Use of this web site signifies your agreement to the terms and conditions. It includes consistent descriptions of conducted electromagnetic phenomena occurring on power systems. This recommended practice describes nominal conditions and deviations from these nominal conditions that may originate within the source of supply or load equipment or may originate from interactions between the source and the load. Tags: Monitoring Electric Power Quality. IEEE BS standards.
New Sale! View larger. Notify me when available. Quantity The minimum purchase order quantity for the product is 1. This Recommended Practice encompasses the monitoring of electrical characteristics of single-phase and polyphase ac power systems. The document describes nominal conditions and deviations from these nominal conditions that may originate within the source of supply or load equipment, or from interactions between the source and the load.
Also, this document discusses power quality monitoring devices, application techniques, and interpretation of monitoring results. Graphical instruments may allow the user to view various waveforms in either visual format or on a hard copy printout.
However, these snapshots are very convenient for setting up the power monitoring instrument and understanding the conditions existing on the electrical distribution system. In many cases, the end user wants to measure the steady-state conditions. This requires an instrument capable of recording and conveniently displaying steady state conditions for the complete monitoring period, which could be weeks or months.
Susceptibility levels for an electronic load may be obtained from the manu- facturer, or from past surveys performed on that particular electronic load. Subclause 8. Monitor thresholds shall be set below more sensitive equipment susceptibility levels to ensure that distur- bances are recorded.
The aging of equipment, discrepancies between equipment and monitor susceptibility nomenclatures, and the accuracy of the monitor are factors that could result in the malfunction of equipment at voltage levels below its expected susceptibility levels. This feature permits users to diagnose current related equipment performance problems such as unwanted circuit breaker tripping, and motor, conductor, and transformer overheating. The proliferation of large, nonlinear loads requires true rms measuring capability and measurement.
An important application of current measurements in power system analysis is to help determine the direc- tion, or origin, of the disturbance. Observing the change in current that occurs simultaneously with the volt- age disturbance can suggest whether the origin of the disturbance is upstream or downstream from the point being monitored.
This technique can help to determine whether a neutral-ground voltage disturbance is grounding conductor-related or power-circuit related. Set monitor thresholds just above these values.
If time permits, repeat the process until no more than one event is recorded in a min period. Three-phase induction motors should be derated when oper- ated with imbalanced voltages [B11]. Conducted Swell 3. Ground current 0. Usually the monitoring period attempts to capture a complete power period, an interval in which the power usage pattern begins to repeat itself.
An industrial plant, for example, may repeat its power usage pattern each day, or each shift. Depend- ing on the monitoring objective, it may be necessary to monitor as little as one shift.
Primary information is steady-state and transient extremes. As the environment changes, repeating the measurements is recommended. Once the problem is found, a corrective action is implemented. After implementation, power monitoring is conducted to ensure the effectiveness of the solu- tion and to verify that no new types of problems have been created. Power studies are conducted for long periods of time, usually a few years, at multiple locations.
Currently there are two more extensive power quality studies. Mehta and J. Smith [B27]. Jerewicz [B24] and by D. Dorr [B21]. These studies have ability to compare data with past and future studies.
This concern was largely based on the growth seen in the number of electronic loads sensitive to power quality disturbances computers, digital clocks, programmable logic controllers and producers of these same disturbances variable speed drives and consumer electronics power supplies. Interpreting power monitoring results 8.
Many problems are solved by carefully examining the load, others by verifying correct wiring and grounding practices, and still others may require the use of power monitoring equipment. No one practice will handle every problem. Similarly, one should not diagnose a power problem simply by looking at only one piece of information.
All of the efforts to obtain information are meaningless unless the investigator has enough knowledge and skill to produce a solution from the available data. This subclause discusses many of the issues which directly impact graph interpretation skills. For further information on analyzing and interpreting data, please refer to [B3] and [B5]. This interval may be anywhere from an hour to a month, but it generally should be at least one business cycle.
See clause 7 for more information on the length of the monitoring period. Looking at the summary of the data will provide an important overview perspective and quickly identify more important data to be examined in greater detail. This is one reason to have clear goals and to properly set up the power monitor. A summary will typically focus on two items. First, data should be placed on a timeline to allow quick chronological correlation. Second, data should be catego- rized by disturbance and time.
This summarizes the data based on particular disturbances. Building a summary may focus on either one or both styles depending on the objectives of the monitoring. Keep in mind that data being produced does not necessarily indicate a power problem. If many reports are produced it could be that the thresholds were set too tight and examining each report in detail may be a waste of time.
On the other hand, no disturbances recorded may indicate that thresholds were not set tight enough, reports were not turned on, or the monitored time did not coincide with the disturbance. All power disturbance recording devices are just tools subject to the skill and knowledge of the user. No matter how careful we are to eliminate wrong data, some may creep in.
No matter how cer- tain we are of an interpretation, it must make sense in the real world. A reality check of the summarized data should be performed before attempting to interpret them. This involves making sure such things as magnitudes are reasonable i.
This may not achieve the desired goal more investigation may be needed , but it will provide needed information to pro- ceed in analyzing the data further.
The timeline summary provides a chronological overview of what occurred during the monitoring interval. If the problem involves equipment malfunctions, isolate the disturbances to which the device is sensitive, if possible. Patterns in time or disturbance characteristics may show the true source of the problem. No consistent failure pattern, such as time of day, is noticed.
Several lock-ups occurred during the one day monitoring time. The timeline summary showed the presence of many transients, L-N sags, and N-G voltage increases. Occa- sionally, an L-N sag and an N-G rise had levels far exceeding the other levels. These excessive levels always occurred simultaneously and at the same time the workstation locked up. Further investigation showed the existence of a laser printer and a photocopier on the same circuit as the workstation.
These two devices constantly caused small transients, L-N sags, and N-G increases in voltage. However, when both the printer and copier were used at the same time they caused an L-N sag and an N-G increase that were much worse. The magnitude of the intermittent N-G increase was found to be the cause of the lock-ups. They should, however, help determine what data needs to be examined more closely. This data is referred to as critical data. In the example of 8.
The transients did not. Thus, the sags and N-G increases would be considered critical data. An event is the electromagnetic phenomena that resulted in one or more reports from the power monitor.
For example, during the short interruption, which happens while a fault is being cleared, the monitor may report an L-N sag or interruption, one or more transients, and a waveshape fault or two.
All of these describe the one event of the interruption. Practically speaking, determining critical events involves collecting all disturbances that appear to describe the same event, and then analyzing each disturbance in light of the whole. If an L-N sag occurred, did an increase in N-G voltage also occur indicating a load change on the monitored circuit?
If an interruption occurred were there any waveshape graphs indicating whether the interruption was local or from the utility? Many times an event will be seen as a group of disturbances, each one providing a valuable piece of informa- tion needed to put the whole puzzle together. Isolating an event is done by correlating each graph or report to others with similar time stamps. Be careful to include graph durations when looking at the time stamps. Notice in 16c that the waveshape disturbance showing the restora- tion of power has a time stamp equal to the initial waveshape disturbance time plus the interruption duration from the sag graph.
This is how events are determined. Figures 17 and 18 demonstrate the need for an event reality check. In reality, it is simply the output voltage of a low-end UPS.
But on closer examination we see that the impulse, with a magnitude of over V, reaches full scale and returns to zero instantly with no overshoot.
It is highly unlikely, even when using mitigating devices, that the normally linear power system would respond to an impulse in this fashion. This impulse fails the reality check and is most likely the result of instrument error. Keep in mind that an event may consist of more than one graph or report. Table 5, shown below, is a reference chart for data interpretation. Each subclause then discusses the char- acteristics and possible causes.
For example, the energization of a certain type of load may consistently generate the same waveshape disturbance. This wave- shape would be called its signature. Many, but by no means all, electromagnetic phenomena have signatures that can be recognized and ana- lyzed. Typically, waveshape analysis is more useful at the facility level or further down- stream as opposed to the utility level.
This impact will be both in magnitude a voltage drop , and in terms of waveshape. For example, if the load is a PC or other electronic load that draws current in large pulses, then the voltage drop will occur at the peak of the voltage waveform. The voltage at low frequencies with zero ground conductor current is directly proportional to the neutral current.
Consequently, the neutral-to-ground waveshapes and voltages can allow conclusions about the current through the neutral. If the neutral-to-ground voltage waveshape shows a large sine wave component, as opposed to the typical pulsed current drawn by electronic loads, there is a non-electronic load sharing the dedicated circuit. It can also be useful in determining the cause of a low-voltage situation at a load. If the neutral-to-ground voltage on a V circuit is less than a few volts, it implies that the voltage drop across the neutral is low, so presumably the drop across the line conductor is low as well.
On the other hand, if the neutral-to-ground voltage is more than a few volts, the voltage drop across the neutral is high, so it is likely that the distribution wiring and connectors are undersized for the load. When dealing with single-phase electronic loads, especially ones with switching power supplies, this conventional wisdom is faulty. Because electronic loads tend to draw all of their current in pulses near the peak of the sine wave, the har- monic currents in each phase fail to cancel even in a perfectly balanced system, and the neutral current can be as much or greater than the current in each phase conductor.
The current waveshape may be roughly sinu- soidal, but at Hz, it is often referred as the third harmonic neutral current. For single-phase electronic loads sharing a common neutral between phases, the neutral conductor should have twice the cross-sectional area of each of the phase conductors. Keep in mind also that not only is the neutral-ground rms voltage proportional to the neutral current, but also its waveshape.
So if the neutral current is Hz, so will be the neutral-ground voltage. Since the neutral current can be very high, this neutral-ground voltage may also become excessive. Also, remember that a change in neutral-ground voltage can occur due to a change in impedance e. Typically, waveshape disturbances are associated with voltages rather than currents since the dynamic load variations in a facility constantly and dramatically alter the current waveshape.
Waveshape disturbances provide information regarding system adequacy and the nature of loads inside a facility. Some faults help determine the appropriate source of disturbances such as interruptions, while oth- ers may identify the cause of distortion. The power system, and all the loads connected to it at any given time, conduct and consume power on a con- tinual basis.
Many loads, such as motors, do not stop instantaneously when supply voltage is inter- rupted. They can regenerate voltage as they spin down, making an interruption take up to several seconds to go to 0 V. This means that instead of looking at the instantaneous waveshape, we examine the rms value of the wave. For example, if a load is energized that has a 1. In fact, the N-G voltage rise will be about one-half the magnitude of the L-N sag.
Recognizing that most sag or swell conditions result from changes in current can help determine the cause of most of these types of disturbances. Whenever both voltage and current are known, it is even easier to iden- tify the possible causes.
Depending on the monitoring location with respect to the entire power system, elec- trical inertia may also contribute to sags and swells. Some of these distur- bances are continuous, low-voltage, high-frequency signals conducted on the power lines.
Others are very brief medium- to high-voltage signals known as transients. When these disturbances are injected into the power system, it responds differently than it would at low frequency. High-frequency models are used when examining transients and other distur- bances with frequency components above about 20 times the fundamental frequency of the power system.
For example, above about 1. Field data has shown that transients can travel from one wire to another, even if the wires are not connected presumably by capacitive coupling. They can travel through open-circuit breakers, and can appear across what appears to be an open circuit at lower frequencies.
The high-frequency characteristics of the power sys- tem need to be considered in the frequency range discussed earlier. Transients are generally caused by adding or removing reactive loads from the line.
A capacitor being added to a power system is typically in its discharged state. This causes a switching transient. As a capacitive load is introduced into the inductive power system, it may also alter the frequency response of the system.
An LC system has resonant frequencies that may be excited by the capacitive transient, lead- ing to a damped oscillatory transient. On the other hand, when an inductor is applied to the power system, not much happens in the transient realm. The inductor, however, causes a tran- sient at de-energization. If the switch controlling the inductive load is opened, three things happen.
This is called inductive kickback. Since this transient is adding energy back into the system, its position on the ac waveform will be in the same polarity. The degree of arcing can also indicate proximity to the source of the transient. Third, depending on the amount of current being interrupted, the switch may bounce. The second provides help in determining what particular type of load may be contributing to harmonic distortion. The last looks at how harmonic data can be used to produce an impedance spectrum of the power system.
The measurement requires both true rms and conventional measuring devices. Some of these pro- vide only the total harmonic distortion THD , while others provide THD and a full harmonic spectrum. Harmonic spectrums can be very useful in gaining insight into the general type of load s which may be con- tributing to the overall distortion.
Three generic harmonic spectrum signatures are described in the following. Keep in mind that these are gen- eral descriptions only.
Each successive set of harmonics will be smaller. See [B13]. This method takes both voltage and current harmonic data and graphs the impedance vs. It provides useful information regarding system frequency response, resonant points, and potential problems due to harmonic distortion. To generate the impedance spectrum, the desired current harmonic data and the difference in voltage har- monic data at the point of interest needs to be measured.
The difference in voltage harmonic data is the dif- ference between the no-load and full-load voltage harmonic data resulting from the load s in question. The no-load data can either be obtained from turning the loads off, or possibly using the harmonic data from some point near the source, say, at the source transformer or service entrance.
With this data, the impedance can be calculated at each harmonic frequency and plotted. The subsequent graph will provide insight into the frequency characteristics of the power system seen at the point of mea- surement. Should a high impedance exist due to resonance at a harmonic frequency, for example, then care should be taken to reduce any harmonic currents of that frequency, and so reduce possible voltage distortion.
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