Page 188 - Power Quality in Electrical Systems

170 Chapter Eleven

■ Constant-voltage transformers (CVs) can be used to supply the
control circuits in the specific equipment—for example, the trans-
former in Figure 11.13. The CV will deliver proper voltage for voltage
sags down to 30–70 percent of nominal voltage, depending on the
loading. However, an AC contactor requires an inrush of about six
times its operating current to close. The CV is load current limited;
it will require an oversize rated CV to supply AC contactors, but not
smaller control relays.
■ Energy storage devices incorporating capacitors or batteries can be
used to hold in a relay or contactor when the supply voltage sags or
is interrupted. However, the opening function of the device is delayed,
which might interfere with its purpose. For example, a motor contactor
must trip rapidly for a fault in the motor.
■ DC contactors and relays can be supplied from rectifiers in AC cir-
cuits. The coils can be shunted with capacitors to provide a short-
time holding current when the source voltage sags or is interrupted.
As in the preceding item listed, the delay in the opening function
might interfere with its purpose.

Summary
The sensitivities of personal computers, controllers, AC contactors and
relays to power quality deficiencies, along with methods of correction,
were addressed in this chapter. The same analysis will be applied to
induction motors and adjustable speed drives (ASDs) in the next chap-
ter as examples of industrial power equipment.

References
[11.1] C. P. Gupta and J. V. Milanovic, “Probabilistic Assessment of Equipment Trips
due to Voltage Sags,” IEEE Trans. on Power Delivery, vol. 21, no. 2, April 2006,
pp. 711–718.
[11.2] S. Z. Djokic, J. Desmet, G. Vaneme, J. V. Milanovic, and K. Stockman, “Sensitivity
of Personal Computers to Voltage Sags and Short Interruptions,” IEEE Trans. on
Power Del., vol. 20, no.1, January 2005, pp. 375–383.
[11.3] IEEE Std 446-1987, “IEEE Recommended Practice for Emergency and Standby
Power Systems for Industrial and Commercial Applications,” (IEEE Orange Book).
[11.4] IEEE Std 1250-1995, “IEEE Guide for Service to Equipment Sensitive to
Momentary Voltage Disturbances,” Art 5.1.1, Computers.
[11.5] M. E. Barab, J. Maclage, A. W. Kelley, and K. Craven, “Effects of Power
Disturbances on Computer Systems,” IEEE Trans. on Power Del., vol. 14, no. 4,
October 1998, pp. 1309–1315.
[11.6] D. O. Koval and C. Carter, “Power Quality Characteristics of Computer Loads,”
IEEE Trans on Ind. Appl., vol. 33, no. 3, May/June 1997, pp. 613–621.
[11.7] IEEE 100, The Authoritative Dictionary of IEEE Standard Terms, seventh edi-
tion, 2000, p. 234.
[11.8] J. G. Truxal, Control Engineers’ Handbook, first edition, McGraw-Hill, New York,
1958, pp. 3–10.