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.

Series and Shunt Capacitors

Why are shunt capacitors preferred over series capacitors for improvement of power factor in distribution systems?

It is because series capacitors are not meant to be used for that purpose since it could increase the fault current level in the system, while shunt capacitors do.

Series capacitors are used to control the power flow within the grid by changing the transmission line’s reactance as well as improve the angular stability of the system after the fault clearance.

Power flow within grid: (a) without series capacitors, (b) with series capacitors

On the other hand, the shunt capacitors are used to provide reactive power needed by the load. By providing the reactive power for the load, the reactive power which is supplied by the grid will thus decrease, hence the power factor is improved.

Reactive Power Loss

Do we have reactive power loss like active power loss?

Yes.

The role and purpose of reactive energy are to maintain the em fields within the electrical network. Although reactive energy only flows within such networks it requires reactive current which together with the active component of current dissipates heat in the conductors and as such it too experiences or creates an active power loss within the transmission and distribution system. Note that the power losses are due to the MAGNITUDE of the current and not just the active component.

So S=|I|2Z=P+j∗Q=|I|2(R+jX)