Smart Grid Solutions Guide - Maxim

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Power-grid monitoringOverviewas well as detection of high-speedfault conditions, such as spikes andbrownouts.Analog-input signal chainDesigners of polyphase power-gridmonitoring systems are increasinglyrelying on precision, multichannelsimultaneous-sampling ADCs. TheseADCs simplify the sampling schemewhen compared to single ADCs thatrequire digital phase compensation.Simplifying the ADC design leads toreduced system cost, since all of thetiming among the ADCs is handledwithin the IC.The simultaneous-sampling ADCmust offer better than 90dB signalto-noiseratio (SNR) in order to meetthe dynamic range requirements formeasuring small voltage fluctuationson large AC signals. Maxim offerstwo simultaneous-sampling ADCfamilies that meet these requirements:the 24-bit MAX11040 with4 channels and 117dB SNR, andthe 16-bit MAX11044/MAX11045/MAX11046 with 4, 6, or 8 channelsand more than 92dB SNR.The transformers drive directly intothe ADC if the input impedance ofthe ADC is high enough; otherwise,a precision low-noise amplifier (LNA)is also needed. Maxim offers acomplete line of amplifiers with lessthan 10nV/√Hz noise and very lowoffsets. These amplifiers minimizemeasurement errors in the system toensure the highest possible accuracy.CommunicationsAside from the analog-input signalchain, power-grid monitoringsystems need specialized communicationscircuits to overcome thechallenges of transmitting andreceiving data in the harsh gridenvironment. Detailed informationabout power-grid communications isavailable on pages 25–40.Electronic calibrationAll practical components, bothmechanical and electronic, havemanufacturing tolerances. The morerelaxed the tolerance, the moreaffordable the component. Whencomponents are assembled into asystem, the individual tolerancessum to create a total system errortolerance. Through the properdesign of trim, adjustment, andcalibration circuits, it is possible tocorrect these system errors, therebymaking equipment more accurateand affordable.Digitally controlled calibration DACs(CDACs) and potentiometers (CDPots)provide quicker and simpler electroniccalibration than mechanical pots.They are also more reliable, andinsensitive to vibration and noise.Using these devices, manufacturerscan compensate for manufacturingtolerances during the final productiontest to meet target specifications.PHASE 1VTTRANSFORMERSECONDARYVTMAX11046CTTRANSFORMERSECONDARYCTADCADCNEUTRALADCLOADADCADCADCADCPHASE 2ADCPHASE 3An example of three-phase power monitoring in a wye configuration.14 Maxim Smart Grid Solutions
Power-grid monitoringOverviewAdditionally, electronic calibrationallows for periodic self-test and calibrationto compensate for environmentalfactors (e.g., temperature,humidity, and drift) in the field.CDACs and CDPots allow both thetop and bottom DAC voltage tobe set to arbitrary voltages, thusremoving excess adjustment range.In the calibration diagram below,a low value of 1V and a high valueof 2V are selected. To achieve a0.0039V step size over the 1V to 2Vrange, only an 8-bit device is needed.This approach reduces cost andincreases accuracy and reliability byremoving any possibility that thecircuit could be grossly misadjusted.The high and low voltages for theCDAC are arbitrary and, therefore,can be centered wherever circuitcalibration is required. Thus, thegranularity of the adjustmentcan be optimized for the specificapplication. In addition, CDACs andCDPots have internal nonvolatile(NV) memory, which automaticallyrestores the calibration settingduring power-up.Precision voltage referencesSensor and voltage measurementswith precision ADCs are only asgood as the voltage reference usedfor comparison. Likewise, outputcontrol signals are only as accurateas the reference voltage supplied tothe DAC, amplifier, or cable driver.Common power supplies are notadequate to act as precision voltagereferences. They are not accurateenough and drift far too much withtemperature.Compact, low-power, low-noise, andlow-temperature-coefficient voltagereferences are affordable and easyto use. In addition, some referenceshave internal temperature sensors toaid in the tracking of environmentalvariations.ORDINARY DAC4VREFIN4V10-BITDACOUT10 BITS, 1024 STEPSGroundFB0.0039Vper stepGROUNDCALIBRATION DAC2V2VREFHI8-BITDAC8 BITS, 256 STEPS1VREFLOOUT1VComparing the calibration range of an ordinary DAC to a CDAC.www.maxim-ic.com/smartgrid 15
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