TABLE OF CONTENTS Most protection systems for distribution networks assume power flows from the grid supply point to the downstream low-voltage network. This approach simplifies the problems associated with controlling the voltages at the loads and helps maintain the quality of supply. Protection is normally based on overcurrent relays with settings selected to ensure discrimination between upstream and downstream relays (see Figure 6.1). (Note that the figure incorporates the IEEE/ANSI designation numbers for a time-delayed phase overcurrent relay (51) and a time-delayed earth fault overcurrent relay (51N) [2, 5]) A fault on a downstream feeder must be cleared by the relay at the source end of the feeder. It must not result in the operation of any of the relays on an upstream feeder unless the downstream relay fails to clear the fault. If this occurs, relays on the immediately adjacent upstream feeders should operate and clear the fault. This will result in a blackout to a part of the network that should not have been affected by the fault [1, 3, 4].
If generation is embedded into the distribution system, the fault current seen by a relay may increase or decrease depending on its location and that of the fault and the generators (see Figure 6.2). To ensure appropriate co-ordination between relays, grading studies must take into account the maximum and minimum infeed from all the embedded generators and the grid supply point. This may create grading problems due to the...
