CN107046296A - Adaptive isolated island detection algorithm based on MPPT - Google Patents

Abstract

The invention relates to an MPPT-based self-adaptive islanding detection algorithm. The method is specifically as follows: according to the voltage amplitude of the grid-connected point before the disturbance starts, the disturbance amount to the duty ratio D of the Boost booster circuit is determined, and the duty ratio D is determined by The disturbance amount realizes the disturbance of MPPT and photovoltaic output active power, and the voltage amplitude of the grid-connected point deviates from the normal range when the grid is powered off, thereby detecting the islanding state. The MPPT-based adaptive island detection algorithm proposed by the present invention will not introduce additional harmonics; it overcomes the shortcomings of existing MPPT-based island detection algorithms that have detection blind spots, and performs adaptive disturbance on the duty cycle D, reducing the Power loss.

Description

Adaptive Island Detection Algorithm Based on MPPT

technical field

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

Background technique

In recent years, distributed photovoltaics have developed rapidly. As more and more distributed photovoltaic power generation systems are connected to the grid, some new problems have arisen. One of the key issues is how to detect the island effect. The so-called island effect means that when the power grid is interrupted due to a fault accident or power outage maintenance, the distributed power supply fails to detect the power outage and disconnects from the grid in time, thus forming a local power supply system that is out of the control of the power grid company. The islanding effect will cause reclosing failure, personal injury of maintenance personnel, and damage to electrical equipment, so the research on islanding detection is of great significance.

Common islanding detection methods include passive and active methods. The passive method only needs to detect the parameters output by the inverter, and will not affect the power quality and system stability. However, the threshold value of this method is difficult to set, and the detection blind zone is relatively large. Active methods mainly include sliding mode frequency drift method, active frequency shift method, active current disturbance method and other methods. The active method effectively reduces the detection blind area, but it needs to disturb the output of the inverter, which affects the power quality and causes power loss. The sliding mode frequency drift method and the active frequency shift method will introduce additional harmonics, and the detection blind zone is greatly affected by the quality factor. The active current disturbance method will not introduce additional harmonics, and the detection blind zone is less affected by the quality factor, but this method will cause an imbalance between the photovoltaic output power and the inverter output power, thereby affecting the stability of the DC bus voltage. Based on the MPPT active power disturbance method, this algorithm does not introduce additional harmonics compared with the sliding mode frequency drift method and active frequency shift method, and the detection blind zone is less affected by the quality factor, because the algorithm reduces the output of the battery panel The power reduces the output power of the inverter, so compared with the active current disturbance method, it has less influence on the DC bus voltage, which effectively makes up for the deficiency of the active current disturbance method, but its disturbance parameter design is unreasonable, resulting in large power loss. And there are detection blind spots.

Contents of the invention

In view of this, the object of the present invention is to provide an adaptive island detection algorithm based on MPPT.

The purpose of the present invention is achieved by the following technical solutions, a MPPT-based adaptive island detection algorithm, the method is specifically:

According to the voltage amplitude of the grid-connected point before the disturbance begins, the disturbance amount to the duty cycle D of the Boost booster circuit is determined, and the disturbance to the MPPT and photovoltaic output active power is realized through the disturbance amount to the duty cycle D. When the grid is powered off The voltage amplitude of the grid-connected point deviates from the normal range, and then the islanding state is detected.

Further, when U _PCC0 <0.88U _N , it is determined to be an island state, U _PCC0 is the grid-connected point voltage, and U _N is the effective value of the grid voltage.

Further, when U _PCC0 >1.1U _N , it is determined to be an island state, U _PCC0 is the grid-connected point voltage, and U _N is the effective value of the grid voltage.

Further, when 0.5Hz<|ff _s |, it is determined to be an island state, f is the voltage frequency of the grid-connected point, and f _s is the rated frequency of the grid, that is, 50Hz.

Further, the duty cycle D and D1 _of the boost circuit before and after the disturbance satisfy:

Among them, U _m is the output voltage of photovoltaic panels at the maximum power point, U _T is the DC bus voltage, and U _PV is the photovoltaic output voltage.

Further, the disturbance period for the duty cycle is 2s.

Owing to adopting above technical scheme, the present invention has the following advantages:

The MPPT-based adaptive island detection algorithm proposed by the present invention will not introduce additional harmonics; it overcomes the shortcomings of existing MPPT-based island detection algorithms that have detection blind spots, and performs adaptive disturbance on the duty cycle D, reducing the Power loss.

Description of drawings

In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be described in further detail below in conjunction with the accompanying drawings, wherein:

Figure 1 Photovoltaic grid-connected system diagram;

Figure 2 output characteristic curve of photovoltaic panel;

Figure 3 is a flow chart of the island detection algorithm;

Figure 4. Disturbance results under normal conditions;

Figure 5 Simulation results when R=R _m ; (a) grid-connected point voltage waveform diagram, (b) duty cycle D, photovoltaic output voltage U _PV , inverter output power P, (c) ratio of U _PCC to U _N , (d) Voltage frequency f of grid-connected point;

Fig. 6 Simulation results when R=1.21R _m ; (a) voltage waveform diagram of grid-connected point, (b) partial enlarged diagram when islanding occurs, (c) voltage frequency f of grid-connected point, (e) local enlarged diagram when islanding is detected, (d ) the ratio of U _PCC to U _N ;

Figure 7 Schematic diagram of the experimental platform;

Experimental results when Fig. 8R=R _m ;

Fig. 9 Experimental results when R=1.21R _m .

detailed description

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

Figure 1 shows the original map of island detection. In the figure, P _PV and Q _PV are the active power and reactive power output by photovoltaics respectively, P and Q are the active power and reactive power output by the inverter respectively, and node a is the grid-connected point , S is the circuit breaker, U _g is the grid voltage, ΔP, ΔQ are the active power and reactive power sent to the grid by node a respectively, R, L, and C are the equivalent loads, P _Load and Q _Load are the load active power respectively , Reactive power. Neglecting the loss of the inverter, the photovoltaic output power is equal to the output power of the inverter, that is, P _PV =P, Q _PV =Q.

It can be seen from Figure 1 that when the circuit breaker S is closed, the amplitude of the grid-connected point voltage U _PCC is equal to the grid voltage amplitude U _g due to the clamping effect of the grid. When the circuit breaker S is turned off, the grid-connected point voltage (R is the load resistance), so in the island state, proper disturbance of the photovoltaic output active power can cause fluctuations in the voltage amplitude of the grid-connected point. If the U _PCC exceeds the over/under voltage protection threshold, the island state can be detected. At present, the photovoltaic grid-connected system is generally connected to the grid with a unit power factor, and has a maximum power tracking system (MPPT), which works at the maximum power point. Therefore, disturbing the MPPT will affect the photovoltaic output power, and will affect the amplitude of the grid-connected point voltage U _PCC in the island state.

Figure 2 is a graph of photovoltaic output characteristics. It can be seen from the figure that when the photovoltaic output voltage _UPV decreases from _Um at the maximum power point, the photovoltaic output current _IPV basically does not change. At this time, the photovoltaic output power _{PPV has a linear relationship with the photovoltaic output voltage UPV ,} that is, P _PV =K* _UPV (K is the slope of the PU curve). Therefore, by reducing the photovoltaic output voltage, the photovoltaic output power can be reduced.

Photovoltaic grid-connected power generation systems can be divided into single-stage and two-stage systems according to the system structure. The inverter of a single-stage photovoltaic grid-connected power generation system needs to realize the functions of maximum power tracking (MPPT) and grid connection at the same time, and the controller design is complicated. To meet the inverter requirements for the output voltage amplitude of the photovoltaic array, a large number of panels need to be connected in series. The two-stage photovoltaic grid-connected power generation system first realizes the boost and MPPT functions through DC/DC conversion, and then realizes grid connection through DC/AC conversion. DC/DC and DC/AC two-stage conversion can be independently controlled, and the controller design is simple. DC/DC conversion can realize the boost function, so it is not necessary to increase the number of series-connected panels to increase the output voltage of the photovoltaic array.

Distributed photovoltaic power generation systems generally have relatively small power, and the number of battery panels connected in series is limited, so it is difficult to reach the grid voltage. times, so it is necessary to boost the voltage first to realize the DC/AC conversion, so the distributed photovoltaic power generation system mostly adopts a two-stage structure. Among them, the front-stage DC/DC conversion circuit mostly adopts Boost boost circuit to realize boost and maximum power tracking functions. Because the DC bus voltage is stable, as shown in formula (1), by increasing the duty cycle D of the Boost circuit, the photovoltaic output voltage can be reduced, thereby reducing the photovoltaic output power.

Among them, D is the duty cycle of the Boost circuit, and U _T is the DC bus voltage. The algorithm proposed by the present invention realizes periodic disturbance to MPPT by periodically disturbing the duty ratio D of the Boost circuit, and affects the voltage of the grid-connected point when the grid is powered off, making it exceed the over/under voltage Protection thresholds to detect islanding conditions.

IEEE Std.92-2000 stipulates that when the grid-connected point voltage U _PCC0 satisfies 0.88U _N ≤ U _PCC0 ≤ 1.1U _N (U _N is the effective value of the grid voltage) after islanding occurs, the islanding state must be detected within 2s. The distributed photovoltaic grid-connected system works in a normal state most of the time, so the longer the disturbance period, the better. The longer the disturbance period, the smaller the power loss per unit time. At the same time, in order to meet the detection of the island state within 2s, the disturbance period set by the present invention is 2s, and the disturbance time of each period lasts for two power frequency periods.

The photovoltaic grid-connected system generally works at the maximum power point, and the disturbance to MPPT can only reduce the photovoltaic output power. Therefore, in the island state, the disturbance to MPPT can only reduce the amplitude of the grid-connected point voltage U _PPC . Therefore, after islanding occurs, when the voltage amplitude of the grid-connected point satisfies 0.88U _N ≤ U _PCC0 ≤ 1.1U _N , the most difficult detection situation is U _PCC0 = 1.1U _N . Disturbance in the case of the situation will cause a large power loss, so the present invention proposes an adaptive island detection algorithm based on MPPT.

(1) After the islanding occurs, if the grid-connected point voltage U _PCC0 amplitude and frequency meet one of the three conditions A, B, and C, it can be determined as the islanding state according to the over-undervoltage and over-underfrequency. The three conditions are as follows :

AU _PCC0 <_0.88UN;

BU _PCC0 >_1.1UN;

C.0.5Hz<|ff _s |.

Among them, f represents the voltage frequency of the grid-connected point, and U _N represents the effective value of the grid voltage.

(2) If the grid-connected point voltage U _PCC0 satisfies 0.88U _N ≤ U _PCC0 ≤ 1.1U _N after the islanding occurs, then the MPPT must be disturbed to detect the islanding state.

After islanding occurs, the load power P _Load satisfies:

P _m =K*U _m (3)

Where P _m represents the photovoltaic output active power at the maximum power point. After the disturbance, the load power needs to meet:

P _PV ＝K*U _PV (5)

The duty ratios D and D1 _of the Boost circuit before and after the disturbance satisfy:

From formula (2)-(7) can get:

In order to ensure no missed detection, take D1 as shown in formula ( ₉ ):

Since the disturbance period is 2s, and the disturbance duration of each period is two power frequency periods, it is impossible to determine whether islanding occurs before the disturbance, so in actual calculation, U _PCC0 in formula (9) is taken as U _100n-2 , then D ₁ can also be Expressed as formula (10):

Under the condition that the illumination and temperature do not change suddenly, the influence of the algorithm proposed by the present invention on the output power is shown in formula (12).

P _PV ＝0.96P _m +0.04P _PV ＝0.96KU _m +0.04KU _PV (11)

Put formula (6), (7), (10) into formula (11) to get:

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

To sum up, after disturbance, the voltage amplitude of the grid-connected point in the island state will drop below 0.88U _N , exceeding the over/under voltage protection threshold, and there is no detection blind zone. The algorithm flow chart is shown in Figure 3.

In order to verify the correctness and effectiveness of this algorithm, the self-adaptive island detection algorithm based on MPPT is simulated and verified on a three-phase grid-connected inverter by using Matlab/Simulink. The simulation parameters are set as follows: the peak value of the grid phase voltage is 311V, the frequency is 50Hz; the DC bus voltage is 800V; the LCL parameters of the filter circuit are L _i =3.5mH, C _f =4.5μF, L _s =0.5mH; the switch tube frequency is 10KHz . In order to fully explain the problem, the present invention simulates the RLC loads in the two cases of R=R _m , that is, U _{PCC0 =} UN and R=1.21R _m , that is, U _PCC0 = _1.1Un . According to the recommended RLC load resonance frequency of IEEE Std.92-2000 is 50Hz, quality factor Q _f =2.5, the RLC load parameters corresponding to the two cases are R=37.2Ω, C=213.9μF, L=47.4mH; R= 45Ω, C=176.8μF, L=57.3mH, the islanding time is 1.5s. In the islanding detection algorithm proposed by the present invention, the disturbance period is 2s, and two continuous power frequency period disturbances are implemented on the MPPT in each period. Because the three-phase voltage and current waveforms are the same under the condition of three-phase balance, the present invention only provides the A-phase voltage and current waveform.

Figure 4 shows the disturbance results under normal conditions. It can be seen from Figure 4 that under normal circumstances, when the duty cycle D of the Boost circuit is disturbed, the duty cycle D increases, the photovoltaic output voltage U _PV decreases, the photovoltaic output current I _PV remains unchanged, and the grid-connected point voltage U _PCC is due to The clamping effect of the grid does not change, but the inverter output current I _a decreases. The changes of these parameters are consistent with the theoretical analysis, that is, by perturbing the duty ratio D of the Boost circuit, the perturbation of the MPPT can be realized and the active power of the photovoltaic output can be reduced.

The simulation results in the case of U _{PCC0 =} UN, that is, R=R _m (R _m is the load resistance when the output power of the inverter is balanced with the active power of the load) are shown in FIG. 5 . It can be seen from Fig. 5(a) and (c) that after 1.5s islanding occurs, the grid point voltage U _PCC amplitude does not change due to the photovoltaic output power P _PV =P _Load . Figure 5(a), (d) shows that because 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 frequency of the grid-connected point voltage are within the normal range, and the system is in an island state Running, the island state can only be detected by disturbing the MPPT. It can be seen from Figure 5(b) and (c) that after 1.96s disturbance to MPPT, the duty cycle D of the Boost circuit increases, the output voltage of the photovoltaic array decreases, and the output power of the inverter decreases, which in turn leads to the voltage of the grid-connected point The amplitude U _PCC decreases rapidly, and the voltage amplitude of the grid-connected point decreases to below 0.88U _N in 1.995s, that is, the islanding state is detected, which only takes 35ms.

The simulation results in the case of U _PCC0 =1.1UN, that is, R= _{1.21R m are} shown in FIG. 6 . It can be seen from Figure 6(a), (b) and (e) that after 1.5s the islanding occurs, because R=1.21R _m , the amplitude of the network point voltage U _PCC increases U _PCC ≈1.1U _N . Due to the existence of the energy storage element, the change of the output power P of the inverter in Fig. 6(b) lags behind the change of the duty cycle D. It can be seen from Figure 6(a) and (c) that because the resonant frequency of the RLC load is 50Hz, the frequency of the grid-connected point voltage does not change significantly. The amplitude and frequency of the voltage at the grid-connected point are within the normal range, and the system is operating in an island state. Only by disturbing the MPPT can the island state be detected. From Figure 6(d) and (e), it can be seen that after the MPPT is disturbed at 1.96s, the voltage amplitude U _PCC of the grid-connected point decreases rapidly, and the voltage amplitude of the grid-connected point decreases below 0.88U _N at 1.995s, that is, the detected In the island state, it only takes 35ms.

In order to further verify the correctness of the theoretical analysis, the present invention builds the experimental platform shown in FIG. 7 .

Among them, the photovoltaic analog power supply adopts the AGP1000 series photovoltaic analog power supply of Shenzhen Jushui Royal Technology Co., Ltd., the DSP adopts the TMS320F28335 control chip of TI Company, and the three-phase parallel RLC load adopts chroma63802 electronic load. The load quality factor Q _f = 2.5, the resonant frequency is 50Hz, which is consistent with the simulation. The load is divided into two cases: R = R _m , that is, U _PPC0 = U _N1 (U _N1 is the voltage rating of the grid connection point in the experiment), R = 1.21 R _m is U _PPC0 ＝1.1U _N1 , the RLC load parameters corresponding to the two cases are R＝15.1Ω, C＝526.2μF, L＝19.3mH; R＝18.3Ω, C＝434.9μF, L＝23.3mH, The peak value of the grid phase voltage is 100V and the frequency is 50Hz. The experimental waveforms are shown in Figures 8 and 9. Because the disturbance of the duty cycle D cannot be expressed on the same time scale as the voltage and current, the present invention uses a disturbance signal to replace the change of the duty cycle D. When the disturbance signal is 1 It means the start of the disturbance, and when the disturbance signal is 0, it means the end of the disturbance. I _a represents the current of phase A output by the inverter, and U _a represents the voltage of phase A of the power grid.

It can be seen from Figure 8 that when R=R _m , the output power of the photovoltaic array is equal to the load power after islanding occurs, the load current does not change, and the load resistance remains unchanged, so the amplitude of the grid-connected point voltage does not change significantly. The resonant frequency of the RLC load is 50Hz, so the amplitude and frequency of the grid-connected point voltage are within the normal range, and the photovoltaic power generation system is operating in an island state. 40ms after the islanding occurs, the MPPT starts to be disturbed, and the islanding state is detected after 38ms.

It can be seen from Fig. 9 that when R=1.21R _m , the output power of photovoltaic is greater than the load power after islanding occurs, the load current increases, and the load resistance remains unchanged, the amplitude of the grid-connected point voltage increases, but still In the normal range, because the resonant frequency of the RLC load is 50Hz, the frequency of the grid-connected point voltage does not change, so the photovoltaic power generation system operates in an island state. 70ms after the islanding occurs, the MPPT starts to be disturbed, and the islanding state is detected after 33ms.

Fig. 8 represents the experimental results under general load conditions, and Fig. 9 represents the experimental results of the algorithm of the present invention under the most severe load conditions. It can be seen from Figures 8 and 9 that in both cases, the algorithm proposed by the present invention can quickly detect the islanding state. The experimental results are consistent with the theoretical analysis and simulation results, verifying the correctness and effectiveness of the algorithm proposed by the present invention.

The self-adaptive algorithm based on MPPT proposed in the present invention, according to the grid-connected point voltage amplitude U _100n-2 before the disturbance, determines the disturbance amount to the duty cycle D of the Boost booster circuit, and realizes the disturbance to the duty cycle D by disturbing the duty cycle D The disturbance of MPPT and photovoltaic output active power will affect the voltage amplitude of the grid-connected point when the island occurs, making it deviate from the normal range, and then detect the island state. Compared with the frequency-based island detection algorithm, the algorithm proposed by the present invention will not introduce additional harmonics; it overcomes the shortcomings of the existing MPPT-based island detection algorithm that has detection blind spots, and performs adaptive disturbance on the duty cycle D, reducing Small power loss. The simulation and experiment are carried out under the worst condition stipulated by IEEE Std.92-2000, and the validity of the algorithm proposed by the present invention is verified.

Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that it can be described in terms of form and Various changes may be made in the details without departing from the scope of the invention defined by the claims.

Claims (6)

1. an adaptive island detection algorithm based on MPPT, is characterized in that: the method is specifically: According to the voltage amplitude of the grid-connected point before the disturbance begins, the disturbance amount to the duty cycle D of the Boost booster circuit is determined, and the disturbance to the MPPT and photovoltaic output active power is realized through the disturbance amount to the duty cycle D. When the grid is powered off The voltage amplitude of the grid-connected point deviates from the normal range, and then the islanding state is detected. 2. a kind of self-adaptive island detection algorithm based on MPPT according to claim 1, is characterized in that: when _UPCC0 ＜ _0.88UN , it is determined to be an island state, _UPCC0 is grid _- connected point voltage, and UN is grid voltage valid value. 3. a kind of self-adaptive island detection algorithm based on MPPT according to claim 1, is characterized in that: when U _PCC0 >1.1U _N , be determined as the island state, U _PCC0 is grid-connected point voltage, and U _N is grid voltage valid value. 4. A kind of self-adaptive island detection algorithm based on MPPT according to claim 1, it is characterized in that: when 0.5Hz<|ff _s |, it is determined to be an island state, f is the voltage frequency of grid-connected point, and f _s is the grid The rated frequency is 50Hz. 5. a kind of self-adaptive island detection algorithm based on MPPT according to claim 1, is characterized in that: the duty cycle D before and after the disturbance of Boost step _- up circuit, D Satisfies: Among them, U _m is the output voltage of photovoltaic panels at the maximum power point, U _T is the DC bus voltage, and U _PV is the photovoltaic output voltage. 6. An adaptive island detection algorithm based on MPPT according to claim 5, characterized in that: the disturbance period for the duty cycle is 2s.