Photovoltaic Inverter Basic Principles and Parameters
Inverter Power Generation Principle
The DC power generated by the photovoltaic modules first passes through a DC filter circuit and a boost circuit to remove current fluctuations and electromagnetic interference, and then boosts the string voltage to the DC voltage required for inverter output control.
The power then enters the inverter circuit, where it is first converted to AC and then rectified into a sinusoidal AC. A filter circuit at the output removes high-frequency interference generated during the inverter process, outputting AC voltage and current at a common frequency. This can then be connected to the grid or directly supply a load.
Input Parameters
1. Maximum Input Power: This represents the maximum power input from the photovoltaic modules to the inverter.
2. Maximum Input Voltage: This represents the maximum voltage of the photovoltaic string during operation (calculated based on the open-circuit voltage at the lowest module temperature).
3. MPPT Voltage Range: The primary function of MPPT is to ensure that the module always outputs the voltage at its maximum power point. Because the voltage of the modules fluctuates with factors like sunlight and temperature, and the number of modules connected in series is designed based on the specific project circumstances, the inverter has a set operating range. As long as it operates within this range, the inverter will function properly. The wider the voltage range, the wider the inverter's applicability.
4. Full-load MPPT voltage range: This is the voltage range within which the inverter can output its rated power. If it is outside this voltage range, the inverter's rated power will be reduced.
5. Starting voltage: Before the inverter starts, the modules are not operating and are in an open-circuit state, resulting in a higher voltage. Once the inverter starts, the modules are operating, and the voltage decreases. To prevent the inverter from restarting repeatedly, the starting voltage of the inverter is set higher than the minimum operating voltage.
Starting the inverter does not mean that it will immediately output power. The inverter's control unit, CPU, and display components will start working first. The inverter will first perform a self-test, then check the modules and the power grid. Once all problems are clear, and the photovoltaic power exceeds the inverter's standby power, the inverter will start outputting power.
6. Rated Input Voltage: Designing the string voltage around the rated voltage will result in high inverter efficiency and, consequently, high power generation. Therefore, when designing the string system, aim for a voltage around the rated operating voltage of the inverter for maximum efficiency. This ensures that the voltage will not exceed the maximum voltage at extremely low temperatures and that the system remains within the full-load MPPT voltage range during operation. This eliminates the need for complex calculations and is extremely simple and practical.
7. Maximum Input Current per MPPT: This is the maximum current allowed by each MPPT. The sum of the string input currents must be less than this value. If the selected PV module Imp exceeds this value, the inverter will not be able to capture the module's maximum power point.
8. Number of MPPTs and Number of Input Strings per MPPT: During system configuration, ensure that all modules connected to multiple strings within each MPPT system are consistent (model, specifications, mounting inclination, azimuth, etc.).
Output Parameters:
1. Rated Output Power: This refers to the power that the inverter can consistently and stably output over a long period of time. 2. Maximum Output Power: Maximum power, also called peak power, refers to the maximum power the inverter can output in a short period of time.
3. Power Factor: In an AC circuit, the cosine of the phase difference (Φ) between voltage and current is called the power factor, represented by the symbol cosΦ. Numerically, power factor is the ratio of active power to apparent power, i.e., cosΦ = P/S. To maximize power generation returns, PV inverters are generally not used to generate reactive power. Therefore, the default power factor setting for inverters is 1 or 0.99.
Insulation Impedance Test:
The inverter measures the voltages of PV+ to ground and PV- to ground, and calculates the resistances of PV+ and PV- to ground, respectively. If the resistance on either side falls below the threshold, the inverter stops operating and displays a "PV Insulation Impedance Low" alarm. Low insulation impedance is a common fault in photovoltaic systems. Damage to components, DC cables, and connectors, as well as aging of the insulation layer, can cause low insulation impedance. When the DC cable passes through the bridge, the outer insulation of the cable may be damaged during threading due to the possibility of barbs on the edge of the metal bridge, resulting in leakage to the ground.