it is a simple calculation procedure to find the maximum length of the cable that can be
demonstrated in 126.96.36.199.
13.3.5 Earthing Rods and Grids
An essential aspect in the design of earthing systems for land-based plants in particular is
From the Worked Example 188.8.131.52 the combined impedance of the rotor and magnetising
branches is Zm2 = 0.00348 + j 0.5362 ohms. Adding the stator impedance gives,
Z1m2 = 0.00878 + j 0.7711 ohms
All the harmonics of th
against the ingress of water and solid particles.
14.1.2 Power Supply
Most power systems in the oil industry do not have variable speed drives and so the AC
a highly dependable and simple source of sinusoidal voltage and current. Little or no
and VHF and UHF radiation will penetrate to the cores of the cable. If it is anticipated that
electronic equipment in a plant may be influenced by VHF or UHF radiation, for example
local transmitting station, then the screens should be made of
nearby. The tabs may be on either the outer or the inner surface, depending on the type of
and its environment. Care must be taken to ensure that water and other liquids cannot pass
Casings and heavy-duty terminal boxes are
The fundamental coefficients from the 180, 120 and 60 waveforms are found to be in the
3:1 respectively. The fundamental coefficient of the supply current is double the magnitude
of the 120 waveform coefficient, which is the desired re
The design of filters to reduce or eliminate harmonics from the system connected upstream
the source of harmonics is a specialised subject and the results will depend on many factors
a) Proximity of the harmonic source to the source of main po
b1 = sin(2 + 2u) sin 2 2u
2[cos cos(u + )]
where and u are in radians.
c1 = _a1
2 + b1
cos1 = a1
u = cos1 _R 3Xc
R + 3Xc
_ radians (15.7)
The fundamental components of the rms current I in the phases of the secon
anu = 0
ia(t) = imax
bju sin jt
It is found from the integration that all even harmonics and those multiples of three are not
present in the waveform. Hence n has the following value
8. Enclosure over-temperatures X
9. Control system faults X X
10. Harmonic filter faults X X
11. Single-phase operation of the
12. Overcurrent of the motor X X X
13. Winding temperature of the
14. Winding temperature of power
0.798 = +236.8 amps
Ii = 761.1
1.834 = 544.2 amps
I = 761.1
(0.76802 + 1.8342)1/2
= 593.46 amps per phase
From (15.13), (15.14) and (15.15) the volt-amperes at the bridge AC terminals are,
Ssec = Psec + jQsec
Psec = 3 346.0 761.1
tape is also frequently used. Paired, tripled and quadrupled conductors are often screened
with similar tapes. However, all these various layers of screening are not very effective
frequency interference from sources such as adjacent power cab
Therefore the magnitude of all the harmonics decrease as u increases, which is a reasonable
expectation since the waveform more closely resembles a sine wave.
The magnitude of the sum of the four cosine terms in (15.18) is 2
3 for all values of k in
29 1.85 89.88 1.78 89.88 0.07 89.96 5.74 0.05
31 1.62 89.89 1.56 89.88 0.06 89.96 5.37 0.04
If the above calculations are made for all the active harmonics then their results can be
and the waveforms synthesised. Table 15.4 summarises the results.
The pump unit must be designed to operate at very onerous levels of pressure, temperature
and in the presence of contaminates such as sand, acids etc. Moreover, the unit must be
lowering into a well-bore which can
stray signals by interference from nearby sources. These stray signals will be called noise
and they occur due to several different forms of coupling: Common circuit conduction.
Electrostatic or capacitive coupling.
Electromagnetic or mutua
Let bn be denoted as bn120 for use in sub-section 15.3.4.
The lowest harmonic present is the fifth.
414 HANDBOOK OF ELECTRICAL ENGINEERING
The value of the fundamental coefficient b1 is,
b1 = 1
15.3.2 Simplified Commutation Delay
1. Albert Kloss, A basic guide to power elements. John Wiley & Sons (1984). ISBN 0 471-90432-5
2. W. Shepherd and L. N. Hully, Power electronics and motor control. Cambridge University Press
ISBN 0 521-31283-3
3. I. K. Dortort, Extended
184.108.40.206 Active and reactive power
The rectifying elements of the bridge are assumed to be free of ohmic power losses.
power input to the DC motor must be equal to the AC power input to the bridge. Hence the
the active power in each p
_ 1_ ohms (13.5)
Where c is the back fill resistivity in ohm-metres
d is the diameter of the back fill or casing in metres.
This equation can also applied to reinforced concrete in which a steel rod is encased. A
rectangular strip of width () bur
of a controlled firing system for the thyristor gates, and in some cases the application of
commutation circuits, see Figure 15.1.
The commutation processes for Mode 1 operation of delay and current transfer are
overhead line compared with that of the earth resistance path back to the source. The
required for determining the fault current and its diverted amounts are shown in Appendix H
of an example, and Figure 13.12.
Figure 13.12 Earthing ci
jM3c Ro I3
Rin + Rc + Ro + Re + jLc
Let the core loop resistance be called Rcc :Rcc = Rin + Rc + Ro + Re ohms (13.9)
Note that the components R3 and L3 of the source are not included in this result because the
interference is a current source
Assume the stator resistance to be constant.
Table 15.3. Peak values of harmonic
Peak value and sign of
the component voltage
Modern fast-acting micro-computers are capable of storing and manipulating a reasonably
detailed mathematical model of the motor. It is therefore possible to compute the model in
with the actual motor and compare the computed variables with those
come under the general heading of pole-changing motors. The method just outlined is
squirrel-cage motors but not to wound rotor motors.
In a multi-pole motor for eight-pole operation, the adjacent poles change in polarity from
North to South ar
added resistance was one of the most common and simplest methods of speed control and
is used for
motors up to 10 MW. The main disadvantage is that the resistance bank is wasteful of
the removal of the heat produced can prove difficult. The st
Let M = M3s M3c
And Msc _ M3s or M3c because of the relative dimensions and separation distances.
The denominator of (13.12) and (13.13) becomes:a12a21 a11a22 = RssRcc + j(RccLs + RssLc) + 2(Ls(Ls Lc)
In which the extreme right-hand term is very small in
110 V or 10 A 300
240 V or 50 A 450
415 V or 500 A 600
3300 V to 33,000 V 1000
Currents above 200 A 1000
EARTHING AND SCREENING 375
Figure 13.13 Mutual coupling between spaced out cables.
In these examples the situation of interest is a combination of a s
of the variable speed drive, see also sub-section 15.1.
The most frequently used reference document based on European practice that makes
on the levels of harmonics that can be tolerated in LV and HV systems is Reference 14,
430 HANDBOOK O
= IdR + Em (15.1)
Where R is the DC circuit resistance.
Em is the emf in the motor armature.
Xc = 2Xl is the commutating reactance.
408 HANDBOOK OF ELECTRICAL ENGINEERING
An alternative expression for Vd in terms of the commutation angle u, is,
(1.155) for n = 1, 5, 7, 11, 13, etc.
And = 4
(2.309) for n = 3, 9, 15, 21 etc.
The rms value of the fundamental phase-to-neutral voltage is 415/
3 = 239.6 volts.
squirrel-cage induction motors.
Examples in this sub-group are: Star-delta stator winding.
Pole-changing motors, e.g. PAM and NS motors.
Special motors that have connections made to their rotor windings.
References 4 and 5 give descriptions of the PAM a
line current. See Reference 12, Chapter 3 which emphasises this aspect. The controlled firing
delta-star bridge Tu cancels the 30 degree phase shift of the transformer. From the Fourier
point of view this can be achieved by adding a + 30 p