Harmonics is defined as the content of the signal whose frequency is an integral multiple of the system frequency of the fundamentals. Harmonics current generated by any non-linear load flows from the load into the power system. These harmonic currents degrade the power system performance and reliability and can also cause safety problem. Harmonics need to be clearly located, sources identified and corrective measures taken to prevent them.
Electrical load is categorised under two categories
i. Linear load: Such load draws voltage and current in essentially sine wave shape but at varied phase shift (power factor). Example: resistors, inductors, capacitors and their combinations are classified as linear load. Linear loads have smooth, straight and predictable response.
ii. Non-linear load: Power supplies in non-linear load draw current in abrupt pulses rather than in smooth sinusoidal wave. It indicates distorted or suddenly changing response. Example-modern electronic/electrical equipments consisting of rectifying, charging /discharging and phase control circuits.
Harmonics: The distortion in a sinusoidal wave is generally defined in terms of various harmonics components. Harmonics are defined as the content of signal whose frequency is an integral multiple of the system frequency of the fundamental. Typical harmonics for a 50 Hz system (fundamental frequency) are the 5th (250 Hz), 7th (350 Hz), 9th (450 Hz).
The harmonics of a periodic wave can be represented by a Fourier series:
f(wt) = AO + A1coswt + A2 cos2wt + B1sinwt + B2 sin2wt + -------
f(wt) = Given non sinusoidal periodic wave form with angular velocity w = 2 Σ f
A0 = Const.
A1, A2, A3 ---------- An coefficient of cosine terms, nth is the order of harmonic.
B1, B2, B3, -------- Bn coefficient of sine terms, nth is the order of harmonic.
Effects of harmonics: Harmonics current generated by any non-linear load flows from the load into the power system. These harmonics currents degrade the power system performance and reliability and could also cause safety problem. Harmonics need to be clearly located, sources identified and corrective measures taken to prevent these problems. THD (Total Harmonic Distortion) can be computed as per IEE-519 standard as:
Where hn is the individual harmonics of nth order.
Source of harmonics:
(1) Transformers under no load and light loads (2) Saturated Reactors (3) Thyrister controlled motor drives (4) Arc Furnaces (5) Arc Welders (6) Conduction Furnaces (7) Gas discharging lighting-low pressure/ high pressure Sodium vapour lamps (8) High-pressure Mercury vapor lamps (9) CFL/fluorescent tube lights (10) Energy conservation devices e.g. soft starters, electronics ballast and fan regulators (11) Rectifiers (12) UPS (13) Static VAR compensator.
Harmonic effects on various components
i. Transformers: Harmonics in transformers cause an increase in the iron and copper losses. Voltage distortion increase losses due to hysteresis and eddy currents and causes overstressing of the insulation material used. The primary effect of power line harmonics in transformer is, thus the additional heat generated. Other problems include possible resonance between the transformer inductance and the system capacitance, thermal fatigue due to temperature cycling and possible core vibrations.
ii. Motor and generators: Harmonic voltage and current cause increased heating in rotating machines due to additional iron and copper losses at harmonic frequencies. This lowers the machine efficiency and affects the torque developed. The flow of harmonic currents in the stator induces current flow in the rotor. This results in rotor heating and pulsating or reduced torque. Rotor heating reduces the efficiency and life of the machinery whereas pulsating or reduced torque results in mechanical oscillation causing shaft fatigue and increased ageing of mechanical parts.
iii. Thyrister drives: AC variable frequency drives with thyrister converter when operated at slow speed, generally result in poor power factor.
iv. Power cable: Normal level of harmonics currents cause heating in cables. However, cables involved under system resonance condition may be subjected to voltage stress and corona, which can lead to insulation failure.
v. Metering equipments: In general, harmonics flowing in induction type metering equipment will generate additional coupling paths thereby increasing the speed of the disc and hence an apparent increase of costs.
vi. Switchgear and relay: Harmonics current increases heating and losses in switchgear there by lowering its normal current capacity and shortening the life due to voltage stress fuses require derating due to the heat generated by harmonics.
vii. Earthing system and computer performance: In a 3 phase and neutral system- when 3rd harmonics and multiples are expected, the neutral conductor size should be the same size as the phase conductor size.
Computer hanging up, loosing instructions, data or misbehaving can be as much attributed to poor quality of power. Eearthing of computer equipment should be independent and be fixed into the mains earthing at one point – preferably at the entry point only. Multipoint earthing introduces coupling to various other equipments.
viii. Communication network: The induction coupling between the AC power transmission lines containing harmonics and the neighbouring communication network causing high noise levels.
ix. Capacitor: Capacitors for power factor correction are always present in industrial installations and are worst affected if harmonics are present. Capacitors do not generate harmonics, but provide network loop for the possible resonance
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