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Chapter 9—Wind Power Plants 9–11 size. The neutral carries a very small current in normal operation, so does not enter into the thermal calculations. A trench is dug, perhaps 18 inches wide by 24 to 36 inches deep. A layer of sand is placed on the bottom, to prevent mechanical damage to the conductor insulation from sharp rocks. The three phase conductors are laid in the trench with a nominal separation of 7.5 inches. The neutral conductor is placed anywhere in the trench. Another layer of sand is placed on top of the four conductors before the trench is backfilled with dirt. If the ampacity of a single circuit is not adequate for a particular installation, a second circuit is added in parallel with the first. The conductors of each circuit must be the same size, so that both the resistance and inductive reactance of each circuit will be the same, so that current will divide evenly between the circuits. The trench for the double circuit case is much wider, approximately 60 inches, to allow a separation of at least 24 inches between circuits. Even with this separation, the heat dissipated by one circuit will raise the temperature of the other circuit. This lowers the allowable ampacity. For example, a 4/0 aluminum circuit will carry 307 A in isolation, but only 283 A when a second circuit is 24 inches away. The table should have a correction factor applied when the ambient temperature is different from 20 o C. At a depth of 36 inches, the ambient soil temperature has a rather small annual variation, being very close to the annual average air temperature above the soil. It makes sense that circuits in Minnesota could carry more current that similar circuits in southern Arizona since the soil temperatures will be different. For example, if the ambient temperature is less than 10 o C, the ampacity is increased by 9 to 12%, while if the ambient temperature is greater than 26 o C, the ampacity is reduced by 10 to 13%. Once we find the rated current of the turbine from Eq. 6, we are ready to find the rated current of the conductors connected to the turbine. These currents are not necessarily identical because of code requirements. The National Electrical Code requires “The ampacity of the phase conductors from the generator terminals to the first overcurrent device shall not be less than 115 percent of the nameplate current rating of the generator”, Article 445-5. One reason for this rule is that generators can be operated above their rating by as much as 15% for extended periods of time, and we would not want the conductors to fail before the generator. This rule only applies to this particular section of wire. The remainder of the windfarm circuits are sized according to actual current flow. For example, a 100 kW, 480 V generator with a power factor of 0.8 will have a rated current of 150 A. We assume that the conditions of Table 9.2 apply. The cable needs a rating of (1.15)(150) = 173 A. This is met by a single circuit of AWG 2 copper at 209 A, or by a single circuit of AWG 1 aluminum at 184 A. It is also met by a double circuit of two AWG 8 copper conductors in parallel, which will carry 2(92) A = 184 A, which would meet the generator requirements. Two AWG 6 aluminum conductors in parallel will have the same ampacity. In smaller wire sizes, the cost of the extra (or wider) trench probably makes a double circuit more expensive than a single circuit. For larger wire sizes, a double circuit may be less expensive and even essential to get the required current. Doubling the wire size does not Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 9—Wind Power Plants 9–12 double the ampacity. For example, a single circuit of 1000 kcmil copper will carry 887 A, while a double circuit of 350 kcmil copper will carry 2(474) = 948 A. The double circuit case requires seventy percent of the copper ((2)(350) = 700 as compared with 1000 kcmil) and will carry seven percent more current in this case. This saving in copper can easily pay for the cost of a second trench. Transformers The next element to be sized is the transformer T 1 . Transformers are always rated in terms of kVA (or MVA) so we have to convert from the generator rating in kW by using the power factor. If the generator is rated at 100 kW with a power factor of 0.8, the kVA rating would be 100/0.8 = 125 kVA. Since N tc generators are connected to a transformer in this cluster network, the transformer rating would be N tc times this value. Actual available ratings may not match this calculated value, of course. Table 9.3 gives the available sizes and 1991 prices for a popular manufacturer of three-phase distribution transformers. These particular transformers are padmounted, that is, mounted on a small concrete pad at ground level. They are built as three-phase transformers rather than individual single-phase transformers connected in three-phase. There are two choices as to the type of connection, either radial feed or loop feed. Radial feed refers to a single path between source and load. If a transformer or distribution line along this path is not operative, then there will not be any electrical service downstream from this point. This connection is cheap and simple, and most residential loads are served from a radial feed for this reason. Table 9.3. Transformer costs, 480Y/12470∆ kVA NLL LL COST 112.5 334 834 $4500 150 353 1170 5000 225 481 1476 6000 300 554 1872 6500 500 817 2982 8000 750 1112 5184 10000 1000 1364 5910 12500 Adders per transformer for: Loop Feed $600 LBOR Switches 500 Lightning Arresters 400 Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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