CN103091578A - Island detection method of photovoltaic grid-connected system - Google Patents

Abstract

The invention relates to an island detection method of a photovoltaic grid-connected system. In views of the problems that independently-adopted passive detection mode is effective only when effective when mismatching degrees of source and load are large and a large non-detection zone (NDZ) can be accordingly provided; and when an active detection mode is independently-adopted, the system is disturbed, and therefore the disturbance results in instability of the system, and accordingly over-voltage / under-voltage is provided. An over-frequency / under-frequency detection method is combined with an improved impedance detection method to conduct island detection to the system, and therefore the NDZ is reduced to a minimum, quality of electric energy is improved and total harmonic distortion (THD) is reduced.

Description

A kind of island detection method of photovoltaic parallel in system

Technical field

The present invention relates to a kind of island detection method of photovoltaic parallel in system, make electrical network abnormal or when disconnecting, the generation of island effect can be detected fast and accurately, belong to the photovoltaic generating system detection field.Relating in particular to a kind of employing the/under-voltage, mistake/under-frequency comes accurately to judge that whether system island effect occurs, and mainly applies in photovoltaic combining inverter in conjunction with improved impedance detection mode.

Background technology

21 century, the needs of problems of the energy is more and more urgent, and sun power has caused countries in the world government and energy expert's pay attention to day by day as one of regenerative resource in the last few years.At home, electric energy is in short supply has been a very severe problem.Photovoltaic generation is used more and more widely as one of forms of electricity generation of desirable environmental protection.When increasing photovoltaic generating system and when receiving on electrical network, just brought new phenomenon---the isolated island phenomenon of electric network protection.So-called isolated island phenomenon refers to: when mains supply barrier accident or when having a power failure maintenance and escape for some reason, the distributed grid-connected electricity generation system of each user side is failed immediately to detect power down mode and self is cut off electricity network, and form the isolated island of a self-energizing that is formed by distribution electric station grid connection electricity generation system and load on every side, as shown in Figure 1.When being in the islet operation state, grid-connected photovoltaic system can produce serious consequence, and as uncontrollable in the voltage in isolated island and frequency, may cause damage to user's equipment; Circuit in isolated island is still charged, may jeopardize maintainer's personal safety etc.As seen, research island detection method and safeguard measure, the harm of eliminating the isolated island generation is of great practical significance.

Isolated island commonly used detects and is divided into two large classes: passive type detection method and active detection method.The passive type detection method comprises voltage, frequency, phase place, power, harmonic wave etc. by detecting the parameter of grid-connected system and electrical network junction PCC, and when parameter fluctuation surpassed setting value, inverter automatically and the electrical network disconnection.But uses separately the passive type island detection method, detect the blind area larger, and it is little to be applied to the bearing power change, and with the unmatched occasion of the output of inverter.The passive type detection method mainly comprised/under-voltage and mistake/under-frequency (OVP/OUP and OFP/UFP), voltage harmonic detect, the voltage-phase sudden change detects, gives harmonic mode identification detection method etc.The active alone island detection method refers to the meritorious reactive power of the copying of inverter output current, frequency, phase place and output is carried out certain interference, thereby island effect detected when grid cut-off.Mainly comprise current interference method, active frequency drift method (AFD), positive feedback intentional frequency offset (AFDPF), variation method, synovial membrane frequency shift (FS) (SMS) etc.But this disturbance will cause the unstable of system, and the time that detects isolated island is long, should not use separately.The deficiency of or active monitoring method passive based on independent use, GB/T19939-2005 stipulates the setting that the isolated island of photovoltaic inverter grid-connected detects, each is a kind of namely to adopt at least the active-passive detection method, and moves in 2S after the electrical network decompression.

Summary of the invention

The objective of the invention is in order to detect fast and accurately the generation of island effect, realization stopped being incorporated into the power networks of inverter before High-Speed Automatic reclosing action, eliminate and detect the blind area and farthest reduce detection method to the distortion of the impact of the quality of power supply and total harmonic wave.Proposed a kind of employing the/under-voltage, mistake/under-frequency and accurately detected the method for island effect in conjunction with the mode of improving impedance detection, mainly applied in photovoltaic combining inverter.The present invention can accurately detect island effect fast, has improved security performance.And made up independent the employing initiatively or the deficiency of passive measure, farthest reduced detection blind area and total harmonic distortion.

The present invention is based on the topological structure of single-phase full-bridge inverter, inverter adopts dead beat to control, and connects electrical network, photovoltaic cell, L-type wave filter, RLC load in parallel, and PWM drives, digital phase-locked loop PLL.Anti-island protect measure adopted/under-voltage, mistake/under-frequency measure and improve the impedance detection measure and jointly complete.

The 1st go on foot, utilized/under-voltage, mistake/under-frequency method detects island effect

First utilize the passive type detection method namely mistake/under-voltage, mistake/under-frequency system carried out isolated island detect, measure the frequency of point of common coupling PCC, as frequency at 49HZ＜f _pccProceed in＜51HZ to detect, otherwise detected island effect, can carry out island protect;

All combining inverters all need to have a mistake/under-voltage, mistake/underfrequency protection.The restriction of most countries to voltage and frequency, as shown in table 1.

Table 1

Title	Measure little value	Maximal value
			Frequency	49Hz	51Hz
Voltage	0.88pu	1.1Hz

Single-phase full-bridge inverter, photovoltaic cell, L-type wave filter, RLC load in parallel and isolating switch have formed topological structure jointly, as shown in Figure 2.The connected node of all parts is called point of common coupling PCC.Voltage and the electric voltage frequency of measuring PCC namely can be used for the isolated island detection, also can be used for the setting parameter of inverter control.For inverter being controlled accurately the output current that also needs to measure inverter.The inductance of inverter is for generation of sinusoidal wave stably.

The 2nd step, in the frequency of PCC at 49HZ＜f _pccIn the time of in＜51HZ, recycling active detection method is namely improved the impedance detection method system is carried out the isolated island detection, inject respectively the current perturbation of 50HZ and 75HZ, then the voltage of point of common coupling PCC is detected and relatively detects island effect with the line voltage of 0.8 times, as U _pcc＜0.8U can carry out island protect, otherwise repeats the first step.

Impedance measurement is a kind of of active detecting, adopts/under-voltage, mistake/under-frequency and impedance detection method jointly detect island effect and can substantially realize without detecting the blind area and the good quality of power supply being arranged.The measurement of electric network impedance need to detect the change of injecting line voltage after a measuring current.This detection method and the Current Control voltage detecting that is incorporated into the power networks is complementary, and guaranteeing need not increase extra circuit devcie in the measuring process of electric network impedance.Inject the electric current of a non-integer harmonic wave, then remove to calculate sine waveform.Produce I with reference to non-integer harmonic current value and base frequency current value sum _refTwo kinds of methods combine, and jointly carry out detection and the realization of island effect.Process as shown in Figure 3.

Advantage of the present invention and beneficial effect:

1): made up independent the employing initiatively or the deficiency of passive measure, farthest reduced detection blind area and total harmonic distortion.

2): accurately detect fast island effect, improved security performance.

Description of drawings

Fig. 1 is isolated island phenomenon analysis structural representation.

Fig. 2 is the system topology schematic diagram.

Fig. 3 is overhaul flow chart.

Fig. 4 is the control system schematic diagram of single-phase inverter.

Fig. 5 is the control system schematic diagram of single-phase inverter after Injection Current.

Fig. 6 is for improving the impedance modulation process flow diagram.

Embodiment

Be described in detail below in conjunction with the island detection method of the drawings and specific embodiments to a kind of photovoltaic parallel in system of the present invention.

1. utilized/under-voltage, mistake/under-frequency method detects island effect, detects the frequency of PCC.For inverter, one pole PWM modulation technique is compared with bipolar PWM modulation technique better harmonic current and less switching loss.Inverter injects the unity power factor electric current and enters electrical network.The photovoltaic system of a 1.5KW is connected with local load with public electric wire net.Photovoltaic system active power of output and reactive power are identical with load.The RLC of load take fixed frequency as 50HZ is model, quality factor q _f=2.5. electrical network is the voltage source of Dai Weinan impedance.Design parameter is set as shown in table 2.

Table 2

Parameter	Size
		P pv	1.5KW
V dc	450V
		f switching	10KHZ
L ac	7.2mH
		R load	31.1Ω
C load	0.256mF
		L load	39.6mH
V grid	220V
		R grid	0.2Ω
X grid	0.2Ω
		f grid	50HZ

All combining inverters all need to have a mistake/under-voltage, mistake/underfrequency protection.The connected node of all parts is called point of common coupling PCC.Voltage and the electric voltage frequency of measuring PCC namely can be used for the isolated island detection, also can be used for the setting parameter of inverter control.For inverter being controlled accurately the output current that also needs to measure inverter.The inductance of inverter is for generation of sinusoidal wave stably.Power-balance PCC is ordered satisfies P _Load=P _DG+ △ P, Q _Load=Q _DG+ △ Q.If △ is P _Load=△ P _DG, △ Q _Load=△ Q _DG, just do not have so the difference of active power and reactive power between photovoltaic system and electrical network.If mains frequency is identical with the fixed frequency of RLC load, linear load is absorbing reactive power not, active power directly and voltage be complementary.After electrical network was disconnected, the active power of load was forced identical with photovoltaic system, so line voltage should be changed into V ₁=KV, wherein

As △ P _DG△ P _Load, voltage magnitude has growth to a certain degree; As △ P _DG＜△ P _Load, voltage magnitude can decrease to some degree.A little △ P causes the amplitude of voltage to only have a very little change; a little △ Q causes electric voltage frequency also to only have a very little change; therefore can make the variation of voltage and frequency in detecting blind area (NDZ), make safeguard measure island effect can not effectively be detected.As detect the frequency f of point of common coupling _pcc＜49HZ or f _pcc51HZ can directly carry out island protect; Otherwise carry out impedance detection.

2. utilize active detecting to send out the method for improving impedance detection that namely adopts.The single-phase inverter control system as shown in Figure 4.The effect of the PI controller in figure is to come balance input and output active power by keeping DC voltage to equal 450V.V _dcBe input voltage, be output as with reference to Injection Current I _refAmplitude.Amplitude with reference to Injection Current is a dc signal source, multiply each other with a sinusoidal signal.The AC signal source requires to enter next control system.In the second layer of control system, the effect of dead beat predictive controller is to cut down output current.Predictive controller calculates the voltage of inverter, and impressed current to follow reference current identical.This measure has guaranteed to obtain more current values accurately with the disturbance of minimum, but needs computing power and the better setting of systematic parameter faster.The dead beat controller obtains I from the PI controller _ref, obtain V from the output of photo-voltaic power supply _acAnd I _acRemove to calculate the inverter output voltage V of next switch circulation _inv(n+1).

${\stackrel{\‾}{V}}_{\mathrm{inv}[n+1]=}\frac{5}{2}{V}_{\mathrm{ac}}\left[n\right]-\frac{3}{2}{V}_{\mathrm{ac}}[n-1]+\frac{\mathrm{Lac}}{\mathrm{Tsw}}\left(i_{\mathrm{ref}}\right[n+2]-{2i}_{\mathrm{ac}}[n]+i_{\mathrm{ac}}[n-1\left]\right)$

In this equation, V _acThe expression line voltage, I _acThe output current of expression inverter, I _refThe expression reference current, n represents the number of times of switch circulation.

Modulation parameter M can be defined as

V _dcThe expression DC bus-bar voltage.

3. harmonic wave between injecting, its reference current represents a disturbance in control system, two of 50HZ and 75HZ are detected electric current.If the amplitude of electric current is higher than designated value, control system can not compensate, and it is unstable that system will become.As shown in Figure 5.PI1, the PI2 regulator is for generation of harmonic current between injecting.Between the harmonic current assignment can be by the integrator coefficient adjustment in PI2.In this method, when control system worked, a harmonic current assignment was controlled.The PI controller parameter is as shown in table 3.

Table 3

?	Scale-up factor	Integral coefficient
			PI1	0.066	0.416
PI2	0.017	0.055

For the electric current that injects simultaneously 75HZ and 50HZ, use the unipolarity modulation can produce some problems.When the switch mode of switch by Q ₁, Q ₄Be transformed into Q ₂, Q ₃The injection of two kinds of frequencies can cause the transformation of electric current from the positive pole to the negative pole, and the time is early than 10ms.During 20ms, this modulation can be not influential to the zero crossing of line voltage, but will go wrong when next semiperiod 30ms, and this problem each semiperiod can occur afterwards.In order to address this problem in this modulation measure, some changes must be arranged.Switch is by Q ₁, Q ₄Become Q ₂, Q ₃Key with index of modulation M, switch is by Q ₂, Q ₃Become Q ₁, Q ₄Key be synchronizing pulse.Synchronizing pulse be when line voltage be that the timing meeting produces, line voltage is 0 when negative.For the total harmonic distortion (THD) of removing to reduce output current, the injection that the simple harmonic wave electric current will be interrupted.K in figure ₇₅Get used to for injecting this electric current in some cycles.Impedance detection when between harmonic current work when being injected into.In order to remove to reduce THD, should keep I in addition _Ref75To I _Ref50Ratio surpass scarcely 5%.Flow process as shown in Figure 6.After the electric current injection is namely modulated and completed, detect the voltage U of point of common coupling _pcc, in conjunction with the IEEI.Std.2000-929 international standard, if U _pcc＜0.8U, U are the electrical network normal working voltage, can judge island effect has occured, and can carry out island protect.

Claims (2)

1. the island detection method of a photovoltaic parallel in system; make electrical network abnormal or when disconnecting; the generation of island effect can be detected fast and accurately; it is characterized in that this detection make method adopted/under-voltage, mistake/under-frequency comes accurately to judge whether system island effect occurs, and concrete steps are in conjunction with improving the impedance detection mode

The 1st go on foot, utilized/under-voltage, mistake/under-frequency method detects island effect

First utilize the passive type detection method namely mistake/under-voltage, mistake/under-frequency system carried out isolated island detect, measure the frequency of point of common coupling PCC, as frequency at 49HZ＜f _pccProceed in＜51HZ to detect, otherwise detected island effect, can carry out island protect;

The 2nd step, in the frequency of PCC at 49HZ＜f _pccIn the time of in＜51HZ, recycling active detection method is namely improved the impedance detection method system is carried out the isolated island detection, inject respectively the current perturbation of 50HZ and 75HZ, then the voltage of point of common coupling PCC is detected and relatively detects island effect with the line voltage of 0.8 times, as U _pcc＜0.8U can carry out island protect, otherwise repeats the first step.

2. method according to claim 1, it is characterized in that the method adopts the topological structure based on single-phase full-bridge inverter, inverter adopts dead beat to control, and connects electrical network, photovoltaic cell, L-type wave filter, RLC load in parallel, PWM drives, digital phase-locked loop PLL.

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