The application of let-through-energy protection in multi-source interconnected utility networks

Abstract

Medium voltage feeders are evolving from traditional radial topologies to interconnected feeders with distributed generation. For a feeder with a source at either end, the circuit breakers at both ends have to open to isolate a fault on the feeder. The type of overcurrent protection applied to these networks are predominantly of the inverse definite minimum time type.

To evaluate if a feeder conductor is protected from the thermal effect of conducting fault current, let-through energy can be considered. Let-through energy refers to the I²t heating effect of the fault current for a certain time period and the absorption of this energy as an adiabatic process. The hypothesis that is tested in this thesis shows that it is possible to evaluate conductor let-through energy in a multi-source interconnected network. To evaluate a conductor’s let-through energy exposure, the network has to be considered from a holistic perspective as a circuit breaker operation in a multi-source interconnected network will result in fault current redistributing. This change in fault current will change the protection operating time. The change in fault current and exposure time creates different let-through energy levels over time and at different positions on a faulted feeder. With this change in measured fault current a discrete version of the traditional inverse definite minimum time relay equations have to be used to determine the relay operating time. An average disk speed relay model was also created in this research.

A full evaluation application was developed for evaluating the hypothesis and feeder let-through energy exposure. A new three-dimensional surface and heat map of let-through energy-to-distance-to-time was created within this full evaluation application. The time component of this surface allows other circuit breakers in the network to operate and the fault current to redistribute. The change in conductor let-through energy exposure is captured in the current that is measured at either end of the feeder and the relay operating time (time to trip). To assist with the evaluation the volume under the let-through energy graph is determined so as to provide a single quantifiable comparable number. The hypothesis was proven by means of case studies. Three case studies were used, the first a radial feeder application where elements that influence let-through energy were shown. The second and third case studies were for a multi-source interconnected feeder; one with a strong and weak source and the other with two similar sources at either end of the feeder. Some of the protection elements that were evaluated are high-set elements, auto reclosing and different operating curves. In the case studies it was shown that this full evaluation method works well for evaluating the conductor exposure in both radial and multi-source interconnected networks. This holistic evaluation method assists with identifying elements that influence let-through energy and supports optimising protection settings with the aim of minimising conductor let-through energy exposure.