Nov 14, 2020
Main technical performance and evaluation selection of inverter
In order to correctly select the inverter used in the photovoltaic power generation system, the technical performance of the inverter should be evaluated. According to the influence of the inverter on the operating characteristics of the main off-grid photovoltaic power generation system and the performance requirements of the photovoltaic power generation system on the inverter, the evaluation content includes the following items:
1. Rated output capacity
Characterize the ability of the inverter to supply power to the load. An inverter with a higher rated output capacity can carry more electrical loads. But when the load of the inverter is not purely resistive, that is, when the output power is less than 1, the load capacity of the inverter will be less than the rated output capacity value given.
2. Output voltage stability
Most inverter products give the deviation% of the inverter output voltage within the allowable fluctuation range of the input DC voltage, which is usually called the voltage regulation rate. A high-performance inverter should also give the deviation% of the inverter output voltage when the load changes from 0%→100%, which is usually called the load regulation rate. The voltage regulation rate of the inverter with good performance should be ≤±3%, and the load regulation rate should be ≤±6%.
3. Overall efficiency
Characterize the magnitude of the power loss of the inverter itself, usually expressed in %. The inverter with larger capacity should also give full load efficiency value and low load efficiency value. The efficiency of the inverter below the kW level should be 80% to 85%, and the efficiency of the 10kW level inverter should be 85% to 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.
4. Protective function
Overvoltage, overcurrent and short-circuit protection are the most basic measures to ensure the safe operation of the inverter. The perfectly functional sine wave inverter also has functions such as undervoltage protection, phase loss protection and temperature over-limit alarm.
5. Starting performance
The inverter should be guaranteed to start reliably under rated load. The high-performance inverter can be started at full load multiple times without damaging the power devices. For their own safety, small inverters sometimes use soft start or current limit start.
Technical performance such as waveform distortion and noise level of inverters in high-power photovoltaic power generation systems and interconnected photovoltaic power generation systems are also very important.
When selecting inverters for off-grid photovoltaic power generation systems, in addition to the above five basic evaluation contents, the following points should be noted:
(1) It should have sufficient rated output capacity and load capacity. The selection of the inverter must first consider having sufficient rated capacity to meet the electrical power requirements of the equipment under the maximum load. For the inverter with a single device as the load, the selection of its rated capacity is relatively simple. When the electrical equipment is a pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter is selected as 1 of the electrical equipment capacity. ．1～1.15 times is enough. When the inverter uses multiple devices as the load, the selection of the inverter capacity should consider the possibility of several electrical devices working at the same time, that is, the "load simultaneous coefficient".
(2) It should have high voltage stability. In off-grid photovoltaic power generation systems, batteries are used as energy storage devices. When the battery with a nominal voltage of 12V is in a floating charge state, the terminal voltage can reach 13.5V, and the short-term overcharge state can reach 15V. The terminal voltage can be reduced to 10.5V or lower when the battery is discharged under load. The fluctuation of the battery terminal voltage can reach about 30% of the nominal voltage. This requires the inverter to have better voltage regulation performance to ensure that the photovoltaic power generation system is supplied with a stable AC voltage.
(3) High efficiency or higher efficiency under various loads. The high efficiency of the whole machine is a significant feature of the inverter for photovoltaic power generation which is different from the general inverter. The actual efficiency of a 10kW general-purpose inverter is only 70% to 80%. When used in a photovoltaic power generation system, it will bring about 20% to 30% of the total power loss. Therefore, the special inverter for photovoltaic power generation system should pay special attention to reduce its own power loss and improve the efficiency of the whole machine in the design. Therefore, this is an important measure to improve the technical and economic indicators of photovoltaic power generation systems. In terms of overall efficiency, the requirements for photovoltaic power generation inverters are: rated load efficiency of inverters below kW ≥ 80% to 85%, low load efficiency ≥ 65% to 75%; rated load efficiency of 10 kW inverters ≥85%～90%, low load efficiency ≥70%～80%.
(4) It should have good over-current protection and short-circuit protection functions. During the normal operation of the photovoltaic power generation system, it is completely possible that the power supply system overcurrent or short circuit caused by load failure, personnel misoperation and external interference, etc.
The inverter is most sensitive to the overcurrent and short circuit phenomenon of the external circuit, and it is the weak link in the photovoltaic power generation system. Therefore, when selecting an inverter, it must have a good self-protection function against overcurrent and short circuit.
(5) Convenient maintenance. It should be a normal phenomenon that a high-quality inverter fails due to component failure after several years of operation. In addition to having a good after-sales service system, manufacturers are also required to have good maintainability in terms of inverter production technology, structure and component selection. For example, the damaged components have sufficient spare parts or are easy to buy, and the interchangeability of the components is good; in the process structure, the components are easy to disassemble and replace, and the replacement is convenient. In this way, even if the inverter fails, it can quickly return to normal.
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