电气专业毕业设计外文翻译----电力变压器工作原理

外文资料翻译

Power Transformer Principles

1. INTRODUCTION

The high-voltage transmission was need for the case electrical power is to be provided at considerable distance from a generating station. At some point this high voltage must be reduced, because ultimately is must supply a load. The transformer makes it possible for various parts of a power system to operate at different voltage levels. In this paper we discuss power transformer principles and applications.

2. TOW-WINDING TRANSFORMERS

A transformer in its simplest form consists of two stationary coils coupled by a mutual magnetic flux. The coils are said to be mutually coupled because they link a common flux.

In power applications, laminated steel core transformers (to which this paper is restricted) are used. Transformers are efficient because the rotational losses normally associated with rotating machine are absent, so relatively little power is lost when transforming power from one voltage level to another. Typical efficiencies are in the range 92 to 99%, the higher values applying to the larger power transformers.

The current flowing in the coil connected to the ac source is called the primary winding or simply the primary. It set s up the flux φ in the core, which varies periodically both in magnitude and direction. The flux links the second coil, called the secondary winding or simply secondary. The flux is changing; therefore, it induces a voltage in the secondary by electromagnetic induction in accordance with Lenz’s law. Thus the primary receives its power from the source while the secondary supplies this power to the load. This action is known as transformer action.

3. TRANSFORMER PRINCIPLES

When a sinusoidal voltage V p is applied to the primary with the secondary open-circuited, there will be no energy transfer. The impressed voltage causes

a small current I θ to flow in the primary winding. This no-load current has two functions: (1) it produces the magnetic flux in the core, which varies sinusoidally between zero and ±φm , where φm is the maximum value of the core flux; and (2) it provides a component to account for the hysteresis and eddy current losses in the core. There combined losses are normally referred to as the core losses.

The no-load current I θ is usually few percent of the rated full-load current of the transformer (about 2 to 5%). Since at no-load the primary winding acts as a large reactance due to the iron core, the no-load current will lag the primary voltage by nearly 90º. It is readily seen that the current component I m = I 0sinθ0, called the magnetizing current, is 90º in phase behind the primary voltage V P . It is this component that sets up the flux in the core; φ is therefore in phase with I m .

The second component, I e =I 0sinθ0, is in phase with the primary voltage. It is the current component that supplies the core losses. The phasor sum of these two components represents the no-load current, or

e m o I I I +=

It should be noted that the no-load current is distortes and nonsinusoidal. This is the result of the nonlinear behavior of the core material.If it is assumed that there are no other losses in the transformer, the induced voltage In the primary, E p and that in the secondary, E s can be shown. Since the magnetic flux set up by the primary winding ，there will be an induced EMF E in the secondary winding in accordance with Faraday’s law, namely, t N E ∆∆⋅=/ϕ. This same flux also links the primary itself, inducing in it an EMF, E p . As discussed earlier, the induced voltage must lag the flux by 90º, therefore, they are 180º out of phase with the applied voltage. Since no current flows in the secondary winding, E s =V s . The no-load primary current I 0 is small, a few percent of full-load current. Thus the voltage in the primary is small and V p is nearly equal to E p . The primary voltage and the resulting flux are sinusoidal; thus the induced quantities E p and E s vary as a sine function. The average value of the induced voltage given by