Voltage Dips (Sags)

Voltage Dips (Sags)

Voltage dips is the aspect of power quality that is responsible for untold production stoppages and losses. Can be tricky to solve, but typically something can be done to better the situation.

Particular note should be given to this problem as it is typically the most troublesome and costly power quality problem in many areas, particularly where long rural supply lines are involved.

A voltage dip is a reduction in voltage for a short time. Different definitions are used but one definition is that a voltage dip is a reduction in voltage of between 10 percent and 90 percent of the nominal voltage, and it persists for any time period from 5 milliseconds (half a cycle) to 1 minute. (A voltage drop of greater than 90 percent is considered to be a supply interruption). Dips can be caused by the customer’s equipment on site (or by equipment on nearby sites) such as direct on-line starting of motors that are large in relation to the supply capability. But equally, dips can be caused on the utility’s network and thereby affect a high number of customers. The most common cause on the utility side is when there are electrical faults on the utility’s network and the dip is experienced for the small period of time that it takes for the fault to be cleared.

Such faults can for example be a digger striking an underground cable, but more commonly are caused by such things as wind blown debris striking overhead power lines during a storm. It depends on the depth and duration of the dip, but voltage dips originating on the utility network are notoriously difficult to compensate for when production processes are being interrupted. One valuable technique is to ensure that all the computerised control equipment and other control circuits in an industrial plant can “ride through” the typical dips. This is often a relatively low cost solution and there are many cases where the power hungry production process will ride through a dip so long as the control equipment can do so. This is unfortunately not always the case and certain sensitive industries (such as semiconductor manufacture and steel mills) invest large quantities of money in sophisticated techniques and equipment to mitigate against voltage dips on the supply.
It must be noted that often there is nothing practical that the utility can do to reduce the number of supply side dips.

However, there are many occasions when they can take action, such as refining protection settings or simply doing better vegetation control near overhead lines. So if you are experiencing problems with voltage dips it is worth a discussion with the local electrical utility. In such a case it is best to be armed with a complete record of all dips you have experienced including the depth and duration of each dip – such information would be obtained from a comprehensive power quality survey.

Voltages swells are not particularly common and are not discussed as a separate section, suffice to say though that where such issues exist they are identified by a standard power quality survey.

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