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How to improve efficiency-high technology in photovoltaic inverters

Mar 10, 2021

How to improve efficiency-high technology in photovoltaic inverters

 Photovoltaic inverter is a very important device of photovoltaic system. Its main function is to convert the direct current from photovoltaic components into alternating current. In addition, the inverter also undertakes to detect the running status of components, power grid and cables, and communicate with the outside world. , System security butler and other important functions. In the photovoltaic industry standard NB32004-2013, the inverter has more than 100 strict technical parameters, and each parameter is qualified to get a certificate. The General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China also conducts random inspections every year for the protection connection, contact current, power frequency withstand voltage of solid insulation, rated input and output, conversion efficiency, harmonics and waveform distortion, and power factor of photovoltaic grid-connected inverter products. 9 items including, DC component and AC output side over/under voltage protection were tested. A brand new inverter, from development to mass production, takes more than two years to come out. In addition to over-voltage protection and other functions, the inverter also has many little-known black technologies, such as leakage current control and heat dissipation. Design, electromagnetic compatibility, harmonic suppression, key device protection, efficiency control, etc., require a lot of manpower and material resources to develop and test.


This article mainly introduces how inverters can improve efficiency


The efficiency of the inverter is directly related to the power generation of the system, so it is an important indicator that customers pay close attention to.


In January 2018, the "Specifications for Photovoltaic Manufacturing Industry" issued by the Ministry of Industry and Information Technology requires that the weighted efficiency of photovoltaic inverters with transformers in China shall not be less than 96%, and the weighted efficiency of photovoltaic inverters without transformers in China shall not be less than 98. % (Relevant indicators for photovoltaic inverters with single-phase two-level topology are not less than 94.5% and 96.8%, respectively), and relevant indicators for micro-inverters are not less than 94.3% and 95.5%, respectively. This standard is not high, it is entry-level, and most manufacturers can reach it. The continuous improvement of efficiency is the goal that inverter manufacturers have been pursuing. The efficiency of centralized inverters averaged about 96% in 2010 and rose to 99% in 2018. The related indicators of the inverter are not less than 94.5% and 96.8%, respectively.” It may be that the efficiency of single-phase transformers is not less than 94.5%, the efficiency of transformers is not less than 96%, and most string inverters are transformer-free. , The single-phase China efficiency can reach 98%.


1. The importance of inverter conversion efficiency


It is of great importance to improve the conversion efficiency of the inverter. For example, if we increase the conversion efficiency by 1%, a 500KW inverter takes an average of 4 hours per day. The inverter can generate nearly 20 kilowatt-hours of electricity per day, so it can generate nearly 7,300 kilowatt-hours of electricity a year, which is more than ten years. 73,000 kilowatt-hours of electricity is emitted. This is equivalent to the power generation of a 5KW inverter. In this way, customers can save a power station with a 5KW inverter. Therefore, in order to improve the best interests of customers, we need to improve the conversion efficiency of the inverter as much as possible.


2. Influencing factors of inverter efficiency


The only way to improve the efficiency of the inverter is to reduce losses. The main losses of the inverter come from power switches such as IGBTs and MOSFETs, as well as magnetic devices such as transformers and inductors. The loss is related to the current and voltage of the components and the process of the selected materials.



 

The loss of IGBT mainly includes conduction loss and switching loss. Among them, the conduction loss is related to the internal resistance of the device and the current passing through, and the switching loss is related to the switching frequency of the device and the DC voltage that the device bears.


The loss of inductance mainly includes copper loss and iron loss. Copper loss refers to the loss caused by the resistance of the inductance coil. When the current flows through the coil resistance to generate heat, part of the electrical energy is converted into heat and is lost. Because the coil is generally wound by insulated copper wire It is called copper loss, and copper loss can be calculated by measuring the short-circuit impedance of the transformer. Iron loss includes two aspects: one is hysteresis loss, and the other is eddy current loss. The iron loss can be calculated by measuring the no-load current of the transformer.


3. How to improve inverter efficiency


There are currently three technical routes: one is to use space vector pulse width modulation and other control methods to reduce loss, the other is to use silicon carbide components to reduce the internal resistance of power devices, and the third is to use three-level, five-level and other multi-electronics. Flat electrical topology and soft switching technology reduce the voltage across the power device and reduce the switching frequency of the power device.


Voltage Space Vector Pulse Width Modulation (SVPWM)


It is a fully digital control method with the advantages of high DC voltage utilization and easy control. It is widely used in inverters. The DC voltage utilization rate is high, and a lower DC bus voltage can be used under the same output voltage, thereby reducing the voltage stress of the power switching device, the switching loss on the device is smaller, and the conversion efficiency of the inverter has been obtained. The promotion. In space vector synthesis, there are a variety of vector sequence combination methods. Through different combinations and sequencing, the effect of reducing the switching times of power devices can be obtained, thereby further reducing the switching losses of the inverter power devices.


Components using silicon carbide materials


The resistance per unit area of a silicon carbide device is only one percent of that of a silicon device. Power devices such as IGBTs (Insulated Gate Bipolar Transistors) made of silicon carbide materials have reduced on-state impedance to one-tenth of that of ordinary silicon devices. Silicon carbide technology can effectively reduce the diode reverse recovery current, thereby reducing power The switching loss on the device and the current capacity required by the main switch can also be reduced accordingly. Therefore, using the silicon carbide diode as the anti-parallel diode of the main switch is a way to improve the efficiency of the inverter.


Compared with the traditional fast recovery silicon anti-parallel diode, the reverse recovery current of the diode is significantly reduced after the silicon carbide anti-parallel diode is used, and the total conversion efficiency can be improved by 1%. After adopting fast IGBT, the switching speed is accelerated, and the conversion efficiency of the whole machine can be improved by 2%. When the SiC reverse diode is combined with a fast IGBT, the efficiency of the inverter will be further improved.


Soft switching and multi-level technology


The soft switching technology uses the principle of resonance to make the current or voltage in the switching device change according to a sinusoidal or quasi-sine law. When the current naturally crosses zero, the device is turned off; when the voltage naturally crosses zero, the device is turned on. Thereby reducing the switching loss, while greatly solving the problems of inductive turn-off and capacitive turn-on. It turns on or off when the voltage across the switch tube or the current flowing through the switch tube is zero, so that the switch tube does not have switching loss.


The main application of the three-level inverter topology is in high-voltage and high-power applications. Compared with the traditional two-level structure, the output of the three-level inverter increases the zero level, and the voltage stress of the power device is halved. Because of this advantage, under the same switching frequency, the three-level inverter can use a smaller output filter inductance than the two-level structure, and the inductance loss, cost and volume can be effectively reduced; and at the same output harmonics In terms of content, the three-level inverter can use a lower switching frequency than the two-level structure, the switching loss of the device is smaller, and the conversion efficiency of the inverter is improved.


to sum up:


The photovoltaic industry cannot blindly rely on government subsidies. It is only possible to achieve parity on the grid. To achieve this goal, one is to reduce costs, and the other is to increase the yield of power generation. At present, the various industrial chains of the photovoltaic industry, including component and inverter manufacturers, are sparing no effort. In order to increase revenue, system design needs to be optimized from the system level, and the efficiency of each component needs to be improved from the equipment level. Behind the 0.1% increase in photovoltaic module efficiency is countless sweat and countless innovations. The same reasoning applies to inverters. Every 0.1% increase in inverter efficiency implies a lot of hard work by R&D personnel.