1、1 Power Quality Monitoring Patrick Coleman Many power quality problems are caused by inadequate wiring or improper grounding. These problems can be detected by simple examination of the wiring and grounding systems. Another large population of power quality problems can be solved by spotchecks of vo
2、ltage, current, or harmonics using hand held meters. Some problems, however, are intermittent and require longer-term monitoring for solution. Long-term power quality monitoring is largely a problem of data management. If an RMS value of voltage and current is recorded each electrical cycle, for a t
3、hree-phase system, about 6 gigabytes of data will be produced each day. Some equipment is disrupted by changes in the voltage waveshape that may not affect the rms value of the waveform. Recording the voltage and current waveforms will result in about 132 gigabytes of data per day. While modern data
4、 storage technologies may make it feasible to record every electrical cycle, the task of detecting power quality problems within this mass of data is daunting indeed. Most commercially available power quality monitoring equipment attempts to reduce the recorded data to manageable levels. Each manufa
5、cturer has a generally proprietary data reduction algorithm. It is critical that the user understand the algorithm used in order to properly interpret the results. 1.1 Selecting a Monitoring Point Power quality monitoring is usually done to either solve an existing power quality problem, or to deter
6、mine the electrical environment prior to installing new sensitive equipment. For new equipment, it is easy to argue that the monitoring equipment should be installed at the point nearest the point of connection of the new equipment. For power quality problems affecting existing equipment, there is f
7、requently pressure to determine if the problem is being caused by some external source, i.e., the utility. This leads to the installation of monitoring equipment at the service point to try to detect the source of the problem. This is usually not the optimum location for monitoring equipment. Most s
8、tudies suggest that 80% of power quality problems originate within the facility. A monitor installed on the equipment being affected will detect problems originating within the facility, as well as problems originating on the utility. Each type of event has distinguishing characteristics to assist t
9、he engineer in correctly identifying the source of the disturbance. 1.1.1 What to Monitor At minimum, the input voltage to the affected equipment should be monitored. If the equipment is single phase, the monitored voltage should include at least the line-to-neutral voltage and the neutral to-ground
10、 voltages. If possible, the line-to-ground voltage should also be monitored. For three-phase equipment, the voltages may either be monitored line to neutral, or line to line. Line-to-neutral voltages are easier to understand, but most three-phase equipment operates on line-to-line voltages. Usually,
11、 it is preferable to monitor the voltage line to line for three-phase equipment. If the monitoring equipment has voltage thresholds which can be adjusted, the thresholds should be set to match the sensitive equipment voltage requirements. If the requirements are not known, a good starting point is u
12、sually the nominal equipment voltage plus or minus 10%. In most sensitive equipment, the connection to the source is a rectifier, and the critical voltages are DC. In some cases, it may be necessary to monitor the critical DC voltages. Some commercial power quality monitors are capable of monitoring
13、 AC and DC simultaneously, while others are AC only. It is frequently useful to monitor current as well as voltage. For example, if the problem is being caused by voltage sags, the reaction of the current during the sag can help determine the source of the sag. If the current doubles when the voltag
14、e sags 10%, then the cause of the sag is on the load side of the current monitor point. If the current increases or decreases 1020% during a 10% voltage sag, then the cause of the sag is on the source side of the current monitoring point. Sensitive equipment can also be affected by other environment
15、al factors such as temperature, humidity, static, harmonics, magnetic fields, radio frequency interference (RFI), and operator error or sabotage. Some commercial monitors can record some of these factors, but it may be necessary to install more than one monitor to cover every possible source of dist
16、urbance. It can also be useful to record power quantity data while searching for power quality problems. For example, the author found a shortcut to the source of a disturbance affecting a wide area by using the power quantity data. The recordings revealed an increase in demand of 2500 KW immediatel
17、y after the disturbance. Asking a few questions quickly led to a nearby plant with a 2500 KW switched load that was found to be malfunctioning. 1.2 Selecting a Monitor Commercially available monitors fall into two basic categories: line disturbance analyzers and voltage recorders. The line between t
18、he categories is becoming blurred as new models are developed. Voltage recorders are primarily designed to record voltage and current strip chart data, but some models are able to capture waveforms under certain circumstances. Line disturbance analyzers are designed to capture voltage events that ma
19、y affect sensitive equipment. Generally, line disturbance analyzers are not good voltage recorders, but newer models are better than previous designs at recording voltage strip charts. In order to select the best monitor for the job, it is necessary to have an idea of the type of disturbance to be r
20、ecorded, and an idea of the operating characteristics of the available disturbance analyzers. For example, a common power quality problem is nuisance tripping of variable speed drives. Variable speed drives may trip due to the waveform disturbance created by power factor correction capacitor switchi
21、ng, or due to high or low steady state voltage, or, in some cases, due to excessive voltage imbalance. If the drive trips due to high voltage or waveform disturbances, the drive diagnostics will usually indicate an over voltage code as the cause of the trip. If the voltage is not balanced, the drive
22、 will draw significantly unbalanced currents. The current imbalance may reach a level that causes the drive to trip for input over current. Selecting a monitor for variable speed drive tripping can be a challenge. Most line disturbance analyzers can easily capture the waveshape disturbance of capaci
23、tor switching, but they are not good voltage recorders, and may not do a good job of reporting high steady state voltage. Many line disturbance analyzers cannot capture voltage unbalance at all, nor will they respond to current events unless there is a corresponding voltage event. Most voltage and c
24、urrent recorders can easily capture the high steady state voltage that leads to a drive trip, but they may not capture the capacitor switching waveshape disturbance. Many voltage recorders can capture voltage imbalance, current imbalance, and some of them will trigger a capture of voltage and curren
25、t during a current event, such as the drive tripping off. To select the best monitor for the job, it is necessary to understand the characteristics of the available monitors. The following sections will discuss the various types of data that may be needed for a power quality investigation, and the c
26、haracteristics of some commercially available monitors. 1.3 Voltage The most commonly recorded parameter in power quality investigations is the RMS voltage delivered to the equipment. Manufacturers of recording equipment use a variety of techniques to reduce the volume of the data recorded. The most common
