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Wed, 08/09/2006 - 10:09am

New technology improves data and service

‘With the large capacity memories available now, storing 52 weeks of a one—minute minimum, maximum, and average value is a simple process.’‘Sales and marketing can benefit from power quality and historical data to provide high levels of customer service and attract new businesses.’With Figure 1(NE Blackout frequency plot): This plot from the August 14, 2003 NE blackout minutes before the actual blackout shows the system frequency in Rochester, NY, swinging from 59.972 to 60.047Hz in less than 13 seconds. Moments later it was much more severe. This is a small portion of a 2—hour plot that is available from the more advanced recorders storing data at a 30Hz rate.With Figure 2: Breaker operations details showing a slow pole operation for phase B.With Figures 3 and 4: Sample displays that are available to replace panel meters that can be accessed locally or remotely.

By John Sperr

In the never—ending drive to get more from less, Electric Utility Engineers are continually looking for ways to get more information from fewer pieces of equipment. To support those needs, innovative vendors are providing utilities and electric power users with devices that have greater capabilities at lower costs than the earlier generation equipment. Transient Recorders are an example of this trend, through which, manufacturers have expanded their functionality well beyond simple electronic oscillograph.

Traditional Methods

In addition to traditional sinusoid recording (for seconds of data), modern transient recorders provide long—term recording for power swings (for several minutes), continuous recording of minimal resolution of data (for days), steady—state loggers (for weeks or months), power quality data, and fault location and information for scheduling breaker maintenance.

A Comparison of Features

        Digital Oscillograph Mid—range recorders High—end recorders Sinusoid data 1㬆kHz sample rates 3200 — 15k Hz 4.8 — 195kHz Disturbance data No In parallel with transient triggers Dedicated disturbance triggers Continuous storage No No 25/30Hz data rate RMS log No 1 minute min/max/avg 1 minute min/max/avg Hz log No 1 ch as 1 minute min/max/avg 2 ch as 1 minute min/max/avg Power log No 1 minute min/max/avg 1 minute min/max/avg Flicker log No No 10 min. Pst Harmonic log No 10 min. min/max/avg 10 min. min/max/avg plus angle Voltage unbalance log No No 10 min. min/max/avg Breaker maintenance No Yes Yes Fault location yes Yes Yes Power Quality No Yes Yes Synchrophasor data No No No Time synchronization IRIG—B (1msec.) IRIG—B (1 msec) Internal GPS (1 µsec) Chart recorder No Yes Yes Panel meters No Yes Yes Sequence—of—Events No Yes Yes

Sinusoid data has been used for years as a tool for verifying the operation of the overall protection scheme from the PTs, CTs, and relay to the breakers and carrier signals. Providing details on faults not only enables the protection engineers to detect faulty settings, relay algorithm problems, failed hardware and other issues to minimize time to investigate problems, but also provide documented data for legal and regulatory requirements.

Need for Change

Since the widespread power outages in the Western United States in the mid—to—late�s, and, more recently, the August 2003 blackout in the Northeastern United States, there has been a focus on the need for longer—term data. This has been reinforced with new requirements from NERC that requires Dynamic Disturbance Recorders (DDRs) at select locations on the power system.

Since recording sinusoid data for multiple minutes would create files >10Mbytes, a method was needed to store power system data at reasonable speeds without sacrificing the information available from the data. Some systems limit the number of channels or calculated values that are stored for disturbance records.p> However, an excellent balance can be obtained by storing submd>cycle phasor data on every channel along with RMS (or DC for appropriate signals) and a frequency channel. By storing triggered data at a 100 or 120Hz rate, slower phenomenon, such as power and frequency swings in the 0.3 md> 15Hz oscillation range, outmd>ofmd>step conditions, generator start up, and other slower, longermd>term problems can be diagnosed.p> This can also be useful in analyzing issues associated with large motor starts and other large load changes. This type of information can also be very useful at generating plants where it can take several minutes for problems to grow to the point of causing a plant trip or other serious issue.p>Recording a disturbance record with the faster transient record is a good compromise for less powerful systems. However, to get the most out of the recorders, it is best to have dedicated triggers that focus on these slower phenomena. By offering triggers over a variety of bands with dedicated time constants, the system is able to differentiate between a real problem and the normal oscillations that occur every day./t2> h3>Utilizing New Technology/h3>t3>The more advanced systems also take this a step further by continuously storing the phasor data at a 25 or 30 Hz data rate. This provides lower resolution data for distant faults that don’t trigger recorders, or where triggers were misapplied. One possible drawback to this is the volume of memory needed at the recorder. In a 32md>analog channel system, storing the RMS, phasor magnitude and angle, and two frequency values requires 6Gb of storage for two weeks worth of data./t3> t2>This is easily accomplished using modern hard drives integral to the recorder. However, streaming this type of data to an external device requires a dedicated, reliable communication link that typically cannot be used for other systems without compromising this primary function.p>(Figure 1)Another function available in the new generation of recorders allows them to be used as circular chart recorder replacements. Most substations had the old penmd>based circular recorders that required monthly paper replacement and annual pen replacement. Typically, these provided a 15md>minute average value of bus voltages for regulatory compliance requirements, were very labor intensive, and of little value beyond steadymd>state values.p>Newer systems are able to compute and store RMS values for not only the bus voltages, but the line currents too. With the large capacity memories available now, storing 52 weeks of a onemd>minute minimum, maximum, and average value is a simple process. This provides superior resolution, more details beyond just steadymd>state values (max and min), and files can be analyzed via computer programs to compute the total time that the system is operating within prescribed limits, such as EN 61000md>4md>30.p>In addition to computing steadymd>state RMS levels, the system frequency can also be recorded in the same type of longmd>term steadymd>state format. This can be used to replace strip chart recorders in control rooms at generating plants or control centers. Additionally, this information (frequency, Irms, Vrms, digital input status) is available on communication ports in industrymd>standard protocols, such as DNP, reducing or eliminating the need for RTUs at some locations.p>The new systems also provide the capability to create and use phase groups and line groups to compute a variety of parameters internal to the recorder. From phase groups, positive, zero, and negative sequence values, are computed, and voltage unbalance obtained. Combining a current phase group and a voltage phase group then provides the capability to compute Watts, VARs, Voltmd>Amps and power factor. These values are then used for triggering and logging steadymd>state values. This provides an overall measure of system loading and capacity for growth./t2>h3>New Challenges and New Solutions/h3>t3>Over the past 15 years, there has been an explosion in the use of switchmd>mode power supplies in everything from PC and consumer electronics to PLCs and other types of industrial controllers. These, along with adjustable speed drives and many other devices, are creating an amount of harmonic pollution on the power system never seen before./t3> t2>To counter this, IEEE and EN have implemented limits on harmonic magnitudes at the point of common coupling (PCC). Some countries have taken this a step further, and have regulatory limits on harmonics at different voltage levels. Because of this, utilities need to know what level of harmonics is present on the power system, and where they are coming from.p> Another phenomenon that has resurfaced after many decades of limited concern is flicker. This term originated from the way incandescent lights would “flicker” in intensity due to a modulation in the magnitude of the voltage. Human perception of this would result in headaches, reduced productivity, and other personnel problems.p> Caused by rapidly fluctuating loads, flicker is measured as an instantaneous, perceived short term (Psub>st/sub>), and perceived long term value (Psub>lt/sub>). The Psub>st/sub> is a 10md>minute value, and the Psub>lt/sub> is a 120md>minute value that can be calculated from the Psub>st/sub>. As with the other parameters noted above, some advanced fault recorders have the ability to compute and store a flicker value. Most use the IEC definition, which is based on 230V/50Hz, and then interpolate the data to 115V/60Hz if necessary. Typically, storing this on one set of voltages at a location is sufficient, since this will propagate through transformers and affect an entire location.p>Voltage unbalance is another parameter that can be computed and stored by these advanced recorders. Two different parameters can be calculated and storedmd> negative sequence unbalance and zero sequence unbalance. As the name would indicate, these are the ratio of the negative or zero sequence voltages to the positive sequence voltage measured as a percent (or fraction). These are excellent measures of the overall efficiency of the power system. Too high of a value, and there will be too many losses, costing the utility or consumer money.p>As with many IEDs in a substation, the modern oscillograph is monitoring a variety of signals that can be used for several different applications. An example of extending the digital transient recorder data to newer applications is breaker maintenance scheduling. By monitoring all three phasemd>currents through the breaker and the A or B contact on the breaker, three critical parameters can be tracked in a database md> total number of operations, duty, and total current. Additionally, data can be presented to maintenance personnel showing the duration of fault current for each individual operation. This can clearly flag them if there is fault current present for a longer time on one or two phases indicating a stuck pole (or an evolving fault).p> (Figure 2)By using an additional analog channel, the trip coil current can also be monitored for every operation. This gives an excellent indication of the travelmd>curve and acceleration within the breaker so deteriorating trends in the mechanical portion of the breaker can be acted upon before a catastrophic failure occurs./t2> h3>Improving Data and Service/h3>t3>Most utilities have used some form of fault location tied to the data from their oscillographs. Early methods consisted of using the current and voltage magnitudes from the recorded data, and correlating those numbers to specific locations from their fault studies. The new systems provide distancemd>tomd>fault data automatically. By using line models and impedancemd> algorithms, an actual fault resistance is calculated and distance computed. Now, without ever having to look at waveforms, this distance value can be obtained by those personnel that need it, but who may not be experts in analyzing oscillographs./t3>t2>In this era of deregulation, remd>regulation and competition, having a variety of data collected for different reports is critical to efficiently operate the overall T&D system to attract and keep customers. By tracking voltage dips and surges over time, it is possible to identify circuits to invest in to improve performance, and to show a beforemd>andmd>after quality.p> Additional graphs and tables can be created showing compliance to a variety of standards such as EN61000md>4md>30, IEEEmd>519, or state and national regulatory requirements./t2>h3>Analysis and Prevention/h3>t3>One of the newest applications for power system data is recording or transmitting Synchrophasor data. This consists of using the three phase voltages, and computing and precisely timemd>tagging the positive sequence phasor data. Using this data over a wide area, utilities are able to detect instabilities before they become a serious problem./t3> t2>The Western Electric Coordinating Council (WECC) has been collecting this data for several years, and the DOE is sponsoring the Eastern Interconnect Phasor Project (EIPP) to install substation devices for computing and transmitting phasor data. Additionally, the EIPP is sponsoring the development of applications to visualize this data, store it, and provide reports. A more recent development is that the IEEE Power System Relaying Committee is now voting on a new standard for transmitting this data to a central location. This should become IEEE C37.118md>2005 very soon./t2>h3>Time and Synchronization/h3>t3>In conjunction with the Synchrophasor capability, the system must have the ability to accurately timemd>tag this data to better than 30micro>sec. However, to minimize error due to timemd>tagging, it is better to minimize this. The more advanced systems have time synchronization accuracy in the range of 50md>100 nanoseconds./t3> t2>Independent of the Synchrophasor capability, with the need to analyze data from a wide geographic area, as witnessed by the recent NE Blackout, it is critical that all recoding and reporting devices are synchronized. This requires some source of UTC for all these distributed devices. Most common today is the GPS clock. These clocks typically output an IRIGmd>B or a pulsemd>permd>second signal.p> With this, it is easy to ensure that all devices are synchronized to plusmn>1msec. Additional considerations are UTC, local time, and daylight savings time. With the merger of utilities, there are many systems that now span several time zones, and it is best to keep all devices on the same time. Using UTC (formerly GMT) is an easy way to do this, but isn’t instantly intuitive to operators and other personnel as they look at the time tags. Typical practice at most utilities has been to settle on leaving all devices on standard timemd> no adjustments for daylight savings timemd> and if across multiple time zones, they will choose one and keep it there.p>For unmanned substations, the need for panel meters has been minimized. By taking advantage of the new technology in fault recorders and many other IEDs, they can be completely eliminated. With a laptop or PC, any analog input and many computed values can be displayed in real time, either locally in the substation or remotely via modem or TCP/IP. RMS current and voltage, Watts, VARs and Volt-Amps, and THD are just some of these parameters.t2>h3>Benefits/h3>t3>Not only do modern transient recorders communicate to vendormd>specific software for traditional date transfers, they now can interface to SCADA systems using DNP3.0, or other standard protocols. This once again saves space, equipment, installation and wiring costs, and engineering and drafting time when designing a new substation. With this interface, analog values, digital status, and frequency magnitudes are available in real time, eliminating the need for analog transducers, and reducing channel counts on the RTU./t3>t2>When all is said and done, a modern transient recorder not only complies with the latest requirements from NERC, but it also provides data for a variety of functions that were discrete devices in the past. Sales and marketing can benefit from power quality and historical data to provide high levels of customer service and attract new businesses.p>Maintenance Departments can use the data for optimizing breaker maintenance schedules, tree trimming or line upgrades, and other regular tasks. Protection and Relay Engineers can use not only the traditional oscillograms for analysis of faults, but verify operations, line models, and use the newer disturbance data for stability analysis. Planning can use the data for load growth and equipment sizing, and the system can be an interface for SCADA personnel./t2>(Figures 3 &4)h3>Conclusion/h3>t3>In summary, a modern transient recorder is a valuable tool used to replace several discrete devices, and the advanced support software provides answers, not just raw data. These systems, when tied to expert systems, provide a quick summary of all faults detailing the faulted circuit and phase(s), magnitude and duration of the fault, and what protection equipment operated. Location of faults, voltage quality information, and data for optimally scheduling breaker maintenance are additional types of answers that are giving utility engineers more for less./t3>bio>John Sperr is an active member of IEEE md> Power System Relaying Committee. He received his BS in Chemical Engineering from Clarkson University in 1984, and has worked for AMETEK Power Instruments in various salesmd>support and marketing roles, focusing on their transient recording and power monitoring products since 1985. AMETEK Power Instruments, 255 North Union Street, Rochester, NY 14605, is a recognized leader in the design and manufacture of power measurement and recording instrumentation used worldwide by the electric power and manufacturing industries. They offer an extensive line of fault recorders, annunciators, and power measurement products in the electric power industry and are a leader in sensor systems for gas turbine engines and for boilers and burners used by the utility, petrochemical, process, and marine industries worldwide. AMETEK Power Instruments is a unit of AMETEK, Inc., a leading global manufacturer of electronic instruments and electric motors with annualized sales of more than $1.6 billion. For more information, contact AMETEK Power Instruments at (585) 263md>7700, or via their website, www.ametekpower.com./bio>

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