Handbook of Electrical Installation Practice - BeKnowledge

Fig. 22.9 Voltage balance stability.
Protective Systems 609
In pilot wire differential schemes it is not practical to connect relays at the electrical
centre point because this would be geographically in the centre of the feeder.
Thus the design of a feeder protective scheme must arrange for tripping contacts at
each end and the relay equipment is therefore duplicated at each end, with both
ends and the communication channel between ends forming the measuring circuit.
Steady-state out-of-balance spill currents are overshadowed by the severe out-ofbalance
during unequal saturation of the CTs caused by the offset transient in the
primary current and the remanence left in the CTs from previous fault conditions.
Considering that these effects are transient in nature, they are related to the
dynamic performance of a protection which must therefore be established in
relation to the primary transient and its effect on the combination of relays, CTs
and connections between them which make up the protective scheme.
These effects are best understood and analysed by computer simulation.
Comprehensive dynamic representation of the power system, CTs and relay circuit
can be set up using standard software packages (e.g. EMTP) which will allow stepby-step
calculation and graphical display of the currents, voltages and fluxes in the
circuits. This is essential for design and development but also allows special
applications and unexplained field experiences to be analysed.
Low and high impedance schemes
Current balance schemes are classified as low or high impedance by the relative
impedance of the relay used. The high impedance differential circulating current
scheme allows for transient unbalance more definitely than any other type of
protection because it assumes complete saturation of the CTs at one end with the
CTs at the other end producing full output. With correct relay design in a high
impedance protection system it should be possible to remove the CT at one end
and replace it by its winding resistance only and still obtain stability.
Low impedance current balance differential schemes can be considered basically
as two CTs feeding an effective short-circuit which is the low impedance relay.
Thus assuming the relay has zero impedance, the excitation current of each CT is