HVDC

High voltage direct current (HVDC):

High voltage direct current (HVDC) is used to convert energy from AC to DC, to transport the energy as DC, and to convert it back from DC to AC. To meet the ever growing demand for bulk power transmission over long distance, one solution is to build HVDC transmission lines.

This is a simple system with two converters C1 and C2, and one DC line.Based on the polarity of DC lines, HVDC systems are classified as monopolar, bipolar, and homopolar systems. a monopolar system, has only one DC line that normally has negative polarity and uses the ground as the current return path. The monopolar system is mainly implemented to reduce the cost of line construction. Bipolar systems uses two DC lines, one positive and the other negative. All lines have the same polarity in the homopolar connection.

The HVDC converter bridge is a three-phase converter bridge circuits as shown in Fig;

Each converter bridge has six branches of valves. The DC terminals of a converter connect to DC lines and the AC terminals to AC lines. The converter transformer is a conventional transformer with on-load tap changers. The turns-ratio of the converter transformer can then vary to manage the converter operation. The ‘‘DC side’’ of the converter transformer is usually delta or Y connected with ungrounded neutral, so that the DC line can have an independent voltage reference relative to the AC network. Harmonic voltages and currents arise during converter operation. The inductance reduces the harmonic voltages and currents on the DC lines, to prevent commutation failure of inverters, to maintain continuous current under light loading, and to curtail short-circuit current in converters during faults.

TRANSMISSION AND DISTRIBUTION

Structure of electric power system:

The modern power system is a complex interconnected network.A power system can be subdivided into four major parts:

1.Generation

2.Transmission and Subtransmission

3.Distribution

4.Loads

The basic components of a power system can be shown as figure below;

DIRECT CURRENT (DC) AND ALTERNATING CURRENT (AC)

DIRECT CURRENT (DC):

Direct current is the electricity flowing in a constant direction, and/or possessing a voltage with constant polarity. It can be expressed in terms of just two variables: polarity (or direction), and magnitude. It is the current that always flows in the same direction, and that does not change in intensity with time. DC is the kind of electricity made by a battery , or the kind of charge generated by rubbing certain types of materials against each other. Batteries and other sources of direct current (dc) produce a constant voltage. This can be represented by a straight, horizontal line on a graph of voltage versus time.

ALTERNATING CURRENT (AC):

In alternating current, the polarity reverses again and again at regular intervals. The magnitude usually changes because of this constant reversal of polarity, although there are certain cases where the magnitude doesn't change even though the polarity does. The rate of change of polarity makes ac so much different from dc. The length of time between one repetition of the pattern, or one cycle, and the next is called the period of the wave. frequency is the cycles per second and is denoted by f, which is the reciprocal of the period. That is, f = 1/T and T = 1/f .unit of frequency is hertz (Hz)

Alternating current (AC) waves are of different types; Such as sine wave, square wave,sawtooth waves,complex and irregular waveforms etc.

CONDUCTOR,INSULATOR AND SEMICONDUCTOR

CONDUCTORS

In some materials, electrons move easily from atom to atom. A conductor is a substance in which the electrons are mobile and that can conduct electricity.The term conductor is applied to any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals.The best conductor at room temperature is pure elemental silver. Copper, aluminum, iron, steel etc are examples for conductors. In most electrical circuits and systems, copper or aluminum wire is used. Silver is impractical because of its high cost.

INSULATORS:

Some substances prevent electrical currents from flowing through it, those substance are known as insulators. An insulator is a material that offers a very low level of conductivity under pressure from an applied voltage source.Most gases are good electrical insulators. Glass, dry wood, paper, and plastics are other examples. Pure water is a good electrical insulator.

SEMICONDUCTORS:

In a semiconductor, electrons flow, but not as well as they do in a conductor. That is semi conductors are partial conductor of electricity. A semiconductor, therefore, is a material that has a conductivity level somewhere between the extremes of an insulator and a conductor. Semiconductors carry electrons almost as well as good electrical conductors like copper or aluminum; others are almost as bad as insulating materials.

ELECTRICITY :BASICS

GENERAL PHYSICS PRINCIPLES BEHIND THE ELECTRICITY:

Scientists revealed 92 different kinds of fundamental substances in nature called elements. Each element is made up of tiny particles known as its atom. Atoms of different elements are always different.

The atom consists of proton , neutron and the electron. The main part of an atom is the nucleus that consists of proton and the neutron. The number of protons in an element’s nucleus gives, the atomic number, The atomic weight of an element is approximately equal to the sum of the number of protons and the number of neutrons in the nucleus. Electrons are the particles having opposite electric charge from the protons are Surrounding the nucleus of an atom .Electrons can move rather easily from one atom to another in some materials. In other substances, it is difficult to get electrons to move. But in any case, it is far easier to move electrons than it is to move protons. Electricity almost always results, in some way, from the motion of electrons in a material.

Different elements join together to share electrons which the results in a chemical compound.There are about thousands of different chemical compounds that occur in nature.When atoms of elements join together to form a compound, the resulting particles are molecules.All matter, whether it is solid, liquid, or gas, is made of molecules.In a solid, the molecules are interlocked in a sort of rigid pattern, although they vibrate continuously . In a liquid, molecules are loosely packed and these molecules slide around. In a gas, molecules are whizzing all over the place, bumping into each other.