The working principle of a solar inverter is that the process of converting direct current (DC) energy into alternating current (AC) energy is called inversion. The circuit that performs the inversion function is called an inverter circuit, and the device that realizes the inversion process is called an inverter device or inverter. So what are the main technical performance indicators of a solar inverter? Let's find out below.
Inverter main technical performance indicators
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 rated output 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 (ratio of reverse sequence component to 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 allowable 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% is allowed for single-phase output).
4. Rated output frequency: The frequency of the inverter's output AC voltage should be a relatively stable value, typically 50Hz (power frequency). 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 rated output current of the inverter within a specified load power factor range. Some inverter products specify the rated output capacity, expressed in VA or kVA. The rated capacity of the inverter is the product of the rated output voltage and the rated output current when the output power factor is 1 (i.e., a purely resistive load).
7. Rated Output Efficiency: The efficiency of an inverter 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 the efficiency at 10% of its rated output capacity is its low-load efficiency.
8. Protection
(1) Overvoltage protection: For inverters without voltage stabilization measures, there should be output overvoltage protection measures to protect the load from damage caused by output overvoltage.
(2) Overcurrent protection: The inverter’s overcurrent protection should be able to operate in a timely manner when the load is short-circuited or the current exceeds the allowable value, so as to protect it from damage by surge current.
9. 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.
10. 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.
For high-power photovoltaic (PV) power generation systems and grid-connected PV power generation systems, the technical performance of inverters, such as waveform distortion and noise levels, is also very important. When selecting inverters for off-grid PV power generation systems, the following points should also be noted:
(1) It should have sufficient rated output capacity and load capacity. 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 rated capacity is relatively simple. When the electrical equipment is a purely resistive load or the power factor is greater than 0.9, the rated capacity of the inverter should be 1.1 to 1.15 times the capacity of the electrical equipment. When the inverter has multiple devices as loads, the selection of the inverter capacity should consider the possibility of several electrical devices operating simultaneously, i.e., the "load simultaneity factor".
(2) It should have high voltage stability. Off-grid photovoltaic power generation systems all use batteries as energy storage devices. When a battery with a nominal voltage of 12V is in float charging mode, its terminal voltage can reach 13.5V, and in a short-term overcharge state, it can reach 15V. When the battery is discharged under load, its terminal voltage can drop to 10.5V or lower. The fluctuation of the battery terminal voltage can reach about 30% of the nominal voltage. This requires the inverter to have good voltage regulation performance to ensure that the photovoltaic power generation system is supplied with a stable AC voltage.
(3) High or relatively high efficiency under various loads. High overall efficiency is a significant feature that distinguishes photovoltaic inverters from general-purpose inverters. The actual efficiency of a 10kW-class general-purpose inverter is only 70% to 80%, which will result in a 20% to 30% power loss when used in a photovoltaic power generation system. Therefore, photovoltaic power generation system-specific inverters should pay special attention to reducing their own power loss and improving overall efficiency in their design. This is an important measure to improve the technical and economic indicators of photovoltaic power generation systems. The requirements for photovoltaic power generation-specific inverters in terms of overall efficiency are: rated load efficiency ≥ 80% to 85% for inverters below kW level, and low load efficiency ≥ 65% to 75%; rated load efficiency ≥ 85% to 90% for 10kW-class inverters, and low load efficiency ≥ 70% to 80%.
(4) It should have good overcurrent and short-circuit protection functions. During the normal operation of a photovoltaic power generation system, it is entirely possible for the power supply system to experience overcurrent or short circuits due to load failures, human error, and external interference. The inverter is most sensitive to overcurrent and short-circuit phenomena in the external circuit and is the weakest link in the photovoltaic power generation system. Therefore, when selecting an inverter, it is essential to ensure that it has good self-protection functions against overcurrent and short circuits.
(5) Easy Maintenance. It is normal for high-quality inverters to malfunction due to component failure after several years of operation. In addition to a robust after-sales service system, manufacturers must also ensure good maintainability in their inverter manufacturing processes, structure, and component selection. For example, there should be sufficient or readily available spare parts for damaged components, and good interchangeability of components; the components should be easy to disassemble and replace. This way, even if the inverter malfunctions, it can be quickly restored to normal operation.
