1. EE 462L dc and ac Power Distribution Systems Fall 2008

2. 2 © Alexis Kwasinski, 2012 History Competing technologies for electrification in 1880s: Edison: dc. Relatively small power plants (e.g. Pearl Street Station). No voltage transformation. Short distribution loops – No transmission Loads were incandescent lamps and possibly dc motors (traction). “Eyewitness to dc history” Lobenstein, R.W. Sulzberger, C. Pearl Street Station: 6 “Jumbo” 100 kW, 110 V generators

3. 3 © Alexis Kwasinski, 2012 History Competing technologies for electrification in 1880s: Tesla: ac Large power plants (e.g. Niagara Falls) Voltage transformation. Transmission of electricity over long distances Loads were incandescent lamps and induction motors. http://spiff.rit.edu/classes/phys213/lectures/niagara/niagara.html Niagara Falls historic power plant: 38 x 65,000 kVA, 23 kV, 3-phase generatods

4. 4 © Alexis Kwasinski, 2012 History Edison’s distribution system characteristics: 1880 – 2000 perspective Power can only be supplied to nearby loads (< 1mile). Many small power stations needed (distributed concept). Suitable for incandescent lamps and traction motors only. Higher cost than centralized ac system. Used inefficient and complicated coal – steam actuated generators (as oppose to hydroelectric power used by ac centralized systems). Not suitable for induction motor. Cannot be transformed into other voltages (lack of flexibility). dc MOTORdc GENERATOR Vdc,1 Vdc,2

5. 5 © Alexis Kwasinski, 2012 History Traditional technology: the electric grid: Parts: generation, transmission, and distribution. Centralized and passive architecture. Extensive and very complex system. Complicated control. Not reliable enough for some applications. Relatively inefficient. Stability issues. Vulnerable. Lack of flexibility.

6. 6 © Alexis Kwasinski, 2012 History Edison’s distribution system characteristics: 2000 – future perspective Power supplied to nearby loads is more efficient, reliable and secure than long power paths involving transmission lines and substations. Many small power stations needed (distributed concept). Existing grid not suitable for dc loads (e.g., computers) or to operate induction motors at different speeds. Edison’s system suitable for these loads. Power electronics allows for voltages to be transformed (flexibility). Cost competitive with centralized ac system. Can use renewable and alternative power sources. Can integrate energy storage. Can combine heat and power generation.

7. 7 © Alexis Kwasinski, 2012 Typical configuration: Total power consumption: > 5 MW (distribution at 208V ac) Conventional (ac) datacenters

8. 8 © Alexis Kwasinski, 2012 Conventional (ac) datacenters Data centers represent a noticeable fast increasing load. Increasing power-related costs, likely to equal and exceed information and communications technology equipment cost in the near to mid-term future. Servers are a dc load 860 W of equivalent coal power is needed to power a 100 W load

9. 9 © Alexis Kwasinski, 2012 Use of 380 Vdc power distribution for: Fewer conversion stages (higher efficiency) Integration of local sources (and energy storage). Reduced cable size New (dc) datacenters

10. 10 © Alexis Kwasinski, 2012 Data centers efficiency comparison dc vs. ac Brian Fortenbery and Dennis P. Symanski, GBPF, 2010 A 380Vdc power distribution standard is currently under study by the IEC

11. 11 © Alexis Kwasinski, 2012 Many “small” distributed data centers powered locally and with a coordinated operation Energy is used more effectively. Generation inefficiencies is energy that is not harvested (i.e. converted), contrary to inefficiencies in conventional power plants which represent power losses. New distributed (dc) datacenters

12. 12 © Alexis Kwasinski, 2012 Utility dc distribution Jonbok Bae, GBPF 2011

13. 13 © Alexis Kwasinski, 2012 Traditional grid availability: Approximately 99.9 % Availability required in critical applications: Approximately 99.999% Traditional Electricity Delivery Methods: Reliability

14. 14 © Alexis Kwasinski, 2012 Traditional Electricity Delivery Methods: Reliability http://www.oe.netl.doe.gov/docs/katrina/la_outage_9_3_0900.jpghttp://www.gismonitor.com/news/newsletter/archive/092205.php http://www.nnvl.noaa.gov/cgi-bin/index.cgi?page=items&ser=109668 Large storms or significant events reveal the grid’s reliability weaknesses: Centralized architecture and control. Passive transmission and distribution. Very extensive network (long paths and many components). Lack of diversity.

15. 15 © Alexis Kwasinski, 2012 dc vs. ac in microgrids Microgrids Microgrids are considered to be locally confined and independently controlled electric power grids in which a distribution architecture integrates loads and distributed energy resources—i.e. local distributed generators and energy storage devices—which allows the microgrid to operate connected or isolated to a main grid

16. 16 © Alexis Kwasinski, 2012 FUEL CELL ENERGY STORAGE PV MODULES AIR CONDITIONER REFRIGERATOR (LOAD) WIND GENERATOR MAIN DC BUS EPA 430-F-97-028 dc Homes dc in homes allows for a better integration of distributed generation, energy storage and dc loads. With a variable speed drive air conditioners can be operated continuously and, hence, more efficiently (about 50%) LED LIGHTS (DC) ELECTRIC VEHICLE

17. 17 © Alexis Kwasinski, 2012 Comparison of ac vs. dc systems Advantages of dc: Higher availability At least 5 % more efficient than ac Enables for more dense systems Easier control Easier to connect in parallel More flexible architectures Most critical loads and future loads are, actually, dc Most local sources are dc (for diverse input) Allows for a simpler and usually direct way to integrate energy storage Power quality control Advantages of ac: Usually tends to be more cost efficient than dc (economics of scale) Simpler circuit protections Adds one more control degree of freedom

18. 18 © Alexis Kwasinski, 2012 dc power architectures in electric ships

19. 19 © Alexis Kwasinski, 2012 dc power architectures in electric ships Circuit protection: conventional approach

20. 20 © Alexis Kwasinski, 2012 dc power architectures in electric ships Circuit protection: power electronics or solid state switches approaches.

21. 21 © Alexis Kwasinski, 2012 Dc systems faults management In power electronic distributed architectures, faults may not be properly detected because, without a significant amount of stored energy directly connected to the system buses, short-circuit currents are limited to the converter maximum rated current plus the transitory current delivered by the output capacitor. If the latter is not high enough, the protection device will not trip and the fault will not be cleared. In this case, the converter will continue operation delivering the maximum rated current but with an output voltage significantly lower than the nominal value. Consider the following situation

22. 22 © Alexis Kwasinski, 2012 Dc systems faults management With C = 600 μF, the fault is not properly cleared and voltage collapse occurs for both loads.

23. 23 © Alexis Kwasinski, 2012 Dc systems faults management To avoid the situation described above, the converter output capacitance has to be dimensioned to deliver enough energy to trip the protection element. One approach is to calculate the capacitance based on the maximum allowed converter output voltage drop. However, this is a very conservative approach that often leads to high capacitance values. Another option is to calculate the capacitance so that it can store at least enough energy to trip the protection device, such as a fuse. Fuse-tripping process can be divided into two phases: pre-arcing Lasts for 90% of the entire process. During this phase, current flows through the fuse, which heats up. arcing the fuse-conducting element melts and an arc is generated between the terminals. The arc resistance increases very rapidly, causing the current to drop and the voltage to increase. Eventually the arc is extinguished. At this point, the current is zero and the voltage equals the system voltage.

24. 24 © Alexis Kwasinski, 2012 Dc systems faults management The energy during pre-arcing is where T F is the total fault current clearing time, R F is the fuse resistance before melting, and I C,F is the limiting case capacitor current during the fault. I C,F equals the fault current less the sum of the converter current limit and other circuit currents. For larger capacitances than the limit case, the converter current may not reach the rated limit value, so I C,F might be slightly higher than in the limit case..If a linear commutation is assumed, the portion of the arcing phase energy supplied by the capacitor is Thus,

25. 25 © Alexis Kwasinski, 2012 Dc systems faults management With V S = V F = 50 V, I C,F = 135 A, R F = 1 mΩ, and considering a typical value for T F of 0.1 s, the minimum value of C is 900 μF. If the previous system is simulated with C = 1mF, then Ringing on R 2 occurring when the fault is cleared can be eliminated by adding a decoupling capacitance next to R 2

26. 26 © Alexis Kwasinski, 2012 Dc systems faults management Additional simulation plots

27. 27 © Alexis Kwasinski, 2012 Series faults in ac systems Series faults occur when a cable is severed or a circuit breaker is opened, or a fuse is blown…. Then an arc is observed between the two contacts where the circuit is being opened. The arc is interrupted when the current is close to zero. Due to cable inductances, voltage spikes are observed when the arc reignites.

28. 28 © Alexis Kwasinski, 2012 Series faults in ac systems Visually, arcs in ac series faults are not very intense

29. 29 © Alexis Kwasinski, 2012 Series faults in dc systems In dc arcs last longer (because there are no zero crossings for the current) but no voltage spikes are generated.

30. 30 © Alexis Kwasinski, 2012 Series faults in dc systems Dc arcs last longer than ac ones, are much more intense and may damage the contacts.