OMICRON Magazine

aDaptIve lINe pROteCtION primary protection 1 With a line differential protection unit, the protec- tion of a “node” is strictly selective. remote backup protection is not possible due to the measurement principle involved. The load current flow is not lim- ited as a result of this measurement principle. primary protection 2 With a line distance protection unit, the voltage and current are measured in one location; the impedance can then be derived from the results. Consequently, selective protection is used for the node and remote backup protection is also implemented for down- stream equipment. The protection algorithm uses an impedance-dependent approach (Z=v/I) and there- fore is dependent on the current. In turn, this means that the starting value is a limiting value for the load current as far as distance protection is concerned. This state of affairs will now be considered below. adaptation nodes within the line protection concept during commissioning, the distance protection units are configured with static starting values for a worst- case scenario. The short-circuit current level is de- pendent on the short-circuit power level. In turn, the short-circuit power is dependent on the impedance of the upstream grid. Even the lowest of short-circuit cur- rents must always start the protection system reliably. For this reason, today’s starting values for distance pro- tection are based on the lowest possible short-circuit power. The protection starting values are hypersensi- tive and limit the maximum possible load current. In light of high power flows in the German transmis- sion grid – particularly in the north and south – the existing infrastructure will require further modifi- cations in order to optimize its new line connec- tions. Making higher power flows available is the purpose of this optimization process. One aspect of that process will be the introduction of an adap- tive form of line protection. As far as distance protection is concerned, the measured current and derived impedance serve as criteria for starting the protection system. The starting values are currently set statically within the protection unit based on the lowest short-circuit power, which has a limiting effect on the load cur- rents. In the future, the starting values could be adapted dynamically based on the current short-cir- cuit power, thereby allowing higher load currents. Design of an eHv overhead-line protection system In the transmission grid, an Ehv overhead-line protection system is designed with redundancy and diversity (primary protection 1 + primary protec- tion 2). When viewed abstractly, an overhead line constitutes a node. during fault-free operation, the load current flows into the overhead line at one end and flows out in equal magnitude (same amount) at the other. When a short circuit occurs in an overhead line – and assuming there is sufficient short-circuit power available – the short-circuit currents flow into the overhead line from both ends towards the fault location. by Stefan Scheerer, Amprion GmbH 40