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Achieve more efficient and reliable solar inverter design

due to the growing demand for energy, it is reported that China will become the number one carbon dioxide emitter by 2010; The scarcity of fossil energy makes renewable energy a global topic. Therefore, the development of solar photovoltaic technology has become the key to China's energy conservation and emission reduction strategy. The market analysis report of "Electronic Engineering album" points out that in 2008, the output of photovoltaic cells in Chinese Mainland has ranked first in the world, with a number of elite photovoltaic cell manufacturers such as Suntech, Jingao, CLP photovoltaic and Trinasolar. The financial crisis has also given China's solar photovoltaic industry a good opportunity to integrate and upgrade

currently, the solar photovoltaic market (including photovoltaic modules and inverters) is growing at an annual cumulative rate of about 30%. The function of solar inverter is to convert the DC voltage that changes with solar radiation and illumination into an electrically compatible AC output; For the majority of electronic engineers, solar inverter is a technical field worthy of high attention. Therefore, the following will introduce the technical points, challenges and corresponding solutions for the design of solar inverter

basic design standard

based on the specificity of solar inverter and maintaining the high efficiency of design, it needs to continuously monitor the voltage and current of solar panel array, so as to understand the instantaneous output power of solar panel array. It also needs a current controlled feedback loop to ensure that the solar panel array works at the maximum output power point to cope with variable high input. At present, solar inverters have a variety of topologies, the most common are half bridge, full bridge and herric (Sunways patent) inverters for single-phase, as well as six pulse bridge and neutral point clamping (NPC) inverters for three-phase; Figure 1 shows the topology of these inverters (MICROSEMI source). At the same time, the design also needs to comply with safety specifications, and can quickly disconnect from the electricity in case of electrical failure. Therefore, the basic design standards of solar inverter include rated voltage, capacity, efficiency, battery energy efficiency, output AC power quality, maximum power point tracking (MPPT) efficiency, communication characteristics and safety

figure 1A: half bridge inverter. Figure 1b: full bridge inverter

figure 1C: herric inverter

figure 1D: three-phase bridge inverter. Figure 1E: NPC three-level inverter

rated voltage: the main function of solar inverter is to convert the variable DC voltage from photovoltaic panel (sometimes stabilized DC voltage) into AC voltage to drive load or power supply. The most commonly used single-phase and three-phase AC voltages are 120v/220v and 208v/380v respectively; For industrial applications, 480v is also very common. For the selected inverter topology, the range of output AC voltage will determine the DC bus voltage and the rated voltage of each semiconductor switch

capacity: it is another term for the rated power of solar inverter. This value ranges from 200W (panel integrated module) to hundreds of kW. The larger the capacity, the larger the volume and the higher the price of the inverter. The cost of solar inverter is measured in USD/watt. For a proper design, surge, overload and continuous working mode must be taken into account when determining the capacity

efficiency: each solar inverter has its own requirements for efficiency (output power/input power). For example, the typical efficiency of a several kilowatt system can reach 95%. Based on the fact that the energy conversion efficiency of the solar array is relatively low (about 15%), so it is of great significance for the smallest solar panel to obtain the most output power

battery capacity: adding a battery pack on the DC side of the inverter acts as an energy buffer, which can stabilize the possible fluctuation of DC voltage and store the unused energy of the load. One advantage of battery capacity is that it can provide energy continuously in the dark of the day. Any solar inverter equipped with batteries requires a battery controller, although it is generally not used when connected to electricity

output power quality: due to the inherent switching mode characteristics of the inverter, its AC output waveform is not an ideal sine wave, and usually includes a wide range of high-frequency harmonics introduced by pulse width modulation (PWM). For many electronic loads, these harmonics are harmful and unhelpful; When combined, these harmonics become pollution sources. Despite these harmonics, the solar inverter can still compensate for the poor power factor of the load and weaken the power quality problems such as voltage sag and fluctuation. A well-designed solar inverter should output an approximate sine wave and reduce the undesirable low-frequency components introduced into the electricity

mppt new scheme will reduce the cost efficiency of metal or traditional composite materials: the output of solar panels will follow a series of characteristic curves under different lighting conditions in the current voltage curve. Therefore, in order to obtain the maximum power output, the voltage needs to be dynamically adjusted. The maximum power point tracking method is similar to the method of obtaining the best efficiency curve of internal combustion engine, in which torque and speed correspond to current and voltage. In the past 10 years, several algorithms have been developed, the most popular of which is the method of disturbing voltage and observing output

communication characteristics: for a several kilowatt solar inverter, it is necessary to build a communication connection for monitoring and data storage. Thanks to such a digital era, microprocessor (MCU) as a general controller is very suitable for this function

security: there are two meanings: 1 When paralleling, carefully observe the waveform and cut off the connection immediately in case of power failure; Anti island protection is critical to this. 2. During maintenance and repair, the staff should have no safety risks

the parallel inverter needs to closely match the phase and frequency of power distribution without reducing the power level. In parallel, the inverter can send back the power that the load cannot use or reflect the SME or same terminal or surface correction technology mentioned in the data sheet to the power without the help of bulky and expensive energy storage devices. Based on safety considerations, the inverter will automatically cut off when it loses power, and generally there is no battery pack for storing energy. At the same time, the solar inverter works in an independent mode and does not need to be synchronized with external AC power. Therefore, it does not need any anti island protection measures

in addition, the difference between the parallel design and the off design of the inverter lies in the output stage. However, in parallel connected systems, in most cases, the dc/ac level is constructed by 600V power MOSFET and/or IGBT, while the off system uses low-voltage output fed as battery level. The main applications include solar street lighting or solar assisted telecommunications systems with 48V voltage rail output. In 48V system, 100V power MOSFET is generally selected to build a full bridge inverter. The use of MOSFET and IGBT in solar inverter will also be introduced in detail below

system efficiency may become the most important design consideration of solar inverter and the distinguishing factor between different competitors. The average output power of a 20kwp installation equipment is 190kwh per day. If its efficiency is increased from 95% to 96%, if the forced feed in tariff is US $0.40/kwh and calculated based on the 10-year life cycle, the savings are about half of the cost of the inverter itself. Therefore, the importance of efficiency is self-evident

once the output power is determined, the highest conversion efficiency and the lowest power device loss are the same thing. Considering that the efficiency of photovoltaic panels converting solar energy into electric energy is very low (generally only 15%), the efficiency of energy inverter is very meaningful in reducing the area of solar panels and the volume of the whole system. In addition, the power loss of the device will generate heat on the bare silicon chip, resulting in temperature rise. Therefore, effective heat dissipation is necessary. The thermal overload caused by these losses must be dealt with by highly reliable designs and radiators must be used. As we all know, radiators are large and expensive; In addition, the reliability of the radiator is not high due to the use of devices such as fans. In other words, the smallest possible power loss can not only save energy, but also improve the reliability of the system, make the system more compact and reduce the cost

since the average time of the first failure of the existing inverter is about 5 years, the solar inverter has become the main cause of many failures of the photovoltaic system. In order to improve the reliability of inverter design, the following factors should be considered and corresponding measures should be taken, including low loss power devices and switching circuits, updated packaging technology, replacement of electrolytic capacitors, over design, redundancy of devices, and in-depth analysis of common failure modes and causes

Qian Chang, the application engineer manager of MICROSEMI semiconductor, pointed out that electrical and thermal overloads are the two reasons for failure. Choosing devices and circuits with higher energy efficiency will reduce the power consumption of the inverter itself, thereby reducing the junction temperature of power devices and reducing the thermal overload at the same time; Over design is another way to make the electrical and thermal stress far lower than the level that the device can withstand; The redundant design makes the devices work alternately, thus sharing and reducing the pressure on each device

however, over design and redundant design will significantly increase costs, which manufacturers do not want. Therefore, it is more feasible and cheaper to study the failure modes and causes, and then feed back this information to the product for redesign. Of course, this requires on-site testing of a large number of products in order to find and verify the failure mechanism and mode

In addition, Qian Chang believes that the mold cooling system with closed solar contour ensures that the test vessel not only has the necessary dimensional accuracy, but also other challenges of inverter reliability design should include: the technical difficulty of electrolytic capacitors with low reliability and finding different types of high-voltage and large capacity capacitors at a reasonable price; Immature manufacturing processes that lack structured methods for product planning and quality control will also damage reliability; In addition, working in harsh environments (extremely low or high temperature, humidity and exposure) also brings challenges to reliability design

Eric Zhang, assistant manager of Fairchild Semiconductor Technology Marketing, also believes that the bus capacitance required by the system has indeed become the most important factor affecting reliability. Therefore, electrolytic capacitors are usually selected in design because they can withstand daily temperature change cycles and operate at high temperatures. The designer must also understand the rated output power of the solar cell to be combined to generate electricity, so as to select the appropriate topology (refer to figure 1), and use the power switching device with sufficient withstand voltage

Jerome Lee, a senior engineer of Infineon, suggested that the service life of the inverter could be extended by reducing the ripple current in the electrolytic capacitor. When the high-frequency operation of the switch conflicts with the goal of high efficiency, it is necessary to consider whether the capacitor bank is too large or there is a polyphase system. In addition to the aging of electrolytic capacitors, he believes that the reduction of voltage rating and heat dissipation effect are also affecting the reliability of solar inverters. How many manufacturers know? The main factor for cost performance optimization is the use of 600V power devices. At this time, the over-voltage protection system or step-down converter can be used as the input stage to reduce the voltage stress below 500V

igbt or MOSFET

the main factors affecting the inverter design of semiconductor devices can be summarized as follows: device breakdown voltage, packaging, thermal resistance (slave junction

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