Potential or Voltage

 Potential or Voltage

Because like charges repel and opposite charges attract, charge has a natural tendency to “spread out.” A local accumulation or deficit of electrons causes a certain “discomfort” or “tension” unless physically restricted, these charges will tend to move in such a way as to relieve the local imbalance. In rigorous physical terms, the discomfort level is expressed as a level of energy. This energy (strictly, electrical potential energy), said to be “held” or “possessed” by a charge, is analogous to the mechanical potential energy possessed by a massive object when it is elevated above the ground: we might say that, by virtue of its height, the object has an inherent potential to fall down. A state of lower energy—closer to the ground, or farther away from like charges—represents a more “comfortable” state, with a smaller potential fall.

The potential energy held by an object or charge in a particular location can be specified in two ways that are physically equivalent: first, it is the work that would be required in order to move the object or charge to that location. For example, it takes work to lift an object; it also takes work to bring an electron near an accumulation of more electrons. Alternatively, the potential energy is the work the object or charge would do in order to move from that location, through interacting with the objects in its way. For example, a weight suspended by a rubber band will stretch the rubber band in order to move downward with the pull of gravity (from higher to lower gravitational potential). A charge moving toward a more comfortable location might do work by producing heat in the wire through which it flows.

This notion of work is crucial because, as we will see later, it represents the physical basis of transferring and utilizing electrical energy. In order to make this “work” a useful and unambiguous measure, some proper definitions are necessary.

The first is to explicitly distinguish the contributions of charge and potential to the total amount of work or energy transferred. Clearly, the amount of work in either direction (higher or lower potential) depends on the amount of mass or charge involved. For example, a heavy weight would stretch a rubber band farther, or even break it. Similarly, a greater charge will do more work in order to move to a lower potential. On the other hand, we also wish to characterize the location proper, independent of the object or charge there. Thus, we establish the rigorous definition of the electric potential, which is synonymous with voltage (but more formal). The electric potential is the potential energy possessed by a charge at the location in question, relative to a reference location, divided by the amount of its charge. Casually speaking, we might say that the potential represents a measure of how comfortable or uncomfortable it would be for any charge to reside at that location. A potential or voltage can be positive or negative. A positive voltage implies that a positive charge would be repelled, whereas a negative charge would be attracted to the location; a negative voltage implies the opposite. Furthermore, we must be careful to specify the “reference” location: namely, the place where the object or charge was moved from or to. In the mechanical context, we specify the height above ground level. In electricity, we refer to an electrically neutral place, real or abstract, with zero or ground potential. Theoretically, one might imagine a place where no other charges are present to exert any forces; in practice, ground potential is any place where positive and negative charges are balanced and their influences cancel. When describing the potential at a single location, it is implicitly the potential difference between this and the neutral location.

However, potential can also be specified as a difference between two locations of which neither is neutral, like a difference in height. Because electric potential or voltage equals energy per charge, the units of voltage are equivalent to units of energy divided by units of charge. These units are volts (V). One volt is equivalent to one joule per coulomb, where the joule is a standard unit of work or energy. Note how the notion of a difference always remains implicit in the measurement of volts. A statement like “this wire is at a voltage of 100 volts” means “this wire is at a voltage of 100 volts relative to ground,” or “the voltage difference between the wire and the ground is 100 volts.” By contrast, if we say “the battery has a voltage of 1.5 volts,” we mean that “the voltage difference between the two terminals of the battery is 1.5 volts.” Note that the latter statement does not tell us the potential of either terminal in relation to ground, which depends on the type of battery and whether it is connected to other batteries. In equations, voltage is conventionally denoted by E, e, V, or v (in a rare and inelegant instance of using the same letter for both the symbol of the quantity and its unit of measurement).

CHARGE

 It was a major scientific accomplishment to integrate an understanding of electricity with fundamental concepts about the microscopic nature of matter. Observations of static electricity like those mentioned earlier were elegantly explained by Benjamin Franklin in the late 1700s as follows: There exist in nature two types of a property called charge, arbitrarily labeled “positive” and “negative.” Opposite charges attract each other, while like charges repel. When certain materials rub together, one type of charge can be transferred by friction and “charge up” objects that subsequently repel objects of the same kind (hair), or attract objects of a different kind (polyester and cotton, for instance).

Through a host of ingenious experiments, scientists arrived at a model of the atom as being composed of smaller individual particles with opposite charges, held together by their electrical attraction. Specifically, the nucleus of an atom, which constitutes the vast majority of its mass, contains protons with a positive charge, and is enshrouded by electrons with a negative charge. The nucleus also contains

neutrons, which resemble protons, except they have no charge. The electric attraction between protons and electrons just balances the electrons’ natural tendency to escape, which results from both their rapid movement, or kinetic energy, and their mutual electric repulsion. (The repulsion among protons in the nucleus is overcome by another type of force called the strong nuclear interaction, which only acts over very short distances.)

This model explains both why most materials exhibit no obvious electrical properties, and how they can become “charged” under certain circumstances: The opposite charges carried by electrons and protons are equivalent in magnitude, and when electrons and protons are present in equal numbers (as they are in a normal atom), these charges “cancel” each other in terms of their effect on their environment. Thus, from the outside, the entire atom appears as if it had no charge whatsoever; it is electrically neutral.

Yet individual electrons can sometimes escape from their atoms and travel elsewhere. Friction, for instance, can cause electrons to be transferred from one material into another. As a result, the material with excess electrons becomes negatively charged, and the material with a deficit of electrons becomes positively charged (since the positive charge of its protons is no longer compensated). The ability of electrons to travel also explains the phenomenon of electric current, as we will see shortly.

Some atoms or groups of atoms (molecules) naturally occur with a net charge because they contain an imbalanced number of protons and electrons; they are called ions. The propensity of an atom or molecule to become an ion—namely, to release electrons or accept additional ones—results from peculiarities in the geometric pattern by which electrons occupy the space around the nuclei. Even electrically neutral molecules can have a local appearance of charge that results from imbalances in the spatial distribution of electrons—that is, electrons favoring one side over the other side of the molecule. These electrical phenomena within molecules determine most of the physical and chemical properties of all the substances we know.

While on the microscopic level, one deals with fundamental units of charge (that of a single electron or proton), the practical unit of charge in the context of electric power is the coulomb (C). One coulomb corresponds to the charge of 6.25 x 10 to power protons. Stated the other way around, one proton has a charge

of 1.6 x 10 to power -19 C. One electron has a negative charge of the same magnitude, 21.6 x 10 to power -19 C. In equations, charge is conventionally denoted by the symbol Q or q.

Power lines

Power lines are made of two materials, copper and an aluminum wire with a steel core. Transmission lines (also all new construction) are usually made of the aluminum variety. This is because while copper is a better conductor, and is stronger, copper is also very expensive and Heavy. In contrast, aluminum is also quite conductive, very light, and not to mention cheap in comparison. One downside to aluminum is it’s quite a lot weaker than copper, which is where the steel core comes in. The combination of aluminum (good conductor, cheap, weak) and steel (only okay conductor, cheap, and strong as heck) make both a cost effective and strong material to make new lines with.

Also worth noting, a big problem in the power line industry is copper theft. Not only is it dangerous to the thief, but those wires are essential to the safety of the utility workers and reliability of the grid. **

** I never understood why people steal copper, they don’t realize how dangerous it really is. In my area alone there has been 3 deaths of thieves in the past 10 years. With the price of copper and the time it takes, you'd make more sweeping the parking lot for some generous small business, a lot safer too.

5 volt DC to 220V AC

 

If we want a 5 volt DC from a 220V AC, what should we do?

Without building anything: buy an AC adapter that uses a universal input (look at the letters stamped into the plastic under “INPUT”).

IF you want to build it yourself, a simple linear AC->DC power supply:

1. AC Transformer with winding ratio to take the voltage down from 220V peak to slightly about 6V peak (e.g., say a 36:1 winding ratio)
2. A “rectifier” or arrangement of four diodes that will turn the positive/negative swinging sine wave to an all-positive wave of ‘camel humps”. You lose about 0.7V here, so that’s why we ended step 1 with a 6V peak voltage, not 5V.
3. A resistor-capacitor circuit (typically using a large eletrolytic or “can” capacitor.
4. A voltage regulator to eliminate any residual voltage > 5V. Some voltage loss here, too.

The resulting circuit and waveforms of each stage look like this:

CT- Current transformer PT- Potential transformer

What are CT- Current transformer PT- Potential transformer ?

CT or Current Transformer & PT or Potential Transformer are measuring devices in AC system. These are also called as instrument transformers. As AC system deals with very high power hence we require Ammeter & voltmeter of humongous sizes to measure such high power which is impractical & expensive too.

A CT has following properties:-

1. It is a step up transformer.
2. It enhances the voltage & thus the current gets reduced for a fixed power, since P= V×I×cosΦ. Thus for a fixed amount of power, current can easily be measured.
3. Primary winding of a CT is always connected in series with the load & secondary winding is connected with the ammeter.

A PT has following properties:-

1. It is step down transformer.
2. It reduces the voltage for a fixed power & thus the voltage can easily be measured, since P= V×I×cosΦ
3. The primary winding of the PT is always connected in parallel to the load & secondary winding is connected to the voltmeter.

CT- Current transformer

PT- Potential transformer

CT and PT form the sub parts of instrument transformer .They are extensively used in power system for metering and protection purpose .These 2 form the most important part of any substation .Since in actual electrical power system we deal with high voltage and high current and it is not feasible and economical to manufacture devices to measure such high values .So to measure high values in power system use CT and PT.The primary of of these transformer is placed in main line and secondary is placed in any meter or relay depending on the application.The turns of primary and secondary are adjusted in such a manner that the current in secondary is small and easy to measure for CT and voltage in case of PT .Also they provide isolation between primary and secondary which is often required in electrical.

Shunt Reactors

 What is the use of a shunt reactor in a power system?

Shunt reactor is an equipment used for voltage control of the line in the power system. During the operation of a power line the load fluctuates from full load, light load or no-load causing voltage variations that need to be controlled either in steady-state or transient In these conditions is normal that the line voltage varies due to the capacitive current of the line. During very light load or no-load the line voltage increases. The phenomena is called Ferranti effect. The shunt reactor is an inductance connected from line to ground rated to absorb the capacitive current of the line thus reducing the voltage to avoid damages due to over-voltage to the customers.

Another type of reactor is the series reactor. This type of reactor is used to limit the short-circuit current of the system increasing the equivalent impedance of the line and thus reducing the short-circuit current.

AIS and GIS Switchgear

What's the different between AIS and GIS switchgear?

Let’s start with some definitions. AIS stands for air insulated switchgear and GIS stands for gas insulated switchgear. So far we can see that both are some sort of insulated switchgear like this.

So the remaining difference is air vs gas. These are the actual insulators in the switchgear. In air insulated switchgear the arc between the contacts is extinguished by the air. In the gas switchgear it is extinguished by the gas.

The differences are that GIS is more compact as the gas, usually sulfur hexafloruide - SF6 has a higher dielectric breakdown voltage than air. So less gas is needed to extinguish the arc. However the gas insulation raises costs. Therefore it tends to be used in areas where space is a premium such as in cities.

Air Insulated Switchgear

Gas Insulated Switchgear

However it should be noted that SF6 is a potent greenhouse gas. It is 23,500 times more potent than carbon dioxide, and can persist in the atmosphere for 1000 years. As the deployment of renewables increases so does the usage of SF6 gas. There are attempts being made to use alternatives to the gas whether its a different kind of gas or a combination of clean air and vacuum technology.