TEMPUS ENERGY: ENERGY SAVING TECHNOLOGIES

Chapter 2: Energy Saving Technologies in transmission, distribution of electrical energy

1: Historical evolution from AC-grids to DC-grids

Although in the early years of electricity production a lot of electrical grids were DC-grids, nowadays the great majority of the electrical grids are AC-grids. This switch from DC-grids to AC-grids has a number of technical reasons.

Due to the need for a commutator, it is very difficult to build DC-generators generating high voltages. Especially when generating higher voltages (of for instance 24 kV), it is easier to construct alternators (AC-generators).

An additional advantage arises when considering electrical motors. In general, constructing AC-motors (like the three phase induction motor) is easier and cheaper than constructing DC-motors. Moreover, a lot of AC-motors require less maintenance.

When using AC, it is possible to use transformers. Due to these transformers, a generated voltage level can be transformed to a higher voltage level of 70 kV, 220 kV or even 380kV. These transformers have a long life expectancy (more than 30 years) and they have a high efficiency (even more than 99.9%). Transformers have a straightforward construction and they require almost no maintenance (no moving parts).

When using a high voltage grid to inject power into a medium or a low voltage grid, transformers can be used to transform the voltage level to a lower level (for instance from 380 kV to 70 kV or from 11 kV to 400 V).

By using transformers, it is possible to supply power at a low voltage level of 230 V and/or 400 V to a customer (reducing the risks related with electricity) and to use high voltage grids to transport large amounts of power over larger distances (reducing the losses in the grid).

It is easier to interrupt AC-currents since these currents have two zero-crossings in one single period whereas a DC-current has no zero-crossings. When opening an electrical contact, the zero-crossing can be used to extinct the arc and it is sufficient to prevent re-ignition of the arc.

When considering AC voltages, the insulation level of cables, appliances and machines must withstand the peak value and not only the RMS value. This peak value equals √2 or 1.41 times the RMS value. 2: HVDC-grids as an emerging technology

Although all these technical aspects explain why nowadays the large majority of the grids are AC-grids, it is wrong to conclude that using DC to transport electrical energy is old-fashioned and the use of DC only has disadvantages. Using DC, can be an emerging and useful technological challenge.

transformer rectifier inverter transformerDC

When transporting electrical energy, it is possible to compare the energy losses between a single phase AC-grid, a three phase AC-grid and a DC-grid. The Joule losses in a three phase grid are only 75% of the Joule losses in a similar single phase grid (insulation which withstands the same voltage level and the same amount of copper is used) transporting the same amount of power. This explains why three phase grids instead of single phase grids are used when transporting electrical energy. When comparing the Joule losses of a DC-grid and a three phase AC-grid. The Joule losses in a DC-grid are only 67% of the Joule losses in a similar three phase AC-grid. This explains why DC-grids are sometimes used when transporting large amounts of electrical energy over large distances (more than 750 km).

2.1: Investment costs related with HVDC-grids

A disadvantage related with the use of DC-grids to transport electrical energy is the lack of transformers allowing to increase or decrease the voltage level. In practice, the situation visualized in Figure 1 combines the use of DC-grids and AC-grids.

Figure 1: Basic structure of a HVDC-grid

Starting from a classical three phase AC-grid, the voltage level is increased using a transformer. A very high AC-voltage is obtained which will be rectified giving a very high DC-voltage (originally mercury-vapor rectifiers were used, nowadays semiconductor rectifiers are used). The very high DC-voltage will be used to transport the electrical energy over a long distance (HVDC: High Voltage Direct Current). At the other side of the DC-grid, an inverter converts this DC-voltage back to an AC-voltage. Using a transformer, the AC-voltage level is decreased and the power is injected into a classical three phase AC-grid.

It is not only possible to transport active power from the left side to the right side. In case the left power electronic converter does not operate as a rectifier but as an inverter and in case the right power electronic converter does not operate as an inverter but as a rectifier, active power is transported from the right side to the left side.

This is an emerging technology since it is not obvious to construct a rectifier and an inverter dealing with such high voltages. Such a rectifier and such an inverter require a considerable investment. This investment can only be justified when the energy is

transported over a very long distance and the transported power is sufficiently large (for instance 2000 MW). Indeed, when preconceiving the energy losses, less copper (or aluminum) is required when using a HVDC-grid instead of a three phase AC-grid. However, only for sufficiently long distances the reduced use of copper is able to justify the installation cost of the rectifier and the inverter.

This situation is visualized in Figure 2 (information provided by ABB). The DC terminal cost is much higher than an AC terminal cost but the DC line cost is lower implying a lower total investment when the distance is sufficiently large.

Figure 2: Comparison investment of a HVDC grid and an AC grid

2.2: Applications of HVDC-grids

HVDC-grids are used to transport large powers over large distances. For instance, a HVDC-grid in Brazil transports power of the Itaipu hydroelectric power plant to the region of Sao Paulo. This installation contains two ±600 kV bipoles having a total rated power of 6300 MW and transporting the power over a distance of 780 km.

Using HVDC-grids, it is also possible to connect two AC-grids having a different frequency. For instance in Japan, the eastern and northern parts of Honshu and Hokkaido have a grid frequency of 50 Hz whereas the western Honshu, Shikoku, Kyushu and Okinawa operate at 60 Hz. The boundary between these two regions contains four back-to-back HVDC-substations operating as frequency converters (in case the rectifier and the inverter which are visualized in Figure 1 are located in the same station, a back-to-back HVDC-system is obtained). For instance the Higashi-Shimizu frequency converter operates with a DC voltage of 125 kV and has a maximum power of 300 MW.

Moreover, DC-grids also allow to interconnect grids having the same nominal frequency but which are not synchronized with each other. For instance, there is a DC-cable interconnecting the British grid and the continental European grid. This HVDC Cross-Channel is a 2000 MW system and has a total length of 73 km containing 46 km submarine cables (an operating voltage of 270 kV).

2.3: DC-grids and renewable energy generation

HVDC allows to transport large amounts of electrical energy over large distances. This is useful to transport the energy of large remotely located hydroelectric power stations (for instance the Three Gorges Dam in China, the Itaipu dam in Brasil) to the industrialized world. More recently and focused to the transportation of powers which are not that extremely high, is the use of HVDC light (http://www.abb.com/hvdc) which is used to transport the energy coming from onshore and offshore wind turbine parks.

When considering the Thornton Bank (offshore, actually farshore wind turbine park) in Belgium, a three phase AC-grid having a voltage level of 150 kV is used to transport the power to land. For instance the Gotland project in Sweden (onshore wind energy) uses the HVDC light technology to transport the generated power.

3: Low voltage DC-grids

Due to the rise of distributed generation (the use of small scale renewable energy sources which inject their power in the low voltage distribution grid) and the importance of a reliable power supply for an increasing number of loads, mainly in the academic world there is a growing interest for the use of low voltage DC distribution grids.

Due to a number of technical evolutions, the importance of the old disadvantages related with use of DC-grids (which stimulated the rise of AC-grids about a century ago) have mainly disappeared when considering low voltage applications.

The power of a lot of distributed renewable energy sources is injected into the grid using a power electronic converter. For instance in case of photovoltaic panels generating a DC voltage, it is possible to use of DC-AC-converter in case of an AC-grid. However, by using a DC-DC-converter it is also possible to inject the power of the photovoltaic panels into a DC-grid. Sometimes, it is even possible to inject the power of photovoltaic panels into the DC-grid without any power electronic converter in case of a DC-grid with an appropriate voltage level.

In general, no power will be generated using a traditional rotating DC-generator. In case of for instance a wind turbine or a generator driven by a combustion engine, an AC-voltage will be generated which will be converted into DC using an AC-DC-converter.

Due to the rise of power electronics, a DC-AC-converter can be used to supply for instance induction machines or permanent magnet synchronous motors (PMSM). The use of a DC-grid does not anymore imply the need to use DC-motors having a commutator and carbon brushes.

Power electronic DC-DC-converters can be used to connect two DC-busses having different voltage levels which compensates the lack of transformers.

Using semiconductor devices, it is also possible to interrupt a DC-current although there are no zero-crossings of the current. Also in case of fault currents, circuit breakers can be used to interrupt overload currents or short circuit currents [Sannino, Gregory] .

3.1: Advantages when using low voltage DC grids

The rise of small scale renewable energy resources (photovoltaic panels, micro wind turbines, small scale CHP installations, …) increases the need to store energy. Especially when working in island mode, the storage capacity is important in order to maintain the power balance. In case the generated power is larger than the consumed power, the excess of power is stored using for instance batteries of fuel cells. In case the generated power is smaller than the consumed power, the batteries of fuel cells provide the additionally required power. Since batteries and fuel cells are DC-sources, the use of a DC-grid instead of an AC-grid can be useful in order the decrease the number of power electronic converters.

Figure 3: Photovoltaic panels mounted on the roof of a private household

The importance of energy storage devices is growing since the number of sensitive loads is increasing. In industry, in commercial premises, in service businesses and also in residential complexes the number of for instance power sensitive computers and communication devices in increasing. In order to obtain a reliable power supply, UPS systems (Uninterruptible Power Supply) are needed. The batteries and other storage devices (fuel cells, ultra capacitors) which are crucial in these UPS systems generally provide a DC voltage which stimulates the use of local DC-grids.

When using photovoltaic panels, loaded batteries, fuel cells, … to supply power, they generate a DC-voltage which is commonly converted into an AC-voltage in order to obtain a 50 Hz or a 60 Hz grid. Internally, a lot of loads do not need an AC-voltage but a DC-voltage implying they are commonly equipped with a transformer in combination with a rectifier or a rectifier in combination with a DC-DC-converter. By using a DC-grid instead of an AC-grid, a lot of converters can be eliminated. Since power electronic converters require an investment, account for power losses (especially giving a low efficiency when the load is low) and account for EMC-related problems, reducing the number of converters is useful.

3.2: The structure of AC and DC local grids

Figure 4 compares the structure of a local low voltage AC-grid (part (a) on the left) with the structure of a local low voltage DC grid (part (b) on the right) indicating the structure of the DC grid is more straightforward.

Figure 4: Comparison between AC-grid and DC-grid (source: Sannino)

The AC-grid in Figure 4 requires DC-AC-converters to inject the powers of solar panels and fuel cells. Natural gas micro turbines generally rotate at a very high speed and produce energy at a high frequency implying a two-stage frequency converter is needed. Notice also the connection between the local AC-grid and the public AC-grid.

Some loads of the AC-grid are fed without the use of a converter, but a lot of loads need a converter. For instance, digital electronic equipment needs a limited DC-voltage which is obtained by cascading two converters i.e. an AC-DC-converter (a rectifier) and a DC-DC-converter. Sensitive loads are fed by a DC-voltage while using an UPS system. The second converter in the UPS system can also be a DC-AC-converter in case the sensitive load requires an AC-voltage.

By choosing an appropriate voltage level and using a DC-grid, solar panels and fuel cells can be able to inject their power without the need for a power electronic converter. The same remark applies for batteries. A micro turbine only needs one single AC-DC-converter. Sensitive loads can be fed by the DC-grid without the need for converters or separate UPS-systems. Only one large battery is sufficient to obtain the UPS-property for all the sensitive loads. The digital electronic equipment is fed by one single step-down DC-DC-converter (instead of two cascaded converters). Some loads are fed by the DC-grid without the need for converters. If there are AC-loads (for instance induction motors), they need a DC-AC-converter which is not needed in case of an AC-grid. When connecting with the public AC-grid also an additional converter is needed. However in general, less converters are needed which accounts for less power losses in these converters.

A similar comparison between an AC-grid and a DC-grid is visualized in Figure 5 and Figure 6. Figure 5 visualizes a local AC-grid which needs a larger number of converters than the local DC-grid visualized in Figure 6.

Figure 5: Structure of a local AC-grid

Figure 6: Structure of a local DC-grid

4: The DC-voltage level

When realizing a local DC-voltage grid, an appropriate choice of the voltage level is important. When using a higher voltage level, the current will be lower in order to supply the same required power. This implies the voltage drops decrease and also the power losses in the grid conductors decrease. At the other hand side, using a higher voltage level has implications concerning safety issues and the number of power electronic converters which are needed.

In order to allow standardization, not all voltage levels are possible. In reality the standardized voltages of 326 VDC, 230 VDC, 120 VDC, 48 VDC, 24 VDC, 12 VDC and finally 6 VDC can be taken into consideration.

Many electronic appliances use a DC voltage which is obtained by rectifying the 230 VAC grid voltage using a diode rectifier. The rectified voltage level equals 326 VDC which is the peak value of the 230 VAC grid voltage. By supplying directly with 326 VDC, the same appliances can be used by simply removing the input rectifier.

When using 230 VDC, the RMS value is the same in comparison with the traditional 230 VAC grid voltage. This is useful when feeding resistance equipment like heating resistors or incandescent lamps (in case they are still used). In such situations, no changes to the resistor appliances are needed.

By using a 120 VDC voltage level, no protection against indirect contacts (contacts with exposed conductive parts which have become live under fault conditions) is required. This makes the system simpler due to the absence of grounding.

By using a 48 VDC voltage level, even no protection against direct contacts (contacts with live parts) is needed. This normalized voltage is also used to supply a lot of communication devices implying a lot of devices adapted to a 48 VDC supply are available on the market which is an important advantage.

Also 24 VDC, 12 VDC and 6 VDC are existing standardized voltage levels which are safe to be used. They are useful when considering small power systems (for instance in a car or to supply pay and display machines) but when considering the powers used in a domestic or a similar situation, they are not useful. Due to the low voltage level, the required currents are too high implying the voltage drops and the copper losses are also too high.

5: Voltage drops and power losses in local AC and DC-grids

A practical application of a distribution system has been realized in the Department of Electric Power Engineering at the Chalmers University of Technology in Gothenburg (Sweden) [Sannino]. The electric diagram of the system is visualized in Figure 7. The system includes 15 offices with a total of 26 people. Mainly typical office loads like computers and lighting are fed. There is also a copy room with a printer, a fax and a

copy machine. There is also a lunchroom equipped with a refrigerator, a cooker, an exhaust fan, a dishwasher, a coffee machine, two microwave ovens and a water boiler.

Figure 7: Electric diagram of the low voltage grid at Chalmers university

In Figure 7, the majority of the loads are single phase loads (230 VAC). Only the high power three-phase loads of the kitchen (like the coffee machine and the cooker) are fed by a three-phase grid (having a 400 VAC line-to-line voltage level). Using this three phase grid instead of a single phase supply, reduces the voltage drops and the copper losses.

By replacing the AC-grid of Figure 7 by a DC-grid where either 326 VDC, 230 VDC, 120 VDC or 48 VDC voltage levels are considered, it is possible to compare the properties of the AC-grid and the DC-grids. Also the three phase connections feeding the high power loads are replaced by a DC-connection.

Figure 7 and Figure 8 give an overview of all the loads with their consumed active powers and their cosφ. This allows to calculate the consumed active and reactive currents in the AC-grid. Figure 7 also visualizes the lengths of the conductors and when the cross sections of these conductors are known, the voltage drops and the copper losses in the AC-grid can be calculated. Information about the cross sections of the cables is given in Figure 9 (the numbers of the feeders are indicated in Figure 7).

Figure 8: Overview of load power consumption and power factor

One “office unit” consists of a computer with a monitor (consuming 180 W) and a table lamp (consuming 30 W). The total power consumed by the system is almost 30 kW. Notice the impedance Z in Figure 7 between the substation transformer and the switchgear. This impedance (0.443 + j0.07 Ω/km at 20°C) represents 20 m of 70 mm2

cables implying a maximum current of 145 A.

The cable between the substation transformer and the switchgear can be considered in AC and DC. When considering DC, the rectifier is placed immediately after the transformer. If the DC cable between transformer/rectifier and switchgear accounts for a voltage drop which is too high, it is also possible to place the rectifier at the switchgear and still use the AC-connection between the transformer and the switchgear.

Figure 9: Load and feeder data of the grid visualized in Figure 7

5.1: Calculating the voltage drops and the copper losses

Consider a single-phase AC-grid having a voltage level E (in the present application, E = 230 V) and a load consuming an active power P and a reactive power Q = P tgφ. The resistance of the cable is given by R (at 80°C) and its reactance is given by X (for instance based on a specific reactance x = 0.1 Ω/km). The voltage drop ΔV is given by

ΔV=2(R PE+X PEtgφ)

and the power losses ΔP are given by

ΔP=2 Rcos2φ

P2

E2

Consider a three phase voltage having a phase-to-ground voltage E (E equals 230 V and the line-to-line voltage equals √3 E). The load consumes a three phase active power P and a three phase reactive power Q = P tgφ (the three-phase load is assumed to be a symmetrical load). In case the cable has a resistance R and an inductance X, the voltage drop ΔV is given by

ΔV= 1√3 (R PE +X P

Etgφ)

and the power losses ΔP are given by

ΔP= R3 cos2φ

P2

E2

Consider a DC-grid implying only active power P and no reactive power Q is consumed. The power P of the DC-load is assumed to be equal to the active power of the AC-load. The voltage drop ΔV and the power losses ΔP are given by

ΔV=2 R PV DC

and

ΔP=2 R P2

V DC2

in case V DC is the DC-grid voltage.

5.2: Simulation results

Figure 10 shows the calculated voltage drops and the calculated power losses (copper losses) when the AC-grid is used to supply all loads. The voltage drops and the power losses are expressed as percent values. In case of the single-phase parts, the voltage drop is referred to the line-to ground rated voltage E. In case of the three-phase parts, the voltage drop is referred to the line-to-line rated voltage. Power losses are referred to the total load power for each feeder. For the cable, the power losses are referred to the system total power.

Columns 5 and 6 show the results (voltage drops and power losses) for the individual feeders. Columns 7 and 8 show the results (voltage drops and power losses) from the substation i.e. the cable connection between the transformer and the switchgear is included.

In Figure 10 (and also in the figures giving similar results when considering the DC-grid), some cells are highlighted. This indicates the feeder either exceeds the allowed 5% limit for the voltage drop or exceeds the maximum current (exceeding the maximum current implies an excessive heating of the cable decreasing the life expectancy of the insulation material). In case of a cable section of 1.5 mm2, a

maximum current of 14 A is allowed. In case of a cable section of 2.5 mm 2, a maximum current of 20 A is allowed. In case of a cable section of 70 mm 2, a maximum current of 145 A is allowed.

Figure 10: Voltage drop and power losses for the AC-supply

Notice in Figure 10 the maximum allowed voltage drop and the maximum allowed current are never exceeded. This situation can be compared with the situation visualized in Figure 11 where a DC voltage level of 326 V is used. Also in Figure 11, the maximum allowed voltage drop is never exceeded. Feeder 4 requires a current of 14.417 A but since a cable section of 2.5 mm2 is used, no problem occurs. The cable requires a current of 91.681 A but since a cable section of 70 mm2 is used, no problem occurs. In Figure 11 with 326 VDC, the maximum allowed current is never exceeded.

By decreasing the voltage level with a factor √2, the required current increases with a the same factor √2. This increases the absolute voltage drop with a factor √2, but it doubles the relative voltage drop. The increase of the current also doubles the power losses in the conductors. Indeed, when comparing the results in Figure 12 with the results in Figure 11, it is obvious that

The relative voltage drops double.

The power losses double. The currents increase with a factor √2.

Figure 11: Voltage drop and power losses for the DC-supply (326 VDC)

When considering the results in Figure 12, the maximum allowed current is slightly exceeded in feeder 4 (20.435 A in a 2.5 mm2 cable allowing 20 A). The maximum allowed voltage drop of 5% is never obtained.

The power losses visualized in Figure 12 in case of 230 VDC are in general slightly lower than the corresponding losses in case of 230 VAC (Figure 10) (when excluding the three phase loads fed by a three-phase cable). Indeed, in case of an AC-grid some loads consume reactive current which increases the copper losses. The lower the power factor, the higher the copper losses (this is for instance the case when considering the dish washer fed by feeder 19).

The results of Figure 11 and Figure 12 show a DC-grid with voltage levels of 326 VDC or 230 VDC can be used and the cables used in the existing AC-grid can be used. By reducing the voltage level to 120 VDC, the voltage drops increase and the copper losses increase as visualized in Figure 13.

When using 120 VDC, the maximum allowed current is exceeded in feeder 3, feeder 4, feeder 22 and the 70 mm2 cable. Notice also in quite a lot of feeders, the voltage drop exceeds the maximum allowed 5% limit. Feeder 3 and feeder 4 are three-phase cables where changing into DC gives significantly larger voltage drops. Feeders 7, 9 and 13 are quite long cables (80 m, 40 m and 30 m) implying quite large voltage drops.

When considering the 120 VDC system, it is useful to place the rectifier at the switchgear and not immediately after the transformer in order to reduce the voltage drop across the 70 mm2 cable. It is also useful to realize a voltage control system in order maintain a constant voltage at the DC-side.

Figure 12: Voltage drop and power losses for the DC-supply (230 VDC)

When reducing the DC voltage level to 48 VDC (see Figure 14), a further increase of the required currents is obtained. This implies in the great majority of the feeders the maximum allowed current is exceeded. Similarly, in almost all feeders the allowed voltage drop of 5% is exceeded. A limited voltage of 48 VDC can only be used in case

The consumed powers are lower. The lengths of the cables are smaller. The cables have cross-sections which are sufficiently large i.e. the existing

conductors must be replaced.

Small DC-voltages like 48 VDC, 24 VDC or even 6 VDC are standardized but they are only applicable in case the distances are sufficiently short and/or the consumed powers are sufficiently small.

Figure 13: Voltage drop and power losses for the DC-supply (120 VDC)

6: Maximum voltage drop and maximum current limitations

Depending on the voltage level and the feeder length, the maximum power which can be fed can be limited by

the maximum allowed current, the voltage drop limitation.

The most restrictive of these two conditions is dominant. The cable section S determines the maximum allowed current Imax which limits the heat dissipation and the temperature in the cable in order to avoid a decreased life expectancy of the cable insulation. In case of a voltage level VDC, the maximum power

Pmax1=ImaxV DC

does not depend on the cable length as visualized in Figure 15 and Figure 16. This cable length independent Pmax1 increases as the cable section S increases and as the voltage level VDC increases.

Figure 14: Voltage drop and power losses for the DC-supply (48 VDC)

The maximum power than can be supplied without exceeding the 5% voltage drop is given by

Pmax2=0.05 SV DC

2

2 ρL

where ρ is the specific resistance at 80°C and L is the cable length. In case of a longer cable, the maximum power Pmax2 decreases. The actual power which can be supplied is the minimum of Pmax1 and Pmax2. In case of smaller cable lengths, Pmax1 is dominant. In case of larger cable lengths, Pmax2 is dominant as visualized in Figure 15 and Figure 16.

Figure 15 visualizes Pmax in case of a DC-grid with a cross-section S = 1.5 mm2 and a cross-section S = 2.5 mm2. The thin line is valid when the rated voltage equals 326 VDC. The thick line is valid when the rated voltage equals 230 VDC. Figure 16 visualizes Pmax in case of a DC-grid with a cross-section S = 1.5 mm2 and a cross-

section S = 2.5 mm2. The thin line is valid when the rated voltage equals 120 VDC. The thick line is valid when the rated voltage equals 48 VDC.

Figure 15: Pmax in case of 326 VDC and 230 VDC

Figure 16: Pmax in case of 120 VDC and 48 VDC 7: Protection of low-voltage DC-systems

Since a DC-current has no natural zero crossings, interrupting a DC-current is more difficult than interrupting an AC-current. This observation is valid in case of a normal load current, in case of an overload current and in case of a short circuit current. Due to this reason there exist dedicated DC-contactors, DC circuit breakers and DC-fuses.

In a low-voltage DC-system, it is also possible to use AC circuit breakers provided that the ratings are adjusted by proper correction factors which are provided by the manufacturers.

A thermal-magnetic circuit breaker [Sannino, Gregory] allows to protect against overload currents using the thermal part of the current-time-characteristic. The tripping time is the same for AC and for DC applications since the bimetal sensor reacts to the RMS value of the current which is the same for AC and DC.

When protecting against short circuit currents, the circuit breakers opens as fast as possible. Tripping occurs due to the electromagnetic force which is proportional with the square of the instantaneous value of the current.

Based on catalogs from Siemens or ABB [Sannino], information about rated voltages, rated currents and also the breaking capacity is available. When considering the project visualized in Figure 7 at Chalmers University of Technology, a sufficient breaking capacity was obtained. A more effective arc interruption can for instance be obtained by mounting poles in series (especially for voltages higher than 250 VDC). 8: Conclusion

Due to the existing dominance of AC-grids, they will not disappear very fast. However, the rise of distributed renewable energy sources and the need of a reliable power supply for an increasing number of sensitive loads will probably encourage the installation of local DC-grids. The competition between AC-grids and DC-grids has a growing attention of the academic world and the situation differs from the situation at the age of Edison and Tesla.

References

Agustoni A., Brenna M. and Tironi E., “High Quality DC local distribution network with photovoltaic generation and storage systems”, 7th Electrical Power Quality and Utilisation Conference (EPQU ‘03), September 17-19, 2003, Cracow, Poland, pp. 623-632.

Brenna M., Tironi E. and Ubezio G. “Proposal of a Local DC Distribution Network with Distributed Energy Resources”, 11th International Conference on Harmonics and Quality of Power ICHQP, 2004, pp. 397-402.

Ciezki J.G. and Ashton R.W., “Optimal Design of a Soft-Switched DC-DC Converter For a Prototype DC Distribution System”, 35th Annual IEEE Power Electronics Specialists Conference, Aachen, Germany, 2004, pp. 1731-1736.

Gregory G.D., “Applying low-voltage circuit breakers in direct current”, IEEE Transactions on Industry Applications, vol. 31, pp. 650-657, July/Augustus 1995.

Karlsson P. and Svensson, “DC bus Voltage Control for a Distributed Power System”, IEEE Transactions on Power Electronics, vol. 18, pp. 1405-1412, November 2003.

Nilsson D. and Sannino A., “Efficiency analysis of low- and medium-voltage DC distribution systems”, IEEE Power Engineering Society General Meeting, PES 2004, vol. 2, pp. 2315-2321.

Sannino A., Postiglione G. and Bollen M.H.J., “Feasibility of a DC Network for Commercial Facilities”, IEEE Transactions on Industry Applications, vol. 39, pp. 1499-1507, September/October 2003.