Page 826 - Industrial Power Engineering and Applications Handbook

System voltage regulation 241781
automatic. as the VAr of the series capacitors will by experimentals to pr~dti~e data to dosign ;in
\ary with a change in the load current. When the appropriate series coinpen\ation schcine.
mltage drops, the line current will rise. to cope 2 Sub-synchronous resonance (SSR)
with the same load demand and so will rise the VAr A series compenuted network \vi11 h;iie its natui-ul
of the capacitors also providing an automatic higher frequency expressed b!
VAr compensation. When the voltage rises, the
current will fall and so will fall the VAr compensation.
No switching sequence, as necessary in shunt
capacitors. is therefore required for series capacitors. The abo\e ciin also be eripic\\ed h\
(ii) They may be connected permanently on the system. .,
;IS they compensate the line reactance. which is fixed,
though the load reactance may be variable. unlike
shuni capacitors. which are to be monitored for their where
addition or deletion during peak and offpeak load ,fi, = natural frequcnc) of thc writ\ circuit
periods respectively. The costs of switching ,f'= nominal frequency 01' the \y+m
cquipment and operational difficulties are therefore L = natural reactance of thc linc pcr I)h;isc.
low in series capacitor\. including that of gcncrator and load
(iii) They also provide the same degree of p.f. improve- X,. 27t . .f . 1-
=
ment as the shunt capacitors and do so by the leading Cs = serie\ c2ipacit;ince pet' phaw
voltage phasor rather than the current phasor.
I
x,, = __-
?E I c\
Liniitritiorzs The frequency.,fl,. will occur I'or only ;I fe~ cqcles
(a) It i5 not advisable to use them on circuits that have during an abrupt change in the line pai-anietcr\.
fluctuating loads or frequent inrush currents. such as such as during a switching operation 01- occiii-reiicc
switching of motor loads. During a start the latter of a fault etc. (Section 17.6.3). To ciisurc that the
will cause an excessive current. I,,, and proportionately circuit remains iiiductive Lindcr all condition.; of
raise the potential difference across the capacitor units load variation5 and Fault. to ;I\ oitl a capacitiw
(I,, . X,) and overload them in addition to causing mode of operation and an crices\i\c charging
higher dielectric stresses. Series capacitors for such voltage the content of X,, mu\t renuin highcr Ihan
installations must he designed for very high voltages. Xcs(XL > Xes). The natural frequcnc);.,f;,. thcrcl'ore
say, up to I,, . Xc/l 5, which will not be economical. has to be necessaril) lo\\ei. than the po\\cr
Also, protection against over\ oltages will still be frequency ofthe system (/i, <f). This i\ an untiwal
essential as a safeguard against ;L similarcontingency, transient phenomenon that (i~~ctith in ;I \cries
a\ discussed later. compensated system which i\ aditisted for its
(b) It is possible that the natural (suh-harmonic) frequency natural frequency to maintain XI > X, and may
( I/?rdLC, Section 17.6.3) of a system with series have far- I-eac h i ii g i in p I c at ions . [ 11 t tic \ u b-
i
capacitors will fall below the fundamental frequency synchronous range of ;I steam ttirhine generator
and render the sptcm more prone to resonance. Now this frequency may causc a i.e\oniince with the
resonance may occur below the fundamental rotating masses of the turbogenerator rotor and
frequency. This may prove fatal under certain loading generate electromechanical oscillations in the rotor.
conditions and influence the source of supply in the This may assume serious \ignificancc in largc
1-ollowing way: generators which have ;I numbcr. of natural
1 Ferro-resonance effects mechanical frequencies of the I-otating ina\\u\. in
An L-C circuit is more prone to ferro-rcsonance the range of 10-25 Hr. 'This trequcncy inay
effects during voltage fluctuations as a result of coincide with the natiiral frcquency ol'the \y\tem
saturation of the iron core. which may be of a during a line disturbance and niagnifj thc
transformer or an inductor coil. On saturation, oscillations of the rotating maws beyond de\irable
the inductance reduces drastically and becomes limits. If unchecked. these oscilla[ions may
more prone to resonance with the capacitance of continue to magnify and result in the shcaring off
the circuit. Voltage fluctuations rnay occur due to of the weakest part of the \haft. Although rare.
switching operations, particularly of an unloaded serious dainage has occulred ill edici- year\. when
line or load fluctuations (Section 20.2.1(2)). the turbine shaft had actually rhearcd off because
Although the line impedance will provide a of this phenomenon. Hydroturbine generators art'
s ii ffi e i en t d a m pe n i n g effect to automatic a I 1 y less prone to such oscillations. 3% theii- natural
attenuate such a state. precautions are mandatory mechanical frequency lies below IO HL. The natural
to avert the same. An inductive compensation, of frequency of a serie.; compensated system ma)
the order of40-50%. may be adequate Lo improve not rcach this level.
the system parameters and also avert a lerro- These oscillations can he dwnpcned M ith the
resonance effect. A more realistic approach to the use of filter circuits or by bypassing all or pari 01'
problem is possible if a mathematical model of the series compensation during ii line disttirbancc.
the ferro-resonance is developed and supplemented Similar techniques are adopted whilc protecting