How can inverters improve efficiency and increase power generation?

Photovoltaic inverters are the core equipment of photovoltaic systems. Their main function is to convert the direct current generated by photovoltaic modules into alternating current that meets the requirements of the power grid. But in fact, inverters are not only inverters, but also the safety stewards of photovoltaic power stations. Inverters also bear system-level functions such as monitoring and protection of photovoltaic arrays and power grids, protection of the external environment, and human-computer interaction.

In the photovoltaic industry standard NB32004-2013, the inverter has more than 100 strict technical parameters, and each parameter must be qualified to obtain the certificate. The General Administration of Quality Supervision, Inspection and Quarantine also conducts spot checks every year, inspecting 9 items of photovoltaic grid-connected inverter products, including protection connection, contact current, power frequency withstand voltage of solid insulation, rated input and output, conversion efficiency, harmonics and waveform distortion, power factor, DC component, and over/under voltage protection on the AC output side.

It takes more than two years for a new inverter to be developed and put into mass production. In addition to functions such as over-voltage and under-voltage protection, the inverter also has many little-known black technologies, such as leakage current control, thermal design, electromagnetic compatibility, harmonic suppression, efficiency control, etc., which require a lot of manpower and material resources to develop and test.

Mainly introduces how inverters 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 "Standardized Conditions for Photovoltaic Manufacturing Industry" issued by the Ministry of Industry and Information Technology required that the weighted efficiency of photovoltaic inverters with transformers must not be less than 96%, and the weighted efficiency of photovoltaic inverters without transformers must not be less than 98% (the relevant indicators of photovoltaic inverters with single-phase two-level topology structure must not be less than 94.5% and 96.8% respectively), and the relevant indicators of micro inverters must not be less than 94.3% and 95.5% respectively. This standard is not high, it is entry-level, and most manufacturers can achieve it. The continuous improvement of efficiency is the goal that inverter manufacturers have always pursued. The efficiency of centralized inverters averaged about 96% in 2010 and rose to 99% in 2018. "The relevant indicators of photovoltaic inverters with single-phase two-level topology structure must not be less than 94.5% and 96.8% respectively", which may be that the efficiency of single-phase with transformer is not less than 94.5%, and the efficiency without transformer is not less than 96%. Most string inverters do not contain transformers, and the single-phase efficiency can reach 98%.

1. Importance of inverter conversion efficiency

Improving the conversion efficiency of the inverter is very important. For example, if we improve the conversion efficiency by 1%, a 500KW inverter can generate nearly 20 more degrees of electricity every day for an average of 4 hours a day, which means that it can generate nearly 7,300 more degrees of electricity in a year and 73,000 more degrees of electricity in ten years. This is equivalent to the power generation of a 5KW inverter. In this way, customers can save a 5KW inverter power station. Therefore, in order to maximize the interests of customers, we need to improve the conversion efficiency of the inverter as much as possible.

2. Factors affecting inverter efficiency

The only way to improve the efficiency of the inverter is to reduce the loss. The main loss of the inverter comes from power switch tubes such as IGBT and MOSFET, 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 losses of IGBT mainly include conduction loss and switching loss. The conduction loss is related to the internal resistance of the device and the current passing through it, while the switching loss is related to the switching frequency of the device and the DC voltage it withstands.

The losses of inductors mainly include copper loss and iron loss. Copper loss refers to the loss caused by the resistance of the inductor coil. When the current passes through the coil resistance and generates heat, part of the electrical energy is converted into heat energy and lost. Since the coil is generally wound with insulated copper wire, it is called copper loss. 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. 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 control methods such as space vector pulse width modulation to reduce losses; the second is to use components made of silicon carbide materials to reduce the internal resistance of power devices; the third is to use multi-level electrical topologies such as three-level and five-level and soft switching technology to reduce the voltage across the power devices and reduce the switching frequency of the power devices.

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, and is widely used in inverters. The high DC voltage utilization rate allows the use of a lower DC bus voltage at the same output voltage, thereby reducing the voltage stress of the power switching device, reducing the switching loss on the device, and improving the conversion efficiency of the inverter to a certain extent. In space vector synthesis, there are a variety of vector sequence combination methods. Through different combinations and sorting, the effect of reducing the number of power device switches can be obtained, thereby further reducing the switching loss of the inverter power device.

Components using silicon carbide materials

The impedance per unit area of ​​silicon carbide devices is only one percent of that of silicon devices. The on-state impedance of power devices such as IGBTs (insulated gate bipolar transistors) made of silicon carbide materials is reduced to one tenth of that of ordinary silicon devices. Silicon carbide technology can effectively reduce the reverse recovery current of the diode, thereby reducing the switching loss on the power device, and the current capacity required for the main switch can also be reduced accordingly. Therefore, using silicon carbide diodes as anti-parallel diodes for the main switch is a way to improve the efficiency of the inverter. Compared with traditional fast recovery silicon anti-parallel diodes, the reverse recovery current of the diode is significantly reduced after using silicon carbide anti-parallel diodes, and the total conversion efficiency can be improved by 1%. After using fast IGBTs, the switching speed is accelerated and the overall conversion efficiency can be improved by 2%. When SiC anti-parallel diodes are combined with fast IGBTs, the efficiency of the inverter will be further improved.

Soft switching and multi-level technology

Soft switching technology uses the resonance principle to make the current or voltage in the switching device change according to the sine or quasi-sine law. When the current naturally passes through zero, the device is turned off; when the voltage naturally passes through zero, the device is turned on. This reduces the switching loss and greatly solves the problems of inductive turn-off and capacitive turn-on. The switch tube is turned on or off only when the voltage across the switch tube or the current flowing through the switch tube is zero, so there is no switching loss in the switch tube. The three-level inverter topology is mainly used in high-voltage and high-power occasions. Compared with the traditional two-level structure, the three-level inverter output adds zero level, and the voltage stress of the power device is halved. Because of this advantage, at the same switching frequency, the three-level inverter can use a smaller output filter inductor than the two-level structure, and the inductor loss, cost and volume can be effectively reduced; and at the same output harmonic content, the three-level inverter can use a lower switching frequency than the two-level structure, the device switching loss is smaller, and the inverter conversion efficiency is improved.

4. Summary

The photovoltaic industry cannot rely solely on government subsidies. It can only develop if it achieves grid parity. To achieve this goal, we must first reduce costs and secondly increase the revenue from power generation. At present, all industrial chains in the photovoltaic industry, including component and inverter manufacturers, are making unremitting efforts. In order to increase revenue, from the system level, it is necessary to optimize the system design, and from the equipment level, it is necessary to improve the efficiency of each component. Behind the 0.1% increase in the efficiency of photovoltaic components is countless sweat and countless innovations, and the same applies to inverters. Every 0.1 percentage point increase in inverter efficiency implies a lot of hard work by R&D personnel.