Polaris solar PV net news: the working principle and characteristics
Core of the inverter, inverter switching circuit is, referred to as the Inverter circuit. The circuit through the power electronic switch turn-on and turn-off, to complete the function of the inverter.
(1) high efficiency is required.
Due to the current high price of solar cells, in order to maximize the use of solar cells, improve system efficiency, we must try to improve the efficiency of inverters.
(2) requires a higher reliability.
Current PV systems are mainly used in remote areas, many power stations unattended operation and maintenance, this requires that the inverter has a reasonable structure, strict selection of components, inverter and a variety of protective functions, such as: input reverse DC polarity protection, output short-circuit protection, overheat protection, overload protection and so on.
(3) require a wide input voltage range.
Due to the solar cell voltage with load and sunshine intensity varies. Especially when the storage battery aging big change of voltage, 12V battery, the voltage may vary between 10V~16V, which requires larger inverter DC input voltage range to ensure normal work.
Classification of photovoltaic grid-connected inverter
About classification of inverter method, for example: according to the inverter’s output AC voltage phase can be divided into single-phase three-phase inverters and inverter; according to the inverter using the semiconductor device types, inverters can be divided into the transistor, thyristor inverter and gate turn-off thyristor inverter and so on. According to the different principles of inverter circuits, also divided into self-oscillation type inverter, ladder wave type inverter and PWM inverter and so on. According to applications in grid-connected system is off grid systems can be divided into grid-connected inverters and off-grid inverters. In order to facilitate optical user selection of inverter, there were only one to classify the different application of the inverter.
1, centralized inverter
Concentrated inverse variable technology is several a parallel of PV group series was even to same Taiwan concentrated inverse variable device of DC entered end, general power big of using three-phase of IGBT power module, power smaller of using field effect transistor, while using DSP conversion controller to improved by output power of quality, makes it very close Yu sine wave current, General for large PV power station (>10kW) of system in the. Greatest feature is that the system of high power, low cost, but because of different PV string output voltage, current, often does not exactly match (especially PV string because of cloudy, shade, stains and other reasons are partially occluded), centralized inverter can cause the reverse process is less efficient and could decline. While PV system reliability of power generation as a whole under the influence of a PV Unit working state side. Latest research direction is using space vector modulation control and the development of topological connection to new inverter to get high efficiency under partial load.
2, string inverter
String inverter is based on a modular concept on the basis of each PV string (1-5kW) through an inverter, with maximum peak power tracking on the DC side, grid-connected parallel on the AC side, has become the most popular on the international market of inverters.
Many large PV power plants use string inverters. Advantages of group difference chuanjian module and hide the shadow does not affect, while reducing the amount of PV components work best and inverters do not match the conditions, thus increasing the generating capacity. These advantages in technology not only reduces costs, increases the reliability of the system. Meanwhile, in Group chuanjian inductive “master-slave” concept, which in a single series of power does not make a single inverter case, several groups of PV string together, one or several work to produce more electricity.
Latest concept for several inverters to form a “team” instead of “master-slave” concepts, making the reliability of the system and goes a step further. At present, the transformer-less string inverters have a dominant position.
In traditional PV systems, each string inverter DC input, will be about 10 pieces of PV panels in series connection. When the panels in series 10, if a does not work well, then this series will be affected. If the inverter input using a MPPT, then the input will also be affected, significantly reduce the efficiency of power generation. In practice, clouds, trees, chimneys, animals, dust, snow and ice can cause blocking factors such as above, is very common. And in the micro-inverter PV system, each piece of access panels respectively a micro-inverter, as solar panels there is a not a good job, only this one will be affected. Other photovoltaic panels will run in top working condition, make the system more efficient, generating greater. In practice, if the string inverter fails, would cause thousands of Watts of solar panels don’t function, micro-inverter fault impact is very small.
4, power optimizer
Solar power system installed power optimizer (OptimizEr) can greatly increase conversion efficiency, and inverter (Inverter) functions make things simple to reduce costs. To achieve intelligent solar power systems, device power optimizer can best make for each solar cell efficiency and monitor the depletion of battery status at any time. Power optimizer is between power system between the inverter and the device, the main task is to replace the inverter, the best power point tracking capabilities. Power optimizer through the line was simplified and a single solar cell that corresponds to a power device or any other means, by analogy-best power point tracking for extremely fast scanning, then let each solar cell can indeed achieve the best power point tracking, in addition, can also take place communications chips anywhere, anytime monitoring battery status, instant return issues related to personnel maintenance as soon as possible.
Function of photovoltaic grid-connected inverter
Inverter not only have direct AC transformation functions, and maximizing the performance of solar cells functional and system failure protection functions. Summed up with an automatic running and stopping, maximum power point tracking control functions, and run the individual functions (used in grid-connected system), automatic voltage adjustment feature (used in grid-connected systems), DC detection feature (used in grid-connected systems), the DC system ground fault detection (grid-connected systems). Here brief introduction of function and maximum power point tracking control of automatic running and stopping capabilities.
(1) automatic running and stopping function
The morning after sunrise, solar radiation intensity increases, solar output increases, when once the output power of the inverter, the inverter automatically starts running. After entering the running, the inverter will always monitor the module output, as long as the module output power output power greater than required for inverter, inverter runs continuously until sunset down, even rainy days inverter can run. When the solar output smaller, inverter output close to 0 o’clock, inverter form standby.
(2) maximum power point tracking control
Solar output is with the intensity of solar radiation and the module temperature (temperature) changes. Because the module has the characteristics of voltage drops as the current increases, so get the best working point of maximum power. The solar radiation is changing, obviously the best work is also changing. With respect to these changes, always let the module working point is at the maximum power point, always obtain maximum power output from the solar modules of the system, which is the maximum power point tracking control. Inverter for solar power generation system’s greatest feature is including a maximum power point tracking (MPPT) this function.
PV grid-connected inverter of the main technical indicators
1. the stability of the output voltage
In photovoltaic systems, solar cells made electricity from a battery store, and then inverse inverter 220V or 380V AC. But under the influence of their charging and discharging of storage battery, changes its output voltage range, such as nominal 12V battery, the voltage values between 10.8~14.4V changes (beyond that may cause damage to battery). For a good inverter, when input voltage in this range, the steady-state output voltage variation should not exceed the nominal value of &Plusmn;5%, and when a mutation occurs when the load, the output should not exceed the rated value of voltage deviation ± 10%.
2. output voltage waveform distortion
Sine wave inverter, it should be provided the maximum allowed distortion (harmonic content). Usually expressed as a total distortion of the output voltage, and its value should not exceed 5% (single-phase output allows l0%). Due to the higher harmonics of the inverter output current will produce Eddy current inductive load such as additional losses if the inverter waveform distortion is too large, will lead to serious load part fever is not conducive to the safety of electrical equipment, and seriously affect the efficiency of the system.
3. rated frequency
Contain loads such as motors, such as washing machines, refrigerators and so on, due to its optimal frequency of motor work points to 50Hz, the frequency is too high or too low will cause the unit is hot, reduce operational efficiency and service life of the system, so the output frequency of the inverter should be a relatively stable value, usually a frequency 50Hz, under normal working conditions, the deviation should be less than &Plusmn;l%.
4. the load power factor
Characterization of inverter with inductive or capacitive load capability. Sine wave power inverters the load power factor is 0.7~0.9, rated value is 0.9. In the case of load power, if the inverter power factor is low, then the inverter capacity required will increase, resulting in increased costs on the one hand, and photovoltaic power system communication circuits increase, loop current increases, the losses will increase, reduced system efficiency.
5. inverter efficiency
Inverter efficiency refers to the provision of working conditions, the ratio of output power to input power, expressed as a percentage, typically, PV inverter nominal efficiency refers to a pure resistance load, 80% efficiency under load. Due to the overall higher cost of PV system, so you should maximize the efficiency of photovoltaic grid-connected inverter, reducing system cost, improving the performance of PV systems. Current inverter nominal efficiency between 80%~95%, requirements for small power inverter efficiency not less than 85%. PV system design in practice, not only to choose a highly efficient inverter, should be accompanied by proper configuration, try to make PV systems working near the best efficiency point.
6, rated output current (or nominal volume)
Specified load power factor within the rated output current of the inverter. Some inverter to rated output capacity of its units are expressed in VA or kVA. Is the rated capacity of the inverter when the power factor is 1 (that is, pure resistance load), the rated voltage for the rated output current of the product.
7, protection measures
A good performance of the inverter, it should also have complete protection or measures to deal with anomalies arising in actual use, the inverter itself and other parts of the system from damage.
(1) input under-voltage policy holders:
When the input voltage drops below the nominal voltage of the 85%, the inverter should be protected and displayed.
(2) input overvoltage policy holders:
When the input voltage is higher than the rated voltage 130%, the inverter should be protected and displayed.
(3) over current protection:
Inverter’s overcurrent protection, should guarantee the load short-circuit or action in a timely manner when the current exceeds the allowed value, against current surge damage. When the current exceeds the rated 150%, the inverter should be automatically protected.
(4) output short circuit of policy holders
Inverter short circuit protection shall be not more than 0.5s.
(5) input reverse polarity protection:
When the input is correct, when negative anti-, should have a protective function and display of the inverter.
(6) protection against Lightning:
The inverter should have lightning protection.
(7) over temperature protection.
In addition, the inverter without voltage stability measures, inverter output overvoltage protection measures, so that load received voltage of damage-free.
8. starting characteristics
Characterization of start-up of inverter load capability and dynamic performance at work. Inverter to ensure reliable starting under load.
Electric and electronic equipment in the transformer, inductor, magnetic switch and fan components will produce noise. During normal operation of the inverter, the noise should not exceed 80dB, small inverter noise should be less than 65dB.
The selection of inverter, first consideration has sufficient capacity to meet the load requirements of equipment for electric power. For a single device for a load the inverter, its rating is simple.
When electrical equipment as a pure resistance load or power factors greater than 0.9, the selected inverter’s rated capacity is 1.1~1.15 times the capacity of electrical equipment will be allowed. While the inverter should have anti-capacitive and inductive load impact ability.
For inductive load, such as motors, refrigerators, air conditioners, washing machines, power pumps, at the start, the instantaneous power may be 5~6 times its rated power, at this point, the inverter will be under a great deal of transient surges. For such systems, the rated capacity of the inverter should have a sufficient cushion to ensure that load reliable starting, high performance of inverter can be done multiple times in a row starting at full capacity without damaging the device. Small inverters for their own safety, sometimes using soft-start or current limited manner.
Installation, and maintenance
1, before installing should first check whether the inverter is damaged during shipping.
2, select an installation site, should ensure that there is no interference with other electronic devices around.
3, before making the electrical connections, must be using translucent material of photovoltaic panels cover or disconnect the DC circuit breaker. Exposure to the Sun, solar array will generate dangerous voltages.
4, all the installation must be done only by professional and technical personnel.
5, PV power generation system used in the cable must be firmly attached, good insulation as well as specification is appropriate.
For solar inverters, and improve power conversion efficiency is an eternal subject, but when systems are more efficient, close to 100%, further efficiency improvements will be accompanied by low price and, therefore, how to keep a very high efficiency, while maintaining very good price competitiveness will be an important topic.
Compared with the inverter efficiency improvement efforts, and how to improve the efficiency of the inverter system as a whole, is becoming another important topic in the solar system. In a solar array, when enriched until the area of the shadow appears, use an inverter of MPPT function, the output power of the system at this time about bad 20% to appear even when power is reduced! in order to better adapt to such single or part of a solar panel, one or multiple MPPT MPPT control function is a very effective method.
Due to inverse variable system in grid run of status, system on to of leakage will caused serious of security problem; also, to improve system of efficiency, solar array column most will was series into is high of DC output voltage using; for, in electrode between for exception status of occurred, is easy produced out DC arc, due to DC voltage high, very not easy out arc, very easy led to fire. With the widespread adoption of Solar Inverter systems, security issues will also be an important part of inverter technology.
In addition, heading for a rapid development of smart grid technology in power systems and popular. A lot of new energy sources such as solar power systems, to the stability of the smart grid system raised new technical challenges. Design can more quickly and accurately and intelligently smart grid compatible inverter system, it will become necessary for Solar Inverter systems.
In General, the development of inverter technology is power electronics technology, and development of modern control theory and the development of microelectronics technology. With the passage of time, inverter technology is advancing to higher frequencies, more power, higher efficiency, smaller size direction.
Original title: photovoltaic grid-connected inverter