Electrical Power Systems Quality, Second Edition phần 10 pptx

Important capabilities for useful harmonic measure-ments include Figure 11.11 Demonstrating the use of a hand-held, three-phase power quality monitoring instrument to quickly evaluate v

1 Nameplates of transformers, motors, etc.

2 Instrumentation setups

3 Transducer and probe connections

4 Key waveform displays from instruments

5 Substations, switchgear arrangements, arrester positions, etc

6 Dimensions of key electrical components such as cable lengthsVideo cameras are similarly useful when there is moving action or ran-dom events For example, they may be used to help identify the loca-tions of flashovers Many industrial facilities will require specialpermission to take photographs and may place stringent limitations onthe distribution of any photographs

11.3.5 Oscilloscopes

An oscilloscope is valuable when performing real-time tests Looking atthe voltage and current waveforms can provide much informationabout what is happening, even without performing detailed harmonicanalysis on the waveforms One can get the magnitudes of the voltagesand currents, look for obvious distortion, and detect any major varia-tions in the signals

There are numerous makes and models of oscilloscopes to choosefrom A digital oscilloscope with data storage is valuable because thewaveform can be saved and analyzed Oscilloscopes in this categoryoften also have waveform analysis capability (energy calculation, spec-trum analysis) In addition, the digital oscilloscopes can usually beobtained with communications so that waveform data can be uploaded

to a personal computer for additional analysis with a software package.The latest developments in oscilloscopes are hand-held instrumentswith the capability to display waveforms as well as performing somesignal processing These are quite useful for power quality investiga-tions because they are very portable and can be operated like a volt-ohm meter (VOM), but yield much more information These are idealfor initial plant surveys A typical device is shown in Figs 11.10 and11.11 This particular instrument also has the capability to analyzeharmonics and permits connection with personal computers for furtherdata analysis and inclusion into reports as illustrated

11.3.6 Disturbance analyzers

Disturbance analyzers and disturbance monitors form a category ofinstruments that have been developed specifically for power qualitymeasurements They typically can measure a wide variety of system

disturbances from very short duration transient voltages to tion outages or undervoltages Thresholds can be set and the instru-ments left unattended to record disturbances over a period of time Theinformation is most commonly recorded on a paper tape, but manydevices have attachments so that it can be recorded on disk as well.There are basically two categories of these devices:

long-dura-1 Conventional analyzers that summarize events with specific

infor-mation such as overvoltage and undervoltage magnitudes, sags andsurge magnitude and duration, transient magnitude and duration,etc

2 Graphics-based analyzers that save and print the actual waveform

along with the descriptive information which would be generated byone of the conventional analyzers

It is often difficult to determine the characteristics of a disturbance

or a transient from the summary information available from tional disturbance analyzers For instance, an oscillatory transientcannot be effectively described by a peak and a duration Therefore, it

conven-Figure 11.10 A hand-held oscillographic monitoring instrument (Courtesy of Fluke Corporation.)

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is almost imperative to have the waveform capture capability of agraphics-based disturbance analyzer for detailed analysis of a powerquality problem (Fig 11.12) However, a simple conventional distur-bance monitor can be valuable for initial checks at a problem location.

11.3.7 Spectrum analyzers and harmonic

analyzers

Instruments in the disturbance analyzer category have very limitedharmonic analysis capabilities Some of the more powerful analyzershave add-on modules that can be used for computing fast Fouriertransform (FFT) calculations to determine the lower-order harmonics.However, any significant harmonic measurement requirements willdemand an instrument that is designed for spectral analysis or har-monic analysis Important capabilities for useful harmonic measure-ments include

Figure 11.11 Demonstrating the use of a hand-held,

three-phase power quality monitoring instrument to quickly

evaluate voltages at the mains.

■ Capability to measure both voltage and current simultaneously sothat harmonic power flow information can be obtained.

■ Capability to measure both magnitude and phase angle of individualharmonic components (also needed for power flow calculations)

■ Synchronization and a sampling rate fast enough to obtain accuratemeasurement of harmonic components up to at least the 37th har-monic (this requirement is a combination of a high sampling rate and

a sampling interval based on the 60-Hz fundamental)

■ Capability to characterize the statistical nature of harmonic tion levels (harmonics levels change with changing load conditionsand changing system conditions)

distor-There are basically three categories of instruments to consider forharmonic analysis:

1 Simple meters. It may sometimes be necessary to make a quickcheck of harmonic levels at a problem location A simple, portablemeter for this purpose is ideal There are now several hand-heldinstruments of this type on the market Each instrument has advan-tages and disadvantages in its operation and design These devicesgenerally use microprocessor-based circuitry to perform the necessarycalculations to determine individual harmonics up to the 50th har-monic, as well as the rms, the THD, and the telephone influence factor(TIF) Some of these devices can calculate harmonic powers (magni-tudes and angles) and can upload stored waveforms and calculateddata to a personal computer

2 General-purpose spectrum analyzers. Instruments in this gory are designed to perform spectrum analysis on waveforms for a

cate-Figure 11.12 Graphics-based analyzer output.

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wide variety of applications They are general signal analysis ments The advantage of these instruments is that they have very pow-erful capabilities for a reasonable price since they are designed for abroader market than just power system applications The disadvan-tage is that they are not designed specifically for sampling power fre-quency waveforms and, therefore, must be used carefully to assureaccurate harmonic analysis There are a wide variety of instruments inthis category.

instru-3 Special-purpose power system harmonic analyzers. Besides thegeneral-purpose spectrum analyzers just described, there are also anumber of instruments and devices that have been designed specifi-cally for power system harmonic analysis These are based on the FFTwith sampling rates specifically designed for determining harmoniccomponents in power signals They can generally be left in the field andinclude communications capability for remote monitoring

11.3.8 Combination disturbance and

One example of such an instrument is shown in Fig 11.13 Thisinstrument is designed for both utility and end-user applications, beingmounted in a suitable enclosure for installation outdoors on utilitypoles It monitors three-phase voltages and currents (plus neutrals)simultaneously, which is very important for diagnosing power qualityproblems The instrument captures the raw data and saves the data ininternal storage for remote downloading Off-line analysis is performedwith powerful software that can produce a variety of outputs such asthat shown in Fig 11.14 The top chart shows a typical result for a volt-age sag Both the rms variation for the first 0.8 s and the actual wave-form for the first 175 ms are shown The middle chart shows a typicalwave fault capture from a capacitor-switching operation The bottomchart demonstrates the capability to report harmonics of a distortedwaveform Both the actual waveform and the harmonic spectrum can

be obtained

Another device is shown in Fig 11.15 This is a power quality toring system designed for key utility accounts It monitors three-phasevoltages and has the capability to capture disturbances and page power

moni-quality engineers The engineers can then call in and hear a voice sage describing the event It has memory for more than 30 events.Thus, while only a few short years ago power quality monitoring was

mes-a rmes-are femes-ature to be found in instruments, it is becoming much morecommonplace in commercially available equipment

11.3.9 Flicker meters*

Over the years, many different methods for measuring flicker have beendeveloped These methods range from using very simple rms meterswith flicker curves to elaborate flicker meters that use exactly tuned fil-ters and statistical analysis to evaluate the level of voltage flicker Thissection discusses various methods available for measuring flicker

Flicker standards. Although the United States does not currently have

a standard for flicker measurement, there are IEEE standards thataddress flicker IEEE Standards 141-19936and 519-19927both contain

Figure 11.13 A power quality monitoring instrument capable of monitoring disturbances, harmonics, and other steady-state phenomena on both utility systems and end-user sys-

tems (Courtesy of Dranetz-BMI.)

*This subsection was contributed by Jeff W Smith and Erich W Gunther.

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Phase C-A Voltage

BMI/Electrotek

Uncalibrated Data

Figure 11.14 Output from combination disturbance and

har-monic analyzer.

flicker curves that have been used as guides for utilities to evaluate theseverity of flicker within their system Both flicker curves, fromStandards 141 and 519, are shown in Fig 11.16.

In other countries, a standard methodology for measuring flicker hasbeen established The IEC flicker meter is the standard for measuringflicker in Europe and other countries currently adopting IEC stan-dards The IEC method for flicker measurement, defined in IECStandard 61000-4-158 (formerly IEC 868), is a very comprehensiveapproach to flicker measurement and is further described in “FlickerMeasurement Techniques” below More recently, the IEEE has beenworking toward adoption of the IEC flicker monitoring standards with

an additional curve to account for the differences between 230-V and120-V systems

Flicker measurement techniques

RMS strip charts. Historically, flicker has been measured using rmsmeters, load duty cycle, and a flicker curve If sudden rms voltage devi-ations occurred with specified frequencies exceeding values found inflicker curves, such as one shown in Fig 11.16, the system was said tohave experienced flicker A sample graph of rms voltage variations isshown in Fig 11.17 where large voltage deviations up to 9.0 V rms (⌬V/V

⫽ ± 8.0 percent on a 120-V base) are found Upon comparing this to theflicker curve in Fig 11.16, the feeder would be experiencing flicker,regardless of the duty cycle of the load producing the flicker, because

Figure 11.15 A low-cost power quality monitor that can page power quality

engi-neers when disturbances occur.

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any sudden total change in voltage greater than 7.0 V rms results inobjectionable flicker, regardless of the frequency The advantage tosuch a method is that it is quite simple in nature and the rms datarequired are rather easy to acquire The apparent disadvantage to such

a method would be the lack of accuracy and inability to obtain the exactfrequency content of the flicker

Fast Fourier transform. Another method that has been used to measureflicker is to take raw samples of the actual voltage waveforms and

Figure 11.16 Flicker curves from IEEE Standards 141 and 519.

Figure 11.17 RMS voltage variations.

implement a fast Fourier transform on the demodulated signal (flickersignal only) to extract the various frequencies and magnitudes found inthe data These data would then be compared to a flicker curve Althoughsimilar to using the rms strip charts, this method more accurately quan-tifies the data measured due to the magnitude and frequency of theflicker being known The downside to implementing this method is asso-ciated with quantifying flicker levels when the flicker-producing loadcontains multiple flicker signals Some instruments compensate for this

by reporting only the dominant frequency and discarding the rest

Flicker meters. Because of the complexity of quantifying flicker levelsthat are based upon human perception, the most comprehensiveapproach to measuring flicker is to use flicker meters A flicker meter

is essentially a device that demodulates the flicker signal, weights itaccording to established “flicker curves,” and performs statisticalanalysis on the processed data

Generally, these meters can be divided up into three sections In thefirst section the input waveform is demodulated, thus removing thecarrier signal As a result of the demodulator, a dc offset and higher-fre-quency terms (sidebands) are produced The second section removesthese unwanted terms using filters, thus leaving only the modulating(flicker) signal remaining The second section also consists of filtersthat weight the modulating signal according to the particular meterspecifications The last section usually consists of a statistical analysis

of the measured flicker

The most established method for doing this is described in IECStandard 61000-4-15.8 The IEC flicker meter consists of five blocks,which are shown in Fig 11.18

Block 1 is an input voltage adapter that scales the input half-cyclerms value to an internal reference level This allows flicker measure-ments to be made based upon a percent ratio rather than be dependentupon the input carrier voltage level

Block 2 is simply a squaring demodulator that squares the input toseparate the voltage fluctuation (modulating signal) from the mainvoltage signal (carrier signal), thus simulating the behavior of theincandescent lamp

Block 3 consists of multiple filters that serve to filter out unwantedfrequencies produced from the demodulator and also to weight theinput signal according to the incandescent lamp eye-brain response.The basic transfer function for the weighting filter is

ᎏᎏᎏ(1⫹ s/␻3) (1⫹ s/␻4)

(See IEC Standard 61000-4-15 for a description of the variables usedabove.)

Block 4 consists of a squaring multiplier and sliding mean filter Thevoltage signal is squared to simulate the nonlinear eye-brain response,while the sliding mean filter averages the signal to simulate the short-term storage effect of the brain The output of this block is considered

to be the instantaneous flicker level A level of 1 on the output of thisblock corresponds to perceptible flicker

Block 5 consists of a statistical analysis of the instantaneous flickerlevel The output of block 4 is divided into suitable classes, thus creat-ing a histogram A probability density function is created based uponeach class, and from this a cumulative distribution function can beformed

Flicker level evaluation can be divided into two categories,

short-term and long-short-term Short-short-term evaluation of flicker severity PST isbased upon an observation period of 10 min This period is based uponassessing disturbances with a short duty cycle or those that produce

continuous fluctuations PSTcan be found using the equation

PST⫽ 兹0.031苶4P苶0.1⫹苶0.0525P苶1s⫹苶0.0657P苶3s⫹苶0.28P 10s苶⫹ 0.08P苶50s苶

where the percentages P0.1, P 1s , P 3s , P 10s , and P 50sare the flicker levelsthat are exceeded 0.1, 1.0, 3.0, 10.0, and 50.0 percent of the time,respectively These values are taken from the cumulative distribution

curve discussed previously A PSTof 1.0 on the output of block 5 sents the objectionable (or irritable) limit of flicker

repre-For cases where the duty cycle is long or variable, such as in arc naces, or disturbances on the system that are caused by multiple loadsoperating simultaneously, the need for the long-term assessment of

fur-flicker severity arises Therefore, the long-term fur-flicker severity PLTis

derived from PSTusing the equation

where N is the number of PSTreadings and is determined by the dutycycle of the flicker-producing load The purpose is to capture one dutycycle of the fluctuating load If the duty cycle is unknown, the recom-

mended number of PSTreadings is 12 (2-h measurement window).The advantage of using a single quantity, like Pst, to characterizeflicker is that it provides a basis for implementing contracts anddescribing flicker levels in a much simpler manner Figure 11.19 illus-trates the Pst levels measured at the PCC with an arc furnace over a

24-h period The melt cycles when the furnace was operating can beclearly identified by the high Pst levels Note that Pst levels greaterthan 1.0 are usually considered to be levels that might result in cus-tomers being aware of lights flickering.

11.3.10 Smart power quality monitors

All power quality measurement instruments previously described aredesigned to collect power quality data Some instruments can send thedata over a telecommunication line to a central processing location foranalysis and interpretation However, one common feature amongthese instruments is that they do not possess the capability to locallyanalyze, interpret, and determine what is happening in the power sys-tem They simply record and transmit data for postprocessing

Since the conclusion of the EPRI DPQ project in Fall 1995, it wasrealized that these monitors, along with the monitoring practice previ-ously described, were inadequate An emerging trend in power qualitymonitoring practice is to collect the data, turn them into useful infor-mation, and disseminate it to users All these processes take placewithin the instrument itself Thus, a new breed of power quality mon-itor was developed with integrated intelligent systems to meet this newchallenge This type of power quality monitor is an intelligent powerquality monitor where information is directly created within theinstrument and immediately available to the users A smart power

quality monitor allows engineers to take necessary or appropriateactions in a timely manner Thus, instead of acting in a reactive fash-ion, engineers will act in a proactive fashion.

One such smart power quality monitor was developed by ElectrotekConcepts, Dranetz-BMI, EPRI, and the Tennessee Valley Authority (TVA)(Fig 11.20) The system features on-the-spot data analysis with rapidinformation dissemination via Internet technology, e-mails, pagers, andfaxes The system consists of data acquisition, data aggregation, commu-nication, Web-based visualization, and enterprise management compo-nents The data acquisition component (DataNode) is designed tomeasure the actual power system voltages, currents, and other quanti-ties The data aggregation, communication, Web-based visualization, andenterprise management components are performed by a mission-specificcomputer system called the InfoNode The communication between thedata acquisition device and the InfoNode is accomplished through serialRS-232/485/422 or Ethernet communications using industry standardprotocols (UCA MMS and Modbus) One or more data acquisition devices,

or DataNodes, can be connected to an InfoNode

The InfoNode has its own firmware that governs the overall tionality of the monitoring system It acts as a special-purpose data-base manager and Web server Various special-purpose intelligentsystems are implemented within this computer system Since it is aWeb server, any user with Internet connectivity can access the dataand its analysis results stored in its memory system The monitoringsystem supports the standard file transfer protocol (FTP) Therefore, adatabase can be manually archived via FTP by simply copying the data-base to any personal computer with connectivity to the mission-specificcomputer system via network or modem Proprietary software can beused to archive data from a group of InfoNodes

func-11.3.11 Transducer requirements

Monitoring of power quality on power systems often requires ducers to obtain acceptable voltage and current signal levels Voltagemonitoring on secondary systems can usually be performed with directconnections, but even these locations require current transformers(CTs) for the current signal

trans-Many power quality monitoring instruments are designed for inputvoltages up to 600 V rms and current inputs up to 5 A rms Voltage andcurrent transducers must be selected to provide these signal levels.Two important concerns must be addressed in selecting transducers:

1 Signal levels. Signal levels should use the full scale of the ment without distorting or clipping the desired signal

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2 Frequency response. This is particularly important for transientand harmonic distortion monitoring, where high-frequency signalsare particularly important.

These concerns and transducer installation considerations will now bediscussed

Signal levels. Careful consideration to sizing of voltage transducers(VTs) and CTs is required to take advantage of the full resolution of theinstrument without clipping the measured signal Improper sizing canresult in damage to the transducer or monitoring instrument

Digital monitoring instruments incorporate the use of ital (A/D) converters These A/D boards convert the analog signalreceived by the instrument from the transducers into a digital signalfor processing To obtain the most accurate representation of the signalbeing monitored, it is important to use as much of the full range of theA/D board as possible The noise level of a typical A/D board is approx-imately 33 percent of the full-scale bit value (5 bits for a 16-bit A/Dboard) Therefore, as a general rule, the signal that is input to theinstrument should never be less than one-eighth of the full-scale value

analog-to-dig-so that it is well above the noise level of the A/D board This can beaccomplished by selecting the proper transducers

Voltage transducers. VTs should be sized to prevent measured bances from inducing saturation in the VT For transients, this gener-ally requires that the knee point of the transducer saturation curve be

distur-at least 200 percent of nominal system voltage

Signature System Architecture

Web Browsers

InfoNodes

DataNodes

Figure 11.20 A smart power quality monitoring system—it

turns data into information on the spot and makes it

avail-able over the Internet (Courtesy of Dranetz-BMI.)

Example 1. When monitoring on a 12.47-kV distribution feeder andmeasuring line-to-ground, the nominal voltage across the primary ofthe voltage transducer will be 7200 V rms.

A VT ratio of 60:1 will produce an output voltage on the VT of 120 Vrms (170 V peak) for a 7200-V rms input Therefore, if the full-rangevalue of the instrument is 600 V rms and the instrument incorporates

a 16-bit A/D board, 13⫹ bits of the A/D board will be used

It is always good practice to incorporate some allowance in the culations for overvoltage conditions The steady-state voltage shouldnot be right at the full-scale value of the monitoring instrument If anovervoltage occurred, the signal would be clipped by the A/D board, andthe measurement would be useless Allowing for a 200 percent over-voltage is suggested This can be accomplished by changing the inputscale on the instrument, or sizing the VT accordingly

cal-Current transducers. Selecting the proper transducer for currents ismore difficult The current in any system changes more often and withgreater magnitude than the voltage Most power quality instrumentmanufacturers supply CTs with their equipment These CTs come in awide range of sizes to accommodate different load levels The CTs areusually rated for maximum continuous load current

The proper CT current rating and turns ratio depend on the surement objective If fault or inrush currents are of concern, the CTmust be sized in the range of 20 to 30 times normal load current Thiswill result in low resolution of the load currents and an inability toaccurately characterize load current harmonics

mea-If harmonics and load characterization are important, CTs should beselected to accurately characterize load currents This permits evalua-tion of load response to system voltage variations and accurate calcu-lation of load current harmonics

Example 2. The desired current signal to the monitoring instrument

is 1 to 2 A rms Assuming a 1-A value, the optimum CT ratio for anaverage feeder current of 120 A rms is 120:1 Manufacturer’s data com-monly list a secondary current base of 5 A to describe CT turns ratiosrather than 1 A The primary rating for a CT with a 5-A secondary rat-ing is calculated as follows:

Thus, a 600:5 CT would be specified

120⭈ 5ᎏ1

Frequency response. Transducer frequency response characteristicscan be illustrated by plotting the ratio correction factor (RCF), which isthe ratio of the expected output signal (input scaled by turns ratio) tothe actual output signal, as a function of frequency.

Voltage transducers. The frequency response of a standard meteringclass VT depends on the type and burden In general, the burdenshould be a very high impedance (see Figs 11.21 and 11.22) This isgenerally not a problem with most monitoring equipment availabletoday Power quality monitoring instruments, digital multimeters(DMMs), oscilloscopes, and other instruments all present a very highimpedance to the transducer With a high impedance burden, theresponse is usually adequate to at least 5 kHz A typical RCF is plotted

in Figs 11.21 and 11.22 for two VT burdens.9

Some substations use capacitively coupled voltage transformers

(CCVTs) for voltage transducers These should not be used for general

power quality monitoring There is a low-voltage transformer in lel with the lower capacitor in the capacitive divider This configurationresults in a circuit that is tuned to 60 Hz and will not provide accuraterepresentation of any higher-frequency components

paral-Measuring very high frequency components in the voltage requires acapacitive divider or pure resistive divider Figure 11.23 illustrates thedifference between a CCVT and a capacitive divider Special-purposecapacitor dividers can be obtained for measurements requiring accu-rate characterization of transients up to at least 1 MHz

Current transducers. Standard metering class CTs are generally adequatefor frequencies up to 2 kHz (phase error may start to become significantbefore this).10For higher frequencies, window-type CTs with a high turns

ratio (doughnut, split-core, bar-type, and clamp-on) should be used

Additional desirable attributes for CTs include

1 Large turns ratio, e.g., 2000:5 or greater

2 Window-type CTs are preferred Primary wound CTs (i.e., CTs in

which system current flows through a winding) may be used, vided that the number of turns is less than five

pro-3 Small remnant flux, e.g., ±10 percent of the core saturation value

4 Large core area The more steel used in the core, the better the quency response of the CT

fre-5 Secondary winding resistance and leakage impedance as small aspossible As shown in Fig 11.24, this allows more of the output sig-nal to flow into the burden, rather than the stray capacitance andcore exciting impedance

Installation considerations. Monitoring on the distribution primaryrequires both voltage and current transducers Selection of the bestcombination of these transducers depends on a number of factors:

■ Monitoring location (substation, overhead, underground, etc.)

■ Space limitations

■ Ability to interrupt circuit for transducer installation

■ Need for current monitoring

Substation transducers. Usually, existing substation CTs and VTs(except CCVTs) can be used for power quality monitoring

Utility overhead line locations. For power quality monitoring on tion primary circuits, it is often desirable to use a transducer that could

distribu-be installed without taking the circuit out of service Recently, ducers for monitoring both voltage and current have been developedthat can be installed on a live line

trans-These devices incorporate a resistive divider-type VT and type CT in a single unit A split-core choke is clamped around the phase

Figure 11.22 Frequency response of a standard VT with 100- ⍀ burden.

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conductor and is used to shunt the line current through the CT in theinsulator This method allows the device to be installed on the crossarm

in place of the original insulator By using the split-core choke, thephase conductor does not have to be broken, and thus, the transducerscan be installed on a live line

Initial tests indicated adequate frequency response for these ducers However, field experience with these units has shown that thefrequency response, even at 60 Hz, is dependent on current magnitude,temperature, and secondary cable length This makes this type ofdevice difficult to use for accurate power quality monitoring Care must

trans-be exercised in matching these transducers to the instruments

In general, all primary sites should be monitored with metering classVTs and CTs to obtain accurate results over the required frequency spec-trum Installation will require a circuit outage, but convenient designscan be developed for pole-top installations to minimize the outage.Another option for monitoring primary sites involves monitoring atthe secondary of an unloaded distribution transformer This will giveaccurate results up to at least 3 kHz This option does not help with thecurrent transducers, but it is possible to get by without the currents atsome circuit locations (e.g., end of the feeder) This option may be par-ticularly attractive for underground circuits where the monitor can beinstalled on the secondary of a pad-mounted transformer

primary winding

secondary winding Zp

Ze

Zs Zb Cs

Frequency in Hertz

Figure 11.24 Frequency response of a window-type CT.

Primary wound CTs are available from a variety of CT turers Reference 2 concludes that any primary wound CT with a sin-gle turn, or very few turns, should have a frequency response up to

manufac-10 kHz

End-user (secondary) sites. Transducer requirements at secondary sitesare much simpler Direct connection for the voltage is possible for120/208- or 277/480-V rms systems This permits full utilization of theinstrument’s frequency-response capability

Currents can be monitored with either metering CTs (at the serviceentrance, for example) or with clamp-on CTs (at locations within thefacility) Clamp-on CTs are available in a wide range of turns ratios.The frequency range is usually published by the manufacturer

Summary of transducer recommendations Table 11.2 describes ent monitoring locations and the different types of transducers that areadequate for monitoring at these locations

differ-Table 11.3 describes the different power quality phenomena and theproper transducers to measure that type of power quality problem.Tables 11.2 and 11.3 should be used in conjunction with each other todetermine the best transducer for a given application

Summary of monitoring equipment capabilities. Figure 11.25 rizes the capabilities of the previously described metering instruments

summa-as they relate to the various categories of power quality variations

Measurement Data

As utilities and industrial customers have expanded their powerquality monitoring systems, the data management, analysis, andinterpretation functions have become the most significant challenges

in the overall power quality monitoring effort In addition, the shift

in the use of power quality monitoring from off-line benchmarking toon-line operation with automatic identification of problems and con-cerns has made the task of data management and analysis even morecritical

There are two streams of power quality data analysis, i.e., off-lineand on-line analyses The off-line power quality data analysis, as theterm suggests, is performed off-line at the central processing locations

On the other hand, the on-line data analysis is performed within theinstrument itself for immediate information dissemination Both types

of power quality data assessment are described in Secs 11.4.1 and11.4.2

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TABLE 11.2 VT and CT Options for Different Locations

capacitive or resistive dividers

Calibrated bushing taps

Special-purpose dividers

TABLE 11.3 VT and CT Requirements for Different Power Quality Variations

*VTs are usually not required at locations below 600 V rms nominal.

Figure 11.25 Power quality measurement equipment capabilities.

11.4.1 Off-line power quality data

assessment

Off-line power quality data assessment is carried out separately fromthe monitoring instruments Dedicated computer software is used forthis purpose Large-scale monitoring projects with large volumes ofdata to analyze often present a challenging set of requirements forsoftware designers and application engineers First, the softwaremust integrate well with monitoring equipment and the large num-ber of productivity tools that are currently available The storage ofvast quantities of both disturbance and steady-state measurementdata requires an efficient and well-suited database Data manage-ment tools that can quickly characterize and load power quality datamust be devised, and analysis tools must be integrated with the data-base Automation of data management and report generation tasksmust be supported, and the design must allow for future expansionand customizing

The new standard format for interchanging power quality data—thePower Quality Data Interchange Format (PQDIF)—makes sharing ofdata between different types of monitoring systems much more feasi-ble This means that applications for data management and dataanalysis can be written by third parties and measurement data from awide variety of monitoring systems can be accessible to these systems.PQView (www.pqview.com) is an example of this type of third-partyapplication The PQDIF standard is described in Sec 11.6

The off-line power quality data assessment software usually forms the following functions:

per-■ Viewing of individual disturbance events

■ RMS variation analysis which includes tabulations of voltage sagsand swells, magnitude-duration scatter plots based on CBEMA, ITI,

or user-specified magnitude-duration curves, and computations of awide range of rms indices such as SARFI, SIARFI, and CAIDI

■ Steady-state analysis which includes trends of rms voltages, rms rents, and negative- and zero-sequence unbalances In addition,many software systems provide statistical analysis of various mini-mum, average, maximum, standard deviation, count, and cumula-tive probability levels Statistics can be temporally aggregated anddynamically filtered Figures 11.26 and 11.27 show the time trend ofphase A rms voltage along with its histogram representation

cur-■ Harmonic analysis where users can perform voltage and current monic spectra, statistical analysis of various harmonic indices, andtrending overtime

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■ Transient analysis which includes statistical analysis of maximumvoltage, transient durations, and transient frequency.

■ Standardized power quality reports (e.g daily reports, monthlyreports, statistical performance reports, executive summaries, cus-tomer power quality summaries)

Samples: 1404 Minimum: 6873.0806

distribu-■ Analysis of protective device operation (identify problems).

■ Analysis of energy use

■ Correlation of power quality levels or energy use with importantparameters (e.g., voltage sag performance versus lightning flashdensity)

■ Equipment performance as a function of power quality levels ment sensitivity reports)

(equip-11.4.2 On-line power quality data

assessment

On-line power quality data assessment analyzes data as they are tured The analysis results are available immediately for rapid dis-semination Complexity in the software design requirement for on-lineassessment is usually higher than that of off-line Most features avail-able in off-line analysis software can also be made available in an on-line system One of the primary advantages of on-line data analysis isthat it can provide instant message delivery to notify users of specificevents of interest Users can then take immediate actions upon receiv-ing the notifications Figure 11.28 illustrates a simple message deliv-ered to a user reporting that a capacitor bank located upstream from adata acquisition node called “DataNode H09_5530” was energized at05-15-2002 at 04:56:11 A.M The message also details the transientcharacteristics such as the magnitude, frequency, and duration alongwith the relative location of the capacitor bank from the data acquisi-tion node

cap-Figure 11.29 shows another example of the on-line power qualityassessment It shows the time trend of a fifth-harmonic current mag-nitude along with its statistical distribution The data and its analysisare displayed on a standard Web browser Here a user can analyze data

up to the current time This on-line system has the capability of forming a full range of transient, harmonic, and steady-state charac-terization along with their statistical distribution analysis comparable

per-to that in off-line assessment analysis

Many advanced power quality monitoring systems are equipped witheither off-line or on-line intelligent systems to evaluate disturbancesand system conditions so as to make conclusions about the cause of theproblem or even predict problems before they occur The applications ofintelligent systems or autonomous expert systems in monitoringinstruments help engineers determine the system condition rapidly

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