Brief Talk about Efficiency Improvement of PV Inverters

1.Importance of Inverter Efficiency

Improving conversion efficiency is of great importance to PV inverters’ performance. If we assume a 500KW inverter generating for an average of 4 hours per day, when its efficiency rises by 1%, nearly 20 KWH more energy will be produced daily and 7,300 KWH more annually. Therefore, in order to maximize the benefit of customers, it's critical to increase inverter efficiency as much as possible.

2.Factors Affecting the Efficiency of PV Inverters

The only measure to improve PV inverter efficiency is to reduce losses that mainly come from power switching tubes such as IGBT and MOSFET, as well as magnetic devices like transformers and inductors.

Energy losses caused by IGBT mainly include conducting loss and switching loss, among which the first one is related to the internal resistance of its components and the current that passes through it, while the switching loss is more relevant to the switching frequency of power devices and the DC voltage/current that they bear.

There are basically two types of inductive losses: Cooper loss and iron loss. Copper loss is the term used to describe the energy dissipated by resistance in the wire used to wind a coil. When the current passes through the wire resistance, part of energy is converted into heating loss. In most of cases the wire is made of insulated copper, so it’s called copper loss, that can be calculated by mean of measuring the transformer’s short-circuit impedance.

Iron loss is from two sources: hysteresis loss and eddy losses, and can be calculated by measuring the no-load current of transformer.

3.Methods to Enhance Inverter Efficiency

There are currently three technical routes to enhance inverter efficiency. First, silicon carbide materials are used to reduce the internal resistance of power devices and the reverse recovery current of diodes. Second, soft switching technology is adopted to limit the voltage at both ends of power devices and the switching frequency. Third, optimized designs of inductor are applied for that purpose. 

The impedance per unit area of silicon carbide devices is only one percent of that of silicon devices. Power devices made of silicon carbide’s on-state impedance represents one-tenth of that of a conventional silicon device, which can effectively reduce the reverse recovery current of diode, thus reducing the switching loss on the diode itself and the IGBT.

4.Soft Switching and Multi-level Topology

Soft switching technology makes use of resonance principle to make the current or voltage in switching devices change according to sinusoidal or quasi-sinusoidal law, turns off the devices when the current crosses zero naturally, and turns on the device when the voltage naturally crosses zero. The coincidence area of voltage and current is significantly limited, thus lowering the switching loss and the problems of inductive and capacitive switching are greatly solved. 

Compared with traditional two-level structure, the output of three-level inverters adds zero level, and the voltage stress of its power devices is halved. Thanks to this advantage, under the same switching frequency, three-level inverters have smaller output filter inductance than two-level equipment, and the inductance loss, cost and volume can be effectively reduced. Under the same output harmonic content, three-level inverters have lower switching frequency than two-level inverters, so as to decrease switching loss and increase conversion efficiency.

TSUN's 5KW energy storage unit uses the most advanced SIC, soft switching and three-level technologies, adopting optimized designs of circuit, drive and inductors, so that the efficiency of inverter breaks through the 98% bottleneck of traditional inverters, and the maximum efficiency reaches 98.5%. In consequence, the inverter performance is significantly improved while the radiator weight has been furtherly reduced.