Answer to Question #157668 in Electrical Engineering for Mina malik

Question #157668

i) the advantage and disadvantage of DC and ac transmission system

ii) the advantage of using high transmission voltage

Expert's answer

1) Advantages of AC power

The question of increasing and reducing the AC voltage at the current level of technical development is solved much easier than direct electric current.

Such transformations are quite simply performed using a relatively simple device-a transformer. The transformer has a high efficiency, which reaches 99 %. This means that no more than one percent of the power is lost when the voltage is increased or decreased. In addition, the transformer allows you to decouple a high voltage with a lower one, which for most electrical installations is a very weighty argument.

The use of a three-phase AC system can further improve the efficiency of the power supply system. To transmit electricity of similar power, fewer wires will be required than with single-phase alternating current. In addition, a three-phase transformer is smaller than a single-phase transformer of equal power.

AC electric machines, in particular asynchronous motors with a closed-loop rotor, have a much simpler design than DC motors. The main advantage of three-phase asynchronous motors is the absence of a collector-brush assembly. This reduces the cost of manufacturing and operating such electric machines. In addition, due to the absence of a collector-brush assembly, asynchronous motors have many times more power than DC motors.

Disadvantages of direct current

The following disadvantages follow from the above.

1. The complexity of raising and lowering the voltage, that is, converting DC electricity. This is primarily due to the complexity of converter designs. Because you need powerful semiconductor switches designed for high voltage. The absence of which leads to a large number of series and parallel connected semiconductor devices. As a result, the reliability of the entire converter decreases, the cost increases and the power loss increases.

2. Electric machines have a more complex design, so they are less reliable and more expensive, both in production and in operation.

3. Difficulties in decoupling high and low voltages.

Disadvantages of alternating current

1. The most important disadvantage of alternating current is the presence of reactive power. As you know, the capacitor and the inductor exhibit their reactive properties only in AC circuits. Simply put, the coil and the capacitor create an AC reactance, but do not consume it. As a result, of the total power given by the alternator, part of the power is not spent on performing useful work, but only circulates uselessly between the generator and the load. Such power is called reactive and is harmful. Therefore, they try to minimize it.

However, most loads – motors, transformers, and the wires themselves-are inductive elements. And the greater the inductance, the greater the proportion of the reactive power of the full and this must be fought.

2. The second main disadvantage of alternating current is that it does not flow through the entire cross-section of the conductor, but is displaced closer to its surface. As a result, the area over which the electric current flows decreases, which in turn leads to an increase in the resistance of the conductor and to an increase in power losses in it.

The higher the frequency, the stronger the current is forced to the surface of the conductor and ultimately, the higher the power loss.

Advantages of direct current

1. The main advantage of DC electric power is the lack of reactive power. This means that all the power generated by the generator is consumed by the load, minus the losses in the wires.

2. Direct current, unlike alternating current, flows through the entire cross-section of the conductor.

These two points lead to the fact that if you transmit the same power at equal voltages with direct and alternating currents, then the power loss of electricity with direct current would be almost two times less than with alternating current.

In addition, if we consider such household electronic devices as laptops, computers, televisions, etc., then they all have power supplies that convert an alternating voltage of 220 V (230 V) to a constant voltage of a lower value. And such transformations are associated with a partial loss of power.

In addition, as mentioned earlier, a three-phase asynchronous motor (AD) can be connected directly to the 380 V network, which is quite justified in the case when it is not necessary to change the operating mode of the motor. But if it is necessary to change the frequency of rotation of its shaft, then it is necessary to apply a voltage to the stator windings, the frequency and amplitude of which must change proportionally, according to Kostenka's law. For this purpose, three-phase autonomous inverters (AI) are used, most often voltage inverters. Such inverters must be powered by a constant voltage source.

It should also be noted that recently began to be very widely used solar panels that produce direct current. In addition, the power of batteries has significantly increased and the capacity of supercapacitors, which also belong to direct current sources and are finding more and more practical applications every day, has increased.

2) Despite the fact that the transmission of electricity on alternating current is the most common, in some cases it is more preferable to use high-voltage direct current transmissions.

Examples of such cases include the following:

Underwater cables whose high capacitance leads to large losses during AC transmission (for example, a 250 km cable line between Sweden and Germany).

Transmission of electricity from the power plant to the consumer over long distances without intermediate branches, for example, to remote areas.

Increase the capacity of existing electrical networks in cases where the installation of additional circuits is difficult or expensive.

Transmission of electricity between unsynchronized AC distribution systems.

Reducing the cross-section of wires and the number of supports for a given transmission line capacity, since the transmission capacity of high-voltage DC transmissions is higher for a given conductor diameter.

Connecting remote power plants to the distribution network.

Increasing the stability of the system without increasing the short-circuit currents.

Reduced corona losses compared to high-voltage AC lines of the same power.

Reducing the cost of power lines, because high-voltage DC transmissions require fewer conductors (for example, a bipolar high-voltage DC transmission requires 2 conductors, and a high – voltage AC line requires 3).

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