CN104037800B - A kind of photovoltaic combining inverter current control method - Google Patents

Abstract

The invention discloses an improved photovoltaic grid-connected inverter current loop control method, which belongs to the technical field of solar photovoltaic power generation and includes the following steps: step 1, obtain the voltage feedforward control value u1 through the mains voltage feedforward; step 2, Obtain the current error Δi through the voltage outer loop control structure; step 3, segment the current error Δi to form multiple current error segments, each current error segment adopts a different gain, and pass through the current inner loop control structure using the P controller After that, the current equivalent voltage duty cycle control value u ₂ is obtained; step 4, the instruction value u_order of the switch comparison register is obtained. The control method of the present invention is simple to implement, does not require complex integral and differential coefficient setting, does not need to introduce virtual orthogonal signals, avoids time-consuming dp coordinate transformation, and can effectively reduce output current distortion and enhance the protection against grid distortion and harmonic interference Resistance.

Description

A current control method for a photovoltaic grid-connected inverter

technical field

The invention relates to the optimal control of a photovoltaic grid-connected inverter, and provides an improved current control method of a photovoltaic grid-connected inverter, which belongs to the field of solar photovoltaic power generation.

Background technique

Solar energy is currently one of the cleanest, most realistic, and most promising renewable energy sources for large-scale development and utilization in the world. Solar photovoltaic grid-connected power generation is the main trend of solar photovoltaic utilization, and it is bound to develop rapidly. Photovoltaic grid-connected inverter is the core of photovoltaic power generation system, and it is the interface circuit between new energy power generation, micro-grid, etc. and large power grid. The quality control of its incoming current is one of the key technologies. Photovoltaic grid-connected inverters usually include a PWM modulation inverter topology based on high-frequency switches, a grid-connected L/LC/LCL low-pass filter, a voltage and current signal detection circuit, an inverter control loop, and a drive circuit. The purpose of current control is to make the inverter deliver current with high power factor and low THD value to the grid. At present, the commonly used inverter current loop control methods include current hysteresis control, proportional integral differential control (PID), proportional resonance control (PR), repetitive control (RC), and current model predictive control (CMPC). These control methods have their own advantages and disadvantages, and some studies have combined multiple control methods to form a composite control method, which has achieved good current control performance.

PID control is the most widely used control method in the feedback control system. It calculates the control quantity by obtaining the error, and the control idea is simple. Parameter design adopts methods such as frequency analysis and pole configuration. In the current control of single-phase grid-connected inverters, the controlled variable is an AC signal, but PID control cannot track the AC signal without static error, and the differential control is very sensitive to disturbance, and it is necessary to introduce a virtual quadrature signal. After dq transformation Use the PID controller to achieve the effect of no static difference in the grid-connected current. In order to ensure the stability of the system, the gain of the PID regulator is limited, the open-loop gain of the system in the low frequency band is small, and the system's ability to suppress the steady-state error and harmonics of the output current is not good. At the same time, this method is also vulnerable to grid voltage distortion and harmonic interference.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the traditional PID current control technology, and propose a photovoltaic grid-connected inverter current control method that adopts segmental proportional control of current error signals. Based on the traditional PID current control, the method removes the differential component that is sensitive to the interference signal, and removes the integral component that cannot achieve zero static error of the AC signal. The invention is easy to operate, reduces the steady-state error of the output current while meeting the stable control requirements of the system, enhances the resistance to grid distortion and harmonic interference, and reduces the THD value of the output current to below 5%.

For realizing above object, the technical scheme that the present invention takes is:

A current control method of a photovoltaic grid-connected inverter, the current control method of the photovoltaic grid-connected inverter adopts a grid voltage feedforward and a double closed-loop control structure of a voltage outer loop and a current inner loop, which includes the following steps:

Step 1. Obtain the voltage feedforward control value u ₁ through the mains voltage feedforward;

Step 2. Obtain the current error Δi by using the voltage outer loop control structure of the PI controller, and the current error Δi is the grid-connected current effective value command I ^* multiplied by sin table, and then compare the difference with the grid-connected current sampling value i _n ;

Step 3. Segment the current error Δi to form multiple current error segments. Each current error segment adopts a different gain. Under the condition that the current control amount is continuous, after the current inner loop control structure using the P controller , to obtain the current equivalent voltage duty cycle control value u ₂ ;

Step 4. Obtain the instruction value u_order of the switch comparison register, where the instruction value u_order=u ₁ +u ₂ .

The method for obtaining the voltage feedforward control quantity u1 in the step ₁ is: feedforward the mains voltage measured by the mains voltage sensor, and under the action of high-frequency switching, the inverter link is regarded as a high-gain and small-inertia link, and the The sampled value of the mains voltage is divided by the sampled value of the bus voltage, and then multiplied by the value of the period register of the PWM module to obtain the voltage feedforward control value u ₁ .

In the step 2, the effective value command I ^* of the grid-connected current is the output of the PI control quantity of the bus voltage stabilizing loop.

The PI control quantity is obtained by controlling the voltage error value between the sampled effective value of the bus voltage and the stabilized value of the bus voltage through a PI controller.

In the step 3, the current equivalent voltage duty cycle control quantity u ₂ is the product of the current control quantity m obtained through P controllers with different gains and the value of the period register of the PWM module.

Said step 3 includes:

Step 31. Obtain the error segmentation nodes of multiple current error segments, denoted as c ₁ , c ₂ ... c _n , where n is the number of error segmentation nodes, and the xth error segmentation node is denoted as c _x , n≥2, x≤n, c _n-1 <c _n ; each error segment node corresponds to a corresponding proportional gain, which is recorded as kp ₁ , kp ₂ ... kp _n , the xth error segment node c _x corresponds to the xth proportional gain kp _x , where kp _n-1 >kp _n ;

Step 32, calculate the current control amount m, the calculation method is:

When |Δi|<c ₁ , m=0;

When c _n-1 ≤Δi<c _n , $m={\mathrm{kp}}_{\mathrm{no}-1}*(\mathrm{\&Delta;i}-c_{\mathrm{no}-1})+\underset{x=3}{\overset{\mathrm{no}}{\mathrm{\&Sigma;}}}[{\mathrm{kp}}_{x-2}*(c_{x-1}-c_{x-2})];$

When Δi≥c _n , $m={\mathrm{kp}}_{\mathrm{no}}*(\mathrm{\&Delta;i}-c_{\mathrm{no}})+\underset{x=2}{\overset{\mathrm{no}}{\mathrm{\&Sigma;}}}[{\mathrm{kp}}_{x-1}*(c_x-c_{x-1})];$

When -c _n < Δi ≤ -c _n-1 , $m={\mathrm{kp}}_{\mathrm{no}-1}*(\mathrm{\&Delta;i}+c_{\mathrm{no}-1})+\underset{x=3}{\overset{\mathrm{no}}{\mathrm{\&Sigma;}}}[{\mathrm{kp}}_{x-2}*({-c}_{x-1}+c_{x-2})];$

When Δi≤-c _n , $m={\mathrm{kp}}_{\mathrm{no}}*(\mathrm{\&Delta;i}+c_{\mathrm{no}})+\underset{x=2}{\overset{\mathrm{no}}{\mathrm{\&Sigma;}}}[{\mathrm{kp}}_{x-1}*(-c_x+c_{x-1})];$

Step 33. Obtain the current equivalent voltage duty cycle control value u ₂ .

In the step 31, the error segmentation node c _x and its corresponding proportional gain kp _x are adjusted.

The tuning method is as follows: first, the current inner loop adopts constant gain proportional control, and then gradually increases the proportional coefficient. When the output current is adjusted to a certain threshold, record the proportional coefficient threshold kp and the maximum absolute value of the current steady-state error| Δi _max |, let kp ₁ =kp, ${\mathrm{kp}}_x=\frac{\mathrm{kp}}{2^{x-1}};c_1=0,c_x=(x-1)*\frac{\left|{\mathrm{\&Delta;i}}_{\max }\right|}{\mathrm{no}}.$

The certain threshold is a critical value at which the output current is between overshoot and non-overshoot.

Compared with the prior art, the advantages of the present invention are reflected in:

1. It is simple to implement, does not need complex integral and differential coefficient setting, does not need to introduce virtual quadrature signals, and avoids time-consuming dp coordinate transformation.

2. There is a static error in the fixed gain proportional control, and an excessively large proportional gain will reduce the stability of the system. According to the magnitude of the current error, the present invention selects a large gain for a large error and selects a small gain for a small error, so that the current THD value can be controlled within a required range.

3. By improving the output current quality, the system's ability to suppress harmonic interference is improved.

Description of drawings

Figure 1 is a typical photovoltaic grid-connected inverter topology and control diagram;

Fig. 2 is the flow chart of a kind of photovoltaic grid-connected inverter current control method of the present invention;

Figure 3 is a block diagram of the inverter current loop control;

Fig. 4 is a flow chart of current loop segmentation control;

Figure 5 shows the tuning method of kp _x and c _x .

detailed description

The content of the present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Example

Figure 1 shows the topology and control diagram of a typical single-phase bipolar non-isolated photovoltaic grid-connected inverter. The pre-stage Boost circuit charges the bus capacitor to realize the DC bus boost and the MPPT control function of the photovoltaic module. The post-stage inverter part adopts double-loop control to realize power conversion. Double-loop control refers to the voltage outer loop for bus capacitor voltage stabilization and the current inner loop for power output. U _{b_ref is selected} according to the withstand voltage of the bus capacitor and actual needs, generally 400V. The voltage outer loop bus voltage regulator adopts PI control, which can realize no static error tracking for U _{b_ref} ; the current feedback signal is sinusoidal, and the current inner loop adopts P control.

Please refer to Fig. 2, the current control method of photovoltaic grid-connected inverter, which includes the following steps:

S10. Mains voltage feedforward. Under the action of high-frequency switching, the inverter link is regarded as a high-gain and small-inertia link. The mains voltage sampling value rel_UAC is divided by the bus voltage sampling value rel_Ubus, and then multiplied by the value of the period register of the PWM module. , get the voltage feed-forward duty cycle control value u ₁ , the inverter generates a voltage with the same frequency and the same amplitude as the mains, and eliminates the interference of the mains distortion on the grid-connected current.

S20. The voltage outer loop refers to the PI voltage stabilization control on the DC side of the bus bar before the inverter, which requires the bus bar to be stable at Ub. The bus voltage sampling effective value Ub_rms and Ub are compared to get the error ΔU. After PI control, the grid-connected current effective value command is obtained. I ^* . The instruction value is multiplied by sin table, and then compare it with the grid-connected current sampling value in to get the current error _Δi .

S30. Segment the current error, adopt different gains for different error segments, and ensure that the current control amount is continuous. After passing through the P controller, the current control amount m is obtained, and then multiplied by the value of the period register of the PWM module to obtain the current equivalent voltage. Air ratio control quantity u ₂ .

Please refer to Figure 3 for the principle of segmental proportional control using current error signal. Figure 3 is a block diagram of the inverter control. I* is the given signal of the grid-connected current; G1(s) is the current regulator; ignoring the nonlinear influence of the voltage drop of the power device and the dead time, the bridge inverter link under the SPWM control mode can be regarded as a high-gain The small inertia link, this link can be expressed by G2(s)=K/(Ts+1), K is the open loop gain, T is the time constant; G3(s)=1/(Ls+R) is the AC inductance The filtering link composed of L and circuit impedance R; Gf(s) is the feedback link of grid-connected current; Gn(s) is the grid voltage feed-forward link used in system control to eliminate the interference of grid voltage on the current loop.

Let Gf(s)=1, then the unit negative feedback system transfer function

$\frac{I_{\mathrm{no}}}{I^{*}}=\mathrm{\&phi;}\left(\mathrm{the\; s}\right)=\frac{G_1\left(\mathrm{the\; s}\right)K}{{\mathrm{TLs}}^2+(\mathrm{TR}+L)\mathrm{the\; s}+G_1\left(\mathrm{the\; s}\right)K+R},$ Typical second-order system.

Here G1(s)=kp, analyze with fixed gain coefficient. Do Taylor expansion on φ(s),

G(s)=a ₁ s+a ₂ s ² +a ₃ s ³ +a ₄ s ⁴ +..., a _n is the Taylor coefficient.

Let r(t)=sin(ωt), ω is the frequency.

Then the system output is $C\left(\mathrm{the\; s}\right)=\frac{\mathrm{\&omega;}}{{\mathrm{the\; s}}^2+{\mathrm{\&omega;}}^2}\&CenterDot;(a_1\mathrm{the\; s}+a_2{\mathrm{the\; s}}^2+a_3{\mathrm{the\; s}}^3+a_4{\mathrm{the\; s}}^4+......)$

The time domain response is c(t)=f ₁ δ(t)+f ₂ sin(ωt)+f ₃ cos(ωt), f _n is a coefficient function that has nothing to do with t. As time goes by, the system cannot eliminate the steady-state error, so the appeal of kp tuning is put forward.

The basic principle of kp tuning proposed by the present invention. The high-frequency inverter link G2(s)=K/(Ts+1) is approximately proportional to the link G2(s)=K, then the open-loop transfer function Analyze the dynamic process of the switch. At a certain switching cycle t ₁ moment, before the switching action, assume that the input current is r(t ₁₀ ), the sampling output is c(t ₁₀ ), and the error e(t ₁₀ ), then the output of the proportional controller is k _p e(t ₁₀ ); after the switch action, the input current is r(t ₁₁ ), and the sampling output is error Traditional fixed-gain P controllers are difficult to meet the current THD down to below 5%, as long as e(t) is reduced as much as possible, the control requirements can be met. r(t ₁₁ )=r(t ₁₀ ) before and after switching, e(t ₁₁ ) is only related to the size of k _p e(t ₁₀ ), in the case of no overshoot, the larger k _p e(t ₁₀ ), The smaller e(t ₁₁ ) is, so when e(t ₁₀ ) is large, use a small equivalent gain (control amount/error) to ensure that k _p e(t ₁₀ ) increases

Fig. 4 is a flow chart of the segmental control method of the current loop proposed by the present invention. The present invention takes three error segment nodes as an example. In the figure, Δi is the current error signal, c ₁ , c ₂ , and c ₃ are error segment nodes, and kp ₁ , kp ₂ , and kp ₃ are the ratios selected for the corresponding error segments. Gain, c ₁ <c ₂ <c ₃ , kp ₁ >kp ₂ >kp ₃ . The control method is specifically:

If |Δi|<c1, then m=0;

If c1<=Δi<c2, then m=kp1*(Δi-c1);

If c2<=Δi<c3, then m=kp2*(Δi-c2)+kp1*(c2-c1);

If Δi>=c3, then m=kp3*(Δi-c3)+kp2*(c3-c2)+kp1*(c2-c1);

If -c2<Δi<=-c1, then m=kp1*(Δi+c1);

If -c3<Δi<=-c2, then m=kp2*(Δi+c2)+kp1*(-c2+c1);

If Δi<=-c3, then m=kp3*(Δi+c3)+kp2*(-c3+c2)+kp1*(-c2+c1).

Figure 5 shows the tuning logic of kp _x and c _x . Initially, the current loop adopts constant-gain proportional control, the proportional coefficient kp, gradually increasing kp, the steady-state error becomes smaller and smaller, when the output current is adjusted to be overshoot but not yet overshoot, record the absolute value of kp and steady-state error at this moment The maximum value |Δi _max |, let kp ₁ =kp, kp ₂ =kp/2, kp ₃ =kp/4; c ₁ takes a small amount or 0 relative to |Δi _max |, c ₂ =|Δi _max | /3, c ₃ =2*|Δi _max |/3. In actual operation, several nodes can be set according to the actual situation, and the current error is divided into several sections, and each section takes a different proportional gain kp _x .

S40. Obtain an instruction value u_order of the switch comparison register, where the instruction value u_order=u ₁ +u ₂ is used to control the current of the photovoltaic grid-connected inverter.

Although the present invention is described through specific embodiments, those skilled in the art should understand that various changes and equivalent substitutions can be made to the present invention without departing from the scope of the present invention. In addition, various modifications may be made to the invention for a particular situation or application without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but should include all implementations falling within the scope of the appended claims.

Claims (7)

1. a photovoltaic combining inverter current control method, described photovoltaic combining inverter current control method adopts the double-closed-loop control structure of voltage feed-forward control and outer voltage and current inner loop, and it is characterized in that, it comprises the following steps:

Step 1, by line voltage feedforward obtain voltage feedforward control amount u ₁;

Step 2, by adopting the outer voltage control structure of PI controller to obtain current error △ i, described current error △ i is grid-connected current effective value instruction I ^*be multiplied by table, then with grid-connected current sampled value i _ndifference relatively;

Step 3, segmentation is carried out to current error △ i, form multiple current error section, each current error section adopts different gains, in guarantee Current Control amount continuous print situation, after adopting the current inner loop control structure of P controller, obtain current equivalence voltage duty cycle controlled quentity controlled variable u ₂;

In described step 3, current equivalence voltage duty cycle controlled quentity controlled variable u ₂for the P controller through different gains, the product of the Current Control amount m obtained and the value of PWM module period register;

Described step 3 comprises:

Step 31, obtain the error segmented node of multiple current error section, be designated as c ₁, c ₂c _n, wherein, n is the number of error segmented node, and an xth error segmented node is designated as c _x, n>=2, x≤n, c _n-1<c _n; Each error segmented node correspondence arranges corresponding proportional gain, is designated as kp respectively ₁, kp ₂kp _n, an xth error segmented node c _xa corresponding xth proportional gain kp _x, wherein, kp _n-1>kp _n;

Step 32, calculating current controlled quentity controlled variable m, its computational methods are:

When | △ i|<c ₁time, m=0;

Work as c _n-1≤ △ i<c _ntime, $m={\mathrm{kp}}_{n-1}*(\mathrm{\&Delta;}i-c_{n-1})+\underset{x=3}{\overset{n}{\&Sigma;}}\&lsqb;{\mathrm{kp}}_{x-2}*(c_{x-1}-c_{x-2})\&rsqb;;$

As △ i>=c _ntime, $m={\mathrm{kp}}_n*(\mathrm{\&Delta;}i-c_n)+\underset{x=2}{\overset{n}{\&Sigma;}}\&lsqb;{\mathrm{kp}}_{x-1}*(c_x-c_{x-1})\&rsqb;;$

As-c _n< △ i≤-c _n-1time, $m={\mathrm{kp}}_{n-1}*(\mathrm{\&Delta;}i+c_{n-1})+\underset{x=3}{\overset{n}{\&Sigma;}}\&lsqb;{\mathrm{kp}}_{x-2}*(-c_{x-1}+c_{x-2})\&rsqb;;$

As △ i≤-c _ntime, $m={\mathrm{kp}}_n*(\mathrm{\&Delta;}i+c_n)+\underset{x=2}{\overset{n}{\&Sigma;}}\&lsqb;{\mathrm{kp}}_{x-1}*(-c_x+c_{x-1})\&rsqb;;$

Step 33, obtain current equivalence voltage duty cycle controlled quentity controlled variable u ₂;

The command value u_order of step 4, acquisition switch comparand register, this command value u_order=u ₁+ u ₂.

2. photovoltaic combining inverter current control method according to claim 1, is characterized in that, obtains voltage feedforward control amount u in described step 1 ₁method be: the feedforward of line voltage that mains voltage sensor measurement is obtained, under HF switch effect, inversion link is considered as the little inertial element of high-gain, by line voltage sampled value divided by busbar voltage sampled value, be multiplied by the value of PWM module period register again, to obtain voltage feedforward control amount u ₁.

3. photovoltaic combining inverter current control method according to claim 1, is characterized in that, grid-connected current effective value instruction I in described step 2 ^*for busbar voltage regulation loop PI controlled quentity controlled variable exports.

4. photovoltaic combining inverter current control method according to claim 3, is characterized in that, described PI controlled quentity controlled variable is that the voltage error value of busbar voltage being sampled between effective value and busbar voltage voltage stabilizing value controls to obtain through PI controller.

5. photovoltaic combining inverter current control method according to claim 1, is characterized in that, in described step 31, to error segmented node c _xand the proportional gain kp of correspondence _xadjust.

6. photovoltaic combining inverter current control method according to claim 5, it is characterized in that, described method of adjusting is: first current inner loop adopts and determines gain scale control, then proportionality coefficient is increased gradually, when adjusting to output current to certain threshold value, record proportionality coefficient threshold value kp this moment and electric current steady-state error maximum absolute value value | △ i _max|, make kp ₁=kp, c ₁=0,

7. photovoltaic combining inverter current control method according to claim 6, is characterized in that, described certain threshold value is the critical value of output current between the non-overshoot of hyperharmonic.