Protective Relaying (Power Line Monitoring)

Overview
With an estimated doubling in global electricity capacity required between 2008 and 2030, the electrical power delivery infrastructure, or "grid" will undergo significant change over the coming years. Specifically, the increased need to better measure, monitor, control and protect the grid will see a fundamental shift away from an essentially electro-mechanical based system to a fully digital based one. The need to accommodate distributed generation from renewable energy sources and provide for an electric car charging infrastructure are just two of the emerging requirements adding to the pre-existing need to minimize energy losses, ensure stability of the network, enable continuity of supply, prevent outages and provide sufficient global electrical power needs for the emerging economies in China, India, South America and elsewhere. While emerging economies are going about building such advanced digital based electricity grid infrastructures from the ground up, the developed economies have the additional challenge of retrofitting and upgrading the already installed grid infrastructure.

Measurement / Sensing
Throughout the grid, from the point of generation through the high voltage transmission system and on down into the distribution network, this so called 'smart-grid' will be characterized by its advanced sensing and signal processing capabilities enabling accurate and real time measurement and monitoring of many more parameters of the grid and at many more points on the grid. Precise and continuous measurement of voltage, current, phase, power (KWatts) and energy (KWhrs) at substations, transformers, feeders, and overhead lines is going to be the heart of such a system with effective and reliable communication of such information being its lifeblood.

Modern digital protective relay systems need to not only quickly detect and respond to faults in transmission lines, sub-stations and transformers but, increasingly, must also be able to monitor harmonics and other leading fault indicators to take action before the fault becomes established. The AD7606/AD7608 is an example of a solution that has been optimally designed for such systems with the ability to simultaneously and accurately sample, using 16 Bits of ADC resolution, and up to 8 channels of I & V (three phases each of current & voltage as well as two neutrals). This superior level of performance, coupled with a high level of functionality, significantly contributes to improved operation, protection, and control within the transmission/distribution infrastructure. Whether it be a 1000KV HVDC system, or a 100KV AC transmission link, performing this function reliably is the entry point to this digital grid.

Distribution Automation (DA) will further drive optimal regulation of the grid, maintaining security, reliability of supply and efficiency right down to the end consumers’ premises. The specific design priority of a particular ‘smart grid sensor’ will be determined by its usage mode, be it Protective Relaying, Quality Monitoring, Metering, or some combination of all three. Analog Devices provides a family of multi-channel, 3-phase analog-to-digital converter frontends (AFEs) optimally designed for power grid, monitoring, protection and metering. The high speed AD7606 enables both precision monitoring and protective relaying for up to 8 simultaneously sampled channels of I & V. The 6-channel, ADE7878, incorporates all the signal processing required to measure and calculates total active, reactive and apparent energy.

Control / Processing
As well as providing optimized ADCs that interface to the various physical sensors including current transformers, voltage transformers, shunt based schemes, active/passive optical architectures and Rogowski coils, Analog Devices also provides the necessary digital processing engines, covering the gamut from ARM® based processors through its family of Blackfin® processors(ADSP-BF53x/54x) to its most high performance SHARC® DSPs (ADSP-21469).

Communications
Aside from the local requirement to speedily actuate the protective relay, all the other data on the condition of the grid must be communicated back to the SCADA system. This will be through a combination of means that will include both wireless and wired communications. In the distribution part of the grid, with many dispersed 10KV level transformer stations, using Analog Devices' Short Range RF transceivers whose high sensitivity and immunity to interferers enable convenient and reliable communication of measured data to the grid’s communication backbone.

Isolation, whether it be for current/voltage sensing of the mains, or interconnection to the signal processing and communications infrastructure, Analog Devices iCoupler® technology is available as both standalone isolator products (ADuM148x) and embedded into a large range of functions including A/D Converters (AD7400A), CAN bus Transceivers (ADM3052/ADM3053) and RS-485/RS-422 Interfaces (ADM248xE/ADM258xE/ADM268xE). Precision, low noise, low power op amps like ADI's Opx177 family completed the signal conditioning demanded by the different current sensor signals while a broad portfolio of power management products ensures IC level power is reliably available for the entire signal processing chain.