CN102914697A - Micro-grid harmonic impedance measuring method based on three-phase symmetrical square wave current injection - Google Patents

Abstract

The invention discloses a micro-grid harmonic impedance measuring method based on three-phase symmetrical square wave current injection. The method comprises the following steps: injecting three-phase symmetrical square wave current ih into a certain node of a micro-grid and measuring the voltage disturbance caused by injection; sampling the square wave current ih and the voltage disturbance delta uh, wherein the sampling ratio and the sampling point number meet the requirement of a base-2FFT algorithm; and performing quick Fourier transformation on the sampled square wave current ih and the voltage disturbance delta uh. The harmonic impedance of the micro-grid is measured by injecting the three-phase square wave current into the certain node of the micro-grid. In a micro-grid simulation model based on PSCAD, the harmonic impedance of the system under a droop control mode is measured by the method; and according to the simulation result, the measuring value basically coincides with the theoretical calculation value, so that the method is correct and practical.

Description

Micro power network harmonic impedance measuring method based on the injection of three-phase symmetrical square wave current

Technical field

The invention belongs to the micro power network technical field, relate to a kind of micro power network harmonic impedance measuring method, especially the micro power network harmonic impedance measuring method that a kind of three-phase symmetrical square wave current injects, specifically, be exactly by inject the harmonic current of certain frequency and amplitude to micro power network system, and by correlation analysis and calculating, obtain the harmonic impedance of system.

Background technology

In short supply along with global energy, environmental pollution is day by day serious, countries in the world all efficiently utilize mode what strive to find a kind of energy.All by powering as the centralized single electric power system of principal character take large unit, large electrical network, high voltage, still large electrical network is owing to the defective of self for the electric load in the present whole world 90%, and its single power supply can not have been satisfied the electric energy requirement of social increasingly stringent.The operating mode that the large electrical network that occurs at present combines with micro power network, thought to improve the main method of security of system and dirigibility by the many energy in the world and electric power expert, it is one of main development direction of 21 century power industry, micro power network is as replenishing the useful of large electrical network, the having a high potential of its widespread use.Compare with traditional electrical network, micro power network contains multiple distributed power source, a large amount of converters and constant power load, therefore, and in the design of micro power network and the stability of necessary taking into account system in service.And the harmonic impedance parameter of Systems balanth and system has close relationship, thus become an important directions of current research based on the stability analysis of impedance, particularly important thereby the Measurement accuracy of micro power network harmonic impedance just seems.

Find by literature search, the research that more existing large electric network impedances are measured, but not yet find the research of measuring for the micro power network harmonic impedance at present.Micro power network structurally has a great difference with the large electrical network of tradition, and it contains a large amount of power electronic equipments and distributed power source, and system stability is relatively poor.Since need to come based on impedance parameter the stability of analytic system, therefore higher for frequency span and the accuracy requirement of impedance measurement.Micro power network current detecting point is many, and therefore simultaneously harmonic impedance and each networking part output impedance of measuring system utilizes recently analytic system stability of impedance, and large electrical network does not then generally adopt such method.The main method of at present system impedance measurement has switched capacitor method, thyristor branch road switching method and harmonic current injection etc.The switched capacitor method is reduced investment outlay, but the harmonic wave that this method produces is uncontrollable, may cause fractional harmonic electric current disappearance, and amplitude is too small, and the radio-frequency component decay is very large; Thyristor branch road switching method can be controlled the size of harmonic electric current, but can not accurately control the harmonic frequency of injected system, signal HFS disappearance; Two kinds of above-mentioned methods all can not the Measurement accuracy micro power network harmonic impedance.Harmonic current injection then utilizes harmonic current source directly to system's harmonic electric current, because harmonic current source can control on each harmonic frequency and amplitude, harmonic current can not occur in some frequency range too small and cause that harmonic impedance measures inaccurate situation.

Summary of the invention

The object of the invention is to overcome the shortcoming of above-mentioned prior art, a kind of micro power network harmonic impedance measuring method of injecting based on the three-phase symmetrical square wave current is provided, the method uses the three-phase square wave current injection method to measure the micro power network harmonic impedance, obtain the very interior system harmonic impedance of broadband, for the micro power network stability analysis provides necessary condition.

The objective of the invention is to solve by the following technical programs:

This micro power network harmonic impedance measuring method of injecting based on the three-phase symmetrical square wave current may further comprise the steps:

1) at first, injects the three-phase square wave current i to a certain node of micro power network _h, and measure the voltage disturbance Δ u that causes thus _h=u ₁-u ₀, u wherein ₁For micro power network behind the injection square wave current injects the voltage of node, u ₀For injecting the voltage of node before the injection square wave current;

2) then, sampling square wave current i _hWith voltage disturbance Δ u _h, wherein sampling rate and sampling number satisfy base-2FFT algorithm requirement;

3) last, to the square wave current i of sampling _hWith voltage disturbance Δ u _hDo Fast Fourier Transform (FFT), system harmonic impedance is determined by following formula:

$Z_h\left(\mathrm{j\ω}\right)=\frac{{\mathrm{\ΔU}}_h\left(\mathrm{j\ω}\right)}{I_h\left(\mathrm{j\ω}\right)}.$

Further, above-mentioned steps 3) in, amplitude and the phase angle of its middle impedance are respectively:

$\left|Z_h\left(\mathrm{j\ω}\right)\right|=\frac{\left|U_h\left(\mathrm{j\ω}\right)\right|}{\left|I_h\left(\mathrm{j\ω}\right)\right|};$

The present invention has following beneficial effect:

The present invention adopts method from the three-phase square wave electric current to a certain node of micro power network that inject, realizes the measurement to the micro power network harmonic impedance.In the micro power network realistic model based on PSCAD, adopt this method, record the harmonic impedance of system under the droop control pattern, can find out from simulation result, measured value and calculated value are substantially identical, and correctness and the feasibility of the method is described.

Description of drawings

Fig. 1 is the micro power network structural representation;

Fig. 2 is for injecting the micro power network impedance measurement simulation architecture figure of three-phase square wave electric current;

Fig. 3 is three-phase square wave current injection device analogous diagram;

Fig. 4 is that micro power network adopts droop control, injects same frequency, during different amplitude square wave electric current, and micro power network harmonic impedance curve; Figure (a) for the Injection Current amplitude be single inverter output current amplitude 5% the time, the system harmonic impedance curve; Figure (b) for the Injection Current amplitude be single inverter output current amplitude 10% the time, the system harmonic impedance curve; Figure (c) for the Injection Current amplitude be single inverter output current amplitude 20% the time, the system harmonic impedance curve; Wherein, blue curve is the impedance theory value, and green curve is impedance measurements.

Fig. 5 is that micro power network adopts droop control, injects identical amplitude, during the different frequency square wave current, and micro power network harmonic impedance curve; When (a) being 50Hz for the harmonic power frequency, the system harmonic impedance curve; When figure (b) is 100Hz for the harmonic power frequency, the system harmonic impedance curve; Wherein, blue curve is the impedance theory value, and green curve is impedance measurements.

Embodiment

Below in conjunction with accompanying drawing the present invention is done and to describe in further detail:

As shown in Figure 1, the square wave current injection device is connected on the micro power network ac bus, corresponding therewith, Fig. 2 has provided the realistic model that the micro power network harmonic impedance is measured under the PSCAD environment, for sake of convenience, micro power network is simplified herein, two distribution type power supplys are connected on the ac bus by inverter, it is the load supplying of two groups of Y-connections, the three-phase square wave electric current injects from a side (A point Fig. 2) of micro power network, suppose that micro power network is operated under the island mode, two groups of inverters all adopt droop control.Fig. 3 has provided the simulation architecture figure of current injection device, injects the square wave current of three-phase symmetrical herein.Concrete steps are as follows:

Step 1: inject three-phase symmetrical square wave current i at a certain node of the net of micro power network (A point among Fig. 2) _hAnd measure electric current and inject the voltage that front and back A is ordered, be designated as respectively u ₀, u ₁, and calculate the caused voltage disturbance of Injection Current, Δ u _h=u ₁-u ₀, because three-phase symmetrical, so locate only to get a wherein phase;

Step 2: sampling Injection Current i _hWith disturbance voltage Δ u _h, wherein sampling rate and sampling number satisfy base-2FFT algorithm requirement;

Step 3: disturbance voltage and Injection Current to sampling are done Fast Fourier Transform (FFT) (FFT), and system harmonic impedance is calculated according to the following formula:

$Z_h\left(\mathrm{j\ω}\right)=\frac{U_h\left(\mathrm{j\ω}\right)}{I_h\left(\mathrm{j\ω}\right)}$

Wherein, the amplitude of impedance and phase angle are respectively: $\left|Z_h\left(\mathrm{j\ω}\right)\right|=\frac{\left|U_h\left(\mathrm{j\ω}\right)\right|}{\left|I_h\left(\mathrm{j\ω}\right)\right|}$

It is 50Hz that Fig. 4 has provided injection square wave current frequency, amplitude be respectively single inverter output current amplitude 5%, 10%, 20% the time system harmonic impedance that records.As seen from the figure, when the Injection Current amplitude increased, it is smooth-out that harmonic impedance is measured curve, but measurement result is had no significant effect.

It is 10% of single inverter output current amplitude that Fig. 5 has provided injection square wave current amplitude, the system harmonic impedance that frequency records when being respectively 50Hz and 100Hz.As seen from the figure, when the Injection Current frequency increased, the harmonic impedance amplitude-versus-frequency curve was smooth-out, and the phase-frequency characteristic precision is improved.

Hence one can see that, and when choosing suitable amplitude and frequency, it is feasible taking the three-phase symmetrical square wave current to inject the method that measures the micro power network harmonic impedance.

In sum, the present invention has provided a kind of harmonic impedance measuring method that is applicable to micro power network, and by build model under the PSCAD environment it has been carried out simulating, verifying.Can see from simulation result, in the situation that take into account measuring accuracy and micro power network is affected, choose the Injection Current of appropriate frequency and amplitude, can realize that the micro power network harmonic impedance is measured in the broad frequency band, the result who comparatively is satisfied with.Emulation experiment has been verified feasibility and the validity of the method.

Claims (2)

1. a micro power network harmonic impedance measuring method of injecting based on the three-phase symmetrical square wave current is characterized in that, may further comprise the steps:

1) at first, injects the three-phase square wave current i to a certain node of micro power network _h, and measure the voltage disturbance Δ u that causes thus _h=u ₁-u ₀, u wherein ₁For micro power network behind the injection square wave current injects the voltage of node, u ₀For injecting the voltage of node before the injection square wave current;

2) then, sampling square wave current i _hWith voltage disturbance Δ u _h, wherein sampling rate and sampling number satisfy base-2FFT algorithm requirement;

3) last, to the square wave current i of sampling _hWith voltage disturbance Δ u _hDo Fast Fourier Transform (FFT), system harmonic impedance is determined by following formula:

$Z_h\left(\mathrm{j\ω}\right)=\frac{{\mathrm{\ΔU}}_h\left(\mathrm{j\ω}\right)}{I_h\left(\mathrm{j\ω}\right)}$

2. the micro power network harmonic impedance measuring method of injecting based on the three-phase symmetrical square wave current according to claim 1 is characterized in that, in the step 3), amplitude and the phase angle of its middle impedance are respectively:

$\left|Z_h\left(\mathrm{j\ω}\right)\right|=\frac{\left|U_h\left(\mathrm{j\ω}\right)\right|}{\left|I_h\left(\mathrm{j\ω}\right)\right|};$

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