Intelligent Current Sensors as Part of Smart Grid Network

Abstract

The continuous development of the ITC systems, especially industrial Ethernet network using the IEC 61850 protocol allows to building a distributed sensor networks for control and monitor temporary states of the power infrastructure. In the paper the construction of the intelligent current sensors, Measurement Units (MU) for the Smart Grid network has been presented. In the paper the circuit diagram of sensor and synchronization method of the analog to digital converter based on the UTC timestamps has been described. The presented method allows to synchronize intelligent current sensors with an accuracy better than 1 μs. The presented innovative synchronization method is characterized by a high level of resistance to disappearance and attack of the reference time signal. The measuring device MU design is based on the embedded Linux-platform and as a sensing element used air core current transducers based on Rogowski coil. The connection between the current transducers and the central unit is made using a fiber optic link. Built-in IEC-61850 server provides a real-time measurement value of the True RMS, harmonics spectrum and samples value. The intelligent current transducers in addition to being used in the MUs create also new possibilities in the substation construction like the possibility to place the Autonomous Sensors in large distance from Intelligent Electronic Devices that are located in the substation. DOI: 10.12736/issn.2300-3022.2015205 Introduction Modern intelligent current sensors are gradually replacing traditional current transformers. The sensors, manufactured with the use of PCB technology, are composed of a current transducer based on the Rogowski coil principle and an electronic circuit integrated with the transducer. The current transducer is characterized by perfect linearity, high dynamic range, wide frequency bandwidth and very repeatable electrical characteristics. Equipped with signal processing circuits, autonomous power supply and communication modules, it allows the realization of the Smart Grid concept related to the monitoring of the temporary state of the power grid. Data transmitted by the intelligent sensor are collected by MU (Merging Unit) which, after time tagging make them available to IEDs (Intelligent Electronic Device), among them the Protection Relay, Power Quality Analyzer and PMU (Phasor Measurement Unit). Construction of the intelligent current sensor The block diagram of the intelligent current sensor is presented in Fig. 1. The main parts are current transducer, signal processing module, synchronization module, communication module and power supply module. The Merging Unit has also been shown because the intelligent current sensor communicates directly with it. The principle of operation of the current transducer has been described in [1]. This article will focus on the electronic parts of the sensor. Signal processing and communication module The signal processing module consists of an anti-aliasing filter, amplifier, A/D converter and additional processing circuits. The amplifier, together with A/D converter, must have a dynamic range high enough to cover the current measurement range of interest which may be in the range of a few amperes to several kA. Providing a high dynamic range is one of the more challenging issues when designing the current sensor. The A/D converter is followed by additional processing circuits responsible for packing the data into a format suitable for transmission. The transmission module changes the data from electrical into optical format. The block diagram of the APSP circuit described above is presented in Fig. 2. A. Nowakowski et al. | Acta Energetica 2/23 (2015) | 48–53