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)

Frequency Synchronization

Can any two buses in an AC power system have different frequencies? Is it possible to send real power from low-frequency bus to high-frequency bus region in an AC power system?

Yes, and it happens all the time. However the frequency deviations are very small and within the range of normal operations. One bus may be at 60.01 Hz and the other at 59.96 Hz and the system is still stable. Larger frequency deviations would be unstable however as the synchronous generators lose synchronization and their protection systems take them offline.

However is the frequency difference is much larger, then the two AC sections cannot be directly connected. In this scenario, an asynchronous connection using either a DC tie or variable frequency transformer is needed to connect the two grid sections.

DC Vs AC Transmission Line

When and why is DC used instead of AC for long-distance electric power lines? Is DC becoming more common now? What are its advantages and disadvantages?

The largest losses in long distance electrical power transmission come from energy lost in the resistance of the power line.
If P is the power transmitted, and R is the resistance of the line:
P=IV$P=IV$
Ploss=I2R=P2R/V2$P_{loss}=I^{2}R=P^{2}R/V^2$
If P is fixed by community demand, then you can reduce lost power dramatically by increasing the transmission voltage. As a result, all long-distance power transmission, AC or DC, is done at high voltage.

The advantage of AC has always been that it is easy to change the voltage up and down with a transformer; DC requires more equipment and some losses to convert.

That being said, transferring AC power between separate grids requires making sure the phase of the power transmitted matches from the two grids (so that the power from the two grids doesn't cancel or ring), which is difficult and expensive. This is not a problem for DC, so DC lines are used in cases such as where power is transferred from another grid to increase the capacity of an existing grid, or between countries that use different frequency power.

Capacitance between the AC phases (usually 3 phases are transmitted at once over a line) or between the line and the surrounding soil or water causes losses that are not a problem with DC. Therefore, undersea high voltage lines tend to be DC.

Overall line loss is also lower per 1,000 km, so very long distance transmission lines sometimes use DC.

Have you wondered what are the cones on power lines?

Have you wondered what are the cones on power lines?

I think you're talking about these vines. I'll explain what is their use and why they are made in this specific manner in layman terms to make sure it's not a boring electrical engineering lecture to you. Let's get into it.

Short answer : These are Insulators

Long answer (with explanation) : Electrical power lines have some voltage associated to it, usually in the order of kilovolts. The potential for the ground is zero.

Why we need an Insulator?

1. To make sure that the power lines are serving their purpose which is supplying electricity. In absense of this Insulator, all the current will be directed towards ground.
2. And we need something to hang those wires to the tower and that thing cannot be a conductor.

Why this specific shape with discs?

An electric spark (even the lighting) happens due to the potential difference between two points which can ionize air.

So, we need to manage this potential difference, and that is done by fragmenting it using multiple discs which create kind of zones with somewhat same potential drop.

The material we use is ceramic.

BIG STORAGE BATTERIES

What limits us from having giant, city-sized battery banks to store sustainable energy like solar?

Batteries that big are BIG

This is the world's biggest battery, near Jamestown north of Adelaide, South Australia.

It's 139 megawatt hours storage and can deliver power at 100 megawatts. So at full delivery, it could supply power for about an hour and 20 minutes. For a size comparison, you can see a car parked to the right.

South Australia (pop about 1.4 million) requires about 3000 megawatts in the middle of summer, so even the world's biggest battery would need to be 30 times larger to supply the whole state (for just an hour and 20 minutes!). It performs a very important function though - because it can be switched on in seconds, it provides a very stable back up for the grid if there is a big power failure elsewhere, and it has brought down the cost of providing peaking power to the interstate grid by a BIG margin. Powering up a steam generating plant can take several hours from cold. And even peaking turbine or diesel generators can take some minutes to start and bring on line.

And it's not just the battery that's big - you have to charge it from something. You can see the associated wind farm in the distance, and that's BIG too. The battery can also buy charging power off the grid at off-peak times too.

The cost - $90 million Aussie Dollars - about $US 65 million. It is expected to pay for itself in 3–4 years.

It's been so successful that another smaller 30 megawatt hour battery has been built near Whyalla to the west, and a third battery is planned for the south east of the state.

This huge battery has been so successful at stabilizing South Australia's power grid and reducing the cost of peak power previously bought from interstate suppliers at "rip-off" prices, they are expanding it from 139 MwH to 170 MwH.