Effect of Harmonics on electrical system
Defines harmonic as a sinusoidal component of a periodic wave or quantity (for example voltage or current) having a frequency that is an integral multiple of the fundamental frequency.For example on 60Hz supply, the 3rd harmonic is 3 x 60Hz (=180Hz); the 5th harmonic is 5 x 60Hz (=300Hz) and so on....When all harmonic currents are added to the fundamental a waveform known as complex wave is formed.
Types of equipment that generate harmonics:
when a nonlinear load (for example computers, variable frequency drives, discharge lighting, Static power converters etc) draws distorted (non-sinusoidal) current from the supply, which distorted current passes through all of the impedance between the load and power source. The associated harmonic currents passing through the system impedance cause voltage drops for each harmonic frequency based on Ohm’s Law The vector sum of all the individual voltage drops results in total voltage distortion, the magnitude of which depends on the system impedance, available system fault current levels and the levels of harmonic currents at each harmonic frequency.
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| Voltage and current wave form non-linear load |
How harmonic are generated in electrical power distribution system:
Static power converters are the equipment that utilize power semiconductor devices for power conversion from AC to DC, DC to DC, DC to AC and AC to AC; and constitute the largest nonlinear loads connected to the electric power systems. These converters are used for various purposes in the industry, such as adjustable speed (or variable frequency) drives, uninterruptable power supplies, switch-mode power supplies etc. These static power converters used in a variety of applications draw non-linear (i.e. non-sinusoidal) currents and distort the supply voltage waveform at the point of common coupling (PCC)Effect of Harmonics on generator, transformer, induction motor, cables, circuit breaker and fuses are given below
1) What is effect of Harmonics on Generators
In comparison with utility power supplies, the effects of harmonic voltages and harmonic currents are significantly more pronounced on generators (esp. stand-alone generators used a back-up or those on the ships or used in marine applications) due to their source impedance being typically three to four times that of utility transformers. The major impact of voltage and current harmonics is to increase the machine heating due to increased iron losses, and copper losses, since both are frequency dependent and increase with increased harmonics.To reduce this effect of harmonic heating, the generators supplying nonlinear loads are required to be derated. In addition, the presence of harmonic sequence components with nonlinear loading causes localized heating and torque pulsations with torsional vibrations.
In comparison with utility power supplies, the effects of harmonic voltages and harmonic currents are significantly more pronounced on generators (esp. stand-alone generators used a back-up or those on the ships or used in marine applications) due to their source impedance being typically three to four times that of utility transformers. The major impact of voltage and current harmonics is to increase the machine heating due to increased iron losses, and copper losses, since both are frequency dependent and increase with increased harmonics.To reduce this effect of harmonic heating, the generators supplying nonlinear loads are required to be derated. In addition, the presence of harmonic sequence components with nonlinear loading causes localized heating and torque pulsations with torsional vibrations.
2) Effect of Harmonics on Transformers
The effect of harmonic currents at harmonic frequencies causes increase in core losses due to increased iron losses (i.e., eddy currents and hysteresis) in transformers. In addition, increased copper losses and stray flux losses result in additional heating, and winding insulation stresses,especially if high levels of dv/dt (i.e.rate of rise of voltage) are present.Temperature cycling and possible resonance between transformer winding inductance and supply capacitance can also cause additional losses.The small laminated core vibrations are increased due to the presence of harmonic frequencies, which can appear as an additional audible noise.The increased rms current due to harmonics will increase the I 2R (copper) losses. The distribution transformers used in four-wire (i.e., three-phase and neutral) distribution systems have typically a delta-wye configuration. Due to delta connected primary, the Triplen (i.e. 3rd, 9th, 15th…) harmonic currents cannot propagate downstream but circulate in the primary delta winding of the transformer causing localized overheating. With linear loading, the three-phase currents will cancel out in the neutral conductor. However, when nonlinear loads are being supplied, the triplen harmonics in the phase currents do not cancel out, but instead add cumulatively in the neutral conductor at a frequency of predominately 180 Hz (3rd harmonic), overheating the transformers and occasionally causing overheating and burning of neutral conductors. Typically, the uses of appropriate “K factor”
rated units are recommended for non-linear loads.
The effect of harmonic currents at harmonic frequencies causes increase in core losses due to increased iron losses (i.e., eddy currents and hysteresis) in transformers. In addition, increased copper losses and stray flux losses result in additional heating, and winding insulation stresses,especially if high levels of dv/dt (i.e.rate of rise of voltage) are present.Temperature cycling and possible resonance between transformer winding inductance and supply capacitance can also cause additional losses.The small laminated core vibrations are increased due to the presence of harmonic frequencies, which can appear as an additional audible noise.The increased rms current due to harmonics will increase the I 2R (copper) losses. The distribution transformers used in four-wire (i.e., three-phase and neutral) distribution systems have typically a delta-wye configuration. Due to delta connected primary, the Triplen (i.e. 3rd, 9th, 15th…) harmonic currents cannot propagate downstream but circulate in the primary delta winding of the transformer causing localized overheating. With linear loading, the three-phase currents will cancel out in the neutral conductor. However, when nonlinear loads are being supplied, the triplen harmonics in the phase currents do not cancel out, but instead add cumulatively in the neutral conductor at a frequency of predominately 180 Hz (3rd harmonic), overheating the transformers and occasionally causing overheating and burning of neutral conductors. Typically, the uses of appropriate “K factor”
rated units are recommended for non-linear loads.
3) How harmonics effect on Induction Motors
Harmonics distortion raises the losses in AC induction motors in a similar way as in transformers and cause increased heating, due to additional copper losses and iron losses (eddy current and hysteresis losses) in the stator winding, rotor circuit and rotor laminations.These losses are further compounded by skin effect, especially at frequencies above 300 Hz.Leakage magnetic fields caused by harmonic currents in the stator and rotor end windings produce additional stray frequency eddy current dependent losses. Substantial iron losses can also be produced in induction motors with skewed rotors due to high-frequency-induced currents and rapid flux changes (i.e., due to hysteresis) in the stator and rotor.Excessive heating can degrade the bearing lubrication and result in bearing collapse. Harmonic currents also can result in bearing currents, which can be however prevented
by the use of an insulated bearing, a very common practice used in AC variable frequency drive-fed AC motors. Overheating imposes significant limits on the effective life of an induction motor. For every 10°C rise in temperature above rated temperature, the life of motor insulation may be reduced by as much as 50%. Squirrel cage rotors can normally withstand higher temperature levels compared to wound rotors. The motor windings, especially if insulation is class B or below, are also susceptible to damage due high levels of dv/dt (i.e., rate of rise of voltage) such as those attributed to line notching and associated ringing due to the flow of harmonic currents.Harmonic sequence components also adversely affect induction motors. Positive sequence components (i.e., 7th, 13th, 19th…) will assist torque production, whereas the negative sequence components (5th, 11th, 17th…) will act against the direction of rotation resulting in torque pulsations. Zero sequence components (i.e., triplen harmonics) are stationary and do not rotate, therefore, any harmonic energy associated with them is dissipated as heat. The magnitude of torque pulsations generated due to these harmonic sequence
components can be significant and cause shaft torsional vibration problems.
Harmonics distortion raises the losses in AC induction motors in a similar way as in transformers and cause increased heating, due to additional copper losses and iron losses (eddy current and hysteresis losses) in the stator winding, rotor circuit and rotor laminations.These losses are further compounded by skin effect, especially at frequencies above 300 Hz.Leakage magnetic fields caused by harmonic currents in the stator and rotor end windings produce additional stray frequency eddy current dependent losses. Substantial iron losses can also be produced in induction motors with skewed rotors due to high-frequency-induced currents and rapid flux changes (i.e., due to hysteresis) in the stator and rotor.Excessive heating can degrade the bearing lubrication and result in bearing collapse. Harmonic currents also can result in bearing currents, which can be however prevented
by the use of an insulated bearing, a very common practice used in AC variable frequency drive-fed AC motors. Overheating imposes significant limits on the effective life of an induction motor. For every 10°C rise in temperature above rated temperature, the life of motor insulation may be reduced by as much as 50%. Squirrel cage rotors can normally withstand higher temperature levels compared to wound rotors. The motor windings, especially if insulation is class B or below, are also susceptible to damage due high levels of dv/dt (i.e., rate of rise of voltage) such as those attributed to line notching and associated ringing due to the flow of harmonic currents.Harmonic sequence components also adversely affect induction motors. Positive sequence components (i.e., 7th, 13th, 19th…) will assist torque production, whereas the negative sequence components (5th, 11th, 17th…) will act against the direction of rotation resulting in torque pulsations. Zero sequence components (i.e., triplen harmonics) are stationary and do not rotate, therefore, any harmonic energy associated with them is dissipated as heat. The magnitude of torque pulsations generated due to these harmonic sequence
components can be significant and cause shaft torsional vibration problems.
4) Cables
Cable losses, dissipated as heat, are substantially increased when carrying harmonic currents due to elevated I 2R losses, the cable resistance, R, determined by its DC value plus skin and proximity effect. The resistance of a conductor is dependent on the frequency of the current being carried. Skin effect is a phenomenon whereby current tends to flow near the surface of a conductor where the impedance is least. An analogous phenomenon, proximity effect, is due to the mutual inductance of conductors arranged closely parallel to one another. Both of these effects are dependent upon conductor size, frequency, resistivity and the permeability of the conductor material. At fundamental frequencies, the skin effect and proximity effects are usually negligible, at least for smaller conductors. The associated losses due to changes in resistance, however, can increase significantly with frequency, adding to the overall I 2R losses.
Cable losses, dissipated as heat, are substantially increased when carrying harmonic currents due to elevated I 2R losses, the cable resistance, R, determined by its DC value plus skin and proximity effect. The resistance of a conductor is dependent on the frequency of the current being carried. Skin effect is a phenomenon whereby current tends to flow near the surface of a conductor where the impedance is least. An analogous phenomenon, proximity effect, is due to the mutual inductance of conductors arranged closely parallel to one another. Both of these effects are dependent upon conductor size, frequency, resistivity and the permeability of the conductor material. At fundamental frequencies, the skin effect and proximity effects are usually negligible, at least for smaller conductors. The associated losses due to changes in resistance, however, can increase significantly with frequency, adding to the overall I 2R losses.
5) Circuit Breakers and Fuses
The vast majority of low voltage thermal-magnetic type circuit breakers utilize bi-metallic trip mechanisms which respond to the heating effect of the rms current. In the presence of nonlinear loads, the rms value of current will be higher than for linear loads of same power. Therefore, unless the current trip level is adjusted accordingly, the breaker may trip prematurely while carrying nonlinear current. Circuit breakers are designed to interrupt the current at a zero crossover. On highly distorted supplies which may contain line notching and/or ringing, spurious “zero crossovers” may cause premature interruption of circuit breakers before they can operate correctly in the event of an overload or fault. However, in the case of a short circuit current, the magnitude of the harmonic current will be very minor in comparison to the fault current.
The vast majority of low voltage thermal-magnetic type circuit breakers utilize bi-metallic trip mechanisms which respond to the heating effect of the rms current. In the presence of nonlinear loads, the rms value of current will be higher than for linear loads of same power. Therefore, unless the current trip level is adjusted accordingly, the breaker may trip prematurely while carrying nonlinear current. Circuit breakers are designed to interrupt the current at a zero crossover. On highly distorted supplies which may contain line notching and/or ringing, spurious “zero crossovers” may cause premature interruption of circuit breakers before they can operate correctly in the event of an overload or fault. However, in the case of a short circuit current, the magnitude of the harmonic current will be very minor in comparison to the fault current.
Fuse ruptures under over current or short-circuit conditions is based on the heating effect of the rms current according to the respective I 2t characteristic. The higher the rms current, the faster the fuse will operate. On nonlinear loads, the rms current will be higher than for similarly-rated linear loads, therefore fuse derating may be necessary to prevent premature opening. In addition, fuses at harmonic frequencies, suffer from skin effect and more importantly, proximity effect, resulting in non-uniform current distribution across the fuse elements, placing additional thermal stress on the device.
6) Lighting
One noticeable effect on lighting is the phenomenon of “flicker” (i.e., repeated fluctuations in light intensity). Lighting is highly sensitive to rms voltage changes; even a slight deviation (of the order of 0.25%) is perceptible to the human eye in some types of lamps.Superimposed interharmonic voltages in the supply voltage are a significant cause of light flicker in both incandescent and fluorescent lamps.
One noticeable effect on lighting is the phenomenon of “flicker” (i.e., repeated fluctuations in light intensity). Lighting is highly sensitive to rms voltage changes; even a slight deviation (of the order of 0.25%) is perceptible to the human eye in some types of lamps.Superimposed interharmonic voltages in the supply voltage are a significant cause of light flicker in both incandescent and fluorescent lamps.
Other negative effects of harmonics
a) Effect of harmonics on power factor correction capacitors:
Power factor correction capacitors are generally installed in industrial plants and commercial buildings. Fluorescent lighting used in these facilities also normally has capacitors fitted internally to improve the individual light fitting’s own power factor. The harmonic currents can interact with these capacitances and system inductances, and occasionally excite parallel resonance which can over heat, disrupt and/or damage the plant and equipment.
b) Effect of harmonics on on power cable:
Power cables carrying harmonic loads act to introduce EMI (electromagnetic interference) in adjacent signal or control cables via conducted and radiated emissions. This “EMI noise” has a detrimental effect on telephones, televisions, radios, computers, control systems and other types of equipment. Correct procedures with regard to grounding and segregation within enclosures and in
external wiring systems must be adopted to minimize EMI.
external wiring systems must be adopted to minimize EMI.
c) Effect of harmonics on telemetry, protection:
Any telemetry, protection or other equipment which relies on conventional measurement techniques or the heating effect of current will not operate correctly in the presence of nonlinear loads. The consequences of under measure can be significant; overloaded cables may go undetected with the risk of catching fire. Busbars and cables may prematurely age. Fuses and circuit breakers will not offer the expected level of protection. It is therefore important that only instruments based on true rms techniques be used on power systems supplying nonlinear loads.
d) Effect of harmonics on installations:
At the installations where power conductors carrying nonlinear loads and internal telephone signal cable are run in parallel, it is likely that voltages will be induced in the telephone cables. The frequency range, 540 Hz to 1200 Hz (9th harmonic to 20th harmonic at 60 Hz fundamental) can be troublesome.
e) There is also the possibility of both conducted and radiated interference above normal harmonic frequencies with telephone systems and other equipment due to variable speed drives and other nonlinear loads, especially at high carrier frequencies. EMI filters at the inputs may have to be installed on drives and other equipment to minimize the possibility of inference.
f) Effect of harmonics on meters,measurement euipments:
Conventional meters are normally designed to read sinusoidal-based quantities. Nonlinear voltages and currents impressed on these types of meters introduce errors into the measurement circuits which result in false readings.
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