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VOLTAGE SAG ITS EFFECTS AND MITIGATION

A voltage sag, or voltage dip, is a reduction of the voltage at a customer position with a duration of

between one cycle and a few seconds. Voltage sags are caused by motor starting, short circuits and fast

reclosing of circuit breakers.

Voltage sags occur in power systems all over the world. They are a nuisance for many industrial and

commercial customers; they are impossible to prevent; but it is often possible to mitigate the impact on

equipment. Like other power quality problems, this one needs co-operation between the utility, the

customer and the manufacturer of the equipment. Before a decision about mitigation can be made, an

assessment of the expected number of sags is needed. Such an assessment can be made through

measurements or by using stochastic prediction methods.

EFFECTS AND MITIGATION OF VOLTAGE SAG

The main reason for the increased interest in voltage sags is that modern equipment is often not able to

withstand such sags.

Computers, process control equipment and power electronics equipment are notorious for their

sensitivity. Trip frequencies of once a week and more have been reported. Also some more classical

pieces of equipment still cause problems due to voltage sags:

CONSUMER ELECTRONICS: equipment like personal computers, compact disc players, electronic alarms,

video recorders and microwave ovens can reset due to a short reduction in voltage. The sensitivity

(maximum permissible sag) varies from 85% during half a cycle up to 50% during 1 s. (A sensitivity of

50%, 10ms, means that the equipment will trip if the voltage is below 50% of its nominal value for

10ms.) From Table 1 it is easy to conclude that the number of trips will vary from two per year up to almost 50 peryear. For household equipment tripping is a nuisance but seldom a very serious problem. The problems are more serious foroffice equipment.Control systems for industrial processes are equally sensitive. Spurious tripping of continuous processes can cause serious

loss of revenue and it can even lead to dangerous situations. In most industrial processes therefore the control system isprotected against sags as much as possible.

Both types of equipment are fed from an internal DC bus through a single-phase rectifier (e.g. a simple diode bridge). The

rectifier operates as a DC voltage source through a capacitor connected to the DC bus. If the voltage at the DC bus getstoo low the equipment (often some kind of digital electronics or microprocessor) will malfunction. The capacitor will keep thevoltage up for some time but often not longer than a few cycles. The acceptable sag duration depends, among other things,on the capacitor size in relation to the DC load.Another thing that both types of equipment have in common is that they are basically low power (the main power is oftentaken by less-sensitive loads like fans and monitors). It is therefore not too difficult to feed them from an uninterruptable

power supply (UPS). A UPS consists of a rectifier, a DC bus with a battery block connected to it, and an invertor (see Fig.

3). If the AC input voltage drops, the DC voltage is kept up with power from the battery and the AC output is not affected. AUPS can ride through sags and even interruptions up to several minutes in duration. The cost of a UPS is low enough tojustify its use for industrial control systems, and for important office equipment. They are, however, seldom justifiable for

home use.Looking at the way a computer is fed through a UPS (AC to DC and DC to AC in the UPS and again AC to DC in thecomputer), one easily concludes that there are too many conversions. It should not be too difficult to manufacture acomputer in which a battery block can be inserted. Every laptop computer has this provision already. A next step would betothink about the feasibility of a DCdistribution system for offices andeventually even at home.Many adjustable-speed drives are toolarge to be fed from a UPS, and they areoften essential for the production

process. Their sensitivity varies from 85%,

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10 ms, up to 50%, 500 ms. If thisequipment needs to ride through a sag,some large energy storage is required, likethe recently proposed superconductingmagnetic energy storage (SMES) devices.However, there has not been muchevidence as to their feasibility.