Abstract: 1 KW and a network inverter design. Detailed introduction of the inverter and the basic design of key technologies, as well as the current network and control strategies. And the experimental device, the inverter verify the stability and reliability of the control strategy.
Foreword
As the ecological environment is deteriorating, people are realizing that we must follow the path of sustainable development, solar energy must be added to complete the transition to alternative energy sources. Photovoltaic and solar energy utilization network is the development trend of photovoltaic power generation system will be used mainly for peak shaving power plant and photovoltaic roof system.
In photovoltaic systems and networks, and network inverter is the core part. At present, network-based system of DC mainly in the DC and AC DC transform the structure of energy levels. DC DC transform part of the work of adjustment photovoltaic arrays to track the maximum power point to point; DC-AC inverter main part of the output current and voltage power grid with the phase, and access to power factor units. AC DC system which is the key to the design.
Solar photovoltaic system and structure of the network as shown in Figure 1. The system uses two-tier design, pre-boost for the chopper, after class for the full-bridge inverter.。 Former level for the maximum power tracking, after-class network and to achieve the current control. 。 DSP chips are controlled by the TMS320F2812 coordination completed.
Inverter design
Solar power inverter and network and network generating system is the core of its main functions is to solar panels issued by the DC reverse a single-phase AC power, and into the power grid. At the same time to achieve stability in the Middle voltage, for the former level chopper to boost the maximum power point tracking. And has a sound and network protection features to ensure safe and reliable system to run. Figure 2 network and is the principle of inverter.
Control system to TI's TMS320F2812 as the core, can achieve the feedback signal processing and A and D converter, DC and DC converter and PWM inverter control of a pulse, the system running the surveillance and control, fault protection and storage, 485 communications and other functions. In the middle of the actual circuit voltage VDC, network pressure, and net current and solar cell voltage and current signals sent to the F2812 sampling after the control board. 。 Control panel include: central controller and its peripheral circuits, signal detection and processing circuit, and driving circuit protection circuit. One signal detection and conditioning units to complete the main strength of isolation, level conversion and signal amplification and filtering capabilities to meet the DSP control system on the scope of all signal levels and signal quality requirements. Driving Circuit raise a pulse of the drive and ability to isolate the role. Logic will guarantee protection to a fault, the system hardware directly from the blockade output pulse.
In achieving the same frequency can be used under the conditions of a vector, can be seen from Figure 3 inverter output there as shown in Figure 3 a vector, the PV inverters and the network has the following basic input-vector relations :
Vac=Vs+j ω L。 Vac = Vs + j ω L. IN+RS。 IN + RS. 在(1)中 (1)
Type in Vac - voltage power grid fundamental amplitude, Vs - inverter output fundamental amplitude.
In the net pressure Vac (t) for certain circumstances, IN (t) amplitude and phase only by the network and photovoltaic inverter output voltage pulses in the fundamental component Vs (t) the amplitude, and its net Pressure Vac (t) of the phase to decide. Change Vs (t) the amplitude and phase can control input current IN (t) and Vac (t) the same phase. PWM Rectifier input side there is a vector triangle, in the actual system in the RS Value of general relatively small, can usually be ignored or shown in Figure 3 b simplified vector triangular relationship, that is, under the formula: the original location
(2) In a switch-on cycle for cycle and assuming the average input current in a switch to track current cycle can be derived under the command of:
(3) where K = L and TC, TC carrier for the cycle.
From the model of the system that can be used by the control shown in Figure 4 frame. This method known as the average model based on improved cycle of fixed-frequency current tracking method.
Figure 4 inverter control diagram
Inverter control diagram in reference Vref photovoltaic cells and the actual output voltage VDC comparison, the error by adjusting the current command PI I *, with the sine-be multiplied sine command Iref, Iref and the actual output of the current phase Comparison, the P-adjusted error by the value of (physical sense, the equivalent inverter output inductance on the side of the voltage) and net pressure Vac (t) by the sum of the wave and triangular wave, it produced a four-way PWM wave inverter control of the on-off switch, this has photovoltaic cells on the output voltage Vref basic work in the vicinity, the system output sine wave current amplitude for the I *.
Programme in the current network and the introduction of a fixed switching frequency control method.。 Fixed switching frequency control is to adjust current error after P-wave modulation as compared with 1.30 a PWM carrier wave. Its shortcomings must be current with the actual existence of bias can produce PWM wave. So at a fixed switching frequency control on the basis of an improvement, adding the exchange side of the net pressure Vac, the error signal that the current Iref after PI-adjusted and Vac added together, to get the value compared with the 1.30-carrier. Δ i in the physical sense, the equivalent inverter output inductance on the side of the voltage. Δ i × P and the Vac and, on impulse equivalent to the output voltage inverter, such a vector and the inverter output vector map line. Improved fixed switching frequency control strategy in maintaining the existing advantages of the current tracking error decreases significantly improve the current tracking rectifiers PWM performance.
The maximum power tracking and detection of anti-island effect
MPPT control of the most dynamic of the aim of locating the solar panels of the maximum power point. Commonly used method of tracking a fixed voltage, the disturbance observer, admittance by law and the occasional micro-track scanning method. This article is based on a final approach. This principle is regularly scan the solar panel array of output power, and then gradually, until the follow-up to the maximum power point. As the largest battery power at the plate the impact of changes in illumination is not very keen, the author of this approach was improved, only need to search near the maximum power of scanning to find the maximum power point. Improvement after the intermittent scanning the track to maintain control of both the control accuracy have been raised to the stability of the system.
The so-called island effect is that when power companies supply system, power cuts due to an accident or breakdown maintenance, and other reasons to stop work, installed in every client's network and photovoltaic power generation system can not be detected rapidly blackout state of their own Cut from the electricity network, creating a network and by the photovoltaic power generation systems to supply a load around the power companies unable to grasp the phenomenon of self-sufficiency in electricity supply island.
Its concrete realization of thinking is: hardware and software systems through the circuit periodically detected two adjacent voltage power grid had 0.1 times, calculated the frequency of voltage power grid f, then this frequency f on the basis of the introduction of offset △ f, Finally, the frequency (f disabilities △ f) and net current output as a given frequency and voltage in the grid so that every time a zero net current output and reset. Then, when the power system failure, load voltage on the grid frequency is the frequency of voltage, the DSP each detected voltage power grid frequency will remain basically unchanged, and when the electricity from the network, the output of photovoltaic arrays and net current Individual role in the load on the output and net current frequency shift-by-cycle, so, DSP detected each time the load voltage frequency will change accordingly, so that the formation of a given output and net current frequency of positive feedback , Making the frequency of load voltage frequency will soon be over the protection of the upper and lower limits, thus allowing the system to effectively detect the city, from network, therefore, take the initiative to shift the frequency of the system with a good anti-island effect function.
The experimental results
According to the above design, the structures have been completed rated power of 1.5 kw photovoltaic and experimental prototype network. Enter to 100-300 V, and net output current of 4.5 A. Power output of about 1 kw, frequency of 50 Hz. And net current and voltage power grid with the same frequency, power factor close to 1. 。 Experimental waveform shown in Figure 5.
Figure 5 1500W of output current and voltage waveforms
Closing remarks
Has produced 2 KW of experimental prototype, and has completed the 1500 W and experimental network. This paper describes the network of small and photovoltaic power inverter with improved fixed switching frequency of the current control and net programmes so that the output power factor close to 1. TMS320F2812 used as a control chip, the system has a good dynamic accordingly, the protection of sound, and improved network efficiency. With the use of the maximum power tracking and anti-island effect of software design, through experiments prove that the system is stable and reliable, good performance.