Why Are Such High Voltages Used for Electricity Transmission?

 And why do we use AC?

Public domain image by FelixMittermeier on Pixabay.com

 

To understand the absolute basics of electricity, you might like to read my guide to electricity basics on the Medium.com website first which is more detailed and has diagrams. This post on Facebook is a summary of that article.

Electricity 101: A Complete Beginner's Guide at this link:

https://medium.com/.../how-to-understand-electricity...

 

Volts and amps 

 

If we call the voltage in a circuit V (measured in volts) and the current I (measured in amps), the power P in watts used by a connected electrical load is simply calculated by multiplying the voltage by the current. I.e:

P = VI

Amps are denoted by the symbol A, volts by V and power in watts by W.

Question. A toaster takes 4 amps (4 A) at 230 V. How much power does it use?

Answer. Power P = VI = 230 x 4 = 920 watts (920 W)

 

Electrical resistance 

 

Electric cables and loads also have a property called resistance, analogous to the resistance in a water hose. When a hose is reduced in diameter or made longer (or someone steps on it), resistance to flow increases. Resistance to electricity flown in electrical cables, or any conductor for that matter (something that carries electricity) results in power dissipation as heat when electricity flows through the cables. (That's why we use cables with greater cross sectional area or CSA to carry heavier current, because it reduces resistance).
If we denote the resistance of a cable or electrical load by R and the current again by I, then the power dissipation in the cable is given by:

P = I²R

So we square the current (multiply it by itself) and multiply by the resistance to calculate the power dissipation. Resistance is measured in ohms.

Question. An incandescent lightbulb has a resistance of 100 ohms. A current of 200 milliamp (200 mA or 0.2 A) passes through it. How much power does it use?

Answer. P = I²R = 0.2 x 0.2 x 100 = 4 W

So current flowing through a resistance produces heat. This isn't of any advantage unless it's a desired effect. Devices such as electric heaters, toasters, elements in kettles, immersion elements and electric blankets make use of the property of resistance to create heat which is useful. Incandescent light bulbs also use an electric current to make a filament white hot. However in this case it's the light produced that's of benefit and the heat is just a wasteful by-product (95% of the energy used is converted to heat).

 

Resistance of electrical cables

 

We saw above that the power dissipated in a resistance is P = I²R. It doesn't matter whether the resistance is that of an electrical device, the resistance of a cable or an electrical component called a resistor (You might have seen these if you've opened a device with electronics inside. They look like cylinders with coloured stripes).

When Eirgrid or ESB Networks transmit power, they want to reduce power losses in cables, i.e. reduce P = I²R). This power loss is known as copper loss (although transmission cables are primarily aluminium to reduce weight). To reduce P, we need to make I and/or R smaller. R can be reduced by sizing cables accordingly. Making them thicker reduces R, but obviously there are practical limits because of cost and weight. A more sensible option is to reduce I, the current. Because of the squared term in the equation, the effect of reducing current is exponential. So for instance if current is made 10 times smaller, for the same resistance R, power loss is reduced to one hundredth of what it was previously. So this is exactly the technique used by transmission networks: Step up voltage by a factor of 100 or more and reduce current proportionately. 

 

So what are transformers for ?

 

Devices called transformers, which are the electrical analog of a mechanical gearbox, convert low voltage high current to higher voltage lower current. (They also do the reverse). This is easy to do when alternating current or AC is used. The lower current then reduces the power loss in cables over distance. It also reduces voltage drop because drop is proportional to current. the third advantage is lower current allows lighter gauge cables to be used, reducing cost and weight on supporting pylons or poles. Very high voltages are used for transmission countrywide and voltage is then reduced or stepped-down to a more practical level at substations. It's then further reduced by additional transformers (often pole mounted) before electricity is delivered to consumers' premises. The transmission line from Moneypoint Power Station on the Shannon Estuary that runs to the substation at Dunnstown near Carnalway operates at 400,000 volts or 400 kV.

 

See this guide for an explanation of the difference between AC and direct current DC

 

https://medium.com/.../whats-the-difference-between-ac...)

 

Also this Wikipedia article about the late 19th century War of the Currents which was a series of events surrounding the introduction of Edison's DC and George Westinghouse's competing AC electric transmission systems. Guess which won out?

https://en.wikipedia.org/wiki/War_of_the_currents

Waiting for the Whistle

© Eugene Brennan

No. 186, which was built in 1879 and worked as a goods engine, but also on passenger trains. Her last operation was hauling beet in 1962/63. Up until a few years ago, she was the oldest engine still operating on the mainline, but is now out of service, probably permanently, and on display at Whitehead Museum, This was from an excursion to Maynooth in 2012. How much of the original locomotive remains I don't know, and it's probably a bit like the proverbial Ship of Theseus or Trigger's Broom (Which had "17 new heads and 14 new handles"). The firebox and boiler tubes have been replaced on several occasions I think at the RPSI's Whitehead works near Carrickfergus. The driver for this trip was Ken Fox. Not sure who the other man is.

© Eugene Brennan

Drought and the Panama Canal

Image courtesy Andrea Spallanzani via Pixabay

The Grand Canal that connects the River Shannon to Dublin is like a series of steps of stairs, descending level by level as it makes its way to Dublin. Locks on the canal allow barges to move from one level to another, the locks filling with water to raise a barge or emptying to lower it as it travels in the Dublin direction. Streams along the route act as feeders for the canal, maintaining water level as it's lost and moves from section to section when locks fill and empty (Pollardstown Fen is one of the feeder sources for the Naas branch). 

 

The Panama Canal 

 

The Panama Canal was a huge feat of engineering, construction starting in 1904 and finally completed in 1914. It allowed ships to travel from the Pacific to the Atlantic, avoiding the long journey around South America and having to negotiate the treacherous waters of Cape Horn at the southernmost tip of Chile. It's similar in principle to the Grand Canal, but incredibly more complex. The canal originally had a series of six locks, three at each end. The locks are arranged in pairs, allowing ships to travel in both directions at the same time. The Panama Expansion Project has increased the number of locks to twelve. Ships climb the canal through three locks at one end to reach Gatun Lake and descend at the other end through another three locks to exit the canal. Gatun Lake and the Chagres River are used for navigation over most of the length of the canal, but the Pacific and Atlantic oceans are also at different levels because of a variety of factors including tides, atmospheric pressure and surges caused by storms. The locks compensate for this difference in levels also, just like the way the locks on the Grand Canal are necessary to cater for the River Shannon being at a higher altitude above sea level than the exit of the canal near Dublin Port.

This episode of The Global Story on the BBC World Service from March of this year examines how drought is threatening the canal and how it could affect shipping.