Passive filters such as the L filter, LC filter, and LCL filters are used for harmonic mitigation in grid-connected inverter systems. Among the passive filters mentioned, LCL filters are cost-effective and perform well.
The advantages of LCL filters are high attenuation, improved performance, cost-effectiveness, and less weight and size. The LCL filter offers good harmonic elimination with low values of inductors and capacitors.
As the LCL filter provides the shunt capacitance feature, it can improve the system power factor when used with motor drives.
The demand for electrical energy is increasing every day.
As the global demand for energy rapidly increases, renewable energy is harnessed to meet residential, commercial, and industrial needs. Even though solar energy is abundant, solar PhotoVoltaic (PV) power is DC in nature. Solar energy needs to be converted into AC before supplying it to the grid. If solar energy is utilized for grid integration, then PV inverters are an inevitable part of the system. The power electronic inverters convert solar DC power to AC power before integrating it into the grid. However, the solar power integration to the grid is always affected by the harmonics produced by PV inverters. Passive filters such as the L filter, LC filter, and LCL filter are employed for harmonic mitigation in grid-connected inverter systems. Among the passive filters mentioned, LCL filters are cost-effective and perform well. Because of LCL filter design, the harmonics generated by PV inverters can be limited from reaching the grid by using them.
LCL Filter Design for Grid-Connected Inverter Systems
In grid-connected inverters for PV applications, filters are essential elements. The filter incorporated in such systems should offer high harmonic attenuation. The simple inductor L filter provides only low harmonic attenuation, and the voltage drop across it is very high. The L filter is also so bulky that it consumes more space, which is a demerit.
LCL filters are a good alternative, as they offer all the merits that are not present in the L and LC filters. The LCL filter effectively smooths the inverter current output, and the filtered harmonic-free current is supplied to the grid. The advantages of LCL filters are high attenuation, improved performance, cost-effectiveness, and less weight and size. The LCL filter offers good harmonic elimination with low values of inductors and capacitors. The figure below shows the LCL filter and grid-connected inverter.
LCL filters are a key element in grid-connected inverters, shown above.
LCL Filter Design Considerations
When designing an LCL filter for grid-connected applications, you need to consider characteristics such as filter size, current ripple, and switching ripple attenuation. The damping is another requirement when the filter is interfacing with the grid. When the inverter system is integrated into the grid, the capacitance may undergo resonance. Active or passive damping is needed to suppress the resonance. The inclusion of a series resistor with the filter capacitor is a kind of damping used in LCL filters.
LCL filter design starts by taking system parameters such as inverter output voltage, rated active power, grid frequency, switching frequency, and resonance frequency into consideration. The rest of the procedure in the LCL filter design involves mathematical calculations. The base impedance and base capacitance are calculated. The capacitance Cf is taken as a certain percentage (usually 5%) of base capacitance. The inverter side inductor L1 value is inversely proportional to the current ripple. By fixing the required current ripple value, the inductor L1 is designed. The grid side inductor L2 is determined by providing an attenuation factor. With the calculated LCL values, the resonance frequency of the filter should be greater than 10 times the grid frequency, and less than 0.5 times the switching frequency. If this condition is not satisfied, the inductor L2 is redesigned until it gets satisfied.
LCL Filters for Motor Drives
Figure 3: LCL filters suppress the current harmonics.
Globally, modern processes in manufacturing industries are anchored on motor drives. The precise speed control and energy conservation achieved by using motor drives have helped industries to flourish with limited machinery. Any failure to motor drive converters can result in quality degradation, end-product defects, or complete shutdown. The current harmonic is a major challenge affecting the motor drive operation. There is a need to filter out the input current harmonics to the drives. The input filter is one of the harmonic mitigation methods used in motor drive systems. LCL filter design is considered to be the best in suppressing the input current harmonics to power electronic converters.
The figure above shows the block diagram of LCL filter placement in motor drive systems. The LCL filters used in motor drives meet the IEEE 519-2014 standards. The LCL filter is capable of suppressing lower order harmonics and prevents the harmonics from reaching the motor. The harmonics in voltage and currents can jeopardize motor functioning, which leads to system instability and, ultimately, financial loss. Considering this aspect, the LCL filter is the most cost-effective way of improving system reliability and efficiency. It also extends the life of the machinery, as the motor stator and rotor windings are not subjected to harmonics due to LCL filters in the system. As resonance is a phenomenon associated with LC circuits, LCL filter design pays more attention to the resonance concepts and makes the filter operation resonance-free.
Motors are one of the main sources responsible for the low power factor in the grid. Connecting a shunt capacitor is the basic power factor correction method. As the LCL filter provides the shunt capacitance feature, it can improve the system power factor as well.
The parallel-series arrangement of inductors and capacitors of suitable value summarizes LCL filter design. Depending on the application, the LCL filter design varies. When in a grid-connected system, the LCL filter design will be focused on reducing the ripple content. When it comes to motor applications in consumer electrical equipment, size and weight reduction is one of the design objectives. However, in both the applications, harmonic elimination is of most concern, and the LCL filter is what addresses this concern the best.