltage detection threshold value scope_{pcc,h}~U'_{pcc,h}。
If step 24, duration exceed the time limit value of setting, judge occur island effect；Otherwise, it is transferred to weight
Step 22 is performed again.
In order to make it easy to understand, being illustrated with reference to a specific example；It should be noted that being adopted in following examples
Numerical value is only for example, and user can do corresponding change according to actual demand.
Step in this example is as shown in fig. 6, in certain parallel network power generation distribution network system, basic parameter to be as follows：
500kW photovoltaic DCtoAC converters access power network, transformer rated capacity S by 0.27/10kV transformers_{N}For 500kW, shortcircuit impedance U_{k}
(%) is 6%；10kV system short circuit capacities S_{d}For 198MVA；500kW photovoltaic DCtoAC converters LC enters the electric capacity C=200 μ of port filter
F, inductance L=0.150mH；10kV system load phasor power factorsCapacity is 5MVA；Inverter IGBT is switched
Frequency is 3kHz.
(1) system initialization.
(2) at least two groups overtone orders are chosen as referring in subharmonic, this example according to inverter IGBT switching frequencies
IGBT switching frequencies are 3kHz, system frequency f_{0}For 50Hz, therefore choose 56 times, 58 times, 62 times and 64 times four groups of harmonic voltages and make
Subharmonic is referred to for isolated island detection.
(3) system impedance model is set up, conversion is to 0.27kV (lowvoltagegrade) side system impedance：
LC enters port filter induction reactance (singlephase)：X_{L}=2 π fL=2 π × 50 × 0.15 × 10^{3}=0.0471 Ω,
LC enters port filter capacitive reactance (singlephase)：
Transformer impedance is：
Load resistance and reactance are：
(4) system harmonic impedance model when isolated island not occurring is set up according to system impedance model, and isolated island does not occur for calculating
When grid entry point each group refer to subharmonic voltage size.In this example, introduced exemplified by choosing h=58 times and calculate grid entry point harmonic voltage
Process.Calculating each harmonic impedance value of 58 subsystems is：
In 0.27kV equivalent systems, gridconnected inverters point a 58 subharmonic voltages are when isolated island not occurring：
Calculating is obtained：
10kV side bus is converted, the subharmonic voltage of PCC points 58 is：
In formula58 subharmonic voltages exported for inverter.
(5) system harmonic impedance model when occurring isolated island is set up according to system impedance model, and grid entry point is each when calculating generation isolated island
Group refers to subharmonic voltage size.58 load minimum impedances
Remaining parameter is consistent with step (4).
Calculate in 0.27kV equivalent systems, gridconnected inverters point a 58 subharmonic voltages are when occurring isolated island：
Calculating is obtained：
10kV side bus is converted, the subharmonic voltage of PCC points 58 is：
(6) according to the calculated value of grid entry point harmonic voltage in the case of two kinds, set 58 subharmonic voltage detection ranges as
6.296×10^{2}U_{58}~0.344U_{58}, used load minimum impedance is calculated by system load impedance is more than, so using
This threshold range detection isolated island is accurately and reliably.
(7) setting synchronized sampling frequency and sampled point.
According to IGBT switching frequencies, setting cutoff frequency f_{h}For 70 fundamental frequencies, sample frequency f_{s}≥2f_{h}, sampled pointThe integral number power that sampled point N is equal to 2 is determined, N=2 is chosen^{8}=256, sample frequency f_{s}For
12800Hz。
(8) when isolated island is detected, sampling is synchronized to inverter output end and grid entry point voltage by PT, turned by A/D
Change, carry out fast Fourier decomposition, obtain 58 times and refer to subharmonic voltage size, be compared with setting range.Other three groups
The calculating comparison procedure of 56 times, 62 times and 64 subharmonic voltages with 58 times similarly.
(8) judge whether the harmonic voltage beyond setting detects comparison range to each group with reference to subharmonic voltage size, such as
Do not exceed and then return to (7) step；
(9) if exceeding detection comparison range, and the duration exceedes the time limit value of setting, judges that generation is lonely
Island, output alarm signal or antiislanding control signal.
The such scheme scope of application of the embodiment of the present invention is the distributed generation system containing gridconnected inverters, and such as photovoltaic is simultaneously
Net electricity generation system.Existing passive detection method when gridconnected inverters power output is matched with bearing power can be solved and there is blind area
(NDZ) the problem of, the power quality problem that active detection method can be avoided to be brought again, and for GB/T 293192012
《Photovoltaic generating system access distribution network technology regulation》In detect that rapidity and accuracy requirement can also be met on isolated island.
The foregoing is only a preferred embodiment of the present invention, but protection scope of the present invention be not limited thereto,
Any one skilled in the art is in the technical scope of present disclosure, the change or replacement that can be readily occurred in,
It should all be included within the scope of the present invention.Therefore, protection scope of the present invention should be with the protection model of claims
Enclose and be defined.
Claims (7)
1. a kind of distributed generator islanding detection method based on grid entry point characteristic harmonics voltage measurement, the distributed power source with it is inverse
Become device to be connected, the inverter is passed sequentially through into port filter and transformer access power network composition Gridconnected Distributed Generation Power System, its
It is characterised by, this method includes：
Choose at least two groups characteristic harmonics number of times as reference subharmonic to carry out isolated island inspection according to inverter IGBT switching frequencies
Survey；
The voltage of grid entry point to inverter output end and between transformer and power network synchronizes sampling, and passes through conversion
After decomposition, the selected corresponding reference subharmonic voltage value of at least two groups characteristic harmonics number of times is obtained；
Judge whether each group exceeds this group of harmonic voltage detection threshold value scope set in advance with reference to subharmonic voltage value size；
If so, and the duration exceed setting time limit value, then judge occur island effect.
2. according to the method described in claim 1, it is characterised in that include before synchronizing sampling：
Sampling analysis cutoff frequency f is determined according to inverter IGBT switching frequencies_{h}With sample frequency f_{s}, wherein, f_{s}≥2f_{h}；
Sampling interval T is set_{s}, sampled point isAnd N is equal to 2 integral number power, f_{0}For fundamental frequency.
3. according to the method described in claim 1, it is characterised in that this method also includes determining harmonic voltage detection threshold value model
Enclose, its step is：
System impedance model is set up, wherein, the parameter in the system impedance model is converted to same voltage class, power network end etc.
Imitate and connected for voltage source with system impedance, inverter is equivalent to harmonic voltage source；
Set up according to the system impedance model and isolated island does not occur with occurring system harmonic impedance model during isolated island, and institute is calculated with this
At least two groups characteristic harmonics number of times chosen corresponding grid entry point when isolated island does not occur with occurring isolated island refers to subharmonic voltage；
According to each group of characteristic harmonics number of times, when isolated island does not occur with occurring isolated island, grid entry point refers to subharmonic voltage to determine
This group of harmonic voltage detection threshold value scope.
4. method according to claim 3, it is characterised in that this method also includes：By the parameter in system impedance model
Convert to same voltage class；
Wherein, voltage class is designated as U_{N}, system short circuit capacity is S_{d}, then system impedance X_{S}For：
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The inductance value and capacitance for entering port filter are designated as L and C respectively, and system frequency is designated as f_{0}, then port filter induction reactance X is entered_{L}With
Capacitive reactance X_{C}Respectively：
X_{L}=2 π f_{0}L；
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System load capacity is designated as S_{L}, phasor power factor is designated asThen equivalent load resistance R_{load}With equivalent reactance X_{load}
For：
5. method according to claim 4, it is characterised in that this method also includes at least two groups features selected by calculating
Overtone order corresponding grid entry point when isolated island does not occur refers to subharmonic voltage, and it includes：
System load is equivalent to open circuit, parameter therein and selected feature in system harmonic impedance model when isolated island not occurring
Overtone order h is related：System impedance X_{S,h}=jhX_{S}；Transformer impedance X_{T,h}=jhX_{T}；Enter the capacitive reactance X of port filter_{C,h}=j
(X_{C}/h)；Enter port filter induction reactance X_{L,h}=jhX_{L}；Load impedanceFor inverter output harmonic wave voltage；
Calculate gridconnected inverters point a harmonic voltage
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<msub>
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<mi>U</mi>
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</mover>
<mi>h</mi>
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By gridconnected inverters point a harmonic voltageBe converted to grid entry point and refer to subharmonic voltage
<mrow>
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Wherein, n is transformer primary side to secondary side noload voltage ratio.
6. method according to claim 4, it is characterised in that this method also includes：Calculate selected at least two groups special
Levy overtone order corresponding grid entry point when occurring isolated island and refer to subharmonic voltage, it includes：
System impedance is separated with system in system harmonic impedance model during generation isolated island, and distributed power source is by inverter to load
Directly power, parameter therein is related to selected characteristic harmonics number of times h：System impedance X_{S,h}=jhX_{S}；Transformer impedance
X_{T,h}=jhX_{T}；Enter the capacitive reactance X of port filter_{C,h}=j (X_{C}/h)；Enter port filter induction reactance X_{L,h}=jhX_{L}；Load minimum impedance
Calculate gridconnected inverters point a harmonic voltage
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Wherein,For inverter output harmonic wave voltage；
By gridconnected inverters point a harmonic voltageBe converted to grid entry point and refer to subharmonic voltage
<mrow>
<msub>
<msup>
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<mi>U</mi>
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</msup>
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<mi>a</mi>
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</mrow>
Wherein, n is transformer primary side to secondary side noload voltage ratio.
7. the method according to claim 3,4,5 or 6, it is characterised in that described to be existed according to each group of characteristic harmonics number of times
Grid entry point determines that this group of harmonic voltage detection threshold value scope includes with reference to subharmonic voltage when isolated island not occurring with generation isolated island：
When not occurring isolated island according to selected characteristic harmonics number of times h with to occur grid entry point during isolated island effective with reference to subharmonic voltage
Value U_{pcc,h}With U'_{pcc,h}It is U to determine corresponding harmonic voltage detection threshold value scope_{pcc,h}~U'_{pcc,h}。
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