Page 887 - Industrial Power Engineering and Applications Handbook

26/838 Industrial Power Engineering and Applications Handbook
ventilation facilities should also be provided, either Avoid any spillovers, leakage or vaporization of liquid
louvres, exhaust fans or by providing an expanded or disposal of a waste unit. One can use Araldite, solder
metal enclosure, for the capacitors to ensure a free air or any other means to stop a leakage. If this is not
circulation around each capacitor unit (Figure 23.32). possible, collect carefully all the chemical PCB and
Concentration of capacitor banks at one point in a carefully dispose it of.
system may cause amplification of harmonics, self- Soaked rags, cloth or papers, must be destroyed in an
excitation of machines, overvoltages due to switching incineration plant at 1000°C. Disposal at a landfill
and unsatisfactory working of audiofrequency remote- area is not advisable for during rain they may be carried
control apparatus. To overcome this, a concentration by storm drains into rivers, canals or ponds.
of capacitor banks at one point must be avoided as far Old units should not be sold to a scrap dealer for
as possible. These may be installed at different locations similar reasons.
in the system.
A loose contact within the circuit, producing sparks,
will generate high-frequency oscillations (harmonics). 26.3 Test requirements
However small these oscillations are, they may raise
the system voltage and frequency. High-frequency Below we discuss briefly recommended tests on a finished
oscillations will also reduce the impedance of the capacitor unit based on various IEC recommendations.
capacitors xch a llfh and cause high currents, leading For standards refer to the list provided at the end of the
to premature failure of the capacitors. Loose contacts chapter.
must therefore be thoroughly checked. It is
recommended that a capacitor circuit particularly be 26.3.1 Routine tests
checked periodically for loose connections or
sparkings. Routine tests are carried out on each capacitor unit.
When the reactive control is not automatic, then during
offpeak periods (such as during the night) it is important (I) Capacitance measurement
that some of the banks are dropped manually to avoid
an overcompensation and a consequent overvoltage. This is to be determined at rated voltage and frequency
It is advisable to periodically check the condition (i.e. and can be carried out between 0.9 to 1.1 times the rated
the capacitance value C) of each capacitor unit to voltage and 0.8 to 1.2 times the rated frequency.
detect failure of one or more of them and the consequent Permissible tolerances in capacitance from the declared
overvoltage across the healthy units. This can be values can be as follows:
measured by any of the methods discussed in Section
26.3.1(1). The capacitance value of each unit must For LT capacitor units up to 1000 V For both self-
fall within the permissible limits as indicated in that healing and non-self-healing types:
section. The failed units, if any, must be replaced S% to + 10% for units and banks up to 100 kVAr
promptly with units of the same rating and brand to 5% to + 5% for units and banks above 100 kVAr.
avoid any performance variation. If this is not possible For HT capacitors
then the banks must be balanced by readjusting the 5% to + 15% for capacitor units or banks containing
units. When the banks are connected through a series one unit per phase
reactor, the value of the reactor must also be readjusted 5% to + 10% for capacitor banks less than 3 MVAr
to match changed parameters. Otherwise the reactor 0% to + IO% for capacitor banks from 3 MVAr to 30
may resonate with the fifth or the seventh harmonic, MVAr
for which it is designed, and cause an overvoltage. 0% to + 5% for capacitor banks above 30 MVAr.
0 For series capacitors
26.2.1 Precautions in handling a capacitor unit These are usually HT capacitor units/banks:
with PCB f7.5% for capacitor units or banks containing one unit
per phase
Although the manufacture of PCB-filled capacitor units * 5% for capacitor banks less than 30 MVAr
has been discontinued, for older installations some may k3% for capacitor banks 30 MVAr and above.
still be used. For the benefit of their users we give below
the precautions that they must take when handling these Methods to determine the capacitance of a
units: capacitor unit
Two common methods to determine this are:
Do not inhale the vapour of this chemical.
Avoid its contact with eyes and skin. 0 Voltage method and
Avoid its contact with birds and animals. 0 Bridge method.
Avoid its contact with animal feed.
Do not re-use contaminated paper and cloth. In the voltage method the normal voltage is applied across
In the event of contact with the human body, wash the the capacitor terminals and line current, I,, is measured.
skin with soap and water and the eyes with clean running The value of capacitance, C, can be determined from the
water. equations provided in Section 23.5.2. In the bridge method