Multiple Choice Questions on Transformer | 14

1. Two 40 kVA 1 - phase transformer are connected in open delta to supply a 230 V balanced 3 phase load. The total load that can be supplied without overloading either transformer will be

The rated secondary transfer current, I2B = 40 × 103 / 230 = 173.9 A
This also be load line current, therefore the load kVA is = √3 × V2BI2B × 10-3 = 69.28 A

2. A 900 kVA load is supplied by three transformer connected in delta - delta. The primaries are connected to a 2300 V supply line, while the secondaries are connected to a 230 V load. If one transformer is removed for repair then the currents in the high voltage side of the transformer is

In delta - delta operation, VL1 and IL1 are the line voltage and line current respectively on HV side, SΔ - Δ = √3 × VL1IL1
⇒ 900 × 103 = √3 × 2300 × IL1
∴ IL1 = 900 × 103 / √3 × 2300 = 225.9 A
Transformer current per phase on HV side, Ip1 = 1 / √3 × line current = 1 / √3 × 225.9 = 130.4 A

3. Two single phase furnace working at 100 V are connected to a 3300 V, 3 - phase supply through scott connected transformer. If there is no transformation loss, then the current in the 3 - phase lines, when the power taken by each furnace is 500 KW at a p.f. of 0.8 lagging will be

a = 3300 / 100 = 33
For teaser transformer, P2t = V2tI2tcosφ2t
⇒ I2t = P2t / V2tcosφ2t = 500 × 103 / 100 × 0.8 = 6250 A
Since,the 2 phase load is balanced, the 3 phase side is also balanced. For the mmf balance of the teaser transformer IA × 2 / √3 × Ts / Tp × I2t = 2 / √3 × 6250 / 33 = 218.7 A
∴ |IA| = |IB| = |Ic| = 218.7 A

4. Two 40 kVA 1-phase transformer are connected in open delta to supply a 230 V balanced 3 phase load. The total load that can be supplied without overloading either transformer will be

The rated secondary transfer current, I2B = 40 × 103 / 230 = 173.9 A
This also be load line current.
Therefore the load kVA is = √3 × V2BI2B × 10 - 3 = 69.28 kVA.

5. A 900 kVA load is supplied by three transformer connected in delta - delta. The primaries are connected to a 2300 V supply line, while the secondaries are connected to a 230 V load. If one transformer is removed for repair then the currents in the high voltage side of the transformer is

In delta - delta operation, If VL1 and IL1 are the line voltage and line current respectively on HV side, SΔ - Δ = √3 × VL1IL1
⇒ 900 × 103 = √3 × 2300 × IL1
∴ IL1 = 900 × 103 / √3 × 2300 = 225.9 A
Transformer current per phase on HV side, Ip1 = 1 / √3 × line current = 1 / √3 × 225.9 = 130.4 A.