DC Microgrids

Christine Chan

CSE 291

5/29/2013

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Introduction What constitutes a DC microgrid?

• Power generation – PV, wind, fuel cells

• Electrical storage – batteries, super capacitors

• Distribution – wiring and electronic control

• Loads – computers, appliances, lighting

Use cases

1. Small-scale residential

2. Remote/sparsely populated areas

3. Commercial/datacenters

http://www.renewableenergyfocus.com/view/3199/dc-microgrids-a-new-source-of-local-power-generation/ Farhangi, Hassan. "The path of the smart grid." Power and Energy Magazine, IEEE 8.1 (2010): 18-28.

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Why DC microgrids?

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• Put small-scale generation closer to load

• Enable demand-side management, cut grid reliance

• Efficient distribution - current in a 380Vdc grid is 61% of current in 230V single phase grid losses are 37% of the AC network

Becker, Dustin J., and B. J. Sonnenberg. "DC microgrids in buildings and data centers." Telecommunications Energy Conference (INTELEC), IEEE, 2011.

Emerge Alliance (2011)

Why DC microgrids? • Many renewable sources generate DC, e.g.: photovoltaic, wind, fuel cells

• Fewer conversions - increase conversion efficiency – DC-to-AC inversion 85%; AC-to-DC rectifying: 90%; DC-to-DC conversion: 95%

• Simpler power-electronic interfaces, fewer points of failure

• Easily stored in batteries

4 Tim Martinson, “380 VDC for Data Center Applications Update: There’s More to the Story than Efficiency Improvements” Universal Electric Corp (2011) Shah, K., et al. "Smart efficient solar DC micro-grid." Energytech, 2012 IEEE. IEEE, 2012.

Wireless residential/commercial use

• Solar PV panel [40W]

• Morningstar PS-15M wireless charge controller

• Sensors for ambient light/heat

• DC ceiling fan motor [24Vdc]

• Fluorescent light with DC dimmable ballast

• 12V battery bank

5 Shah, K., et al. "Smart efficient solar DC micro-grid." Energytech, 2012 IEEE. IEEE, 2012.

Rural electrification • Sparsely populated areas with no/little grid connectivity

• Heavy reliance on PV from existing solar home systems (SHS), mixture of consumers with/without generation

• 120V grid lines; 24V loads

• Humanitarian potential, e.g. solar trailers used in Haiti as mobile charging stations for residents

Sarker, Md Junayed, et al. "DC micro-grid with distributed generation for rural electrification." Universities Power Engineering Conference (UPEC), 2012 47th International. IEEE, 2012.

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Commercial/standardized use

The "Zero Energy Building Model“

• Occupied Space: [24Vdc] Commercial interiors

• Data/Telecom Centers: [380Vdc] Hybrid use of AC and DC

• Outdoor: [24/380Vdc] Exterior lighting, signage, and electric vehicles

• Building Services: [380Vdc] HVAC, motor loads and high bay/industrial applications

7 Emerge Alliance (2011)

Voltage levels • Dependent on allowable/normal voltages for

ICT equipment

• Guide testing for abnormal operating conditions and stress (inrush currents, load steps)

8 Computer Business Equipment Manufacturers Association – CBEMA, Electric Power Research Institute (EPRI), Nippon Telegraph and Telephone (NTT), Power Standards Lab (PSL)

Voltage tolerance envelope for traditional AC-powered computers vs. equivalent DC curve

Voltage levels

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Voltage levels

• LVDC: 5/48/380Vdc

• HVDC: electric ships and HV dc transmission (thousands of Vdc)

Multiple efforts to standardize:

• Emerge Alliance + EPRI task force

– 380Vdc standard to cover telecom and building distribution.

• European Telecommunications Standards Institute (ETSI)

– Standard for 400V dc distribution standard for telecom

• lEC SG4: 400Vdc (LVDC) distribution – goes up to 1500Vdc

10 Becker, Dustin J., and B. J. Sonnenberg. "DC microgrids in buildings and data centers." Telecommunications Energy Conference (INTELEC), IEEE, 2011.

Protection • Voltage stability calculations

and challenges are exacerbated as DC/AC distributions coexist

• Connected to the AC grid at point of common coupling (PCC)

• With converters with bidirectional power flow

11 D. Thukaram, L. Jenkins, and K. Visakha, “Optimum allocation of reactive power for voltage stability improvement in ac-dc power systems,” IEE Proc. Gener. Transm. Distrib., vol. 153, pp. 237-246, Mar. 2006.

Protection

• Local sources are often low-voltage, have different voltage amplitude and frequencies

• Need power-electronic converters

• Grounding

– Ungrounded, high resistance grounded, or low resistance grounded

– Grounded to +/- poles or to middle point of the converter

• Protection devices

– Fuses: appropriate for residential/small scale use

– Circuit breakers: longer breaker operation time than AC

– Power-electronic protection devices: faster than mechanical switches but higher losses

• Protective relays and measurement

12 Salomonsson, Daniel, L. Soder, and Ambra Sannino. "Protection of low-voltage DC microgrids." Power Delivery, IEEE Transactions on 24.3 (2009): 1045-1053.

References

• N. Hatziargyriou, H. Asano, R. Iravani, and C. Marnay, “Microgrids,” IEEE Power Energy Mag., vol. 5, no. 4, pp. 78–94, Jul./Aug. 2007.

• B. Patterson, "Improved Efficiency & Renewable Energy Adoption via LVDC Microgrid Power Distribution," NEMA LVDC Workshop 201 1, Washington DC

• Y. Ito, Y. Zhongqing, and H. Akagi, “DC micro-grid based distribution power generation system,” in Proc. IEEE IPEMC, 2004, vol. 3, pp. 1740–1745.

• H. Kakigano, Y. Miura, T. Ise, and R. Uchida, “DC micro-grid for super high quality distribution—System configuration and control of distributed generations and energy storage devices,” in Proc. IEEE IPEMC, 2004, vol. 3, pp. 1740–1745.

• D. Thukaram, L. Jenkins, and K. Visakha, “Optimum allocation of reactive power for voltage stability improvement in ac-dc power systems,” IEE Proc. Gener. Transm. Distrib., vol. 153, pp. 237-246, Mar. 2006.

• Uriarte, Fabian M., et al. "Development of a series fault model for dc microgrids." Innovative Smart Grid Technologies (ISGT), 2012 IEEE PES. IEEE, 2012.

• Shah, K., et al. "Smart efficient solar DC micro-grid." Energytech, 2012 IEEE. IEEE, 2012. • Balog, Robert S., Wayne W. Weaver, and Philip T. Krein. "The load as an energy asset in a

distributed DC smartgrid architecture." Smart Grid, IEEE Transactions on 3.1 (2012): 253-260. • Becker, Dustin J., and B. J. Sonnenberg. "DC microgrids in buildings and data

centers." Telecommunications Energy Conference (INTELEC), 2011 IEEE 33rd International. IEEE, 2011.

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Guerrero – Hierarchical control

• Large conventional power systems: high inertia, inductive loads

• Power-electronics-based microgrids: no inertia, mainly resistive loads

• 1) The primary control is based on the droop method, including an output-impedance virtual loop;

• Inner control of distributed generation, adding virtual inertias and controlling output impedance

• Control loops applied to connect voltage source inverters (VSI) in parallel in uninterruptible power supply (UPS) systems to avoid mutual control wires while obtaining good power sharing. However, although this technique achieves high reliability and flexibility, it has several drawbacks that limit its application.

• 2) Secondary control restores the deviations produced by the primary control

• 3) Tertiary control manages the power flow between the MG and the external electrical distribution system.

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