Inverters convert DC energy (battery, storage battery) into AC (typically 220V/50HZ sine or square wave). Off-grid inverters are often used in new energy systems, not connected to the public grid, and are often used in homes. Or the emergency power supply in the office is generally to reverse the DC battery into 220V AC. There are also some remote area lodges or cars, the cabin will also be used. The output of the inverter is also divided into sine wave and square wave output. Sine The wave inverter outputs a sine wave AC power that is as good as or better than the grid we use on a daily basis because it does not have electromagnetic pollution in the power grid. The square wave inverter output is a square wave AC power with poor quality, Positive maximum to negative maximum values are produced almost at the same time. This causes severe instability effects on the load and the inverter itself. The objects to be tested in this paper are sine wave off-grid inverters. Like inverters, off-grid inverters also have a series of technical requirements and national standard test specifications. Idex IT8615 AC/DC electronic load combined with IT6500 high-power DC power supply to build the test system architecture shown in Figure 1, Complete all of the following test items off-grid sine wave inverter.
Figure 1. Test diagram of the inverter
Inverter efficiency
The inverter efficiency index is very important. The inverter with higher efficiency has less wasted energy, and more power is used for electrical appliances. Especially when using low-power systems, the importance of efficiency is more obvious. Therefore, small inverters The efficiency test of the device 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 It is 90%.
GB/T 19064-2003 states 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%. The inverter with larger capacity should also give full-load efficiency and low-load efficiency. The efficiency of the inverter below 10kW class should be 80%? 85%, and the efficiency of the 10kW class inverter should be 85%? 90%. The efficiency of the inverter has an effect on the photovoltaic power generation system to increase the effective power generation and reduce the cost of power generation. Significant influence.
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. It matches with the DC power supply IT6500 at the previous stage, without power meter, ie The efficiency of the inverter can be calculated.
Harmonic distortion
When the off-grid inverter output waveform is a square wave and a correction wave, the output current of the inverter has higher harmonics in addition to the fundamental wave. Higher harmonic currents will generate additional losses such as eddy currents on the inductive load. , Causes serious heating of components, which is not conducive to the safety of electrical equipment. Harmonic distortion of sine-wave inverter is less than 3%, its waveform quality is better than the quality of the power grid, and can be applied to all AC power loads. Measure up to 50 voltage harmonics at the output of the inverter.
Output voltage stability
It refers to the voltage regulator capacity of the inverter output voltage. In the off-grid solar PV system, the battery terminal voltage fluctuates greatly during charge and discharge. Generally, the fluctuation of the lead-acid battery terminal voltage is up to about 30% of the nominal voltage. This requires that The inverter has good voltage regulation performance and can guarantee normal operation within a large DC input range. A high-performance inverter should also give the inverter output voltage when the load varies from 0% to 100%. The % deviation, often referred to as load regulation, ie dynamic response. The voltage regulation rate of an inverter with good performance should be< ±3% , 负载调整率应< ±6% . IT8615可工作在CC, CP, CR的模式下, 调节不同的拉载电流, 电阻或功率, 可在面板或上位机软件上实时查看逆变器的输出电压.
Starting performance
General inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power pumps, etc. At start-up, the power may be 5 to 6 times the rated power. Therefore, usually when the inductive load is started, the inverter will withstand large transients. Surge power. The inverter should be guaranteed to start reliably under rated load. The high-performance inverter can be started several times at full load without damaging the power devices. Small inverters sometimes require soft start for their own safety. Or current limiting starter. IT8615 can adjust cf up to 5, ie instantaneous power with 5 times the rated power of the load. Customers do not need to purchase high power load models according to instantaneous power.
System output voltage and frequency
The inverter output voltage to the load, single-phase inverter is generally 110V, 220V, 3-phase inverter 380V; GB/T 19064-2003 standard output frequency should be between 49Hz to 51Hz. IT8600 The series supports 15 ~ 420V input, frequency 45 ~ 500Hz, single, three-phase input can be, in full compliance with test requirements.
Maximum output power
The rated output power represents the ability of the PV inverter to supply power to the load. A photovoltaic inverter with a high rated output power can bring more power loads. When selecting a PV inverter, first consider that it has enough rated power to meet the maximum The electrical power requirements of the equipment under load, as well as the expansion of the system and the access of some temporary loads. When the electrical equipment is made of purely resistive loads or the power factor is greater than 0.9, the rated output power ratio of the photovoltaic inverter is generally selected. The total power consumption of electrical equipment is 10% to 15%.
The GB/T 19064-2003 standard requires that the inverter be capable of continuous operation for a certain period of time under a specific output power condition.
(1) The input voltage and output power are rated values, and the inverter should work continuously for more than 4h.
(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 for more than 10s.
IT8600 series electronic load has data recording function. The panel has a shortcut key log to record the current measurement data and can be stored in U disk and other peripheral storage devices.
Figure II, energy flow diagram
Inverters can be divided into coal-fired inverters, solar inverters, wind energy inverters, nuclear energy inverters, hydro-electric inverters, fuel cell inverters, etc., depending on the power generation system. The 220V electricity obtained by the device is 220V 50HZ. The high-end point is a sine wave, and the cheaper one is generally a square wave.
In addition to inverters, UPS uninterruptible power supplies, frequency converters, alternators, AC power supplies, etc. can also be tested using the inverter output test method. IT8600 series AC/DC electronic load can adjust the CF and PF values to simulate inductive or capacitive Sex load makes testing easier.
