Page 782 - Industrial Power Engineering and Applications Handbook

Power capacitors. behaviour, switching and improvement of power factor 231739

line from the systcm or bq preventing tripping ot a healthy line through hlocking \ignal\ during a \\.\tern di\turhancr. hence
retaining continuity of upp ply of the healthy lines on ir \y\tem disturbance as fw. a\ po\\ihle.
Carrier relaying permih high-speed clearing 01. faults.
For protection \\here the direction of the current 01- thc phase angle of the two end\ of the line are coiiipni-ctl to tlccide ~hctliei-
there i\ ii fa~ilt on a particular line.
The tran\mi\.;ion of data to the load dispatch centre\ is conducted at different frequencies. The ciii-rier ti-equenq I\ dccidcd hy
the user and may fall in the range 30 500 kHi. A typical communication and relaying ichcmc i\ illu\trnted in Fipire 23.9tb). For
more detail\ on the PLCC. refer to the further reading at the end of the chapter.
,A PLCC \hould ensure a good quality of speech and tranjmission of vital data at ii carrier frequency for cIc;ir and rin;rnibtfiiiou\
communication between tu0 \tation\. It should also avoid falw signalling so that
Higli-frcqueiicy dt~tnrbonccs ai-e ii~t traiisfei-1-cd to the carrier equipmcnt thrvugh the coupling uqutpnicnl.
Di\turhances do not mutilate the tfirnsfer oi' vital data or send el-roneow \igiials that may lead to ni~rlfLtiicti~~titii~ or \(\ITlr I itiil
relay\ or prevent them from taking timely corrccti\e action.
The coupling of ii PLCC with the main lines i\ carried out through line traps. and coupling capacitor\ or CL'T. 4 C\ 7' I\ u\cd
when it IS also required lor measurement and protection. For detail\ on a CVT refer to Section 1.5.1.1.
The PLCC ciin he connccted between a phn\e and the ground ofthe tran\niission line ihelf. the ground forming the return pnth. ll\e
ot only one conductor saves on cost and makes the a hole syvtem economical. But this arrangement i\ not reliable duu to dependence
on a single conductor which during a problem or fiult on this phase may render the whole communication sy\tcm inoperative. Good
practice i\ therefore to connect the PLCC between any two phases.A typical nelaork is shown in Figure 23.O(b). U\e ofdiyitnl PLCC
quiprnent can provide :I more reliable communication sy\tem and more speech and data communication ch;inncl\.
Optical fibre cables (OFCs)
Thc optical fibre conimunication system employ\ the latest technology in the held of coniniunication dnd data trnn\mi\sioii ml
ireplace$ the use of conventional copper cables and ovei-head lines to underground lihre optical COIIIIII~IIIIC~~~~OI~ \!,htcmi. The
principle of thi\ technique i\ that the transmission of light signals by internal reflection\ through transparent fitires 01 :lass. cm be
carried over long distances uithout losing clarity or strength.
The optical fibre i\ an extremely pure core of glass fibre. surrounded by ir cladding to contain the light beam\ within the core.
Metallic shedding ofnon-magnetic material is aI\o provided over the gin\\ fibre to protect the cable from damage dut-ing trm\portiition
;ind handling.
The electrical vgnals (pulses) of the iiudio wave\ are converted into light pulses optical energy through trmsducm This nptic;il
energy i\ then ainied At cine end ofthe thin arid transparent optical fihre. The thickness of the fihre 19 even less than ;I human hair. \:I>~
up to 25 pm. The corc of tihre glass transforms the optical (light) energy into a jingle wabe ( I330 x IO" or 15.50 x I Of' kin wavelength)
of intense light. Thi\ wa\e posscsscs much iiiioie capacity fur cat-rying information than coppcr cable\. At the recciviny cnd :I decoder
is used to convet-t the optical energy back to the original audio wa~es. A hunch of optical fibre cables can transmit nitIlion\ of ;iudio
\iynals at the sime time. each fibre being capable of transmitting as much information as a thick copper cable with ii number of core\.
Thi\ technique has completely transformed traditional overhead communication systems. As there is no u\r of coppet- or
clcctricity in the optical fihre the optical waves are not subject to any electrical interference a\ di\cussed aha\ c. Corning Glas\
Work\. USA. were the fir\^ to produce \uch cable\ in 1970.

D Influence of harmonics on other electrical and and overhead lines etc.. connected on the systcm due
electronic circuits to the higher effective current (equation (23.2)).
2 The electromagnetic relays, which have a magnetic
core and are matched to the fundamental frequency,
By the hririiioiiic idttigrs may be sensitive to such disturbances and may
I ,411 elcctrical equipirierit and devices connected on malfunction. sending erroneous signals.
the system are subject to higher dielectric stresses 3 They may cause magnetic disturbance\ and noise in
diie to a higher effective voltage (equation (23.1 )). communication networks in the bicinity as discuswd
2 High harmonic voltages may give rise to pulsating abo\ie.
and transient lorqucs in a motor oI a generator, in 3 They may also affect the pertormancc of electronic
square proportion to the voltage (7;, oc v; ). At higher equipment operating on the same system. such ax a
amplitudes of such harmonics, it is possible that the computer or a static power factor correction relay.
driving or the driven shafts may even shear off as a 5 Increased errors in all types of measuring instruments,
consequence of transient torques. Transient torques. which are calibrated at a fundamental frequcncy.
up to 20 times the rated torque, have been experienced 6 Possible resonance and ferro-resonance effects (Section
with oversiLed capacitor units connected on the system, 20.2.1(2)) between the reactances of the generator
causing wide voltage fluctuations. and the transformer windings and the line capacitances.
3 Additional noise from inductive equipment such as In addition to the line capacitors. it is possible that
moton. generators and transformers and even overhead the circuit is completed through the ground coupling
lines. capacitances (Section 20. I and Figure 20.2) and give
3 Flickering of GLS lumps in homes and offices. rise to high to very high system voltages. The effect
will be the same. whe~her the syslem is grounded or
By the hcrrrizonic currents not. The ground coupling capacitances providing the
I Overheating of the windings of an inductive load, return path, as through a grounded neutral. This
such ;IS a transfornmer and rotating machines. cables phenomenon would normally appl) to an EHV ry\tt'in.