Page 780 - Industrial Power Engineering and Applications Handbook

Power capacitors: behaviour, switching and improvement of power factor 23/737
the distortion level. even for their (harmonics) small contents (Table 23. I ). During a phase-to-ground Fault. for iii\taiice. the ground
potential may rise (Section 20.1 ) and cause much higher inductive interferences (electrostatic induction). that may jeopardix the
conimunication sy\tems particularly those serving essential services such as railways. defence installationh, power generation and
transnii\sion. The ground conductor now acts like a large-diameter solenoid producing high induced currents in the grounded
communication network and raises its ground potential. It gives rise to noise disturbances and affect\ its audio quality. It may also
distort the carrier n,aves (-30-500 kHz) such as those u\ed in a power line carrier communication (PLCC) network. and mutilate
the tran5fer of vital data or \end$ out wrong signals. To achieve better reliability, \uch as on a critical powei- net\vork. coupling
between phnsc to phaw is uwd. We provide the layout of ruch ;I \y\tein in Figut-e 23.9(b).
2 Electroniagnetic induction (a current effect)
This may occur as a result of electromagnetic induction between the power and the communication line\ due to their- proxiinit!. Thc
magnetic tield produced by the main power conductors may infringe upon nearby existing communication lines. As such. it ma) not
c;iusc \trious disturbance due to adequate \pacing (>305 mm. Section 28.8) between the power and the comniunication line\ But
in ii grounded communication network the situation may deteriorate. during a single phase-to-ground fault in the power linea. The
grounded conductor which will carry the ground fault current will no\\ act a\ a vel-y lai-ge-diameter solcnoid and produce high
magnetic fields. inducing heavy overvoltages in the coiniiiunication lines significantly affecting audio quality. Whci-e ii more reli.iblc
i~iiiiiiiiitiiciiti(iii network i': considered imperative. this is achieved by connecting the network hetween ph;i\r IO phaw than pha\e
ti) ground.
All effects caud by electrostatic or electromagnetic inductions are termed Inductive Interferences. With the u\e of gla\\ optical
fibre cables in new installation\. this effect is werconie automatically. Optical fibre cable\. as discussed later, ha\e no metal content
and carrq no electrical \ignals. Therefore the above discussion is more appropriate for existing in\tallations and rilso to pro\ idc a
theoretical a\pcct atid more clarity on the phenomena of inductive interference\. These can also be applied to other field\ rather than
communications alonc.
In the e:irIicr installations sensitiYc to such interferences the normal practice wa\ coordination between the gcnei-atrng and powci-
ti-an\niis\ioti agencies and the authorities of e\senrial rervices (such as public telephones, defence services and railway\). who
pro\ idc their own communicotion system<. to rrlnciire their telephone lines to mitigate this problem at thc planning stage.
3 Poor joints
In addition IO the above, sparking in the main line\ due to poorjoint\ or old and dirty inwlators and corona discharge\. llashoter\
stid arcings between the making contacts of a switching device during a switching operation. also may generate high-lreqtiency
wives imin IO kHc to snmc MH7 arid distort the cnrrier aid radio frequency wave\.
Reliable communication services
I\ reliable telephone system for public communication. a defence installation. railways or a power generating and transmittin,o
network i\ an r\\rntial requirement between any two stations, and must he free of \uch disturbances. The proxiinit) of HV and EHV
power lines influence\ their performance. Some interference may be due to lightning and \witching surge\ or corona discharges at
\cry high frequencies. All these may cause disturbances when their frequencies coincide with those of the carrier haves (-30-501)
iH/) and radio waves (in MHz). These disturbances may also di\tort the transmittal of vital data. when wch \enices :?re ;tlso
cmployed for transmitting of information \uch a\ hy a power line carrier communication (PLCC) network.
We gi\ e below for a general reference brief details of a PLCC network a\ used lor power gerieiating and ti ansmitting communication
wrvicc\. A PLCC use\ coupling equipment. filter and blocking circuits, which have also been discussed \epal.atel! in thi\ chapter.

Power system communication through a PLCC
Power qysteni communication i\ a complex wbject. As power generation and tran\mission capacity prow\. \o doe\ thc neceshit! tor
exchmge of iiifoi-mation at high speed. between the generating stations. local di\patch centres and the load control ccnlres.
Powerhouse communication services are different from normal telephone services. They are regarded a\ hlghly essential and irequire
tar more reliability to maintain continuity. They should remain free of disturbance. particularly during fault conditions or line
disturbances. Reliable communication helps in monitoring all the feeding stations and generating units that arc operating in tandem
on that network tor their operating coiidilivir\ and load balancing and then taking prompt remedial action. when required. hy load
balancing or load shedding as may be necessary. It also provide\ protection signalling to isolate the faulty section from the system
and enhance the security level of the system. All this is achieved with the uw of a PLCC network. which is connected at both end\
i1f the lines a\ illustrated in Figure 23.9(h). It can carry out a number of important functions for a power generating arid transmitting
network. hehidm monitoring its health.
The main function\ of ;I PLCC can he
0 Voice communication (telephone service\) between the generating stations. sub-stations and the load dispatching centre\.
Fax and telex services
Telcmetering.
(a) To transmit messages and data between two stations and to monitor and take preventive measures in the operating condilion\
of the entire power network, to achieve a more etticient and reliable power sy\tem.
(h) To trarivnit data records or voltage, frequency. kW. kWh and kVAr or any other relevant information on the \y\tem'\
operating conditions. Thew data are generally wperimposed on the speech channels.

,Yore
There can bc dedicated channels tor data transmi\\ion and \<)ice communication. Thc data channel can alw bc \uperimpo\ed over
,I voice on rhe same channel.
Remote wpei-vision and automatic load-control of generating units. load dispatch centres and \ub-station\ to maintain the dehircd
opei-ating parameters. including load sharing.
Protection signalling betueen two ends of the line, This accelerate\ operation ofthe protective relays either by i\olating the firult)