Protection of high-voltage industrial networks
When designing high-voltage networks, a protection plan must be drawn up to ensure the safety of people but also to preserve the continuity of operation in the event of a fault in the electrical installation.
For this purpose, it is necessary to determine for each point of the installation, the maximum and minimum prospective short-circuit currents for each type of fault: three-phase, two-phase, two-phase with earthing and single-phase. The international standard IEC 60909-0, “Short-circuit currents in three-phase a.c. systems” uses the symmetrical components method. It is based on the introduction of an equivalent voltage source at the short-circuit point, with all network power supplies, synchronous and asynchronous machines being replaced by their internal impedances. In order to calculate the values of the balanced and unbalanced short-circuit currents, the currents in each phase are obtained by adding the currents relative to the three symmetrical components of the network (positive-sequence, negative sequence and zero-sequence). This requires knowledge of the impedances of all the installation components in each of the three systems.
Unlike low voltage where overcurrent detection and cut-off are generally managed on the same device, in high voltage the functions are split. The overcurrent detection is performed via current transformers connected to a protection relay which will give the tripping order to the breaking device. The knowledge of the short-circuit currents will make it possible to define the tripping thresholds to be set on the protection relay(s).
The functions of the protection relays are standardized and described in the ANSI / IEEE C37.2 standard:
Figure 1: IEEE C37.2 excerpt