Centralised Loss of Mains Protection using IEC 61850

In power distribution systems possessing a significant penetration of DG, there are many issues to be taken into account. Providing adequate loss of mains (LOM) protection is a primary concern. LOM is the situation in which a section of the distribution system is isolated from the remainder of the power system (the 'mains'), but continues to be energised by one or more DG units, effectively forming an islanded subsystem. Current practice is that all utilities require DG to be disconnected from the network as soon as possible in the case of islanding. In the UK, the regulations that govern the connection of such generation, G59 and G75, require that a LOM protection system be fitted to all DG units at the interface between the DG unit and the utility distribution system. There are several LOM detection techniques, which can be divided into remote and local techniques. Local techniques can further be divided into passive and active techniques. Presently, local passive techniques are predominant, such as rate of change of frequency (ROCOF), vector shift, under voltage, under frequency, etc. Unfortunately, these passive LOM techniques are prone to false tripping when settings which enable fast disconnection of the generation for real islanding conditions are applied. Furthermore, when system conditions are such that generator output matches local demand at the time of islanding, detecting the loss of mains condition can be problematic as the perturbation on voltage and frequency may not be significant. This presentation discusses a centralised area based technique, which overcomes false tripping issues because it is conceptually different from the passive techniques in that it does not operate based on measuring any electrical system parameters, such as voltage and frequency. This technique can be incorporated with new automation and control systems without further costs to utilities. A central controller communicates with all the intelligent electronic devices in the area using IEC61850 to monitor the position of circuit breakers and switches. LOM events are detected running an algorithm in the central controller to check the connection to the mains of any DG in a region of the network. When there is a LOM event the central controller has the possibility to disconnect the islanded generators or allow islanding operation when it is possible, which may be attractive in the future. The system may act alongside traditional single-measurement LOM relays, with each protection system acting as backup to the other. A prototype of the proposed LOM algorithm has been implemented on the ABB COM600 controller and tested using the model of a 33kV meshed network example of Scottish Power. This technique is believed to have the potential to offer the most direct and effective method for LOM detection. It can substitute the existing inter-tripping schemes and protect a network from LOM together with existing local LOM protections, such as ROCOF, under frequency and under voltage relays.