Current Inrush Phenomenon ( Switching Transients )

Current Inrush Phenomenon ( Switching Transients ) 

AU : May-15

In the steady state operation, both V₁ and ϕ are sinusoidal and ϕ lags V₁ by 90^o as shown in the Fig. 6.27.1. 

• 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 transients is severe when voltage wave passes through origin.

• In the inductive circuit flux can start with zero value. But the steady state value of flux at start is - ϕ_m, as shown in the Fig. 6.27.1, at t = 0. Thus during transients flux called off-set flux, ϕ_t = ϕ_m originates such that at t=0, ϕ_t + ϕ_ss  is zero at the instant of switching. This transient flux & t 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 2 ϕ_m Such effect is called doubling effect. This is shown in the Fig. 6.27.2.

• Due to the doubling effect, core flux achieves a value of 2 ϕ_m 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 exciting current can be as large as 100 times the normal exciting current. To withstand electromagnetic forces developed due to large current, the windings of transformer must be strongly braced. This large current drawn during transients is called current inrush phenomenon.

• Practically initial core flux cannot be zero due to the residual flux or present, due to retentivity property of core. The transient resultant flux goes through ϕ_r = ϕ'_r+2 ϕ_m and there is heavy inrush current in practice. The effect of transient is even severe in practice.

• Such high transient current gradually decreases and finally acquires a steady state. It can last for several seconds. The transient flux ϕ_t and exciting current both are unidirectional during transients. In steady state, exciting current becomes sinusoidal. The Fig. 6.27.3 shows the oscillogram of the inrush current.

Review Question

1. Explain the current inrush phenomenon in transformers.AU May-15, Marks 6