CN103207335B - Island detection method based on phase-frequency positive feedback - Google Patents

Abstract

The invention provides an islanding detection method based on phase-frequency positive feedback, and relates to the technical field of grid-connected inverter control. The method includes: collecting the voltage signal of the grid-connected point of the inverter through the AD converter, and using the phase-locked loop to perform phase-locking to obtain the grid frequency f and the voltage phase θ; and calculating the grid frequency average value F by accumulating the grid frequency f for N times ; Calculate the compensation angle dθ according to the grid frequency average value F, and compensate the phase to obtain the compensated output phase According to output phase Measure the frequency f' of the grid-connected point of the inverter to determine whether the inverter is in an island operation state. Compared with the traditional algorithm, the islanding detection method proposed by the present invention is not affected by current, power, and load impedance, and there is no detection blind area, no artificial frequency, phase and other disturbances, and no harmonics are introduced into the power grid. The detection method is simple, rapid, reliable and accurate, and improves the speed of island detection.

Description

An Islanding Detection Method Based on Phase-Frequency Positive Feedback

technical field

The invention relates to the technical field of grid-connected inverter control, in particular to an islanding detection method based on phase-frequency positive feedback.

Background technique

With the increasingly serious energy shortage and environmental problems, the utilization of renewable energy such as solar energy and wind energy has become a research hotspot all over the world. The function of the distributed power inverter is to convert the electric energy generated by wind turbines, photovoltaic panels and other devices into alternating current that meets the national grid access standards. When the inverter is connected to the grid, an island phenomenon may occur in the power system. The so-called isolated island phenomenon means that when the large power grid loses power, the inverter still transmits electric energy to the large power grid, fails to detect the power failure state of the large power grid and separates from it, and the distributed power supply and the local load form a self-sufficient voltage and frequency. Power supply system controlled by large grid. After the island phenomenon occurs, if the load capacity is greater than the capacity of the distributed power supply, it may cause load shedding in the microgrid or burning of power generation systems such as photovoltaics and wind turbines; It is likely to cause personal injury to maintenance personnel. At present, relevant organizations and institutions at home and abroad have clearly stipulated that distributed grid-connected systems must have island detection capabilities. It can be seen that island detection is related to the personal safety of personnel, and the safe and reliable power supply of the power system is an essential link in the grid-connected power generation system.

Existing islanding detection methods mainly include passive detection and active detection methods. Passive detection refers to the detection of changes in parameters such as inverter output voltage, frequency, phase, and harmonics to perform islanding detection. This method mainly determines whether the inverter is in the islanding operation state by setting the upper limit and lower limit of the voltage change at the voltage monitoring point and the upper limit and lower limit of the frequency change at the frequency monitoring point, and checking the actual voltage and frequency for exceeding the limit. However, passive detection has a large dead zone. When the local load matches the output power of the inverter, the grid transmits almost no power to the inverter and the local load. Whether the grid is powered off or not has little effect on the output voltage and frequency of the inverter. At this time, passive detection is invalid.

The active detection method refers to controlling the inverter so that the output power changes according to a certain rule, the current waveform is distorted at a certain position, and the frequency or phase is disturbed. Since the power grid is strong enough, various parameters of the power grid will not be disturbed by the disturbance of the inverter, and the islanding detection device will not detect these disturbances. Once the islanding phenomenon occurs, the output disturbance of the inverter will quickly accumulate and exceed the set limit, thereby detecting the islanding. These methods have small working dead zone, fast detection speed and high precision, but the algorithm is complex, requires high chip operation speed, and brings harmonic problems to the power grid.

Therefore, for the above deficiencies, the present invention provides a phase-based. Islanding detection method for frequency positive feedback.

Contents of the invention

(1) Solved technical problems

Aiming at the deficiencies of the prior art, the present invention proposes a phase-based. The islanding detection method of frequency positive feedback can detect the islanding phenomenon simply, quickly and accurately, and avoid detection blind spots, without bringing artificial transient disturbance and harmonic pollution to the power grid.

(2) Technical solution

To achieve the above purpose, the present invention provides a phase-based. The island detection method of frequency positive feedback, it comprises the following steps:

S1. Collect the voltage signal of the grid-connected point of the inverter through the AD converter, and use the phase-locked loop to perform phase-locking to obtain the grid frequency f and voltage phase θ;

S2. Calculate the grid frequency average value F by accumulating the grid frequency f described in S1 N;

S3. Calculate the compensation angle dθ according to the grid frequency average value F described in S2, and compensate the voltage phase θ to obtain the compensated output phase

S4, according to the output phase in S3 Measure the frequency f' of the grid-connected point of the inverter to determine whether the inverter is in an island operation state.

Wherein the phase-locked loop described in S1 is the phase-locked loop based on second-order generalized integrator; Among the S3, the calculation formula of compensation angle dθ is:

dθ=k·(F-50)+d

Wherein, k and d are proportional coefficients and constant coefficients which are linearly fitted according to the phase-frequency characteristic curve of the second-order generalized integrator;

Compensated inverter output phase The calculation formula is:

Among them, _θi is the voltage phase in the i-th S1, dθ is the linear compensation angle for the phase-locked loop output phase, F is the average value of the power grid frequency calculated in S2, F _min is the lower limit of the linear compensation frequency, and F _max is the frequency upper limit of linear compensation, and d is the fixed compensation angle after the frequency exceeds the limit.

The formula for calculating the frequency f' of the grid-connected point of the inverter in S4 is:

Among them, Δβ is the phase-frequency response curve relation of the phase-locked loop, τ is the engineering error of the inverter operation, dθ is the compensation angle, PI is the PI control function in the phase-locked loop, and _{the initial setting value of} f is the starting value of the inverter. The set frequency during operation, _θi is the phase-locked loop output phase calculated after the i-th AD converter sampling, is the output phase of the inverter after the i-1th AD converter sampling, and Δt is the interval time between two samplings.

As an optimization of this program, steps after S3 are included: every time the AD converter samples, steps S1 to S3 are executed once, and after execution i times, the frequency average value F described in S2 is used as the reference frequency, and S1 The voltage phase θ _i calculated in the i-th time is used as the initial phase, and the reference phase Φ is obtained by using the second-order generalized integrator.

The process of constructing the reference phase Φ is as follows:

The relationship between the frequency average F, the voltage phase _θi calculated by the ith AD converter sampling and the reference phase Φi+ _j when calculating the i+j AD converter sampling is:

Φ _i+j =θ _i +2π·F·Δt·j

Among them, i is the number of repeated executions of S1 to S3, and j represents the number of sampling executions of the AD converter after the initial phase θ _i is obtained.

Finally calculate the reference phase Φ and the output phase after compensation phase difference Determine whether the inverter is in the islanding state based on the phase difference.

(3) Beneficial effects

The present invention provides a phase-based. The island detection method of frequency positive feedback adopts the method of linear fitting for phase compensation, and the calculation is simple. Compared with the passive detection method, the detection parameters are the voltage amplitude, frequency, and phase information of the grid-connected point, and do not include information such as power and current, and there is no dead zone problem caused by the matching of load power and inverter output power. Compared with the active detection method, this method does not add any disturbances such as phase and waveform. On the contrary, this method reversely compensates the phase shift characteristics of the second-order generalized integrator with frequency, corrects the unreasonable shift of the inverter output phase, and reduces the phase difference caused by the inverter and the power grid. harmonic.

Description of drawings

Fig. 1 is a kind of embodiment circuit diagram when the island detection of bidirectional energy storage inverter;

Figure 2a is based on the phase of the present invention. An algorithm flow chart of the frequency positive feedback islanding detection method;

Figure 2b is based on the phase of the present invention. Another algorithm flow chart of the island detection method of frequency positive feedback;

Fig. 3 a is the phase-frequency characteristic curve of the second-order generalized integrator;

Fig. 3b is the phase angle compensation curve obtained by linear fitting of the phase-frequency characteristic curve of the second-order generalized integrator;

Figure 3c is the phase-frequency characteristic curve of the phase-locked loop after phase angle compensation;

Figure 4 is a comparison diagram of grid voltage and output voltage before and after islanding;

Figure 5 is the frequency measurement value of the grid-connected point voltage when islanding occurs;

Figure 6 shows the phase difference between the output phase of the grid-connected point and the reference phase when islanding occurs. ;

Wherein reference numeral 41 is grid voltage, and numeral 42 is output voltage. the

Detailed ways

The following is a phase-based method proposed by the present invention. The islanding detection method of frequency positive feedback is described in detail with reference to the accompanying drawings and embodiments.

The invention is suitable for a digital control inverter system with grid-connected function. The method of the present invention is based on the phase. The island detection method of frequency positive feedback is mainly to modify the phase-frequency characteristics of the second-order generalized integrator, and perform phase compensation on the output of the phase-locked loop, so that the positive feedback of the phase-locked loop occurs when the inverter works in the island state, and the engineering error is infinite. Zoom in and shift the frequency until it is determined that islanding occurs.

The bidirectional energy storage inverter is taken as an example below, and other grid-connected photovoltaic power generation systems such as photovoltaics can still use the present invention, which is not considered as a limitation of the present invention.

Figure 2a is based on the phase of the present invention. An algorithm flow chart of the frequency positive feedback islanding detection method;

S1. Collect the voltage signal of the grid-connected point of the inverter through the AD converter, and use the phase-locked loop based on the second-order generalized integrator to perform phase-locking to obtain the grid frequency f and voltage phase θ.

As shown in Figure 1, it is the circuit diagram of the islanding detection of the bidirectional energy storage inverter, and the system adopts the inverter with the conventional power frequency transformer structure.

The main parameters of the inverter are as follows:

Rated power: 6kW

Grid voltage: 230V

Grid frequency: 50Hz

Transformer ratio: 1:8

Battery voltage: 48V

AD sampling frequency: 12kHz

When the grid frequency is in the range of 48-52Hz, when the inverter is connected to the grid, the output phase of the phase-locked loop after phase compensation has almost no phase difference with the grid. The upper limit and lower limit of the frequency protection in this embodiment are set to 50.5 Hz and 49.5 Hz respectively. The voltage detection circuit amplifies the voltage of the grid-connected point, and the AD converter in the DSP chip samples the voltage signal;

The grid frequency f is obtained through the phase-locked loop based on the second-order generalized integrator, and the grid phase θ is obtained by integrating the grid frequency f.

The calculation process is: the measured voltage waveform constructs two quadrature signals 'υ' and qv' through the second-order generalized integrator, where υ' is a sinusoidal signal with the same phase as the voltage waveform, and qv' is the phase difference with the voltage waveform 90° cosine signal. The relationship between the reactive component q in the dq rotating coordinate system and the quadrature signal υ', qv', and the voltage phase θ is:

q=υ'·cosθ+qυ'·sinθ

Using q as the input of the PI regulator, calculate the frequency compensation value df, and the grid frequency is

f=df+f ₀

Among them, the initial frequency f ₀ =50Hz;

The voltage phase θ can be obtained by integrating the frequency f:

θ _i+1 = θ _i +2πf·Δt

Among them, θi ₊₁ is the phase of the voltage at the i+1th measurement, _θi is the phase of the voltage at the ith measurement, f is the grid frequency, and Δt is the time difference between two measurements.

Since the PI regulator has a negative feedback function, the phase-locked loop realizes the phase-locking function, and obtains the grid frequency f and the voltage phase θ.

S2. Calculate the grid frequency average value F by accumulating the grid frequency f mentioned in S1 for N times.

Every 240 points of sampling, calculate the average frequency F once:

The procedure for calculating the frequency average is as follows:

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}^{\mathrm{<span\; class="notranslate">\; N</span>}}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; N</span>}$

S3. Calculate the compensation angle dθ according to the average value F of the power grid frequency described in S2, and compensate the phase to obtain the compensated output phase

Calculate the phase compensation angle dθ according to the average frequency and the phase-frequency characteristic curve fitting formula of the second-order integrator. The calculation formula is:

dθ=k·(F-50)+d

Among them, k and d are proportional coefficients and constant coefficients obtained by linear fitting according to the phase-frequency characteristic curve of the second-order generalized integrator, respectively.

It should be noted that the method to obtain the phase-frequency characteristic curve of the second-order integrator is to perform Z-transform on the second-order generalized integrator, and then draw a Bode diagram. In practical applications, the DSP can also be used to output different frequencies and compare the phase difference. No matter which method is used, the phase-frequency characteristic curve needs to be fitted. Since the linearity of the curve is better in the range of 48-52 Hz, the present invention adopts a linear fitting method, and others such as quadratic polynomials have higher fitting precision.

As shown in Figures 3a-3c, after performing phase compensation for the second-order generalized integrator, the phase offset of the phase-locked loop is maintained near 0° or has a small positive correlation with the frequency. Inverter output phase

Among them, _θi is the voltage phase in the i-th step S1, dθ is the linear compensation angle of the phase-locked loop output phase, F is the calculated average value of the power grid frequency, F _min is the lower limit of the frequency of linear compensation, and F _max is The frequency upper limit of linear compensation, d is the fixed compensation angle after the frequency exceeds the limit.

S4, according to the output phase in S3 Measure the frequency f' of the grid-connected point of the inverter to determine whether the inverter is in an island operation state.

The formula for calculating the frequency f' of the grid-connected point of the inverter is:

Among them, Δβ is the phase-frequency response curve relation of the phase-locked loop, τ is the engineering error of the inverter operation, dθ is the compensation angle, PI is the PI control function in the phase-locked loop, and _{the initial setting value of} f is the starting value of the inverter. The set frequency during operation, _θi is the phase-locked loop output phase calculated after the i-th AD converter sampling, is the output phase of the inverter after the i-1th AD converter sampling, and Δt is the interval time between two samplings.

As shown in Figure 4, the inverter line impedance is very large, and the grid-connected current is zero at this time. Before the islanding occurs, it is due to the clamping effect of the grid on the frequency. The output frequency and phase of the inverter are the same as those of the grid, and do not contain any artificial disturbance, only engineering errors, calculation truncation errors, etc. exist. After the island occurs, the inverter loses the power grid signal, and the initial phase error enters the phase-locked loop as a phase difference. If it is detected that the output phase of the phase-locked loop is ahead of the output voltage phase, the phase difference is used as the PI input value of the phase-locked loop. This reduces the frequency of the phase-locked loop. Since the engineering error still exists, the direction does not change, so the error accumulates continuously and forms a phase. Frequency positive feedback.

In order to ensure positive feedback acceleration operation, the phase can be modified. The slope of the frequency curve fitting formula such that phase is positively correlated with frequency.

As shown in Figure 5, the frequency f' cross-limit check is one of the conditions for judging the occurrence of islanding.

Figure 2b is based on the phase of the present invention. Another algorithm flow chart of the frequency positive feedback island detection method is shown in the following figure:

As above based on phase. The improvement of the island detection method of frequency positive feedback, the present invention also provides a detection method, after step S3 of the above method, it also includes a step: every time the AD converter samples, steps S1 to S3 are executed once, and after i times , taking the frequency average value F described in S2 as the reference frequency, taking the voltage phase θ _i calculated for the ith time in S1 as the initial phase, and using the second-order generalized integrator to obtain the reference phase Φ.

The process of constructing the reference phase Φ is as follows:

The relationship between the frequency average F, the voltage phase _θi calculated by the ith AD converter sampling and the reference phase Φi+ _j when calculating the i+j AD converter sampling is:

Φ _i+j =θ _i +2π·F·Δt·j

Wherein, i is the number of repeated executions of S1 to S3, and j represents the number of sampling executions of the AD converter after the initial phase θ _i is obtained in claim 4.

Finally calculate the reference phase Φ and the output phase after compensation phase difference Determine whether the inverter is in the islanding state based on the phase difference.

As shown in Figure 6, the reference phase takes the phase-locked loop output phase at a certain time point as the initial phase, and integrates the grid frequency. If the difference between the reference phase and the actual output phase exceeds the limit, it means that it enters the island operation state.

What needs to be explained here is: the structure of the reference phase is suitable for the environment where the grid frequency is very stable. The reference phase is based on the phase and frequency at a certain time point, and the phase calculation is performed at each sampling time in the future. When the time is very long, the error between the reference phase and the actual output phase will be large. At this time, it is necessary to use the current phase and frequency to reconstruct the reference phase, and reset the previous phase difference to zero. When the engineering error is small, the frequency shift is slow and the phase shift is large.

Implementation Effect:

Once islanding occurs, the engineering errors of the inverter will accumulate rapidly, and the frequency will decrease or increase rapidly. According to the size of the engineering error and the different setting limits, the above detection methods can detect the islanding operation state within the range of 0.02s ~ 1s. Compared with the existing technology, the island detection method proposed by the present invention is not affected by current, power, and load impedance, and there is almost no detection blind area, no artificial frequency, phase and other disturbances, and no harmonics are introduced into the power grid. The detection method is simple, rapid, reliable and accurate, especially the phase difference detection is more sensitive, and the speed of island detection is improved.

The above embodiments are only used to illustrate the present invention, but not to limit the present invention. Those of ordinary skill in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all Equivalent technical solutions also belong to the category of the present invention, and the scope of patent protection of the present invention should be defined by the claims.

Claims (5)

1. an islanding detection method based on phase-frequency positive feedback, is characterized in that, comprises the following steps: S1. Collect the voltage signal of the grid-connected point of the inverter through the AD converter, and use the phase-locked loop to perform phase-locking to obtain the grid frequency f and voltage phase θ; S2. Calculate the grid frequency average value F by accumulating the grid frequency f described in S1 N; S3. Calculate the compensation angle dθ according to the grid frequency average value F described in S2, and compensate the voltage phase θ to obtain the compensated output phase S4, according to the output phase after compensation in S3 Measure the frequency f' of the grid-connected point of the inverter to determine whether the inverter is in an island operation state; The formula for calculating the frequency f' of the grid-connected point of the inverter described in S4 is: Among them, Δβ is the phase-frequency response curve relation of the phase-locked loop, τ is the engineering error of the inverter operation, dθ is the compensation angle, PI is the PI control function in the phase-locked loop, and _{the initial setting value of} f is the starting value of the inverter. The set frequency during operation, θ _i is the voltage phase calculated after the ith AD converter sampling, is the output phase after compensating the voltage phase calculated after the i-1th AD converter sampling, and Δt is the interval time between two samples. 2. a kind of island detection method based on phase-frequency positive feedback as claimed in claim 1, is characterized in that, the phase-locked loop described in S1 is the phase-locked loop based on second-order generalized integrator; Described in S3 The calculation formula of compensation angle dθ is: dθ=k·(F-50)+d Among them, k is the proportional coefficient linearly fitted according to the phase-frequency characteristic curve of the second-order generalized integrator, and d is the fixed compensation angle after the frequency exceeds the limit; The output phase after the compensation of the voltage phase calculated after the i-th AD converter sampling The calculation formula is: Among them, θ _i is the voltage phase in the i-th S1, F is the average value of grid frequency calculated in step S2, F _min is the lower limit of the frequency of linear compensation, and F _max is the upper limit of frequency of linear compensation. 3. A kind of islanding detection method based on phase-frequency positive feedback as claimed in claim 1, it is characterized in that, after S3 also comprises the step: every time AD converter sampling, all execute step S1 to S3 once, carry out i times Finally, take the grid frequency average value F described in S2 as the reference frequency, and use the voltage phase θ _i calculated for the ith time in S1 as the initial phase, and use the second-order generalized integrator to obtain the reference phase Φ. 4. A kind of island detection method based on phase-frequency positive feedback as claimed in claim 3, is characterized in that, the process of constructing reference phase Φ is: The relationship between the grid frequency average value F, the voltage phase _θi calculated by the ith AD converter sampling and the reference phase _Φi +j during the i+j AD converter sampling calculation is as follows: Φ _i+j ＝θ _i +2π·F·Δt·j Among them, i is the number of repeated executions of S1 to S3, and j represents the number of sampling executions of the AD converter after the initial phase θ _i is obtained. 5. A kind of island detection method based on phase-frequency positive feedback as described in claim 3 or 4, it is characterized in that: calculate reference phase Φ and output phase after compensation phase difference Determine whether the inverter is in the islanding state based on the phase difference.