Electrical Engineering

Transformer oil is a much better electrical insulator than air. So, in large transformers, oil is used to reduce the flashover distances between live internal parts, thus reducing the overall bulk of the transformer. At the same time, oil helps cool the transformer either by natural convection, or by forced circulation.

'Dry transformers' are transformer that do not use oil for insulation or cooling purposes. They tend to be transformers with lower voltage and power ratings, where flashover and cooling is far less relevant.

DC versus AC.

The vast majority of electric power transmissions use threephase

alternating current. The reasons behind a choice of

HVDC instead of AC to transmit power in a specific case

are often numerous and complex. Each individual

transmission project will display its own set of reasons

justifying the choice.

A. General characteristics

The most common arguments favouring HVDC are:

1) Investment cost. A HVDC transmission line costs

less than an AC line for the same transmission

capacity. However, the terminal stations are more

expensive in the HVDC case due to the fact that

they must perform the conversion from AC to DC

and vice versa. On the other hand, the costs of

transmission medium (overhead lines and cables),

land acquisition/right-of-way costs are lower in the

HVDC case. Moreover, the operation and

maintenance costs are lower in the HVDC case.

Initial loss levels are higher in the HVDC system,

but they do not vary with distance. In contrast, loss

levels increase with distance in a high voltage AC

system

Above a certain distance, the so called "break-even

distance", the HVDC alternative will always give

the lowest cost. The break-even-distance is much

smaller for submarine cables (typically about 50

km) than for an overhead line transmission. The

distance depends on several factors, as transmission

medium, different local aspects (permits, cost of

local labour etc.) and an analysis must be made for

each individual case (Fig. 3).

2) Long distance water crossing. In a long AC cable

transmission, the reactive power flow due to the

large cable capacitance will limit the maximum

transmission distance. With HVDC there is no such

limitation, why, for long cable links, HVDC is the

only viable technical alternative.

3) Lower losses. An optimized HVDC transmission line

has lower losses than AC lines for the same power

capacity. The losses in the converter stations have of

course to be added, but since they are only about 0.6

% of the transmitted power in each station, the total

HVDC transmission losses come out lower than the

AC losses in practically all cases. HVDC cables also

have lower losses than AC cables.

4) Asynchronous connection. It is sometimes difficult or

impossible to connect two AC networks due to

stability reasons. In such cases HVDC is the only

way to make an exchange of power between the two

networks possible. There are also HVDC links

between networks with different nominal

frequencies (50 and 60 Hz) in Japan and South

America.

5) Controllability. One of the fundamental advantages

with HVDC is that it is very easy to control the

active power in the link

6) Limit short circuit currents. A HVDC transmission

does not contribute to the short circuit current of the

interconnected AC system.

7) Environment. Improved energy transmission

possibilities contribute to a more efficient utilization

of existing power plants. The land coverage and the

associated right-of-way cost for a HVDC overhead

transmission line is not as high as for an AC line.

This reduces the visual impact. It is also possible to

increase the power transmission capacity for

existing rights of way. There are, however, some

environmental issues which must be considered for

the converter stations, such as: audible noise, visual

impact, electromagnetic compatibility and use of

ground or sea return path in monopolar operation.

In general, it can be said that a HVDC system is

highly compatible with any environment and can be

integrated into it without the need to compromise on

any environmentally important issues of today.

There is a misconception that a step-up transformer 'steps up voltage, but steps down current'. Nothing could be further from the truth!

The secondary current of any transformer is determined by the secondary voltage and the load which it supplies. It is NOT determined by the primary current!

In fact, it is the primary current that is determined by the secondary current -not the other way around.

So, to answer your question, your step-up transformer has a voltage ratio of 1:2.3, so the primary current that results from a secondary current of 150 A must be 2.3 x 150 = 345 A.

A transformer is a device in which two circuits are coupled by a magnetic field that is linked to both. There is no conductive connection between the circuits, which may be at arbitrary constant potentials. Only changes in one circuit affect the other. The circuits often carry at least approximately sinusoidal currents, and the effect of the transformer is to change the voltages, while transferring power with little loss. Sinusoidal excitation is not necessary, and transformers may handle arbitrary signals, in which the action can be considered as a transformation of impedances. The magnetic field coupling the circuits can be in air, but is usually in a ferromagnetic material, the core, in which the field can be thousands of times greater than it would be in air, making the transformer efficient and small. The transformer is an honorary electrical "machine" in which the flux changes occur by variation in currents with time, instead of by motion.

Most transformers with iron cores can be considered as ideal when you use them. An ideal transformer has no losses, an aim that is closely attained in practice, so the energy transfer from the primary circuit to the secondary circuit is perfect. The diagram represents such a transformer, showing the core with magnetic flux φ, the primary winding of N1 turns, and the secondary winding of N2 turns. The reference directions for the voltages and currents at the terminals are shown. All of these quantities are to be considered as phasor amplitudes, varying sinusoidally with time. Note the dots at one or the other of the terminals of each winding. Currents entering the dotted terminals produce flux in the same direction, the direction shown. The current and voltage ratios are equal to the turns ratio. This means that the power factor (cosine of the phase angle), and the power, are the same at input (primary) and output (secondary). These things you probably already know, and we will not explore their consequences further.

The diagram shows the usual schematic way to represent a transformer. In an actual transformer, the windings are wound on top of each other, not on separate legs, to reduce leakage flux. In the usual shell-type transformer, both primary and secondary are on one leg, and are surrounded by the core. A core-type transformer has windings covering the core legs.

In order to design a transformer, or to examine in more detail how it departs from ideality, it is necessary to understand how a transformer works, not just how to express its terminal relations in an approximate way. It is also important to know how the properties of the iron core affect the performance of the transformer. A real transformer becomes hot because of losses, and the ouput voltage may vary with load even when the primary voltage is held constant.

The mutual flux φ is the means of transfer of energy from primary to secondary, and links both windings. In an ideal transformer, this flux requires negligibly small ampere-turns to produce it, so the net ampere-turns, primary plus secondary, is about zero. When a current is drawn from the secondary in the positive direction, ampere-turns decrease substantially. This must be matched by an equal increase in primary ampere-turns, which is caused by an increase in the current entering the primary in the positive direction. In this way, the back-emf of the primary (the voltage induced in it by the flux φ) equals the voltage applied to the primary, as it must. This fundamental explanation of the operation of a transformer must be clearly understood before proceeding further.

Yes, with an auto transformer. It is a transformer with multiple taps on the winding. Connect the 208 volts across the common tap and 208 volt tap. Then connect the load across the common tap and the 240 volt tap.

208 VAC usually comes from one floating side of a Delta 3 phase transformer.

240 VAC usually comes from a grounded center tapped transformer.

Using an autotransformer would be very hazardous and violate code. Probably tripping the 208 VAC breakers.

The only way to do it correctly would be with a transformer with a 208 VAC floating primary and a 240 VAC grounded center tapped secondary.