Page 44 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH002.3D ± 34 ± [31±81/51] 17.11.2001 9:49AM

34 Power systems engineering ± fundamental concepts

Table 2.1 Typical voltage fluctuation standards
Load Limits of voltage fluctuation

Large motor starts 1±3% depending on frequency
Mine winders, excavators, large motor drives 1±3% at distribution voltage level
1 ¤ 2 ±1 ¤ 2 % at transmission voltage level
1
Welding plant 1 ¤ 4 ±2% depending on frequency
Induction furnaces Up to 1%
1
Arc furnaces < ¤ 2 %

Table 2.2 Factors to consider in specifying compensating equipment
1. Continuous and short-time reactive power requirements.
2. Rated voltage and limits of voltage variation.
3. Accuracy of voltage regulation required.
4. Response time of the compensator for a specified disturbance.
5. Maximum harmonic distortion with compensator in service.
6. Performance with unbalanced supply voltages and/or with unbalanced load.
7. Environmental factors: noise level; indoor/outdoor installation; temperature, humidity, pollution,
wind and seismic factors; leakage from transformers, capacitors, cooling systems.
8. Cabling requirements and layout; access, enclosure, grounding; provision for future expansion;
redundancy and maintenance provisions.
9. Protection arrangements for the compensator and coordination with other protection systems,
including reactive power limits if necessary.
10. Energization procedure and precautions.

motor starts can also be avoided by using a `soft starter', that is, a phase-controlled
thyristor switch in series with the motor, which gradually ramps the motor voltage
from a reduced level instead of connecting suddenly at full voltage.
Standards for the quality of supply. One very noticeable effect of supply voltage
variations is flicker especially in tungsten filament lamps. Slow variations of up to
3% may be tolerable, but rapid variations within the range of maximal visual
sensitivity (between 1 and 25 Hz) must be limited to 0.25% or less. A serious
consequence of undervoltage is the overcurrent that results from the fact that AC
motors run at a speed which is essentially determined by the frequency, and if the
voltage is low the current must increase in order to maintain the power. On the other
hand, overvoltage is damaging to insulation systems.
Table 2.1 gives an idea of the appropriate standards which might be applied in
different circumstances, but local statutes and conditions should be studied in each
individual case.
Specification of a load compensator. Some of the factors which need to be con-
sidered when specifying a load compensator are summarized in Table 2.2.

2.2 Conventions used in power engineering

In power engineering it is helpful to have a set of conventions for symbols. Unfortu-
nately many people disregard conventions, and this causes confusion. There is no
universal standard, but the simple conventions given in Table 2.3 are widely used,
practical, and consistent with most classic textbooks.