EFFECTS OF VOLTAGE SAGS IN PROCESS INDUSTRY APPLICATIONS

This paper describes the causes of voltage sags in affecting process industries, their impacts on equipment operation, and possible solutions. The definition proposed focuses on system faults as the major cause of voltage sags. The sensitivity of different types of process industry equipment; including adjustable speed drive controls, programmable logic controllers, and motor contactors; is analyzed. Available methods of power conditioning for this sensitive equipment are described. A voltage sag is a momentary (i.e. 0.5-60 cycles) decrease in the rms voltage magnitude [1,2], usually caused by a remote fault somewhere on the power system (Figure 1). Voltage sags are the most important power quality problem facing many process industry customers. Equipment used in modern industrial plants (process controllers, programmable logic controllers, adjustable speed drives, robotics) is actually becoming more sensitive to voltage sags as the complexity of the equipment increases and the equipment is interconnected in sophisticated processes. Even relays and contactors in motor starters can be sensitive to voltage sags, resulting in shut down of a process when they drop out. It is important to understand the difference between an interruption (complete loss of voltage) and a voltage sag. Interruptions occur when a protective device actually interrupts the circuit serving a particular customer. This will normally only occur if there is a fault on that circuit. Voltage sags occur during the period of a fault for faults over a wide part of the power system. Faults on parallel feeder circuits or on the transmission system will cause voltage sags but will not result in actual interruptions. Therefore, voltage sags are much more frequent than interruptions. If equipment is sensitive to these voltage sags, the frequency of problems will be much greater than if the equipment was only sensitive to interruptions. This paper describes the voltage sag characteristics and the sensitivity of equipment. With this information, the range of fault locations on the power system that can cause problems can be estimated (area of vulnerability). Options for improving equipment performance in the presence of voltage sags include power conditioning or equipment design modifications. Both of these options are described. Figure 1. Voltage sag waveform caused by a remote fault condition (7 cycles) Voltage sags are typically caused by fault conditions. Motor starting can also result in undervoltages, but these are typically longer in duration than 60 cycles and the associated voltage magnitudes are not as low. Motor starting voltage variations are often referred to as "voltage flicker", especially if the motor starting can occur frequently. Faults resulting in voltage sags can occur within the plant or on the utility system. The voltage sag condition lasts until the fault is cleared by a protective device. In the plant, this will typically be a fuse or a plant feeder breaker. On the utility system, the fault could be cleared by a branch fuse or a substation breaker. If reclosing is used by the utility, the voltage sag condition can occur multiple times, with varying durations (Figure 3). Also, faults on the distribution system can result in voltage sags that are difficult to characterize with simple magnitude/duration information because the fault characteristics change with time (Figure 4). INTRODUCTION CAUSES OF VOLTAGE SAGS