HBFEM Used in Renewable Energy Systems and Microgrids

Harmonics in Renewable Energy Systems and Microgrids

The uses of nonlinear loads connected to electric power systems include static power converters, arc discharge devices, saturated magnetic devices and, to a lesser degree, rotating machines. Static converters of electric power are the largest nonlinear loads, and are used in industry for a variety of purposes, such as electrochemical power supplies, adjustable speed drives, uninterruptible power supplies, the EV charging system and vehicle to grid (V2G), and energy storage system used in Renewable Energy Systems and Microgrid. The harmonic currents injected by DC/AC inverters (power electronic devices), as used for renewable energy systems and microgrids, might be able to cause harmonics. The magnitude and the order depend on the technology of the inverter and mode of operation, as described in IEEE Std. 519-2014 [20].

Nonlinear loads change the sinusoidal nature of the AC power current (and consequently the AC voltage drop), thereby resulting in the flow of harmonic currents in the AC power system that can cause interference with communication circuits and other types of equipment. These harmonic currents also lead to increased losses and heating in numerous electromagnetic devices (motors, transformers, etc.). When reactive power compensation is used, in the form of power factor improvement capacitors, resonant conditions can occur, which may result in high levels of harmonic voltage and current distortion when the resonant condition occurs at a harmonic associated with nonlinear loads.

In order to analyze harmonic problems, the network models that are often used for system harmonic analyses in existing works include impedance of the lines and cables,

Figure 6.24 Block diagram of harmonics current flowing through a transformer in the microgrid with nonlinear load and DG and DS

transformers, and filter components. The inverter is modeled as either a simple harmonic source, or a harmonic source with output impedance in the form of a Thevenin or Norton equivalent circuit [7,14]. However, as discussed in Chapter 2.4 (in the microgrid with a grid connected case), the current harmonics generated by power electronics and drives will be injected into the main grid through a PCC to the transformer, as shown in Figure 6.24. The voltage waveform applied to the transformer at the PCC, and current waveforms from the inverter, include a larger number of harmonics if poor DC/AC inverters are used.

Generally, current harmonics and voltage-frequency imbalances increase losses in the LV transformer and AC power lines. The main power quality parameters, including voltage, frequency, and harmonic distortion, have been improved by adopting Statcom, Static VAR Compensator (SVC) and the Shunt Active Power Filter (SAPF) in [23]. The voltage and frequency control mode is adopted in the microgrid under grid tied or islanded, in order to regulate voltage and frequency in the microgrid [24, 25].