I. Working Principle and Characteristics:
Working principle: The core of an inverter device is the inverter switching circuit, or simply the inverter circuit. This circuit performs the inversion function by turning on and off power electronic switches.
Features:
(1) High efficiency is required.
Because solar cells are currently expensive, in order to maximize their utilization and improve system efficiency, it is necessary to find ways to improve the efficiency of inverters.
(2) High reliability is required.
Currently, photovoltaic power station systems are mainly used in remote areas, and many power stations are unattended and unmaintained. This requires inverters to have a reasonable circuit structure, strict component selection, and various protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheat protection, and overload protection.
(3) The input voltage is required to have a wide range of adaptability.
The terminal voltage of solar cells varies with load and solar radiation intensity. In particular, the terminal voltage of batteries can vary greatly as they age; for example, a 12V battery may have a terminal voltage that varies between 10V and 16V. This requires inverters to operate normally over a wide range of DC input voltages.
II. Classification of Photovoltaic Inverters
There are many ways to classify inverters. For example, based on the number of phases of the output AC voltage, they can be divided into single-phase inverters and three-phase inverters. Based on the type of semiconductor devices used, they can be divided into transistor inverters, thyristor inverters, and turn-off thyristor inverters. Based on the different circuit principles, they can also be divided into self-oscillating inverters, stepped-wave superposition inverters, and pulse-width modulation inverters. Based on whether they are used in grid-connected or off-grid systems, they can be divided into grid-connected inverters and off-grid inverters. To facilitate inverter selection for photovoltaic users, this classification only considers the different application scenarios of inverters.
1. Centralized inverter
Centralized inverter technology connects several parallel photovoltaic (PV) strings to the DC input of a single centralized inverter. High-power systems typically use three-phase IGBT power modules, while lower-power systems use field-effect transistors (FETs). A DSP (Digital Signal Processor) converter controller is used to improve the quality of the generated power, making it very close to a sinusoidal current. This technology is generally used in large-scale PV power plants (>10kW). Its main advantages are high system power and low cost. However, because the output voltage and current of different PV strings are often not perfectly matched (especially when PV strings are partially shaded due to clouds, trees, or dirt), centralized inverter technology can lead to reduced inverter efficiency and decreased power consumption. Furthermore, the overall reliability of the PV system is affected by the poor operating condition of any single PV unit. The latest research focuses on using space vector modulation control and developing new inverter topologies to achieve high efficiency under partial load conditions.
2. String inverter
String inverters are based on the modular concept. Each photovoltaic string (1-5kW) is connected to an inverter. They have maximum power point tracking on the DC side and are connected in parallel to the grid on the AC side. They have become the most popular inverters in the international market.
Many large-scale photovoltaic (PV) power plants use string inverters. The advantages are that they are unaffected by differences in modules between strings and shading, while also reducing the possibility of mismatch between the optimal operating point of the PV modules and the inverter, thus increasing power generation. These technological advantages not only reduce system costs but also increase system reliability. Furthermore, introducing the "master-slave" concept between strings allows the system to connect several PV strings together, enabling one or more of them to operate, thereby producing more electricity, even when a single string of power is insufficient to operate a single inverter.
3. Micro inverter
In traditional PV systems, each string inverter's DC input is connected to approximately 10 photovoltaic panels in series. If even one of these 10 panels malfunctions, the entire string is affected. If multiple inverter inputs use the same MPPT (Multi-Phase Power Transmission Tablet), all inputs will be affected, significantly reducing power generation efficiency. In practice, various factors such as clouds, trees, chimneys, animals, dust, and snow can all cause these issues, and this is quite common. However, in a micro-inverter PV system, each panel is connected to a separate micro-inverter. If one panel malfunctions, only that panel is affected. The other panels will operate at their optimal performance, resulting in higher overall system efficiency and greater power generation. In practical applications, a string inverter failure can cause several kilowatts of panels to become unusable, while the impact of a micro-inverter failure is considerably smaller.
4. Power Optimizer
Adding a power optimizer (OptimizEr) to a solar power system can significantly improve conversion efficiency and simplify the inverter's functions, reducing costs. To achieve a smart solar power system, the power optimizer ensures that each solar cell performs at its optimal performance and monitors cell degradation status in real time. The power optimizer is a device positioned between the power generation system and the inverter, its primary task being to replace the inverter's original optimal power point tracking (OPT) function. By simplifying the wiring and assigning one power optimizer to each solar cell, the power optimizer performs extremely fast analog OPT scanning, ensuring that each solar cell achieves optimal OPT. Furthermore, by incorporating a communication chip, it can monitor cell status anytime, anywhere, and report problems immediately for prompt repair.
III. Functions of Photovoltaic Inverters
Inverters not only have DC-AC conversion capabilities, but also the ability to maximize solar cell performance and provide system fault protection. These can be summarized as follows: automatic operation and shutdown functions, maximum power point tracking (MPPT) control, anti-isolation operation function (for grid-connected systems), automatic voltage regulation function (for grid-connected systems), DC detection function (for grid-connected systems), and DC grounding detection function (for grid-connected systems). This section briefly introduces the automatic operation and shutdown functions and the MPPT control function.
(1) Automatic operation and shutdown function
After sunrise, as solar radiation intensifies, the output of the solar cells increases accordingly. Once the output power required for the inverter to operate is reached, the inverter automatically begins operation. Once operational, the inverter continuously monitors the output of the solar cell modules. As long as the output power of the solar cell modules exceeds the inverter's required output power, the inverter continues to operate until sunset, even on cloudy or rainy days. When the output of the solar cell modules decreases and the inverter output approaches zero, the inverter enters standby mode.
(2) Maximum power point tracking control function
The output of a solar cell module varies with the intensity of solar radiation and the module's own temperature (chip temperature). Furthermore, because the voltage of a solar cell module decreases as current increases, there exists an optimal operating point where maximum power can be obtained. Since solar radiation intensity is variable, the optimal operating point also changes. To keep the solar cell module's operating point at its maximum power point relative to these changes, ensuring the system consistently draws maximum power from the module, is called maximum power point tracking (MPPT) control. A key feature of inverters used in solar power systems is the inclusion of MPPT functionality.
IV. Main Technical Specifications of Photovoltaic Inverters
1. Output voltage stability
In photovoltaic systems, the electrical energy generated by solar cells is first stored in batteries and then converted into 220V or 380V AC power by an inverter. However, due to the effects of its own charging and discharging, the output voltage of a battery varies considerably. For example, a nominal 12V battery can have a voltage range of 10.8 to 14.4V (exceeding this range may damage the battery). For a qualified inverter, when the input voltage varies within this range, the change in its steady-state output voltage should not exceed ±5% of the rated value. Furthermore, when the load changes abruptly, the output voltage deviation should not exceed ±10% of the rated value.
2. Output voltage waveform distortion
For sinusoidal inverters, the maximum permissible waveform distortion (or harmonic content) should be specified. This is typically expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (10% for single-phase output). Because the high-order harmonic currents output by the inverter will generate additional losses such as eddy currents in inductive loads, excessive waveform distortion can lead to severe overheating of load components, jeopardizing electrical equipment safety and significantly impacting system operating efficiency.
3. Rated output frequency
For loads containing motors, such as washing machines and refrigerators, the optimal operating frequency of the motor is 50Hz. Too high or too low a frequency will cause the equipment to overheat, reduce system operating efficiency and lifespan. Therefore, the output frequency of the inverter should be a relatively stable value, usually the power frequency of 50Hz, and its deviation should be within ±1% under normal operating conditions.
4. Load power factor
The power factor of an inverter is characterized by its ability to handle inductive or capacitive loads. The load power factor of a sinusoidal inverter is typically 0.7 to 0.9, with a rated value of 0.9. Given a fixed load power, a lower inverter power factor necessitates a larger inverter capacity. This increases costs, increases the apparent power and current in the photovoltaic system's AC circuit, leading to increased losses and reduced system efficiency.
5. Inverter efficiency
Inverter efficiency refers to the ratio of its output power to its input power under specified operating conditions, expressed as a percentage. Generally, the nominal efficiency of a photovoltaic (PV) inverter refers to the efficiency under purely resistive load conditions at 80% load. Since the overall cost of PV systems is relatively high, the efficiency of PV inverters should be maximized to reduce system costs and improve the cost-effectiveness of the PV system. Currently, the nominal efficiency of mainstream inverters is between 80% and 95%, and for low-power inverters, an efficiency of no less than 85% is required. In the actual design of PV systems, not only should high-efficiency inverters be selected, but also reasonable system configuration should be used to ensure that the PV system load operates near its optimal efficiency point.
6. Rated output current (or rated output capacity)
This indicates the inverter's rated output current within a specified load power factor range. Some inverter products specify the rated output capacity, expressed in VA or kVA. The inverter's rated capacity is the product of the rated output voltage and rated output current when the output power factor is 1 (i.e., a purely resistive load).
7. Protective Measures
A high-performance inverter should also have complete protection functions or measures to deal with various abnormal situations that may occur during actual use, so as to protect the inverter itself and other system components from damage.
(1) Enter the undervoltage customer information:
The inverter should have protection and display functions when the input voltage is lower than 85% of the rated voltage.
(2) Input overvoltage protection account:
The inverter should have protection and display functions when the input voltage exceeds 130% of the rated voltage.
(3) Overcurrent protection:
The inverter's overcurrent protection should be able to operate promptly in the event of a short circuit in the load or when the current exceeds the allowable value, protecting it from damage by surge current. The inverter should also be able to automatically protect itself when the operating current exceeds 150% of the rated current.
(4) Output short circuit protection
The short-circuit protection of the inverter should operate within 0.5 seconds.
(5) Input reverse connection protection:
The inverter should have protection functions and a display when the positive and negative input terminals are reversed.
(6) Lightning protection:
Inverters should have lightning protection.
(7) Over-temperature protection, etc.
In addition, for inverters without voltage stabilization measures, the inverter should also have output overvoltage protection measures to protect the load from overvoltage damage.
8. Starting characteristics
Characterizes the inverter's ability to start under load and its performance during dynamic operation. The inverter should guarantee reliable starting under rated load.
9. Noise
Components in power electronic equipment, such as transformers, filter inductors, electromagnetic switches, and fans, all generate noise. During normal operation, the noise level of an inverter should not exceed 80 dB, and for small inverters, it should not exceed 65 dB.
V. Selection Techniques
When selecting an inverter, the first consideration should be having sufficient rated capacity to meet the power requirements of the equipment under maximum load. For inverters with a single device as the load, the selection of their rated capacity is relatively simple.
When the electrical equipment is a purely resistive load or has a power factor greater than 0.9, the rated capacity of the inverter should be 1.1 to 1.15 times the capacity of the electrical equipment. The inverter should also have the ability to withstand capacitive and inductive load surges.
For typical inductive loads, such as motors, refrigerators, air conditioners, washing machines, and high-power water pumps, the instantaneous power during startup can be 5 to 6 times their rated power. In this case, the inverter will experience a significant surge. For such systems, the inverter's rated capacity should have sufficient margin to ensure reliable load startup. High-performance inverters can perform multiple consecutive full-load starts without damaging power devices. For their own safety, small inverters sometimes require soft starting or current-limiting starting methods.
VI. Installation Precautions and Maintenance
1. Before installation, you should first check whether the inverter has been damaged during transportation.
2. When selecting an installation site, ensure that there is no interference from any other power electronic equipment in the vicinity.
3. Before making any electrical connections, be sure to cover the photovoltaic panels with an opaque material or disconnect the DC-side circuit breaker. Exposure to sunlight will cause the photovoltaic array to generate dangerous voltages.
4. All installation operations must be performed by qualified technicians only.
5. The cables used in the photovoltaic power generation system must be securely connected, well insulated, and of appropriate specifications.
