The inverter converts DC power (battery, storage battery) into AC power (generally 220V/50HZ sine or square wave). Off-grid inverters are often used in new energy systems and are not connected to the public grid. They are often used as emergency power supplies at home or in offices. They generally invert DC batteries into 220V AC. It is also used in some remote area lodges or in cars and boat cabins. The output of the inverter is also divided into sine wave and square wave output. The sine wave inverter outputs sine wave alternating current that is the same as or even better than the power grid we use every day because it does not have electromagnetic pollution in the power grid. The square wave inverter outputs square wave alternating current of poor quality, and its positive maximum value to negative maximum value are generated almost at the same time, which causes severe instability effects on the load and the inverter itself. The objects under test discussed in this article are all sine wave off-grid inverters. Like grid-connected inverters, off-grid inverters also have a series of technical requirements and national standard test specifications. ITECH IT8615 AC/DC electronic load is combined with IT6500 high-power DC power supply to build a test system architecture as shown in Figure 1, which can complete all the following test items of off-grid sine wave inverter.
Inverter efficiency
The efficiency index of the inverter is very important. The higher the efficiency, the less electric energy is wasted by the inverter, and more electric energy is used for electrical appliances. Especially when using a low-power system, the importance of efficiency is more obvious. Therefore, the efficiency test of small inverters is particularly important. The efficiency of the inverter is the ratio of the inverter output power to the input power. For example, if an inverter inputs 100 watts of DC power and outputs 90 watts of AC power, its efficiency is 90%.
GB/T 19064-2003 stipulates that when the output power of the inverter is greater than or equal to 75% of the rated power, the efficiency should be greater than or equal to 80%. Inverters with larger capacity should also give full load efficiency values and low load efficiency values. The efficiency of inverters below 10kW should be 80%~85%, and the efficiency of 10kW inverters should be 85%~90 %. The efficiency of the inverter has an important impact on the photovoltaic power generation system’s ability to increase effective power generation and reduce power generation costs.
IT8615 comes with an AC power meter, which can simultaneously measure parameters such as Vrms, Vpk, Vdc, Irms, Ipk, Idc, W, VA, VAR, CF, PF, Freq, etc., with the front-end DC power supply IT6500, no power meter is required, that is The efficiency value of the inverter can be calculated.
harmonic distortion
When the output waveform of the off-grid inverter is a square wave or a modified wave, in addition to the fundamental wave, the output current of the inverter also contains high-order harmonics. The high-order harmonic currents will produce additional losses such as eddy currents on the inductive load. , causing serious heating of components, which is not conducive to the safety of electrical equipment. The harmonic distortion of the sine wave inverter is less than 3%, its waveform quality is better than that of the mains power grid, and it can be applied to all AC power loads. IT8615 can measure voltage harmonics up to the 50th order at the inverter output.
Output voltage stability capability
It refers to the voltage stabilization ability of the inverter output voltage. In off-grid solar photovoltaic systems, the battery terminal voltage fluctuates greatly during the charge and discharge process. Usually the fluctuation of the lead-acid battery terminal voltage can reach about 30% of the nominal voltage. This requires the inverter to have good voltage regulation performance and be able to Guaranteed normal operation within a large DC input range. A high-performance inverter should also provide the deviation % of the inverter’s output voltage when the load changes from 0% to 100%, which is usually called the load regulation rate or dynamic response. The voltage regulation rate of an inverter with good performance should be <±3%, and the load regulation rate should be <±6%. IT8615 can work in CC, CP, CR modes, adjust different load current, resistance or power, and the output voltage of the inverter can be viewed in real time on the panel or host computer software.
Starting performance
For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power water pumps, etc., the power may be 5 to 6 times the rated power when starting. Therefore, when an inductive load is usually started, the inverter will bear a large instantaneous surge power. The inverter should ensure reliable starting under rated load. A high-performance inverter can start at full load multiple times without damaging the power device. For its own safety, small inverters sometimes need to use soft start or current limiting start. IT8615 can adjust the cf up to 5, that is, the instantaneous power that is 5 times the rated load power. Customers do not need to purchase high-power load models based on the instantaneous power.
System output voltage and frequency
The operating voltage output from the inverter to the load is generally 110V or 220V for single-phase inverters and 380V for 3-phase inverters; the output frequency specified in the GB/T 19064-2003 standard should be between 49Hz and 51Hz. IT8600 series supports input from 15 to 420V, frequency from 45 to 500Hz, and both single and three-phase inputs are available, fully meeting the test requirements.
Maximum output power
The rated output power indicates the ability of the photovoltaic inverter to supply power to the load. Photovoltaic inverters with high rated output power can carry more electrical loads. When selecting a photovoltaic inverter, you should first consider having sufficient rated power to meet the electrical power requirements of the equipment under maximum load, as well as system expansion and the connection of some temporary loads. When the electrical equipment relies on purely resistive loads or the power factor is greater than 0.9, the rated output power of the photovoltaic inverter is generally selected to be 10% to 15% greater than the total power of the electrical equipment.
The GB/T 19064-2003 standard requires that the inverter can continue to work for a certain period of time under specific output power conditions.
(1) The input voltage and output power are rated values, and the inverter should work continuously and reliably for more than 4 hours.
(2) The input voltage and output power are 125% of the rated value, and the inverter should work continuously and reliably for more than 1 minute.
(3) The input voltage and output power are 150% of the rated value, and the inverter should work continuously and reliably for more than 10 seconds.
The IT8600 series electronic load has a data logging function. The panel has a shortcut key log to record the current measured data with one click, which can be saved in peripheral storage devices such as U disks.
Figure 2. Simple diagram of energy flow
Inverters are divided into coal power inverters, solar inverters, wind power inverters, nuclear power inverters, hydropower inverters, fuel cell inverters and so on according to different power generation sources. The 220V electricity obtained by the inverter on the car is 220V 50HZ. The high-end ones are sine wave, and the cheap ones are generally square wave.
In addition to inverters, UPS uninterruptible power supplies, frequency converters, alternators, AC power supplies, etc. can also be tested using the test method at the inverter output end. IT8600 series AC/DC electronic loads can adjust the CF and PF values to simulate inductive or capacitive loads, making testing easier.
Job Wang（Mr）
Shenzhen Lolsolar New Energy Technology Co.,Ltd
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Leading Manufacturer of Lithium Battery / Solar Inverter / Solar House System
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