CN103412207B - Based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects - Google Patents

Based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects Download PDF

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CN103412207B
CN103412207B CN201310291637.6A CN201310291637A CN103412207B CN 103412207 B CN103412207 B CN 103412207B CN 201310291637 A CN201310291637 A CN 201310291637A CN 103412207 B CN103412207 B CN 103412207B
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voltage
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negative
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inverter
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戴志辉
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华北电力大学(保定)
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    • YGENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02EREDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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    • Y02E10/56Power conversion systems, e.g. maximum power point trackers

Abstract

Based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects, belong to relay protection of power system and automatic control technology field.The method starts inverter and injects negative-sequence current when voltage harmonic aberration rate exceedes definite value, detect points of common connection voltage unbalance factor and voltage harmonic aberration rate, when the two all exceedes respective definite value and the duration time definite value that exceedes setting can judge that isolated island occurs, realize detecting isolated island fast.Present invention reduces the difficulty of adjusting of criterion, be conducive to improving detection speed; Ensure that the interference decreased most possibly under normal operating conditions electrical network; By unbalanced grid faults impact and without check frequency.Carried out simulating, verifying when being most disadvantageous in islanding detect, the present invention still quick and precisely can detect island operation state.For distributed power source during isolated island is out of service or proceed to islet operation control mode safe operation of power system etc. by being incorporated into the power networks and detect and provided method.

Description

Based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects
Technical field
The present invention relates to a kind of method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current, belong to relay protection of power system and automatic control technology field.
Background technology
Along with the aggravation of earth environment pollution and petering out of conventional energy resources, the exploitation of new forms of energy become current research focus gradually.As wherein a kind of important forms of electricity generation, still there are three large main bugbears in photovoltaic generation (Photovoltaic, PV): maximal power tracing controls, interconnection technology and islanding detect.Island effect refers to when electrical network is due to reason interruption of power supply such as electric fault, maloperation or natural causes, distributed power generation (Distributed Generation, DG) system fails to detect power down mode and departs from electrical network, still peripherad load provides electric energy, thus forms a uncontrollable self-energizing isolated island of Utilities Electric Co..Potential safety hazard may be brought to subscriber equipment during DG islet operation, unplanned islet operation also can form potential threat to the personal safety of maintainer simultaneously, fast and effeciently detect island operation state and take rational control strategy, being conducive to the utilization ratio improving power supply reliability and distributed power source.
At present, islanding detect mainly can be divided into based on communication island detection method and local island detection method.Based on the island detection method communicated by status information that is wireless or wire communication transmission isolating switch, and then judge whether isolated island.Local island detection method can be roughly divided into passive type islanding detect and active detection.Wherein, distributed power source and electrical network points of common connection (Point of Common Coupling when passive type detection method utilizes isolated island, PCC) voltage, phase place, frequency, harmonic content etc. detect isolated island, mainly contain " mistake/under-voltage " (Over Under Voltage, OUV), " mistake/under-frequency " (Over UnderFrequency, OUF) method, voltage harmonic content detection method (Harmonics Detection, HD), voltage-phase abrupt climatic change method (Phase Jump Detection, PJD) etc.The advantage of passive type detection method does not need to increase additional hardware circuit, can not impact load, shortcoming there is larger check frequency (Non-Detection Zone, NDZ), when main grid cut-off and PV power mates with load power time, electric voltage frequency will maintain in normal operation range, and the island detection method based on OUV and OUF may lose efficacy.Active detection method regularly injects disturbing signal to judge whether isolated island occurs to points of common connection when distributed power source is incorporated into the power networks.Electrical network normally runs the equilibrium activity due to electrical network phaselocked loop, and these disturbances can not be detected, and during isolated island, these injection disturbances of inverter will be accumulated, and just can trigger circuit for detecting island and detect that isolated island occurs when exceeding the scope of setting.Active alone island detection method is also mainly based on variation, frequency shift (FS) and phase offset principle, and its advantage is that check frequency is little, and shortcoming can cause certain disturbance to electrical network when distributed power source is normally incorporated into the power networks.
Passive type islanding detect method and active alone island detection method are complementary in some aspects, and negative-sequence current injects and can produce harmful effect to relevant devices such as generators, and the island detection method simultaneously based on negative-sequence current may by the impact of grid side asymmetric fault.Therefore the present invention proposes the pouring-in island detection method of a kind of negative-sequence current based on percent harmonic distortion, has little to grid disturbance, that strong interference immunity, detection speed are fast feature compared with other active alone island detection methods.
The existing islanding detect theory based on voltage harmonic is at present:
For PV system, possesses anti-islanding effect capability extremely important.The key of anti-islanding is the detection with main grid cut-off, and detection time is shorter, and Detection results is generally better.GB/T19939-2005 specifies that isolated island effect prevention protection should action in 2s, and photovoltaic system and electrical network are disconnected, in addition, when line voltage is not in this scope of 85%-110%, photovoltaic combining inverter should disconnect with electrical network.
Table 1 Voltage Harmonic limit value (Tab.1Grid voltage harmonic limits)
It is by detecting points of common connection voltage V that voltage harmonic detects pCCtotal harmonic distortion factor (Total HarmonicDistortion, THD) judge whether to occur isolated island.It calculates such as formula shown in (1).
THD = Σ n = 2 M V n 2 V 1 × 100 % - - - ( 1 )
Wherein V nfor nth harmonic effective value.GB/T14549-1993 defines the voltage limits of mains by harmonics shown in table 1.
Always comprise certain harmonic component in the output voltage of inverter, electric current, but in being incorporated into the power networks, electrical network can be approximately the very low voltage source of impedance, harmonic current flows into electrical network, and the voltage harmonic aberration rate of points of common connection will become very low, now, and V pCCtHD generally lower than the limit value of table 1 correspondence.And when islet operation, on the one hand, the nonlinear-load in isolated island, such as substation transformer, under the excitation of current harmonics, voltage there will be high distortion, is generally third harmonic; On the other hand, current harmonics flows into the larger load of impedance, makes V pCCproduce larger harmonic wave.Therefore, selected suitable threshold value in theory, the island detection method based on THD can be constructed, but have difficulties in practical application: be difficult to selected can ensure to be incorporated into the power networks or system jam time do not judge by accident, can the threshold value of sensitive differentiation when can ensure again islet operation.And when the load in isolated island has very strong low-pass characteristic, the input harmonic current required for nonlinear-load mates with inverter output current, and when inverter has the situations such as high-quality low distortion output, the method also may lose efficacy.
The output of solar module changes with intensity of solar radiation and solar module own temperature (chip temperature), and increase and the characteristic of decline with electric current because solar module has voltage, therefore there is the best operating point that can obtain peak power.And power inverter radiation intensity varies, best operating point is also change.Relative to these changes, allow the working point of solar module be in maximum power point all the time, obtain control mode and maximal power tracing control (MPPT) of maximum power output all the time from solar module.
The instantaneous conductance of photovoltaic array and the rate of change of conductance mainly compare and realize MPPT maximum power point tracking by conductance increment method, ultimate principle can be sketched and be: the output P-U family curve of photovoltaic array is single peak curve, and dP/dU=d (UI)/dU=0 is had at maximum power point place, obtain dI/dU=-I/U, wherein P is output power, and U is output voltage I is output current.As dP/dU > 0, photovoltaic array is operated in the left side of maximum power point, and as dP/dU < 0, photovoltaic array is operated in the right side of maximum power point, so just can the peak power of real-time follow-up photovoltaic cell.
UL1741 is North America distribution inverter, frequency converter, controller and system interconnection equipment standard, which specify the grid-connected requirement of distributed power source through grid-connected inverters.
IEEE1547 is the distributed power source access standard that IEEE-USA formulates, and which specify the impact restrictions such as distributed power source access electrical network points of common connection voltage, frequency, harmonic wave.
Summary of the invention
The object of this invention is to provide a kind of method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current, the method does not inject negative-sequence current when voltage THD is less than given definite value, inject negative-sequence current when voltage THD is greater than given definite value, the voltage unbalance factor then by detecting points of common connection judges island state.This method reasonably drop into negative-sequence current inject, reduce the interference to electrical network, not by unbalanced grid faults impact and without check frequency.Mate with load power at photo-voltaic power supply power, points of common connection voltage and frequency be not when skew occurs substantially, simulating, verifying has been carried out to method, even if result show when this be most disadvantageous in islanding detect, method still can detect island operation state rapidly and accurately.
Based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects,
Suppose that existing electrical network is equivalent to:
The DC voltage of the electric energy that photovoltaic battery array sends after DC/DC conversion makes conversion meets grid-connected requirement, in DC/DC conversion, take the maximal power tracing of conductance increment method to control (MPPT) simultaneously, by inverter, direct current is converted to alternating current grid-connected, when PV is grid-connected, takes unity power factor Current Control;
Wherein, R f, L ffor resistance and the inductance of inverter ac side filter, R s, L sbe main electrical network equivalent resistance and inductance, load model can be equivalent to RLC parallel connection; i abc=[i ai bi c] tfor the three-phase current phasor that photovoltaic DC-to-AC converter exports;
The method step is:
(1) by the three-phase voltage V at voltage measuring transformer PCC place av bv c,
(2) the three-phase voltage V at this PCC place is calculated av bv ctotal harmonic distortion factor (THD);
Described total harmonic distortion factor (Total Harmonic Distortion, THD) computing formula is such as formula (1):
THD = &Sigma; n = 2 M V n 2 V 1 &times; 100 - - - % ( 1 )
Wherein, V nfor nth harmonic effective value;
(3) judge whether THD exceeds the threshold value of setting;
GB/T14549-1993 defines the voltage limits of mains by harmonics shown in table 1 (threshold value).(threshold reference table 1.)
As do not exceeded, return (2) step;
As exceeded, starting and negative-sequence current injected Driven by inverter control circuit:
Measure the output voltage U of photovoltaic cell component dCwith output current I dCits output power P=I of real-time calculating dCu dC, because the power of inverter output AC side is equal with DC side power, therefore, u aCfor electrical network AC rated voltage, I * abcfor input inverter current effective value; Electric current for three-phase reference current during Current Control;
When negative-sequence current injects, the output current i of inverter abcthe given current three-phase reference current that follow current controls and i abc * = i abc ( 1 ) * + i abc ( 2 ) * = i a * i b * i c * T ; By injecting different forward-order currents i abc ( 1 ) * = i a ( 1 ) * i b ( 1 ) * i c ( 1 ) * T And negative-sequence current i abc ( 2 ) * = i a ( 2 ) * i b ( 2 ) * i c ( 2 ) * T , Control the size injecting PCC positive sequence, negative-sequence current; Wherein, C is the ratio of given forward-order current amplitude and given negative-sequence current amplitude; V pCCfor the three-phase voltage of points of common connection, comprising each harmonic component, fundamental positive sequence, first-harmonic negative sequence component and zero-sequence component;
(4) then detect the voltage with the PCC place of grid contact by voltage transformer (VT), calculate PCC place three-phase voltage V av bv ctotal harmonic distortion factor (THD), positive-negative sequence voltage and voltage unbalance factor VI,
Three-phase voltage vector U abc = U . a U . b U . c T Can be analyzed to positive sequence voltage U ( 1 ) = U . a ( 1 ) U . b ( 1 ) U . c ( 1 ) T , negative sequence voltage U ( 2 ) = U . a ( 2 ) U . b ( 2 ) U . c ( 2 ) T And residual voltage U ( 0 ) = U . a ( 0 ) U . b ( 0 ) U . c ( 0 ) T
Be the positive-sequence component of special phase, negative sequence component and zero-sequence component and three-phase voltage vector U with A phase abcrelation such as formula shown in (4):
U . a ( 1 ) U . a ( 2 ) U . a ( 0 ) = 1 3 1 a a 2 1 a 2 a 1 1 1 U . a U . b U . c - - - ( 4 )
Because PCC has the Injection Current of inverter, so PCC voltage contains harmonic component, first must carry out filtering process before each order components of extraction PCC voltage, and obtain fundametal compoment.Discrete Fourier transform (DFT) (DFT) has good filter effect, but often having calculated a DFT computing needs N2 complex multiplication and N (N-1) secondary complex addition, and calculated amount is comparatively large, and wherein N is the sampling number of every cycle; Recommendation fast fourier transform (FFT) algorithm carrys out filtering, and each FFT computing only needs secondary complex multiplication and secondary complex addition operations, can reduce operand when n is large greatly;
Described non-equilibrium among three phase voltages (Voltage Imbalance, VI) formula such as formula shown in (2), three-phase current unbalance degree (Current Imbalance, CI) such as formula shown in (3);
VI = V ( 2 ) V ( 1 ) &times; 100 % - - - ( 2 )
CI = I ( 2 ) I ( 1 ) &times; 100 % - - - ( 3 )
Wherein, V (2)for the amplitude of negative sequence voltage, V (1)for the amplitude of positive sequence voltage, I (2)for the amplitude of negative-sequence current, I (1)for forward-order current amplitude;
UL1741 or IEEE1547 all can be used as the testing standard of islanding detect; This sentences the standard that UL1741 standard will be observed as islanding detect;
Judge whether non-equilibrium among three phase voltages and three-phase voltage total harmonic distortion factor all exceed respective threshold value,
Describedly judge whether non-equilibrium among three phase voltages and three-phase voltage total harmonic distortion factor all exceed respective threshold value, be the reliability and sensitivity ensureing isolated island monitoring, VI threshold value is chosen as the value a little less than C;
As do not exceeded, return (2) step;
If all exceeded, and the time continuing to exceed be greater than the time definite value of setting, then judge there is islet operation situation.
This method is injected negative-sequence current by starting inverter when voltage harmonic aberration rate exceedes definite value, is detected points of common connection voltage unbalance factor, and the time definite value exceeding setting when PCC voltage unbalance factor can judge that isolated island occurs.
The principle of described step (4) is:
If G1 is PV power supply, the given three-phase current of setting inverter determine the size injecting positive sequence, negative-sequence current; Grid is main electrical network, and can equivalence be a voltage source, its voltage be expressed as U grid; Z dG (1), Z dG (2)for the equivalent impedance of positive-negative sequence power supply, Z l (1), Z l (2)for load is to the positive-negative sequence equivalent impedance of PCC, Z s (1), Z s (1)be the positive-negative sequence equivalent impedance of main electrical network to PCC;
When K is closed there is not isolated island time PCC positive-negative sequence voltage calculate such as formula shown in (5) formula (6);
U . PCC ( 1 ) = Z L ( 1 ) U . Grid + I . G ( 1 ) Z S ( 1 ) Z L ( 1 ) + Z S ( 1 ) - - - ( 5 )
U . PCC ( 2 ) = I . G ( 2 ) Z S ( 2 ) Z L ( 2 ) Z S ( 2 ) + Z L ( 2 ) - - - ( 6 )
Due to Z s (1), Z s (2)value respectively relative Z l (1), Z l (2)very little, therefore , the voltage unbalance factor VI=0 of PCC;
When isolated island appears in K disconnection, PCC positive sequence voltage, negative sequence voltage calculate such as formula shown in (7) formula (8);
U . PCC ( 1 ) = I . G ( 1 ) Z L ( 1 ) - - - ( 7 )
U . PCC ( 2 ) = I . G ( 2 ) Z L ( 2 ) - - - ( 8 )
Generally Z l (1)=Z l (2), therefore have
VI = V PCC ( 2 ) V PCC ( 1 ) &times; 100 % = I G ( 2 ) I G ( 1 ) &times; 100 % - - - ( 9 )
Wherein, U pCC (1), U pCC (2), I g (1), I g (2)be respectively the positive and negative sequence voltage of PCC and the positive and negative sequence electric current phasor modulus value of G1 injection PCC;
Also may produce the negative sequence voltage exceeding Islanding detection threshold values at PCC place when grid side generation unsymmetrical short-circuit, it is equivalent to access a negative sequence voltage source G2 at PCC, now iff dependence voltage unbalance factor, islanding detect erroneous judgement may occur; When noticing generation unsymmetrical short-circuit, only short trouble there is moment and failure removal transient voltage percent harmonic distortion very high, and the damping time constant of harmonic wave is usually very little, therefore here using voltage harmonic aberration rate as the start-up criterion of islanding detect, when voltage harmonic aberration rate reaches the definite value of setting, to inverter control circuit given value of current value add negative sequence component, the time definite value exceeding setting when PCC voltage unbalance factor can judge that isolated island occurs.
Beneficial effect of the present invention is: the present invention using voltage harmonic aberration rate as the start-up criterion of islanding detect but not islanding detect criterion, reduces the difficulty of adjusting of this criterion, is conducive to a certain extent improving detection speed.And the strategy of negative-sequence current is just injected when voltage harmonic aberration rate exceedes definite value, ensure that the interference decreased most possibly under normal operating conditions electrical network.This method reasonably drop into negative-sequence current inject, reduce the interference to electrical network, not by unbalanced grid faults impact and without check frequency.Mate with load power at photo-voltaic power supply power, points of common connection voltage and frequency be not when skew occurs substantially, simulating, verifying has been carried out to method, even if result show when this be most disadvantageous in islanding detect, method still can detect island operation state rapidly and accurately.For distributed power source during isolated island is out of service, or proceed to islet operation control mode by being incorporated into the power networks and provide effective reference signal.For safe operation of power system and detection provide a kind of good method.
Accompanying drawing explanation
Fig. 1 is islanding detect electrical network theory structure schematic diagram.
Fig. 2 is negative-sequence current injecting principle schematic diagram.
Fig. 3 is negative sequence network schematic diagram.
Fig. 4 is positive sequence network schematic diagram.
Fig. 5 is electric network fault principle schematic.
Fig. 6 is islanding detect theory diagram.
Fig. 7 is voltage oscillogram before and after isolated island.
Fig. 8 is points of common connection change in voltage schematic diagram.
Fig. 9 is points of common connection frequency diagram.
Figure 10 is voltage harmonic aberration rate schematic diagram.
Figure 11 is that DG injects PCC current unbalance factor schematic diagram.
Figure 12 is PCC voltage unbalance factor schematic diagram.
Figure 13 is islanding detect result schematic diagram.
Figure 14 is islanding detect result close-up schematic view.
Figure 15 is schematic flow sheet of the present invention.
Embodiment
The present invention is further illustrated below in conjunction with the drawings and specific embodiments.
A kind of method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current, Fig. 1 is islanding detect electrical network theory structure schematic diagram, as shown in Figure 1, the DC voltage of the electric energy that photovoltaic battery array sends after DC/DC conversion makes conversion meets grid-connected requirement, in DC/DC conversion, take the maximal power tracing of conductance increment method to control (MPPT) simultaneously, by inverter, direct current is converted to alternating current grid-connected, when PV is grid-connected, takes unity power factor Current Control.Wherein, R f, L ffor resistance and the inductance of inverter ac side filter, R s, L sbe main electrical network equivalent resistance and inductance, load model can be equivalent to RLC parallel connection.I abc=[i ai bi c] tfor the three-phase current phasor that photovoltaic DC-to-AC converter exports.Negative-sequence current injecting principle figure as shown in Figure 2, wherein, i abc * = i abc ( 1 ) * + i abc ( 2 ) * = i a * i b * i c * T Three-phase reference current given during for carrying out Current Control, the output current i of inverter abctracing preset electric current by given different forward-order current i abc ( 1 ) * = i a ( 1 ) * i b ( 1 ) * i c ( 1 ) * T And negative-sequence current i abc ( 2 ) * = i a ( 2 ) * i b ( 2 ) * i c ( 2 ) * T , Can control the size injecting PCC positive sequence, negative-sequence current, wherein C is the ratio of given forward-order current amplitude and given negative-sequence current amplitude.Combining inverter exports and is connected at PCC with electrical network, and the folding condition of K switch reflects the generation of isolated island, V pCCfor the three-phase voltage of points of common connection, comprising each harmonic component, fundamental positive sequence, first-harmonic negative sequence component and zero-sequence component.
GB/T15543-2008 specifies, electric system PCC normal working voltage degree of unbalancedness permissible value is 2%, must not more than 4% in the short time, and therefore arranging C is 0.04, and the current unbalance factor namely injecting PCC is 4%.
Fig. 2 is negative-sequence current injecting principle schematic diagram.Combining inverter exports and is connected at points of common connection PCC with main electrical network Grid, is reflected and simulate the generation of isolated island by the folding of K switch.UL1741 or IEEE1547 all can be used as the testing standard of islanding detect.Islanding detect is carried out herein with UL1741 standard.The basic ideas of institute's extracting method do not inject negative-sequence current when voltage THD is less than given definite value, injects negative-sequence current when voltage THD is greater than given definite value, and the voltage unbalance factor then by detecting points of common connection judges island state.
Based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects, the method step is:
(1) by the three-phase voltage V at voltage measuring transformer PCC place av bv c,
(2) the three-phase voltage V at this PCC place is calculated av bv ctotal harmonic distortion factor (THD);
Described total harmonic distortion factor (Total Harmonic Distortion, THD) computing formula is such as formula (1):
THD = &Sigma; n = 2 M V n 2 V 1 &times; 100 % - - - ( 1 )
Wherein, V nfor nth harmonic effective value;
(3) judge whether THD exceeds the threshold value of setting;
Threshold value described in described threshold reference table 1(is the mains by harmonics voltage limits as shown in table 1 of GB/T14549-1993 regulation);
As do not exceeded, return (2) step;
As exceeded, starting and negative-sequence current injected Driven by inverter control circuit:
Measure the output voltage U of photovoltaic cell component dCwith output current I dCits output power P=I of real-time calculating dCu dC, because the power of inverter output AC side is equal with DC side power, therefore, u aCfor electrical network AC rated voltage, I * abcfor input inverter current effective value; Three-phase reference current when this electric current is Current Control;
Inject the negative phase-sequence equivalent network of measurement PCC voltage unbalance factor as shown in Figure 3 based on negative-sequence current, positive sequence equivalent network is (Fig. 3 is negative sequence network schematic diagram, and Fig. 4 is positive sequence network schematic diagram) as shown in Figure 4.Wherein, G1 is PV power supply, is equivalent to a current source, can by the given three-phase current of setting inverter determine the size injecting positive sequence, negative-sequence current.Grid is main electrical network, and can equivalence be a voltage source, its voltage be expressed as U grid.Z dG (1), Z dG (2)for the equivalent impedance of positive-negative sequence power supply, Z l (1), Z l (2)for load is to the positive-negative sequence equivalent impedance of PCC, Z s (1), Z s (1)be the positive-negative sequence equivalent impedance of main electrical network to PCC.
When negative-sequence current injects, the output current i of inverter abcthe given current three-phase reference current that follow current controls i abc * = i abc ( 1 ) * + i abc ( 2 ) * = i a * i b * i c * T ; By injecting different forward-order currents i abc ( 1 ) * = i a ( 1 ) * i b ( 1 ) * i c ( 1 ) * T And negative-sequence current i abc ( 2 ) * = i a ( 2 ) * i b ( 2 ) * i c ( 2 ) * T , Control the size injecting PCC positive sequence, negative-sequence current; Wherein, C is the ratio of given forward-order current amplitude and given negative-sequence current amplitude; V pCCfor the three-phase voltage of points of common connection, comprising each harmonic component, fundamental positive sequence, first-harmonic negative sequence component and zero-sequence component;
(4) then detect the voltage with the PCC place of grid contact, calculate PCC place three-phase voltage V av bv ctotal harmonic distortion factor (THD), positive-negative sequence voltage and voltage unbalance factor VI,
Three-phase voltage vector U abc = U . a U . b U . c T Can be analyzed to positive sequence voltage U ( 1 ) = U . a ( 1 ) U . b ( 1 ) U . c ( 1 ) T , negative sequence voltage U ( 2 ) = U . a ( 2 ) U . b ( 2 ) U . c ( 2 ) T And residual voltage U ( 0 ) = U . a ( 0 ) U . b ( 0 ) U . c ( 0 ) T
Be the positive-sequence component of special phase, negative sequence component and zero-sequence component and three-phase voltage vector U with A phase abcrelation such as formula shown in (4);
U . a ( 1 ) U . a ( 2 ) U . a ( 0 ) = 1 3 1 a a 2 1 a 2 a 1 1 1 U . a U . b U . c - - - ( 4 )
Because PCC has the Injection Current of inverter, so PCC voltage contains harmonic component, first must carry out filtering process before each order components of extraction PCC voltage, and obtain fundametal compoment.Discrete Fourier transform (DFT) (DFT) has good filter effect, but often having calculated a DFT computing needs N2 complex multiplication and N (N-1) secondary complex addition, and calculated amount is comparatively large, and wherein N is the sampling number of every cycle; Here adopt fast fourier transform (FFT) algorithm to carry out filtering, each FFT computing only needs secondary complex multiplication and secondary complex addition operations, can reduce operand when n is large greatly;
Described non-equilibrium among three phase voltages (Voltage Imbalance, VI) formula such as formula shown in (2), three-phase current unbalance degree (Current Imbalance, CI) such as formula shown in (3);
VI = V ( 2 ) V ( 1 ) &times; 100 % - - - ( 2 )
CI = I ( 2 ) I ( 1 ) &times; 100 % - - - ( 3 )
Wherein, V (2)for the amplitude of negative sequence voltage, V (1)for the amplitude of positive sequence voltage, I (2)for the amplitude of negative-sequence current, I (1)for forward-order current amplitude;
UL1741 or IEEE1547 all can be used as the testing standard of islanding detect; This sentences UL1741 standard is according to carrying out islanding detect;
Judge whether non-equilibrium among three phase voltages and three-phase voltage total harmonic distortion factor all exceed respective threshold value,
VI threshold value elects the value a little less than C as;
As do not exceeded, return (2) step;
If all exceeded, and the time continuing to exceed be greater than the time definite value of setting, then judge there is islet operation situation.
The time definite value of described setting: can not be mistaken for isolated island when should ensure grid disturbance, ensures that the reliability of islanding detect is not judged by accident, meets the requirement that UL1741 specifies again.
For step (4), when K is closed there is not isolated island time PCC positive-negative sequence voltage calculate such as formula shown in (5) formula (6):
U . PCC ( 1 ) = Z L ( 1 ) U . Grid + I . G ( 1 ) Z S ( 1 ) Z L ( 1 ) + Z S ( 1 ) - - - ( 5 )
U . PCC ( 2 ) = I . G ( 2 ) Z S ( 2 ) Z L ( 2 ) Z S ( 2 ) + Z L ( 2 ) - - - ( 6 )
Due to Z s (1), Z s (2)value respectively relative Z l (1), Z l (2)very little, therefore , the voltage unbalance factor VI=0 of PCC.
When isolated island appears in K disconnection, PCC positive sequence voltage, negative sequence voltage calculate such as formula shown in (7) formula (8).
U . PCC ( 1 ) = I . G ( 1 ) Z L ( 1 ) - - - ( 7 )
U . PCC ( 2 ) = I . G ( 2 ) Z L ( 2 ) - - - ( 8 )
Generally Z l (1)=Z l (2), therefore have
VI = V PCC ( 2 ) V PCC ( 1 ) &times; 100 % = I G ( 2 ) I G ( 1 ) &times; 100 % - - - ( 9 )
Wherein, U pCC (1), U pCC (2), I g (1), I g (2)be respectively the positive and negative sequence voltage of PCC and the positive and negative sequence electric current phasor modulus value of G1 injection PCC.
In addition, also may produce the negative sequence voltage exceeding Islanding detection threshold values at PCC place when grid side generation unsymmetrical short-circuit, it is equivalent to access a negative sequence voltage source G2 at PCC, and (Fig. 5 is electric network fault principle schematic.), as shown in Figure 5, now iff dependence voltage unbalance factor, islanding detect erroneous judgement may be there is.When noticing generation unsymmetrical short-circuit, only short trouble there is moment and failure removal transient voltage percent harmonic distortion very high, and the damping time constant of harmonic wave is usually very little, therefore herein using voltage harmonic aberration rate as the start-up criterion of islanding detect, when voltage harmonic aberration rate reaches definite value (definite value select can reference table 1) of setting, to inverter control circuit given value of current value add negative sequence component, the time definite value exceeding setting when PCC voltage unbalance factor can judge that isolated island occurs, and islanding detect theory diagram as indicated with 6.(Fig. 6 is islanding detect theory diagram.)
Using voltage harmonic aberration rate as the start-up criterion of islanding detect but not islanding detect criterion, reduce the difficulty of adjusting of this criterion, be conducive to a certain extent improving detection speed.And the strategy of negative-sequence current is just injected when voltage harmonic aberration rate exceedes definite value, ensure that the interference decreased most possibly under normal operating conditions electrical network.
(Fig. 1 is islanding detect electrical network theory structure schematic diagram to this method realistic model of concrete enforcement as shown in Figure 1.), major network system voltage is 35kV, as there is line to line fault at PCC place, analyzes unsymmetrical short-circuit to this paper island detection method with or without impact.Transition resistance 20 Ω arranged, emulate total duration 10s, AB line to line fault is there is in the 4s moment, through 0.5s failure removal, disconnect at 6s moment isolating switch K, PV islet operation, consider the situation of least favorable islanding detect simultaneously, namely PV capacity mates with load capacity, and PV power factor is identical with load power factor.
Before and after isolated island, voltage waveform as shown in Figure 7 (Fig. 7 is voltage oscillogram before and after isolated island), and (Fig. 8 is points of common connection change in voltage schematic diagram to the change in voltage of points of common connection as shown in Figure 8.), as shown in Figure 8 when PV capacity mates with load capacity, points of common connection voltage drop when breaking down, know that isolated island moment points of common connection voltage has momentary fluctuation by Fig. 7, Fig. 8, the original level of basic maintenance is constant later, is difficult to detect that isolated island occurs as seen when the power that PV sends mates with load power based on the method for variation.
Fig. 9 is points of common connection frequency diagram.As shown in Figure 9, when the known power sent as PV mates with load power, during isolated island mains frequency have small size change but electric system allow frequency departure within the scope of, the island detection method based on frequency shift (FS) may lose efficacy.
Figure 10 is voltage harmonic aberration rate schematic diagram.Normally running time-harmonic wave voltage distortion rate is as shown in Figure 10 0, rise in short-term when grid collapses and failure removal transient voltage percent harmonic distortion have, but very fast decay to 0, when knowing isolated island by Fig. 7 there is distortion harmonic content and rise in PCC voltage, and voltage harmonic aberration rate is instantaneous to be risen to 2.7% and remain unchanged.
Figure 11 is that DG injects PCC current unbalance factor schematic diagram.The negative-sequence current degree of unbalancedness of PV injection PCC only has instantaneous rising then to drop to less than 1% very soon when the grid collapses as shown in Figure 11, when failure removal, negative-sequence current degree of unbalancedness has to raise in short-term and then drops to 0 very soon, and during isolated island, PV injects PCC current unbalance factor and rises to 4% at once.
Figure 12 is PCC voltage unbalance factor schematic diagram.Know when there is unbalanced fault by Figure 12, points of common connection voltage unbalance factor rises to the level being greater than 4%, and failure removal voltage unbalance factor reduces to 0, and during isolated island, voltage unbalance factor rises to 4%, it equals the number percent that negative-sequence current injects, and meets formula (9).Visible simple dependence voltage unbalance factor detects the impact that isolated island is subject to unbalanced grid faults, produces erroneous judgement.According to Fig. 6, propose to exceed initiation value based on percent harmonic distortion herein and inject negative-sequence current, for ensureing that the reliability of islanding detect reduces again that can to arrange this definite value to the interference of electrical network be 2% as far as possible, judges that the PCC voltage unbalance factor definite value of isolated island is as 3%.Islanding detect time delay t is set to 0.1s herein, and islanding detect result as shown in figure 13.
Figure 13 is islanding detect result schematic diagram.This method avoid the erroneous judgement of electric network fault islanding detect as seen from Figure 13, namely during 4s, electric network fault can not be mistaken for isolated island, Figure 14 is islanding detect result close-up schematic view, and during real isolated island, 0.13s can detect, lower than GB/T19939-2005 official hour as shown in Figure 14.
Figure 15 is schematic flow sheet of the present invention.
The present invention is directed to the deficiency of traditional non-blind area island detection method, the island detection method that a kind of negative-sequence current based on percent harmonic distortion injects is proposed, it does not inject negative-sequence current under electrical network normal operation, start inverter to inject negative-sequence current when voltage harmonic aberration rate exceedes definite value, detect points of common connection voltage unbalance factor, the time definite value exceeding setting when PCC voltage unbalance factor can judge that isolated island occurs.There is, strong interference immunity, islanding detect fireballing feature little to grid disturbance compared with other active alone island detection methods.Also fast and effeciently can detect isolated island when PV power mates with load power, now line voltage mains frequency is all in the normal operation range of regulation.For distributed power source during isolated island is out of service, or proceed to islet operation control mode by being incorporated into the power networks and provide effective reference signal.

Claims (7)

1., based on the method for detecting island of photovoltaic grid-connected inverter that negative-sequence current injects, it is characterized in that,
Suppose that existing electrical network is equivalent to:
The DC voltage of the electric energy that photovoltaic battery array sends after DC/DC conversion makes conversion meets grid-connected requirement, in DC/DC conversion, take the maximal power tracing of conductance increment method to control (MPPT) simultaneously, by inverter, direct current is converted to alternating current grid-connected, when PV is grid-connected, takes unity power factor Current Control;
Wherein, R f, L ffor resistance and the inductance of inverter ac side filter, R s, L sbe main electrical network equivalent resistance and inductance, load model can be equivalent to RLC parallel connection; i abc=[i ai bi c] tfor the three-phase current phasor that photovoltaic DC-to-AC converter exports;
The method step is:
(1) the three-phase voltage V at PCC place is measured a, V band V c,
(2) the three-phase voltage V at this PCC place is calculated a, V band V ctotal harmonic distortion factor (THD);
Described total harmonic distortion factor (Total Harmonic Distortion, THD) computing formula is such as formula (1):
THD = &Sigma; n = 2 H V n 2 V 1 &times; 100 % - - - ( 1 )
Wherein, V nfor nth harmonic effective value;
(3) judge whether THD exceeds the threshold value of setting;
As do not exceeded, return (2) step;
As exceeded, starting and negative-sequence current injected Driven by inverter control circuit:
Measure the output voltage U of photovoltaic cell component dCwith output current I dCits output power P=I of real-time calculating dCu dC, because the power of inverter output AC side is equal with DC side power, therefore, u aCfor electrical network AC rated voltage, I * abcfor input inverter current effective value; Three-phase reference current when this electric current is Current Control;
When negative-sequence current injects, the output current i of inverter abcthe given current three-phase reference current that follow current controls and i abc * = i abc ( 1 ) * + i abc ( 2 ) * = i a * i b * i c * T ; By injecting different forward-order currents i abc ( 1 ) * = i a ( 1 ) * i b ( 1 ) * i c ( 1 ) * T And negative-sequence current i abc ( 2 ) * = i a ( 2 ) * i b ( 2 ) * i c ( 2 ) * T Control the size injecting PCC positive sequence, negative-sequence current; Wherein, C is the ratio of given forward-order current amplitude and given negative-sequence current amplitude; V pCCfor the three-phase voltage of points of common connection, comprising each harmonic component, fundamental positive sequence, first-harmonic negative sequence component and zero-sequence component;
Be the positive-sequence component of special phase, negative sequence component and zero-sequence component and three-phase voltage vector U with A phase abcrelation such as formula shown in (4):
U . a ( 1 ) U . a ( 2 ) U . a ( 0 ) = 1 3 1 a a 2 1 a 2 a 1 1 1 U . a U . b U . c - - - ( 4 )
Because PCC has the Injection Current of inverter, so PCC voltage contains harmonic component, first must carry out filtering process before each order components of extraction PCC voltage, and obtain fundametal compoment; Described filtering algorithm is fast fourier transform (FFT);
(4) then detect the voltage with the PCC place of grid contact, calculate PCC place three-phase voltage V a, V band V ctotal harmonic distortion factor (THD), positive-negative sequence voltage and voltage unbalance factor VI,
Three-phase voltage vector U abc = U . a U . b U . c T Can be analyzed to positive sequence voltage U ( 1 ) = U . a ( 1 ) U . b ( 1 ) U . c ( 1 ) T , Negative sequence voltage U ( 2 ) = U . a ( 2 ) U . b ( 2 ) U . c ( 2 ) T And residual voltage U ( 0 ) = U . a ( 0 ) U . b ( 0 ) U . c ( 0 ) T ;
Described non-equilibrium among three phase voltages (Voltage Imbalance, VI) formula is such as formula shown in (2), and three-phase current unbalance degree (Current Imbalance, CI) is such as formula shown in (3);
VI = V ( 2 ) V ( 1 ) &times; 100 % - - - ( 2 )
CI = I ( 2 ) I ( 1 ) &times; 100 % - - - ( 3 )
Wherein, V (2)for the amplitude of negative sequence voltage, V (1)for the amplitude of positive sequence voltage, I (2)for the amplitude of negative-sequence current, I (1)for forward-order current amplitude;
This sentences UL1741 standard is according to carrying out islanding detect;
Judge whether non-equilibrium among three phase voltages and three-phase voltage total harmonic distortion factor all exceed respective threshold value,
As do not exceeded, return (2) step;
If all exceeded, and the time continuing to exceed be greater than the time definite value of setting, then judge there is islet operation situation.
2. the method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current according to claim 1, is characterized in that, the three-phase voltage V at described measurement PCC place a, V band V cby voltage measuring transformer.
3. the method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current according to claim 1, it is characterized in that, described step (3) judges whether THD exceeds the threshold value of setting, and described threshold value is the mains by harmonics voltage limits that GB/T14549-1993 specifies.
4. the method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current according to claim 1, it is characterized in that, judge whether non-equilibrium among three phase voltages and three-phase voltage total harmonic distortion factor all exceed respective threshold value in described step (4), for ensureing the reliability and sensitivity of isolated island monitoring, VI threshold value elects the value a little less than C as.
5. the method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current according to claim 1, it is characterized in that, judge whether non-equilibrium among three phase voltages and three-phase voltage total harmonic distortion factor all exceed respective threshold value, and described threshold value is 3% in described step (4).
6. the method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current according to claim 1, it is characterized in that, the time definite value of described setting is: can not be mistaken for isolated island when should ensure grid disturbance, ensure that the reliability of islanding detect is not judged by accident, meet the requirement that UL1741 specifies again.
7. the method for detecting island of photovoltaic grid-connected inverter injected based on negative-sequence current according to claim 6, it is characterized in that, the time definite value of described setting is: 0.1s.
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