Voltage variations, such as voltage sags and momentary interruptions are two of the most important power quality concerns for customers. Customers understand that interruptions cannot be completely prevented on the power system. However, they are less tolerant when their equipment misoperates due to momentary disturbances which can be much more frequent than complete outages. These conditions are characterized by short duration changes in the rms voltage magnitude supplied to the customer. The impact to the customer depends on the voltage magnitude during the disturbance, the duration of the disturbance, and the sensitivity of the customer equipment.
Voltage variations and interruptions are inevitable on the power system. The most important of these variations occur during fault conditions on the power system. Since it is impossible to eliminate the occurrence of faults, there will always by voltage variations. This section describes some of the concerns associated with short duration voltage sags and interruptions. Voltage swells can also be associated with fault conditions but these short duration overvoltages are usually not severe and problems are uncommon.
Power quality complaints occur either when the customer has equipment which is very sensitive to these voltage sags (e.g., waveform below) and is critical to the overall process or when the frequency of occurrence of the interruptions or sags is interpreted as being unacceptable.
On the utility system, protective systems are designed to limit damage caused by unusual events like faults or lightning strikes, and to localize the impact of such events to the smallest number of customers. This is accomplished with overcurrent protection devices, such as reclosers, sectionalizers, and fuses.
Voltage Sag during a Remote Fault
IEEE Std. 1159-1995, Recommended Practice on Monitoring Electric Power Quality, provides definitions to label an rms voltage disturbance based upon its duration and voltage magnitude. Short duration rms variations are divided into the instantaneous, momentary, and temporary time periods, while the voltage magnitude of the disturbance characterizes it as a sag, swell, or interruption. A long duration rms voltage variation is defined to last longer than one minute, and can be classified as a sustained interruption, undervoltage, or overvoltage depending upon its magnitude.
Voltage sags and momentary interruptions have always existed on the power system. In the past, there were not many complaints about these conditions because residential customers had analog clocks and industrial customers had standard induction motors. Now, residential customers have digital clocks, VCRs, electronic coffee makers, and many other electronic gadgets that rely on continuous power to operate correctly. Every time there is a momentary interruption, many of these devices lose their settings and must be reset manually.
Industrial customers also have numerous loads that can be sensitive to voltage sags and momentary interruptions. Voltage sags are the most important power quality problem experienced by most industrial customers. Adjustable-speed drive (ASD) controls, robotics, programmable logic controllers, and even contactors for motor controllers and other control applications will have problems with voltage sag conditions. Whether or not a problem exists depends on the magnitude and duration of the voltage sag. Much of this equipment is used in applications that are critical to an overall process, resulting in very expensive downtime whenever the voltage sag condition occurs.
RMS Variation Magnitude Duration Scatter Plot
It is important for customers to understand the sensitivity of their equipment to momentary interruptions and voltage sags. The trip thresholds of sensitive equipment can often be modified, either with available settings in the controls or by manufacturer design changes. Once the sensitivity of equipment is known, an area of vulnerability for faults on adjacent feeders can be identified. This will help in evaluating the likelihood of problems due to utility system faults. The area of vulnerability concept can also be applied to customers supplied from the transmission system. Figure 6-8 shows an area of vulnerability diagram for a customer supplied from a transmission system bus. The figure shows that the area of vulnerability is dependent on the sensitivity of the equipment. Contactors that drop out at 50% voltage will have a relatively small area of vulnerability while ASDs that drop out at 90% voltage may be sensitive to faults over a wide range of the transmission system.
Customer Area of Vulnerability
There are three levels of possible solutions to voltage sag and momentary interruption problems:
Voltage sag (dip) analysis is the process of determining the number (and severity) of voltage sags that affect end-use equipment. It takes into account utility fault performance, the utility network, transformations to end-use equipment, and equipment connections in predicting the voltage sags that will occur.
Voltage sag analysis is useful to electric utilities for predicting power quality levels at new industrial/commercial sites being constructed. The utility can predict in advance the expected power quality level, allowing their customer to be more proactive in specifying solutions to voltage sags as the facility is being built. It also allows electric utilities to determine the effect of network changes and their impact (potential improvement) on customer power quality levels. For example, adding generation can result in fewer (less severe) voltage sags at nearby customers. Changing the configuration of tie switches, or adding transmission lines can also impact performance.
Voltage sag analysis is useful to large industries and commercial customers to evaluate investments in power quality solutions. Many of the most economical power quality solutions are solutions that address the most common problem, voltage sags. Examples of these solutions are Dynamic Voltage Sag Correctors DySC™, Dynamic Voltage Restorer DVR™, and PureWave™. To evaluate the value of these investments an annual profile of the sites voltage sag performance is necessary. Voltage sag analysis provides this vital information (example illustrated below).
Voltage sag analysis is done by compiling the electric utility fault performance (faults per 100 miles per year) of various classes (voltage levels) of network. A complete system short circuit analysis is done to determine the voltage sag resulting from faults across the network. A computerized analysis determines the annual profile of voltage sags at a given utilization point – taking into consideration various transformations and equipment connections.
Expected Sag Performance
Voltage Sag Performance