DC Machine & Transformer Parallel Operation of Transformer Excitation Phenomenon in Transformer Harmonics in Transformer Made By: Name Enrollment No. Abhishek Chovatia 1304201009508 Nehal Desai 1304201009509 Farzan Todiwala 1304201009510 Jay Kheni 1304201009513 Khushbu Naik 1304201009522

Content Parallel Operation of 1-Phase Transformer Need for Parallel Connection Condition for Parallel Connection Parallel Connection of 3 Phase Transformer Advantage of Parallel Connection Excitation Phenomenon in Transformer Harmonics in Transformer

Introduction Parallel Operation of 1-Phase Transformer By parallel operation we mean two or more transformers are connected to the same supply bus bars on the primary side and to a common bus bar/load on the secondary side. Such requirement is frequently encountered in practice.

Need for Parallel Connection Non-availability of a single large transformer to meet the total load requirement. The power demand might have increased over a time necessitating augmentation of the capacity. More transformers connected in parallel will then be pressed into service. To ensure improved reliability. Even if one of the transformers gets into a fault or is taken out for maintenance/repair the load can continued to be serviced.

Need for Parallel Connection To reduce the spare capacity. If many smaller size transformers are used one machine can be used as spare. If only one large machine is feeding the load, a spare of similar rating has to be available. The problem of spares becomes more acute with fewer machines in service at a location. When transportation problems limit installation of large transformers at site, it may be easier to transport smaller ones to site and work them in parallel.

Condition for Parallel Connection The voltage ratio must be the same. The per unit impedance of each machine on its own base must be the same. The polarity must be the same, so that there is no circulating current between the transformers. The phase sequence must be the same and no phase difference must exist between the voltages of the two transformers.

Same Voltage Ratio: If two transformers of different voltage ratio are connected in parallel with same primary supply voltage, there will be a difference in secondary voltages. Now say the secondary of these transformers are connected to same bus, there will be a circulating current between secondaries and therefore between primaries also. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss. Same Percentage Impedance: The current shared by two transformers running in parallel should be proportional to their MVA ratings. Again,  current  carried by these transformers are inversely proportional to their internal impedance. From these two statements it can be said that,  impedance of transformer running in parallel are inversely proportional to their MVA ratings. In other words, percentage impedance or per unit values of impedance should be identical for all the transformers that run in parallel.

Same Polarity: Polarity of all transformers that run in parallel, should be the same otherwise huge circulating current that flows in the transformer but no load will be fed from these transformers. Polarity of transformer means the instantaneous direction of induced emf in secondary. If the instantaneous directions of induced secondary emf in two transformers are opposite to each other when same input power is fed to both of the transformers, the transformers are said to be in opposite polarity. If the instantaneous directions of induced secondary emf in two transformers are same when same input power is fed to the both of the transformers, the transformers are said to be in same polarity. Same Phase Sequence: The phase sequence or the order in which the phases reach their maximum positive voltage, must be identical for two parallel transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

Parallel Connection of 3 Phase Transformer Single-phase transformers can be connected to form 3-phase transformer banks for 3-phase power systems. Four common methods of connecting three transformers for 3-phase circuits are ∆- ∆, Y-Y, Y-∆, and ∆-Y connections.

Parallel Connection of 3 Phase Transformer An advantage of ∆-∆ connection is that if one of the transformers fails or is removed from the circuit, the remaining two can operate in the open-∆ or Y connection. This way, the bank still delivers 3-phase currents and voltages in their correct phase relationship. However, the capacity of the bank is reduced to 57.7 % of its original value. In the Y-Y connection, only 57.7% of the line voltage is applied to each winding but full line current flows in each winding. The Y-Y connection is rarely used The ∆-Y connection is used for stepping up voltages since the voltage is increased by the transformer ratio multiplied by 3 . The Y-∆ connection may be used for stepping down voltages.

Advantage of Parallel Connection It enables an existing transformer to be upgraded to meet an increase in load, without having to remove that transformer and replace it with a 'larger' (expressed in volt amperes) one. It provides an useful way of using up a stock of smaller transformers. Ex. We are currently using 55kVA transformer but there is a need of 110kVA in future then we don’t need to replace it by a bigger one, but we can connect another 55kVA transformer and we can get the 110kVA output.

Excitation Phenomenon It is known that though secondary is open, the transformer draws current from supply, when primary is excited by rated voltage. This current is not load current and is basically required to produce core flux. But due to non-linearity of core material such as hysteresis and saturation, the no load current is not sinusoidal in nature. Let us study the effect of hysteresis and saturation on the waveform of no load current which is also called exciting current.

Current inrush phenomenon ( Switching transient )  In the steady state operation, both V1 and Φ are sinusoidal and Φ lags V1 by 90°as shown in the Figure  When the primary voltage V1 is switched on to the transformer, the core flux and the exciting current undergo a transient before achieving the steady state. They pass through a transient period. The effect of transient is severe when voltage wave pass through origin.       

In the inductive circuit flux can start with zero value In the inductive circuit flux can start with zero value. But the steady state value of flux at start is -Φm , as shown in the Fig. 2, at t = 0. Thus during transients a transient flux called off-set flux, Φt = Φm originates such that at t = 0, Φt +Φss  is zero at the instant of switching. This transient flux Φt then decays according to circuit constants i.e. ratio L/R. This ratio is generally very small for transformers.        Thus during transients, the total flux goes through a maximum value of 2Φm. Such effect is called doubling effect. This is shown in the Fig.3.        Due to the doubling effect, core flux achieves a value of  2Φm due to which transformer draws a large exciting current. This is due to the fact that core goes into deep saturation region of magnetisation. Such a large exiting current can be as large as 100 times the normal exiting current. To withstand electromagnetic forces developed due to large current, the windings of transformer must be strongly braced. This large current drawn during transient is called inrush phenomenon

Doubling effect

Harmonics in Transformer

Harmonics increase both load and no-load losses due to increased skin effect, eddy current, stray and hysteresis losses. The most important of these losses is that due to eddy current losses in the winding; it can be very large and consequently most calculation models ignore the other harmonic induced losses. The precise impact of a harmonic current on load loss depends on the harmonic frequency and the way the transformer is designed. In a transformer that is heavily loaded with harmonic currents, the excess loss can cause high temperature at some locations in the windings. This can seriously reduce the life span of the transformer and even cause immediate damage and sometimes fire. Reducing the maximum apparent power transferred by the transformer, often called de-rating.

Problems Caused By Harmonics Excessive heating in distribution transformer. Increased cooling load on buildings. Increased heating in motors and generators. Increased heating in cables. Nuisance tripping of breakers. Malfunction or failure of communication and data processing equipments.

Benefits Reduces the K-Factor ratio of transformer. Saves energy. Prevents distribution system apparatus from overheating. Saves Money. Balances Phase currents and voltages on primary. Reduces voltage distortion. Increases system capacity and reliability. Reduces apparatus vibration and noise. Prevents electronic circuit breaker malfunction. Provides healthier environment for the loads.

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