ES2C6 – Power Electronics 4 – Power Systems
Power Systems
The power system is the largest engineering project tin the world. There are thousands of
synchronous generators driven by a variety of energy sources, and they all must be perfectly
synchronised. The whole power grid is star to star connected, this is because a neutral point is
needed, which doesn’t occur in a delta connection. In order to transport electricity from one grid to
another, for instance from France to the UK, a high voltage DC (HVDC) system is used, where the AC
in France is transferred to DC and then changed back to AC in the UK. This is also used in wind
turbines where the AC power is turned to DC and then back to AC, so the frequency of the electricity
can be synchronised with the grid.
The power system is comprised of four major stages: the generation subsystem, the transmission
system, the distribution system and the load subsystem. The voltages through each system varies
and is also different depending on the distance to be transmitted. The electrical power is generated
at around 12𝑘𝑉 and is stepped up by three-phase transformers to around 400𝑘𝑉, giving a line to
line voltage of 132𝑘𝑉 or higher. After being stepped down, it is transmitted over shorter distances
at a line to line voltage of 11𝑘𝑉 or 33𝑘𝑉, before being stepped down again to 230𝑉 single phase –
or 400𝑉 line to line – for consumer use.
Electricity Generation
Both of these fuels must be burnt in order to turn a turbine. However, coal must be burnt and
evaporate water to form pressurised steam which then rotates a generator, but gas can be used to
power the turbine directly, so has a much quicker pick-up rate. Rapid load changes are a big problem
for the grid, whether this is turning appliances on or off. Currently the grid uses spinning reserves, a
method in which extra generators are creating electricity, but are only connected to the grid when
demanded. A newer method is to store the generated electricity, sometimes water is pumped to a
higher altitude and released through generators when there is demand – storing electricity in this
way is more common with renewable resources.
Nuclear power plants are often cleaner than other plants, although they leave unwanted nuclear
waste. A lot of people are hesitant about nuclear power as, when things go wrong, they often go
very wrong.
Hydro-electric power plants are very clean and generate a huge amount of power. The Three Gorges
Dam in China is the largest in the world, although this kind of plant relies on a country’s natural
landscape and rivers.
Power System Components
Generators are usually synchronous and generate AC power at a fixed frequency - 50𝐻𝑧 or 60𝐻𝑧.
They are generally connected to a mechanical source of power.
High voltage transformers are used for stepping voltages up or down. They are stepped up before
transmission over long distances to reduce losses in the lines. The voltages are then stepped down
before distribution to load centres, such as homes.
Circuit breakers are used to interrupt the flow of current in the event of a fault. It is necessary to
isolate or divert an excessively high current to protect the system from surges and contain blackouts.
Capacitor banks are used for power factor correction, or reactive power compensation. They can
stabilise voltage surges and dips as well as control reactive power.
17
, ES2C6 – Power Electronics 4 – Power Systems
Fault current limiters are used for interrupting very high currents called fault currents. These occur
when is an unintentional short circuit which can damage equipment such as transformers.
Shunt reactors are also used for reactive power compensation – to stabilise line voltage. They
improve the power factor by absorbing VARS from the system when there is under-consumption.
Transmission Lines
The resistance of a cable is proportional to its length, so in high distance transfer, the resistance is
high, so a larger voltage is required to reduce losses.
Transformers
A single-phase transformer takes a primary current and voltage, and outputs a secondary current
and voltage.
The wires wrapped around the core are insulated but create a magnetic field which follows the rules
of the right-hand-law. The strength of the magnetic field is denoted by 𝐻. Electromagnetic fields are
functions of current, so the electromagnetic field strength is controlled by the current passed
through it.
As the current flowing through the conductor is linked to the magnetic field strength:
𝐼 = ∫ 𝐻 𝑑𝑙 (6)
This is Amperes Law.
The flux density can be calculated as:
𝐵 = 𝜇0 𝜇𝑟 𝐻 (7)
where 𝜇0 is the magnetic permittivity in free space and 𝜇𝑟 is the permittivity of the material the
magnetic field is passing through.
18
Power Systems
The power system is the largest engineering project tin the world. There are thousands of
synchronous generators driven by a variety of energy sources, and they all must be perfectly
synchronised. The whole power grid is star to star connected, this is because a neutral point is
needed, which doesn’t occur in a delta connection. In order to transport electricity from one grid to
another, for instance from France to the UK, a high voltage DC (HVDC) system is used, where the AC
in France is transferred to DC and then changed back to AC in the UK. This is also used in wind
turbines where the AC power is turned to DC and then back to AC, so the frequency of the electricity
can be synchronised with the grid.
The power system is comprised of four major stages: the generation subsystem, the transmission
system, the distribution system and the load subsystem. The voltages through each system varies
and is also different depending on the distance to be transmitted. The electrical power is generated
at around 12𝑘𝑉 and is stepped up by three-phase transformers to around 400𝑘𝑉, giving a line to
line voltage of 132𝑘𝑉 or higher. After being stepped down, it is transmitted over shorter distances
at a line to line voltage of 11𝑘𝑉 or 33𝑘𝑉, before being stepped down again to 230𝑉 single phase –
or 400𝑉 line to line – for consumer use.
Electricity Generation
Both of these fuels must be burnt in order to turn a turbine. However, coal must be burnt and
evaporate water to form pressurised steam which then rotates a generator, but gas can be used to
power the turbine directly, so has a much quicker pick-up rate. Rapid load changes are a big problem
for the grid, whether this is turning appliances on or off. Currently the grid uses spinning reserves, a
method in which extra generators are creating electricity, but are only connected to the grid when
demanded. A newer method is to store the generated electricity, sometimes water is pumped to a
higher altitude and released through generators when there is demand – storing electricity in this
way is more common with renewable resources.
Nuclear power plants are often cleaner than other plants, although they leave unwanted nuclear
waste. A lot of people are hesitant about nuclear power as, when things go wrong, they often go
very wrong.
Hydro-electric power plants are very clean and generate a huge amount of power. The Three Gorges
Dam in China is the largest in the world, although this kind of plant relies on a country’s natural
landscape and rivers.
Power System Components
Generators are usually synchronous and generate AC power at a fixed frequency - 50𝐻𝑧 or 60𝐻𝑧.
They are generally connected to a mechanical source of power.
High voltage transformers are used for stepping voltages up or down. They are stepped up before
transmission over long distances to reduce losses in the lines. The voltages are then stepped down
before distribution to load centres, such as homes.
Circuit breakers are used to interrupt the flow of current in the event of a fault. It is necessary to
isolate or divert an excessively high current to protect the system from surges and contain blackouts.
Capacitor banks are used for power factor correction, or reactive power compensation. They can
stabilise voltage surges and dips as well as control reactive power.
17
, ES2C6 – Power Electronics 4 – Power Systems
Fault current limiters are used for interrupting very high currents called fault currents. These occur
when is an unintentional short circuit which can damage equipment such as transformers.
Shunt reactors are also used for reactive power compensation – to stabilise line voltage. They
improve the power factor by absorbing VARS from the system when there is under-consumption.
Transmission Lines
The resistance of a cable is proportional to its length, so in high distance transfer, the resistance is
high, so a larger voltage is required to reduce losses.
Transformers
A single-phase transformer takes a primary current and voltage, and outputs a secondary current
and voltage.
The wires wrapped around the core are insulated but create a magnetic field which follows the rules
of the right-hand-law. The strength of the magnetic field is denoted by 𝐻. Electromagnetic fields are
functions of current, so the electromagnetic field strength is controlled by the current passed
through it.
As the current flowing through the conductor is linked to the magnetic field strength:
𝐼 = ∫ 𝐻 𝑑𝑙 (6)
This is Amperes Law.
The flux density can be calculated as:
𝐵 = 𝜇0 𝜇𝑟 𝐻 (7)
where 𝜇0 is the magnetic permittivity in free space and 𝜇𝑟 is the permittivity of the material the
magnetic field is passing through.
18
