Modern inverters use solid-state components and high-quality transformers, and the peak power efficiency can reach 95%. Efficiency is the ratio of output power to input power. Research and experiments show that if the inverter efficiency reaches 96% and the rated output power is 960W, the required input power is at least 1000W. According to the rule of thumb of high-quality sine wave inverter, the designer can estimate according to. This is only a quick estimation method. If accurate calculation is required, refer to the manufacturer’s specifications. The efficiency of all electrical equipment is less than 100%. If the efficiency of inverter increases by only 1% ~ 2%, it will bring significant long-term benefits in saving system cost. For example, if the efficiency of a 20kW system is increased by 1%, 200W photovoltaic panels and equipment can be installed less. If a 75kW system improves the same efficiency, 750W photovoltaic panels and equipment can be installed less. If the average cost of the equipment is $5 / W, it can save $1000 and $3750 respectively.
Assuming that a system has three electrical units, the connection mode and efficiency are shown in, the total efficiency of the system is the product of the efficiency of the three units: obviously, the efficiency of each unit is very high, but the efficiency of the system is low. Part of the energy entering the system is used to drive the circuit, and part of the energy becomes heat and is lost. The challenge for scientists and engineers is to develop their own components and circuits with lower energy consumption, while reducing their calorific value. Designers and installers of renewable energy systems face similar challenges, that is, the power system and controller must be compatible, the design for specific applications must be correct, and the wiring, placement of components and safety must be carried out in an optimal way.
Generally, the rated efficiency of the inverter is the primary factor determining the system performance, but this may also cause misunderstanding. The efficiency will vary with DC input voltage, AC output voltage, the proportion of rated load in operation and other factors. Efficiency parameters are very important, but the performance of the system is a function of the overall design and use.
- Harmonic distortion
Relevant research shows that a periodic (repetitive) non sine wave can be superimposed by a series of sine waves with different frequencies from the basic waveform. A sine wave with the same frequency as the basic waveform is also called the first harmonic. The actual AC voltage and current waveforms include waveforms of various frequencies, which are called harmonics. The harmonic frequency is several times the fundamental frequency (equal to the frequency of the first harmonic). For example, if the basic frequency of a waveform is 5KHz, its possible harmonic frequencies are 10kHz, 15KHz, 20kHz, etc. A pure sinusoidal waveform contains only the fundamental frequency and no other harmonics. However, non sine wave shape includes the basic frequency waveform and harmonics of sine wave. For example, a square wave with a frequency of 5KHz contains a basic frequency of 5KHz and other odd multiple harmonics, such as 15KHz, 25kHz and 35kHz harmonics. A 50Hz square wave is formed by superposition of the following sine waves: first harmonic = 50Hz, third harmonic = 150Hz, fifth harmonic = 250Hz, etc. Square waves contain only odd harmonics, but other waveforms contain even harmonics or both odd and even harmonics. Even harmonics are even times of the fundamental frequency. If a 2KHz waveform is composed of even harmonics, it includes harmonics of basic frequency and frequencies of 4kHz, 8kHz and 12khz.
For a waveform containing basic frequency and second harmonic, when a pure sine wave generator drives some loads, pulse current may appear, so as to introduce harmonic into sine wave. Such loads that may cause pulse current include fluorescent lamp ballasts, computers and motor controllers. If a sine wave is superimposed by odd harmonics, distortion will occur and begin to look like a square wave.
If a sine wave is generated by pulse width modulation, high-frequency sine wave harmonics will be introduced into the waveform. These harmonics must be eliminated before the power generation system outputs pure sine wave voltage to the power grid. The total harmonic distortion (THD) of high-quality grid connected inverter is less than 5%. When calculating the THD of the waveform, you can first calculate the sum of the power of all harmonics except the basic waveform, and then divide it by the power of the fundamental wave.
Harmonics can cause overheating of power cables, connecting lines, motors and transformers. Overheating will cause the protection device to start frequently and reduce the power factor. Static noise in audio system is usually caused by harmonics in signal. We can use filters to eliminate harmonics. In fact, the equipment called “sine wave inverter” can not produce “pure” sine wave, but the THD of a typical high-quality sine wave inverter should be less than 5%.