A case study of mitigating harmonics in electrical systems

The past decade has seen considerable changes in industrial as well as domestic loads in terms of demand and appliance electrical characteristics. Modern loads are considerably different to those found 10 to 15 years ago. Equipment which has become common place in most facilities including computer power supplies, solid state lighting ballasts, adjustable speed drives (ASD), and uninterruptible power supplies (UPS) are examples of non-linear loads. Even, Locomotives, few large traction motors often supplied by thyristor controlled choppers or PWM converters have high distortion due to harmonics injected. Therefore, it is necessary to evaluate and analyse the power quality issues due to various non-linear loads to give a clear picture of the current scenario so that future technologies can be accommodated in the distribution systems while coping with existing power quality issues.

From Power Quality point of view, the most important evolution in the non-linear loads has been the proliferation of single-phase drives powered by Switch Mode Power Supplies or other power electronic systems. The evolution of non-linear loads in terms of magnitude and characteristics into a significant distorting load and as such a potential source of significant power quality disturbances is worth of detailed study. Harmonic analysis of the distribution system is essential to study the behaviour of equipment connected in the non-sinusoidal system environment for designing and optimal location of filters.

Harmonic currents generated by these non-linear loads can cause problems in the power systems and the distribution transformers as they are vulnerable to overheating and premature failure. Most of the transformers are designed, manufactured and calibrated for using only sinusoidal shape current. Also, with their rated power within tens of MW, electric locomotives of metro substations are one of the biggest appliances in the electrical network. At the same time, they represent a potential source of huge interferences that can be transmitted into feeding network and irradiate into their surroundings.

This paper provides a detailed analysis in tackling non-sinusoidal loads with few case studies of a steel plant where special K-Factor transformer is analysed that has the potential to deliver large energy savings. A case study of harmonic analysis of a Metro rail power distribution system is also presented to understand the various types of harmonic filters available in market, selection and sizing of harmonic filter, suitable for Locomotives, Traction Motors.