Photovoltaic power generation system multi-inverter island detection method based on wavelet transform

Abstract

The present application relates to the technical field of photovoltaic power generation system smart grid, especially to a wavelet transform photovoltaic power generation system multi-inverter island detection method, through monitoring the switching transient of capacitor bank, the photovoltaic power generation system island multi-inverter system is researched and analyzed, and the characteristics of the wavelet transform processing non-steady-state, transient signal advantage are used, the nonlinear multi-inverter system t-f model is established, the Thevenin equivalent circuit of the system with capacitor bank is designed, the component reacts a capacitor bank connected to PCC bus and a circuit breaker S1, wherein S1 is used as a switch. The present application is reasonable in design, the wavelet transform photovoltaic power generation system multi-inverter island detection method provides a new idea for the design of multi-inverter fault diagnosis system under island mode, and has good engineering application prospect.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of photovoltaic power generation system intelligent power grids, in particular to a photovoltaic power generation system multi-inverter island detection method based on wavelet transform. BACKGROUND

[0002] With the rapid development of photovoltaic power generation technology, the popularity rate of distributed grid-connected power generation systems based on renewable energy is increasing year by year. The distributed grid-connected power generation system improves the flexibility and reliability of power supply, but at the same time, it also causes islanding effect, which seriously endangers electrical equipment and personal safety. A fast and reliable island detection method is of great significance for the distributed grid-connected power generation system to have the anti-islanding capability. Therefore, the island detection method of the photovoltaic grid-connected power generation system is studied.

[0003] Island detection and protection of unplanned islanding are considered important. Island operation of a distributed power generation system will involve many risks, such as impacting the power grid, damaging electrical equipment, and endangering personal safety. Photovoltaic grid-connected inverters must have the function of anti-islanding effect. Therefore, studying island analysis technology and protection measures with accurate identification and high-speed detection capability is of great significance for the large-scale development and application of distributed power generation.

[0004] The wavelet transform optimizes the tedious feature layering and detection range, determines the disadvantage of process complexity, and shortens the detection time. Compared with the active island detection method applied to multiple inverters, the method does not have the problem of dilution effect and interference with power quality. Compared with other detection methods of similar principles, the method can also effectively identify false islands.

[0005] The detection method in the prior art has the problems of dilution effect and interference with power quality, the detection time is relatively long, and false islands cannot be effectively identified. SUMMARY

[0006] In order to effectively solve the problems in the background art, the application provides a photovoltaic power generation system multi-inverter island detection method based on wavelet transform.

[0007] The specific technical scheme is as follows.

[0008] A photovoltaic power generation system multi-inverter island detection method based on wavelet transform, comprising the following steps:

[0009] Step 1. According to the moment when the power grid stops working, the switching transient of the capacitor bank is monitored, the photovoltaic power generation system multi-inverter system is researched and analyzed, and a nonlinear multi-inverter system is established by using wavelet transform t-f Model, wherein t t represents a time parameter, f f represents a frequency;

[0010] Step 2. Designing the Thevenin equivalent circuit of the system with capacitor banks according to the nonlinear multi-inverter system of step 1 above t-f Model, connecting the capacitor banks to the PCC bus and a circuit breaker S1, wherein S1 is used as a switch;

[0011] Step 3. Designing the Thevenin equivalent circuit of the system with capacitor banks according to the nonlinear multi-inverter system of step 1 above t-f Model, selecting the difference between the actual positive sequence value and the nominal positive sequence value of the PCC voltage as the signal for discrete wavelet transform (DWT) analysis, selecting db4 wavelet as the mother wavelet for feature extraction, calculating the fourth-level wavelet coefficients, and deriving the energy entropy using the wavelet coefficients wherein, is the energy entropy, represents the frequency parameter during detection, and if the actual frequency is greater than or less than its threshold value (= 50 Hz), the island phenomenon will be detected; Z

[0012] Step 4. Simulating in the Simulink environment to verify the effectiveness of the wavelet transform-based multi-inverter island detection method for photovoltaic power generation systems.

[0013] When S1 is closed, the capacitor will be turned on.

[0014] In island mode t-f The mathematical model is: , wherein, is the mother wavelet, is a fixed real value, m and k are positive integers representing the frequency and time parameters, respectively; and and are the frequency parameter during detection and the threshold value, respectively. ,

[0015] wherein, represents the actual positive sequence value of the PCC voltage, represents the nominal positive sequence value of the PCC voltage.

[0016] ​Compared with the prior art, the beneficial effects of the present application are: by researching the island detection method under the condition of multiple inverters grid-connected, a multiple-inverter island detection method for photovoltaic power generation system based on wavelet transform is proposed, the common point voltage and frequency are detected, the reactive power is taken as the reference, the entropy value is calculated as the additional judgment condition, to carry out island detection, and a photovoltaic grid-connected simulation model is established, and the feasibility of the detection method is verified under different conditions. The wavelet transform optimizes the tedious feature layering and detection range, determines the disadvantage of process complexity, and shortens the detection time. Compared with the active island detection method applied to multiple inverters, the method does not have the problem of dilution effect and interference to power quality. Compared with other detection methods with similar principles, the method can also effectively identify false islands. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a multiple-inverter photovoltaic grid-connected power generation system equivalent model diagram;

[0018] Figure 2 is a Thevenin equivalent circuit diagram;

[0019] Figure 3 is a discrete wavelet transform implementation schematic diagram;

[0020] Figure 4 is a waveform diagram of energy entropy and frequency in time series. DETAILED DESCRIPTION

[0021] For ease of description, spatial relative terms such as "over", "above", "upper surface", "upper", etc. can be used herein to describe the spatial positional relationship of one device or feature with other devices or features as shown in the drawings. It should be understood that the spatial relative terms are intended to include different orientations in use or operation in addition to the orientation of the device described in the drawings. For example, if the device in the drawing is inverted, the device described as "above" or "over" other devices or structures will be positioned "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below" orientations. The device can also be positioned at 90 degrees or in other orientations in other different ways, and the spatial relative descriptions used herein are interpreted accordingly.

[0022] The specific embodiments of the present application will be described in detail below in conjunction with the drawings and preferred embodiments. A multiple-inverter island detection method for photovoltaic power generation system based on wavelet transform is in the like Figure 1The equivalent model of the multi-inverter grid-connected photovoltaic power generation system is shown. It is composed of the main grid, the grid-connected photovoltaic power generation system, the parallel RLC load and the circuit breaker S. The local loads are usually connected to the power generation system, and the transmission line between them is very short. Therefore, the line loss is minimal.

[0023] The island detection process must be effective under various operating conditions, and if normal events occur under normal circumstances, such as capacitor switching, motor starting or lightning strikes, resulting in transient interruptions, the frequency may exceed the threshold limit, resulting in false tripping. Therefore, under certain conditions, an additional constraint must be added to improve the efficiency of island detection.

[0024] As Figure 2 The Thevenin equivalent circuit of the system with capacitor banks is shown, in order to better understand the problem of island detection, by studying the switching transient of the capacitor bank. Due to the time-varying load, the capacitor bank can be switched multiple times a day to compensate for reactive power. During the energization of the capacitor bank, unnecessary transient overvoltage and high-frequency inrush current will be generated. The process of energizing one capacitor bank while the other capacitor bank is still energized is called back-to-back switching.

[0025] Neglecting the source resistance R g , the circuit is composed of source inductance L g and capacitance C 1 in series, the voltage at the PCC point can be obtained:

[0026]

[0027] where, R g is the source inductance, C 1 is the capacitance, V PCC is the voltage at the PCC point, v g (t) is the grid voltage.

[0028] V PCC is transformed by Laplace transform:

[0029]

[0030] where, V m and are the magnitude and initial phase of the PCC voltage, is the resonance frequency, is the initial voltage of the capacitor when energized.

[0031] Figure 3 is the implementation of the discrete wavelet transform, and the voltage VAs the signal of discrete wavelet transform analysis, db4 wavelet is selected as the mother wavelet for feature extraction.

[0032] The dbN wavelet has the following characteristics: if

[0033]

[0034] In the formula is the binomial coefficient, then:

[0035] ,

[0036] wherein,

[0037] ,

[0038] The db wavelet support length and vanishing moment order are 2N-1 and N respectively, the db4 wavelet support length is 7, and the vanishing moment is 4th order. The dbN wavelet is an orthogonal wavelet base, which satisfies the orthogonality. The larger the order N is, the better the stability and accuracy of wavelet analysis and reconstruction are, and the better the localization analysis can be performed. Therefore, the db4 wavelet is selected.

[0039] The fourth-level wavelet coefficients are calculated, and the energy entropy contained in the decision criterion is derived from the wavelet coefficients. The energy entropy proposed is calculated from the approximation coefficients:

[0040]

[0041] Wherein, E(V) is the energy entropy, which represents the frequency parameter during detection.

[0042] When the actual frequency is greater than or less than the threshold value during normal operation, the island phenomenon will be detected. In this scenario, the distributed power supply is in the fault detection zone (FDZ).

[0043] The entropy difference factor EDF between the entropy E and the entropy threshold EThreshold=0 is defined to evaluate the probability of falling within the FDZ:

[0044]

[0045] When the EDF value is small, it indicates that the entropy value under the island condition is similar to the entropy value under the normal condition, which increases the probability of entering the free solution area.

[0046] Figure 4 Method a adopts a fuzzy control method, method b adopts an stm32 control method, and method c adopts a wavelet transform control method. Through simulation tests of zero active power mismatch and capacitor bank switching, the performances of the three methods are compared. The experimental conditions are set as follows: the capacitor bank is opened at 0.1 seconds, closed at 0.2 seconds, and island occurs at 0.3 seconds.

[0047] It can be seen that when the capacitor is opened, the three methods participating in the test have a large fluctuation in frequency, and the threshold is crossed up and down, thereby generating a false island effect. However, the method proposed in the present application also has a second determination condition, as can be seen from figure (c), the entropy does not exceed the threshold, so as to avoid the false island phenomenon. When the island occurs, the methods participating in the test all respond, at this time the frequency of the method proposed in this chapter is greater than the threshold, and the entropy changes positively, so it can be judged that the island phenomenon is detected. In this case, the island detection time is 40 ms, which is much lower than the detection standard, thereby verifying the stability and effectiveness of the proposed method.

[0048] The above only describes the preferred embodiments of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A method for island detection of a photovoltaic power generation system with multiple inverters based on wavelet transform, characterized in that: Comprising the following steps: Step 1. According to the grid stop working moment, the switching transient of the capacitor bank is monitored, the photovoltaic power generation system island multi-inverter system is researched and analyzed, and the nonlinear multi-inverter system t-f model is established by using wavelet transform, wherein t represents the time parameter, and f represents the frequency; Step 2. According to the nonlinear multi-inverter system t-f model of the above step 1, the Thevenin equivalent circuit of the system with the capacitor bank is designed, and the capacitor bank is connected to the PCC bus and a circuit breaker S1, wherein S1 is used as a switch; Step 3. According to the nonlinear multi-inverter system t-f model of step 1 above, the difference between the actual positive sequence value and the nominal positive sequence value of the PCC voltage is selected as the signal for discrete wavelet transform (DWT) analysis, db4 wavelet is selected as the mother wavelet for feature extraction, the fourth level wavelet coefficient is calculated, and the energy entropy is derived from the wavelet coefficient Wherein, E(V) is the energy entropy, f' represents the detection frequency parameter, if the actual frequency is greater than or less than its threshold f0=50Hz Z , the island phenomenon will be detected; Step 4. Simulation is carried out in the Simulink environment, and the effectiveness of the wavelet transform photovoltaic power generation system multi-inverter island detection method is verified; The t-f mathematical model in the island mode is: Wherein, ψ(.) is called the mother wavelet, a0>1, b0>1 are fixed real values, m and k are positive integers, respectively representing the frequency and time parameters, f', f0 are the frequency parameters and threshold values respectively, x(n) is a discrete input signal, and n is the length of the discrete signal; a selection voltage V as the signal for the discrete wavelet transform analysis, wherein, denotes the actual positive sequence value of the PCC voltage, denotes the nominal positive sequence value of the PCC voltage.

2. The method according to claim 1, wherein the method is a method for detecting an island of a plurality of inverters of a photovoltaic power generation system using a wavelet transform. When S1 is closed, the capacitor will be turned on.

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