Sim's Electrical Companion
Transformer Construction, Types & Calculations
Single Phase Transformers
A transformer is a simple device that changes a primary voltage / current into a secondary voltage / current. The method of achieving this is so simple, a primary source conductor current produces a magnetic fields and is wound around an iron core to increase its subsequent magnetic field. The lines of flux cut across a secondary winding which induces an emf in the secondary circuit. The secondary voltage will be dependant upon the ratio of windings primary and secondary. If there are an equal amount of windings on both sides the transformers output and input will be the same (excluding loses). If the secondary windings are lower the transformer is considered to be a step down transformer. If the secondary windings are greater than the primary windings it is a step up transformer.
UK Power Generation
In the UK we consume between 30 and 45 GW of electrical energy. We generate as close to our demand as possible to reduce waste. The power is distributed on to a grid system, where most points of generation can feed into the supply. The UK national grid was the first of its kind. Previous to the grid system each major town or suburb had its own power station and distribution methods. The grid was designed to create uniformity in supply so electrical equipment designed for the UK would work without alteration anywhere in the country.
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Internationally the two most common frequencies & voltages for Low Voltage supply are:
60 Hertz (Hz) 110 Volts (V) Americas & parts of Asia
50 Hertz (Hz) 230 Volts (V) Europe, Africa & parts of Asia
Generation of electrical supplies in the UK is achieved by many methods the chart below shows a snapshot.
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Methods of electricity generation in the UK (June 2017 midday)
NOTE: Short of updating this Pie chart - Differences in 2023 include increased wind and solar production and a reduction in coal. To the point that coal is often at Zero. Wind can supply over 40% on the right day. Our main use when it is not sunny or windy is Gas which is still a major use of fossil fuel. The UK is planning to create more nuclear and Biomass production to reduce the use of fossil fuels.
Below is a snapshot of Grid Watch a free website showing how power is being generated in real time - Click the dials to visit the site
Generators
Turbines
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Most methods of generation rely on the turning of a turbine which is connected to generator which converts mechanical energy to electricity by the use of a rotating magnetic field (See Fleming's generator rule for more detail - Science - tab). Other methods of generation include chemical reactions, semiconductors, pressure & heat transfer.
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Coal, biomass, gas and nuclear stations use fuel to heat water causing the expansion of steam through a turbine which drives a generator. No matter how complex the method, essentially we are still in the steam age!
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Photovoltaic, biomass, wind & hydro generation along with methods of conserving energy are covered in the environmental Technology tab.
Combined Cycle Gas Turbine (CCGT)
This accounts for up to 45% of UK electricity generation. Combined cycle gas turbines are a two part generation system. Fueled with natural gas it is not a renewable method but is the most efficient of the fossil fuel powered plants. One half of the system uses the expanding burning gas to turn a gas turbine connected to an electrical generator. The other half uses a heat recovery system to compress heated water and pass it through a steam turbine also connected to a generator. A far more efficient system compared to the older gas turbine stations where the heat was a waste by product. This method is being adopted in other methods of energy production like Combined heat and power & Biomass generators.
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Natural gas supplies for the UK come from North & Irish Sea production, Interconnection pipelines, & Liquefied Natural Gas (LNG) from international sources. Current interconnections are from Belgium, Netherlands and two from Norway which are in greater use due to decline of North & Irish sea gas production. This makes it a less effective power production strategy for the UK in the long term. Unless more reserves are tapped in our territories we will become more dependant on other nations for our current primary source of fuel for electricity production.
Filtered air (1 & 2) and gas (3) is fed to the Gas turbine (4) the rotary kinetic motion produced is fed to a generator (9). The by product heat is collected by a heat recovery system (5) which converts water (6) to steam to drive a steam turbine (8). The used steam is condensed (7) and cooled at the cooling towers (6) to repeat the cycle. The rotary motion from both turbines is fed to two separate generators (9) which are linked to transformers (10) which increase the voltage for more efficient transmission onto the grid (11).
Coal Power Generation
Coal is one of the most environmentally damaging methods of generating electricity. The UK has plans to phase out the use of coal by 2025. To achieve this and still meet national demand some coal stations are being converted to use biomass.
In 2020 we had our first day of generation without burning coal, since the start of the industrial revolution. I noticed that we did import some energy so we were not entirely self sufficient sans coal! This was achieved during summer when peak demand is lower than winter where heating & lighting account for an extra 10 - 15 Gw.
The primary problem with coal powered stations is the direct burning of fossil fuels creates carbon dioxide that is released to atmosphere and is considered to be the primary gas responsible for global warming. The effects are being experienced world wide and will have disastrous effects within this generation. It has also been proven that returning warmed water to rivers can adversely affect the river ecology so to prevent this more expense and time is required for post processing. Finally the solid waste and fly ash from coal is mostly sent to dry landfill and contains a variety of carcinogens and heavy metals depending upon where it was mined. Some headway has been made in creating building materials out of the waste that can in some cases outperform the properties of Ordinary Portland Cement (OPC).
Carbon capture & sequestration or CCS and is being researched and in some cases effectively employed where the carbon is stored so it cannot be released to atmosphere. Coal is still a heavy polluter even if all the carbon is captured from burning. Mining operations, storage, waste removal and transport have a large environmental impact & it is a finite resource. It makes more sense to invest in research and development of renewable energy sources that do not require so many ancillary services.
A large volume of water (1) is required to operate a coal fired station so they are sited near large rivers or the sea. The filtered water (2) & Coal (3) is fed into a furnace (4) where water is boiled by a heat exchanger (5). The resultant steam is directed to a turbine (6) which is connected to a 25kV electromagnetic generator (7). The resultant energy is then transformed / stepped up (8) for more efficient grid transmission (9). By products or waste include flue gases (10) particulates & ash (12) which are removed from the furnace and mostly end up in land fill. The low pressure steam is condensed back into liquid (13) cooled and reused or returned to source (1).
Nuclear Power Generation
Nuclear fuelled stations work much the same way as a gas or coal powered station. The major difference is the fuel and method of producing and controlling the heat. Typically the fuel is Uranium though sometimes Plutonium is used. This method of energy production is extremely efficient by weight of material needed but the environmental impact of failure or damage to containment of waste materials can be catastrophic. Examples of where it has gone wrong are Fukushima Daiichi (Japan) nuclear disaster in 2011 and Chernobyl (Soviet Union) in 1986. Both were caused by very different reasons but the environmental effects of both were far reaching and will remain so for decades.
Spent nuclear fuel is currently stored or reprocessed into usable fuel and some High level waste. A permanent safe method of storage has not yet been created so spent fuel is being stored in holding centres until a more permanent answer can be proven to be effective for the required thousands of years!
Nuclear fuelled stations work much the same way as a gas or coal powered station. The major difference is the fuel and method of producing and controlling the heat. Typically the fuel is Uranium though sometimes Plutonium is used. This method of energy production is extremely efficient by weight of material needed but the environmental impact of failure or damage to containment of waste materials can be catastrophic. Examples of where it has gone wrong are Fukushima Daiichi (Japan) nuclear disaster in 2011 and Chernobyl (Soviet Union) in 1986. Both were caused by very different reasons but the environmental effects of both were far reaching and will remain so for decades. Spent nuclear fuel is currently stored or reprocessed into usable fuel and some High level waste. A permanent safe method of storage has not yet been created so spent fuel is being stored in holding centres until a more permanent answer can be proven to be effective for the required thousands of years!
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Uranium is an extremely heavy element and can produce through nuclear fission (splitting) 3.7 million times more energy than coal by weight. As uranium is a very large element it is easily hit at speed with a neutron this splits the uranium nucleus releasing a great amount of energy and another neutron which can continue the fission process. This chain reaction is why control rods need to be used to maintain a desired reaction rate and the use of a moderator often water or graphite to slow down the neutrons. In the process of fission a large amount of energy is released and this is used to heat water & create steam. Once again were still in the steam age albeit a rather complex form! Nuclear energy production counts for approximately 10% of global electricity production. In the UK it makes up 20-25% of our energy production so is an important contributor. In France they have a capacity of approximately 70% of their energy requirements nearly 60 Giga watts provided by nuclear stations.
Nuclear power stations require a very large amount of water to operate so they are all situated near the coast or large bodies of water (1) The filtered water (2) is pumped into a pressure cylinder. (3) where water is boiled by a heat exchanger (4) which is a separate closed loop that is heated in the heavily shielded reaction chamber (5). Fuel rods produce heat (6) which is monitored by control rods (7) & a moderator medium normally water or graphite. The resultant steam (8) is directed to a turbine or series of high and low pressure turbines (9) which is connected to an electromagnetic generator (10) Usually producing 25kV. The resultant voltage is then transformed / stepped up (11) for more efficient grid transmission (12). By products or waste include low pressure steam which is condensed (13) and cooled by cooling towers (14) and reused or returned to it's source (1).
UK - Non Renewable Capacity
The images above gives a good representation of the UK's nuclear, coal and gas capacity in 2023. The centre of the circles represent the power plants location & the size of the circle is relative to its power output. Since the production of the above diagrams some of the coal production has been converted to biomass or closed down. Increase in wind, hydro, nuclear & solar capacity is in production or planned.
Currently (2023) the capacity figures are approximately:
Nuclear: 6.5 Giga Watt 12 % of maximum demand
Coal: 2.5 Giga watt 5 % of maximum demand
Gas: 28 Giga Watt 50 % of maximum demand
This leaves 33% if we were at capacity to be taken up by interconnections or alternative energy.
Thankfully we rarely use coal to capacity as other provisions are in place. Coal is used more extensively in winter but is due to be phased out by 2025 where it is intended to be replaced with a combination of nuclear, gas, biomass, wind, hydro and photovoltaic.
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The most consistent method of generation is nuclear that provides approximately 6 - 7 GW all year round. Nuclear power plants are hotly contested in the UK and development in the area is complex. Currently 8 nuclear sites are in construction or planned for 2030.
Over the next decade the proposed or in construction phase sites are: Bradwell, Essex, Hartlepool,, Heysham, Lancashire, Hinkley point, Somerset, Oldbury, South Gloucestershire, Sellafield, Cumbria, Sizewell, Suffolk and Wylfa, Anglesey.
UK - Renewable Capacity
The UK's Hydro, Wind & Solar Capacity
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The above images show the UK capacity for hydro, Solar and Wind power. The distribution is not surprising as all three require specific locations to get the best out of the technology.
Hydro plants can only be located where the geology and location fits the scheme etc. The large Dinorwig hydro power scheme in Snowdonia is mentioned in greater detail in the Environmental technology section of the book (Part 9). Note the large Blue circle in the North West of Wales.
Wind production is gaining in popularity and some very large offshore schemes have been produced recently e.g. The 400 MW Rampion offshore project East Sussex.
Solar farms are less common the further north you go for obvious reasons. Many are abreast of motorways as the land is not ideal for food production due to traffic fumes. Mountainous areas are also quite limiting in the setting out of solar farms so Wales is not as densely populated as central England is.
The diagrams on these two pages are a snapshot of the generation capacity of the UK in 2015. The images were created by The Carbon Brief . The carbon brief is a is a UK-based website covering the latest developments in climate science, climate policy and energy policy.
Transmission - National Grid
Brief History & Overview​
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In 1926 Conservative PM Stanley Baldwin introduced the Electricity Supply Act. Legislation was put in place to nationalise the generation and transmission of electricity in the UK. Previously towns and cities had individual power stations that were operating at different voltages and frequencies. The consequence of this is if a town was not producing enough power it would suffer outages and equipment designed for use in one part of the country may not work elsewhere without modification. Before the Act was put in place less than 10% of the country had access to electricity and the cost was so great many simply could not afford it.The agreed national frequency was set at 50Hz and is highly regulated to improve performance and reduce losses or damages.
The common voltages for Generation, transmission & distribution are:
Generation:
25 kV - Mostly non renewable but not all
Transmission:
132 kV Original grid system & some remaining
275 kV Enhanced grid (lower losses) after 1950
400 kV Super grid (lower transmission losses) >1965
Distribution:
66 kV Heavy Industry
33 kV Industrial
25 kV Rail networks
11 kV Commercial / light industrial
400V Light commercial / Large domestic E.g. flats or HMO's
230V Domestic / light commercial
Transmission - Substations
Substations are designed for a variety of purposes including control, protection, diversion, and most commonly for changing the voltage type or value as shown on the opposite page. Depending upon their purpose substations have a variety of classifications.
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1 Transformer - Used to step up or step down voltage as required. In general stepping up is to reduce transmission losses over distance.
2 Switching Substations - Can be used to control how a grid is fed and can isolate sections as required for maintenance or repairs.
3 Frequency Change Stations - Can be used when feeding to another country or to an industrial purposes with specific supply needs.
4 Power Factor Substation - To adjust power factor to reduce loses in the system.
5 Industrial Substations - Serve an individual company or industrial estate. Often these are end of line and do not provide a ring main.
6 Converting Substation - Can change AC to HVDC and back for long distance transmission e.g. on interconnections. May also be used for specific industrial purposes like large scale electroplating e.g. motor vehicle industry.
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Substations will have some if not all of the following components, transformers, instruments, cut outs, circuit protection, switches, insulators and a method of Earthing. Larger subs may have additional provision for welfare and emergency services.
Substations (Subs) can be found indoors and outdoor but most commonly indoor substations will be supplying up to 11kV and are near or within the confines of the building or complex it is supplying. Once higher voltages are encountered outdoor stations are the most economic due to their size. They may also be pole mounted but these will often be close to what they are supplying and not likely to be above 33kV.
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Pole mounted subs are used commonly in rural areas supplying farms for example. Underground subs may be encountered where land is at a premium or for underground purposes like rail or a super-villains lair!
Transmission - Overland
Most of the transmission of the national grid is done overland as it is far easier to install and maintain compared to subterranean systems. Sub subterranean systems of distribution are costly to install and maintain and are mostly reserved for areas of outstanding natural beauty, locations near airfields or urban areas. Though there are many types of pylon used for this purpose most of them have common elements including, earthing, conductors, insulators, spreaders (for multiple cables) & arc protection. There are three major types of steel pylons that are easily recognised by their features.
Terminal Pylons
These are used where the grid meets a substation or joins to a subterranean cable e.g. where transmission meets a town or power station. These are easily recognised by the twin insulators carrying cables from suspension to the ground. They are essentially deviation pylons at the end of a run.
Deviation Pylons
Used where an overland route changes direction. These can easily be noticed by the change in direction or the fact that there are two insulators linked by a fly lead to avoid the other conducting parts of the tower.
Suspension Towers
When the overland route is a straight line these pylons are used. There are fewer parts to install so they are cheaper to install with regards to time, maintenance & material compared to a route that deviates often. Notable by their vertical insulators that hold the conductors away from all other conductive parts of the tower.
Junction Tower
Use to create two or more paths for the conductors.
Pole Mounted
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Are wooden, composite, concrete or steel structures that are used for carrying currents up to 33kV but more commonly ≤11kV. Pole transformers have similar features to steel pylons and in some cases support transformers and protective devices for voltage reduction at the end point of transmission as can be seen in the picture below.
Transmission - Pylons
Typical pylons in the UK have common features that are applicable to most if not all of the pylons. Some features of their construction are indicated in the picture and described below.
In the UK the most common types of pylon are a steel galvanised lattice structure. The tower types are chosen to fit their environment, transmission requirements and in some cases for their aesthetic. As smart grid applications are increasing some design additions are required for communications and control. Common transmission voltages are 132kV, 275kV and 400kV. This can be determined by their size & distance between the phase conductors.
1 - Earthing Conductor
Located at the top most point of the tower to help protect against atmospheric effects like lightning. The earthing conductor also ensures all exposed conductive parts are at earth potential in the case of fault conditions.
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2 - Insulators
Often made from ceramic or glass / polymer materials in multiple sections to allow some flexibility and provide a capacitive effect. The size and quantity of the insulators give an indication of the voltage being carried as well as the distance from all other conductive parts. They have a shiny glaze finish and bell shape for self cleaning and shedding water in poor weather.
3 - Spreaders
These are used where a phase is carried by more than one conductor. These help strengthen the cables and keep them from clashing in high winds which can damage the individual conductors. The amount of cables used per phase can give an indication of the operating voltage. Example if there are 4 conductors per phase it is likely you are looking at a modern 400kV pylon.
4 - Phase Conductors
The live conductors are commonly made from a steel core for strength and outer layers of aluminium strands. Aluminium is used due to its low weight and cost and acceptable current carrying capacity. Modern cables are now incorporating a composite central core made of glass / carbon fibre with far superior tessellated aluminium strands to reduce air gaps and associated losses. These modern cables are lighter and have far better current carrying capacities. Common configurations have 2 arms per phase with 1 - 4 conductors supported by each arm. So a large 400kV pylon may well be supporting 24 live conductors.
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5 - Surge / Lightning Arresters
There are many types of surge or lightning arresters but their operation in short is to divert transient voltages to earth for example in the case of lightning strike or equipment fault. On pylons the arresters can be seen as a metal conductor attached to the line and ground across the insulators with a gap. The gap is to allow higher than standard running currents to arc and be taken away from the line conductor and fed safely to earth through the steel lattice structure of the pylon.
Grid loading - Demand
As grid generation needs to meet varying demand to allow for fast response to peaks in demand we have water based storage systems that can be activated far more rapidly than fuel based turbines like gas and coal. Pumped storage systems are a highly effective system for rapid response to unexpected peaks.
Dinorwig power station in Snowdonia is an example of this system and is buried deep within a mountain to reduce the visual impact on the national park. This plant is mainly used to meet unexpected peaks like TV pick up or extreme weather conditions. Dinorwig alone can add up to an extra 1800 MW to the grid with a very short response time compared to gas or coal stations that can take up to eight hours to initiate a turbine.
During peak loads pumped storage systems can release water from an upper reservoir (1) pass it through a filter (2) and gate (3) down a Penstock (4) that compresses the water and focuses it on to a turbine (5). The turbine is connected via a gear box to a generator and the voltage is stepped up by a transformer (6) for efficient grid transmission (7). During low peak time water from the lower reservoir (8) is filtered (9) and pumped (10) back into the upper reservoir (1). The energy available = height or head (11) x gravity x mass of water.
TV Pick up
TV pick up is a scenario where a large proportion of the country may be watching the same thing and breaks can often cause peaks in demand as high load kettles are being used all at the same time. The popular soap EastEnders has caused large peaks in demand on special episodes. An example could also be England playing in a world cup final, it is far fetched I know!
At half time it is considered that the nation will turn to their favourite tipple the good old cup of tea. I'm sure the reality is more likely they will light up their faces briefly opening the fridge to get a beer!
The strain on our grid was shown during the London 2012 Olympics when the 100m male sprint damaged transformers in Lambeth. Additional loading during the Olympics also left travellers stranded when a transformer blew at London's Kings cross station. It is suggested that if every home turned on their kettles and heating all at the same time the effect could be catastrophic failure of substation equipment.
Smart Grid
The next great leap in improving the performance of the national grid is smart grid technology. Currently we are undergoing a revolution to incorporate renewable technologies and maximising the use of our generation capacities and delivering power when and where it is needed. As renewable energy is not consistent and reliable the grid needs to be balanced and overuse of non-renewable forms of generation can be limited by a more accurate and responsive grid.
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One area that most of us have come into contact with is internet connected smart metering that has reduced costs in meter reading and customer complaints and issues with estimated billing. Advanced meters can log trends in power usage and inform distribution network operators (DNO) where power peaks are a regular occurrence. Large scale communications networks are being installed to monitor output and demand across the major points of generation and substations. By utilising fibre optic communications the connections can be made using existing pylons as magnetic effects do not affect fibre optics.
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With more than 10 million photovoltaic panels on domestic properties two way generation is another method of balancing out grid requirements with localised generation becoming a more common feature in modern homes. Distribution network operators are in the process of upgrading the existing infrastructure.
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Other features of smart grid control is in the high load switching and power factor balancing of distribution systems. By improving power factor heat losses are reduced by keeping supplies in phase and as close to parity as possible. With changing demands smart monitoring can effectively switch in capacitors to respond to increased inductive loading. For example on a day where there is a sharp drop in temperature inductive loading will sharply increase with the use of pumps and motors running heating and circulation systems.
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Smart buildings with building management systems can also speed up & improve the deployment of smart grids as their usage can be monitored and information shared with DNO's to improve efficiency
and reduce over generation.
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Smart controls and detection systems can automatically change the grid dynamics with regard to weather and other unexpected peaks. Weather is a key factor in current generation strategy as wind farms and photo-voltaic systems become more common place and account for over half of our generation on a favourable day. The provision for wind and solar farming is on the rise and many more installations are being built or planned for future implementation. The intention is to get 50GW of wind generation by 2030.
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One more feature that will vastly improve grid efficiency is information on poor transmission, equipment degradation or failure. Automatic insulation and fault monitoring reduces down time or complete failure by informing grid operators as faults are developing as opposed to response to failures or power outages. The maintenance factor of grid tied equipment will be greatly enhanced by monitoring its performance and logging when performance drops below a desirable level. For example insulation monitoring can identify when cables are approaching their operational lifetime and planning can be used to pro-actively maintain the supply and upgrade the cable or piece of equipment that is beginning to degrade. Implementing an information driven planned preventative maintenance schedule will make total failures rare events so less time is spent on reactive, time consuming and disruptive maintenance.
Distribution Network
The grid itself is operated by Transmission operators. In the UK there are currently three at the time of writing. Their role is to ensure that power is made available to the main grid from source and to transmit it to regional DNO's. They regulate fair pricing across the DNO's & ensure they spend sufficient profits on maintaining and upgrading their areas for predicted future requirements and implementation of continual upgrading to meet the growing demand for improvements in grid technologies for greater stability and efficiency.
The diagram below shows the regions for Transmission operators in the UK & Ireland. The operators need to be in constant communication to maintain an active efficient grid that serves all the countries.
Distribution Network Operators (DNO's)
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Electricity distribution to end users is operated and maintained by regional operators called Distribution Network Operator's (DNO's). The main purpose of the DNO is to oversee effective distribution to consumers. They are required to maintain the networks and perform upgrades in line with the requirements of OFGEM (see overleaf). They do change quite often so it is worth checking which operator is currently in control of a particular region.
In the case of supply fault or failures they are the first point of contact for information and requests for repairs.
The DNO's set pricing for electricity tariffs but are highly regulated to prevent unfair treatment of consumers in particular regions.
The UK's & Ireland’s DNO's at the time of writing are:
Scottish & Southern Electricity Networks (SSE)
SP Energy Networks
Northern Ireland Electricity Networks
Electricity North West
Northern Power Grid
Western Power Distribution
UK Power Networks
ESB Networks
Ofgem - Regulatory Body
Overall the entire network is regulated by OFGEM who work with the government and EU to ensure sustainability, security, value for money and delivering on new initiatives. They are responsible for ensuring compliance with regulations and conduct investigations for alleged breaches. Ofgem also ensure continuation of service if a DNO goes bankrupt or unexpectedly ceases trading. In addition to this they publish information on their website to aid consumers and inform on what they are doing and future projects. As well as inland regulation Ofgem works with the EU to ensure interconnections are regulated between nations.
Example schemes that are being overseen by OFGEM
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Warm Home Discount (WHD)
This is for people suffering fuel poverty. Eligible persons can receive a £140 rebate to alleviate heating costs. Eligibility can be determined through the www.gov.uk website or from their energy supplier.
Feed in Tariff (FIT)
This applies to properties that have a method of generating their own low carbon electricity. The tariff is a payment for energy produced by the consumer. For most renewables the tariff is paid over a period of 20 years or 10 years for Combined Heat & Power (CHP). The tariff applies to wind, hydro, solar (PV) anaerobic digestion and CHP.
Grants
Ofgem can assist with grants to assist with improving the thermal performance of homes for example loft or cavity wall insulation or more efficient boilers & control for example. It is worth checking before improvement work is undertaken to see if it qualifies for a grant.
Renewable Heat Incentive (RHI)
This is a tariff rate to encourage converting to alternative methods of heating using low carbon renewable methods including biomass, solar thermal, air source heat & ground source heat pumps.
Interconnectors
These are international links that allow electricity to be shared across national borders.
Currently the UK has interconnections between France, Netherlands, Isle of man, Belgium & Ireland.
Proposed and current developments include connections between France, Norway, Denmark, Iceland, Morocco & Ireland.
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Existing Interconnectors
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IFA France 2 GW
IFA2 France 1000 MW
East West Republic of Ireland 500 MW
BritNed Netherlands 1 GW
Moyle Northern Ireland 500 MW
Manx Isle of man 40 MW
Nemo Belgium 1 GW
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Interconnections are a great way of saving on building extra power stations. Over production of renewable energy can be shared over national borders if it is surplus to requirements. This is a cheaper method of meeting demand than traditional generation.
ADS - Automatic Disconnection of Supply
To prevent damage to the supply infrastructure, or danger to life, devices are in place to automatically disconnect the supply in the case of a fault. Grid connected pylons have an earthing connection that connects all of the exposed conductive parts of the installation. The purpose of this is to provide an alternative path for fault current to flow and disconnect the system that is in fault. For lightning protection of grid connected supply systems, like transmission pylons and substations, surge or lightning arrestors are used to take excess current to the body of earth and protect the system from damage like over heating, arc damage or imbalances on the phases.
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At the substations due to the large voltages and currents involved specialist methods of protection are used to prevent damage or danger in the case of faults. For overload or short circuit protection special care needs to be taken to minimise the destructive power of arcs. Arcs can cause massive overheating and in some cases highly destructive explosions. The common methods of reducing the risk is to break the arc into several more manageable arcs or encouraging the arc to occur in a location that is designed to extinguish it. Common methods of arc extinguishing, are the use of inert gases or non flammable oils, both remove oxygen and work as a coolant at the arc location thus extinguishing it.
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In a healthy delta connected transmission system each phase will have equal resistance voltage, current, frequency, power and will be at 120 degrees between L1, L2 & L3.
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In dual transmission systems (most pylons) the paired phases should also be equal.
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Relays
Most types of circuit breaker on transmission systems are initiated by relays. The relays will detect imbalances and operate the circuit breakers when their pre determined set point is reached. If the phase angle varies it is indicative that part of the system is in fault. The relays work in several ways but the most common features they will have is a method of communication between substations to correctly determine which line cables are in fault. The need for communication introduces a latency in communication which needs to be allowed for so time clocks are regulated to work with the latency taken account for. The relays at both ends of the transmission line need to work in unison with very high speed reaction to faults. In many cases more than one option is available to maintain a more reliable system of control and reduce the chance of mistakenly opening or closing the circuit breakers. Communications between both ends of the transmission line is done by one or more of these methods:
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Fibre Optic
This method of communication uses photons of light that are unaffected by high voltage electromagnetic effects and travel at 30% the speed of light. Photons of light travel far faster than electron transmission and over longer distances.
Global Positioning Satellite (GPS)
This can regulate the clocks of both relays at the same time and is unaffected by local faults. May be affected by atmospherics or other signal faults.
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Power Line Carrier
The transmission cables them selves can have a carrier signal superimposed onto their sine. This can be affected quite seriously in the case of electromagnetic disturbances during fault conditions. It is not uncommon for this method for communication of simple sets of data. This is by far the cheapest as the cables are already there!
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Microwave
Micro wave is used as a point to point communication system that must have line of sight to operate. So this method is highly dependant on the location of both ends of the protected transmission cables.
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