Depending where you are travelling in the world, the mains voltage available from the wall plate will be 50Hz or 60Hz AC (Alternating Current) with a nominal voltage of around 120VAC or 230VAC. Unless you are plugging in a hair dryer, kettle or a lamp, you will probably need an adaptor to convert the high voltage AC supply down to a low voltage DC (Direct Current) to be useful, for example to charge your phone or power your laptop. Considering that all electronic equipment runs natively on DC power, you might think why is the mains power always AC? And while we are on the subject, who chose 50/60Hz or 120V/230VAC as the “correct” numbers for the mains supply anyway?
Back in the nineteenth century, when public power distribution networks were first being developed, the choice was much wider. Both AC and DC mains supplies were offered, with the standard AC frequency ranging from as low as 16⅔Hz up to as high as 133Hz. Electronic appliances had not yet been invented, so the most common use of electricity was for lighting or heating, both of which worked equally well with either AC or DC supplies, so the AC frequency was not so important. The most common value was 42Hz and in America, Edison patented DC power distribution and heavily promoted it as being as safe as and more reliable than AC1. To a certain extent, this was true, as early electrical generators were less than reliable and the banks of batteries both stabilized the output voltage and bridged any short duration generator faults with the DC supply. This was not the case with AC generators which needed very good speed regulators to maintain the correct output voltage with changes in demand and had no back-up supply possibility in the event of a generator fault.
AC eventually won over DC distributed networks for three main reasons: the simplicity of the first AC generators which led to a rapid improvement in reliability, the ease in which the voltage could be changed up or down using transformers and the advantages of multiple-pole alternators to reduce the rotation speed of more powerful generators.
The simple electrical generators used at the time converted mechanical energy into electrical energy by rotating a magnet within coils of wire (figure 1.1).
Note that there are no moving electrical contacts.
Fig. 1.1: Principle of operation of an alternator
The arrangement shown in figure 1.1 is more commonly called an alternator because the current flows alternately in one direction and then in the other as the magnet spins. If a non- alternating output is desired, then a mechanical switch called a commutator is needed to reverse the connections every half cycle:
Fig. 1.2: Commutator Action
In this arrangement, the coil of wire rates within a fixed magnetic field instead of rotating a magnet within the coils of wire, but the generating effect is the same. The commutator switching action is typically performed by a split slip ring on the shaft of the generator which reverses the connections every half turn:
Fig. 1.3: Split slip ring commutator
As the power distribution network developed and the demand increased, the current flowing through the commutator brushes increased and made the DC system more unreliable than the simple AC generators used at the time which needed no slip rings.
The second reason for the demise of the DC power transmission scheme was the increasing losses as more and more houses were connected to the system. The power losses in a cable with resistance R are proportional to the square of the current , I , flowing through it (i²R loss), so if the voltage can be doubled to halve the current (Power = VI), then the same power cable can carry the current four times further. This principle applies to both DC and AC power transmission, but it was much easier to use transformers to step up the AC supply voltage for long-distance transmission and to step it back down again at the far end again using transformers. Edison tried to compete with his DC system by using generator sets (a DC motor connected to a dynamo to step up or down the supply voltage) but although a low voltage DC motor for the step-up part was easy to make, a high-voltage DC motor for the corresponding step-down part was not so reliable and the system broke down often. In the end, even Edison abandoned the DC distribution concept and changed to alternating current power distribution.
Although most mains power sockets are single phase, electrical AC power stations generate three phases at 120° from each other. The advantage of this is that 3x120° = 360°. In other words, the phases cancel out when connected to a common point.
Fig. 1.4: Three Phase waveform. The sum of all three phases added together is always zero.
This means that unlike in DC power distribution where the current flows equally in the positive and negative cables which therefore both need to be equally massive, an AC power distribution grid can be made with three heavy duty phase cables and a light gauge neutral wire which is only needed to carry any imbalance current if the loads on the three phases are not exactly equal. If you look at an electricity pylon, you can see the thick power cables suspended from the cross beams with a single, thinner cable running across the tops of the pylons. This is theneutral return wire. The earth (or ground) connection is for safety only. It carries no current in normal conditions. If a current flows from any phase to earth then it is due to a fault and a protective device (fuse or residual current trip) should cut off the power.
The following simplified diagram illustrates this arrangement when applied to whole streets in a town.
Fig. 1.5: Diagrammatic representation of a three-phase power distribution system. The neutral wire will carry no current if the load on each phase is balanced.
Why three phases and not two? Well, two-phase power distribution is still used in some parts of the USA (2x120VAC at 180° so that 240VAC equipment for heavier loads such as ovens and washing machines could be used on a 120V system), but the big advantage of an odd number of phases is for use with AC motors. It does not matter where the rotor sits, a three-phase motor will always start up in the same direction and as the load is equally balanced on all three phases, a neutral wire is not required (L1, L2, L3 and Earth). An AC motor with an even number of phases could either not start if the rotor was exactly in line with the poles, or worse, start up in the wrong direction. Additionally, a two phase system delivers power at twice the fundamental frequency and this pulsating supply must be smoothed out by the inertia of the motor, making a two phase motor larger and heavier than a 3-phase motor of the same power.
Fig. 1.6: Principle of operation of a three-phase motor. As each phase peaks, the rotor is pulled around to line up with that set of windings. The rotor then follows the rotating magnetic field.
But the final nail in the coffin for DC distribution was the popularity of electrical lighting. As more and more houses, public buildings and streets switched from gas lighting to electric lamps, the demand for electrical power increased rapidly. The lower cabling cost of threephase transmission compared to DC became the deciding factor when raising the investment needed to electrify whole towns (a 3-phase system uses 50% more copper than a 2-phase system, but delivers three times the power).
More powerful and larger generators were manufactured to meet this demand. These generators were very heavy and the slower a very massive generator rotor can rotate, the less stress on the bearings and framework. This is why there were originally so many different AC frequencies used: a smaller generator spinning at 2500 RPM created a 42Hz output, while a larger one spinning at 1000 RPM created a 16⅔Hz output (note that “a nice whole number” of revolutions per minute (RPM) was more often used, an indication that mechanical engineers built the alternators, not electrical engineers. 16⅔Hz is still used by the railways because if a commutator is fitted to both the stator and rotor windings, an electric motor will run with either DC or AC at this low frequency). However, while an incandescent filament may not flicker much at 42Hz, at 16⅔Hz it was disturbingly visible. The AC flicker was even more pronounced with arc lighting which became increasingly used in theatres, open spaces and for street lighting.
The solution for high frequency AC output with slower rotation speeds was to use multiple pole alternators: instead of two windings, four windings could be used wired alternately in series. Then instead of one AC cycle per rotation, two cycles would be generated. For the same output AC frequency, the rotor speed could be halved, significantly reducing the stress on the generator.
In the meantime, the mechanical problems with slip rings had been solved and multiple windings could be wound on the rotor with multiple magnetic poles built into the stator. This meant that the optimum rotation speed could be chosen for the physical size of the generator and almost any output frequency could be generated by selecting the appropriate number of rotor windings and stator poles. The original Niagara Falls power station in the USA used 12 pole, low speed (250 RPM) generators to output 25Hz AC, but this was later doubled up to 50Hz by simply rearranging the windings while keeping the original low RPM which was optimally matched to the water turbines.
Fig. 1.7: Example of a Multiple Pole Alternator.
By this time (mid 1850’s), AEG was the leading electrical equipment manufacturer in Europe. 50Hz was supposedly chosen as the standard AC frequency in Europe because it was an even number of 100 peaks per second, which appealed to the Teutonic mind. In America, Westinghouse chose 60Hz, supposedly because 50Hz flicker was still just about visible with arc lighting and therefore Nicola Tesla (who licenced his AC generation patents to Westinghouse) had recommended a higher mains frequency, but equally probably to protect their home market from foreign competition. Either way, commercial interests decided that 50Hz in some regions and 60Hz in other regions should eventually become standard.
Fig. 1.8: Waveforms of 230VAC/50Hz and 120VAC/60Hz single-phase supplies.
The effective voltage (dotted line) is the square root of the mean of the squares of the AC voltage (RMS), in other words, the DC voltage that would have the same heating effect as the AC voltage.
So, protectionism could explain why the AC mains is 50Hz in some countries and 60Hz in others, but why the different supply voltages of 120VAC or 230VAC? Originally, 110-120VAC was a pretty-much universal standard (also in pre-war Europe) because the influential Edison used 110V for his DC distribution system. The competition therefore also chose similar voltages so that any heating or lighting equipment designed to run on Edison’s system could also be used with their own power supply network. As the number of domestic appliances per household increased, the I²R losses of the 120VAC supply became more and more significant, but the wealth of post-war US citizens meant that so many refrigerators, air conditioners and televisions with 120VAC input were already in use that an increase in mains supply voltage in the USA was impractical. Europe, on the other hand, was recovering from the war with no such legacy problems and realizing that the demand for electrical power would only increase in the future chose to double the 110/120VAC voltage (220VAC in continental Europe, 240VAC in the UK) to halve the current and quarter the losses. Eventually, in 1994, the EU decided to harmonize throughout Europe on 230VAC which was within the operating range of both 220VAC and 240VAC equipment. In practice, however, the allowable voltage tolerance limits were wide enough so that UK could stay at 240V and the rest of Europe remain at 220V and both say that they delivered a nominally 230VAC supply. A typical European solution to the problem! In the meantime, the power stations have all been adjusted to deliver 230VAC on average, although my colleague in the UK still measures 240V on his supply as he is very close to a substation.
There are still several countries that for various reasons have “non-standard” mains voltages. Many ex-Commonwealth countries still use the original British 240VAC supply voltage. Japan has opted for 100VAC supplies for safety reasons, but because the South Island was supplied with generators from Westinghouse and the north island from AEG, they have either 60Hz or 50Hz supply frequencies depending where you are in Japan. Four frequency converter substations have now been built to balance out the load between the islands by transferring 50Hz and 60Hz power back-and-forth. In the USA, many large buildings use 115/277VAC split supplies. The higher voltage is primarily used for lighting to increase the overall building efficiency, as lighting can account for 40% of the total power consumption of a large office block. Aircraft quickly settled on a 400Hz AC standard to reduce the weight and size of the motors and transformers used in aeroplanes.
Fig. 1.9: Map of world mains voltages and frequencies
As mentioned previously, the advantage of three phases over a single phase or two phases at 180° is that a motor wound with three windings will automatically start to rotate following each phase peak in turn and always in the same direction. This makes three-phase motors very simple, robust and reliable and therefore popular in industrial automation applications. Three-phase motors do not use the neutral connection and very often have a four-wire cable of just the three phases and earth. As there is no neutral wire, any auxiliary power supply must be connected across two phases as any supply connection between a phase and earth is not allowed. The phase-to-phase voltage is higher than the phase-to-neutral voltage because the two phases add up to a higher combined sine wave (figure 2). The multiplication factor is √3 or about x1.7 – the voltage between two 220VAC RMS single phases will be around 380VAC RMS.
Fig. 1.10: Waveform of a 220VAC phase-to-phase supply
What all this means for a modern AC/DC power supply designer is that an universal input single-phase power supply will need to accommodate an AC input voltage range (including ±10% tolerance) of 90 – 264VAC for world-wide use (covering 100/120/230/240VAC nominal) or 90-305VAC to also accommodate 277VAC supplies sometimes used in the USA. AC supply frequency should ideally be 45-440Hz to cover supply variations.
Table 1.1: Mains voltage ranges.
* TÜV specifies +15% tolerance because 230VAC + 15% is the same as 240VAC + 10%.
Footnote: Modern Power Distribution
Today, technologies exist that allow the conversion of AC to DC in either direction with very high power and efficiencies. Although AC mains voltages will remain standard for the near future, there are several advantages in going back to DC power distribution. One reason is our increasing dependence on electrical power. In order to guarantee supply, power distribution is not just from one generator to the consumer, but from many sources connected together to form a power grid. It is more efficient and cheaper to transmit power over long distances (>500km) using high voltage DC as there are no impedance losses and the generators do not need to be all synchronized to the same frequency or even the same voltage. For example, a 2000MW high voltage DC power link connects England and France to allow the two countries to exchange power according to domestic demand.
In the home, a DC power distribution network that links photovoltaic solar cells on roof with fixed battery or in an electric car allows reliable, high efficiency, low running-cost electrical supply which can be mains independent (off-grid). There are many advantages connecting together groups of homes to share energy sources (Photovoltaic, house external grid supply) make very efficient localized grid. See https://www.isea.rwth-aachen.de for one such concept.
Back in the nineteenth century, when public power distribution networks were first being developed, the choice was much wider. Both AC and DC mains supplies were offered, with the standard AC frequency ranging from as low as 16⅔Hz up to as high as 133Hz. Electronic appliances had not yet been invented, so the most common use of electricity was for lighting or heating, both of which worked equally well with either AC or DC supplies, so the AC frequency was not so important. The most common value was 42Hz and in America, Edison patented DC power distribution and heavily promoted it as being as safe as and more reliable than AC1. To a certain extent, this was true, as early electrical generators were less than reliable and the banks of batteries both stabilized the output voltage and bridged any short duration generator faults with the DC supply. This was not the case with AC generators which needed very good speed regulators to maintain the correct output voltage with changes in demand and had no back-up supply possibility in the event of a generator fault.
AC eventually won over DC distributed networks for three main reasons: the simplicity of the first AC generators which led to a rapid improvement in reliability, the ease in which the voltage could be changed up or down using transformers and the advantages of multiple-pole alternators to reduce the rotation speed of more powerful generators.
The simple electrical generators used at the time converted mechanical energy into electrical energy by rotating a magnet within coils of wire (figure 1.1).
Note that there are no moving electrical contacts.
Fig. 1.1: Principle of operation of an alternator
The arrangement shown in figure 1.1 is more commonly called an alternator because the current flows alternately in one direction and then in the other as the magnet spins. If a non- alternating output is desired, then a mechanical switch called a commutator is needed to reverse the connections every half cycle:
Fig. 1.2: Commutator Action
In this arrangement, the coil of wire rates within a fixed magnetic field instead of rotating a magnet within the coils of wire, but the generating effect is the same. The commutator switching action is typically performed by a split slip ring on the shaft of the generator which reverses the connections every half turn:
Fig. 1.3: Split slip ring commutator
As the power distribution network developed and the demand increased, the current flowing through the commutator brushes increased and made the DC system more unreliable than the simple AC generators used at the time which needed no slip rings.
The second reason for the demise of the DC power transmission scheme was the increasing losses as more and more houses were connected to the system. The power losses in a cable with resistance R are proportional to the square of the current , I , flowing through it (i²R loss), so if the voltage can be doubled to halve the current (Power = VI), then the same power cable can carry the current four times further. This principle applies to both DC and AC power transmission, but it was much easier to use transformers to step up the AC supply voltage for long-distance transmission and to step it back down again at the far end again using transformers. Edison tried to compete with his DC system by using generator sets (a DC motor connected to a dynamo to step up or down the supply voltage) but although a low voltage DC motor for the step-up part was easy to make, a high-voltage DC motor for the corresponding step-down part was not so reliable and the system broke down often. In the end, even Edison abandoned the DC distribution concept and changed to alternating current power distribution.
Although most mains power sockets are single phase, electrical AC power stations generate three phases at 120° from each other. The advantage of this is that 3x120° = 360°. In other words, the phases cancel out when connected to a common point.
Fig. 1.4: Three Phase waveform. The sum of all three phases added together is always zero.
This means that unlike in DC power distribution where the current flows equally in the positive and negative cables which therefore both need to be equally massive, an AC power distribution grid can be made with three heavy duty phase cables and a light gauge neutral wire which is only needed to carry any imbalance current if the loads on the three phases are not exactly equal. If you look at an electricity pylon, you can see the thick power cables suspended from the cross beams with a single, thinner cable running across the tops of the pylons. This is theneutral return wire. The earth (or ground) connection is for safety only. It carries no current in normal conditions. If a current flows from any phase to earth then it is due to a fault and a protective device (fuse or residual current trip) should cut off the power.
The following simplified diagram illustrates this arrangement when applied to whole streets in a town.
Fig. 1.5: Diagrammatic representation of a three-phase power distribution system. The neutral wire will carry no current if the load on each phase is balanced.
Why three phases and not two? Well, two-phase power distribution is still used in some parts of the USA (2x120VAC at 180° so that 240VAC equipment for heavier loads such as ovens and washing machines could be used on a 120V system), but the big advantage of an odd number of phases is for use with AC motors. It does not matter where the rotor sits, a three-phase motor will always start up in the same direction and as the load is equally balanced on all three phases, a neutral wire is not required (L1, L2, L3 and Earth). An AC motor with an even number of phases could either not start if the rotor was exactly in line with the poles, or worse, start up in the wrong direction. Additionally, a two phase system delivers power at twice the fundamental frequency and this pulsating supply must be smoothed out by the inertia of the motor, making a two phase motor larger and heavier than a 3-phase motor of the same power.
Fig. 1.6: Principle of operation of a three-phase motor. As each phase peaks, the rotor is pulled around to line up with that set of windings. The rotor then follows the rotating magnetic field.
But the final nail in the coffin for DC distribution was the popularity of electrical lighting. As more and more houses, public buildings and streets switched from gas lighting to electric lamps, the demand for electrical power increased rapidly. The lower cabling cost of threephase transmission compared to DC became the deciding factor when raising the investment needed to electrify whole towns (a 3-phase system uses 50% more copper than a 2-phase system, but delivers three times the power).
More powerful and larger generators were manufactured to meet this demand. These generators were very heavy and the slower a very massive generator rotor can rotate, the less stress on the bearings and framework. This is why there were originally so many different AC frequencies used: a smaller generator spinning at 2500 RPM created a 42Hz output, while a larger one spinning at 1000 RPM created a 16⅔Hz output (note that “a nice whole number” of revolutions per minute (RPM) was more often used, an indication that mechanical engineers built the alternators, not electrical engineers. 16⅔Hz is still used by the railways because if a commutator is fitted to both the stator and rotor windings, an electric motor will run with either DC or AC at this low frequency). However, while an incandescent filament may not flicker much at 42Hz, at 16⅔Hz it was disturbingly visible. The AC flicker was even more pronounced with arc lighting which became increasingly used in theatres, open spaces and for street lighting.
The solution for high frequency AC output with slower rotation speeds was to use multiple pole alternators: instead of two windings, four windings could be used wired alternately in series. Then instead of one AC cycle per rotation, two cycles would be generated. For the same output AC frequency, the rotor speed could be halved, significantly reducing the stress on the generator.
In the meantime, the mechanical problems with slip rings had been solved and multiple windings could be wound on the rotor with multiple magnetic poles built into the stator. This meant that the optimum rotation speed could be chosen for the physical size of the generator and almost any output frequency could be generated by selecting the appropriate number of rotor windings and stator poles. The original Niagara Falls power station in the USA used 12 pole, low speed (250 RPM) generators to output 25Hz AC, but this was later doubled up to 50Hz by simply rearranging the windings while keeping the original low RPM which was optimally matched to the water turbines.
Fig. 1.7: Example of a Multiple Pole Alternator.
By this time (mid 1850’s), AEG was the leading electrical equipment manufacturer in Europe. 50Hz was supposedly chosen as the standard AC frequency in Europe because it was an even number of 100 peaks per second, which appealed to the Teutonic mind. In America, Westinghouse chose 60Hz, supposedly because 50Hz flicker was still just about visible with arc lighting and therefore Nicola Tesla (who licenced his AC generation patents to Westinghouse) had recommended a higher mains frequency, but equally probably to protect their home market from foreign competition. Either way, commercial interests decided that 50Hz in some regions and 60Hz in other regions should eventually become standard.
Fig. 1.8: Waveforms of 230VAC/50Hz and 120VAC/60Hz single-phase supplies.
The effective voltage (dotted line) is the square root of the mean of the squares of the AC voltage (RMS), in other words, the DC voltage that would have the same heating effect as the AC voltage.
So, protectionism could explain why the AC mains is 50Hz in some countries and 60Hz in others, but why the different supply voltages of 120VAC or 230VAC? Originally, 110-120VAC was a pretty-much universal standard (also in pre-war Europe) because the influential Edison used 110V for his DC distribution system. The competition therefore also chose similar voltages so that any heating or lighting equipment designed to run on Edison’s system could also be used with their own power supply network. As the number of domestic appliances per household increased, the I²R losses of the 120VAC supply became more and more significant, but the wealth of post-war US citizens meant that so many refrigerators, air conditioners and televisions with 120VAC input were already in use that an increase in mains supply voltage in the USA was impractical. Europe, on the other hand, was recovering from the war with no such legacy problems and realizing that the demand for electrical power would only increase in the future chose to double the 110/120VAC voltage (220VAC in continental Europe, 240VAC in the UK) to halve the current and quarter the losses. Eventually, in 1994, the EU decided to harmonize throughout Europe on 230VAC which was within the operating range of both 220VAC and 240VAC equipment. In practice, however, the allowable voltage tolerance limits were wide enough so that UK could stay at 240V and the rest of Europe remain at 220V and both say that they delivered a nominally 230VAC supply. A typical European solution to the problem! In the meantime, the power stations have all been adjusted to deliver 230VAC on average, although my colleague in the UK still measures 240V on his supply as he is very close to a substation.
There are still several countries that for various reasons have “non-standard” mains voltages. Many ex-Commonwealth countries still use the original British 240VAC supply voltage. Japan has opted for 100VAC supplies for safety reasons, but because the South Island was supplied with generators from Westinghouse and the north island from AEG, they have either 60Hz or 50Hz supply frequencies depending where you are in Japan. Four frequency converter substations have now been built to balance out the load between the islands by transferring 50Hz and 60Hz power back-and-forth. In the USA, many large buildings use 115/277VAC split supplies. The higher voltage is primarily used for lighting to increase the overall building efficiency, as lighting can account for 40% of the total power consumption of a large office block. Aircraft quickly settled on a 400Hz AC standard to reduce the weight and size of the motors and transformers used in aeroplanes.
Fig. 1.9: Map of world mains voltages and frequencies
As mentioned previously, the advantage of three phases over a single phase or two phases at 180° is that a motor wound with three windings will automatically start to rotate following each phase peak in turn and always in the same direction. This makes three-phase motors very simple, robust and reliable and therefore popular in industrial automation applications. Three-phase motors do not use the neutral connection and very often have a four-wire cable of just the three phases and earth. As there is no neutral wire, any auxiliary power supply must be connected across two phases as any supply connection between a phase and earth is not allowed. The phase-to-phase voltage is higher than the phase-to-neutral voltage because the two phases add up to a higher combined sine wave (figure 2). The multiplication factor is √3 or about x1.7 – the voltage between two 220VAC RMS single phases will be around 380VAC RMS.
Fig. 1.10: Waveform of a 220VAC phase-to-phase supply
What all this means for a modern AC/DC power supply designer is that an universal input single-phase power supply will need to accommodate an AC input voltage range (including ±10% tolerance) of 90 – 264VAC for world-wide use (covering 100/120/230/240VAC nominal) or 90-305VAC to also accommodate 277VAC supplies sometimes used in the USA. AC supply frequency should ideally be 45-440Hz to cover supply variations.
| Nominal Supply (RMS) | Phase-to-Neutral | Phase-to-Phase | |||
|---|---|---|---|---|---|
| RMS (10% tolerance)* | Peak Voltage | RMS (Nominal) | RMS (10% tolerance) | Peak Voltage | |
| 100 VAC | 90-110 V | 141 V | 173 V | 156-190 V | 245 V |
| 120 VAC | 108-132 V | 170 V | 208 V | 187-229 V | 360 V |
| 230 VAC | 207-253 V | 325 V | 400 V | 360-440 V | 693 V |
| 240 VAC | 216-264 V | 340 V | 415 V | 373-457 V | 588 V |
| 277 VAC | 249-305 V | 392 V | 480 V | 432-528 V | 831 V |
Table 1.1: Mains voltage ranges.
* TÜV specifies +15% tolerance because 230VAC + 15% is the same as 240VAC + 10%.
Footnote: Modern Power Distribution
Today, technologies exist that allow the conversion of AC to DC in either direction with very high power and efficiencies. Although AC mains voltages will remain standard for the near future, there are several advantages in going back to DC power distribution. One reason is our increasing dependence on electrical power. In order to guarantee supply, power distribution is not just from one generator to the consumer, but from many sources connected together to form a power grid. It is more efficient and cheaper to transmit power over long distances (>500km) using high voltage DC as there are no impedance losses and the generators do not need to be all synchronized to the same frequency or even the same voltage. For example, a 2000MW high voltage DC power link connects England and France to allow the two countries to exchange power according to domestic demand.
In the home, a DC power distribution network that links photovoltaic solar cells on roof with fixed battery or in an electric car allows reliable, high efficiency, low running-cost electrical supply which can be mains independent (off-grid). There are many advantages connecting together groups of homes to share energy sources (Photovoltaic, house external grid supply) make very efficient localized grid. See https://www.isea.rwth-aachen.de for one such concept.