Principles and Selection Techniques of Photovoltaic Inverters

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 switching power electronic switches on and off.

Characteristics:

(1) High efficiency is required.

Due to the current high price of solar cells, in order to maximize the utilization of solar cells and improve system efficiency, it is necessary to find ways to improve the efficiency of the inverter.

(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 the inverter 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) A wide range of input voltage adaptability is required.

Because the terminal voltage of solar cells varies with the load and solar radiation intensity. In particular, when batteries age, their terminal voltage varies greatly. For example, the terminal voltage of a 12V battery may vary between 10V and 16V. This requires the inverter to operate normally within a wide range of DC input voltage.

There are many ways to classify inverters, such as:

* Based on whether they are used in grid-connected or off-grid systems, inverters can be divided into grid-connected inverters and off-grid inverters.

To facilitate inverter selection for photovoltaic users, this classification is based solely on the different application scenarios of inverters.

Centralized inverter technology connects several parallel photovoltaic (PV) strings to the DC input of a single centralized inverter. Generally, high-power systems use three-phase IGBT power modules, while lower-power systems use field-effect transistors (FETs). A DSP (Digital Signal Processor) conversion controller is used to improve the quality of the generated power, making it very close to a sinusoidal current. This type of inverter is typically used in large-scale PV power plants (>10kW).

Its advantages include 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 inverters can lead to reduced inverter efficiency and decreased power consumption.

Furthermore, the reliability of the entire 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.

String inverters are based on a modular concept. Each photovoltaic string (1-5kW) is connected to an inverter, featuring peak power point tracking (MPPT) on the DC side and parallel grid connection on the AC side. They have become the most popular inverter type on the international market.

Many large-scale photovoltaic power plants use string inverters. Their advantages include being unaffected by differences in modules between strings and shading, while reducing the mismatch between the optimal operating point of the photovoltaic modules and the inverter, thus increasing power generation. These technological advantages not only reduce system costs but also increase system reliability.

Furthermore, the introduction of a “master-slave” concept between strings allows the system to connect several photovoltaic strings together, enabling one or more of them to operate and thus producing more electricity, even when a single string’s power is insufficient to power a single inverter.

In traditional PV systems, each string inverter’s DC input is connected in series with approximately 10 photovoltaic panels. If one of these 10 panels malfunctions, the entire string is affected.

If multiple inputs of an inverter use the same MPPT (Multi-Phase Power Transmission Tablet), all inputs will be affected, significantly reducing power generation efficiency. In practical applications, various factors such as clouds, trees, chimneys, animals, dust, and snow can all cause these issues, and this is very common. However, in a micro-inverter PV system, each solar panel is connected to a separate micro-inverter. If one panel malfunctions, only that panel will be 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 solar panels to become unusable, while the impact of a micro-inverter failure is considerably smaller.

Adding a power optimizer (OptimizEr) to a solar power system can significantly improve conversion efficiency and simplify the inverter’s functionality, reducing costs.

To achieve a smart solar power system, a power optimizer ensures that each solar cell performs at its best and monitors cell degradation status in real time.

A power optimizer is a device positioned between the power generation system and the inverter. Its main task is 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 OPT scanning in an analog manner, ensuring that each solar cell can reliably achieve optimal OPT. In addition, it can monitor the battery status anytime, anywhere via an embedded communication chip, and report problems in real time so that relevant personnel can carry out repairs as soon as possible.

Inverters not only have DC-AC conversion functions, but also functions to maximize the performance of solar cells and system fault protection functions.In summary, they include:

DC grounding detection functions (for grid-connected systems).

Here, we briefly introduce the automatic operation and shutdown functions and the maximum power point tracking (MPPT) control functions.

(1) Automatic Operation and Shutdown Functions
After sunrise, the intensity of solar radiation gradually increases, and the output of solar cells also increases accordingly. When the output power required for inverter operation is reached, the inverter automatically starts operating.

Once in operation, the inverter constantly monitors the output of the solar cell modules. As long as the output power of the solar cell modules is greater than the output power required for inverter operation, 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 0, the inverter enters standby mode.

(2) Maximum Power Point Tracking (MPPT) 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.

Solar radiation intensity is variable, and obviously, 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 obtains maximum power output from the solar cell module, is called maximum power point tracking (MPPT).

The most significant feature of inverters used in solar power generation systems is that they include maximum power point tracking (MPPT) functionality.

1. Output Voltage Stability
In a photovoltaic system, 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, the output voltage of batteries varies considerably due to their own charging and discharging. CFor example, a nominal 12V battery can fluctuate between 10.8V and 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. Simultaneously, 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 allowable waveform distortion (or harmonic content) should be specified. This is usually 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 generate additional losses such as eddy currents in inductive loads, excessive waveform distortion in the inverter can lead to severe overheating of load components, compromising electrical safety and significantly impacting system efficiency.

3. Rated Output Frequency
For loads including motors, such as washing machines and refrigerators, the optimal operating frequency for these motors is 50Hz. Frequencies that are too high or too low will cause overheating, reducing system efficiency and lifespan. Therefore, the inverter’s output frequency should be relatively stable, typically the industrial frequency of 50Hz, with a deviation within 1% under normal operating conditions.

4. Load Power Factor
This characterizes the inverter’s ability to handle inductive or capacitive loads. The load power factor for a sinusoidal inverter is 0.7–0.9, 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 and, consequently, the apparent power in the photovoltaic system’s AC circuit, leading to increased circuit current, 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 inverter refers to the efficiency under purely resistive load at 80% load.

Due to the high overall cost of photovoltaic systems, the efficiency of photovoltaic inverters should be maximized to reduce system costs and improve the cost-effectiveness of the photovoltaic 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.

TAICO inverters achieve 98%, making them top-tier benchmark products in the industry. In the actual design of photovoltaic systems, not only should high-efficiency inverters be selected, but also reasonable system configuration should be used to ensure that the photovoltaic system load operates near its optimal efficiency point.

6. Rated Output Current (or Rated Output Capacity)
This indicates the rated output current of the inverter within a specified load power factor range. Some inverter products provide the rated output capacity, expressed in VA or kVA.

The rated capacity of an inverter is the product of its rated output voltage and rated output current when the output power factor is 1 (i.e., a purely resistive load).

7.Protection Measures
A high-performance inverter should also have comprehensive protection functions or measures to cope with various abnormal situations that may occur during actual use, protecting the inverter itself and other system components from damage.

(1) Input undervoltage protection

(2) Input overvoltage protection

(3) Overcurrent protection

(4) Output short-circuit protection

(5) Input reverse connection protection

(6) 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
Characteristics the inverter’s ability to start under load and its performance during dynamic operation. The inverter should ensure reliable starting under rated load.

9. Noise
Components such as transformers, filter inductors, electromagnetic switches, and fans in power electronic equipment will generate noise. When the inverter is operating normally, its noise should not exceed 80dB, and the noise of a small inverter should not exceed 65dB.

When selecting an inverter, the first consideration should be sufficient rated capacity to meet the power requirements of the equipment under maximum load. For inverters with a single device as the load, selecting the rated capacity is relatively simple.

When the electrical equipment is a purely resistive load or has a power factor greater than 0.9, a rated capacity of 1.1 to 1.15 times the capacity of the electrical equipment is sufficient. The inverter should also have the ability to withstand capacitive and inductive load surges.

For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, and high-power water pumps, the instantaneous power during startup may be 5 to 6 times its rated power. In this case, the inverter will withstand a large instantaneous 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 need to use soft starting or current-limiting starting methods. TAICO has its own inverter production line, and its products are adapted to the global energy storage market.

5. All cables used in the photovoltaic power generation system must be securely connected, well-insulated, and of appropriate specifications.

1. Universal Voltage Compatibility ▸ US Standard: 120V/240V Phase-by-Phase Adaptive (TKPV Series) ▸ European Standard: 230V/400V Wideband Compatibility (SPS Series)

2. Military-Grade Protection ▸ Dual Short-Circuit Protection: Circuit Breaker + Fuse (TKPV3000 Specification Sheet) ▸ Operation in Extreme Environments from -30℃ to 60℃ (Verified at the Arctic Research Station)

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