It is a temporary voltage drop below 90% of the nominal voltage level. Voltage sags and momentary power interruptions are probably the most important power quality problems affecting industrial and large commercial customers.
This paper describes the causes of voltage sags, their impacts on equipment operations and possible solutions. This paper focuses on system faults as the major cause of voltage sags.
The sensitivity of different types of equipments including programmable logic controllers and motor contactors is analyzed. Then the range of fault locations on the power system that can cause problems is estimated (area of vulnerability).
Available methods of power conditioning for these sensitive equipments are also described in this paper.
The problems of voltage sags can be tackled in two ways. The customer will have to improve the ride through capability of their sensitive equipment. If system power conditioning is expensive it may be economical in the long term to improve the design of the equipment.
- Mitigation of voltage sags requires careful inspection of the characteristics of the process and of the nature and origin of events.
- The installation of mitigation devices (normally the only choice for the customers) can be seen as a short term solution. The mitigation capability of these devices is mainly limited by the energy storage capacity.
- Only improvement of system performance (for long deep sags) and of equipment tolerance (for short, shallow sags) can solve the problem in the long term.