Handbook of Electrical Installation Practice - BeKnowledge

608 Handbook of Electrical Installation Practice
Often data are only questioned when engineering judgement indicates that the final
result is unusual. This means that the computer should be used to assist and not
replace the engineer.
Graphics workstations provide interactive means of grading by displaying the
relay co-ordination curves and thus allowing the engineer to study options and
compromises. If the workstation is interfaced with a CAD/CAM system, hard copy
of the final results may be produced for protection audits.
UNIT PROTECTION
Unit protection operates for faults which it measures as being within the protected
unit and is stable for faults which it measures as being external to the protected
unit. Differential protection, which forms the majority of unit protection, can be
classified by the means used to obtain stability. In the current balance system,
current is circulated in the secondaries between the sets of CTs at each end of the
protected zone and the relay is fed by the summation of the currents which is
theoretically zero under external fault conditions. The voltage balance system has
voltages derived from and proportional to the currents which are balanced under
external fault conditions, so that theoretically no current flows in the relays which
are connected in series between ends. The basic arrangement of both systems is
shown in Figs 22.8 and 22.9.
Each type of balance has limitations and the means of overcoming these limitations
characterises relay design. Originally, the use of distributed air-gap CTs
favoured the voltage balance scheme because this type of CT naturally produced a
voltage output proportional to current. However, as can be seen in Fig. 22.9, significant
pilot capacitance causes spill current in the relays of a voltage balance pilot
wire differential scheme and thus provision must be made for this in the relay
design. Similarly, in a current balance scheme (Fig. 22.8) the voltage drop due to the
circulating current dictates that the relays must be connected at the electrical centre
point.
Fig. 22.8 Current balance stability.