DC MACHINE AND tRANSFORMER TOPIC : THREE PHASE TRANSFORMER CONNECTION Prepared by : Swati Chotaliya(130830109016) Swati Gajera (130830109025) Nidhi Ganatra (130830109026) Nidhi Kami(130830109049) Guide : Prof.Yashit D.tita

Three-Phase Circuits Delta-delta connection Wye-wye connection Wye-delta connection Delta-wye connection Open-delta connection Scott connection Zig zag connection

star and delta connections

Delta-delta connection Fig shows the delta delta connection of three phase transformer. In delta delta connection primary and secondary both are in delta connection. In delta-delta transformer, 2suffixed terminals of each phase primary winding will be connected with 1suffixed terminal of next phase primary winding. If primary is HV side, then A2 will be connected to B1, B2will be connected to C1 and C2 will be connected to A1. Similarly in LV side 2 suffixed terminals of each phase winding will be connected with 1suffixed terminals of next phase winding. That means, a2will be connected to b1, b2will be connected to c1and c2will be connected to a1.

Voltage and current relation In a delta system, the line current is higher than the phase current by a factor of the square root of 3 (1.732). ILine = IPhase x 1.732 IPhase = ILine / 1.732 In a delta system, the line voltage is equal to the phase voltage . Line Voltage VL= Vph

ADVANTAGES VL= Vph so each winding will have more number of turns. ILine = IPhase x 1.732 so due to small winding current thin wires can be used . so connection is economical for low voltage application. The core flux is always perfectly sinusoidal. No distrotion in the secondary voltage because the delta connection provides the closed path for circulation of harmonic component of current. The phase voltage remains constant even if the load is unbalanced type.so this connection can be used for the unbalanced load as well.

DISADVANTAGES This type of connection is suitable only for large low voltage transformer. Neutral point is absent so this type of connection is not suitable for 3 phase 4 wire system.

Wye-wye connection Star-star transformer is formed in a 3 phase transformer by connecting one terminal of each phase of individual side, together. The common terminal is indicated by suffix 2 in the figure . If terminal with suffix 2in both primary and secondary are used as common terminal, voltages of primary and secondary are in same phase. That is why this connection is called zero degree connection or 0° - connection. If the terminals with suffix 2is connected together in HV side as common point and the terminals with suffix 1in LV side are connected together as common point, the voltages in primary and secondary will be in opposite phase. Hence, star-star transformer connection is called 180°-connection, of three phase transformer.

Voltage and current relation In a wye system, the line voltage is higher than the phase voltage by a factor of the square root of 3 (1.732). Line Voltage VL= 1.73 Vph In a wye system, the line current is equal to the phase current. Line current IL = Iph

ADVANTAGES Primary and secondary circuits are in phase. Due to neutral, it is possible to use it for 3 phase 4 wire system. In this connection Line Voltage VL= 1.73 Vph .hence the number of turns required to wind primary and seconday is less.so connection is economical one. IL = Iph so windings have to carry large currents .

DISADVANTAGES With an unbalanced load on the secondary, the neutral point gets shifted.This degrades the performance of the star-star conncetion.This problem can be solved by connecting the primary neutral to the generator neutral. Distortion in the the secondary voltage can result due to the presence of third harmonic voltage in the alternator output.

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