The grid simulation device for photovoltaic power generation

Solar photovoltaic power generation is the solar radiant energy directly into electricity method, namely the sun radiant energy by solar cells is converted into electrical energy, again through energy storage, control, and transformation, and into people’s dc or ac power that can be used directly. The so-called grid-connected network is simply said to be realized by using the principle of simultaneous operation of voltage, phase, phase sequence, and frequency. The simulation device of its inverter grid-connected is designed, the scheme of simulating grid-connected is given, the drive control system and filter system are designed, and the implementation scheme of the drive control system is introduced in detail. The design and parameter selection of the main circuit and the filter system are given in detail. The hardware and software implementation scheme and circuit structure of the system are given based on DSP. The entire system can meet the requirements of microgrid simulation operation.


Introduction
At present, all countries in the world are actively developing all kinds of renewable energy, in order to reduce their dependence on coal, oil, and other traditional energy.Solar Energy is one of the most promising, clean, practical, and large-scale development and utilization of renewable energy in the world.The utilization of photovoltaics power has attracted worldwide attention, and the photovoltaics gridconnected generation is the main development trend of the utilization of photovoltaics power and is bound to develop rapidly.In addition, the advent of high-performance digital signal processing (DSP) chips has made it possible to apply some advanced control strategies to solar inverters. [1]Against this background, this paper makes deep research on the maximum power control and its core device solar inverter in the grid-connected solar power generation system, which has great practical significance.

System Structure and Analysis
Figure 1 shows the principle diagram of the solar photovoltaic grid-connected simulation device. [2]As for the solar panel voltage (photovoltaic conversion) Us, after DC-AC power conversion, and then through the L-C sine wave filter to filter out higher harmonics, we get sine wave fundamental voltage uo1, after step-up and the grid transformer is coupled to the grid and provides the voltage u o to the load.In the Figure, Rs is the equivalent internal resistance of the solar panel, u F is the grid-connected synchronization voltage that is connected to the grid-connected transformer as input to the control circuit, u REF is the given voltage value of the control circuit, and it is used as a DC-AC converter adjustment [3] , namely the voltage command value.Figure 2 shows the architecture of a solar cell parallel system simulation device.The DC voltage U of the photovoltaic panel is inverted by an AC-AC inverter with IGBT.The output signal of the system is filtered by LC sinusoidal filter, and a more stable sinusoidal signal is obtained.DSP constitutes the core control circuit, and completes the system protection and photovoltaic maximum power point tracking function [5] .
The undervoltage and overcurrent protection is composed of the input and output current of the single-chip microcomputer acquisition system.The output signal is collected and the tracking of the maximum output signal is realized.At the same time, we can use the detection circuit to input the reference voltage into the MCU and use it as the reference voltage to realize the grid-connected control, and then realize the frequency and phase tracking of the grid-connected.The system has a complete simulation program, complete control, and driving functions, which can be a stable and reliable simulation operation.The main losses of the inverter circuit are switching losses and conduction losses.The essence of efficiency improvement is to reduce losses.We have adopted the following methods in combination with the software level and the hardware level to reduce the loss improvement efficiency.

Sampling
(1) The large capacitor on the DC side connects the 1000 uF large capacitor in parallel on the DC side so that the voltage on the DC side is almost free of pulsation.
(2) We select suitable controllable devices and drive circuits.Because the breakdown voltage of the MOS transistor is low and the operating current is small, in the three-phase inverter circuit, an IKW15N120H3 type IGBT with a breakdown voltage of up to 600 V and an operating frequency of up to 20 kHz is used as a switching device to reduce the circuit loss and increase the efficiency.
(3) The use of a new SVPWM algorithm.SVPWM is a relatively novel control method developed in recent years.It is a pulse-width modulation wave generated by a specific switching pattern composed of six power switching elements of a three-phase power inverter, which can make the output current waveform as close to the ideal sine waveform as possible.SVPWM technology compared with SPWM, the harmonic wave of the winding current waveform is small, making the motor torque ripple reduced, the rotating magnetic field more rounded, and the utilization of the DC bus voltage has been greatly improved, and easier to achieve digitally.

Inverter and filter main circuit
The main loss of inverter circuits is switching loss and conducting loss, and the essence of improving efficiency is to reduce loss.We have adopted the following several methods to combine the software level and hardware level to reduce loss and improve efficiency.
(1) the DC side parallel large capacitor makes the DC side parallel 1000 uF large capacitor so that the DC side voltage basically has no pulsation.
(2) We select suitable controllable devices and driving circuits.Because of the low breakdown voltage and low working current of the MOS transistor, we choose the IGBT of the IKW15N120H3 model with a breakdown voltage of up to 600 V and working frequency of up to 20 kHz as a switch device in a three-phase inverter circuit to reduce circuit loss and improve efficiency.
(3) We use the new SVPWM algorithm.SVPWM is a novel control method developed in recent years.It is a pulse width modulation (PWM) wave generated by a specific switching mode composed of six power switch elements of a three-phase power inverter, which can make the output current waveform as close as possible to the ideal SINUSOIDAL waveform.Compared with SPWM, SVPWM technology has a smaller harmonic component of winding current waveform, which makes the motor torque ripple lower, the rotating magnetic field more approximate to the circle, and improves the utilization of DC bus voltage greatly, and is easier to realize digital.
The L-C filter circuit is used in the filter circuit.For the single-phase electric power, L-C upper and lower symmetrical connection is used for a better filtering effect.The impedance of the high-order harmonic inductor is larger, but the capacitance and reactance of the capacitor are smaller so that the high-order harmonic can hardly pass through the filter, the fundamental signal can pass through the filter smoothly, and the output of the sine wave is stable and smooth.Two parallel capacitors (one electrolytic capacitor, and one high-frequency film capacitor) are between the DC bus to smooth the DC voltage of the bus in order to filter the DC ripple.
As shown in Figure 3, the main purpose of filtering is to filter the high frequency switching frequency component.The switching frequency in this system is 15 Khz, and the useful period signal is 50 Hz.In order to reduce the attenuation of useful signal output, the filter needs to output amplitude attenuation at a low frequency, so we choose an LC low-pass filter.Its 3 dB bandwidth frequency ITRIP: Overcurrent detection input, using the current detection signal output signal to complete overcurrent protection and turn off the driver output; H01~H03: Drive's 3-way high-end drive output; L01~L03: The driver's 3-way low-side driver output; VS1~VS3: High-end floating power ground, respectively corresponding to the three high-end drive power grounds; FAULT: Over-current, through short-circuit and over-voltage, under-voltage protection output, can be used for fault indication or user system lockout, active low level.
FLT-CLR: The fault input is cleared to clear the fault memory so as to achieve the purpose of resetting the IR2133.

IR2133 drive circuit
IR2133 is a driver chip, and must have a pulse width modulation pulse to output.IR2133 is inputted by 16-bit MCU TMS320F2803.The input of IR2133 is matched with the output of CMOS and SV TTL.The maximum output voltage of IR2133 can reach 20 V, which greatly improves the driving performance of IGBT and realizes the reliable driving of IGBT.In addition, a new design method is adopted to prevent two power supply units on the same bridge arm from being powered at the same time when switching.Channel failure can improve the reliability of the excitation pulse [4] .

System detection circuit
The system consists of five detection circuits: over-current protection, phase sampling, under-voltage protection, maximum power point tracking, and frequency tracking.Sampling circuit design is very important and directly related to the accuracy of MCU sampling data.The frequency tracing and monitoring circuit can convert the sine wave into the same frequency square wave whose amplitude is less than 3.3 V.Among them, the choice of measurement loop has a great effect on the accuracy of measurement results.A signal processing method based on signal processing is used to process the signal processing signal.In this method, the phase error is analyzed by using the method of phase crossing, then the phase error is obtained, and the phase error is compensated.Based on the characteristics of maximum power, a tracking sampler with maximum power consumption is designed.The maximum output power of the system is obtained when the equivalent resistance of the system is equal to the internal resistance of the system.That is, using the sample voltage value matching method, if the supply voltage is 60 V (analog solar panel output voltage), then associated with the load, and the use of a full-bridge inverter regulator, can make the load equivalent resistance equal to the internal resistance of the power supply.That is, when the load reaches Maximum power point tracking (MPPT), the output voltage UD from the load should be equal to one-half of the power supply without load.

Frequency Detection Circuit
In the frequency detection circuit for the AC voltage detected in the input, the one operating amplifier includes an integrated differential amplifier (TLO82).Based on the impedance shown in Figure 5, the calculated amplification of the fretting signal is 50.The OP amp has a voltage of 50 (u1-u2).Because TL082 has a supply voltage of one plus or minus 15 volts, the maximum value of this amplifier is one plus or minus 15 volts.When the input wave crest reaches the two-volt high reference voltage Uref, it is suppressed to make the real square wave output.When the square wave flows through the IN4148 diode, the negative half-cycle of the original waveform is removed and only the positive half-cycle is removed.Via 1.After forming by the Helmut Schmidt switch, the 5 V adjusting tube can effectively suppress the square wave inhomogeneity and make the square wave waveform tend to be stable, which provides a basis for the accurate measurement of the square wave.The method makes the output voltage of the system equal to the reference voltage in the system so that the system can track the voltage in the system automatically.

Voltage and Current Detection Circuits
The methods of detecting voltage and current are similar.In Figure 6, a voltage detection circuit is shown.Differential voltage input is used in the circuit.The output voltage signal is transmitted to MCU by proportional calculation.Differential voltage input is used in the circuit.The output voltage signal is transmitted to MCU by proportional calculation.Because of its high voltage, usually in tens of volts or more, a step-down operation is adopted to reduce the output voltage to less than 5 V, so that the MCU can sample accurately.Because of its high voltage, usually in tens of volts or more.In this case, a stepdown operation is adopted to reduce the output voltage to less than 5 V, so that the MCU can sample accurately.The sampling circuit of MPPT (Maximum Power Point Tracking, MPPT) is essentially the same as the voltage detection circuit mentioned above [5] .The current-detecting circuit is connected in series to the current-detecting ring by a lower resistor, such as a MEGOHM resistor.Similarly, a microprocessor is used on a microprocessor to detect and adjust a voltage signal on a microprocessor [5] .Through the MCU carrying the computation, we can carry on the corresponding computation to this current value.Figure 7 shows the current detection loop.TL082 single-chip integrated OP amp is used in the system, which can effectively eliminate the influence of each sampling circuit and improve the precision and anti-interference of the system.Before inputting the detected information into the MCU, it is necessary to test the collected data several times to ensure the correctness of the collected data and the accuracy of the control [6] .When the current at both ends of the load changes, the voltage across the load changes

Figure 1
Figure 1 Schematic diagram of the composition of the photovoltaic grid-connected simulator

Figure 2
Figure 2 System Structure of Grid-connected Simulation Device

Figure 3 Figure 4
Figure 3 LC low-pass filter circuit4.Drive circuitThe drive circuit uses optocoupler drive.The optocoupler model adopted this time is HCPL-3120, used to drive the IGBT in the main circuit.As shown in Figure4, the input signals of the HCPL-3120 are

Figure 5
Figure 5 Frequency Detection Circuit

Figure 6
Figure 6 AC voltage sampling circuit

Table 1
Voltage and frequency at both ends of the load