Harmonic frequencies in electric railway power systems affect the quality of power transmission in the system. Non-linear loads and power-electronic energy conversion cause harmonics. They result in power quality issues such as overheating of equipment and conductors, quicker aging of insulation systems, and overall energy reduction.
In this research, the effects of the use of single-tuned passive filters on harmonic mitigation were studied. This was done by simulating and studying the impact of harmonics on the power electric substation (ESS) of the city line Airport Rail Link and comparing them to the IEEE 519-2014 standard.
The ARL power system uses 1 x 25 kV, 50 Hz ESS and it consists of ESS and overhead catenary systems (OCS). The effects of harmonics were studied by calculating the Total Harmonic Distortion of voltage (THDv) and current (THDi). The ESS used to study this phenomenon is the one located on the route between Suvarnabhumi Airport and Bangkok’s center, 8 kilometers from Phaya Thai station.
After establishing the railway system and methodology employed, calculations of the train’s movement, forces acting on the train, and power flow in the AC electric train systems were made. To calculate the train’s movement, Newton’s second law was used. The forces opposing the train’s movements were summarized and calculated using Davis’s equation. Power flow in the electric train system was then calculated using the current injection method. Once the overhead catenary system was computed, the Total Harmonic Distortion (THD) was calculated.
Both the voltage harmonic (THDv) and current harmonic (THDi) were considered and compared with the IEEE 519-2014 standard. This harmonic calculation resulted in the conclusion that specific harmonics must be largely reduced while the reduction in the superior order harmonics must be kept to a minimum to keep the distortion within the standard limits. Passive filters are employed for this purpose.
For this study, in particular, Single Tune Harmonic Filters were applied. STF is made up of a capacitor, a reactor, and a resistor. They are connected parallel to the non-linear load, allowing harmonics at a specified frequency to pass through low-impedance circuits and the other harmonic currents to be absorbed.
THDv and THDi were once again calculated, and the filtering effect of the STF was observed and studied by comparing it with the IEEE 519-2014 standard. The study concluded that the characteristics of these harmonics were different. The THDi at the 35th to 50th harmonics was exceeding the standard. After studying the 3rd, 5th, 7th, and 11th harmonic, it was found that the 11th could be used for filtering as it kept THDi within standard limits.