Published by Emerich Energy Private Limited
Power quality is of prime importance in deciding the efficiency of any motor.
Some of the critical parameters of power quality are
• Voltage Unbalance
• Voltage Fluctuations
In an alternating current (AC) system, the voltage potential and the current through the load circuit are described in frequency and amplitude. The current frequency will be identical to the frequency of the voltage as long as the load resistance/impedance does not change. In a linear load, like a resistor, capacitor or inductor, current and voltage will have the same frequency. As long as the characteristics of the load components do not change, the frequency component of the current will not change. When we deal with non-linear loads such as switching power supplies, transformers which saturate, capacitors which charge to the peak of the supply voltage, and converters used in drives, the characteristics of the load are dynamic. As the amplitude of the voltage changes and the load impedance changes, the frequency of the current will change. That changing current and resulting complex waveform result from these load changes.
Harmonics are voltage and current frequencies riding on top of the normal sinusoidal voltage and current waveforms. Usually, these harmonic frequencies are in multiples of the fundamental frequency, which is 50 hertz (Hz). Harmonics are created by these “switching loads” (also called “nonlinear loads,” because current does not vary smoothly with voltage as it does with simple resistive and reactive loads). Each time the current is switched on and off, a current pulse is created. The resulting pulsed waveform is made up of a spectrum of harmonic frequencies, including the 50Hz fundamental and multiples of it. The higher-frequency waveforms collectively referred to as total harmonic distortion (THD), perform no useful work and can be a significant nuisance. The operation of nonlinear loads causes the distorted current, which is path-dependent; the effect of current distortion on loads within a facility is minimal. Therefore, harmonic currents can’t flow into equipment other than the nonlinear loads that caused them. However, the effect of current distortion on distribution systems can be serious, primarily because of the increased current flowing in the system. Therefore current harmonics cause increased losses in the customer and utility power system components.
Sources of harmonics Following are some of the non-linear loads which generate harmonics:
• Static Power Converters and rectifiers, which are used in UPS, Battery chargers, etc.
• Arc Furnaces
• Power Electronics for motor controls (AC /DC Drives)
• Television receivers
• Saturated Transformers
• Fluorescent Lighting
• Telecommunication equipment
Effects of harmonics
The harmonics have a multifold effect on various network elements present in a system. Whenever a harmonic current flows through equipment,
• It causes additional losses due to its higher frequency, devices such as motors, transformers, etc. which have a laminated core have higher losses due to the higher frequency of the harmonic current.
• In cables, the harmonic current trend to flow through the outer skin of the conductor due to the skin effect and results in the heating of these conductors.
• Harmonics can cause nuisance tripping of the relays and failure of capacitors installed in the distribution system for power factor improvement
• Certain harmonic currents (e.g. 5th harmonic) have the reverse phase sequence which means any electro-mechanical device used for metering will not register true values. Similarly, in a polluted network, a normal induction motor may not develop the necessary torque because of harmonic current generating torque in the reverse direction
• Higher order harmonics interfere with the telecommunication systems also. Whenever a telephone line runs parallel to a power line having harmonics, noise is introduced in the telephone line. This phenomenon is known as telephonic interference
• A highly polluted voltage may lead to the mal-operation of devices such as thyristors, the operation of which depend on the zero crossing of the voltage waveform. This may result in commutation failure in thyristors
• A high harmonic content also results in a low power factor. The angle between the fundamental component of current and voltage gives the Displacement Power Factor, whereas, the same between the voltage and RMS current (fundamental and harmonic) gives the total Power Factor. In a linear load, the P.F. and D.P.F. are the same, whereas, for the loads which generate a lot of harmonics, the P.F. is much lower than the D.P.F.
• Some of the harmonic currents which are zero sequence currents (3rd harmonic current) tend to flow in the neutral in a 3-phase, 4-wire system. In most of the domestic and commercial loads, which are non–linear in nature generate a substantial amount of 3rd harmonic current, the neutral conductor gets overheated and may lead to melting of the same. It has been observed that in extreme cases, the neutral current can exceed 1.5 times the normal line current
• The harmonic current affects the generator also, as most of the big generators operate at maximum capacity and they do not have the excessive margin to accommodate heating losses resulting due to the flow of harmonic current into it. All such heating losses result in the deterioration of insulation used in electrical equipment.