Residual voltage

When the primary of a three-phase two-winding transformer, having its secondary wound for a three-phase open delta, is connected across an unbalanced supply system, a residual voltage across the open delta will appear. This is the principle on which this transformer is based (Figure 15.4(a)). As discussed in Section 21.2.2, and illustrated in Figure 21.7, the phasor sum of all the three line to ground voltages in a three-phase balanced system is zero, i.e. 

Residual voltage The residual voltage appearing across the secondary windings will be three times the zero sequence voltage, if it is found in the primary... 

Residual voltages under different operating conditions To extend the ease of application of this device, consider the following circuit conditions to determine the quantum of residual voltages ... 

In this case, all the three phases would be balanced and the residual voltage, Ve, will be zero. The three voltage phasors in the open della windings will be as illustrated in Figure 15.5(a). The phasor sum of these phasors is zero. Therefore W,. = 0. 

Consider a ground fault on phase R. The voltage across this phase will become zero and the phasor diagram will be as illustrated in Figure 15.5(b). The other two phasors will remain the same as in a healthy system and add to give the residual voltage F, i.e. 

The armature of the machine will normally have a residual voltage of around 8 V (for LT machines) across the terminals when running at the synchronous speed. If not, as when the generator is operated after a long shutdown, a d.c. voltage of 12 V can be applied through a battery for a few seconds to obtain the required residual voltage. 

Figure 16.16(b) Residual voltage across the internal circuit of generators (3) and G2... 

Figure 16.31(a) Residual voltage across each phase when / is not equal to /t,... 

Figure 16.32 Residual voltage is zero when E, is in phase with I b...

Figure 16.33 Residual voltage is maximum when , is phase apart with I t,...

Both of these give rise to higher residual voltage. f. which may lead to hunting). 

The auto-synchronizing relay monitors AV, Af and phase shift (Ad), between the two voltage phasors. In other words it monitors the residual voltage, It also ensures that G is slightly ahead of the existing source at the instant of synchronization. 

Figure 18.7 Protective characteristics for arresters type EXLIM Q (maximum residual voltage in per cent ol residual voltage at 10 kA, 8/20 tis)...

V, - Prospective voltage surge that may appear on the primary side. If an arrester is provided on the primary side, this voltage is limited to the residual voltage of the arrester In both cases, consider the... 

The arrester on the primary side may be selected with a lower residual voltage (LreJ. or... 

System voltage Rated voltage Max. MCOy cont. as in operating ANSI voltage tests (coy) TOV capability for Maximum residual voltage with current wave ... 

The residual voltage may also be measured by a residual voltage transformer (RVT) (Figure 20.10). Refer to Section 15.4.1 for more details. 

Residual voltage transformer (star/open delta transformer)... 

This is not a method of providing an artificial neutral, as in the previous case, but to detect an unbalance or residual voltage (zero sequence voltage) in a three-phase three-wire or a three-phase four-wire ungrounded system. The residual or zero sequence voltage that may appear across the open delta will be the reflection of an unbalance or a ground fault in the system (Figure 20.10). Also refer to. Section 15.4.1 for more details. 

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