Design and Analysis of Different Digitizer Circuits for Non-Intrusive AC Voltage Measurement

Abstract

In electrical systems the two most important parameters which are monitored continuously are voltage and current. They are monitored to locate fault, to understand the load flow, to calculate the power, for performing smart energy metering etc. Conventionally the measurement was done with contact but if it is possible that the measurement can be done without any contact then that measurement is preferred. Reason behind choosing a non-contact measurement is that we are measuring any quantity with providing minimum obstruction to the circuit i.e. the measurement process is done without breaking the connection of the live cable. This thesis provides a detailed account on non-contact AC voltage measurement system. Additionally a model of non-contact AC voltage sensor has been developed in the laboratory and it was tested for various input voltages and frequencies. The processing was done on a LabVIEW installed computer.After this the measurement process was simplified by directly taking the readings in digital form. This process reduced the cost and the complexity of equipment as NI-USB DAQ 6341 and LabVIEW component were not required in the new scheme. Hence, a low cost, reliable digitizer is now connected in capacitive non-contact AC voltage probe which provides a digital value and this value is directly proportional to the unknown voltage value. This digitizer is based on the principle of dual slope ADC hence over the full scale range it provides measurements with a very high accuracy. The digitizer-1 circuit was designed in NI Multisim 13.0 software and the hardware circuit for it was developed in the laboratory. The previously developed digitizer-1 circuit had errors due to inherent design flaw and it was mathematically proved the error was structural rather than due to inaccuracies in instruments. To overcome the errors some modifications are implemented in the digitizer-1 circuit and a new digitizer-2circuit is proposed in the thesis. The accuracy of the proposed scheme is higher than the previously mentioned scheme.