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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 determ
6、ine 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 fr
7、equently 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 st
8、udies 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 th
9、e 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 v
10、oltages. 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 voltagesare easier to understand, but most three-phase equipment operates on line-to-line voltages. Usually, it
11、 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 usua
12、lly 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 AC
13、 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 voltage s
14、ags 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 environmental
15、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 disturb
16、ance. 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 immediately a
17、fter 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 the c
18、ategories 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 may af
19、fect 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 recor
20、ded, 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 switching,
21、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 wil
22、l 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 capacitor
23、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 curre
24、nt 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 current du
25、ring 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 chara
26、cteristics 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 comm
27、on method of data reduction is to record Min/Max/Average data over some interval. Figure 1.1 shows a strip chart of rms voltages recorded on a cycle-by-cycle basis. Figure 1.2 shows a Min/Max/Average chart for the same time period. A common recording period is 1 week. Typical recorders will use a re
28、cording interval of 25 minutes. Each recording interval will produce three numbers: the rms voltage of the highest 1 cycle, the lowest 1 cycle, and the average of every cycle during the interval. This is a simple, easily understood recording method, and it is easily implemented by the manufacturer.
29、There are several drawbacks to this method. If there are several events during a recording interval, only the event with the largest deviation is recorded. Unless the recorder records the event in some other manner, there is no time-stamp associated with the events, and no duration available. The mo
30、st critical deficiency is the lack of a voltage profile during the event. The voltage profile provides significant clues to the source of the event. For example, if the event is a voltage sag, the minimum voltage may be the same for an event caused by a distant fault on the utility system, and for a
31、 nearby large motor start. For the distant fault, however, the voltage will sag nearly instantaneously, stay at a fairly constant level for 310 cycles, and almost instantly recover to full voltage, or possibly a slightly higher voltage if the faulted section of the utility system is separated. For a
32、 nearby motor start, the voltage will drop nearly instantaneously, and almost immediately begin a gradual recovery over 30180 cycles to a voltage somewhat lower than before. Figure 1.3 shows a cycle-by-cycle recording of a simulated adjacent feeder fault, followed by a simulation of a voltage sag ca
33、used by a large motor start. Figure 1.4 shows a Min/Max/Average recording of the same two events. The events look quite similar when captured by the Min/Max/Average recorder, while the cycle-by-cycle recorder reveals the difference in the voltage recovery profile.FIGURE 1.1 RMS voltage strip chart,
34、taken cycle by cycle.FIGURE 1.2 Min/Max/Average strip chart, showing the minimum single cycle voltage, the maximum single cycle voltage, and the average of every cycle in a recording interval. Compare to the Fig. 1.1 strip chart data. Some line disturbance analyzers allow the user to set thresholds
35、for voltage events. If the voltage exceeds these thresholds, a short duration strip chart is captured showing the voltage profile during the event. This short duration strip chart is in addition to the long duration recordings, meaning that the engineer must look at several different charts to find
36、the needed information. Some voltage recorders have user-programmable thresholds, and record deviations at a higher resolution than voltages that fall within the thresholds. These deviations are incorporated into the stripchart, so the user need only open the stripchart to determine, at a glance, if
37、 there are any significant events. If there are events to be examined, the engineer can immediately “zoom in” on the portion of the stripchart with the event. Some voltage recorders do not have user-settable thresholds, but rather choose to capture events based either on fixed default thresholds or
38、on some type of significant change. For some users, fixed thresholds are an advantage, while others are uncomfortable with the lack of control over the meter function. In units with fixed thresholds, if the environment is normally somewhat disturbed, such as on a welder circuit at a motor control ce
39、nter, the meter memory may fill up with insignificant events and the monitor may not be able to record a significant event when it occurs. For this reason, monitors with fixed thresholds should not be used in electrically noisy environments.FIGURE 1.3 Cycle-by-cycle rms strip chart showing two volta
40、ge sags. The sag on the left is due to an adjacent feeder fault on the supply substation, and the sag on the right is due to a large motor start. Note the difference in the voltage profile during recoveryFIGURE 1.4 Min/Max/Average strip chart of the same voltage sags as Fig. 1.3. Note that both sags
41、 look almost identical. Without the recovery detail found in Fig. 1.3, it is difficult to determine a cause for the voltage sagsFIGURE 1.5 Typical voltage waveform disturbance caused by power factor correction capacitor energization1.3.1 Voltage Waveform Disturbances. Some equipment can be disturbed
42、 by changes in the voltage waveform. These waveform changes may not significantly affect the rms voltage, yet may still cause equipment to malfunction. An rms-only recorder may not detect the cause of the malfunction. Most line disturbance analyzers have some mechanism to detect and record changes i
43、n voltage waveforms. Some machines compare portions of successive waveforms, and capture the waveform if there is a significant deviation in any portion of the waveform. Others capture waveforms if there is a significant change in the rms value of successive waveforms. Another method is to capture w
44、aveforms if there is a significant change in the voltage total harmonic distortion (THD) between successive cycles. The most common voltage waveform change that may cause equipment malfunction is the disturbance created by power factor correction capacitor switching. When capacitors are energized, a
45、 disturbance is created that lasts about 1 cycle, but does not result in a significant change in the rms voltage. Figure 1.5 shows a typical power factor correction capacitor switching event.FIGURE 1.6 RMS stripcharts of voltage and current during a large current increase due to a motor start downst
46、ream of the monitor point.1.4 Current Waveshape Disturbances Very few monitors are capable of capturing changes in current waveshape. It is usually not necessary to capture changes in current waveshape, but in some special cases this can be useful data. For example,inrush current waveforms can provi
47、de more useful information than inrush current rms data. Figure 1.7 shows a significant change in the current waveform when the current changes from zero to nearly 100 amps peak. The shape of the waveform, and the phase shift with respect to the voltage waveform, confirm that this current increase w
48、as due to an induction motor start.Figure 1.7 shows the first few cycles of the event shown in Fig.1.6. 1.5 Harmonics Harmonic distortion is a growing area of concern. Many commercially available monitors are capable of capturing harmonic snapshots. Some monitors have the ability to capture harmonic
49、 strip chart data. In this area, it is critical that the monitor produce accurate data. Some commercially available monitors have deficiencies in measuring harmonics. Monitors generally capture a sample of the voltage and current waveforms, and perform a Fast Fourier Transform to produce a harmonic spectrum. According to the Nyquist Sampling Theorem, the input waveform must be sampled at least twice the highest frequency
