0 Preface
With the vigorous development of the photovoltaic industry, photovoltaic grid-connected power generation has become the latest highlight of research and development in the field of photovoltaic power generation. As the core of photovoltaic power generation system, photovoltaic grid-connected inverter is essential to improve the efficiency and reliability of the entire photovoltaic power generation system, as well as the service life of the whole system and reduce the cost.
Traditional photovoltaic grid-connected inverters can be divided into two types: isolated and non-isolated according to the presence or absence of isolation transformers. Non-isolated photovoltaic grid-connected inverters are small in size, light in weight, simple in structure, and low in cost, but because they are not isolated from the grid, it is easy to feed DC components and harmonics into the grid, increasing system conduction loss, and It is prone to electric shock and poses harm to people. Isolated photovoltaic grid-connected inverters can be divided into power frequency isolation type and high frequency isolation type. Due to the addition of an isolation transformer, the system guarantees that no DC component is fed into the grid and is safer. However, power frequency transformers are costly, bulky and cumbersome, so power-frequency isolated photovoltaic grid-connected inverters are not conducive to miniaturization and widespread use. Although the high-frequency isolated photovoltaic grid-connected inverter solves the problem of large and bulky of the former, it increases the complexity of the system, increases the number of components, increases the cost, and the overall efficiency is not high.
Based on the above considerations, this paper proposes a high-frequency photovoltaic grid-connected inverter design scheme that uses LLC resonant circuit for isolation, combining the advantages of isolated and non-isolated photovoltaic grid-connected inverters, reducing weight. The volume is reduced, the cost is reduced, and the power quality and safety are improved. Moreover, since the LLC resonant circuit can realize the soft switching of the DC-DC power device, the switching loss of the power device can be greatly reduced, thereby significantly improving the conversion efficiency and the service life of the entire system.
1 Photovoltaic grid-connected inverter structure and basic principles
1.1 System Design Structure
The structure of the photovoltaic grid-connected inverter with LLC isolation is shown in Figure 1. It consists of a DC-DC DC boost stage and a DC-AC inverter stage. The front stage is responsible for the rise of the DC power transmitted from the solar array. Pressure and maximum power tracking, the latter stage is responsible for inverting the DC power transmitted from the previous stage, and finally through the filter circuit and then connected to the grid.
1.2 Working principle
The photovoltaic grid-connected inverter controls the transmission of power by making the power device turn on and off regularly. The power-off and turn-off of the power device is controlled by pulse width modulation (PWM). The DC power generated by the solar cell is first sent to the DC-DC circuit, and the DC-DC stage performs the Maximum Power Point Tracking (MPPT) algorithm to keep the solar cell operating at the maximum power point.
After the maximum power point tracking control, the DC-DC circuit boosts the power of the solar cell into a direct current suitable for the DC-AC level, and then sends it to the DC-AC stage to convert the direct current into alternating current. The controller performs tracking calculation on the grid voltage or current phase taken by the sampling circuit, and then adjusts the output current of the inverter to the same frequency as the grid voltage by adjusting the DC-DC power device switch, and finally the electric energy is output through the output filter circuit or the isolation transformer. Delivery to the grid. In this paper, DC-DC input 200~300 V, output 400 V DC voltage, output power 500 W, power factor not less than 94% at full load. DC-AC input DC voltage 400 V, power level 600 W, power factor is 1.
2 LLC circuit analysis
This article uses LLC resonant circuit instead of power frequency transformer for isolation, which is different from traditional photovoltaic grid-connected inverter, and its advantages. The traditional power frequency isolation transformer is bulky, cumbersome and costly. The isolation by the LLC resonant circuit can greatly reduce the volume of the inverter system and improve the efficiency and power density. The LLC resonant circuit is based on a traditional series resonant circuit. The transformer magnetizing inductance Lm is connected in series in the resonant circuit to form an LLC resonant circuit [4]. Compared with the traditional series resonant circuit, due to the addition of a resonant inductor, the resonant frequency of the circuit is reduced, and the zero-voltage switching of the switching transistor in the full load range can be realized without using an additional auxiliary network. Secondly, the secondary side rectifier diode of the transformer can be conditionally The work is turned off at zero voltage, which reduces the loss caused by diode reverse recovery; and it is suitable for operation over a wide voltage input range. The higher the input voltage, the higher the efficiency, and the 97 points are optimal. % conversion efficiency.
This article uses a half-bridge LLC series resonant circuit, as shown in Figure 2. The half-bridge LLC series resonant circuit includes input capacitors C1 and C2, MOSFETs Q1 and Q2, resonant inductor Lr, resonant capacitor Cr, transformer T1, output rectifier diodes D1 to D4, and output capacitor C3.
Due to the addition of a resonant inductor, the LLC resonant circuit has two resonant frequencies, one is the resonant frequency fr of the resonant inductor Lr and the resonant capacitor Cr, and the other is the resonant frequency fm of Lm plus Lr and Cr. Calculated as follows:
In the series resonant circuit, the operating frequency fs is higher than fr to ensure that the switching transistor operates in the ZVS state. In the LLC circuit, as long as fs is higher than fm, the ZVS of the switching transistor can be realized. Simple Analysis.
The LLC circuit can be divided into four modes according to the switching frequency range. This paper only discusses the working principle of fr "fs" fm mode. The whole working process in one switching cycle is as follows, the working waveform is shown in Figure 3, PS1 and PS2 respectively. Drive pulse waveform for Q1, Q2:
[t0 - t1 ] Phase: At the time t0, the resonant current is negative, the Q1 body diode is turned on, and the voltage across Q1 is clamped at 0. At this time, Q1 is turned on and the voltage is turned on. The energy flows from the positive pole of the power source to the midpoint of C1 and C2, and the resonance of Lr and Cr. The resonant current ILr gradually rises in a sinusoidal form through the switching transistor Q1. The current flowing through the primary side of the transformer IT1 is the difference between the resonant current ILr and the exciting current ILm. The polarity of the primary side is positive and negative, and the polarity of the secondary side is also positive and negative. Therefore, D1 and D4 are naturally turned on, the primary voltage of the transformer is clamped at nVo (n is the transformer ratio), and the excitation current rises linearly. .
After half a cycle of harmonic 165 modern electronic technology, the 36th volume of the vibration of Q1 is still in conduction. After half a cycle, the resonant current begins to decrease, the excitation current continues to rise linearly, and the resonant current is equal to the exciting current at time t1.
[ t1 - t2 ] Stage: At the time t1, the resonant current ILr is equal to the excitation current ILm, the primary voltage of the transformer is 0, the secondary voltage is also 0, the secondary rectifier diodes are all cut off, the primary side no longer supplies energy to the secondary side, and the magnetizing inductance Lm begins to participate in resonance. Since Lm is much larger than Lr, the LLC resonant period is significantly longer, so the resonant current is substantially unchanged. At time t2, Q1 turns off.
[ t2 - t3 ] Phase: At time t2, Q1 turns off. At this time, Q2 is also in the off state, and the circuit enters the dead time. The resonant current ILr discharges the junction capacitance of Q2. When its voltage drops to zero, the body diode is turned on, the polarity of the primary winding of the transformer becomes upper and lower, and the secondary rectifier diodes D2 and D3 are naturally turned on. The Lm voltage is clamped by the output voltage and no longer participates in resonance. The resonant current begins to decrease with a 2Ï€ LrCr cycle sinusoidal law, and the excitation current decreases linearly. T3 Time Q2 Zero voltage is turned on.
[t3 - t4 ] Stage: At time t3, Q2 zero voltage is turned on. Similar to the first stage, Lr and Cr resonate, the resonant current decreases in a sinusoidal form, and the excitation current decreases linearly. The resonant current at t4 is equal to the excitation current.
[ t4 - t5 ] Stage: At the time of t4, the primary voltage of the transformer is 0, the secondary rectifier diodes are all cut off, the primary side no longer supplies energy to the secondary side, and the magnetizing inductance is no longer clamped by the output voltage and begins to participate in the resonance. The LLC resonant current is essentially unchanged.
[t5 - t6 ] Phase: Similar to the [ t2 - t3 ] phase, the circuit enters the dead time, Q1 and Q2 are all turned off, and the resonant current ILr charges the junction capacitance of Q1. When its voltage is equal to the power supply voltage, the body diode When conducting, the polarity of the primary winding of the transformer is positive and negative, and the secondary rectifier diodes D1 and D4 are naturally turned on. The excitation inductor Lm voltage is clamped by the output voltage and no longer participates in resonance.
The resonant current begins to increase with a sinusoidal law of 2Ï€ LrCr as a cycle, and the excitation current increases linearly. At time t6, Q1 turns on zero voltage and starts to enter the next cycle.
In the [ t1 - t2 ] phase and the [ t4 - t5 ] phase, assuming that the resonant current does not change and is set to Im, the output voltage Uo can be expressed as:
Where: Ui is the input voltage; T is the switching period; Ts is the resonant period when Lr and Cr are resonant. It can be seen from the equation that when T = Ts or fr = fs, the [ t1 - t2 ] phase and the [ t4 - t5 ] phase will not exist in this case, the resonant current is a pure sine wave, and the secondary rectifier circuit outputs The current is critically continuous, the rms value is the smallest, the conduction loss of the switch is the smallest, and the circuit efficiency is the highest [8]. Therefore, when the LLC circuit operates at the resonant frequency, the efficiency is highest. The main function of the LLC circuit in this paper is isolation. The highest efficiency is required on the basis of ensuring isolation. Therefore, the switching frequency of the switching transistor is equal to the resonant frequency.
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