CN103311949A - Low voltage ride through control method for high-power photovoltaic inverter - Google Patents

Abstract

The invention discloses a low voltage ride through control method for a high-power photovoltaic inverter. The method comprises the following steps: firstly, considering a low voltage ride through situation under the condition of grid voltage sag, particularly unbalanced sag on the basis of a high-power photovoltaic inverter structure of an LCL filter, calculating reference given values of positive sequence current and negative sequence current specific to the control aim of restraining the negative sequence current, and compensating idle work of filter capacitor absorption via a control algorithm to ensure high-power-factor grid connection while running at different power levels; secondly, designing to add a filter capacitor current feed-forward signal into a modulation signal on the basis of positive and negative sequence double current loop control, and adjusting a feed-forward coefficient to realize active damping control under unbalanced control; and lastly, designing a disturbance quantity feed-forward coefficient to restrain transient current impact. Due to the adoption of the method, current impact at the moment of grid voltage sag can be restrained effectively, three-phase balanced output of current before and after grid voltage sag is ensured, and the control effect meets the electric energy quality requirement of a power grid on the grid connection of the photovoltaic inverter.

Description

A kind of low voltage traversing control method of high-power photovoltaic inverter

Technical field

The present invention relates to a kind of low voltage traversing control method of high-power photovoltaic inverter, be used for keeping photovoltaic DC-to-AC converter uninterruptedly to be incorporated into the power networks during the electric network fault.

Background technology

Along with a large amount of new forms of energy access electric power system, the excision of high-power photovoltaic energy moment will cause the disappearance of electric network active, and this stable recovery on electric power system causes very important impact.For keeping the safety and stablization operation of the lower electric power system of the high infiltration of photovoltaic energy, the new big-and-middle-sized photovoltaic DC-to-AC converter of grid integration rule request should possess low voltage crossing (Low Voltage Ride Through, LVRT) ability namely keeps photovoltaic DC-to-AC converter uninterruptedly to be incorporated into the power networks during electric network fault.When adopting traditional two closed-loop vector control strategy, line voltage falls the impact that moment can cause grid-connected current, may cause the hardware protection tripping operation, even burn power device; Especially when imbalance is fallen, will cause the height of grid-connected current uneven, thereby cause the power of feed-in electrical network to vibrate, affect the safe operation of photovoltaic DC-to-AC converter and electrical network.Be the safety of protection photovoltaic plant, safeguard the stable of electric power system, need the grid-connected control method of photovoltaic DC-to-AC converter during the low voltage crossing is studied.

The service conditions normal according to line voltage and three-phase equilibrium is fallen, (Zheng Fei, Zhang Junjun, Ding Mingchang. based on the photovoltaic generating system low voltage crossing modeling and control strategy of RTDS. Automation of Electric Systems, 2012,36 (22): 19-24) adopt conventional control and the strategy that LVRT control is switched mutually to realize low voltage crossing, do not analyze but imbalance is fallen, be difficult to realize that photovoltaic generating system is in line voltage low voltage crossing requirement during different faults is fallen.And study for the control strategy that imbalance is fallen, classical control is by symmetrical component method amount of unbalance to be carried out positive and negative order to decompose, and set up positive and negative ordered pair and claimed power equation under the condition, according to the control target of choosing, calculate required current-order, realize the floating of instruction current is followed the tracks of by controller, to realize the control under the uneven condition.There is power fluctuation owing to being incorporated into the power networks on the reactor, can not ignore fully, therefore (Liao Yong, Zhuan Kai, Yao Jun. the control strategy of total power wind-electricity integration current transformer during unbalanced source voltage. electric power network technique .2012,36 (1): 72-78) considered the power fluctuation on the reactor and set up new voltage equation, introduce simultaneously the pusher side current feed-forward and realize stable control under the uneven condition, but experiment is only for slight uneven condition, and can the low voltage crossing for satisfy imbalance and fall the time require to be still waiting checking; In addition, adopt inductance filter in the experiment, and normal filter effect and the better LCL filter of economic performance of adopting under the high-power condition, therefore also need analyzing based on the control method of LCL filter.At present, the control mode of research LCL filter is studied based on the line voltage symmetry mostly, in the low voltage crossing process, must consider the stability of system's operation when line voltage falls asymmetric falling especially, therefore existing method is difficult to solve the problem when line voltage is asymmetric to be fallen, in sum, existing control algolithm can't satisfy the high-power photovoltaic inverter grid-connected system (based on the LCL filter and net topology) fall low voltage crossing control requirement under the condition at different line voltages.

Summary of the invention

Technology of the present invention is dealt with problems: overcome the problem that transient current that existing high-power photovoltaic inverter can not the establishment line voltage brings when falling impacts, a kind of low voltage crossing voltage control method has been proposed, realized that line voltage falls front and back Systems balanth operation, and satisfied the reactive power support requirement in the rule that is incorporated into the power networks.

Technical solution of the present invention is: a kind of low voltage traversing control method of high-power photovoltaic inverter the steps include:

1. consider that line voltage is falling, especially under imbalance is fallen condition, the low voltage crossing situation of photovoltaic DC-to-AC converter, calculate forward-order current and negative-sequence current with reference to set-point, for guaranteeing that photovoltaic DC-to-AC converter has high power factor when being incorporated into the power networks, choose the reactive power that absorbs on the inverter compensation filter electric capacity simultaneously; Wherein, forward-order current and negative-sequence current with reference to the calculation expression of set-point are:

$\left\{\begin{array}{c}{i}_{\mathrm{inv}\_d}^{p^{*}}=\frac{{2e}_d^p{P}_0+e_q^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_q}^{p^{*}}=\frac{{2e}_q^p{P}_0-e_d^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_d}^{n^{*}}=0\\ i_{\mathrm{inv}\_q}^{n^{*}}=0\end{array}\right.$

P in the formula ₀, Q ₀Be respectively meritorious, the idle DC component of output;

Be respectively the current reference set-point of positive sequence dq axle and negative phase-sequence dq axle;

Be respectively line voltage positive sequence dq axle component.

The calculation expression of choosing the reactive power that absorbs on the inverter compensation filter electric capacity is:

$\mathrm{\ΔQ}=\mathrm{\ω}{C}_f[{\left(u_{\mathrm{cd}}^p\right)}^2+{\left(u_{\mathrm{cq}}^p\right)}^2+{\left(u_{\mathrm{cq}}^n\right)}^2+{\left(u_{\mathrm{cd}}^n\right)}^2]$

Δ Q is idle for what compensate in the formula; ω is the mains frequency that detects; C is the filter capacitor on the LCL filter;

Be respectively the positive-negative sequence dq axle component of filter capacitor.

2. based on positive-negative sequence double-current ring control strategy, design adds the filter capacitor current feed-forward in modulation signal, by regulating the current feed-forward coefficient k _c, realize the damping control under the uneven condition;

3. for the photovoltaic inverter grid-connected system, consider the transient process when voltage falls, design disturbance electric voltage feed forward coefficient k _e, the current transient when the inhibition line voltage falls is impacted;

4. the current feed-forward coefficient k that 2. forward-order current that 1. calculates according to step and negative-sequence current set-point, step are set _cAnd the disturbance electric voltage feed forward coefficient k that 3. designs of step _e, carry out the control of positive-negative sequence double-current ring, in order to realize the low voltage crossing of high-power photovoltaic inverter.

The k of described step in 2. _cSelection range is 0.2～0.7, and wherein said active damping control is a kind of unbalance control algorithm based on positive-negative sequence double-current ring, by add filter capacitor current feed-forward signal in the SPWM modulating wave, generates final PWM waveform with carrier wave ratio.

The disturbance electric voltage feed forward coefficient k of described step in 3. _eExpression formula be:

$k_e=\frac{1}{G_{\mathrm{inv}}}$

G _InvEquivalent transfer function for the three-phase photovoltaic DC-to-AC converter.

Principle of the present invention is:

The low voltage traversing control method of high-power photovoltaic inverter of the present invention is applicable to non-isolation type stage photovoltaic single grid connected structure, and its topological structure is by photovoltaic array, combining inverter, LCL filter and composition of the control system, as shown in Figure 2, and L among the figure _InvBe the inverter side inductance value; L _gBe the grid side inductance value; C _fBe filter capacitor.Set up Mathematical Modeling:

$u_{\mathrm{inv}\_x}=L_{\mathrm{inv}}\frac{{\mathrm{di}}_{\mathrm{inv}\_x}}{\mathrm{dt}}+u_{c\_x}$

i _{inv_x}＝i _{g_x}+i _{c_x}

$u_{c\_x}=L_g\frac{{\mathrm{di}}_{g\_x}}{\mathrm{dt}}+e_x---\left(1\right)$

In the formula: x=a, b, c, wherein a, b, c represent three-phase; u _{Inv_x}, i _{Inv_x}, e _x, i _{G_x}, u _{C_x}, i _{C_x}Represent respectively the voltage and current on a certain phase voltage of inverter outlet side and electric current, line voltage and electric current, the filter capacitor.

The complicated imbalance of consideration is fallen low voltage crossing situation under the condition, and whole method control block diagram as shown in Figure 3.Wherein, by phase-locked loop and positive-negative sequence separation module input variable is processed needed each variable when obtaining control; Control loop is comprised of outer voltage and current inner loop, when electrical network is normal, controls to realize the maximum power point tracking of photovoltaic battery panel by outer voltage; When voltage falls, excise in real time outer voltage according to testing conditions and adopt current inner loop control; Behind power system restoration, the cut-in voltage outer shroud, the reference of Voltage loop is given to return to value before the fault in the slope mode.Control method mainly is divided into three parts, and first is the calculating of current instruction value, and second portion is the implementation of positive-negative sequence double-current ring active damping, and filter capacitor current feed-forward coefficient k _cDetermine, third part is disturbance electric voltage feed forward coefficient k _eDetermine.

For first, the power that we can obtain inverter output from Fig. 2 can be expressed as:

$S=\frac{3}{2}(u_{\mathrm{inv}\_\mathrm{dq}}^p{e}^{\mathrm{j\ωt}}+u_{\mathrm{inv}\_\mathrm{dq}}^n{e}^{-\mathrm{j\ωt}})\left(\stackrel{\‾}{i_{\mathrm{inv}\_\mathrm{dq}}^p{e}^{\mathrm{j\ωt}}+i_{\mathrm{inv}\_\mathrm{dq}}^n{e}^{-\mathrm{j\ωt}}}\right)---\left(2\right)$

Active power of output and reactive power can be expressed as:

P＝(P ₀+P _c2cos2ωt+P _s2sin2ωt) (3)

Q＝(Q ₀+Q _c2cos2ωt+Q _s2sin2ωt) (4)

In the formula: P ₀, Q ₀Be respectively meritorious, the idle DC component of output;

Be respectively inverter outlet side voltage positive-negative sequence d axle, q axle component;

Be respectively inverter outlet side electric current positive-negative sequence d axle, q axle component; P _C2, P _S2, Q _C2, Q _S2For exporting meritorious, idle wave component; ω is the line voltage angular frequency.Real component and idle component are expanded into:

$\left[\begin{array}{c}{P}_0\\ P_{c2}\\ P_{s2}\\ Q_0\\ Q_{c2}\\ {\mathrm{<figure-callout\; id="2"\; label="Q\; s"\; filenames="HSA0000088483480000023.png"\; state="\{\{state\}\}">Q</figure-callout>}}_s\end{array}\right]=\frac{3}{2}\left[\begin{array}{cccc}{u}_{\mathrm{inv}\_d}^p& u_{\mathrm{inv}\_q}^p& u_{\mathrm{inv}\_d}^n& u_{\mathrm{inv}\_q}^n\\ u_{\mathrm{inv}\_d}^n& u_{\mathrm{inv}\_q}^n& u_{\mathrm{inv}\_d}^p& u_{\mathrm{inv}\_q}^p\\ u_{\mathrm{inv}\_q}^n& -u_{\mathrm{inv}\_d}^n& {-u}_{\mathrm{inv}\_q}^p& u_{\mathrm{inv}\_d}^p\\ u_{\mathrm{inv}\_q}^p& -u_{\mathrm{inv}\_d}^p& u_{\mathrm{inv}\_q}^n& -u_{\mathrm{inv}\_d}^n\\ u_{\mathrm{inv}\_q}^n& -u_{\mathrm{inv}\_d}^n& u_{\mathrm{inv}\_q}^p& -u_{\mathrm{inv}\_d}^p\\ -u_{\mathrm{inv}\_d}^n& -u_{\mathrm{inv}\_q}^n& u_{\mathrm{inv}\_d}^p& u_{\mathrm{inv}\_q}^p\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{inv}\_d}^p\\ i_{\mathrm{inv}\_q}^p\\ i_{\mathrm{inv}\_d}^n\\ i_{\mathrm{inv}\_q}^n\end{array}\right]---\left(5\right)$

In order to utilize this known measuring amount of line voltage, obtain accurate instruction current, need formula (5) is carried out conversion.Carry out coordinate transform by formula (1), and carry out positive-negative sequence and separate the Mathematical Modeling can get under the positive-negative sequence component and can be expressed as:

Can be got by formula (6), under the systematic steady state condition, the pass between inverter side voltage and the line voltage is:

$u_{\mathrm{inv}\_d}^p=e_d^p-{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^p-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^p$

$\begin{array}{c}{u}_{\mathrm{inv}\_q}^p=e_q^p+{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^p+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^p\\ u_{\mathrm{inv}\_d}^n=e_d^n+{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^n+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^n\end{array}---\left(7\right)$

$u_{\mathrm{inv}\_q}^n=e_q^n-{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^n-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^n$

Bringing formula (7) into formula (5) can get

$\left[\begin{array}{c}{P}_0\\ Q_0\\ P_{c2}\\ {\mathrm{<figure-callout\; id="2"\; label="P\; s"\; filenames="HSA0000088483480000023.png"\; state="\{\{state\}\}">P</figure-callout>}}_s\end{array}\right]=\frac{3}{2}\left[\begin{array}{cccc}{e}_d^p& e_q^p& e_d^n& e_q^n\\ e_q^p& {-e}_d^p& e_q^n& {-e}_d^n\\ e_d^n& e_q^n& e_d^p& e_q^p\\ e_q^n& {-e}_d^n& {-e}_q^p& e_d^p\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{inv}\_d}^p\\ i_{\mathrm{inv}\_q}^p\\ i_{\mathrm{inv}\_d}^n\\ i_{\mathrm{inv}\_q}^n\end{array}\right]+---\left(8\right)$

$\frac{3}{2}\left[\begin{array}{cccc}{-\mathrm{\&omega;}}_{\mathrm{inv}}{\mathrm{Li}}_{\mathrm{inv}\_q}^p-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^p& {\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^p+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^p& {\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^n+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^n& {-\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^n{-\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^n\\ {\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^p+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^p& {\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^p+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^p& -{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^n-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^n& {-\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^n-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^n\\ {\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^n+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^n& -{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^n-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^n& {-\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^p-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^p& {\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^p+{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^p\\ -\mathrm{\&omega;}{L}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^n-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^n& -\mathrm{\&omega;}{L}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^n-\mathrm{\&omega;}{L}_g{i}_{\mathrm{gq}}^n& -{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_d}^p-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gd}}^p& -{\mathrm{\&omega;L}}_{\mathrm{inv}}{i}_{\mathrm{inv}\_q}^p-{\mathrm{\&omega;L}}_g{i}_{\mathrm{gq}}^p\end{array}\right]\left[\begin{array}{c}{i}_{\mathrm{inv}\_d}^p\\ i_{\mathrm{inv}\_q}^p\\ i_{\mathrm{inv}\_d}^n\\ i_{\mathrm{inv}\_q}^n\end{array}\right]$

Formula (8) is the relation between known measuring amount and the inverter power output.For eliminating negative-sequence current namely with in the formula (8)

Be target, when considering Operation at full power, under the operational factor of design, coupling terms shared ratio in formula (8) right side is very little, calculates for simplifying, and ignore this impact, so the current reference set-point is

$\left\{\begin{array}{c}{i}_{\mathrm{inv}\_d}^{p^{*}}=\frac{{2e}_d^p{P}_0+e_q^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_q}^{p^{*}}=\frac{{2e}_q^p{P}_0-e_d^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_d}^{n^{*}}=0\\ i_{\mathrm{inv}\_q}^{n^{*}}=0\end{array}\right.---\left(9\right)$

In the formula (9),

Be the current instruction value of inverter outlet side, under low-power was incorporated into the power networks condition, filter be can not ignore the impact of the power factor that is incorporated into the power networks.For guaranteeing that photovoltaic plant keeps higher power factor when being incorporated into the power networks, need the reactive power that absorbs on the compensation filter electric capacity, absorbing reactive power Δ Q can choose:

$\mathrm{\&Delta;Q}=\mathrm{\&omega;C}[{\left(u_{\mathrm{cd}}^p\right)}^2+{\left(u_{\mathrm{cq}}^p\right)}^2+{\left(u_{\mathrm{cq}}^n\right)}^2+{\left(u_{\mathrm{cd}}^n\right)}^2]---\left(10\right)$

When steady operational status, make Q ₀=Δ Q brings the interior circular current set-point of formula (7) can obtain unity power factor and be incorporated into the power networks the time into.During low voltage crossing, fall the degree of depth according to the voltage that detects, real time modifying Q ₀Size to regulate reactive power output.For guaranteeing that grid-connected current is no more than safe limit value, meritorious, the reactive current component size of output should satisfy

$i_{\mathrm{inv}\_d}^{p^{*}}\&le;\sqrt{{\left(i_{\max }\right)}^2-{\left(i_{\mathrm{inv}\_q}^{p^{*}}\right)}^2}---\left(11\right)$

In the formula (11), i _MaxThe maximum limit tentering value of expression output current.Stable control after voltage falls and reactive power support are realized by dynamic change d axle, q shaft current command value, and are chosen suitable controller parameter to guarantee Systems balanth.

For second portion, under high-power condition, the LCL filter can significantly reduce the filter inductance design load, reduces volume and the loss of system, but has resonance problems.Under the prerequisite that does not affect photovoltaic inverter grid-connected efficient, often adopt active damping to suppress resonance.Under the line voltage equilibrium condition, active damping current loop control block diagram as shown in Figure 4.Wherein, G _InvBe the Mathematical Modeling of inverter, G _i(s) be the current controller transfer function, 3s/2r, the conversion of 2r/3s denotation coordination, the dotted line frame partly is the equivalent mathematical model of LCL filter.But owing to adopting positive-negative sequence double-current ring control mode under the uneven condition, voltage, electric current all are separated into positive sequence component and negative sequence component, and the filter capacitor current i _cBe mainly high fdrequency component, positive-negative sequence separate mode commonly used all can't obtain positive sequence and the negative sequence component under the high frequency, therefore adopt the active damping control mode under the equilibrium condition, can't realize damping control under uneven condition, even affect Systems balanth.Based on positive-negative sequence double-current ring control strategy, the current loop control block diagram that the present invention adopts as shown in Figure 5.In Fig. 5, G _InvBe the Mathematical Modeling of inverter, G _i(s) be the current controller transfer function, 3s/2r, the conversion of 2r/3s denotation coordination, dotted line frame 1 is the control block diagram of unbalance control algorithm, dotted line frame 2 is the equivalent mathematical model of LCL.Feed-forward mode shown in Figure 5 has effectively been avoided feedforward amount i _cCarry out positive-negative sequence and separate, and feed-forward coefficients k _cAdjusting, do not affect the transfer function of active damping algorithm among Fig. 4, but the resonance that establishment LCL filter exists.

For third part, fall moment at voltage, to stable be incorporated into the power networks with low voltage crossing running status handoff procedure in the rush of current phenomenon, must be controlled to guarantee that photovoltaic DC-to-AC converter can enter safely the traffic coverage after falling.Loop shown in Figure 5 is simplified, as shown in Figure 6, and is obtained the closed loop transfer function, of system:

$I_g=\frac{G_i\left(s\right)G_1\left(s\right)G_2\left(s\right)}{1+G_i\left(s\right)G_1\left(s\right)G_2\left(s\right)}{I}_{\mathrm{inv}}^{*}-\frac{G_2\left(s\right)(k_e{G}_1\left(s\right)-1)}{1+G_i\left(s\right)G_1\left(s\right)G_2\left(s\right)}E---\left(10\right)$

In the formula, k _eBe disturbance electric voltage feed forward coefficient.

$G_1\left(s\right)=\frac{G_{\mathrm{inv}}}{L_{\mathrm{inv}}{C}_f{s}^2+(G_{\mathrm{inv}}{G}_i\left(s\right)-k_c{G}_{\mathrm{inv}})C_{f}s+1}---\left(11\right)$

$G_2\left(s\right)=\frac{L_{\mathrm{inv}}{C}_f{s}^2+(G_{\mathrm{inv}}{G}_i\left(s\right)-k_c{G}_{\mathrm{inv}})C_{f}s+1}{L_{\mathrm{inv}}{C}_f{L}_g{s}^3+(G_{\mathrm{inv}}{G}_i\left(s\right)-k_c{G}_{\mathrm{inv}})L_g{C}_f{s}^2+s(L+L_g)}---\left(12\right)$

By formula (12) as can be known, control output variable I _gWith input variable I ^* _InvAll there is contact with line voltage E.E is considered as disturbance term, and voltage falls moment, and grid-connected current is with proportional increase.Therefore, in order to eliminate disturbance quantity to the impact of grid-connected current, introduce voltage feed-forward control, shown in Fig. 6 dotted portion.

If eliminate disturbance term fully, can get

$k_e=\frac{1}{G_1\left(s\right)}=\frac{1}{G_{\mathrm{inv}}}(L_{\mathrm{inv}}{C}_f{s}^2+(G_{\mathrm{inv}}{G}_i\left(s\right)-k_c{G}_{\mathrm{inv}})C_{f}s+1)---\left(13\right)$

The disturbance electric voltage feed forward coefficient k of employing formula (13) _eCan realize the fully inhibition to disturbance quantity E, i.e. the distortion of E can not affect I _gOutput.Have differential term and second-order differential item in the factor (13), wherein, differential term and second-order differential item have inhibitory action to high fdrequency component, and proportional then affects the effect of low frequency component.E falls for line voltage, and when especially falling in e moment, the existence of micro component will cause feed-forward signal to be tending towards infinitely great, make system unstable.Fall moment at line voltage, in order to suppress the grid-connected current transient impact, the safe operation of protection grid-connection device adapts to the LVRT requirement, and formula (13) is simplified, and ignores differential term and second-order differential item in the feed-forward coefficients, can get

$k_e=\frac{1}{G_{\mathrm{inv}}}---\left(14\right)$

Be about to feed-forward coefficients and be reduced to proportional component.Simultaneously, add low pass filter in acquisition channel, eliminating the high frequency content that exists among the feed-forward signal e to the impact of the output current wave that is incorporated into the power networks, the introducing of feedforward amount will be fallen the transient current that causes to line voltage and impact and play good inhibitory action.

The method of the invention is fallen when detecting voltage when practical application, and falling flag bit will enable, and control strategy will excise the outer voltage that realizes maximum power point tracking, takes current inner loop control.Reduce simultaneously meritorious givenly, and on-the-fly modify idle givenly, qualifications is suc as formula (11), to realize the idle output during the low voltage crossing.Regulate by the control parameter, choose optimum parameter with stable state and the dynamic property of assurance current loop, thereby realize the stable control of being incorporated into the power networks before and after the low voltage crossing.

The present invention's advantage compared with prior art is:

Low voltage traversing control method for high-power photovoltaic inverter of the present invention, for the high-power photovoltaic inverter based on the LCL filter, consider that falling especially electrical network at dissimilar line voltage serious imbalance occurs and fall situation, by introducing the negative-sequence current control ring, and the current instruction value that calculates simplification to be suppressing negative-sequence current, thereby realizes grid-connected current three-phase equilibrium output; For positive-negative sequence double-current control mode, the filter capacitor current feed-forward mode under the adaptation unbalance control of design has guaranteed the stable of grid-connected system; Fall moment at line voltage; but the voltage feed-forward control establishment transient current that adopts impacts; thereby protection inverter safe transition is to the low voltage crossing running status; in conjunction with positive-negative sequence component extraction and accurate mains voltage signal are synchronous fast; the real-time of signal acquisition in can guaranteeing to control; and the according to demand output of dynamic adjustments reactive current; the control effect has guaranteed that voltage falls front and back Systems balanth operation, and satisfies electric power system for the low voltage crossing requirement of photovoltaic combining inverter.

Description of drawings

Fig. 1 is high-power photovoltaic inverter low voltage traversing control method flow chart of the present invention.

Fig. 2 is the topology diagram of stage photovoltaic single grid-connected system.

Fig. 3 is the positive-negative sequence double-current ring control block diagram of the method for the invention.

Fig. 4 is the active damping current loop control block diagram under the equilibrium condition.

Fig. 5 is the LCL active damping control block diagram under the low voltage crossing control strategy.

Fig. 6 is the simplified block diagram of system's control

The platform hardware structure chart that Fig. 7 adopts for using the inventive method.

Fig. 8 is the experimental waveform of line voltage under balance is fallen, wherein Fig. 8 a is the voltage current waveform of traditional control method, Fig. 8 b is under the 500kW condition, the voltage current waveform of the voltage current waveform when line voltage falls 20%, Fig. 8 c are the three phase network balance of voltage when dropping to 20% moment.

To be line voltage fall experimental waveform under the condition in imbalance to Fig. 9, and wherein Fig. 9 a falls the voltage current waveform that is incorporated into the power networks under the condition for the outlet side a phase (single-phase) that is incorporated into the power networks, and Fig. 9 b is that the outlet side b-c two-phase that is incorporated into the power networks is fallen the voltage current waveform that is incorporated into the power networks under the condition.

Embodiment

A kind of low voltage traversing control method of high-power photovoltaic inverter, as shown in Figure 1, the steps include: 1. to consider that line voltage falls especially under imbalance is fallen condition, the low voltage crossing situation of photovoltaic DC-to-AC converter, calculate forward-order current and negative-sequence current with reference to set-point, for guaranteeing that photovoltaic DC-to-AC converter has high power factor when being incorporated into the power networks, choose the reactive power that absorbs on the inverter compensation filter electric capacity simultaneously; Wherein, forward-order current and negative-sequence current with reference to the calculation expression of set-point are:

$\left\{\begin{array}{c}{i}_{\mathrm{inv}\_d}^{p^{*}}=\frac{{2e}_d^p{P}_0+e_q^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_q}^{p^{*}}=\frac{{2e}_q^p{P}_0-e_d^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_d}^{n^{*}}=0\\ i_{\mathrm{inv}\_q}^{n^{*}}=0\end{array}\right.$

P in the formula ₀, Q ₀Be respectively meritorious, the idle DC component of output; Be respectively the current reference set-point of positive sequence dq axle and negative phase-sequence dq axle;

Be respectively line voltage positive sequence dq axle component.

The calculation expression of choosing the reactive power that absorbs on the inverter compensation filter electric capacity is:

$\mathrm{\&Delta;Q}=\mathrm{\&omega;C}[{\left(u_{\mathrm{cd}}^p\right)}^2+{\left(u_{\mathrm{cq}}^p\right)}^2+{\left(u_{\mathrm{cq}}^n\right)}^2+{\left(u_{\mathrm{cd}}^n\right)}^2]$

Δ Q is idle for compensation; ω is the mains frequency that detects; C is the filter capacitor on the LCL filter;

Be respectively positive-negative sequence d axle, the q axle component of filter capacitor.

2. based on positive-negative sequence double-current ring control strategy, in modulation signal, add the filter capacitor current feed-forward, by regulating the current feed-forward coefficient k _c, realize the damping control under the uneven condition; k _cSelection range is 0.2～0.7, in invention, and preferred k _c=0.5, when can effectively suppressing resonance, also can reduce the impact on the stability of a system.

3. for the photovoltaic inverter grid-connected system, consider the transient process when voltage falls, design disturbance electric voltage feed forward coefficient k _e, the current transient when the inhibition line voltage falls is impacted;

4. the current feed-forward coefficient k that 2. forward-order current that 1. calculates according to step and negative-sequence current set-point, step are set _cAnd the disturbance electric voltage feed forward coefficient k that 3. designs of step _e, carry out the control of positive-negative sequence double-current ring, in order to realize the low voltage crossing of high-power photovoltaic inverter.

The k of described step in 3. _eExpression formula be:

$k_e=\frac{1}{G_{\mathrm{inv}}}$

G _InvEquivalent transfer function for the three-phase photovoltaic DC-to-AC converter.

In order to show the beneficial effect of the method for the invention, the experiment porch hardware configuration as shown in Figure 7, be 500kW high-power photovoltaic inverter experiment porch hardware structure diagram, comprise that mainly electrical network 1, electrical network simulator 7 and high-power photovoltaic inverter major loop 6 form.Electrical network simulator 7 does not control rectifier bridge 3 by three-phase and three-phase PWM inverter 4 forms; The photovoltaic DC-to-AC converter major loop is comprised of three-phase inverter, LCL filter inductance and DC side filter capacitor; Wherein, connected by transformer 2 between electrical network and the electrical network simulator, the electrical network simulator is connected by isolating transformer 5 with photovoltaic DC-to-AC converter.

The simulating grid programmable power supply that the electrical network simulator adopts Taiwan Ai Pusi to produce, model is ACST-L-33800, and its ac output end three-phase voltage, the phase angle is independent adjustable, and the line voltage in the time of can simulating low voltage crossing falls.Electrical network simulator input gets access to grid through transformer, its dc bus is attached to the direct-flow input end of photovoltaic DC-to-AC converter, the output of photovoltaic DC-to-AC converter is bound up on the ac output end of electrical network simulator by isolating transformer, like this, photovoltaic DC-to-AC converter institute energy requirement is provided by the electrical network simulator, and be fed back to electrical network simulator output, realize energy recycling.

For realizing the control of photovoltaic DC-to-AC converter, need in major loop, to add dissimilar transducer, wherein, need to add 2 voltage sensors in the net side, measure voltage on line side e _AbAnd e _BcAdd 2 current sensors at filter inductance, measure the filter inductance current i _{C_a}, i _{C_b}Add 2 current sensors at the inverter outlet side, measure output current i _{Inv_a}, i _{Inv_b}The transducer collection signal is through the sampling modulate circuit, send into DSP (the present invention adopt be TMS320LF28335), the control algolithm of DSP is the design's invention, and the pulse signal that control algolithm calculates is delivered to switch tube driving circuit through optical fiber transmission, controls its conducting and shutoff.

Adopt the 500kW photovoltaic combining inverter, the parameter of system is as shown in table 1.

Table 1 500kW photovoltaic synchronization inverter system parameter list

Suppose and take traditional restriction amplitude protection control measure, fall moment by reducing current instruction value to realize LVRT at line voltage, set test condition (the 50kW service conditions is chosen in experiment) to guarantee sufficient safety allowance.Line voltage falls and is set as three-phase symmetrical and drops to 20% rated voltage, and test waveform is shown in Fig. 8 (a).Adopt control method of the present invention control grid-connected system to operate under the 500kW condition, drop to 20% test waveform shown in Fig. 8 (b), fall moment under this condition and launch waveform shown in Fig. 8 (c).From Fig. 8 (a) as can be known, voltage falls moment, i _cThe current over pulse phenomenon is very obvious, and the amplitude of overshoot is much larger than the stable operation value.If fall under the full power condition, overshoot current will cause hardware protection so that net side circuit breaker trip, thereby disconnects with electrical network, even the danger of burning inverter is arranged.Compare with Fig. 8 (a), in Fig. 8 (b), the stable state and passing through between the state of being incorporated into the power networks switches moment, and the rush of current phenomenon is suppressed fully, the meritorious recovery according to the rate recovery of 80kW/s (greater than the 10%) service conditions to the fault in the experiment.Fall the waveform unfolds of moment shown in Fig. 8 (c), this process has guaranteed the safe and stable operation of inverter during the low voltage crossing.

Adopt the method for the invention also to carry out line voltage and fall experiment under the condition in imbalance.The electrical network simulator falls fault by regulating three-phase output voltage to simulate imbalance in the experiment porch.Test point in the experiment selects to be incorporated into the power networks dotted line voltage e _AbAnd e _BcAnd the output current i that is incorporated into the power networks _aAnd i _bTest condition is set to respectively single-phase falling (c drops to 20% rated voltage mutually) and falls (a drops to 20% mutually with b mutually) with two-phase, and test waveform is respectively shown in Fig. 9 (a) (single-phase falling), Fig. 9 (b) (two-phase is fallen).From Fig. 9 (a) as can be known, electrical network simulator c phase voltage drops to 20% rated voltage, through isolating transformer, and in voltage on line side, e _AbAnd e _BcFalling in various degree all arranged.When two-phase was fallen, it is more obvious that voltage on line side falls the degree of depth, shown in Fig. 9 (b) voltage waveform.Fall under the condition at different voltage, the grid-connected current impact of falling moment all can be suppressed fully.When operating in the low voltage crossing state, the negative sequence component of grid-connected current is eliminated, and electric current keeps three-phase equilibrium output, and a certain amount of idle output is arranged.When line voltage return to rated voltage 90% after, output current will increase gradually, active power satisfies the test request of industry with the rate recovery of 80kW/s (greater than the 10%) service conditions to the fault.Therefore, the control strategy that proposes guarantees that photovoltaic DC-to-AC converter passes through safely the subnormal voltage operation interval, and the control effect meets the functional requirement such as the output quality of power supply and reactive power support when being incorporated into the power networks.

The content of describing that do not elaborate in the specification of the present invention belongs to the known prior art of this area professional and technical personnel.

Claims (3)

1. the low voltage traversing control method of a high-power photovoltaic inverter, it is characterized in that: the steps include: 1. to consider that line voltage is under imbalance is fallen condition, calculate based on the forward-order current under the LCL filter conditions and negative-sequence current with reference to set-point, for guaranteeing that photovoltaic DC-to-AC converter has high power factor when being incorporated into the power networks, come the reactive power that absorbs on the compensation filter electric capacity by inverter control algorithm simultaneously; Wherein, forward-order current and negative-sequence current with reference to the calculation expression of set-point are:

$\left\{\begin{array}{c}{i}_{\mathrm{inv}\_d}^{p^{*}}=\frac{{2e}_d^p{P}_0+e_q^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_q}^{p^{*}}=\frac{{2e}_q^p{P}_0-e_d^p{Q}_0}{3[{\left(e_d^p\right)}^2+{\left(e_q^p\right)}^2]}\\ i_{\mathrm{inv}\_d}^{n^{*}}=0\\ i_{\mathrm{inv}\_q}^{n^{*}}=0\end{array}\right.$

P in the formula ₀, Q ₀Be respectively meritorious, the idle DC component of output;

Be respectively the current reference set-point of positive sequence d axle, q axle and negative phase-sequence d axle, q axle;

Be respectively line voltage positive sequence d axle and q axle component.

The calculation expression of choosing the reactive power that absorbs on the inverter control algorithm compensation filter electric capacity is:

$\mathrm{\&Delta;Q}=\mathrm{\&omega;C}[{\left(u_{\mathrm{cd}}^p\right)}^2+{\left(u_{\mathrm{cq}}^p\right)}^2+{\left(u_{\mathrm{cq}}^n\right)}^2+{\left(u_{\mathrm{cd}}^n\right)}^2]$

Δ Q is idle for compensation; ω is the mains frequency that detects; C is the filter capacitor on the LCL filter;

Be respectively positive-negative sequence d, the q axle component of filter capacitor.

2. based on positive-negative sequence double-current ring control strategy, design adds filter capacitor current feed-forward signal in modulation signal, by regulating the current feed-forward coefficient k _c, to realize the active damping control under the unbalance control condition;

Transient current when 3. falling for voltage impacts, design disturbance electric voltage feed forward coefficient k _e, transient suppression impacts.

4. the current feed-forward coefficient k that 2. forward-order current that 1. calculates according to step and negative-sequence current set-point, step design _cAnd the disturbance electric voltage feed forward coefficient k that 3. designs of step _e, carry out the control of positive-negative sequence double-current ring, in order to realize the low voltage crossing of high-power photovoltaic inverter.

2. the low voltage traversing control method of high-power photovoltaic inverter according to claim 1 is characterized in that: the current feed-forward coefficient k of described step in 2. _cSelection range be 0.2～0.7.

3. the low voltage traversing control method of high-power photovoltaic inverter according to claim 1 is characterized in that: described step 3. in disturbance electric voltage feed forward coefficient k _eAccount form

$k_e=\frac{1}{G_{\mathrm{inv}}}$

G _InvEquivalent transfer function for the three-phase photovoltaic DC-to-AC converter.

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