Design of 5kW off-grid photovoltaic inverter using DSP power inductor

Solar photovoltaic power generation is the most promising new energy technology in the world today. Solar photovoltaic power generation systems can be divided into off-grid photovoltaic power generation systems, grid-connected photovoltaic power generation systems and hybrid photovoltaic power generation systems according to different system operation modes. With the rapid development of my country's photovoltaic power generation systems, especially the implementation of the photovoltaic roof plan, the domestic demand for off-grid photovoltaic inverters will increase. The off-grid photovoltaic power generation system is mainly composed of photovoltaic cell arrays, controllers, inverters, energy storage devices and other links, as shown in Figure 1. Among them, the inverter is one of the important devices in the photovoltaic system. Its reliability and conversion efficiency are crucial to the promotion of photovoltaic systems and the reduction of system costs.

At present, similar domestic products have the following shortcomings: a. Most of them use single-chip microcomputer control, which has poor real-time performance and limited data processing and communication capabilities; b. They use transformers, which are large and bulky; c. The output voltage accuracy is not high and cannot meet the needs of social development. This paper proposes a design scheme for a 5kW photovoltaic controller, which can be widely used in off-grid photovoltaic power generation systems and wind-solar hybrid power generation systems. It has the characteristics of small size, light weight, high output voltage accuracy, good waveform, and intelligent monitoring achieved by field bus.

Basic structure of 5kW off-grid photovoltaic inverter

The structure of the photovoltaic inverter is shown below, which consists of a primary circuit and a secondary circuit. The primary circuit consists of an input filter circuit, a Boost circuit, a full-bridge inverter circuit, and an output filter circuit, and the secondary circuit consists of a TMS320Fz812 controller circuit, a signal detection circuit, a human-computer interaction circuit, and a communication circuit. The following is a design of the hardware main circuit and control strategy of a 5kW off-grid photovoltaic inverter.

5kW off-grid photovoltaic inverter hardware design

At present, there are three main topologies of off-grid inverter circuits: power frequency isolation single-stage inverter, high frequency isolation two-stage inverter and non-isolated two-stage inverter. After theoretical calculation and practical verification, a circuit topology that is more suitable for use in photovoltaic power generation systems is used: non-isolated two-stage inverter, also called Boost inverter, as shown in Figure 3.

The 48V DC power input from photovoltaic solar energy is filtered through the input filter circuit, then boosted through the Boost circuit, inverted using a full-bridge inverter, output SPWM wave, and finally filtered through an LC low-pass filter to output a 50Hz frequency sine wave.

1. Design of input filter circuit

The input filter circuit is composed of filter capacitors to reduce the ripple of the input voltage. Assuming that the maximum power transmitted by the converter is Pmax, the energy provided by the input filter capacitor in one cycle is approximately

2. Boost circuit

The Boost circuit is shown in Figure 4, where Q is a fully controlled power device IGBT. The Boost circuit is a non-isolated DC conversion circuit with an output voltage equal to or higher than the input voltage. When the input voltage of the photovoltaic controller fluctuates within the allowable range, the output voltage is kept stable by controlling the conduction ratio D of the power switch device Q.

According to whether the inductor current in the Boost circuit is continuous, it can be divided into three working modes: continuous inductor current, discontinuous inductor current and critical continuous inductor current. When working in the critical working mode, the value of the inductor satisfies formula (3).

3. Single-phase full-bridge inverter circuit

The driving waveform of the single-phase full-bridge inverter circuit in this article is obtained by modulation. The generation and modulation of the signal wave and carrier are realized by DSP2812. SPWM has three modulation modes: synchronous modulation, asynchronous modulation and segmented synchronous modulation. The output frequency of this design is 50Hz, which is not too low, so synchronous modulation is used.

4. LC low-pass filter

The SPWM wave contains integer multiples of the carrier frequency and harmonic components near it. In order to obtain a good output voltage waveform, an LC low-pass filter must be used to eliminate high-order harmonics. As the carrier ratio increases, the lowest harmonic is farther away from the fundamental wave, and it is easier to filter. Increasing the carrier ratio will effectively improve the output voltage quality, but the increase in the carrier ratio is subject to factors such as the switching speed and switching loss of the power switching device. The selection of the LC low-pass filter mainly considers several factors, including noise, suppression ability, output impedance, and inverter current stress.

The design also needs to consider the size, weight and production cost of the filter circuit. Usually the cutoff frequency is selected between 1/10 and 1/20 of the switching frequency. In this design, the system switching frequency is selected as 18kHz, the inverter output AC power frequency is 50Hz, and the cutoff frequency is preliminarily determined to be 1kHz. There are two parameters to be determined in the filter, namely the filter inductor and the filter capacitor.

The structure of the LC low-pass filter is shown in Figure 5. The control strategy of 3.5kW off-grid photovoltaic inverter SPWM control technology is widely used in inverter circuits. This paper adopts a digital control strategy that combines PID control with closed-loop negative feedback control. 1. Generation of control pulses

This article uses TI's TMS320F2812 as the main control chip. F2812 has two event managers EVA and EVB, each of which can generate 8 pulse outputs, of which the full comparison unit outputs 3 pairs of complementary signals. The delay time of each pair of complementary signals can be generated by the corresponding dead zone timer. The event manager uses the internal timer and comparison unit to generate the corresponding pulse. In this article, EVA outputs a pair of complementary SPWM pulse signals and an independent output PWM signal to control the Boost circuit and the inverter circuit respectively.

2. Calculation of output frequency

The frequency of the inverter output SPWM pulse signal is 50Hz. The number of points outputted by the SPWM waveform in each sine wave cycle mainly depends on the frequency of the target output sine wave and the carrier frequency of the SPWM pulse wave. For example, if the carrier frequency of SPWM is 18kHz, the frequency of the sine wave to be output is 50Hz, and the number of points N of the required sine table is 3.3. The closed-loop negative feedback control DSP2812 detects the voltage and current values ​​of the output and input in real time, and feeds them back to the DSP. After PI adjustment, the relevant register parameters are changed to control the waveform of the drive pulse to achieve real-time closed-loop control. The control block diagram of the system is shown in Figure 6. The system adopts two closed-loop negative feedback adjustments. According to the different feedback signals, the output is adjusted in real time to make the output stable. In addition, when the output current signal suddenly increases to exceed the maximum allowable current, the PWM output is turned off to protect the inverter from damage.

5kW off-grid photovoltaic inverter software design

SPWM control program

This design uses a full comparison unit of the event manager to output a pair of complementary PWM pulses. The clock is provided by the general timer 1, and the counter is set to work in a continuous increase and decrease mode. The power switching device has a certain turn-off delay. When the upper tube of the same bridge arm is turned off, the lower tube cannot be turned on immediately, otherwise it will be broken down due to a short circuit. The dead zone controller in the full comparison unit of the DSP event manager is used to insert a dead zone time between the opening and closing of the same bridge arm to prevent short circuits and protect power devices. The SPWM program mainly includes: EV initialization, related variable initialization, sine table generation and CMPR1 reload. The first three functions are completed in the main program. The sine table generation statement is as follows:

2. A/D conversion interrupt

Service program R》The trigger source of A/D conversion is set to the event source trigger in EV. When the AD unit receives the trigger signal, it automatically starts A/D conversion and automatically stores the conversion result in the result register ADC-RESULT. When the conversion end signal arrives, it enters the ADCINT interrupt service program for corresponding processing. In the interrupt service program, the conversion result is first read, and the arithmetic mean filter algorithm is used to digitally filter the conversion result, which is converted into the corresponding actual voltage and current according to a certain relationship. The effective value of the current and voltage is calculated and passed to the main program for judgment and harmonic analysis and displayed through the LCD. The program flow chart is shown in Figure 8.

Test verification

The primary and secondary circuits of the 5kW photovoltaic inverter were assembled and tested, and the output waveforms were shown in Figure 9 in combination with the software compilation environment CCS3.3. The experimental results of the inverter circuit in steady-state operation are given in the results. In steady-state operation, the measured voltage RMS fluctuates between 216V and 226V, and the frequency fluctuates between 49.6 and 50.5Hz. The test results show that this design meets the design requirements.