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Electrical measurements

Electrical measurements are the methods, devices and calculations used to measure electrical quantities. Measurement of electrical quantities may be done to measure electrical parameters of a system.

Electrical measuring instruments can be divided into three classes: indicating instruments, recording instruments and integrating instruments.

Indicating instruments, such as ammeters, voltmeters and wattmeters, constitute the largest of the three classes. They are fitted with a pointer which moves over a fixed scale and they give an immediate indication of the value of the current, voltage or other quantity being measured. Such an instrument might be compared with a weighing machine or a barometer giving an immediate reading of the weight or pressure existing at any instant, but making no permanent record of such a measurement.

Recording instruments, or graphers, as they are sometimes called, instead of being fitted with a pointer and scale, carry a pencil or pen, which presses on to a travelling ribbon of paper, and thus makes a continuous chart or record of the values measured.

Such an instrument could be compared with the recording barometer often exhibited in an instrument maker's window. It will be noted that these two types do not differ in principle, since they are both used to measure the same kinds of things; but in the one case the indication is momentary and must be read by an observer on the spot, whilst in the other case the values are recorded on a chart for future observation and reference.

The third group, consisting of integrating instruments or electricity supply meters, differs fundamentally from the other two groups, since instead of indicating or recording, these instruments add up the total amount consumed over any given period. Thus, instead of reading the current or the power flowing at any instant, they measure the product of current and time (in ampere-hours) or of power and time (in watt-hours), and so add up the electrical quantity or energy consumed.

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Conductors and insulators

Conductors are materials having a low resistance so that current easily passes through them. The lower the resistance of the material, the more current can pass through it.

The most common conductors are metals. Silver and copper are the best of them. The advantage of copper is that it is much cheaper than silver. Thus copper is widely used to produce wire conductors. One of the common functions of wire conductors is to connect a voltage source to a load resistance. Since copper wire conductors have a very low resistance, a minimum voltage drop is produced in them.

It should be taken into consideration that most materials change the value of resistance when their temperature changes. Metals increase their resistance when the temperature increases while carbon decreases its resistance when the temperature increases. Thus metals have a positive temperature coefficient of resistance while carbon has a negative temperature coefficient. The smaller is the temperature coefficient or the less the change of resistance with the change of temperature, the more perfect is the resistance material.

Materials having a very high resistance are called insulators. Current passes through insulators with great difficulty. The most common insulators are air, paper, rubber, plastics.

Any insulator can conduct current when a high enough voltage is applied to it. Currents of great value must be applied to insulators in order to make them conduct. The higher the resistance of an insulator, the greater the applied voltage must be.

When an insulator is connected to a voltage source, it stores electric charge and a potential is produced on the insulator. Thus, insulators have the two main functions:

- to isolate conducting wires and thus to prevent a short between them;

- to store electric charge when a voltage source is applied.

 

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Substations

A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.

Common substations comprise isolators, switchgear buses, circuit breakers, fuses, power and instrument transformers and reactors. Substations may be described by their voltage class, their applications within the power system, the method used to insulate most connections, and by the style and materials of the structures used.

A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control/power factor correction devices such as capacitors, reactors or static compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.

A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a level suitable for local distribution.

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