Lines lengths

CTfilcs originated in the time of punched cards and therefore their format is quite restrictive. For example, blanks usually arc significant and several consecutive spaces cannot simply be replaced by a single one. Spaces may correspond to missing entries, empty character positions within entries, spaces between entries, or 2cros in the case of numerical entries. Thus, eveiy piece of data has a precise and fixed location within a line in a data file. Moreover, the line length of CTfilcs is restricted to 80 characters. 

The time of travel from the point the lightning strikes on the overhead lines to the arrester, considering a line length of approximately 420 km and speed of propagation of the surge as 0.3 km/,us,... 

Figure 24.15 The line length effect even when the sending-end and receiving-end voltages and currents are maintained at unity p.f.

This phase shift (6) for a particular line length can be calculated as follows ... 

This product can be reduced by reducing X, using series capacitance with a reactance X, which will reduce to - Xq. This is where reactive control plays a major role. By meticulous reactive power manage-ment, the Ferranti effect can be controlled and the electrical line length increased to the desired level. It is a different matter that the electrieal length of the line cannot be raised infinitely, for reasons of stability, as discussed later. 

Table 24.4 Far-end voltage, due to the Ferranti effect, in a 400 kV TZ type line, at different line lengths...

In the first case, if we had considered a safe line length of 250 km, this would become 500 km for a symmetrical line. Figure 24.18 illustrates such a condition. Depending upon the length and type of line, a line length compensation may be required. Most transmission lines are seen to be within permissible lengths and only a few may require such a compensation. Nevertheless, it may be worth reducing the phase displacement between E, and to less than 15° electrical, to further iinprove the quality and stability level of power transmission. 

Line length effect or Ferranti effect, sin 6, that will determine the optimum line length which will also depend upon whether it is a radial or a symmetrical line. 

Ey = phase voltage at the transmiliing-end Vy = phase voltage at the receiving-end By = line length effect or Ferranti effect at the end of the line, in degrees /i = load current P-JQ Vj... 

After identifying the likely line lengths we can then study the most appropriate p.f. at vv hich the load must be transmitted to maintain the highest level of stability. For... 

Figure 24.21 A comparative study of load transfers for different line lengths at different p.fs. for an uncompensated line...

For each p.f. and line length the curve V,. versus P describes a certain trajectory. Maximum power can be transferred only within these trajectories. Each line length has a theoretical optimum level of power transfer, P,nax. which is defined by PoFtn 6. In Table 24.5 we have worked out these levels for different line lengths, for the system considered in Example 24.1. 

Reactive control can alter the line length ( f LC) to the level at which the system will have the least possible swings. It is evident from these curves that an uncompensated line of a much shorter length may not be able, to transfer even its natural load (Pq) successfully. This is due to the steeply drooping characteristics of the voltage profile at about this load point, which may subject the... 

To decide on the best reactive control one should choose the most appropriate electrical line length from the load characteristics drawn already. Choose the one that can transmit the optimum power, and then compensate this to obtain the required line length. For the 400 kV, 50 Hz system considered, we can choose a radial line with an electrical line length of 200-250 km. The compensation is provided so that the P ,aj point, which lies far from the natural power transmission point Pq, shifts within a stable region, i.e. near the Pq region. Then from equation (24.8),... 

While it is possible that P may be further raised by a still closer compensation, this is not advisable to retain the stability level of the system. The above compensation is even higher than the line length compensation considered earlier and will further improve the electrical line length. 

Adding shunt capacitors would also reduce Zq but would raise the electrical line length hence it is not considered. Moreover, on EHVs, the charging shunt capacitances, Cq, as such require compensation during light loads or load rejections to limit the voltage rise (regulation) at the far end or the midpoint. Hence no additional shunt compensation is recommended. 

The rest of the procedure, even the inferences drawn above, would remain the same as for a radial line. The only differenee now is that the system would beeome suitable for twiee the lengths of the radial lines as a result of the midpoint effect which doubles the line length. 

Series eapaeitors These are used for line length compensation to help transmit power over long distances and also improve the stability level of the network. They are usually installed at the line ends or at the selected locations. They reduce Zq and enhance SIL. Pq, and boost the receiving-end voltage. 

Xl = inductive reactance of the wliole line length Influence of line length... 

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