What is voltage sag, how does voltage sag occur, and what are its characteristics

Overview of voltage sag:

With the rapid development of modern power industry and significant changes in the power load structure of the system, the dependence of factories and office automation on electronic equipment is growing rapidly. For power users, voltage sag is becoming a major issue. During the whole process of power generation and use, it must span a wide geographical area, where the entire transmission system generally encounters lightning, rainstorm, gale, construction, personnel misoperation and other accidents, which cause voltage sag caused by short circuit fault, some of which are enough to affect the normal operation of sensitive equipment.

At present, voltage sag/sag is a common voltage problem in the power grid. An abnormal increase in the effective voltage value can directly damage the equipment. The temporary drop in mains voltage that can cause electrical equipment failure or shutdown occurs more than 10 times more frequently than a complete main grid outage. According to authoritative data from EPRI (Electric Power Research Institute), over 92% of power quality events are voltage dips and spikes, while other power quality events account for less than 8%. Voltage sag has been identified by numerous international research institutions as the most common event in power systems. Therefore, it is particularly important to provide voltage sag (sag) protection for sensitive equipment.

Shenghong Electric can provide a comprehensive and three-dimensional solution for voltage sag, from various degrees of sag control to power quality testing and data analysis, comprehensively solving the economic and production losses caused by voltage sag

What is voltage sag?

The IEEE standard and national standard "Voltage sag and short-time interruption of power quality" (GB/T30137-2013) define voltage sag: Voltage sag refers to the phenomenon in which the effective value of the power frequency voltage at a certain point in the power system is temporarily reduced to 10%~90% of the rated voltage (i.e. amplitude of 0.1~0.9 (p.u.)), and lasts for 10ms~1min, during which the system frequency remains at the nominal value and then returns to normal level. Voltage sag is measured by the percentage of remaining voltage. Short term interruption refers to a decrease in voltage frequency of one or more phases to below 0.1p.u., with a duration of 10ms-1min.

Reasons for voltage sag:

Voltage sag is generally caused by faults in the power grid or substation facilities, or sudden changes in load. During long-distance transmission, electrical energy inevitably encounters various situations, such as power system failures, lightning strikes, large motor startups, capacitor switching, and other events in the distribution system.

Natural causes:

Lightning, lightning, rainstorm, gale, snow, etc.

Power system reasons:

Short circuit faults, large motor start-up, line switching, transformer and capacitor switching, power distribution equipment faults, induction motor (high power) start-up, etc.

Unpredictable accidental events:

Traffic accidents, damage to transmission lines caused by construction, human error, and small animals entering the distribution room.

Characteristics of voltage sag events:

The characteristics of voltage sag events include the following aspects:

2. The duration of voltage sag events is generally less than 2 seconds;

3. Single phase and two-phase voltage dips are more common;

4. It is rare for the three-phase voltage to be below 20%, but it does occur from time to time.

The following figure shows the investigation of voltage dips by the Electric Power Research Institute (EPRI) in the United States. After two power quality studies conducted at 22 sites across the Americas, a total of 1076 voltage dip events were recorded. Among them, the number of voltage dips above the green line (SEMI) standard accounted for 93% of the total, with only 6 events being power outages and only 3 events occurring beyond 2 seconds.

The following figure shows the immunity requirements of various standards for electrical equipment. From the immunity curve, it can be seen that general equipment has good immunity to short-term moderate voltage dips and long-term small amplitude voltage dips. However, it does not have short-term deep voltage sag immunity, so voltage sag events with large drop amplitudes are the main cause of voltage sag hazards.

Note: Electrical equipment has immunity to voltage dips above the curve, but not to voltage dips below the curve.