CN107046296B - MPPT-based adaptive island detection algorithm - Google Patents

Abstract

The invention relates to an MPPT-based self-adaptive island detection algorithm, which specifically comprises the following steps: and determining the disturbance amount of the duty ratio D of the Boost booster circuit according to the voltage amplitude of the grid-connected point before disturbance begins, realizing the disturbance of MPPT and photovoltaic output active power through the disturbance amount of the duty ratio D, and detecting the island state when the voltage amplitude of the grid-connected point deviates from the normal range when the power grid is powered off. The MPPT-based adaptive island detection algorithm provided by the invention does not introduce extra harmonic waves; the defect that an existing MPPT-based island detection algorithm has a detection blind area is overcome, self-adaptive disturbance is carried out on the duty ratio D, and power loss is reduced.

Description

MPPT-based adaptive island detection algorithm

Technical Field

The invention relates to the field of distributed photovoltaic power generation, in particular to an MPPT-based adaptive island detection algorithm.

Background

In recent years, distributed photovoltaic is rapidly developed, and with more and more distributed photovoltaic power generation systems connected to a power grid, new problems are caused, and one of the key problems is how to detect an islanding effect. The islanding effect means that when the power supply of a power grid is interrupted due to a fault accident or power failure maintenance, a distributed power supply cannot detect a power failure state and timely disconnects the power grid, and a local power supply system which is controlled by a power grid company is formed. The island effect can cause reclosing failure, the personal injury of maintainers and the damage of electrical equipment, so that the research of island detection has important significance.

Common island detection methods include both passive and active methods. The passive method only needs to detect parameters output by the inverter, and cannot affect the quality of electric energy and the stability of the system, but the threshold value of the method is difficult to set, and the detection blind area is large. The active method mainly includes a sliding mode frequency drift method, an active frequency shift method, an active current disturbance method and the like. The active method effectively reduces the detection blind area, but the output of the inverter needs to be disturbed, the quality of electric energy is influenced, and power loss is caused. The sliding mode frequency drift method and the active frequency shift method introduce extra harmonic waves, and the detection blind area is greatly influenced by the quality factor. The active current disturbance method does not introduce extra harmonic waves, and the detection blind area is less influenced by the quality factor, but the method causes unbalance between the photovoltaic output power and the inverter output power, so that the stability of the direct current bus voltage is influenced. Compared with a sliding mode frequency drift method and an active frequency shift method, the MPPT-based active power disturbance method has the advantages that extra harmonic waves cannot be introduced, and the detection blind area is less influenced by quality factors.

Disclosure of Invention

In view of the above, the present invention provides an adaptive islanding detection algorithm based on MPPT.

The invention aims to realize the self-adaptive island detection algorithm based on MPPT by the following technical scheme, and the method specifically comprises the following steps:

and determining the disturbance amount of the duty ratio D of the Boost booster circuit according to the voltage amplitude of the grid-connected point before disturbance begins, realizing the disturbance of MPPT and photovoltaic output active power through the disturbance amount of the duty ratio D, and detecting the island state when the voltage amplitude of the grid-connected point deviates from the normal range when the power grid is powered off.

Further, when U is turned_PCC0<0.88U_NThen, it is determined as an islanding state, U_PCC0To grid point voltage, U_NIs the effective value of the voltage of the power grid.

Further, when U is turned_PCC0>1.1U_NThen, it is determined as an islanding state, U_PCC0To grid point voltage, U_NIs the effective value of the voltage of the power grid.

Further, when 0.5Hz < | f-f_sWhen is, determining the island state, and f is the voltage frequency of the grid-connected pointRate, f_sThe rated frequency of the power grid is 50 Hz.

Further, the duty ratio D, D before and after the Boost circuit disturbance₁Satisfies the following conditions:

wherein, U_mIs the output voltage of the photovoltaic cell panel at the maximum power point, U_TIs a DC bus voltage, U_PVIs the photovoltaic output voltage.

Further, the period of perturbation to the duty cycle is 2 s.

Due to the adoption of the technical scheme, the invention has the following advantages:

the MPPT-based adaptive island detection algorithm provided by the invention does not introduce extra harmonic waves; the defect that an existing MPPT-based island detection algorithm has a detection blind area is overcome, self-adaptive disturbance is carried out on the duty ratio D, and power loss is reduced.

Drawings

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of a photovoltaic grid-connected system;

FIG. 2 is a graph of photovoltaic panel output characteristics;

FIG. 3 is a flow chart of an island detection algorithm;

FIG. 4 shows the perturbation results under normal conditions;

fig. 5R ═ R_mTime simulation results; (a) voltage waveform diagram of grid connection point, (b) duty ratio D and photovoltaic output voltage U_PVInverter output power P, (c) U_PCCAnd U_N(d) grid-connected point voltage frequency f;

fig. 6R ═ 1.21R_mTime simulation results; (a) voltage waveform diagram of grid connection point, (b) local enlarged diagram when island occurs, (c) grid connection pointVoltage frequency f, (e) partial enlarged view in island detection, (d) U_PCCAnd U_NThe ratio of (A) to (B);

FIG. 7 is a schematic illustration of an experimental platform;

fig. 8R ═ R_mReal-time test results;

fig. 9R ═ 1.21R_mThe results are tested.

Detailed Description

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 shows an original island detection graph, P_PV、Q_PVRespectively photovoltaic output active power and reactive power, P, Q respectively inverter output active power and reactive power, a node a is a grid connection point, S is a circuit breaker, U is a power supply_gFor the voltage of the power grid, delta P and delta Q are respectively active power and reactive power transmitted into the power grid from a node a, R, L, C is an equivalent load, and P is_Load、Q_LoadRespectively load active power and reactive power. Neglecting the inverter loss, the photovoltaic output power is equal to the inverter output power, i.e. P_PV＝P、Q_PV＝Q。

Fig. 1 shows that when the circuit breaker S is closed, the grid-connected point voltage U is clamped by the grid_PCCAmplitude of and grid voltage amplitude U_gEqual, grid point voltage when the circuit breaker S is open

(R is load resistance), therefore, in an island state, the active power of the photovoltaic output is properly disturbed, which can cause the fluctuation of the voltage amplitude of the grid-connected point, if U is adopted_PCCAnd if the over/under voltage protection threshold value is exceeded, the islanding state can be detected. At present, a photovoltaic grid-connected system is generally a unit power factor grid-connected system, is provided with a Maximum Power Point Tracking (MPPT) system and works at a maximum power point. The MPPT is disturbed by the disturbance, the photovoltaic output power can be influenced, and the grid-connected point voltage U can be influenced in an island state_PCCThe amplitude of (c).

Fig. 2 is a graph of photovoltaic output characteristics. As can be seen from the figure, the photovoltaic output voltage U_PVFrom the maximum power pointU_mReduced, photovoltaic output current I_PVSubstantially unchanged when the photovoltaic output power P is_PVAnd the photovoltaic output voltage U_PVIn a linear relationship, i.e. P_PV＝K*U_PV(K is the slope of the P-U curve). The power of the photovoltaic output can be reduced by reducing the photovoltaic output voltage.

The photovoltaic grid-connected power generation system can be divided into a single-stage type and a two-stage type according to the system structure. The single-stage photovoltaic grid-connected power generation system inverter needs to realize the functions of Maximum Power Point Tracking (MPPT) and grid connection at the same time, and the design of a controller is complex. The output voltage amplitude of the photovoltaic array meets the inversion requirement, and a large number of series-connected cell panels are needed. The two-stage photovoltaic grid-connected power generation system firstly realizes the functions of boosting and MPPT through DC/DC conversion and then realizes grid connection through DC/AC conversion. The DC/DC and DC/AC two-stage conversion can be independently controlled, and the controller is simple in design. The DC/DC conversion can realize the boosting function, so that the output voltage of the photovoltaic array is improved without increasing the number of series-connected battery boards.

Distributed photovoltaic power generation system is generally low in power, the number of battery plates connected in series is limited, and the voltage of a power grid is difficult to reach

And the DC/AC conversion can be realized only by boosting, so that the distributed photovoltaic power generation system adopts a two-stage structure. The front-stage DC/DC conversion circuit adopts a Boost circuit to realize the functions of boosting and maximum power tracking. Because the direct current bus voltage is stable, as shown in formula (1), the duty ratio D of the Boost voltage boosting circuit is increased, so that the photovoltaic output voltage can be reduced, and further the photovoltaic output power is reduced.

Wherein D is the duty ratio of Boost circuit, U_TIs the dc bus voltage. The algorithm provided by the invention realizes the periodic disturbance of MPPT by periodically disturbing the duty ratio D of the Boost circuit, and influences the voltage of a grid-connected point when the power grid is cut off, so that the MPPT is periodically disturbedIt exceeds the over/under voltage protection threshold to detect an islanding condition.

IEEE Std.92-2000 stipulates that the voltage U of a grid-connected point after islanding_PCC0Satisfies 0.88U_N≤U_PCC0≤1.1U_N(U_NEffective value of the grid voltage), the islanding state needs to be detected within 2 s. The distributed photovoltaic grid-connected system works in a normal state most of the time, so that the longer the disturbance period is, the better the disturbance period is. The longer the disturbance period, the smaller the power loss caused in the unit time, and meanwhile, in order to detect the island state within 2s, the disturbance period set by the invention is 2s, and the disturbance time per period lasts for two power frequency periods.

The photovoltaic grid-connected system generally works at the maximum power point, the disturbance to the MPPT can only reduce the photovoltaic output power, so that in an island state, the disturbance to the MPPT can only reduce the voltage U of a grid-connected point_PPCThe amplitude of (c). Therefore, after the island occurs, the voltage amplitude of the grid-connected point meets 0.88U_N≤U_PCC0≤1.1U_NThe most difficult case to detect is U_PCC0＝1.1U_NIn order to ensure that detection is not missed, disturbance is required to be carried out according to the most difficult detection condition every time, so that the loss power is large, and therefore the invention provides an MPPT-based self-adaptive island detection algorithm.

(1) After island, if the voltage U of the grid-connected point occurs_PCC0The amplitude and the frequency meet one of A, B, C three conditions, namely, the islanding state can be determined according to the over-voltage, the under-voltage and the over-frequency and the under-frequency, and the three conditions are as follows:

A.U_PCC0<0.88U_N；

B.U_PCC0>1.1U_N；

C.0.5Hz＜|f-f_s|。

wherein f represents the grid-connected point voltage frequency, U_NRepresenting the effective value of the grid voltage.

(2) If island occurs, the voltage U of the grid-connected point_PCC0Satisfies 0.88U_N≤U_PCC0≤1.1U_NThen the MPPT must be disturbed to detect the islanding state.

Load work after island generationRate P_LoadSatisfies the following conditions:

P_m＝K*U_m(3)

wherein P is_mRepresenting the photovoltaic output active power at the maximum power point. The load power after disturbance needs to satisfy:

P_PV＝K*U_PV(5)

duty ratio D, D before and after Boost circuit disturbance₁Satisfies the following conditions:

the following formulae (2) to (7) can be used:

to ensure that the detection is not missed, take D₁Is represented by formula (9):

because the disturbance period is 2s, the disturbance duration of each period is two power frequency periods, whether an island occurs cannot be judged before disturbance, and therefore, during actual calculation, U in the formula (9)_PCC0Is taken as U_100n-2Then D is₁Can also be represented by formula (10):

under the condition that the illumination and the temperature do not change suddenly, the influence of the algorithm on the output power is shown as the formula (12).

P_PV＝0.96P_m+0.04P_PV＝0.96KU_m+0.04KU_PV(11)

Bringing formulas (6), (7) and (10) into formula (11):

as shown in formula (12), the power loss caused by the island detection algorithm provided by the invention is less than 1.5%.

In summary, the grid-connected point voltage amplitude in the islanding state is reduced to 0.88U by disturbance_NHereinafter, the over/under voltage protection threshold is exceeded, no detection blind area exists, and the algorithm flow chart is shown in fig. 3.

In order to verify the correctness and the effectiveness of the algorithm, the self-adaptive island detection algorithm based on MPPT is subjected to simulation verification on the three-phase grid-connected inverter by utilizing Matlab/Simulink. The simulation parameters are set as follows: the peak value of the grid phase voltage is 311V, and the frequency is 50 Hz; the voltage of the direct current bus is 800V; the LCL parameter of the filter circuit is L_i＝3.5mH、C_f＝4.5μF、L_s0.5 mH; the frequency of the switching tube is 10 KHz. In order to fully explain the problem, the present invention deals with R ═ R_mNamely U_PCC0＝U_N、R＝1.21R_mNamely U_PCC0＝1.1U_NRLC loads in both cases were simulated separately. The recommended RLC load resonant frequency is 50Hz and the quality factor Q is according to IEEE Std.92-2000_fThe RLC loading parameters corresponding to the two cases are respectively R37.2 Ω, C213.9 μ F, L μ 47.4 mH; r is 45 Ω, C is 176.8 μ F, L is 57.3mH, and the islanding time is 1.5 s. According to the island detection algorithm provided by the invention, the disturbance cycle is 2s, and each cycle implements continuous two power frequency cycle disturbances on MPPT. Because the three-phase voltage current waveforms are the same under the three-phase balance condition, the invention only provides the A-phase voltage current waveform.

Fig. 4 shows the perturbation result in the normal case. As can be seen from FIG. 4, under normal conditionsWhen the duty ratio D of the Boost circuit is disturbed, the duty ratio D is increased, and the photovoltaic output voltage U is output_PVReduction of the photovoltaic output current I_PVConstant, grid-connected point voltage U_PCCThe clamping effect of the power grid is not changed, and the output current I of the inverter is_aAnd decreases. The change of the parameters is consistent with theoretical analysis, namely, the MPPT can be disturbed by disturbing the duty ratio D of the Boost circuit, and the active power of photovoltaic output is reduced.

U_PCC0＝U_NI.e. R ═ R_m(R_mLoad resistance when power is balanced with load active power for inverter output) as shown in fig. 5. From fig. 5(a), (c) it can be seen that after the 1.5s islanding, due to the photovoltaic output power P_PV＝P_LoadVoltage of net point U_PCCThe amplitude does not change. Fig. 5(a) and (d) show that the resonant frequency of the RLC load is 50Hz, the frequency of the grid-connected point voltage does not change significantly, that is, the amplitude and the frequency of the grid-connected point voltage are both within a normal range, the system is in an islanding state, and the islanding state can be detected only by disturbing the MPPT. From fig. 5(b) and (c), it can be seen that after the MPPT is disturbed for 1.96s, the duty ratio D of the Boost circuit is increased, the output voltage of the photovoltaic array is decreased, the output power of the inverter is decreased, and the voltage amplitude U of the grid-connected point is further caused_PCCDecreasing rapidly, 1.995s grid connection point voltage amplitude decreases to 0.88U_NIn other words, it takes only 35ms to detect the islanding state.

U_PCC0＝1.1U_NI.e. R1.21R_mThe simulation results in the case are shown in fig. 6. As can be seen from fig. 6(a), (b), and (e), after the 1.5s island occurs, R is 1.21R_mVoltage of net point U_PCCAmplitude increase U_PCC≈1.1U_N. The change in inverter output power P in fig. 6(b) lags behind the change in duty ratio D due to the presence of the energy storage element. It can be seen from fig. 6(a), (c) that the frequency of the grid-connected point voltage does not change significantly because the resonant frequency of the RLC load is 50 Hz. The amplitude and the frequency of the grid-connected point voltage are within a normal range, the system is in an island state operation, and the MPPT is disturbed to detect the island state. As can be seen from fig. 6(d) and (e), after the MPPT is disturbed for 1.96s,grid connection point voltage amplitude U_PCCDecreasing rapidly, 1.995s grid connection point voltage amplitude decreases to 0.88U_NIn other words, it takes only 35ms to detect the islanding state.

In order to further verify the correctness of theoretical analysis, the experimental platform shown in fig. 7 is built.

The photovoltaic analog power supply adopts AGP1000 series photovoltaic analog power supply of Shenzhen chrysanthemum water royal science and technology Limited company, the DSP adopts TMS320F28335 control chip of TI company, and the three-phase parallel RLC load adopts chroma63802 electronic load. Load quality factor Q_f2.5, a resonant frequency of 50Hz, and a load consistent with the simulation into two cases, R ═ R_mNamely U_PPC0＝U_N1(U_N1For grid connection point voltage rating in experiment), R ═ 1.21R_mNamely U_PPC0＝1.1U_N1The RLC loading parameters for the two cases are respectively 15.1 Ω, 526.2 μ F, L μ 19.3 mH; r18.3 Ω, C434.9 μ F, L23.3 mH, peak grid phase voltage of 100V, and 50 Hz. The experimental waveform is shown in fig. 8 and 9, and the disturbance condition of the duty ratio D, the voltage and the current cannot be represented in the same time scale, so the invention replaces the change of the duty ratio D with the disturbance signal, and when the disturbance signal is 1, the disturbance starts, and when the disturbance signal is 0, the disturbance ends. I is_aIndicates that the inverter outputs phase A current, U_aRepresenting the grid a-phase voltage.

From fig. 8, it can be seen that R ═ R_mWhen the photovoltaic power generation system is in an island state, the output power of the photovoltaic array is equal to the load power, the load current is unchanged, and the load resistance is unchanged, so that the amplitude of the grid-connected point voltage is not obviously changed, and the resonance frequency of the RLC load is 50Hz, so that the amplitude and the frequency of the grid-connected point voltage are in a normal range. After the island occurs, disturbance is carried out on MPPT 40ms later, and the island state is detected 38ms later.

From fig. 9, it can be seen that R ═ 1.21R_mIn the process, after the island occurs, because the photovoltaic output power is greater than the load power, the load current is increased, and under the condition that the load resistance is not changed, the amplitude of the grid-connected point voltage is increased but still in a normal rangeAnd the resonance frequency of the RLC load is 50Hz, and the frequency of the voltage of the grid-connected point is not changed, so that the photovoltaic power generation system operates in an island state. And after the island is generated, 70ms, the MPPT starts to be disturbed, and after 33ms, the island state is detected.

Fig. 8 represents the experimental results under the general load condition, and fig. 9 represents the experimental results under the most severe load condition of the algorithm of the present invention. In both cases, as can be seen from fig. 8 and 9, the algorithm of the present invention can rapidly detect the islanding state. The experimental result is consistent with the theoretical analysis and simulation result, and the correctness and the effectiveness of the algorithm provided by the invention are verified.

The MPPT-based self-adaptive algorithm provided by the invention is based on the voltage amplitude U of the grid-connected point before disturbance begins_100n-2And determining the disturbance amount of the duty ratio D of the Boost circuit, realizing the disturbance of MPPT (maximum power point tracking) and photovoltaic output active power through the disturbance of the duty ratio D, influencing the voltage amplitude of a grid-connected point after an island occurs, enabling the grid-connected point to deviate from a normal range, and further detecting the island state. Compared with an island detection algorithm based on frequency, the algorithm provided by the invention does not introduce extra harmonic waves; the defect that an existing MPPT-based island detection algorithm has a detection blind area is overcome, self-adaptive disturbance is carried out on the duty ratio D, and power loss is reduced. The effectiveness of the algorithm provided by the invention is verified by carrying out simulation and experiment under the worst condition specified by IEEE Std.92-2000.

Finally, it is noted that the above-mentioned preferred embodiments illustrate rather than limit the invention, and that, although the invention has been described in detail with reference to the above-mentioned preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims (5)

1. An adaptive island detection algorithm based on MPPT is characterized in that: the method specifically comprises the following steps:

determining the disturbance amount of a duty ratio D of a Boost booster circuit according to the voltage amplitude of a grid-connected point before disturbance begins, realizing the disturbance of MPPT and photovoltaic output active power through the disturbance amount of the duty ratio D, and detecting the islanding state when the voltage amplitude of the grid-connected point deviates from the normal range when the power grid is cut off; wherein the period of perturbation to the duty cycle is 2 s;

if island occurs, the voltage U of the grid-connected point_PCC0Satisfies 0.88U_N≤U_PCC0≤1.1U_NThen, the MPPT must be disturbed to detect the island state; in order to ensure that the detection is not missed, the duty ratio of the Boost circuit after disturbance is as follows:

wherein, U_PCC0To grid point voltage, U_NThe effective value of the voltage of the power grid is;

the output power is:

wherein, P_mRepresenting the photovoltaic output active power at the maximum power point.

2. The MPPT-based adaptive island detection algorithm of claim 1, wherein: when U is turned_PCC0<0.88U_NThen, it is determined as an islanding state, U_PCC0To grid point voltage, U_NIs the effective value of the voltage of the power grid.

3. The MPPT-based adaptive island detection algorithm of claim 1, wherein: when U is turned_PCC0>1.1U_NThen, it is determined as an islanding state, U_PCC0To grid point voltage, U_NIs the effective value of the voltage of the power grid.

4. The MPPT-based adaptive island detection algorithm of claim 1, wherein: when 0.5Hz < | f-f_sWhen is, determining the island state, f is the voltage frequency of the grid-connected point, f_sFor rating the frequency of the gridThe rate is 50 Hz.

5. The MPPT-based adaptive island detection algorithm of claim 1, wherein: duty ratio D, D before and after Boost circuit disturbance₁Satisfies the following conditions:

wherein, U_mIs the output voltage of the photovoltaic cell panel at the maximum power point, U_TIs a DC bus voltage, U_PVIs the photovoltaic output voltage.

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