Page 442 - Handbook of Electrical Engineering

HARMONIC VOLTAGES AND CURRENTS 431

which at the time of preparing this book was still in its original form. In addition the Reference 15
is used. When an oil company owns or operates a power system, which is independent of other
power systems such as a ‘national grid’, it may prepare its own specifications to cover the tolerable
levels of harmonic voltages and currents. Such specifications may be more or less as strict as the
recommendations made in References 14 and 15. In Reference 12 there is a comprehensive descrip-
tion of national standards that apply in various European countries, USA, Scandinavia, Australia
and New Zealand. Most of these standards refer to six-pulse and 12-pulse converters and applied in
1985 when the reference was published. Some but not all of these standards have been revised since
1985, e.g. IEEE519 was revised in 1997. The standards described in Reference 12 are not directly
comparable with each other because different criteria are used e.g. current in amps, current in percent,
kVA converter ratings, individual harmonics, odd and even harmonics, total harmonic distortion and
short-circuit rating. These criteria have been applied to public or ‘national grid’ power systems. For
smaller self-contained power systems such as those used in offshore platforms the criteria that use
actual current or kVA levels, rather than percentage levels, may prove to be too generous for the HV
parts of the system. The G5/3 document offers recommendations for both actual currents in amps and
voltages in percent. The recommendations based on percentage voltage are a popular choice in the oil
industry. Table 15.7 summarises these recommendations to cover typical voltages in the oil industry.
The design of filters to reduce or eliminate harmonics from the system connected upstream of
the source of harmonics is a specialised subject and the results will depend on many factors such as,
a) Proximity of the harmonic source to the source of main power, e.g. an HV converter connected
onto the main generation busbars of an oil and gas gathering plant.
b) The type of converter i.e. six or 12 pulse. (Drilling rigs usually have six-pulse converters.)
c) The number of converters that will be operating at the same time.
d) The likely variations in the fundamental frequency during typical operating conditions of the plant.
e) Whether there are long HV feeder cables, e.g. an offshore platform supplied by power from the
shore base or another platform some reasonable distance away. The cable capacitance may be
sufficient to accentuate the effect of one or more of the lower order harmonics, see Reference 16.
f) Power dissipation from the filter may be a significant factor if it is to be placed indoors in a
confined space.

Factors a) and d) are interrelated due to the scheduling for the number of generators that will be
needed to operate for a particular plant condition. Generators and motors can be represented by their
sub-transient impedance when harmonic studies are being carried out. These impedances and those of

Table 15.7. Recommended harmonic levels in relation to the system volt-
age level

Switchgear rated Total harmonic voltage Individual harmonic
voltage (volts) distortion (%) voltage distortion (%)
Odd Even
300 to 1000 5.0 4.0 2.0
1000 to 15,000 4.0 3.0 1.75
5000 to 40,000 3.5 2.5 1.5
40,000 to 80,000 3.0 2.0 1.0
80,000 to 132,000 1.5 1.0 0.5