Protection of MV industrial networks

The successful application of protection relays involves achieving an acceptable compromise between the requirements of speed, dependability and security at an economic cost. The advantages of isolating a system fault as quickly as possible include: health and safety of personnel and public; minimisation of effects on plant operation and system stability; and minimisation of damage to plant. Discrimination refers to the ability of the overall protection scheme to isolate the minimum of plant necessary to remove the effect of the fault. Security is essentially freedom from maloperation, and again is concerned with correct application (e.g. selection of coordinated relay settings). The dependability of a protection relay or scheme refers not only to the relay itself, but also to the correct selection and design of the peripheral equipment necessary for its operation (e.g. current and voltage transformers). The author discusses overcurrent relays, earth fault relays, directional control of overcurrent and earth fault relays, and protection coordination