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论文作者:留学生论文论文属性:短文 essay登出时间:2011-04-19编辑:点击率:20848
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关键词:Supply HarmonicsElectricity sinusoidal voltagesvoltages
having allowable harmonic content, but the extra heating typically reduces performance to
about 95% of that which would be experienced with pure fundamental sine waves [1]. It
must be stressed, however, that the above statements do not apply to the rating and
performance of electrical motors (induction or synchronous) fed from a variable speed
inverter when, depending on the type of variable speed system, the harmonic effects can be
very significant.
7.1.9 Telephone interference
One of the earliest expressed concerns about harmonics in the electricity supply system
was related to interference with telephone communications. This was particularly the case
when both telephone lines and power distribution line were mounted on common overhead
lines. For any given physical arrangement, the disturbance is a function of both the
amplitude and frequency of the harmonic components. The disturbance to the
communication system can be reduced by a number of means including minimising ground
return currents, filtering and other harmonic elimination methods.
S7: Supply Harmonics
112 EEE8044: Fundamentals
7.2 Total Harmonic Distortion
The most widely used measure or index of harmonic distortion of a power system voltage
is the Total Harmonic Distortion (THD) defined by
100%
square of the amplitude of the fundamental component
sumof the squares of the amplitudes of all harmonic components
THD = × (7.1)
or
1
50
n 2
2
n
V
V
THD
Σ=
= (7.2)
where Vn is the value of the nth harmonic voltage drop and V1 is the value of the
fundamental supply voltage.
The value of the THD is usually of interest at the point of common coupling (pcc) with
other users previously defined as the point in the public utility at which other user loads
are, or may be, connected. For example, in Figure 7.1, the pcc for users 1 and 2 is the 11
kV busbar and the pcc for users 3 and 4 is the 33 kV busbar and for customer 5, it is the
132 kV busbar.
132 kV
33 kV
11 kV
user 1 user 2
user 3 user 4
user 5
pcc
pcc
pcc
Fig. 7.1 Point of common coupling
S7: Supply Harmonics
113 EEE8044: Fundamentals
7.3 Standards and Harmonic Limits
A number of national and international standards and recommendations provide guidelines
on the amount of harmonics and harmonic distortion that can be produced or should be
allowed to flow into the power system. These standards usually specify how much of each
harmonic a customer is allowed to inject into the power distribution system as well as
setting limits for the total harmonic distortion that should be allowed anywhere on the
system.
Permitted levels of harmonics in the UK are defined by Engineering Recommendation
G5/4 which sets current and voltage harmonic levels that may exist for any one customer at
the point of common coupling to other users.
In the UK, Engineering Recommendation G5/4 “Limits for Harmonics in the United
Kingdom Electricity Supply System” has been used as the national standard for many
years. The method adopted in G5/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。