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Solar inverters are the core equipment of photovoltaic systems, and their working principles involve multiple aspects such as power electronic conversion, control algorithms, and grid interaction. The following provides a detailed analysis from three aspects: core function implementation, key technical principles, and differences in the operation of different types of inverters:
1、 Core function: Conversion from direct current (DC) to alternating current (AC)
The direct current generated by solar panels cannot be directly connected to the power grid or used by most appliances (the power grid and appliances usually require alternating current), and the inverter needs to complete the following key steps:
1. DC-AC inverter: the fundamental transformation from DC to AC
The switching function of power semiconductor devices
By using switching devices such as Insulated Gate Bipolar Transistor (IGBT) or Metal Oxide Semiconductor Field Effect Transistor (MOSFET), direct current is converted into high-frequency alternating current.
Taking a single-phase inverter as an example, IGBT modules conduct alternately at a specific frequency (such as 50Hz or 60Hz), generating positive and negative alternating voltage waveforms, forming the basic framework of AC power.
For three-phase inverters, three sets of IGBT modules are used to control the voltage of phases A, B, and C, with a phase difference of 120 °, and output stable three-phase AC power.
Work logic:
PWM modulation technology
Pulse width modulation (PWM) is one of the core control technologies for inverters, which controls the amplitude and frequency of the output voltage by adjusting the conduction time (duty cycle) of the switching devices.
Example: If you need to output 220V, 50Hz AC power, the inverter generates a series of adjustable width pulses through PWM algorithm, and after filtering, synthesizes AC power that is close to a sine wave.
2. Filtering and waveform optimization: Generate high-quality AC power
High frequency harmonic filtering
High frequency harmonics are generated during the operation of switching devices (such as IGBT switching frequency usually ranging from 1-20kHz), which need to be filtered by LC filtering circuit (inductor L+capacitor C) to make the output waveform close to a sine wave.
Sine wave synthesis
Monopolar frequency doubling PWM: By modulating the positive and negative half cycles of pulses, it reduces switching losses and improves waveform quality.
Multi level technology (commonly used in high-power inverters): By superimposing multiple DC voltage sources, a stepped waveform is generated, which is closer to a sine wave and reduces harmonic content.
3. Grid connected control: key technology for synchronizing with the power grid
For grid connected inverters, it is necessary to ensure that the output AC power is completely synchronized with the frequency, phase, and voltage amplitude of the grid to avoid any impact on the grid. The core technologies include:
Phase Locked Loop (PLL) Technology
By detecting the frequency and phase of the grid voltage, generating synchronization signals, and controlling the inverter to output AC power consistent with the grid.
Power grid protection function
It has functions such as over/under voltage protection, over/under frequency protection, and islanding effect protection. For example, when the power grid experiences a power outage, the inverter needs to detect an "island" within 2 seconds and shut down to avoid endangering the safety of maintenance personnel.
2、 Key Technical Principles: Energy Management and Efficiency Optimization
In addition to basic transformation, the inverter also needs to achieve the following core functions:
1. Maximum Power Point Tracking (MPPT)
Principle: The output power of solar cells varies with factors such as light intensity and temperature. The MPPT algorithm adjusts the operating voltage in real time to ensure that the battery always operates at the maximum power point (Pmax).
Common algorithms:
Disturbance observation method (P&O): Periodically adjust the voltage slightly, compare the direction of power changes, and determine the maximum power point.
Conductivity incremental method (INC): By detecting the rate of change of current and voltage, the trend of power change can be determined with higher accuracy.
Hardware implementation: MPPT is achieved by adjusting the voltage through DC-DC converters (such as Boost circuits).
2. Energy bidirectional flow (the core of energy storage inverters)
Scenario of optical storage system:
During the day, photovoltaics are given priority for power supply, and excess electricity is charged into the battery through the inverter (at this time, the inverter acts as a charger and operates in rectification mode).
At night/on cloudy days: The battery is discharged through the inverter to supply power to the load or connect to the grid (the inverter returns to inverter mode).
Bidirectional inverter technology:
To achieve bidirectional power flow through the same IGBT module, a battery management system (BMS) needs to be configured to monitor the battery status and avoid overcharging/overdischarging.
3. Thermal management and reliability design
Power loss sources: conduction loss and switching loss during IGBT switching process, Joule loss of inductance/capacitance.
Heat dissipation technology:
Natural heat dissipation: Small inverters conduct heat through aluminum heat sinks.
Forced air cooling/liquid cooling: High power inverters (such as centralized inverters) use fans or liquid cooling pipelines to ensure that the IGBT junction temperature is below the safety threshold (usually ≤ 125 ℃).
3、 The operational differences of different types of inverters
According to the application scenario, the working logic and technical focus of inverters vary:
1. Centralized inverter
Applicable scenarios: Large ground power stations (power ≥ 500kW).
characteristic:
Multiple photovoltaic groups are connected in series and parallel to the same inverter, with only 1-2 MPPT channels, suitable for scenarios with uniform lighting.
It needs to be equipped with a combiner box for centralized input of DC, which has low cost but poor flexibility. If a certain string fails, it may affect the overall efficiency.
2. String inverter
Applicable scenarios: Home, commercial and industrial roofs (power 1-100kW).
characteristic:
Each photovoltaic string is independently connected to the MPPT channel of the inverter (usually 2-12), which can cope with local occlusion (such as leaves and shadows).
Support "plug and play", modular design for easy maintenance, and single unit failure does not affect other strings.
3. Micro inverter
Applicable scenarios: high-end residential buildings, complex occlusion environments (power ≤ 1kW/unit).
characteristic:
A single photovoltaic panel is paired with an inverter to completely solve the mutual influence between the series, and even if a single panel fails, it does not affect the overall system.
Output AC power directly in parallel without the need for DC cables, reducing the risk of electric shock, but at a higher cost (about 2-3 times that of string type).
4. Off grid inverter (independent system)
Applicable scenarios: remote areas without power grids, emergency power sources.
characteristic:
No need for grid control, directly supplying power to local loads, requiring a built-in battery charging and discharging controller.
The output frequency and voltage stability depend on the inverter's own control (such as using constant voltage constant frequency (CVCF) control strategy).
summarize
The working principle of solar inverters is essentially the combination of power electronic conversion and intelligent control, and its core challenge lies in balancing efficiency, reliability, and grid compatibility. With the application of new materials and technologies, inverters will develop towards higher efficiency, intelligence, and adaptability to new power systems in the future, becoming a key support for the photovoltaic industry to move from "alternative energy" to "main energy".
