Currently, the traditional single-stage or multi-stage structure of solar photovoltaic grid system deficiencies: grid inverter switch operating frequency is relatively high, a larger loss; work properly for the inverter, photovoltaic cells need the series to a sufficient voltage level, not only to make the switch voltage stress, and reduce the reliability of the system, the system the anti-Battery local shadow diminished capacity; centralized MPPT strict large photovoltaic grid system, in order to improve the reliability of the system and efficiency, avoid more solar photovoltaic cells connected in series, the literature suggests that a step wave and current instantaneous value feedback cascade inverter for hybrid control to achieve grid-connected PV. On this basis, the control of the the cascade inverter photovoltaic power generation system based on hybrid control strategy, double loop control parameters of the instantaneous value of the feedback unit design is based on the hybrid control cascade inverter PV system design theory and implementation of reference, the theoretical analysis and experimental verification,power inverter.
2 photovoltaic power generation system control strategy
the control system
2.1 Structure
two cascaded inverter PV grid power system topology shown in Figure 1. Figure previous stage is a DC / DC dual tube interleaved Boost circuit, the rear stage is composed of from 2H bridge cascaded inverter. The circuit consists of N + M a 2H bridge unit cascade. 1 ~ N 2H bridge for the the ladder wave control, N +1 to N + M a 2H bridge unit for the current instantaneous value feedback octave phase-shift the SPWM control or current hysteresis tracking control. Due to space limitations, here only ladder wave and current hysteresis hybrid control system analysis.
2 ladder wave and current hysteresis tracking mixed control block diagram. S 1 to N +1 of DC / DC converter unit to the voltage of the photovoltaic cell array to boost and to complete the photovoltaic cell MPPT. 1
N2H bridge unit the trapezoidal wave control, N +1 2H bridge unit for voltage and current double-loop control, current inner hysteresis control. Phase-locked loop (PLL) to network current iL and grid voltage ug same phase reference of the PLL circuit is given multiplied by the voltage output of the outer ring,power inverter, the inner ring is the current input reference current. The current iL comparison error signal is amplified by current amplifier via hysteresis control, generating the control signal of the N +1 2H bridge unit. A first N +1 2H bridge unit output voltage us (N +1) should be 1 ~ N 2H bridge unit, the output of the staircase wave voltage in phase with the PLL,http://www.gridtieinverter.de/index.asp, DSP calculated conduction pulse, so that before the inverter N ~ 1 The unit width of the output voltage respectively (θN ~ π-θN), (θN-1 ~ π-θN-1), ..., (θ1 ~ π-θ1), i.e. operating in different modes.
2.2 cascade inverter unit control strategy
in Figure 2, the calculated error of the actual voltage value of the DC bus voltage reference value and the sampled amplitude command Iref of the DC / AC inverter output current is obtained through a proportional-integral link, then Iref with the grid voltage synchronization unit sinusoidal signal sin (ωt) is obtained by multiplying the output current command value Irefsin (ωt), then Irefsin (ωt) with the sampled value of the error through the current regulator, the hysteresis comparator through the drive circuit to control the first N +1 2H bridge unit DC / AC inverter switch.
when ug IL noninverting,Grid tie inverter, the inverter output voltage in phase ahead ug. Therefore the 1 ~ N2H bridge unit output the staircase wave voltage with the instantaneous value of the output voltage of the feedback control of the 2H bridge unit us (N +1) the same phase, gives us (N +1) by the PLL is in phase with the ladder Boji Bo The voltage is then sampled by DSP real s 1 to the N2H bridge unit DC bus voltage, the calculated conduction pulse each 2H bridge unit.
3 instantaneous value the feedback unit bicyclic control analysis
3.1 instantaneous value of the the feedback cascade inverter unit current inner loop design
current inner tri-state the DPM the current hysteresis control, control block diagram shown in Figure 3.
type I system design current regulator. In addition to considering the fast-track the performance of the current inner grid-connected inverter the bicyclic control system, the inner current loop bandwidth of at least 5 to 10 times the voltage loop bandwidth. When the output inductor 25 mH, inductor equivalent resistance is taken as 0.4 Ω take 0.1 ms, Ts, ki is 16. Single P-loop direct current control current inner loop frequency response and the step response is shown in Figure 4.
be seen from Figure 4a, the current closed-loop bandwidth is greater than 10 times the fundamental frequency high the tracking steady accuracy of the fundamental current; seen from Figure 4b, the current closed-loop response is fast, less than 1 ms. Outer voltage, the current inner loop can be equivalent to a controlled current source, and its transfer function can be expressed as FIG. 3B.
3.2 instantaneous value feedback the the cascade inverter unit voltage outer Design
feedback means and the DC input power
set the instantaneous value of the total output power of the cascaded inverters, that is fed into the grid power λ (λ <1) times, the inverter loss is ignored and that the input power of the inverter DC is equal to the output power, namely:.
Figure 5 is an outer voltage control block diagram.
The first-order low-pass filter in
figure GLPF be used to filter out the harmonic components in the DC bus voltage, so that it does not participate in the feedback. PI regulator is set to: Gv = Kpv (τpvs +1) / s, low-pass filter is set to: the GLPF = 1 / (TLPFs +1), the voltage open-loop transfer function is:
The inverter is designed for a typical type II system: take TLPF = T2 = 10ms, λ = 0.435, the Udc (N +1) = 160V, KIF = 0.2, CN +1 = 5600μF, then have Kd ≈ 0.42, Kpv ≈ 3.2, PI regulator proportionality coefficient is 0.16. The design parameters are substituted into the formula (3), we can obtain the frequency response of the voltage of the outer ring and the step response shown in Figure 6,Grid tie inverter. Seen from Figure 6a, the system is equivalent to a cutoff frequency of 18.8 Hz (the closed-loop bandwidth) of the low-pass filter, harmonic components of bus voltage can be well suppressed to ensure the quality of the steady-state input current. By the the Figure 6b bus voltage unit step response shows that the voltage adjustment time of about 80ms.
4 experimental results
design of a single-phase inverters based on a cascade of 3 2H bridge unit 10 kWp photovoltaic power generation system. Two 2H bridge unit for the trapezoidal wave control, the input DC bus voltage in the standard light and the ambient temperature, the inverter unit is respectively 80 V, 160 V, 3 2H bridge unit tristate DPM current hysteresis for tracking control, The input DC voltage is 160 V. Grid current filter inductor L = 2.5 mH. Figure 7 cascade inverter-based photovoltaic grid system current closed-loop control of the experimental results. Seen from the figure, the grid current phase with the grid voltage. In the light intensity changes, and net current amplitude command change. Figures 7a, b, respectively, for the current amplitude command experimental waveform 40 A and 60 A, respectively corresponding grid power was 6.22 kW, 9.33 kW. With the light intensity and the increase in net current and power PV output power point tracking.
5 Conclusion
here by controlling the cascaded inverter input DC bus voltage value is constant, to achieve the input voltage control (power control loop) and photovoltaic systems and network control method of the current control (power quality control loop) decoupling verified through simulation and experimental results with theoretical analysis. System to achieve the maximum power point tracking for photovoltaic output to verify the effectiveness of the proposed control method.
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