Whether you have a question or just want to browse, take a few moments to read over our collection of commonly asked technical questions and help tips. You might find what you're looking for without even contacting our support staff. These can help you get started and provide on-going reference.
PU stands for Per Unit. It is a term that electric utility engineers frequently use, and it was incorporated into IEEE 1159.
PU is the percentage formed by dividing the actual voltage by the nominal. For example, a sag to 0.8 pu on a 480V circuit would be a sag to 366V, and an interruption to 0.1 pu on a 120V circuit would be 12V.
The idea of "per unit" is to express a voltage without regard to the nominal, since through the various transformers in an electric utility system, the percent change will be similar (less losses in the transformer). If the 138kv transmission line sags to 0.7 pu, then the 13kv distribution line will see a similar sag, as will the end user voltage. (Of course, if there are delta-wye or wye-delta transformers in between, this will change the values, but the concept remains the same.)
Most people in the power engineering field have encountered the question about which direction the electrical harmonic power is flowing, from the source to the load, or, from the load to the source. While this is still a controversial topic for some people, the most commonly accepted practice for determining this is to look at the harmonic watts phase angle, or the relationship between the voltage and current for a particular harmonic. The same rules that would be applied to a pure sine wave of voltage and current (which has only a fundamental frequency component) would be applied here.
The phase relationship between voltage and current with a pure resistive load is zero degrees, or a power factor of 1. If the load is a pure inductor, then the current lags the voltage by 90 degrees, which is normally displayed as a +90 degrees. If the load is pure capacitor, the current leads the voltage by 90 degrees, so the phase angle is called -90 degrees. This is so that the power factor of an inductive and resistive load would be a positive number between 0 and 1, while a capacitive and resistive load would be a negative number.
If the phase angle between the voltage and current is more than 90 degrees apart, this usually means that the current probe used with a power/harmonic meter or analyzer is placed in opposite direction of the assumed power flow. Most current probes have an arrow that should be pointing in the direction from the source to the load, which is the normal direction of power flow. When the phase angle of the harmonic voltage and current is between 90 degrees and 270 degrees (270 is also referred to as -90 degrees) on a properly installed CT, then it is assumed that this harmonic power flow is in the opposite direction of the fundamental power flow, or from load to source.
In some Dranetz products, this is indicated by the words SOURCE or LOAD next to the printout of each harmonic watt. In other products, you have to look at the phase angle of the harmonic watts to determine where it falls. The user must be cautioned that in a number of measurements, the harmonic current and voltage levels are so low that the harmonic watts number is so small that it may be meaningless, as would be the direction of power flow information. For example, if on a 120V/30A circuit, there is a 5th harmonic voltage of 0.05 V and harmonic current of 0.2 A, 0.01W is really insignificant and the accuracy of the direction is very low.
by Jack King & Greg Rauch
When it downloads data from an 8010 PQNode, the PASS software package looks at the calibration expiration date stored in the instrument. If the date is expired it reports "Uncalibrated Data".
Some users find this inconvenient, so there is a flag for controlling this in the PASS.INI file in the [PASS] section:
If you edit your PASS.INI file to set this flag to 1, PASS will not report "Uncalibrated Data," regardless of the expiration date stored in the instrument.
Dranetz determines recommended recalibration intervals for these instruments based on worst-case conditions and standard industry practices. The expiration date stored in the instrument is based on these recommendations. By setting this HIDEUNCALIBRATION flag, you will be overriding these recommendations, based on your own knowledge of the environment, instrument application, etc. It is important not to lose sight of the need for periodic maintenance/calibration checks.
"Stiffness" of the source is kind of an engineering slang term for the ability of the source to provide a nearly constant voltage level even under heavy current loading conditions. In technical terms, it is related to the equivalent source impedance. Ohm's and Kirchoff's Laws are the keys here.
For example, if the source has a 1 ohm impedance and is supplying 100V, then if the load draws 1A, then the load will see 99V, with 1V dropped across the source impedance. If the load draws 10A, then the load will see only 90V, with 10V dropped across the source impedance. However, if the source impedance were 0.1 ohms, 10A draw by the load would still give the load 99V, as only 1V would be dropped across the source impedance. Hence, the source with the 0.1ohm source impedance is a much "stiffer" source than the 1ohm source. This is true of harmonic source impedances as well.
Typically, the stiffer the source, the less likely is it that the user will have power quality problems, whether they be harmonic, RMS variations like sags, etc. Though like any rule, there are exceptions.
Is your instrument installed out in the middle of nowhere? And now you need to get the data out of it remotely?
Are you having trouble getting PES to dial out? Does it appear to be dialing but you never hear the phone dialed? It's probably because it's not issuing the correct commands for your modem. Installing a secondary INI file in your Windows directory may help solve your problems.
All of the commands PES issues to your modem are accessible through an INI file called pesmodem.ini.. By changing settings in this file you are able to monitor what commands are sent to your modem and how it responds. You can also change the commands and the timing of the commands. The sections below show each line of pesmodem.ini with a brief description. Note that PES interprets the "^M" sequence as a carriage return.
This is an INI file section heading. This section contains entries that control the use of this file.
Determines whether the contents of this file are used during modem communications:
Determines what will be written to the debugging file.
FileName=c:\pes\modem.txt Specifies the full path and filename of the debugging file. You MUST create the debug file before attempting to use pesmodem.ini. Use Windows Notepad to create the file PRIOR to running PES with the ENABLE= flag set to one. The file may be empty, but it must exist.
This is an INI file section heading. This section exposes all of the commands sent to the modem.
This is the very first command issued by PES. Its purpose is to clear any pending modem commands. The default is a carriage return.
This command is issued when PES is trying to "find" the modem. PES is looking for an "OK" response.
When two modems are connected they are said to be online. In this state, anything a computer sends to its modem is simply sent on to the other modem. The computer needs some way of telling its modem to switch from online mode to command mode so that it may issue other commands. This is known as the escape sequence and is almost always three plus signs, "+++".
Unless you are very familiar with modem operation, it's best to leave this string alone.
Unless you are very familiar with modem operation, it's best to leave this string alone.
This is the last PES attempt to find the modem. If all PES attempts to receive an OK response have failed it issues this command as a last resort. The default simply sets the result code level (X4) and sets the modem to give word result codes instead of numbers (V1. i.e. OK instead of 0)
The command issued by PES to reset the modem to its power-on state. Some modems require a number after the "ATZ". If yours does then add it to this line. It's usually a zero or a one
PES issues this command to ensure that the modem is providing word responses instead of numeric responses(V1). OK instead of 0. The default string also sets the result code level..
The factory command to return the modem to its factory (not necessarily power-on) state. Some modems require a number after the "ATF". If yours does then add it to this line. It's usually a zero or a one.
This command is seldom used by PES since the modem manufacturer factory definition is not usually known.
Once the modem is "found", PES must initialize it based on its requirements. You can control the initialization string from within PES. Enter the Site/Setup/Name screen. At the bottom of the name screen is a field called the "PC Modem Initialization String". You may enter any valid modem commands in this field. PES will issue this command in an attempt to initialize the modem.
If the modem string you enter contains an error, PES will use the "Init=" string as the fallbaack initialization string.
PES requires the modem to monitor certain hardware states at all times. The carrier detect state is monitored (&C1) so that PES knows when a modem connection is lost. Either as a result of a hang-up command or because of a poor connection. The data terminal ready (DTR) state is monitored (&D2) to ensure that the physical connection betwen the modem and the computer is good.
This field and the Site/Setup/Name "PC Modem Initialization String" field are the two places you should put any specific protocol requirement commands you may choose to use.
You enter a site's telephone number in the PES Setup/Site/Name screen. PES defaults to dialing using tone dialing. However some phone systems still require pulse dialing. If yours does, I feel sorry for you. Aside from that you'll have to change the DialPrefix entry to specify pulse dialing. This is done by changing "ATDT" to "ATDP".
As with all of these commands, be careful. If you make a mistake in this entry your modem won't dial.
This command is issued by PES when you press the callback button on the PES left toolbar. The command tells the modem not to echo commands (E0), and to answer on the first ring (S0=1). You can change the number of rings before answering by changing the number after "S0=" to the number of rings you desire. Careful, though. If you set the number to zero, 0, the modem won't answer.
This is an INI file section heading. This section exposes all of the timeouts used by PES. A timeout is the amount of time PES waits before concluding that a command has failed. All of the value are entered in seconds.
In most cases, two seconds is more than enough time for the modem to respond to a command. Don't make the timeouts too long. PES retries most commands a few times so long timeouts can make for long delays before PES responds with an error.
You'll notice a one-to-one correspondence between the [TIMEOUTS] section and the [STRINGS] section.
Determines the number of seconds PES waits before it considers the start command to have failed.
Determines the number of seconds PES waits before it considers the Attention command to have failed.
Determines the number of seconds PES waits before it considers the Escape command to have failed. There shouldn't be any need to play with this timeout. The timing on an escape sequence is well defined. Two seconds more than covers the timing requirements.
Determines the number of seconds PES waits before it finally gives up trying to talk to the modem.
Determines the number of seconds PES waits before it considers the Reset command to have failed. This timeout is usually a little longer since some modems take a fair amount of time to reset.
Determines the number of seconds PES waits before it considers the Verbose command to have failed.
Determines the number of seconds PES waits before it considers the Factory command to have failed. This commands requires the modem to do some significant work. Therefore the response can take longer than most other commands. This is reflected in the default value.
Determines the number of seconds PES waits before it considers the Init command to have failed.
Determines the number of seconds PES waits before it considers the Dial command to have failed. A successful dial command results in a "CONNECT" message of some kind or a modem error message. Since the response can come only after two modems have negotiated and connected, all telephone system latencies must be taken into account. In other words, you have to wait for the call to go through. This can take some time. Especially in the case of international calls.
If when you try to connect to an instrument it seems like the modems start to negotiate and then just disconnect, it may be because this timeout is too short. Try making it longer. Of course, it could also be because the two modems cannot agree on a protocol to use.
Determines the number of seconds PES waits before it considers the CallBack command to have failed.
Dick Piehl & Alex McEachern
For remote monitoring, you'll often want to connect your Dranetz/BMI instrument to a telephone line. Here's some useful information that answers questions that we often hear.
Analog vs. Digital Phone Lines
There are two types of phone lines commonly available in commercial and industrial buildings: analog and digital. To figure out which one you're working with, look at the connector. If there are four little grooves in the end, you have probably have an analog line. If there are more than four grooves (typically 6 or 8), you definitely have a digital line. Analog phone lines typically use only the center two conductors (the outer two conductors, if used, carry a second analog phone line). The two conductors are called "tip" and "ring".
Digital phone lines use all of the conductors to carry various digital signals. Digital phone lines connect to telephones with multiple lines, status indicators, etc. Occasionally, you will find a digital phone line that uses only 4 conductors, but that's unusual. (If the phone it connects has more than two lines, or has a digital status display, you should suspect that it's a digital phone line, even if there are only four conductors.)
All modems -- modems in an IBM/PC, modems in Dranetz/BMI equipment -- require an analog phone line. You cannot use a digital phone line.
Phone lines in a commercial or industrial building usually come from a "telephone switch" or PBX located in the building. These telephone switches always support analog phone lines (they usually support digital phone lines as well). The local telephone system administrator can always arrange for an analog phone line from the local telephone switch; sometimes it will be necessary to add a card to the switch, but it is always possible.
Many digital phone manufacturers offer an inexpensive adapter box that plugs into the digital phone line and converts it to an analog line. These adaptors usually work best at lower modem speeds.
Cellular phone lines
Analog phone lines are available through the cellular phone system; however, a special adaptor that generates the "telephone ring" and "dial tone" signals for a modem, and accepts the "on/off hook" signal from a modem, is required. You can also purchase special cellular phones commonly called "cellular bricks" that have this adaptor built in. These cellular bricks have a standard analog phone connector, a 12-volt supply connector, and an antenna connector -- that's it.
Once you have an analog phone line, you still have some work to do. Unlike standard wired analog phone lines, cellular analog phone lines can have brief "drop-outs" (short periods of time when the analog transmit and/or receive signals are blocked, typically while transferring from one cell to another). A special modem protocol, MNP-10, has been developed to tolerate these drop-outs. For this protocol to be active, the modems at each end of the connection must both support the MNP-10 protocol, and it must be specifically enabled. This requires an initialization string; consult the modem manual.
Line-sharing devices allow one analog phone line to be shared by multiple analog phones or modems. Typically, you place a call to the line-sharing device, then send a DTMF (TouchTone) code to tell it which line you want. The line-sharing device then generates a "ring signal" for that line, and connects the incoming telephone to that line when that line goes off-hook.
The standard "ring signal" is a 20 Hertz, 90 Volt pk-to-pk signal. All modems will recognize and respond to a standard ring signal. However, some inexpensive line-sharing devices don't generate a standard ring signal; they may generate one at a lower voltage, or at an incorrect frequency, or one that is a square wave instead of a sine wave. Modems, including Dranetz/BMI modems, may or may not respond to non-standard ring signals. This is a problem with the line-sharing device, not with the modem.
Noise on phone lines
50 Hertz or 60 Hertz noise on phone lines is especially common at substations. The reason: the phone system is referenced to earth at the switching office, often miles away, and there is considerable local ground current at the substation. There's a quick and easy test for this problem. Using a hand-held voltmeter set for AC RMS measurements, measure from each of the two telephone lines to local earth (not from line to line). Typically, you should only see a few millivolts; anything over a volt is a sign of a high-noise environment, and you may have difficulties with modems.
Two of the most frequently asked questions by PP1, PQ Plus, and PP4300 users are:
How can I increase the analyzer's monitoring period using auto-transfer?
How does auto-transfer work?
Extending the monitoring period is frequently accomplished by programming threshold parameters to be more "tolerant." The use of the auto-transfer feature offers an alternative method that retains tighter thresholds. Significant increases in effective monitoring period can be achieved by adding a memory card, selecting auto-transfer to update continuously and changing memory type to "wrap" (also known as overwrite). For example, the trend recording period of the PP1 (with no other external events) is approximately 30 days based on the internal memory. It's 9 days for the PP4300. If a 2MB memory card is used, the effective recording period for both analyzers becomes approximately 65 days.
When auto-transfer is enabled, a database (memory allocation) is created on the memory card. Also, any events in the analyzer's internal memory are copied to the newly created database at this time. Database size is determined by the size memory card used: 1, 2, or 4MB. An important point to remember is that the database is simply space allocated to store events. For example, when using the 2MB card (2048K) the analyzer will create a database of two megabytes leaving 48K of (free space. Using the 1MB card (1024K), a one megabyte database is created leaving 24K of free space. Databases will appear as DOS files with the first 8 characters of the analyzer's site name with the "MDB" extension when viewed on the PC.
(When formatting a memory card with auto-transfer enabled, only the free space is formatted. To format the total card space, auto-transfer must be disabled prior to formatting.)
When events are recorded in the analyzer's internal memory, the memory card's database is updated continuously. Internal memory for the PP1/PQ Plus is 750K and 250K for the PP4300. The internal memory being smaller, will fill first. At this point, data in the memory card's database is a mirror image of the analyzer's internal memory. If memory type is set to "overflow" (also known as fill and stop), monitoring is turned off. If the memory type is set to "wrap" (overwrite), oldest events in internal memory are overwritten with new events that are saved in the remaining space of the card's database, effectively expanding the monitoring the period. When the card's database becomes full the analyzer will stop saving events to the card.
Reading auto-transferred data can be accomplished on the analyzer itself or via Dran-View Windows PC software.
Can't find any Double Density (720K) diskettes? You can use readily available High Density (1.44MB) diskettes in your Model 658 by following this easy trick.
Take a look at a diskette. The old Double Density (720K) type had only one square hole along the back end (the write-protection slot). The 658 was designed for this older diskette.
However, the newer High Density diskettes have two square holes; one is the write-protection slot, and the other identifies the diskette to the drive as High Density.
To make the newer disks usable in the 658, simply cover the second hole with a piece of masking tape. (Don't use clear tape, as it might prevent the optical sensor in the drive from properly recognizing the disk.) Be sure to format the diskette in the 658 before use.
Several Dranetz/BMI instruments can store their recorded data on SRAM PCMCIA cards for later transfer to your Windows PC, including the PP1 and the 4300. You can use Dranv-View software to inspect the stored data.
Configuring the PCMCIA slot on your computer for SRAM is easy. Just pull down the Help menu and click the Index tab. Type in the keyword SRAM and follow the easy instructions. Windows 95 checks for IRQ conflicts and indicates the device statements to add to the config.sys file. If you accept the default values, everything should work well.
Are you getting unexpected reports every hour from your Model 3030a? You can fix this problem easily.
You know you can set up your 3030a for hourly, daily, weekly, and/or monthly reports with the buttons at the lower right corner. But what should you do if you've got hourly reports turned off, but you still get a report every hour?
The problem is probably that you have "summaries" turned on. Summaries are useful if you're storing data on your disk and want to use it with a spreadsheet later, but they're not so useful if you have your printer turned on.
To turn summaries off, press the menus key, press the down arrow 10 times, and you'll be at the correct menu item. Turn summaries off.
Does your Model 130 answer the first time you call it, then refuse to answer again? This is a problem with your external modem, and it's easy to fix.
Just turn off flow control in your external modem (see your modem manual). It's easiest to do this by connecting the modem to your PC's serial port, then sending it the correct commands to turn off flow control. Make sure you store the modem setup in its memory so that it is retained when the modem is turned off. Then re-attach the modem to your 130. The problem will be fixed.
Harmonics are often displayed as a harmonic spectrum, which is either a list or bar graph showing the magnitude of each harmonic, for voltage and current. The magnitudes are a good clue as the source of the harmonics.
It is usually the harmonic currents that are of concern, as they can cause "harmonic pollution" to spread to other equipment. Just like Current * Impedance = Voltage for fundamental frequency, Ohm’s Law also applies to harmonic current, impedance and voltage.
A common statistical number used is called THD, or total harmonic distortion. This is a mathematical process where: the magnitude of the harmonics for voltage or current are squared; summed; the square root is taken on the sum; and the result is the divided by either the fundamental RMS or the total RMS value; and lastly, multiplied by 100%.
Harmonics are typically defined as "frequencies that are integer multiples of the fundamental frequency". For 60Hz power systems, these means that the 2nd harmonic is 120Hz, the third harmonic is 180Hz, the fourth is 240 Hz, ", the nth harmonic is n*60. Frequencies are found that are actually in between these harmonic frequencies are called interharmonics (such as 185Hz), but are generally much less common than harmonic frequencies themselves. Frequencies below the fundamental frequencies are called subharmonics (such as 9Hz), and often contribute to the phenomena of light flicker.
Variations in the RMS value are often used to trigger capturing of PQ data. The most common type of RMS variation is the sag, (or dip in European lingo). Some studies show over 60% of the PQ disturbances are sags, which is when the RMS value goes below 90% of the nominal value. On a typical office or residential outlet, that would be dropping from 120Vrms to 108Vrms. If the voltage goes down below 10% of nominal, we call that an interruption. Conversely, if it increases above 110% of nominal, that is a swell.
The most common type of electromagnetic phenomena that cause power quality related problems are changes in the basic waveshape of the voltage — the sine wave. One mathematical number used to represent this complex shape with a single number is RMS --- root mean squared. It takes each of the sample points (typically 128) in one cycle of the waveform, multiplies the value by itself (squares it), adds them all up and takes the average (mean), and then takes the square root of that number. This is different than the peak value, which is the largest sample value in a cycle. For different waveshapes, there are different relationships between the peak and RMS. For sine waves, the peak is 1.414 times larger than the RMS value, or the RMS value is 0.707 times smaller than the peak. This relationship doesn’t hold with distorted waveforms, such as when harmonics are present, which is why you should use a "true RMS" meter, not one that multiplies the peak value times 0.707 to determine RMS.
If you have taken a distribution panel off for whatever reason, it is a good time to use one of the most important power quality tools, the screwdriver. Be sure to wear the proper safety equipment and to follow all necessary safety precautions. In most facilities, the current flows during only part of the day. Today, this current is often contains heat-generating harmonic currents. The heating/cooling/heating/cooling cycle and resulting expansion and contraction of the wires can cause the connections to loosen over time. This loosening increases the impedance of the connection, which further increases the heating effects. Tightening loose connections with the screwdriver can help reduce voltage drops and minimize the fire potential.
Many power quality problems will require the use of some type of measuring or monitoring equipment, and some will be nearly impossible to solve without. Having the right tool at your disposal can shorten the time to uncovering the source, and getting the process running smoothly again. But the tools of observation and common sense should be the first that are brought out.
The InfoNode optionally provides for an on-board GPS receiver capable of receiving time signals from the GPS system and utilizing those signals to continuously update the system clock. If there is no GPS present or the GPS is present but a signal is not available, then you can specify whether the Time Manager uses the Internal clock or an Internet clock source (NTP).
To use the Internet clock source (NTP), you should specify the IP address of the NTP Server Host. The procedure below describes how to set up an InfoNode to communicate with the NTP server.
1. Go to the tree branch "Regional Settings" and set the InfoNode for the region it is in.
2. Go to the Tree branch "Time".
a. Click on the drop down list for "Alternate time source" and choose "NTP server".
b. Enter the IP address (in numeric form - ###.###.###.###) of the NTP server the InfoNode is to synchronize with under the "NTP server host".
NOTE: You may use a name such as "bitsy.mit.edu" for NTP server but then you must enter the IP address of a DNS server in the InfoNode. The DNS server address is entered under Setup > Communications > Network.
Harmonics are typically grouped by their harmonic number, either odd or even. Odd are the 3, 5, 7, 9, etc and even being the 2,4,6,8,"and so on. There is another grouping used, called the triplens, which are the 3, 6, 9, 12, etc. The triplens are so grouped because triplen harmonic currents will add in the neutral of a three-phase, four wire wye circuit, as opposed to canceling out. This has resulted in the need to make the neutral conductor equal to or up to 1.73 times as large in current-carrying-capability as the phase conductors.
Even harmonics are grouped together as they typically aren’t found in systems with properly functioning equipment, unless there are half-wave rectifiers present as loads. If half of the input rectifiers aren’t functioning properly in a full wave rectifier, then the load will draw current as if it is a half wave rectifier, and the current waveform will be rich in even harmonics. This is a clue that something maybe broken. Even harmonics are recognizable in a waveform as they cause a loss in symmetry within the halves of the waveform.
Power factor is another parameter that is affected by power quality phenomena, particularly distortion and imbalance. This creates even more confusion about the term "true power factor." Power factor is a measure of how efficiently a load uses the electricity, or, how much energy is consumed by the load versus how much the electricity provider must deliver. This has been defined as real power divided by apparent power, watts / volt-amperes, or W / VA.
Until the onslaught of rectified input type loads (also called switching power supplies, or electronic or non-linear loads), most electrical loads were resistive and/or inductive loads, such as heaters, incandescent lights, and electrical motors. Whereas the voltage and current may not have been exactly in phase, both were nearly sinusoidal in their waveshapes, having only fundamental frequency components present. Hence, real power being equal to Vrms * Irms * cos (angle between V & I called theta) and apparent power being Vrms * I rms reduced down to Power Factor equal to Cos (angle theta). People then assumed that this was the "real" formula for PF, and revenue meters used such for billing purposes.
As the rectified input loads began to become the norm, the current waveshape in particular lost its sinusoidal shape, becoming rich in other harmonic frequency components. SCR-gated loads conducted current only during part of the voltage waveform. Even if the fundamental frequency components were in-phase, the real power was no longer just the Vrms * I rms * cos (theta), since each of the harmonic voltages and currents could have a different value for theta. Not surprising, watts become a lower value, since the purpose of the rectified input switching power supplies and SCR-gated loads was to reduce the real power being consumed. But the apparent power, Vrms * Irms, was still the same. So, surprising as it may be to some people, the power factor become smaller. In one example, a utility person replaced an old electromechanical meter with a new one that called PF with the traditional W/VA method and now the customer owed for a PF penalty (which the customer refused to pay since they hadn’t changed their loads).
The presence of a high percentage of even harmonics usually indicates that there is a significantly large half-wave rectifier on the circuit, or that a full-wave rectifier is damaged and is acting like a half-wave rectifier.
Odd harmonics are usually make up the majority of the spectrum on most electrical circuits. Even harmonics in significant proportions are usually not found, except where current is being drawn on only half the cycle. The Fourier expansion of a half-wave rectified signal is composed entirely of even harmonics, whereas the typical electronic loads, such as PCs, laser printers, or ASDs, have predominately odd harmonic spectrums. Even harmonics are often detectable by the lack of quarter-wave symmetry of the waveform. This means that the part of the waveform up to the first peak of the sine wave doesn’t look like the mirror image of the 2nd part of the waveform, as it goes from the peak back to the zero axis. The same dis-symmetry will show up between the 3rd and the 4th parts in the negative half cycle of the waveform.
Transients are very short duration disturbances, less than 1/4 cycle of power frequency and more often, measured in microseconds. They used to be referred to as impulses, surges, spikes or glitches. But those terms can have ambiguous meanings, so the term "transient" was adopted by the IEEE and other standards groups.
Common causes of voltage transients are power factor capacitor banks being switched on or off, lightning striking a conductor or adjacent to a conductor, arcing from a phase conductor coming in contact with some sort of ground potential (such as a tree), and the notches resulting from the commutation period of the SCRs on rectified input 3 phase power supplies (such as in ASDs).
Possible effects of transients include data corruption on memory devices, equipment damage, data transmission errors, intermittent equipment operation, reduced equipment life, and irreproducible problems. Transients are often "sneaky", in that they happen very quickly and randomly, and many power quality monitors will not capture them, especially the higher frequency transients.
Swells are increases in the voltage, typically above 110% of the nominal. Though must less common than sags, swells can cause catastrophic failures in equipment if the voltage exceeds the safe input level of the equipment for too long. Swells can be caused when a large load is suddenly turned off (opposite of the cause of sags). The voltage will increase for 30-60 cycles, until the automatic tap changers can bring the voltage back into normal regulation limits.
Sags are often caused by sudden, large increases in current, which causes a proportional voltage drop in the wiring, leaving less voltage remaining for the loads. If it is a fault on the electric distribution system, such as a phase-to-ground short circuit caused by lightning, animals, tree branches, or accidents, then the direction of the sag is called upstream or source side, or towards the generating source. If a load starts up, such as a large HP motor, then the direction of the sag is said to be downstream or load side. If the remaining voltage during the sag is too low for the equipment to operate properly, the process can be interrupted or corrupted. Though equipment is usually not damaged during such, the product being produced often has to be scrapped, and there may be a significant restart time to get the operation running smooth again.
Some changes or modulations of the voltage that aren’t large enough to be consider sags may not seem to effect equipment operation. However, these anomalies can result in quality variations in extrusion and textile processes and in flickering lights that can cause human discomfort. In the case of flicker, the frequency of the modulation is critical as to whether it will be noticeable to the particular susceptibilities of the human eye and brain. For example, it would only take about 0.3 volts of modulation at 9Hz on a 120V system for most people to notice the flicker in a 60W light bulb. However, at 1 Hz, it would take nearly 10 times the modulation to be noticeable.