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About the test of off-grid inverter

Apr 02, 2021

About the test of off-grid inverter

Introduction: 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 office. Generally, they convert DC batteries into 220V AC.

The inverter (inverter) converts direct current (battery, storage battery) into alternating current (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 sources in homes or offices. Generally, they convert DC batteries into 220V AC. It is also used in lodges, cars and cabins in some remote areas. The output of the inverter is also divided into sine wave and square wave output. The output of the sine wave inverter is the same or better sine wave AC power as the grid we use daily, because it does not have electromagnetic pollution in the grid. The output of the square wave inverter is poor quality square wave alternating current, and its maximum value in the positive direction to the maximum value in the negative direction are generated almost at the same time, thus causing severe and unstable 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.


Inverter efficiency


Inverter efficiency index is very important. The higher the efficiency of the inverter, the less power is wasted, and the more power is used for electrical appliances. Especially when using low-power systems, 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 output power of the inverter to the input power. If an inverter inputs 100 watts of direct current and outputs 90 watts of alternating current, then 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%. Larger capacity inverters 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 influence on the photovoltaic power generation system to increase the effective power generation and reduce the power generation cost.


IT8615 comes with an AC power meter, which can measure Vrms, Vpk, Vdc, Irms, Ipk, Idc, W, VA, VAR, CF, PF, Freq and other parameters at the same time. With the front-level DC power supply IT6500, no power meter is needed. The efficiency value of the inverter can be calculated.


Harmonic distortion


When the output waveform of the off-grid inverter is square wave and modified wave, the output current of the inverter has higher harmonics in addition to the fundamental wave. The higher harmonic currents will generate additional losses such as eddy currents on the inductive load. , Resulting in 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%, and 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 output of the inverter.


Output voltage stability


It refers to the ability to stabilize the output voltage of the inverter. In the off-grid solar photovoltaic system, the battery terminal voltage fluctuates greatly during the charging and discharging process. Usually, the lead-acid battery terminal voltage fluctuates up to about 30% of the nominal voltage. This requires the inverter to have better voltage regulation performance. Ensure normal operation within a larger 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 the inverter with good performance should be <±3%, and the load regulation rate should be <±6%. IT8615 can work in CC, CP, CR modes to adjust different current, resistance or power, and the output voltage of the inverter can be viewed in real time on the panel or PC software.


Starting performance


In general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power water pumps, etc., the power may be 5-6 times the rated power when starting. Therefore, usually when the inductive load starts, the inverter will withstand large instantaneous surge power. The inverter should be guaranteed to start reliably under the rated load. A high-performance inverter can be started at full load multiple times without damaging the power devices. For its own safety, small inverters sometimes need to use soft start or current limit start. IT8615 can adjust cf up to 5, that is, the instantaneous power that is 5 times the rated power. Customers do not need to purchase high-power load models based on instantaneous power.


System output voltage and frequency


The working voltage that the inverter outputs to the load is generally 110V, 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 15-420V input, frequency 45-500Hz, single-phase and three-phase input, which fully meets the test requirements.


Maximum output power


The rated output power represents 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 choosing a photovoltaic inverter, you should first consider having sufficient rated power to meet the electrical power requirements of the equipment under the maximum load, as well as the expansion of the system and the access of some temporary loads. When the electrical equipment is a pure resistive load 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% larger than the total power of the electrical equipment.


The GB/T 19064-2003 standard stipulates 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 min.


(3) The input voltage and output power are 150% of the rated value, and the inverter should work continuously and reliably for more than 10s.


The IT8600 series electronic load has a data recording function, and the panel has a shortcut key log to record the current measured data with one key, which can be stored in peripheral storage devices such as U disks.


Inverters are divided into coal power inverters, solar inverters, wind power inverters, nuclear power inverters, hydropower inverters, fuel cell inverters, etc. according to different power sources. The 220V electricity obtained by the inverter in the car is 220V 50HZ, the high-end ones are sine waves, and the cheap ones are generally square waves.


In addition to inverters, UPS uninterruptible power supplies, frequency converters, alternators, AC power supplies, etc. can also be tested using the test method of the inverter output. The IT8600 series AC/DC electronic load can adjust the CF and PF values to simulate inductive or capacitive loads, making testing easier.