Phase sequences

Protection against voltage unbalance or negative phase sequence 12/291... 

Impedanceofa motor during a normal running condition to a negative phase sequence voltage will be almost the... 

Such a relay is connected on the supply side as shown in Figure 12.23(a). The arrangement is such that unless the relay closes, the motor switching circuit will not energize and ihe motor will not start. The contact closes only when the supply voltage is normal and the phase sequence positive. Even for undervoltage conditions, ihe torque developed by the relay may not be adequate to close the circuit. Such relays are. therefore, effective against... 

Figure 12.23(a) Schematic (or a negative phase sequence (NPS) relay... 

Negative Phase Sequence Relay, M/s English Electric Co. of India Ltd, Catalogue Ref. PR 0I 306 A. 

The magnitudes of symmetrical and non-symmetrical fault currents, under different conditions of fault and configurations of faulty circuits, can be determined from Table 13.5, where Z] = Positive phase sequence impedance, measured under symmetrical load conditions. The following values may be considered ... 

Zj = Negative phase sequence impedance Zq = Zero phase sequence impedance... 

Table 13.6 Approximate negative and 2ero phase sequence impedances compared to positive phase sequence impedances...

Note The impedances marked on Figure 13.1.6 refer to positive phase. sequence impedances only. Faults that are non-symmelrical alone... 

X2 and afo are negative and zero phase sequence reactances of the generator respectively, in ii/phase. 

When this balance is disturbed, due to either an unbalance in the loads or due to a ground fault, a residual or zero phase sequence voltage in the neutral circuit will appear. When one of the phases in the secondary of a three-phase transformer is open circuited and a three-phase supply is applied to its primary windings, there will appear... 

The phase sequence of the incoming machine, must be the same as that of the existing source (Figures 16.27(a) and (b)). 

This can be checked with the help of a phase sequence indicator. This is a simple instrument that houses a small 30 motor, which rotates a pointer connected to the motor through a gearbox. The direction of rotation of the pointer will determine the phase. sequence of the system. 

Z( = zero phase sequence or residual impedance. This is measured between the three-phase terminals of a star winding shorted together and the neutral (Figure 20.9(b)) and is calculated by... 

The naturai zero phase sequence inductive reactance of the grounded neutral may be considered to be too small compared to this and ignored for ease of calculations. Thus the resistance Rq of the grounded neutral circuit may be considered as its impedance, i.e. 

Generators that are solidly grounded have a different grounding practice from others due to their zero phase sequence reactance, which is much less than its positive or negative phase sequence reactances (Section 13.4. ((. i))... 

For many low alloy steels, therefore, the scale phase sequence is as shown in Fig. 7.5 and the governing equations for individual layer growth are similar to those for pure iron, with the addition of ... 

The first report on the liquid crystalline properties of these compounds was published by Gray and Mosley [44] in 1976. The series of 4 -n-alkyl-4-cyanobiphenyls (CBn) have been widely studied by different methods due to their readily accessible nematic ranges around room temperature. The compounds have the phase sequences crystal-nematic-isotropic for CBS, CBIO, and monotropic nematic for CBS, CB4 crystal-smectic A-nematic-isotropic for CB9 crystal-smectic A-isotropic for CBll. The lower homologous CB2 is nonmesogenic. The general chemical structure of the compounds CBn is presented in Fig. 1. 

In this section, we will describe the crystal structures of the mesogenic A -n-alkoxy-4-cyano-biphenyls (Fig. 1) [53-57]. The compounds have the phase sequences crystal-nematic-isotropic for CB05—CB07 and monotropic nematic for CB01-CB04 crystal-smectic A-nematic-isotropic for CB08. The crystal and molecular data of the investigated compounds CBOn are summarised in Table 2. 

Table 6. Crystal data and phase sequences of mesogenic trans,trans-4,4 -n-m-dia kyl-il,l -bicyclohexyl)-4 -carbonitriles (CCNnm)...

Kurogoshi and Hori [ 104] determined the crystal structures of the mesogenic ethyl and butyl 4-[4-(4-n-octyloxybenzoyloxy)benzylidene]aminobenzoates. The compounds have different phase sequences crystal-smectic A-nematic-isotropic and crystal-smectic C-smectic A-nematic-isotropic for the ethyl and butyl compounds, respectively. Both compounds have layer structures in the solid phase. The butyl compound contains two crystallographically independent molecules. Within the layers, adjacent molecules are arranged alternately so as to cancel their longitudinal dipole moments with each other. In the ethyl compound the core moieties are almost perpendicular to the layer plane, while in the butyl compound these moieties are tilted in the layer. 

Table 13. Crystal data and phase sequences of mesogenic carboxylate compounds ...

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