Harmonics are prevalent in electrical distribution system where personal computers, printers, copiers, medical test equipment, adjustable speed drives, etc, are present in large numbers. Besides, harmonics also degrade the level of power quality and efficiency in a commercial building or industrial facility. This article highlights the techniques to reduce the effects of harmonics.
In electrical terms, harmonics are AC voltage or currents whose frequency is some integral multiple of the fundamental frequency. Personal computers and variable speed drives are the main causes of high harmonic content in the power supply. The existing or latent power quality problems derive from harmonics.
In addition, there is inherent electrical inefficiency in high harmonic environments. But high harmonic content need not result either in equipment breakdown or electrical inefficiency. The fundamental frequency (the lowest possible frequency in a complex wave) in India is 50 Hz. An integral multiple is simply a whole number, eg, 3 x 50 Hz = 150 Hz and 5 x 50 Hz = 250 Hz.
To avoid problems, one simple principle must be followed - the impedance of the electrical installation must be kept low at all frequencies both to reduce the potential for overheating and to minimise the magnitude of induced harmonic voltages.
Origin of harmonics
Non-linear load elements connected to an electrical system draw current in an abrupt fashion distorting current wave shapes and causing harmonic currents to flow throughout the power system. Power supplies for office machines are typically AC to DC converters (rectifiers), which are non-linear and draw current of high harmonic content from the source. Major sources of harmonic distortion include:
· Electronic switching power converters
· Adjustable-speed motor drives (AC and DC)
· Switch mode power supplies [SMPS)
· Uninterruptible power supplies (UPS)
· Electronic lighting ballasts
· Electronic process control equipment, PLCs, etc
· Personal computers and printers
· Reduced voltage motor controllers
· Solid state rectifiers
· Ferromagnetic devices
· Transformers operating near saturation level
· Ballasts (saturated iron core) of fluorescent lights
· Induction heating equipment
· Arcing devices
· Arc-discharge lighting, eg, fluorescent, HID
· Sodium and mercury vapour
· Electric arc furnaces
· Electric welding equipment
Effects of harmonics on equipment
The growing use of power electronic applications has increased the fraction of harmonic currents and voltages in building and utility networks. The purpose of this paper is to summarize the state-of-knowledge of the effects of power system harmonics on equipment.
Do not generate harmonics but provide network loops for possible resonant conditions. If the addition of the capacitors tunes the system to resonate near a harmonic frequency present in the load current or system voltage, large voltages or currents will be produced at that frequency.
Circuit breakers and fuses:
If the distortion results in a higher level of di/dt at zero crossing, load interruption can be more difficult than for a sinusoidal waveform. Fuses are inherently thermally actuated devices and, in most cases, are not affected by distorted current.
Skin effect and proximity effect alter the effective DC resistance of conductors. Abnormally, high currents in the neutrals of three- phase four-wire systems feeding single-phase electronic loads are also possible. The third harmonic neutral phase current can be 1.7 times the phase current.
Electronic equipment may be affected by multiple zero crossings caused by voltage distortion that interferes with electronic timing circuits. Distortion that flattens the waveform increases vulnerability to voltage sags. The high dv/dt association with voltage notching can couple through the power supply and falsely trigger components.
Modern RMS responding voltmeters and ammeters are relatively immune to the influences of waveform distortion. For an induction watt-hour meter, registration errors of as much as 20% are possible with highly distorted voltages and currents.
Every relay performs differently in the presence of waveform distortion. Even relays of the same type and model, from one manufacturer, may respond differently.
The primary harmonic distortion problem associated with rotation machines is rotor overheating.
The most common interference mechanism between power system harmonics and the telephone circuit is induction coupling by the power line magnetic field and the loop formed by the telephone conductor and ground.
The primary effect of harmonics on transformers is heating caused by additional losses generated by the harmonics. The limit on current distortion factor is 5 per cent. Methods are available to increase the limit on dating the transformer.
There are two major categories of effects of harmonics on equipment. In the first category, heating effects in power handling equipment such as motors and transformers reduce equipment-operating life. The second category is disruption that includes electronically controlled equipment. Of the two, the documentation for the former is substantial whereas documentation for the latter is deficient.
To determine a harmonic problem on the system:
· Check for evidence of overheated neutral conductors (typically, white insulation turning brown), particularly on three-phase, four-wire circuits, eg, 120/208 V circuits feeding PCs, laser printers. 277 V lighting circuits, etc
· The noise normally associated with the 50 Hz hum is different when harmonics are present. Resonance, vibration, humming of transformers and panel-boards often indicate harmonics on the system
· Frequent and often unexplained blown fuses on circuits containing adjustable frequency drives, rectifiers, UPSs, adjustable DC voltage drives, power factor capacitor banks and diode/thyristor controlled loads
· Malfunctioning electronic controls (current-wave zero crossing point has shifted and offset has affected the operating sequence)
· Poor picture quality on display monitors, VCRs, PCs, TVs, etc, due to voltage distortion
· Overheated motors, generators or transformers
· Premature tripping of peak sensing electronic trip CBs (conversely, if the peak is lower than normal, the breaker may fail to trip at all)
· Meter reading error - wattmeters, kWh, etc. Typically, on the high side penalizing the customer
· Higher than normal voltage drop (more than 4 or 5 V) between the neutral conductor to ground at 120 V receptacles often signifies harmonics presence or an unbalanced system
· Overloaded and, consequently, overheated neutral connected bus bars and connecting lugs
Minimising the effect of harmonics
Techniques used to reduce the effects of harmonics include:
· Over sizing the neutral conductors to 150-200 per cent of phase conductor current ratings
· Placing isolation transformers as close as physically possible to the load
· De-rating transformers, generators and motors
· The use of RMS sensing meters, relays and circuits breaker trip units
· Investigate and confirm all control units will operate properly with non-linear loads
· When practical, provide line filters to remove the harmonic loads from the source
For more efficient power
· Do not use the pipe as the ground. Install a ground wire conductor in each computer and electronic branch circuit. This provides for a true ground path return for electronic equipment thereby eliminating mechanical conduit connections as the only ground path
· Each computer connection or electronic branch circuit should have individual phase, neutral, and ground conductors. The use of shared neutral conductors as with multi-wire branch circuits is not recommended
· Install transient voltage surge suppression (TVSS) devices at the main electrical service panel to protect in from utility power problems and lightning strikes
· Install individual TVSS devices at each computer system or at any other sensitive electronic equipment load. This is inexpensive protection for power problems generated within the building electrical distribution system. Do not forget to protect copiers, printers, and fax machines
· Connect laser printers and heavy-duty copiers on individual 20 A branch circuits. Laser printers and heavy-duty copiers produce high current surges, which cause an increase in the neutral to ground voltage. This action can potentially damage other computer and electronic equipment if they are on the same branch circuit
· Ensure proper installation of grounding electrode to metal water main, building steel, and deep driven ground rod.
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|Posted : 8/29/2005|