Fault Location in an Electrical Energy Distribution Infrastructure with a Wireless Sensor Network

Authors: C Fortunato and M. Santos, EDP - Energias de Portugal, Portugal, A. Casaca and A. Grillo, INESC-ID/ INOV/ IST, Portugal

Introduction

Nowadays, the electrical energy distribution is considered a critical infrastructure in industrially developed societies and its protection regarding safety and security threats is being more and more used.
The fact that this infrastructure is geographically spread across huge areas brings difficult technological challenges for the real-time prevention, detection and precise localization of anomalies.
The protection and localization of faults in the electrical energy distribution is a key task for any operator of such an infrastructure. Securing the main infrastructure components through the deployment of secure wireless sensor networks (WSN) providing remote monitoring and alarm capabilities specifically in the detection of the fault location is an attractive option for achieving this goal.
This work considers the planned architecture and high-level implementation solutions for the WSN system, including a description of the protected equipment and the hardware and software architecture to be used to detect anomalies. For their detection, the proposed approach in the case of the Medium Voltage (MV) overhead lines is based on the analysis of the increased current.

A current transformer is used to measure its value and to drive a parasitic power source for the wireless sensor, eliminating the need for batteries and at the same time posing no power constraints on the wireless protocols. The sensors will be located in close relation with each tower and each of them monitors one power line.
We also used this wireless communication link to upload a video/image of the MV/LV power transformer of the network. At the same time an infrared thermo sensor attached to the camera sweeps the power transformer critical elements (such as the main switch board) for hotspots. The detection of a hotspot will trigger an alarm into the network.
This feature improves the remote MV/LV power transformer physical security.

Connected to this interface there will be also a switched on by movement camera to show the access to the equipment by intruders. This methodology will:

  • Improve the quality of the service
  • Increase the electrical system reliability
  • Maximize the operational efficiency

The wireless technology assures a cost-effective solution to the upgrade of the existing asset base.

Fault Locator System
The fault detection systems are important tools for the improvement of the Service Continuity in an electrical network. They allow the reduction of the time lost in the search of defects and therefore, in the total time of the network break.
In addition to the benefits associated with the reduction of the energy interruption periods for the electrical companies and their clients, these systems have several virtues, enabling the reduction of the operation costs, the identification of the problematic network zones and the avoidance of the equipment deterioration related to the network reconnection over permanent defects during the traditional process of fault localization.

There are several methodologies for the detection and localization of faults used by the Electrical Companies:

  • From systems based on the implementation of Fault Locator in the distribution networks,
  • To the adoption of analytic methods supported by the Supervision Control And Data Acquisition (SCADA) systems

By using analytic methods we can detect faults, through wave transmission, the high frequency components of the currents and tensions and the fundamental frequency of the currents and tensions, measured in the line terminals (also called Impedance Based Methods).
The Impedance Based Methods, consisting in the lines' impedance calculation in their terminations and in the estimation of the distance to the defects are, due to the easiness of implementation, the most adopted ones by the Electrical Companies. These methods can use the measurements of one of the lines' terminations, or the measurements of the two lines' terminations.
The impedance based methods have a precision of 2 to 3% of the total length of the line.
The precision of this method is influenced by several factors, including the combined effect of the charge current and the resistance of the defect (reactance effect), the imprecision in the identification of the type of defect (phases in fault) and the uncertainty about the parameters of the lines, particularly the homopolar impedance.
Nowadays, MV protection units usually have the function of fault locator incorporated, which is a well established function for transmission networks.

However, distribution networks have some differences that must be taken into consideration.
The main factor is that distribution networks are radial and constituted by many branches, which may have different impedances. In that way, it is not possible to use this information as it is commonly used in higher voltage levels.
In High Voltage (HV) networks each line has a protection unit capable of comparing impedances of the line and fault and transmitting the fault's location data.
However, in MV networks that is not possible because, despite being radial, there are many ramifications with different electrical characteristics.

An option is to use the wireless sensors to communicate from the local network to the SCADA system, which processes the signals sent by the sensors, allowing the operator to know the location of the fault. For example, a sudden variation of current measurements is usually considered a fault indication.
If, for a pair of sensor nodes that are adjacent in the same branch of the electricity distribution network, only one of them is able to measure that sudden variation, then there is a high probability that the fault is located precisely between those sensor nodes.
This solution can be easily implemented with reduced costs.

Relion advanced protection & control.
BeijingSifang June 2016