Page 815 - Industrial Power Engineering and Applications Handbook

Switching Capacitor bank Total switched
steps kVAr
1st 5 kVAr 2nd 10 kVAr 3rd 20kVAr 4th 40kVAr 5th 80 kVAr 6th 160 kVAr
~~~
5
10
15
35
55
75
95
1 15
135
155
175
195

43 X - 215
47 X - 235
51 - X 255
55 - X 215
59 X X 295
63 X X 315
*Modified binary for a four-stage scheme
Note: It is possible to achieve any correction within a step of one unit only (5 kVAr in the above case).

system. This is a universal practice to measure the p.f. of The p.f. correction relays are normally available in
a system to economize on the cost of relay. The actual three versions, i.e.
p.f. of the circuit may therefore be less than measured by
the relay. But one can set the relay slightly higher (less Electromagnetic (being quickly outdated). They are
than unity), to account for the harmonics, when harmonics very slow, and may take up to 2 minutes or more to
are present in the system. initiate a correction.
From this phase displacement, a d.c. voltage output is Solid state-based on discrete ICs.
produced by a transducer circuit. The value of the d.c. Solid state-based on micro-controllers (micro-
voltage depends upon the phase displacement, Le. the processors).
p.f. of the circuit. This d.c. voltage is compared with a
built-in reference d.c. voltage, adjustable by the p.f. setting A delayed correction may not be desirable in applications
knob or by selecting the operating band provided on the that have a fast variation of loads such as rolling mills,
front panel of the relay, as shown in Figure 23.38. induction furnaces, arc furnaces, all types of welding
Corrective signals are produced by the relay to switch loads, power presses, hammers and elevators etc. Such
ON or OFF the stage capacitors through a built-in type of loads draw reactive power from the supply source.
sequencing circuit to reach the desired level of p.f. Since the loads change rapidly, unless they are
A little lower p.f. than set would attempt to switch compensated promptly they may cause voltage
another unit or bank of capacitors, which may overcorrect fluctuations, low p.f. and high harmonics. A rapid voltage
the set p.f. Now the relay would switch off a few capacitor drop may even trip the load, which may cause serious
units or banks to readjust the p.f. and so will commence damage to machines in operation. For instance, in a rolling
a process of hunting, which is undesirable. To avoid mill, the mill may be damaged as the process ingots may
such a situation the sensitivity of the comparator is made be half-way through between the rolls when the mill
adjustable through the knob on the front panel of the stopped.
relay. The sensitivity control can be built in terms of With application of solid-state technology, this
phase angle (normally adjustable from 4 to 14 degrees shortcoming of an electromagnetic relay is automatically
electrical) or percentage kVAr. Thc sensitivity, in terms overcome. The solid-state relays are available with
of an operating band, helps the relay to avoid a marginal switching stages as many as from 2 to 16. For special
overcorrection or undercorrection and hence the hunting. applications, they can be designed for even higher numbers
As soon as the system’s actual p.f. deviates from the of steps.
pre-set limits, the relay becomes activated and switches A time delay is built in to allow discharge of a charged
in or switches out capacitor units one by one, until the capacitor up to 90% before it is reswitched. This is
corrected p.f. falls within the sensitivity limit of the relay. achieved by introducing a timer into the relay’s switching