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3. Repeat Example 7.8, using a 100 kVA transformer and #3/0 AWG and #2 AWG cables for the SLs and SDs, respectively Show the procedure to arrive at the following answers (a) 0.72 pu A (b) 0.050 pu V Example 7.8 residential secondary distribution system. Assume that the distribution trans- Figure 7.11 shows former capacity is 75 kVA (use Table 7.1), all secondaries and services are single-phase three wire, nominally 120/240 V, and all SLs are of #2/0 A1/Cu XLPE cable, and SDs are of #1/0 AI/Cu XLPE cable (use Table 7.2). All SDs are 100 ft long, and all SLs are 200 ft long. Assume an average lagging-load power factor of 0.9 and 100% load diversity factors and determine the following: a a. Find the total load on the transformer in kilovoltamperes and in per units. b. Find the total steady-state voltage drop in per units at the most remote and severe tomer's meter for the given annual maximum system loads. cus- 3 kVA 2 kVA 5 kVA 6 kVA 6 kVA 7KVA 100 ft SL SL 6 kVA 7 kVA 4 kVA 8 kVA 10 kVA 8kVA 200 ft 200 ft A residential secondary distribution system FIGURE 7.11 TABLE 7.1 Single-Phase 7200-120/240-V Distribution Transformer Data at 65°C Core Loss (kW Copper Loss (kW) Excitation Current (A) Rated (kVA, kW) R (pu)X(pu) 0.0130 15 0.083 0.194 0.0094 0.014 0.115 25 0.309 0.015 0.0123 0,0138 0.170 37.5 0.400 0.0107 0.0126 0.014 50 0.178 0.537 0.0107 0,0139 0.014 0.755 0.014 75 0.280 0.0101 0.0143 100 0.335 0.975 0.0098 0.0145 0.014 At rated voltage and frequency. At rated voltage and kilovoltampere load. TABLE 7.2 Twin Concentric Al/Cu XLPE 600 V Cable Data R (0/1000 ft) per Conductor Direct Burial Ka X (Q/1000 ft) per Phase Neutral Size Conductor 80°C Phase Conductor Ampacity (A) Conductor 90°C 90% PF 50% PF 2 AWG 0.334 0.56 0.0299 180 0,02613 0.01608 0.0305 1 AWG 0.265 0.419 205 0.02098 0.01324 1/0 AWG 0.210 0.337 0.0297 230 0,01683 0.01089 2/0 AWG 0,167 0.259 0,0290 265 0.01360 0.00905 3/0 AWG 0.132 0.211 0,0280 300 0,01092 0.00752 4/0 AWG 0.105 0.168 0.0275 340 0.00888 0.00636 250 kcmil 0.089 0.133 0.0280 370 0.00769 0.00573 0,0270 350 kcmil 0.063 0.085 445 0.00571 0,00458 0.044 500 kemil 0,066 0.0260 540 0,00424 0,00371 Per unit voltage drop per 104 A ft (amperes per conductor times feet of cable) based on 120 V line-to-neutral or 240 V -wire loading lne line. Valid for the two power factors shown and for perfectly balanced thre Solution a. Assuming a diversity factor of 100%, the total load on the transfomer is S(3+2+8+ 6)+(5+6+7+ 4)+(6+7+8+10 -19+ 22+31 -72 kVA or, in per units S Sa 72kVA 75KVA 0.96puA b. To find the total voltage drop in per units at the most remote and severe customer's meter, calculate the per unit voltage drops in the transformer, the service line, and the SD of the most remote and severe customer. Therefore VD NR cnse+ X sin) 0,9610.0101x 0.90+0,0143x0.4359 =0.0147puV VDg-K 10t 129.17A 200 10 0.0136 0.03513puV IxI VDan K 10 41.67 Ax100ft 683 0.0168) 10 = 0.0070puV Therefore, the total voltage drop is VD- VD +Dy VDn 0.0147+0.03513+0.0070 -0.0568puV

Answer 1

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