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A web based distributed measurement system for electrical power quality assessment q Juan Jos Gonz lez de la Rosa a b 1 Antonio Moreno Mu oza c 1 Aurora Gil de Castroa c V ctor Pallar s L pez a c Jose Antonio S nchez Castillejoc aResearch Group PAIDI TIC 168 Computational Instrumentation and Industrial Electronics ICEI Andalusia Spain bUniversity of C diz Area of Electronics EPSA Av Ram n Puyol S N E 11202 Algeciras C diz Spain cUniversity of C rdoba Area of Electronics Campus de Rabanales Leonardo da Vinci building E 14071 C rdoba Spain a r t i c l ei n f o Article history Received 17 June 2008 Received in revised form 4 January 2010 Accepted 17 February 2010 Available online 21 February 2010 Keywords Distributed measurement system Power Quality PQ Smart sensors a b s t r a c t Industries systems are shifting toward almost entirely sophisticated electronic devices Power Quality PQ analysis is getting importance for the economy because this equipment is highly sensitive to PQ events Control and supervision of an industrial process has mainly been focused on the electrical protection and little attention has been paid to the quality of the electrical supplies Nowadays measurement and communications systems have advanced to enable the installation of web based sensors within a PQ assessment scenario In this sense this paper presents an innovative low cost measurement system as well as investigates the challenges and trends in the development of distributed PQ measurement systems using smart sensors 2010 Elsevier Ltd All rights reserved 1 Introduction As more and more electronic equipment enter the resi dential and business environment the subjects related to Power Quality PQ and its relationship to vulnerability of installations is becoming an increasing concern to the users The two main aspects of PQ are Technical PQ which includes Continuity of supply or reliability sustained interruptions and Voltage Quality VQ thatisvoltagelevelvariationsandvoltage disturbances Commercial services associated to the wires are regu lated such as the delay to get connected to the grid etc as well as commercial services for energy retail to regulated customers Sustained interruptions which occur when voltage falls to zero for more than one minute are the reliability prob lem with which most electricity consumers have the great estdirectexperienceandarethekeyphenomena measured in traditional utility service quality and reliabil ity statistics Indices such as and System Average Interrup tionFrequencyIndex SAIFI andCustomerAverage Interruption Duration Index respectively CAIDI do not capture PQ perturbations Assessment of VQ and power disturbances involves looking at electromagnetic deviations of the voltage or cur rent from the ideal single frequency sine wave of constant amplitude and frequency VQ problems commonly faced by facilities operations include transients sags swells surges outages harmonics and impulses that vary in quantity or magnitude of the voltage 1 A consistent set of defi nitions can be found in 2 The quality of the power 0263 2241 see front matter 2010 Elsevier Ltd All rights reserved doi 10 1016 j measurement 2010 02 009 q This work was funded and supported by the Andalusian Government towards the Research Unit PAIDI TIC 168 and by the Spanish Ministry of Science and Innovation via the project TEC2009 08988 Corresponding author Address University of C diz Research unit PAIDI TIC 168 Area of Electronics EPSA Av Ram n Puyol S N E 11202 Algeciras C diz Spain Tel 34 956028020 fax 34 956028001 E mail address juanjose delarosa uca es J J G de la Rosa URL http www uca es grupos inv TIC168 J J G de la Rosa 1 Main Researchers of the Research Unit PAIDI TIC 168 Measurement 43 2010 771 780 Contents lists available at ScienceDirect Measurement journal homepage supply delivered by utilities varies considerably and de pends on a number of external factors Things like light ning industrial premises which apply and remove large loads non lineal load stresses inadequate or incorrect wir ing and grounding or short circuits caused by animals branches vehicular impact and human accidents involving electric lines Some power providers seek to determine compliance with limits established in standards such as 3 Among all categories of disturbances the voltage sag dip and momentary interruption are the nemeses of the automated industrial processes 4 Voltage sag is com monly defi ned as any low voltage event between 10 and 90 of the nominal RMS voltage lasting between 0 5 and 60 cycles On the other hand voltage swells which are not so common do not normally disrupt sensitive load but can cause harm to equipment Momentary voltage interruption is any low voltage event of less than 10 of the nominal RMS voltage lasting between 0 5 cycles and 3 s Voltage sags can be caused by natural events e g trees falling on power lines or lightning striking lines or trans formers utility activities e g routine switching opera tionsorhumanerror orcustomeractivities e g starting of large motors Voltage sags at a customer bus are different depending to his location in the electrical net work 5 Because of the short duration of these PQ events residential customers are rarely aware that a VQ event has taken place However for many industrial customers they pose a far more signifi cant problem than outages because of their much greater frequency of occurrence and overall because of that their incidence can cause hours of manu facturing downtime 6 It has been discovered that the 85 of power supply malfunctions attributed to poor VQ are caused by voltage sag or interruptions under one sec ond of duration Some major problems associated with unregulated line voltages in particular long term voltage sags include equipment tripping stalling overheating and complete shutdowns of sensitive equipment if it is designed to oper ate within narrow voltage limits or it does not have ade quate ride through capabilities to fi lter out fl uctuations in the electrical supply These subsequently lead to lower effi ciencies higher power demand higher cost for power electromagnetic interference to control circuits excessive heating of cables and equipment and increased risk of equipment damage The need for line voltage regulation still remains a necessity to meet demands for high indus trial productivity There are several conditioning solutions to voltage regulation which are currently available in the marketplace Among the most common are Un interrupt ible Power Supply systems UPS Recently new technolo gies like Custom Power devices based on power electronic concepts have been developed to provide protection against PQ problems 7 However the most signifi cant barrier to improving the PQ is the lack of contemporaneous and historical PQ data 8 The norm IEC 61000 4 30 9 has been established to defi ne the methods for measurement and interpretation of VQ parameters 10 For each relevant type of perturba tion a measurement method is established Two different classes of measurement devices are defi ned according to the accuracy Class A devices are used when precise mea surements are necessary For example for contractual applications verifying compliance with standards resolv ing disputes etc Class B measuring instruments are used to determine statistical values and regulate correct errors troubleshooting A proper monitoring system that is capable of observing PQ phenomena on a continuous basis is needed since the occurrence of a power disturbance is highly unpredictable 10 The main characteristics and trends in advanced PQ measuring instruments can be found in 11 13 Last years some experimental solutions have been proposed In 14 the system is implemented by the use of standard LabView TCP IP technology on a partial building low voltage net work that could be a simplifi ed representative model of larger networks As before the system hardware in 15 consists of several PCs each one hosting a data acquisition DAQ board linked to the power system by a voltage transducer The software implements different Virtual Instruments using the well known NI LabView graphical programming language Moreover many utilities presently utilize dedicated PQ monitoring devices on the high and medium voltage stations to detect power quality events 16 One of the milestone approaches was originally developed for the Electric Power Research Institute in the last decade 17 However the most widely extended infra structure installed up to now is 18 On the other hand many automated factories presently utilize dedicated PQ measurement devices but on the point of common cou pling PCC and with the exclusive purpose of determining the utility responsibility in PQ events Little empirical research however is available from the perspective of integrating distributed measurement inside the plant In brief is needed a low cost system capable of observing PQ phenomena on a continuous ba sis This is imperative since most of the hidden cause come from unlikely sources such as operation of existing factory equipment or incorrect wiring schemes While all major events can be captured this could result in exces sive data from non critical events The maintenance engi neer must then sort through this data to analyze the PQ disturbance Manual methods are expensive time con suming and error prone Custom software developed for each site for analyzing data again can be very expensive to develop maintain and depending on their underlying architecture diffi cult to expand Thus design ing a system that has the ability of analyzing condensing and interpreting voluminous raw data so that levels can be assessed against limits and can be easily expanded would become the most signifi cant challenge in the PQ measurement arena This paper introduces an innovative and extremely low cost PQ web based measurement sys tem which is suitable for continuous PQ measurement in an industrial or commercial plant The paper is structured as follows Section 2 summa rizes the strategy developed in the design Sensor confi g urationisdescribedinSection3 Theinvolved measurement errors are detailed in Section 4 Timing protocols compatibility is justifi ed in Section 5 System operation is described in Section 6 Finally conclusions are drawn in Section 7 772J J G de la Rosa et al Measurement 43 2010 771 780 2 Design strategy 2 1 Requirements The web based measurement system acts as a sentinel that relies on widespread deployment of a number of low cost sensors termed PQ watch throughout the distri bution network of the plant They continuously monitor energy consumption as well as measure detailed data on PQ events including outages blackouts brownouts inter ruptions and short duration disturbances such as voltage sags and swells In addition to these features advanced functions for energy savings and improving Energy Use Diagnostics can be implemented It is also possible to gather all measured PQ data from various sites inside the plant Data from PQ watch sen sors allows to identify PQ events and determine their im pact on individual machines In addition because the data from all monitors are uploaded via the internet we can correlate data from multiple monitors to deter mine whether events recorded at one site were detected or had impact elsewhere From this information we will determinate the origin and cause of the event and the ef fect of the event on company processes downtime costs drawing conclusions about the responsibility of the utility PQ disturbances can range from high frequency im pulses caused by lightning strikes to long term sustained over voltages resulting from poor voltage regulation The capabilities of the measurement device will also infl uence the choice of which parameters to measure Broadly speak ing these sensors will meter the following quantities volt age and current active power apparent power total reactive power fundamental frequency reactive power en ergy use cost of power power factor total harmonic dis tortion of the supply voltage THD also the nine most important harmonic voltages frequency of the supply volt age and voltage unbalance between the three phases Addi tionalmeasurementsarevoltageswells sagsand interruptions The recording density could be set as some seconds minutes or hours The simplest version of the PQ monitor would record only aforementioned powers voltage and current variations Online and off line analysis have to be considered as implementation options Online analysis is devoted to the incidents that need immediate attention for example sags or interruptions The online data analysis can be per formed within the instrument itself or immediately upon collection of the information at a central processing loca tion In order to reduce the overburden data the event are grouped in categories transients interruptions or volt age sags and swells As exist the possibility of receiving several incident alarm at the same time it could be neces sary the use of some kind of distribution scoreboard algorithm that automatically classifi es events ranking importance This could be done by the use of different cri teria e g maximal time duration maximal magnitude var iation or maximal energy variation Regarding the off line PQ analysis the following infor mation is usually included in a report Statistics of the steady state variations voltage level harmonics unbalance or fl icker Statistics of the Event resulting from incidents sags short interruptions swells or transients Identifi cationofthelikelycauseofthepower disturbances Characterization of the electromagnetic compatibility level of equipments and installation Recommendationsforcost effectivemitigationand maintenance solutions In addition it is necessary a scaled approximation in this reporting activity differentiating between the detailed local approximation to a global point of view Thus the development of systematic procedures for PQ data man agement supports in general the reporting of distribution PQ surveys Site report e g detailing suffi ciently the quality reports planning of compensation of reactive power instruc tions for the use of various equipment Plant network report e g giving simple site indices for all measurement point for investment plans and man agement of voltage drops and other disturbances and offering the ability to correlate events recorded at geo graphically dispersed locations In all of the cases the report is different for variations and events with variations site indices being a kind of average and event site indices being a kind of event count 2 2 The method The method implemented in the web based system fol lows the three steps First to obtain sampled voltages with a certain sampling rate and resolution For every event voltage samples are obtained with a sampling rate estab lished as samples per second or per cycle and with a res olution determined stated as the number of bits Many commercial PQ measurement instruments have sampling rates of 256 samples per cycle since the majority of PQ events have frequency contents below 5 kHz Oscillo graphic data provides the most detailed information for engineering troubleshooting with the handicap of memory storage Secondly it calculates event features as a function of time from the sampled voltages This characteristics can include RMS voltages RMS currents negative and zero se quence un biases real and reactive power harmonic dis tortion levels and individual harmonic components For three phase measurements the three voltages will typi cally give different characteristics versus time for the three phases From these three functions additional characteris tics are needed Several methods have been proposed to achieve this goal Although 10 is mainly based on sin gle phasemeasurements formulti channelmeasure ments the worst phase is taken for further analysis when calculating single event indices This will be analyzed be low As a complement the event duration measurement can be started on one channel and terminated on a J J G de la Rosa et al Measurement 43 2010 771 780773 different channel this is the method proposed for sags characterization and can be extended to interruptions and swells too Both procedures reduce over counting and can be considered as a phase aggregation technique Finally the method calculates single indexes from the event and variations characteristics The key point consists of retrieving data in the time needed for the analysis Then PQ data can be recalculated and packed into indices that could be gathered and stored by the SCADA This includes voltage sags and swells in magnitude duration tables transients tables which include maximum voltage tran sient durations and transient frequency or magnitude duration scatter plots based on CBEMA ITIC or user spec ifi ed magnitude duration curves 19 In addition it could be provided statistical analysis of various minimum aver age maximum standard deviation count cumulative probability levels concretely this is necessary for the har monic indices Single indices give the end user an accurate diagnosis of what has occurred on the system for the off line analysis 2 3 Aggregation An aggregate PQ event is the collection of measure ments associated with a single occurrence which is labeled for computing purposes The aggregate event associated with a fault gathers the associated measurements magni tude duration etc Many customer devices and processes trip or miss operate on the initial sag associated with a fault Subse quent circuit protection operations would have not caused subsequent miss operations because the process had not yet been restarted Considering multiple events resulting from the same power system occurrence individually may distort the representation of the true state of the sys tem service quality Thus an aggregation period should be chosenrelativethetimingsequencesofprotection schemes The time length chosen for aggregation is arbi trary A 5 min period concurs with the maximum length of a