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What is a grid-tie inverter?

A grid-tie inverter (GTI for short) is a special inverter. In addition to converting direct current to alternating current, the output alternating current can be synchronized with the frequency and phase of the mains. The AC power can go back to the mains. Grid-tied inverters are commonly used in applications where some DC voltage sources (such as solar panels or small wind turbines) are connected to the grid.

In many countries, homes or businesses with grid-connected power systems can resell the electricity they generate to power companies. Electricity sent back to the grid is subsidized in several different ways. Net metered electricity price refers to a household or company with renewable energy equipment that receives a subsidy based on the net energy it sends back to the grid. If the electricity is 100 kW-hours, the subsidy will be 400 kW-hours. In the United States, net energy policies vary by jurisdiction. Another policy is the feed-in tariff subsidy policy, where each kilowatt-hour of electricity generated is subsidized in accordance with the subsidy method listed in the contract with the transmission company.

In the United States, grid-connected energy systems are within the scope of the National Electric Code (NEC), so there are some mandatory requirements for grid-connected inverters.

Mode of operation

The inverter converts DC power to AC power for feeding back into the grid. The frequency of the output voltage of the grid-connected inverter needs to be the same as the grid frequency (50 or 60 Hz), which is usually achieved by the oscillator in the machine, and also limits the output voltage to not exceed the grid voltage. For modern high-quality grid-connected inverters, the output power factor can be 1, which means that the output voltage and current phase are the same, and the phase difference between the grid voltage and the grid voltage is within 1 degree. There is a microprocessor in the inverter that senses the AC waveform of the grid and generates voltage based on this waveform to send back to the grid. However, the electricity sent back to the grid needs to have a certain proportion of reactive power, so that the power of the nearby grid is within the allowable limit. noon) its voltage may rise too high.

If the power of the grid is cut off, the grid-connected inverter needs to be disconnected from the grid quickly. This is a requirement of the US National Electric Code (NEC) to ensure that in the event of a power outage, the grid-connected inverter will not provide power to the grid, and workers who repair the grid will not be electrocuted.

Properly configured, grid-tied inverters allow a home to use its own alternative energy source (such as solar or wind power) without the need for cumbersome wiring or batteries. If the alternative energy is insufficient, the shortfall will still be provided by electricity from the grid.

Technology

Inside of SWEA 250W grid-tied inverter, with transformer
Grid-connected inverter architectures can use newer high-frequency transformers, traditional power frequency transformers, or transformerless inverter architectures. The high-frequency transformer does not directly provide 120 V or 240 V AC power, but has a computer-controlled multi-step program that converts the power into high-frequency alternating current, then converts it into direct current, and finally converts it into the voltage and frequency required by the power supply. Transformerless inverters are lighter and more efficient than those with transformers, and are popular in Europe. However, because there is no electrical isolation between the DC circuit and the AC circuit, it is worried that when there is a fault, the short-circuit current of the DC terminal will directly jump into the mains circuit, so it enters the US market later. However, since 2005, the National Electrical Code of the National Fire Protection Association of the United States (NFPA) has eliminated the requirement that all solar systems require negative grounding, and added new safety requirements, thus allowing transformerless (or no electrical isolation) Inverter entered the US market. VDE 0126-1-1 and IEC 6210 have also allowed and defined the security mechanisms required for such systems. First, a residual current or ground circuit is required to detect abnormal short-circuit conditions, and an insulation test is also required to confirm the separation between the DC circuit and the AC circuit.

Characteristic

Inverters print data sheets for their product lines. The terms and content of each manufacturer’s data sheet may vary, but generally include the following:

Rated output power: The unit can be watts or kilowatts. Some inverters may have different output power ratings for different output voltages. For example, the inverter can be set to output 240 VAC or 208 VAC, and the rated output power of the two will be different.

Output voltage: refers to the grid voltage to which the inverter can be connected. If it is a small household grid-connected inverter, its output voltage will probably be 240 VAC. In the case of a commercial inverter, the voltage will be 208, 240, 277, 400, 480 or 600 VAC, and it may be a three-phase output.

Peak efficiency: Peak efficiency refers to the best efficiency the inverter can output (efficiency is its output AC power divided by the inverter input DC power). As of July 2009, the peak efficiency of most grid-connected inverters on the market can reach 94%, and some can be as high as 96%. The dissipated energy is mainly dissipated as heat energy. In order for the inverter to output the rated power, the input electric power must be greater than the rated power. For example, a 5000 W inverter is 95% efficient at full power and therefore requires 5,263 W of input power (rated power divided by efficiency). If the inverter can output power at several different voltages, it will also have its peak efficiency at each voltage.

CEC weighted efficiency: The calculation method of this efficiency is based on the algorithm published by the California Energy Commission (CEC) on its GoSolar website. Unlike peak efficiency, the CEC weighted efficiency is the average efficiency, and the comparison can represent the situation when the inverter is operating. If the inverter can output power at several different voltages, it will also have its CEC-weighted efficiency at each voltage.

Maximum input current: It is the maximum DC current that the inverter can use. If the current output by the system (eg solar cell) exceeds this upper limit, the inverter cannot use this current.

Maximum output current: refers to the maximum AC current that the inverter can continuously provide. This value is typically used to identify the minimum current rating of overcurrent protection devices (fuses or unfused switches) and the minimum current rating of the associated equipment to disconnect the circuit. If the inverter can output at several different AC voltages, it is also possible to have different maximum output currents at each voltage.

Peak power tracking voltage: This represents the DC voltage range over which the inverter’s Maximum Power Point Tracking (MPPT) can operate. The system designer needs to confirm that the series connection of the system has been optimized, and the voltage generated by the series connection is within the above range for most of the year. Because voltages can fluctuate and change with temperature, this can be a difficult job.

Start voltage: Not all inverters will indicate this value. This value refers to the minimum DC voltage required for the inverter to start and start running. It is particularly important in solar energy applications because system designers need to Confirm that there are enough PV modules connected in series, and their voltage can exceed the starting voltage. If the manufacturer does not indicate this value, the system designer generally uses the lower limit of the peak power tracking voltage as the minimum voltage of the inverter.

International protection level certification (IPxx rating): The protection level or IP number classification is for the product’s ability to protect against solid foreign objects (indicated in the first code of the protection level) and water (indicated in the second code of the protection level). The higher the number, the better the protection. US NEMA enclosure type uses are also similar to international protection ratings. Most inverters used outdoors are IP45 (not dustproof) or IP65 (dustproof), or NEMA 3R (no dust protection) or NEMA 4X (dustproof, direct splash water, and extra in the US). corrosion protection).

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