Typical architecture and principle description of solar inverter

Generally, the process of converting AC power into DC power is called rectification. Phase-controlled rectification is the most common AC-DC conversion process; and the process of converting DC power into AC power is called inversion, which is rectification. Reverse process. In the inverter circuit, according to the different characteristics of the load, the inverter is divided into active inverter and passive inverter. Assume that the AC side of the circuit is connected to the AC power supply, and the DC power is converted through DC-AC conversion, and is inverted into AC power with the same frequency as the AC power supply and sent back to the power grid, which is called active inversion. The corresponding equipment is called an active inverter, and a phase-controlled rectifier with a control angle greater than 90° is a common active inverter. A circuit that converts DC power into AC power and directly supplies power to non-power loads is called a passive inverter circuit, also known as a frequency converter.

Inverter types include separately excited inverters, self-excited inverters, and pulse width modulation (PWM) inverters. Among them, the separately excited inverter requires an external AC voltage source to supply the rectified voltage to the thyristor. Separately excited inverters are mainly used in high-power grid-connected situations; for photovoltaic power generation systems with power below 1MW, self-excited inverters are mainly used. The self-excited inverter does not require an external AC voltage source. The rectified voltage is supplied by some energy storage components of the inverter (such as capacitors) or by increasing the resistance value of the rectifier valve to be turned off (such as MOSFET or IGBT).A self-excited inverter whose output voltage is pulse modulated is called a pulse inverter. This kind of inverter reduces the harmonic content of voltage and current by increasing the number of pulse switching times in a cycle; the harmonic content is proportional to the number of pulse switching times. Nowadays, there are two main output control methods of grid-connected inverters: voltage control method and current control method. The principle of the voltage-type control method is to use the output voltage as the controlled quantity, and the system outputs a voltage signal with the same frequency and phase as the grid voltage. The entire system is suitable for a controlled voltage source with very small internal resistance; the principle of the current-type control method is Taking the output inductor current as the controlled target, the system outputs a current signal with the same frequency and phase as the grid voltage. The entire system is suitable for a controlled current source with large internal resistance.

Today, there are many topologies for solar inverters, the most common ones are half-bridge, full-bridge and Heric (Sunways patented) inverters for single-phase, and six-pulse bridge and neutral-point clamp for three-phase Bit (NPC) inverter. The typical architecture of a solar inverter usually uses a full-bridge topology with four switches, as shown in Figure 1.

In Figure 1, Q1 and Q3 are designated as high-side IGBTs, and Q2 and Q4 are low-side IGBTs. The inverter is used to generate a single-phase sinusoidal voltage waveform under the frequency and voltage conditions of its target market. Some inverters are used in residential installations connected to the grid with net metering benefits. This is one of the target application markets. This application requires the inverter to provide low harmonic alternating sinusoidal voltage so that the power can be written into the grid. Essentially, in order to keep the harmonic weight low and the power loss minimal, the high-voltage side IGBT of the inverter uses pulse width modulation (PWM), and the low-voltage side IGBT changes the current direction at a frequency of 60Hz. By allowing the high-side IGBT to use a PWM frequency of 20kHz or above and a 50/60Hz modulation scheme, the output inductors L1 and L2 can be made very small in the example and still filter the harmonic components efficiently. Compared with flexible and standard speed planar devices, ultra-sensitive channel IGBTs with a switching speed of 20kHz provide the lowest total conduction losses and switching power losses. Similarly, for low-voltage side switching circuits, IGBTs operating at the standard speed of 60Hz can provide the lowest power losses.

The switching technology in this plan has the following advantages: high power is achieved by allowing the high-side and low-voltage IGBTs to be independently optimized; the high-side, same-package soft recovery diodes have no freewheeling time, thus eliminating unnecessary switching losses; low-voltage The switching frequency of terminal IGBTs is only 60Hz, so conduction losses are the main factor in these IGBTs; there is no cross-conduction because the switching at any point in time occurs on two diagonal devices (Q1 and Q4 or Q2 and Q3) ; There is no possibility of bus pass-through, since IGBTs on the same side of the bridge are permanently unable to switch in a complementary manner; Same-package, ultra-sensitive, soft-recovery diodes across the low-voltage side IGBTs are optimized to enable freewheeling and reverse recovery The loss during the period is minimized.