Summary of the invention
The present invention, in order to solve the problem of existing photovoltaic combining inverter poor stability, has proposed the virtual reactance control method of photovoltaic combining inverter.
The virtual reactance control method of photovoltaic combining inverter comprises the following steps:
Step 1, according to actual electric network impedance Z
g, photovoltaic combining inverter output impedance Z
o(s) and the amplitude-frequency characteristic of photovoltaic combining inverter output impedance stability criteria and phase-frequency characteristic obtain virtual inductor amount L
v, virtual inductor amount equivalent internal resistance r
vwith simulated capacitance amount C
v;
Step 2, according to virtual inductor amount L
vequivalent internal resistance r with virtual inductor amount
v, and in conjunction with full-bridge gain K
pWM, the equivalent feedforward path transfer function G of acquisition virtual inductor
lV(s);
Step 3, according to the capacitance C of simulated capacitance
v, and in conjunction with full-bridge gain K
pWM, actual filter capacitor C
ac, filter inductance L
acand dead resistance r
ac, electric current loop transfer function G
iand feedback filtering ring transfer function G (s)
a(s) the equivalent feedforward path transfer function of acquisition simulated capacitance
Step 4, the equivalent feedforward path transfer function of the equivalent feedforward path transfer function of virtual inductor and simulated capacitance is carried out to discretization, obtains difference equation:
z(n)=K
1v
O(n)+K
2v
O(n-1)-K
3v
O(n-2),
Wherein, T
sfor the sampling period, K
1, K
2, K
3be coefficient;
Step 5, the output variable x (n) of y (n), z (n) and electric current loop is superposeed.
Beneficial effect: the control method that the present invention proposes is by adding virtual reactance control method, instead of the method that increases filter inductance actual capacity improves the output impedance of current mode photovoltaic combining inverter, connect with original filter inductance by increase a simulated capacitance in control loop simultaneously, the equivalent filter electric capacity of inverter will reduce, the output impedance of inverter is increased, and then reach the enhancing stability of a system.
Embodiment
Embodiment one, in conjunction with Fig. 1-Fig. 6, this embodiment is described, the virtual reactance control method of photovoltaic combining inverter described in this embodiment comprises the following steps:
Step 1, according to actual electric network impedance Z
g, photovoltaic combining inverter output impedance Z
o(s) and the amplitude-frequency characteristic of photovoltaic combining inverter output impedance stability criteria and phase-frequency characteristic obtain virtual inductor amount L
v, virtual inductor amount equivalent internal resistance r
vwith simulated capacitance amount C
v;
Step 2, according to virtual inductor amount L
vequivalent internal resistance r with virtual inductor amount
v, and in conjunction with full-bridge gain K
pWM, the equivalent feedforward path transfer function G of acquisition virtual inductor
lV(s);
Step 3, according to the capacitance C of simulated capacitance
v, and in conjunction with full-bridge gain K
pWM, actual filter capacitor C
ac, filter inductance L
acand dead resistance r
ac, electric current loop transfer function G
iand feedback filtering ring transfer function G (s)
a(s) the equivalent feedforward path transfer function of acquisition simulated capacitance
Step 4, the equivalent feedforward path transfer function of the equivalent feedforward path transfer function of virtual inductor and simulated capacitance is carried out to discretization, obtains difference equation:
z(n)=K
1v
O(n)+K
2v
O(n-1)-K
3v
O(n-2),
Wherein, T
sfor the sampling period, K
1, K
2, K
3be coefficient;
Step 5, the output variable x (n) of y (n), z (n) and electric current loop is superposeed.
The present invention, taking LC type single-phase photovoltaic grid-connected inverter as example, Figure 1 shows that the combining inverter equivalent structure figure adding after virtual inductor, the inductance L in figure in dotted line frame
vbe the filter inductance composition newly increasing, this inductance and original filter inductance L
acseries connection, the general principle increasing according to inductance series inductance value, increases inductance L
vthe filter inductance total amount of rear inverter becomes L
ac'=L
ac+ L
v; In order not increase the volume and weight of original photovoltaic combining inverter, inductance L
vit is not an actual inductance, but in control system, increase a through path, add system linear model after virtual inductor as shown in Figure 2, in Fig. 2, dotted line frame part is the controlling unit newly increasing, this part can be regarded a proportion differential link as, from the angle of controlling, be equal to filter inductance from L after increasing this through path
acincrease to L
ac+ L
v;
Reducing inverter output capacitance is also to improve one of effective measures of the stability of a system in light current net situation, but reducing electric capacity tends to cause system output ripple to become large, and network electric energy quality reduces, can produce harmful effect to system in actual applications, for this reason, the present invention connects with original filter inductance by increase a simulated capacitance in control loop, the equivalent filter electric capacity of inverter will reduce, the output impedance of inverter is increased, and then reaching the object that strengthens the stability of a system, in this system, equivalent filter capacitor becomes
Above-mentioned virtual inductor and simulated capacitance control method are merged and be virtual reactance control method proposed by the invention, can find out as shown in Figure 6, the method that the present invention proposes is equivalent on original system current loop control basis, two control loops are added, obtain exporting control signal y (n) and z (n), and the output signal x of this control signal and electric current loop (n) is superposeed, finally export synthetic control signal through SPWM modulation rear drive power switch pipe, realize the closed-loop control to system.
The difference of the virtual reactance control method of photovoltaic combining inverter described in embodiment two, this embodiment and embodiment one is, the actual electric network impedance Z described in step 1
gto obtain by testing impedance equipment or electric network impedance online test method.
The difference of the virtual reactance control method of photovoltaic combining inverter described in embodiment three, this embodiment and embodiment two is, the photovoltaic combining inverter output impedance Z described in step 1
o(s) expression formula is:
Wherein, ω
cfor the cut-off angular frequency of second-order low-pass filter, Q is quality factor, C
acfor actual filter capacitor, L
acfor actual filter inductance, r
acfor L
acdead resistance, K
pWMfor full-bridge gain, v
ofor photovoltaic combining inverter output voltage, i
ofor photovoltaic grid connection inverter output current,
for inductive current set-point, K
pfor electric current loop proportionality coefficient, K
ifor electric current changes integral coefficient.
The difference of the virtual reactance control method of photovoltaic combining inverter described in embodiment four, this embodiment and embodiment three is, the expression formula of the photovoltaic combining inverter output impedance stability criteria described in step 1 is:
Wherein, GM is gain margin, and PM is Phase margin, when actual electric network impedance Z
gwhen constant, the amplitude-frequency characteristic of photovoltaic combining inverter output impedance meets | Z
g|+GM<|Z
o|, its phase-frequency characteristic does not need to meet-180 °+PM< ∠ Z
g-∠ Z
o<180 °-PM can ensure that network system is stable; When the amplitude-frequency characteristic of photovoltaic combining inverter output impedance does not meet | Z
g|+GM<|Z
o|, its phase-frequency characteristic must meet-180 °+PM< ∠ Z
g-∠ Z
o<180 °-PM, to ensure system stability.
The difference of the virtual reactance control method of photovoltaic combining inverter described in embodiment five, this embodiment and embodiment four is, described in step 1 according to actual electric network impedance Z
g, photovoltaic combining inverter output impedance Z
o(s) and the amplitude-frequency characteristic of photovoltaic combining inverter output impedance stability criteria and phase-frequency characteristic obtain virtual inductor amount L
v, virtual inductor amount equivalent internal resistance r
vwith simulated capacitance amount C
vprocess be: ensure actual electric network impedance Z
gconstant, according to gain margin GM, Phase margin PM and photovoltaic combining inverter output impedance Z
o(s) expression formula obtains virtual inductor amount L
vwith simulated capacitance amount C
v, virtual inductor amount L
vequivalent internal resistance r
vto obtain according to the equivalent internal resistance equal proportion of actual inductance.
Embodiment six, in conjunction with Fig. 1-Fig. 7, this embodiment is described, this embodiment is with the difference of the virtual reactance control method of photovoltaic combining inverter described in embodiment five, described in step 2 according to virtual inductor amount L
vequivalent internal resistance r with virtual inductor amount
v, and in conjunction with full-bridge gain K
pWM, the equivalent feedforward path transfer function G of acquisition virtual inductor
lV(s) process is:
Step 2 one, will feed back output current information i
lequivalent feedforward path transfer function G with virtual inductor
lV(s), after multiplying each other, superpose with electric current loop output, to determine the position of equivalent feedforward path in whole control loop, and by the linear model of this position acquisition virtual inductor feedforward path;
Step 2 two, definite final control target increasing after feedforward path, equivalent target transfer function is
Step 2 three, according to the structure chart simplifying method of Automatic Control Theory, the linear model of virtual inductor feedforward path is carried out to abbreviation, obtain transfer function
this transfer function equates with equivalent target transfer function, thus the equivalent feedforward path transfer function of acquisition virtual inductor
Simulated capacitance equivalence feedforward path transfer function G
cV(s) procurement process and G
lV(s) procurement process principle is similar.
The difference of the virtual reactance control method of photovoltaic combining inverter described in embodiment seven, this embodiment and embodiment six is, described in step 4
Be illustrated in figure 8 and add virtual reactance front and back inverter output impedance comparison diagram, as can be seen from the figure after adding virtual reactance, inverter output impedance amplitude-frequency characteristic improves, and can ensure that in medium and low frequency section inverter output impedance amplitude-versus-frequency curve meets impedance stability criterion; Figure 9 shows that and add open-loop transfer function pole distribution figure before and after virtual reactance control method, increase the limit of system open-loop transfer function after virtual reactance further from the imaginary axis, the stability of a system improves; Figure 10 shows that and add the front system Nai Shi curve chart of virtual reactance, system open loop passes letter Nai Shi curve and surrounds (1, j0) point, and open loop passes letter and do not have RHP limit, and now system does not meet Nai Shi stability criterion, therefore system is unstable; Figure 11 shows that and add system Nai Shi curve chart after virtual reactance, system open loop passes letter Nai Shi curve and does not surround (1, j0) point, and open loop passes letter and does not have equally RHP limit, so time can judge system stability, thereby further proved that virtual reactance control method proposed by the invention can improve the stability of system.