2.1 STANDARD TRANSMISSION VOLTAGES
Voltages adopted for transmission of bulk power have to conform to standard specifications formulated in all countries and internationally. They are necessary in view of import, export, and domestic manufacture and use. The following voltage levels are recognized in India as per IS-2026 for line-to-line voltages of 132 kV and higher.
Nominal System
Voltage kV 132 220 275 345 400 500 750
Maximum Operating
Voltage, kV 145 245 300 362 420 525 765
There exist two further voltage classes which have found use in the world but have not been accepted as standard. They are: 1000 kV (1050 kV maximum) and 1150 kV (1200 kV maximum). The maximum operating voltages specified above should in no case be exceeded in any part of the system, since insulation levels of all equipment are based upon them. It is therefore the primary responsibility of a design engineer to provide sufficient and proper type of reactive power at suitable places in the system. For voltage rises, inductive compensation and for voltage drops, capacitive compensation must usually be provided. As example, consider the following cases.
Example 2.1. A single-circuit 3-phase 50 Hz 400 kV line has a series reactance per phase of 0.327 ohm/km. Neglect line resistance. The line is 400 km long and the receiving-end load is 600 MW at 0.9 p.f. lag. The positive-sequence line capacitance is 7.27 nF/km. In the absence of any compensating equipment connected to ends of line, calculate the sending-end voltage. Work with and without considering line capacitance. The base quantities for calculation are 400 kV, 1000 MVA
Solution. Load voltage V = 1.0 per unit. Load current I = 0.6 (1 – j0.483) = 0.6 – j0.29 p.u.
Base impedance Zb = 4002 /1000 = 160 ohms. Base admittance Yb = 1/160 mho. Total series reactance of line
X = j0.327 × 400 = j130.8 ohms = j 0.8175 p.u.
Total shunt admittance of line Y = j 314 × 7.27 × 10–9 × 400
= j 0.9136 × 10– 3 mho = j 0.146 p.u.
When considering the line capacitance, one half will be concentrated at load end across the load and the other half at the entrance to the line at the sending end, as shown in Figure 2.1. Then, the receiving-end current is
Ir = 0.6 – j0.29 + j0.073 = 0.6 – j0.217 p.u
The sending-end voltage will be
Es = 1 + j (0.6 – j0.217) 0.8175 = 1.1774 + j0.49
= 1.2753 Ð 22.6° = 510 Ð 22.6°, kV
When line capacitance is omitted, the sending-end voltage is
Es = 1 + j (0.6 – j0.29) 0.8175 = 1.33 Ð 21.6° = 532 Ð 21.6°, kV
Note that in both cases, the sending-end voltage, that is, the generating station h.v. bus voltage exceeds the IS limit of 420 kV.
Example 2.2. In the previous example, suggest suitable reactive compensation equipment to be provided at the load end to maintain 400 kV (1 p.u. voltage) at both ends of line.
Solution. Since the load is drawing lagging (inductive) current, obviously we have to provide capacitive compensating equipment across the load in order to reduce the line current. Figure 2.1 (b) shows the overall arrangement. If Ic is the current drawn by this compensating equipment, considering line capacitance, the total receiving-end line current will be Ir = 0.6 – j0.217 + j Ic , p.u., and the resulting sending-end voltage will be
Es = 1 + j (0.6 – j0.217 + j Ic ) 0.8175 = (1.1774 – 0.8175 Ic ) + j0.49.
Now, since |Es| = 1 p.u. also, there results Ic = 0.374 p.u. The resulting rating of the compensating capacitor is 374 MVAR.
When the presence of line capacitance is neglected, Ic = 0.447 p.u. and the required compensation is 447 MVAR, which is of course higher than 374 MVAR by 73 MVAR.
Detailed discussion of line compensation for voltage control at the sending- and receivingend busses will be considered in Chapter 12. We note in passing that voltage control in e.h.v. systems is a very expensive proposition. In addition to switched capacitors which provide variable capacitive reactive power to suit variation of load from no load to full load, variable inductive compensation will be required which takes the form of thyristor-controlled reactors (TCR) which are also known as Static VAR Systems. Unfortunately, these give rise to undesirable harmonics which are injected into the line and may cause maloperation of signalling and some communication equipment.
Source: Extra High Voltage AC Transmission Engineering. Rakosh DB.NEW AGE INTERNATIONAL (P) LIMITED, PUBLISHERS. 2006.
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