Smart Grid Solutions Guide - Maxim

Power-grid monitoringOverviewPower-grid monitoringOverviewAdvanced power-grid monitoringsystems combine power-supplymonitoring, load-balancing, protection,and metering functions toenable safe and efficient powerdelivery. They also reduce operationand maintenance costs for utilities.These systems protect transformers,circuit breakers, and other equipmentat substations; they enable predictivemaintenance by detecting andresponding to fault conditions; theydynamically balance loads toconserve energy; and they monitorand control power quality. Theseadvanced capabilities are critical toensuring uninterrupted powerdelivery and supporting intelligentgrid-management applications.Accuracy requirementsThe rollout of advanced power-gridmonitoring systems is complicated bythe varying international standardsthat specify the accuracy requiredfor energy measurement. Real-timepower-delivery monitoring, faultdetection, fault protection, anddynamic load balancing requirestringent accuracy. As an example,the European Union IEC 62053standard for Class 0.2 equipmentrequires measurement precision tobe 0.2% of the nominal current andvoltage. For power-factor measurementaccuracy, sample-time phasematching should be 0.1% or better.Polyphase measurementsPower companies distribute threephase(polyphase) power using a“wye” connection. The term wyerefers to the arrangement of threetransformer windings that join ata common point, the junction ofthe Y. The line voltages are offsetin phase from each other by 120°,one-third of a cycle. If loads on eachof the three phases are equal, thesystem is balanced and no currentflows through the neutral line. Afourth, neutral, wire connects to thejunction of the wye. It accommodatesimbalanced loads across theline connections.Power-grid monitoring systems trackthe voltage and current on multiplephases with ADCs. The convertersmust be synchronized in order tomeet stringent standards requirementsand to accurately measurepower factor. In a typical scheme,each phase’s power parameters aremeasured with a current transformer(CT) and a voltage transformer (VT).The complete system comprises foursuch pairs: one pair for each of thethree phases, plus a neutral pair. TheADCs simultaneously measure thethree phases and neutral voltagesand currents. The active, reactive,apparent-energy, and power-factorparameters can be calculated fromthe sampled data, often by a DSP.International and local standardsalso dictate the necessary samplerate. These applications typicallyrequire accurate, simultaneous multichannelmeasurement over a widedynamic range of 90dB or better witha sample rate of 16ksps or higher.These capabilities enable analysis ofmultiple harmonics of the AC supplyPT/CTTRANSFORMERSOPAMPACTIVEFILTEROPTIONALVOLTAGEREFERENCEµPSUPERVISORTEMPSENSOROPTIONALMULTICHANNELADCRTC WITHNV RAMµCUARTTOCOMMUNICATIONSBLOCK(SEE pp. 25-40)OPAMPOPTIONALACTIVEFILTERDIGITALPOTENTIOMETEROPTIONALDACTOUCH-SCREENCONTROLLERPOWERMANAGEMENTOPTIONALMAXIMSOLUTIONBlock diagram of a typical power-grid monitoring system. For a list of Maxim's recommended power-grid monitoring solutions, please go to: www.maxim-ic.com/grid-monitoring.www.maxim-ic.com/smartgrid 13