NxtPhase Optical Current and Voltage Sensors are a new type of measurement transducer. Conventional measurement transducers are instrument transformers, both current transformers (CTs) and voltage transformers (VTs), that step down power system current and voltage to levels that are usable by secondary devices (such as meters and protective relays) of all types: electro-mechanical, solid state, and micro-processor based. Conventional CTs and VTs, while providing reasonable performance, share well known operating probems of mutually coupled devices (i.e. magnetic core saturation, ferroresonance, phase error of the inductive circuit, and drift of components). NxtPhase Optical Current and Voltage Sensors measure current and voltage on the power system, by precisely identifying the effect of the magnetic and electric fields on the propagation of light signals traveling through a fiber optic conductor. Current and voltage can be measured very accurately, and since the optical sensor is a passive device, problems synonymous with conventional inductive instruments (such as core saturation, and component shift) are effectively eliminated. Conventional instrument transformers are entirely analog devices, so they only produce analog interface signals. The secondary devices on the power system, both meters and protective relays, are traditionally designed to work with these analog interface signals. Even micro-processor based meters and protective relays, that operate digitally, are designed to work with analog input signals. ANSI/IEEE and IEC standards define instrument transformer performance, including the nominal and peak levels of the resulting analog current and voltage signals. In ANSI influenced regions, CTs are normally a 5A nominal secondary current, while VTs are typically 69V, 115V, or 120V nominal secondary voltage. However, NxtPhase Optical Current and Voltage Sensors produce an inherently digital signal. Standards exist that define the requirements for digital communications between optical measurement transducers and microprocessorbased Intelligent Electronic Devices (IEDs) such as meters and protective relays. A logical choice is to take only digital current and voltage measurements, and use IEDs that accept only digital input signals. This eliminates the problems of analog-to-digital conversion required in contemporary IEDs. Though beta versions of digital input IEDs exist, none are in commercial production today. For compatibility with conventional secondary devices. NxtPhase devices convert and amplify the digital measurements to provide 2 different types of analog output interfaces. This application note describes the uses of these two types of analog output signals, and the requirements for connecting to conventional measurement devices.
Interface, Low energy, High energy
