CN108414838A - A kind of inverter parallel system line impedance measurement method - Google Patents

Abstract

A kind of inverter parallel system line impedance measurement method, by sampling the output current of busbar voltage and Ge Tai inverters in microgrid central controller, the highest specific order harmonic components of extraction wherein content estimate line impedance.It include mainly following six step：First, the virtual output impedance of each inverter is arranged in system initialization；Second, sample busbar voltage and Ge Tai inverter output currents；Third, to busbar voltage carry out Fast Fourier Transform (FFT) or other similar to algorithm, determine the wherein highest overtone order h of harmonic content；4th, extract the h order harmonic components in busbar voltage and each inverter output current；5th, calculate the line impedance of each inverter；6th, result of calculation is filtered.This method will not generate any disturbance to original system, will not increase system Construction cost, computational methods are simple, and good solution is provided for engineer application.

Description

A Method for Measuring Line Impedance of Inverter Parallel System

technical field

The invention belongs to the grid line impedance measurement technology, in particular to a line impedance measurement method of an inverter parallel system.

Background technique

In recent years, with the development of renewable energy power generation technology, distributed power generation mode has been widely used. Primary energy sources such as photovoltaics and wind power usually transmit electric energy to the grid through power electronic interface devices such as inverters. In order to increase the capacity of the system and improve the reliability of the system, each inverter usually adopts a parallel operation mode. Therefore, the distributed power generation system (also called a microgrid) can be simplified into a structure in which multiple inverters are connected in parallel. For such a system, droop control is usually used to realize the coordinated control among parallel inverters. Because droop control can achieve synchronization and power sharing between inverters without relying on communication, it is very suitable for the application background of distributed power generation. However, the droop control has the following disadvantages: the reactive power is affected by the line impedance difference and cannot be shared among the inverters; the output power has control coupling when the line impedance is not purely inductive or purely resistive; nonlinear load Resulting in lower bus voltage quality. Many defects of the above-mentioned droop control are caused by the line impedance, so if the line impedance in the system can be accurately obtained, these problems can be solved.

The existing line impedance estimation methods can be roughly divided into three categories: the first category, through the fluctuation of the steady-state operating point of the system, to measure the output voltage and current of the inverter to estimate the line impedance; the second category, through the system's The inverter injects some additional high-frequency voltage (current) signal, and then measures the response of the frequency signal in the inverter output current (voltage) to estimate the line impedance; the third category is based on specific impedance measuring instruments to The system injects a disturbance and measures the response to calculate the line impedance. All these methods will bring some disturbance or distortion to the original system, so they are all intrusive measurement methods, which affect the performance and stability of the system to a certain extent.

Contents of the invention

In view of the above problems, the object of the present invention is to provide a method for measuring line impedance of an inverter parallel system. This method utilizes the original equipment and signals in the system, and will not bring any disturbance and distortion to the original system.

To achieve the above object, the present invention adopts the following technical solutions:

A method for measuring the line impedance of an inverter parallel system, by sampling the bus voltage and the output current of each inverter in the central controller of the microgrid, extracting the specific sub-harmonic component with the highest content, and estimating the line impedance; Each inverter adopts droop control.

A further improvement of the present invention is to specifically include the following steps:

Step 1, system initialization, setting the virtual output impedance Z _vi of each inverter;

Step 2, sampling the bus voltage and the output current of each inverter in the central controller of the microgrid;

Step 3, calculate the bus voltage, and determine the harmonic order h with the highest harmonic content;

Step 4. After determining the harmonic order h with the highest harmonic content in step 3, extract the h-order harmonic component in the bus voltage and the output current of each inverter; the h-order harmonic component includes the h-order harmonic voltage and h subharmonic current;

Step 5: Calculate the line impedance Z _Li of each inverter according to the h-order harmonic voltage and h-order harmonic current extracted in step 4, where i represents the i-th inverter, i=1,2,... ,n, assuming there are n inverters;

Step 6, filtering the calculation result of step 5 to eliminate high-frequency noise generated in the sampling and calculation links.

The further improvement of the present invention is that in step 2, the bus voltage and the output current of each inverter are sampled in the central controller of the microgrid through the voltage sensor and the current sensor.

A further improvement of the present invention lies in that in step 3, fast Fourier transform is used for calculation.

A further improvement of the present invention is that in step 3, the harmonic order h with the highest harmonic content is determined by the load characteristics.

A further improvement of the present invention is that the harmonic order h with the highest harmonic content is -1, 5, 7, 11 or 11.

The further improvement of the present invention is that in step 4, a signal extraction method based on multiple second-order generalized integrators is used to extract the h-order harmonic components in the bus voltage and the output current of each inverter.

A further improvement of the present invention is that in step 5, the line impedance Z _Li of each inverter is calculated by the following formula:

Among them, Z _ti represents the calculated total impedance, that is, the sum of the line impedance and the virtual output impedance of the inverter, Z _vi represents the virtual output impedance of the inverter, including virtual resistance R _vi and virtual inductance L _vi , and ω represents the basic The angular frequency of the wave voltage, that is, 2π*50, h represents the harmonic order with the highest harmonic content; v _αh represents the α-axis component of the h-order harmonic voltage, v _βh represents the β-axis component of the h-order harmonic voltage, and i _αhi represents h The subharmonic current α-axis component and i _βhi represent the h-th harmonic current β-axis component.

A further improvement of the present invention is that in step 6, filtering is performed by a first-order low-pass filter, and the first-order low-pass filter transfer function G(s) is as follows:

Among them, ω _c represents the cutoff frequency of the filter, and s represents the Laplacian operator.

Compared with the prior art, the present invention has the following beneficial effects:

The above-mentioned technical solution disclosed by the present invention aims to use the existing equipment in the distributed power generation system to measure the line impedance through a simple calculation method. The invention estimates the line impedance by sampling the bus voltage and the output current of each inverter in the central controller of the microgrid, and extracting a certain harmonic component therein. Compared with the prior art, the technical solution of the present invention mainly has the following advantages: First, the measurement method is a non-invasive method, which uses the harmonic signal generated by the load in the system to estimate the line impedance, and does not need to inject other signals , will not have any impact on the original system; second, this method utilizes the original equipment in the system, including the micro-grid central controller, etc., so there is no need to add other additional equipment, and will not increase the system construction cost; third, The calculation method is simple, and can be completed by ordinary industrial-grade digital controllers, and has low requirements for system configuration. The invention provides a good solution for engineering application. This method is applicable to both three-phase and single-phase systems.

Description of drawings

Fig. 1 is the structural diagram of a kind of inverter parallel connection (distributed power generation) system that this invention is applicable;

Fig. 2 is the flow chart of the method that realizes line impedance measurement among the present invention;

Fig. 3 is a structural diagram of the measuring method of the present invention;

Fig. 4 is a schematic diagram of a signal extraction method illustrated in the present invention.

Detailed ways

Herein, the detailed content and technical description of the present invention will be further described with a preferred embodiment, but it should not be construed as a limitation to the implementation of the present invention.

A method for measuring the line impedance of an inverter parallel system, by sampling the bus voltage and the output current of each inverter in the central controller of the microgrid, extracting the specific sub-harmonic component with the highest content, and estimating the line impedance; Each inverter adopts droop control, which is a non-invasive measurement method because it utilizes the existing equipment and signals in the system.

For the distributed generation system shown in FIG. 1 , the present invention provides a method for estimating the line impedance (Z _L1 , Z _L2 , . . . , Z _Ln ) of each inverter therein. It is worth noting that all inverters in this system adopt the droop control method, and there is a certain amount of unbalanced or nonlinear load in the system.

Referring to Fig. 2 and Fig. 3, the present invention mainly comprises following 6 steps:

Step S1, system initialization, setting the virtual output impedance Z _vi of each inverter;

Step S1 initializes relevant parameters and variables in the system, and sets the virtual output impedance _Zv of the inverter at the same time. The purpose of setting the virtual output impedance of the inverter is to realize other functions such as power sharing, which is not directly related to the line impedance measurement of the present invention, and Z _v =0 can be directly set if not needed. Considering _Zv here is because the result obtained by the line impedance calculation method described in this invention is the sum of the line impedance and the virtual output impedance of the inverter, so it is necessary to determine the size of the virtual output impedance of the inverter in advance, and then use the calculated result to subtract The devirtualized output impedance is the size of the line impedance. It should be noted that the command value of the virtual output impedance is transmitted from the central controller to the local controllers located in each inverter through communication. This communication mechanism is generally necessary in the microgrid system, but for the present invention The line impedance measurement method does not rely on this communication, because all steps of the line impedance measurement are completed in the central controller of the system.

Step S2, sampling the bus voltage and the output current of each inverter through a voltage sensor and a current sensor in the central controller of the microgrid;

Step S2 mainly realizes the sampling of the bus voltage and the sampling of the output current of each inverter. The voltage sampling is realized by the voltage sensor installed on the bus, which can sample phase voltage or line voltage, and the present invention is not limited thereto. The current sampling is realized by the current sensor installed in the parallel connection between each inverter and the busbar. It is worth noting that these voltage sensors and current sensors generally exist in the actual distributed power generation system, and are mainly used for the central controller to detect the working status of each inverter, so the present invention does not require additional installation of sensors.

Step S3, perform Fast Fourier Transform (FFT) or other similar algorithms on the bus voltage to determine the harmonic order (indicated by h) with the highest harmonic content, h may be -1 (ie, the negative sequence of the fundamental wave), 5 , 7, 11 and other harmonics in any one, determined by the load characteristics.

Step S3 performs FFT and other operations on the bus voltage sampled in step S2, calculates the amplitude of the lower harmonic voltage (generally lower than the 17th order), and selects the order h of the harmonic voltage with the highest amplitude as the specific value for calculating the line impedance subharmonic. It is worth noting that the main purpose of step 3 is to determine the order of the harmonic voltage with the highest amplitude among the harmonics contained in the bus voltage. FFT calculation is only one of the calculation methods, and other similar algorithms can also be used, as long as the amplitude can be determined The highest harmonic voltage order is sufficient. Therefore, the present invention does not limit that the FFT operation must be used in step 3, and other similar algorithms should also be covered within the scope of protection.

Step S4, after determining the harmonic order h with the highest harmonic content in step S3, extract the h-order harmonic component in the bus voltage and the output current of each inverter; the h-order harmonic component includes the h-order harmonic voltage and h subharmonic current;

Step S4 mainly realizes the signal extraction function. After determining the order h of the harmonic voltage with the largest amplitude in step S3, step S4 extracts the h order harmonic components in the bus voltage and the output current of each inverter. There are many extraction methods, and the present invention does not limit which extraction method to use, as long as the h-order harmonic components in the voltage and current can be accurately and quickly extracted. In order to clearly illustrate the content of the invention, the present invention is exemplified by the signal extraction method based on multiple second-order generalized integrators (MSOGI) proposed by Pedro Rodríguez in "Multiresonant Frequency-LockedLoop for Grid Synchronization of Power Converters Under Distorted GridConditions" Description, its principle is shown in Figure 4. The hth harmonic components of the extracted voltage and current are expressed as hth harmonic voltage α axis component v _αh , h harmonic voltage β axis component v _βh , h harmonic current α axis component i _αhi and hth Harmonic current β axis component i _βhi .

Step S5, calculate the line impedance Z _Li of each inverter according to the h-order harmonic voltage and h-order harmonic current extracted in step S4, i represents the i-th inverter, i=1,2,...,n , (assuming there are n inverters in the system);

Step S5 uses the hth harmonic voltage and current components extracted in step S4 to calculate the magnitude of the line impedance, including line resistance and line inductance. Calculated as follows:

Among them, Z _ti represents the calculated total impedance (that is, the sum of the line impedance and the virtual output impedance of the inverter), Z _vi represents the virtual output impedance of the inverter, including two parts of the virtual resistance R _vi and the virtual inductance L _vi , and ω represents The angular frequency of the fundamental voltage, that is, 2π*50, h represents the harmonic order with the highest harmonic content determined in step 4.

Step S6, filtering the above calculation results to eliminate high-frequency noise generated in the sampling and calculation links.

Step S6 filters the line impedance Z _Li of each inverter as a result of the calculation. The purpose of this step is to filter out the high-frequency noise interference introduced in the sampling link and the calculation link, and the filter form should not be limited. Here, the first-order low-pass filter (LPF) is used as an example to illustrate, the first-order low-pass filter The filter transfer function is as follows:

Where ω _c represents the cut-off frequency of the filter, and s represents the Laplacian operator. After passing through the filter, accurate line impedance measurement results can be obtained.

In order to verify the effectiveness and correctness of this method, the simulation system shown in Figure 1 was built in the PSCAD software environment for verification. The simulation only includes two inverters, and the real line impedance is consistent with Figure 1. The measured values of line impedance obtained in the simulation are 1mH+0.21Ω and 2mH+0.19Ω respectively, which are basically consistent with the real line impedance, which proves the validity of the method.

Claims (9)

1. a kind of inverter parallel system line impedance measurement method, which is characterized in that by the central controller of microgrid The output current of busbar voltage and Ge Tai inverters is sampled, the extraction wherein highest specific order harmonic components of content estimate circuit Impedance；Wherein each inverter is all made of droop control.

2. a kind of inverter parallel system line impedance measurement method according to claim 1, which is characterized in that specific packet Include following steps：

Step 1, the virtual output impedance Z of each inverter is arranged in system initialization_vi；

Step 2, busbar voltage and Ge Tai inverter output currents are sampled in the central controller of microgrid；

Step 3, busbar voltage is calculated, determines the wherein highest overtone order h of harmonic content；

Step 4, after step 3 determines the highest overtone order h of harmonic content, busbar voltage and each inverter output electricity are extracted H order harmonic components in stream；H order harmonic components include h subharmonic voltages and h subharmonic currents；

Step 5, according to the h subharmonic voltages and h subharmonic currents of step 4 extraction, the line impedance Z of each inverter is calculated_Li, Wherein, i indicates i-th inverter, i=1,2 ..., n, it is assumed that there are n platform inverters；

Step 6, the result of calculation of step 5 is filtered, eliminates sampling and calculates the high-frequency noise that link generates.

3. a kind of inverter parallel system line impedance measurement method according to claim 2, which is characterized in that step 2 In, busbar voltage is sampled by voltage sensor and current sensor in the central controller of microgrid and Ge Tai inverters export Electric current.

4. a kind of inverter parallel system line impedance measurement method according to claim 2, which is characterized in that step 3 In, it is calculated using Fast Fourier Transform (FFT).

5. a kind of inverter parallel system line impedance measurement method according to claim 2, which is characterized in that step 3 In, the highest overtone order h of harmonic content is determined by load characteristic.

6. a kind of inverter parallel system line impedance measurement method according to claim 2 or 5, which is characterized in that humorous The highest overtone order h of wave content is -1,5,7,11 or 11.

7. a kind of inverter parallel system line impedance measurement method according to claim 2, which is characterized in that step 4 In, using the method for extracting signal based on more Second Order Generalized Integrators, extract in busbar voltage and each inverter output current H order harmonic components.

8. a kind of inverter parallel system line impedance measurement method according to claim 2, which is characterized in that step 5 In, the line impedance Z of each inverter_LiIt is calculated by the following formula：

Wherein, Z_tiIndicate the total impedance being calculated, i.e. the sum of line impedance and the virtual output impedance of inverter, Z_viIndicate inversion The virtual output impedance of device, including virtual resistance R_viWith virtual inductor L_viTwo parts, ω indicate the angular frequency of fundamental voltage, i.e. 2 π * 50, h indicate the highest overtone order of harmonic content；v_αhIndicate h subharmonic voltage α axis components, v_βhIndicate h subharmonic voltage β axis Component, i_αhiIndicate h subharmonic current α axis components and i_βhiIndicate h subharmonic current beta -axis components.

9. a kind of inverter parallel system line impedance measurement method according to claim 2, which is characterized in that step 6 In, it is filtered by low-pass first order filter, low-pass first order filter transmission function G (s) is as follows：

Wherein, ω_cIndicate that the cutoff frequency of filter, s state Laplace operator.