5 as discussed in section 1581 the performance is

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Unformatted text preview: the circuit resistive in effect. Inductive reactance would tend to reduce stability, whereas the action of capacitance is to block the unidirectional transient component of fault current and so raise the stability constant. An alternative technique used in some relays is to apply the limited spill voltage principle shown in Equation 15.4. A tuned element is connected via a plug bridge to a chain of resistors; and the relay is calibrated in terms of voltage. 15.9 LOW IMPEDANCE BIASED D I F F E R E N T I A L P R OT E C T I O N The principles of low impedance differential protection have been described in Section 10.4, including the principle advantages to be gained by the use of a bias • 246 • Network Protection & Automation Guide The principles of a check zone, zone selection, and tripping arrangements can still be applied. Current transformer secondary circuits are not switched directly by isolator contacts but instead by isolator repeat relays after a secondary stage of current transformation. These switching relays form a replica of the busbar within the protection and provide the complete selection logic. 15.9.1 Stability With some biased relays, the stability is not assured by the through current bias feature alone, but is enhanced by the addition of a stabilising resistor, having a value which may be calculated as follows. The through current will increase the effective relay minimum operating current for a biased relay as follows: IR = IS + BIF where: IR = effective minimum oprating current IS = relay setting current IF = through fault current B = percentage restraint As IF is generally much greater than IS, the relay effective current, IR = BIF approximately. From Equation 15.4, the value of stabilising resistor is given by: RR = I f (R LH It must be recognised though that the use of any technique for inhibiting operation, to improve stability performance for through faults, must not be allowed to diminish the ability of the relay to respond to internal faults. 15.9.2 Effective Setting or Primary Operating Current For an internal fault, and with no through fault current flowing, the effective setting (IR) is raised above the basic relay setting (IS) by whatever biasing effect is produced by the sum of the CT magnetising currents flowing through the bias circuit. With low impedance biased differential schemes particularly where the busbar installation has relatively few circuits, these magnetising currents may be negligible, depending on the value of IS. The basic relay setting current was formerly defined as the minimum current required solely in the differential circuit to cause operation – Figure 15.15(a). This approach simplified analysis of performance, but was considered to be unrealistic, as in practice any current flowing in the differential circuit must flow in at least one half of the relay bias circuit causing the practical minimum operating current always to be higher than the nominal basic setting current. As a result, a later definition, as shown in Figure 15.15(b) was developed. Conversely, it needs to be appreciated that applying the later definition of relay setting current, which flows through at least half the bias circuit, the notional minimum operation current in the differential circuit alone is somewhat less, as shown in Figure 15.15(b). Using the definition presently applicable, the effective minimum primary operating current [ = N I S + B ∑ I eS + R CTH ) where: IR R LH + R CTH B It is interesting to note that the value of the stabilising resistance is independent of current level, and that there would appear to be no limit to the through faults stability level. This has been identified [15.1] as ‘The Principle of Infinite Stability’. = The stabilising resistor still constitutes a significant burden on the current transformers during internal faults. An alternative technique, used by the MBCZ system described in Section 15.9.6, is to block the differential measurement during the portion of the cycle that a current transformer is saturated. If this is achieved by momentarily short-circuiting the differential path, a very low burden is placed on the current transformers. In this way the differential circuit of the relay is prevented from responding to the spill current. Network Protection & Automation Guide • 247 • Busbar P rotection technique. Most modern busbar protection schemes use this technique. N = CT ratio Iop Iop • IS ) IS s Bia e Lin (B% ) B% e( in as L I'S Bi IB IB IB IS IS IR = S I + BIB IR = I + I' = I' (a) Superseded definition Figure 15.15: Definitions of relay setting current for biased relays I'S B⎞ ⎥ (b) Current definition 15 • Unless the minimum effective operating current of a scheme has been raised deliberately to some preferred value, it will usually be determined by the check zone, when present, as the latter may be expected to involve the greatest number of current transformers in parallel. A slightly more onerous condition may arise when two discriminating zones are coupled, transiently or otherwise, by the...
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