CN102095932B - Detection method of voltage phase at access point of photovoltaic inverter - Google Patents

Abstract

The invention discloses a detection method of voltage phase at the access point of a photovoltaic inverter, which comprises the following steps of: measuring the frequency of alternating-current voltage input signals at the access point of the photovoltaic inverter by using a phase lock loop so as to obtain a corresponding frequency period T; according to the period T, setting the sampling interval of the alternating-current voltage input signals to obtain the adopted alternating-current voltage input signals; converting the adopted alternating-current voltage input signals into digital signals; and conducting Fourier analysis on the digital signals with the period T as an expansion peroid, and obtaining the phase of voltage at the access point. According to the invention, the detection method of voltage phase at the access point of the photovoltaic inverter is simple and feasible, and can measure the output phase of the photovoltaic inverter accurately.

Description

Detection method of voltage phase at access point of photovoltaic inverter

Technical field

The present invention relates to a kind of detection method of voltage phase at access point of photovoltaic inverter.

Background technology

Photovoltaic DC-to-AC converter is generally operational in maximal power tracing (MPPT) state, and now its output current is followed the tracks of the phase place of grid ac voltage and kept synchronous, and ideal power factor is 1.For ensureing the peak power output state of photovoltaic DC-to-AC converter, its output current must accurately be followed the tracks of the phase place of line voltage.Photovoltaic inversion unit is generally realized the phase-locking of electric current and voltage by detecting the mode of Zero Crossing Point for Three Phase Voltage, have real-time good, follow the tracks of the advantages such as rapid.But actual track inductance is also non-vanishing, and for high-frequency power electronic on-off circuit, its induction reactance, much larger than resistance, can be approximately pure induction reactance.Photovoltaic inversion unit is except containing outputting inductance, and between electrical network, also has connection line inductance, because electric current is on off state, will on line inductance, form switching voltage, causes AC sampling voltage no longer to have smooth sinusoidal waveform.Especially in the time that the output current of photovoltaic DC-to-AC converter is larger, switching voltage peak-to-peak value even exceedes voltage on line side peak-to-peak value, and may have multiple voltage over zero in one-period.Now adopt the method that detects voltage over zero position to determine that voltage-phase is no longer valid, can not eliminate the impact of switching voltage completely even if increase lowpass pre-filter, and cause voltage-phase to change, detect error larger.Also the photovoltaic inversion unit having adopts phaselocked loop (PLL) technology for detection electric network voltage phase, although which can accurately be followed the tracks of the frequency of electrical network, can not realize the accurate tracking of voltage-phase.

Phaselocked loop has the characteristic of accurate tracking signal frequency, can accurately define the work period of signal, but because the phase differential of phaselocked loop utilization input and output signal regulates the output frequency of voltage controlled oscillator, between input, output signal, certainly exist phase differential, thereby can not accurately follow the tracks of the phase place of input signal.In addition, phase locked-loop unit itself also has certain phase-locked time delay.

Just current document finding, for the coupling interaction impact of line inductance and switching process, not yet has very effective electric network voltage phase detection method to realize the accurate synchronization of photovoltaic DC-to-AC converter output current and voltage.

Summary of the invention

The present invention is in order to overcome the detection method complexity of existing voltage phase at access point of photovoltaic inverter and the defect of poor accuracy, a kind of detection method of the voltage phase at access point of photovoltaic inverter based on phaselocked loop and Fourier analysis is provided, the method is simple, and output phase that can Measurement accuracy photovoltaic DC-to-AC converter.

The present invention is by the following technical solutions:

The detection method of this invention voltage phase at access point of photovoltaic inverter, it comprises the following steps:

A) application phaselocked loop is measured the frequency of the alternating voltage input signal of photovoltaic DC-to-AC converter access point, and then, draw the cycle T of corresponding described frequency;

B) the described cycle T of foundation is set the sampling interval of described alternating voltage input signal, to obtain the alternating voltage input signal of employing;

C) convert adopted alternating voltage input signal to digital signal;

D) taking described cycle T as the expansion cycle, described digital signal is carried out to Fourier analysis, draw access point voltage-phase.

Photovoltaic DC-to-AC converter is linked in the electrical network that contains line inductance, need to output current be synchronizeed with electric network voltage phase by measuring access point voltage-phase.But because of the impact of switching harmonics voltage, the access point voltage that causes sampling to obtain exists multiple zero crossings near voltage zero-crossing point of power grid, be difficult to determine voltage-phase.According to technical scheme of the present invention, PHASE-LOCKED LOOP PLL TECHNIQUE and Fourier analysis method are combined, can the power frequency period of Measurement accuracy tracking signal and the feature of frequency by phaselocked loop, re-use the accurately phase place of calculating voltage signal of fourier series.For the interconnection technology of the photovoltaic DC-to-AC converter that contains larger line inductance, this programme has thoroughly overcome the reciprocal effect of line voltage distribution and photovoltaic DC-to-AC converter switching process, solve a difficult problem that is difficult to accurately detect voltage-phase because of many zero crossings phenomenon, be easy to realize the accurate synchronization of photovoltaic DC-to-AC converter output current and line voltage, to ensure that photovoltaic generation unit is operated in maximum power output state.

Above-mentioned detection method, described steps d) in the access point voltage-phase that draws of Fourier analysis be:

$A_{1m}=\frac{2}{T}\underset{i=0}{\overset{N-1}{\mathrm{\Σ}}}\left(\cos {\mathrm{\ωt}}_i\right){\∫}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}$

$B_{1m}=\frac{2}{T}\underset{i=0}{\overset{N-1}{\mathrm{\Σ}}}\left(\sin {\mathrm{\ωt}}_i\right){\∫}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}$

${\∫}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}=U_{\mathrm{ac}}=U_{1m}{\cos \mathrm{\ωt}}_i$

Wherein, N is a switch periods number in power frequency period, A _1mand B _1mfor the fourier coefficient to described digital signal Fourier expansion formula fundametal compoment, U _acfor photovoltaic cell group output voltage, ω _ifor described photovoltaic cell group output voltage independent variable.

Brief description of the drawings

Below in conjunction with Figure of description in detail technical scheme of the present invention is described in detail, wherein:

Fig. 1 photovoltaic inversion unit equivalent electrical circuit;

Fig. 2 a is access point sampled voltage and line voltage figure in access point sampled voltage and line voltage simulation waveform;

Fig. 2 b is access point sampled voltage and the line voltage in single switch periods in access point sampled voltage and line voltage simulation waveform;

Fig. 3 a is access point voltage and output current wave figure in the voltage of grid integration point and electric current experimental waveform, takes from instrument, and figure is with gray scale;

Fig. 3 b is access point voltage and the switching current oscillogram in single switch periods in the voltage of grid integration point and electric current experimental waveform, takes from instrument, and figure is with gray scale.

Fig. 4 is the workflow of phaselocked loop in preferred embodiment of the present invention;

Fig. 5 is the basic waveform that phaselocked loop is realized frequency-tracking;

Fig. 6 is the output of phaselocked loop floating voltage phase place experiment-comparer and phaselocked loop output, takes from instrument, and figure is with gray scale;

Fig. 7 is invention detection method process flow diagram, wherein in dotted line frame, has digital signal controller to realize;

Fig. 8 is the Phase Tracking lab diagram of grid-connected unit, takes from instrument, and figure is with gray scale.

Embodiment

Below in conjunction with accompanying drawing and embodiment, the present invention will be further described.

While considering line inductance, the equivalent electrical circuit of photovoltaic inversion unit as shown in Figure 1, adopts typical half-bridge topology.In accompanying drawing 1, PV is photovoltaic cell group, C ₁and C ₂for DC bus capacitor, Q ₁and Q ₂for IGBT switching tube, V ₁for the voltage detecting point of photovoltaic DC-to-AC converter access power distribution network, I ₁for photovoltaic DC-to-AC converter output electric current measure point, L ₁for the outputting inductance of inverter, L ₂for line inductance, photovoltaic DC-to-AC converter passes through L ₂voltage node V is connected to the grid _aC.

If flow through inductance L ₁and L ₂electric current be i ₁, V ₁the voltage of point is u ₁.Because the switching frequency of power device is far above power frequency, in a switch periods, photovoltaic cell group output voltage can be approximately a constant U _dc, ac bus voltage U _acalso can think constant.Switching tube Q ₁and Q ₂switching signal complementation, they are alternate conduction in a switch periods.

Phaselocked loop has the characteristic of accurate tracking signal frequency, can accurately define the work period of signal, but because the phase differential of phaselocked loop utilization input and output signal regulates the output frequency of voltage controlled oscillator, between input, output signal, certainly exist phase differential, thereby can not accurately follow the tracks of the phase place of input signal.In addition, phase locked-loop unit itself also has certain phase-locked time delay.

As shown in Figure 4, wherein COMP is comparer to the workflow of phaselocked loop tracking signal frequency, and PD is phase detector, and LF is low-pass filter, VCO voltage controlled oscillator, and DIVIDER is frequency divider.

The sampled signal of photovoltaic DC-to-AC converter access point voltage is first output as multiple pulse signals, then carries out AND operation by the feedback signal of phase detector and frequency divider after comparer, and the output signal in Fig. 4 becomes the square wave of spending with comparator output signal phase differential 90.If the output signal of PD is u _d(t), the output signal after low-pass filter LF is:

u _c(t)＝K _I∫u _d(t)dt (1)

U under stable state _c(t) ideal waveform should be a horizontal linear, and VCO output frequency just can not fluctuate like this, but actual u _c(t) on waveform, be superimposed with triangle ripple, as shown in Figure 5.The combined influence of these intermediate links, causes the phase place of frequency divider DIVIDER output signal and the output signal of comparator C OMP can not keep 90 degree phase differential, also makes the output signal in Fig. 4 can not ensure as square wave simultaneously.

The present invention utilizes phase-locked loop chip CD4046 to follow the tracks of line voltage, and as shown in Figure 6, the phase place of phase detector PD output signal is not into desirable phase quadrature with input signal to the experimental waveform of acquisition.In Fig. 6, comparer output voltage is 100V/ lattice, and phaselocked loop output voltage is 200V/ lattice.

Although PHASE-LOCKED LOOP PLL TECHNIQUE exists phase displacement error, can accurately follow the tracks of the mains voltage signal frequency of slow variation to determine its work period T, if adopt on this basis the Fourier analysis can accurate Calculation voltage-phase.Ac voltage signal can be decomposed into infinite series containing first-harmonic and higher hamonic wave:

$u\left(t\right)=A_0+\underset{k=1}{\overset{\∞}{\mathrm{\Σ}}}(A_{\mathrm{km}}\cos \mathrm{k\ωt}+B_{\mathrm{km}}\sin \mathrm{k\ωt})---\left(2\right)$

Because the output current of photovoltaic inversion unit is followed the variation of voltage fundamental, therefore, fundametal compoment that can a modus ponens (2) when analysis, is made as u ₁(t), obtain:

u ₁(t)＝A _1mcosωt+B _1msinωt (3)

Wherein:

$A_{1m}=\frac{2}{T}{\∫}_0^{T}u\left(t\right)\cos \mathrm{\ωtdt}---\left(4\right)$

$B_{1m}=\frac{2}{T}{\∫}_0^{T}u\left(t\right)\sin \mathrm{\ωtdt}---\left(5\right)$

Accurately asked for the precondition of voltage fundamental signal phase by the fourier coefficient method of formula (3)～formula (5), need to accurately obtain signal period T, this can be completed by phaselocked loop.For this reason, the present invention combines the two, forms the new method of detectable voltage signals phase place, and its flow process as shown in Figure 7.

In view of the existence of switch component in line inductance and voltage waveform, may bring impact to the phase calculation based on Fourier coefficient, for the output current signal shown in Fig. 3 b, this patent has adopted hysteresis current Tracking Control Strategy, and establishing its hysteresis band is Δ I _g, single switch cycle length is Δ t _i.Suppose in a power frequency period and have N switch periods, because switching frequency is far above work frequency, can think cos ω t within the single switch cycle _iwith sin ω t _ifor definite value, formula (4), formula (5) can turn to:

$A_{1m}=\frac{2}{T}\underset{i=0}{\overset{N-1}{\mathrm{\Σ}}}\left(\cos {\mathrm{\ωt}}_i\right){\∫}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}---\left(6\right)$

$B_{1m}=\frac{2}{T}\underset{i=0}{\overset{N-1}{\mathrm{\Σ}}}\left(\sin {\mathrm{\ωt}}_i\right){\∫}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}---\left(7\right)$

Now, need only prove in power frequency period

voltage-phase is calculated without impact.As Fig. 3 b, shown in a switch periods is decomposed into electric current decrement phase Δ t _awith Current rise phase Δ t _b, and can think that interior line voltage of this period is a constant U _ac=U _1mcos ω t _i, the equivalent voltage of establishing in this period is u _eq, meet:

${\∫}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}=u_{\mathrm{eq}}\·{\mathrm{\Δt}}_i=u_{12}\·{\mathrm{\Δt}}_a+u_{11}\·{\mathrm{\Δt}}_b---\left(8\right)$

Through deriving, can obtain:

${\mathrm{\&Delta;t}}_a=\frac{{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="HSA00000369969800022.png,HSA00000369969800031.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L_2}{\frac{{\mathrm{<figure-callout\; id="2"\; label="U\; dc"\; filenames="HSA00000369969800022.png,HSA00000369969800031.png"\; state="\{\{state\}\}">U</figure-callout>}}_{\mathrm{dc}}}{2}+U_{\mathrm{ac}}}\&CenterDot;I_g---\left(9\right)$

${\mathrm{\&Delta;t}}_b=\frac{{\mathrm{<figure-callout\; id="1"\; label="L"\; filenames="HSA00000369969800022.png,HSA00000369969800031.png"\; state="\{\{state\}\}">L</figure-callout>}}_1+L_2}{\frac{U_{\mathrm{dc}}}{2}-U_{\mathrm{ac}}}\&CenterDot;I_g---\left(10\right)$

Finally solve:

${\&Integral;}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}=U_{\mathrm{ac}}=U_{1m}\cos {\mathrm{\&omega;t}}_i---\left(11\right)$

In each switch periods, the integral result of formula (11) is definite value, when this explanation adopts hysteresis current Tracking Control Strategy, the ripple voltage that line inductance and switching process cause is on the electric network voltage phase computing method based on Fourier analysis without any impact, and the method can be applied in phase-locking detection preferably.By in formula (11) substitution formula (6) and (7), can obtain photovoltaic DC-to-AC converter access point electric network voltage phase.

The experiment of photovoltaic DC-to-AC converter current tracking:

Because of the impact of line inductance and switching process, only adopt traditional voltage over zero method for detecting phases, can not realize the phase-locking of photovoltaic inversion unit access point voltage and output current, this is by the experimental result of Fig. 3 a in is above confirmed.The micro-electrical network experiment porch of 10kW based on set up, the voltage-phase new detecting method that concrete application proposes herein, carry out experimental study and obtained the waveform of many groups access point voltage and output current, it shown in Fig. 8, is one group of experimental waveform wherein, wherein access point voltage is 100V/ lattice, and output current is 10A/ lattice.

Can find out, the voltage-phase detection method that adopts this patent to propose, inverter output current can be followed the tracks of the phase place of alternating voltage exactly, is beneficial to the maximum power output of realizing photovoltaic inversion unit.

Claims (1)

1. a detection method for voltage phase at access point of photovoltaic inverter, is characterized in that it comprises the following steps:

A) application phaselocked loop is measured the frequency of the alternating voltage input signal of photovoltaic DC-to-AC converter access point, and then, draw the cycle T of corresponding described frequency;

B) the described cycle T of foundation is set the sampling interval of described alternating voltage input signal, to obtain the alternating voltage input signal of employing;

C) convert adopted alternating voltage input signal to digital signal;

D) taking described cycle T as the expansion cycle, described digital signal is carried out to Fourier analysis, draw access point voltage-phase;

Wherein, by described steps d) in Fourier analysis draw:

$A_{1m}=\frac{2}{T}\underset{i=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\left(\cos {\mathrm{\&omega;t}}_i\right){\&Integral;}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt};$

$B_{1m}=\frac{2}{T}\underset{i=0}{\overset{N-1}{\mathrm{\&Sigma;}}}\left(\sin {\mathrm{\&omega;t}}_i\right){\&Integral;}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt};$

Get ${\&Integral;}_{t_i}^{t_{i+1}}u\left(t\right)\mathrm{dt}=U_{\mathrm{ac}}{\mathrm{\&Delta;t}}_i=\left(U_{1m}\cos {\mathrm{\&omega;t}}_i\right){\mathrm{\&Delta;t}}_i;$ And bring above-mentioned two formulas into and draw access point voltage-phase;

Wherein, N is the switch periods number in one-period T, U _1mfor ac bus amplitude voltage, A _1mand B _1mfor the Fourier coefficient to described digital signal Fourier expansion formula fundametal compoment, U _acfor ac bus voltage, ω t _ifor photovoltaic cell group output voltage independent variable;

And $u\left(t\right)=A_0+\underset{k=1}{\overset{\&infin;}{\mathrm{\&Sigma;}}}(A_{\mathrm{km}}\cos \mathrm{k\&omega;t}+B_{\mathrm{km}}\sin \mathrm{k\&omega;t})$ That alternating voltage input signal is decomposed into infinite series containing first-harmonic and higher hamonic wave.

Images