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impossible to define with any certainty the types of responses that protective relay devices

might exhibit in the presence of harmonics. The Power System Relay Committee of the

IEEE Power Engineering Society has produced a report entitled ‘Sine Wave Distortion on

Power Systems and the Impact on Protective Relaying’. The report states:

“Protective relays generally do not respond to any one identifiable parameter such as the

rms value of a primary quantity or the fundamental frequency component of that quantity.

As a related consideration, the performance of a relay to a range of single frequency inputs

is not an indication of how that relay will respond to a distorted wave containing those

frequencies. Superposition does not apply. Multi-input relays may be more unpredictable

than single input relays in the presence of wave distortion. Relay response under distorted

conditions may vary among relays having the same nominal fundamental frequency

characteristics, not only among different relay manufacturers, but also among different

vintages of relays from the same manufacturer.”

In general, distortion factors of 10-20% are required to cause problems in relay operation

[1].

7.1.7 Static converters

Static converters are usually the villains when it comes to harmonics and harmonic

distortion of the distribution network, being the source of most of these undesirable current

components. It is ironic, therefore, that static converters themselves may fall victim to

harmonics, either self-inflicted or more often than not as a result of similar equipment

connected in parallel on the same ac supply.

Diode bridge rectifiers are usually insensitive to the presence of harmonics, but other

common forms of controlled static converters such as thyristor bridges or sinusoidal

S7: Supply Harmonics

111 EEE8044: Fundamentals

current converters are much more susceptible to harmonics. Most converter control circuits

depend in one form or another on the characteristics of the ac supply voltages (such as zero

crossing points) for their correct operation. If the supply is severely distorted, the converter

may suffer misfiring, commutation failure or loss of control causing equipment failure or

even component destruction.

7.1.8 Electrical motors

Stator current harmonics increase the copper loss in the machine. Also, at harmonic

frequencies the skin effect becomes dominant, increasing the effective resistance of the

machine windings, especially rotor resistance. This extra heating must be taken into

account when choosing the frame size of a motor for a given application.

Machine torque pulsation is also produced as the result of the interaction of air gap

magnetic flux waves of different frequencies, which can cause or enhance machine

cogging (problems with smooth starting) and crawling (operation at high slip) in induction

machines. If the pulsating torque frequency is near the mechanical resonance frequency of

the system, severe hunting and shaft vibrations may occur.

The net effect of these harmonic effects is a reduction in efficiency and life span of the

machine. Neither of these is particularly significant when operating on distributio本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。