I. Analysis of the main technical performance of the inverter
1. Rated output voltage
Within the specified allowable fluctuation range of the input DC voltage, it represents the rated voltage value that the inverter should be able to output. The stability and accuracy of the output rated voltage value are generally specified as follows:
(1) During steady-state operation, the voltage fluctuation range should be limited, for example, its deviation should not exceed ±3% or ±5% of the rated value.
(2) Under dynamic conditions of sudden load changes (rated load 0%→50%→100%) or other interference factors, the output voltage deviation should not exceed ±8% or ±10% of the rated value.
2. Output voltage imbalance
Under normal operating conditions, the three-phase voltage imbalance (the ratio of the reverse-sequence component to the positive-sequence component) of the inverter output should not exceed a specified value, usually expressed as a percentage, such as 5% or 8%.
3. Output voltage waveform distortion
When the inverter output voltage is sinusoidal, the maximum permissible waveform distortion (or harmonic content) should be specified. It is usually expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (10% permissible for single-phase output).
4. Rated output frequency
The frequency of the inverter's output AC voltage should be a relatively stable value, typically the power frequency of 50Hz. Under normal operating conditions, its deviation should be within ±1%.
5. Load power factor
Characterizes the inverter's ability to drive inductive or capacitive loads. Under sinusoidal conditions, the load power factor is 0.7 to 0.9 (lagging), with a rated value of 0.9.
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. Rated output efficiency
Inverter efficiency is the ratio of its output power to its input power under specified operating conditions, expressed as a percentage (%). The efficiency of an inverter at its rated output capacity is its full-load efficiency, and its efficiency at 10% of its rated output capacity is its low-load efficiency.
II. Inverter Types
1. Small and medium power
Small and medium power inverters are a crucial component of residential independent AC photovoltaic systems. Their reliability and efficiency are essential for promoting photovoltaic systems, effectively utilizing energy, and reducing system costs. Therefore, photovoltaic experts from various countries have been working hard to develop inverters suitable for residential use in order to promote the better and faster development of the industry.
2. Multiple series connection type
Multiple series inverters offer numerous advantages for electric vehicles. The series structure significantly increases the variety of output voltage vectors, enhancing control flexibility and accuracy; it also reduces fluctuations in the motor neutral point voltage. The inverter's bypass capability improves the flexibility of charging and regenerative braking control.
With increasing public concern for the urban environment, the development of electric vehicles has received a rare opportunity. In urban transportation, electric buses, due to their large carrying capacity and high overall efficiency, have become a priority for development. Most electric buses use three-phase AC motors. Because of the high motor power, the components in the three-phase inverter need to withstand high voltage and high current stress. The high dv/dt also results in significant electromagnetic radiation and requires good heat dissipation.
High-power inverters employing multiple series-connected structures reduce the voltage stress on individual components, lowering the requirements for components; they also reduce the dv/dt value, decreasing electromagnetic radiation and significantly reducing component heat generation; and their control performance is improved due to the increased variety of output levels.
Multiple series inverters are suitable for high-power electric vehicle drive systems. The multiple series structure reduces the hazards associated with multiple batteries connected in series, lowers switching stress on components, and reduces electromagnetic radiation. However, it doubles the number of batteries required.
The multi-series series structure significantly increases the variety of output voltage vectors, thereby enhancing control flexibility and improving control accuracy; it also reduces fluctuations in the motor neutral point voltage. To maintain a balanced charge level among each battery bank, it is necessary to ensure consistent battery discharge times during operation. Bypassing allows for flexible charging of the battery banks and control of regenerative braking torque.
