Evaluating the performance of solar inverter control strategies requires starting from multiple dimensions such as functionality, efficiency, stability, and compatibility, combining industry standards and actual operating scenarios, and making comprehensive judgments through quantitative indicators and experimental verification. The following are specific evaluation dimensions, core indicators, and methods:
1、 Power Quality Assessment
The core function of a solar inverter is to convert direct current into alternating current that meets the requirements of the power grid or load. Therefore, the quality of the output electrical energy is the basic indicator and must comply with international (such as IEC) or national (such as GB) standards.
Core indicators:
Total Harmonic Distortion (THD)
Definition: The ratio of the root mean square of harmonic components in output current/voltage to the root mean square of fundamental components (usually dominated by current THD and supplemented by voltage THD).
Standard requirements: The THD of grid connected inverters should generally be ≤ 5% (GB/T 19964), and the THD of off grid inverters should be ≤ 3%~5% (depending on the load type).
Test method: Use a power analyzer (such as Yokogawa WT3000) to measure at rated power, 50% power, and other operating conditions, and compare with the standard threshold.
Voltage/frequency stability
Indicators: Output voltage deviation (≤± 5% rated voltage), frequency deviation (≤± 0.5Hz, China's power grid is 50Hz).
Test method: During sudden load changes (such as from 20% to 100% rated load) or light fluctuations, use an oscilloscope to record the voltage/frequency fluctuation curve and calculate the deviation range.
Power factor (PF)
Definition: The ratio of active power to apparent power reflects the inverter's ability to regulate active/reactive power in the power grid.
Standard requirement: Grid connected inverters typically need to support adjustable PF within the range of 0.9 lead to 0.9 lag (GB/T 37408).
Testing method: Set different reactive power requirements through a power grid simulator and measure the actual output PF value of the inverter.
2、 Conversion efficiency evaluation
Efficiency directly affects the power generation and economy of photovoltaic systems, and is the core optimization goal of control strategies.
Core indicators:
Maximum efficiency (η _max)
Definition: The conversion efficiency of an inverter under optimal operating conditions (such as rated power, standard lighting) (η=output active power/input DC power).
Current situation: The maximum efficiency of mainstream inverters can reach 98.5%~99.5%.
Weighted efficiency (η w_weighted)
Definition: Consider the frequency of occurrence of different power points in actual operation (such as low power in the morning/evening and high power at noon), and calculate the average efficiency through weighted calculation (such as European efficiency and California efficiency).
Testing method: Test the efficiency at 25%, 50%, 75%, and 100% rated power points respectively, and calculate the weighted value according to the preset weight (such as European efficiency weight: 5%/10%/20%/65%).
MPPT tracking efficiency
Definition: The maximum power point tracking (MPPT) algorithm tracks the accuracy of the maximum power point of a photovoltaic panel (η=actual output power/theoretical maximum power).
Test method: Under different lighting conditions (200~1000W/m ²) and temperature conditions (-20~60 ℃), compare the output power of the inverter with the theoretical maximum power of the photovoltaic panel (obtained through an IV curve tester), and calculate the deviation rate (generally ≤ 3%).
3、 Dynamic response performance evaluation
Photovoltaic systems often face sudden changes in illumination (such as cloud layer obstruction), load fluctuations, or grid disturbances. The control strategy needs to quickly and stably output to avoid overshoot or instability.
Core indicators:
Dynamically adjust time
Definition: The time from a sudden change in operating conditions (such as a sudden increase in light from 500W/m ² to 1000W/m ²) to a stable output power/voltage (usually requiring ≤ 100ms).
Test method: Simulate a sudden change in illumination using a photovoltaic simulator, record the output power curve with a high-speed oscilloscope, and count the time from the sudden change to a fluctuation amplitude of ≤ 2% of the rated value.
overshoot ratio
Definition: The maximum proportion of output power/voltage exceeding steady-state values during dynamic processes (generally required to be ≤ 5%).
Test method: As above, calculate the percentage of the difference between the peak value and the steady-state value to the steady-state value.
anti-interference ability
Definition: The ability of an inverter to maintain stable output when there are harmonics (such as 3rd and 5th harmonics) or voltage fluctuations in the power grid.
Test method: Inject 2%~5% characteristic harmonics into the power grid simulator and measure whether the output current THD still meets the standard (≤ 5%).
4、 Grid compatibility assessment
Grid connected inverters need to meet the requirements of the power grid for safe and stable operation, especially in terms of fault ride through and reactive power support capabilities.
Core indicators:
Low Voltage Ride Through (LVRT)/High Voltage Ride Through (HVRT)
Requirement: When the grid voltage drops (such as dropping to 0% rated voltage) or suddenly rises, the inverter must remain connected to the grid and provide reactive power support (such as injecting ≥ 1.5% rated current/reactive power per percentage point voltage drop during the drop period).
Test method: Use a power grid simulator to simulate voltage drop (such as 30% rated voltage for 2 seconds), record whether the inverter is disconnected from the grid, and whether the reactive power output meets the GB/T 37408 or EN 50549 standards.
Island effect detection capability
Definition: When the power grid is cut off, the inverter must detect the islanding and disconnect from the grid within a specified time (such as 2 seconds) to avoid endangering maintenance personnel.
Test method: Simulate power grid outage and measure the islanding detection time of the inverter (in accordance with the joint verification of "passive+active" detection in GB/T 19964).
Reactive power regulation response speed
Requirement: When dispatching reactive power commands in the power grid, the inverter must respond within 100ms and reach the target value within 3s (GB/T 35694).
Test method: Send a stepped reactive power command (such as from 0 var to 50% rated reactive power), record the response time and steady-state error.
5、 Reliability and Durability Assessment
The control strategy needs to work stably in long-term operation, especially in response to extreme environments and faults.
Core indicators:
Success rate of fault handling
Definition: The success rate of triggering protection mechanisms (such as shutdown and current limiting) by control strategies in the face of faults such as overvoltage, overcurrent, short circuit, and overheating (must be ≥ 99.9%).
Testing method: Conduct fault injection experiments (such as artificially creating DC side overvoltage) to calculate the response time (≤ 10ms) and successful times of the protection mechanism.
Long term operational stability
Definition: The decay rate of efficiency, THD, and other indicators (required to be ≤ 2%) after continuous operation for 1000 hours in harsh environments such as high temperature (50 ℃) and high humidity (90% RH).
Testing method: Simulate extreme working conditions in the environmental chamber, regularly measure key indicators, and compare initial values.
6、 Indirect economic evaluation
The performance of the control strategy ultimately needs to be reflected in terms of economy, which can be judged through indirect indicators:
LCOE: Calculate the cost per unit of electricity generated by combining inverter efficiency, lifespan (affected by control strategy stability), and maintenance costs (efficient control strategies can reduce LCOE by 5% to 10%).
Annual power generation gain: Comparing the actual annual power generation of different control strategies (such as traditional disturbance observation method vs model predictive control), high-quality strategies can increase by 2% to 5%.
Summary of Evaluation Methods
Simulation verification: Build an inverter model using MATLAB/Simulink and PSCAD to simulate working conditions such as lighting, load, and grid disturbances, and preliminarily evaluate the dynamic response and stability of the control strategy.
Experimental testing: Build a prototype system in the laboratory, use photovoltaic simulators, power grid simulators, power analyzers and other equipment, and conduct quantitative testing according to the above indicators; If necessary, conduct on-site network testing to verify actual operational performance.
Standard Benchmarking: Strictly compare with GB/T 19964, IEC 61727, EN 50530 and other standards to ensure that the indicators meet the requirements for grid connection or off grid.
In summary, the performance evaluation of the control strategy for solar inverters needs to take into account both "meeting technical indicators" and "adapting to actual scenarios". Through multidimensional quantitative analysis and long-term verification, it ensures that it efficiently, stably, and reliably serves the photovoltaic system.
