Special Feature: Power quality

Monitoring power
for the future
Power quality has no standard definition. IJyou can accomplish what you want
with the power available then it is generally considered ‘good power quality’.
Simple availability is only one determinant of power quality. Howeve? when
problems prevent reliable equipment operalion, the malfunction is often attributed
to a power quality problem.

by Afroz K. Khan

    n general, the generated electricity we         cnstoiner hcilities have also changed. The
    receive is quite ‘good’ from an electrical      emergence of 24 hours per clay, 7 clays per week
    perspective. With the exception of a few        operations are commonplace and required to
    weather-related environinent issues and         meet a new competitive marlietplacc. Today, a
random accidents, most of the power quality         clear example of this technological growth is
problems are as a result of the interaction         the global emergence of Internet data centres.
between the connected load and facility             The level o l reliability demandccl/neetledl
electrical system. The iinpact ol thesc loads       required is now changing with the type of
results in adverse electrical characteristics and   facility and its needs. To ensure continual
account for inany ‘power quality’ problems. To      productivity and growth, the electrical
better understand these issues, monitoring 01       environment needs to be pure and clean.
power quality has become very important.            However, how good is good enough?
                                                       Table 1 illustrates the concept of the ‘9s of
The ‘9s’of reliability                              reliability’. The acceptable nuinber of dis-
The electric power system environmeiit is in        ruptions per year is a way to determine how
a state of constant flux. Customer loads have       critical the facility operations is and what level
evolved from simple linear devices, such as         of electrical power delivery is desired. Power
inotors and incandescent lighting to inore          quality, can now be used as a metric for solving
sophisticated, power electronics based loads        the reliability concerns associated with the
such as computers, coinpact fluorescent             sensitive nature of a customer Facility
lighting etc. The nature of these new loads has
created the need to closely examine the             Power quality monitoring evolution
existing electrical infrastructure. In addition     The philosophy of power quality monitoring
to this new market, traditional facilities are      has evolved from a strictly troubleshooting
managing business and inanulacturing opera-         objective to a multitude of purposes. These
tions through sophisticated power-electronics       purposes include: assuring compliance to a
based devices for continuous performance.           contractual oc legal standard; determining
   Besides the load changes, the types of           preventive practices; ancl establishing a

Table 1 The 9s of reliability in power delivery

POWER ENGINEERING JOURNALAPRIL 2001                                                                      81
Special Feature: Power quality

1 Power quality
                                         voltmeter technology          oscilloscope                       digital signal processing
                                         paper tape output             graphics                           integration of PC
                                                                                                          data storage
                                                                                                          communications (ethernet)


                                         field service engineers   0   field service engineers            utility companies
                                                                       utility power quality groups   0   industrial/plant/facility
                                                                                                      0   engineersiconsuitants


                      predictive baseline. In conjunction with the                    collected has increased. Early monitors offered
                      shifts in objective, monitoring equipment                       a low sample rate of 16 samples per cycle. With
                      has incorporated numerous technological                         this limited resolution, details on the variations
                      advancements in the areas oi communication,                     in the waveform are difficult to capture. Today,
                      data capture, physical size and cost. As data is                sampling rates as high as 640 samples per cycle
                      being made available more quickly, the desire                   are now available. The sample rate, combined
                      to use that information to meet defined                         with the quality and performance level of
                      objectives is only natural. With the increasing                 the A/D converter, helps determine the resolu-
                      emphasis on reliability, the need for ensuring                  tion of the waveform. Table 2 details specific
                      both the accuracy and the depth of measure-                     areas of advancement in the power quality
                      ments has become paramount. Fig. 1illustrates                   instrumentation market.
                      how the monitoring objectives and the market
                      have evolved from the 1970s to the present day.                 Power quality monitoring essentials
                         As seen in the tiineline in Fig. 1,advances in               Data is information without context. Taking
                      digital signal processing technology, micro-                    data, converting it into usable information and
                      processors and storage devices have changed                     sharing it in a user-friendly environment is
                      the way in which power quality measurements                     required for malting decisions. Most power
                      are made and presented. Older technology did                    quality instruments collect data continuously
                      not have the speed, storage or computational                    at high-speed sampling rates. These databases
                      ability to calculate and sort power disturbances                can grow to over lO0Mbytes in size. A very
                      in real time. In many cases, power engineers                    small percentage of this data is actually mined
                      wish to use spreadsheet programs to perform                     for information. The capability to analyse and
                      engineering calculations, plot trends or                        distil information for presentation is essential
                      perform statistical studies. Previously available               to understand the translation from data to
                      data formats restricted engineers froin easily                  decisions.
                      transferring measurement data into computers.                      In determining a power quality monitoring
                      In addition to being able to analyse the data                   system for the critical needs of the changing
                      collected. the breadth of the information                       market, there arc two key areas of con-

                      Table 2 Power quality monitoring equipment achievements

                       Communication speed           1200

82                                                                                       POWER ENGINEERING JOURNALAPRIL 2001
                                                                         Special Feature: Power quality

sideration; the instrument hardware and the          the end of the event is designated. A maxiinurn
software capability for data collection and          number of 128 individual wave shape cycles
analysis. These elements together are essential      can be stored and displayed. If RMS has not
for determining the optimum configuration for        slabilised after two seconds, the wave shape
a ‘comprehensive monitoring system’ (Fig. 2).        information is discarded ancl then the RMS
                                                     values are uscd to display the event. RMS
Instrument hardware                                  events are also graded by calculating the
Power qnality instrumentation has made               product of the square of the RMS deviation and
incredible advances over time. Advanced              the duration. The periods during which the
power quality monitors are designed to accept        RMS values and the wave shape change lroin
voltage and current quantities. Typical instru-      one stable value to a new value are defined as
ment input h i t s are around 6OOV RMS for           transition events. The instrument classilies
voltage and 5A RMS for current. To obtain            events either as steady-state events, or
usable voltage ancl current signal levels, voltage   transition events. Two wave shape transition
transducers (PTs) and current transformers           events d e h e the start and stop of a RMS event.
(CTs) are uscd. Digital monitoring instrumenls       Low, sensitive thresholds are necessary for fnll
also utilise A D converters, to convert the          disclosure monitoring.
analogue signals from the signal conditioning           However, if a high level of activity is
circuits into numeric values to process in           generating many events the instrument
the instrument. To get the best resolution the       memory could become saturated. In a Cull
A/D converter should have a high order of            disclosure monitor, thresholds start at very low
bits, somewhere between 16 and 20. After             values. If the rate of incoming events will cause
the signals have been digitised, the processor       the memory to overflow before the monitoring
operates on the fundamental measurements of          period is over, software internal to the monitor
time, voltage and current to derive many power       raises the thresholds in 0.1 25V increments
quality parameters.                                  on successive cycles to regulate the rate of
   Some of the issues to consider when looking       capture. If event activity slows down, the
at power quality hardware are:                       thrcsholds begin to lower in 0. I25V
                                                     increments; the threshold levels arc constantly     2 Comprehensive
  Backup of data collected even when the             readjusting to match the event activity             monitoring system
  power fails.
  Enclosure-depends on portable or perma-
  nent application.
  Sampling rate-is it fast enough for RMS,
  transients, energy and harmonics measure-
  men IS?
  Ease of use, set up and programming.
  Processing power-how         much data is
  reduced by instrument?
  Communications-current       state of the art
  uses Ethernet. TCP/Il?

For most users, the most important item to
consider is capturing the power quality event
and not running out of memory while
monitoring. For event capture, a common
method for triggering RMS variations is to set
manual thresholds. Typical settings are + I 0%
of nominal voltage.
   An alternative method is to provide
full disclosure monitoring with adaptive
thresholds. If the RMS value changes by more
than 2% of the RMS value of the previous cycle,
the wave shape change is triggered and stored.
Each o l the RMS cycle values is stored ancl
when the wave shape stabilises for two cycles,

POWER ENGINEERING JOURNALAPRIL 2001                                                                                          a3
Special Feature: Power- quality

                         and rate of capture to inemory capacity             having the capability to extract information
                            Adaptive thresholds inanage the instrument       from dill'erent databases for comparison into
                         ineniory without the operator needing to            another non-proprietary database structure is
                         be present. They prevent the monitor from           essential for correlation purposes.
                         coming to a halt in noisy, high-activity               With the advanced tools available, coii-
                         situations, and ensure that the monitor             tinuous monitoring is possible. This data is
                         completes its monitoring cycle.                                         iieecled to deteriniiic how and
                         In such a scenario the monitor,                                         where events were caused,
                         in eflect, captures the Worst         Harmonics must leacling to an understaiidiiig
                         case, most severe events, and                                           of how they may be avoided
                         also provides a continuous         be monitored            On     a     in the future, and how the
                         cycle-by-cycle record ol RMS         continuous basis effects may be mitigated
                         voltage and current history,                                            Other benefits ol continuous
                         plus power consumption for          to ensure that the monitoring are
                         the entire monitoring period.
                                                             i.ncrementa'          loads            fiarinonics      must     be
                         Dala collection atid analyTi5           do     not cause                   inonitored on a con-
                         tools                                                                      tinuous basis to ensure
                         The evolution of the coinputer     eXCeSSiVe heating,                      that, over time, the
                         industry has enabled sop-                                                  incremental addition of
                         histicated tools to emerge that
                                                                     leading to                     loads does not cause
                         facilitate the data collection       premature failure                      excessive heating that can
                         and analysis process. Advanced                                             lead to the prelim Lure
                         monitoring software tools for                                              failure of transformers,
                         the future require the following capabilities:        conductors and circuit hreakers.
                                                                               Continuous tracking of power consumption
                           A n open database architecture (ODBC).              and RMS quantities provide uscful data
                           Automation of data collection a i d report/         for planning future plant expansion
                           alarm generation.                                   and for ensuring that existing and future utility
                           Text and graphic export into other applica-         feeder and substation capacities are adequate. It
                           tions (i.e. Microsoft Excel and M7oi-d).            is also uselul lor planning the addition of new
                           Report writer capability                            discrete loads or new back-up power systems.
                                                                               The data is also useful in determining whether
                         One of the most important of the items above          an adequate level of investment in the
                         is using an ODBC compliant software tool.             infrastructure is being made and iC the money is
                         Depending on the power quality monitoring             being spent in the most appropriate areas.
                         objective, different instruments may be used          Archiving power survey reports containing
                         for differing monitoring situations. As a result,     all aspects of power in detail in a database

                                                                                                      ase A RMS Current

3 Sophisticated
software tools used to
overlay parameters

                                                                                 POWER ENGINEERING JOURNALAPRIL 2001
                                                                             Special Feature: Power quality

                                                                                                                   4 Future technology
                                                                                                                   incorporating all modes
                                                                                                                   of technology transfer
                                                   access to data

 provides the ability to compare existing                Technology advances have enablcd signili-
 conditions with historical coiitlitioiis and         cant improvements in the collection and
 to predict when a failure will occur. This           sharing of data across all media; the latest
 concept is called ‘predictive maintenance’.          being the Internet. The possibilities in this new
 Advance warning of a deteriorating power             medium arc extraordinary. Mission-critical
 situation provides the ability to correct            facilities are now able to deliver information
 the situation bcforc disaster strilics. In           to concerned individuals by simply sending
 mission critical operations, data centres            a web address via e-mail, page or fax. This
 and financially vulnerable applications,             link allows an investigator to determine
 prevention yields enormous economic                  immediately-and remotely-the information
 benefits by ensuring continuous system               required to apply predictive, preventive and
 reliability aiid full availability. Predictive       iiiaintenance measures.
 techniques provide the information required
 to achieve 100%‘up time’.                            Conclusions
 Keg personnel can be alerted immediately              Power quality monitoring is moving from
 when a power problem is detected by issuing           standards monitoring and troubleshooting
 alarms or by sending messages to pagers or            monitoring-, to focusing 011 performance
 PC screens. Immediate notification allows             objectives.
 action to he taken to isolate a problem               Monitoring systems allow a variety of topol-
 aiid prevent a doinino effect that would              ogies, including single-vendor or niultiple-
 jeopardise the entire facility.                       vendor solutions with various hardware and
                                                       software platrorms available, ensuring that
Sophisticated soltware tools also allow useful         any facility can build a system optimisetl for
comparisons ol various parameters. Fig. 3              their requiremeiits that will adapt and grow
illustrates an examplc or the RMS voltage and          with the facility.
voltage flicker on phase A overlaid on top of
                                                      O I F F 2001
each other. Software tools available for data
                                                      Ms. Afloz K. K h a n is a Senior Power Quality Coiisulrant
analysis after monitoring provide thc capability
                                                      with lleliahlc Power Meters located in Los Gatos,
to overlay dilferent quantities on top of each        California. She is the representative or her lirin
other for correlation of parameters with known        externally in many international standard organisations,
events.                                               task lorccs and trade shows.


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