FEASIBILITY OF DC-MICROGRID
FOR OFF-GRID COMMUNITIES
ELECTRIFICATION
Presented by
Dr. HS Che UM Power Energy Dedicated Advanced Centre (UMPEDAC)
University of Malaya
Malaysia
The 2nd Smart Villages Workshop – 27 Jan 2015
Background
Electricity supply will be an empowering infrastructure for off-grid
communities.
Due to the remote location, and low population density, power supply
via grid is not economically viable.
Diesel generator is too costly for most rural household.
Local generations (particularly renewable sources such as solar, wind,
hydro etc) should be used.
Forms a self-sustaining micro-grid system for rural community.
Operated by the community, for the community!
The 2nd Smart Villages Workshop – 27 Jan 2015
Research Question
The research team sees that:
1) Most of the off-grid communities do not have existing ac
grid infrastructure, and
2) Renewable energy source (like PV) and battery storage
systems are inherently dc in nature.
Can the use of dc micro-grid
actually be more feasible and
advantageous than ac micro-
grid?
The 2nd Smart Villages Workshop – 27 Jan 2015
Ac micro-grid (Standalone)
Genset
PV
Dc/dc
Inverter
ESS
Dc/dc
Inverter
House
(Load)
Pico
Hydro
Voltage
Regulator
House
(Load) …
Ac grid
50 Hz
230 V
Fig 1. Example of a standalone ac micro-grid topology.
The 2nd Smart Villages Workshop – 27 Jan 2015
Dc micro-grid (Standalone)
Genset PV
Dc/dc Rectifier
ESS
Dc/dc
House
(Load)
Pico
Hydro
House
(Load) …
Rectifier
Dc grid
110 V / 380 V?
Fig 2. Example of a standalone dc micro-grid topology. (Assuming dc household loads)
The 2nd Smart Villages Workshop – 27 Jan 2015
Dc micro-grid (Standalone)
Genset PV
Dc/dc Rectifier
ESS
Dc/dc
House
(Load)
Pico
Hydro
…
Rectifier
Dc grid
110 V / 380 V?
Inverter
House
(Load)
Inverter
Fig 3. Example of a standalone dc micro-grid topology. (Assuming ac household loads)
The 2nd Smart Villages Workshop – 27 Jan 2015
Developments in Dc-micro Grid
Dc grid system has gained much interest recently, not only from the academia, bur in the industry as well.
• Hierarchical control (lower level controlled by higher level). Inner - v & i control, primary - virtual Z control for two or more sources working in parallel, secondary - Restore f & v deviation, tertiary - power flow control among multiple microgrid. [1][3][4][5]
• Variation are using fuzzy logic [2] and artificial neural network [10]
• The control strategy for DC microgrid varies from each other due to the types of element in the grid i.e.
• 3 operation modes which are utility, storage, generation mode [3]
• 4 operation modes. quite similar to 3 operation mode but in addition, generation mode seperates into 2 modes. [4]
• 8 operation modes with 23 transition [5]
The 2nd Smart Villages Workshop – 27 Jan 2015
Fig 4: (a) DC grid for data center (b) operation modes transition [5]
The 2nd Smart Villages Workshop – 27 Jan 2015
Disadvantages
Safety and Protection Issues
• Generally, AC is safer but safety standards are available. i.e. IEC-309 switchboard is for rated up to 450 Vdc 10 A arc flash. [7]
• IEC 23E WG 2 has data to back up the logic that 380 Vdc is as safe as up to 250 Vac. [7]
• Protection equipments are mainly designed for ac systems. Devices for dc systems may be more costly.
Unfamiliar Operation
• lack of rotational inertia - frequency instability [13]
• high share of renewable – unpredictable.
• resistive line - voltage deviation is undesirable [11][14] and higher short-circuit current.
The 2nd Smart Villages Workshop – 27 Jan 2015
Advantages – higher efficiency
Higher Efficiency over ac grid
• comparison between 2 phase +-380 V DC grid & 3phase 230 V/400 V AC grid
• Higher rated AC/DC rectifier has higher efficiency. i.e. eff. of <50 W rectifier is 95 %,
eff. of >300 W rectifier is 98% [6]
• 2 % of power saving for cable power loss and 56 % of copper saved at similar eff.
[6]
• 3 % of power saving when PV & battery connect to DC [6]
(a) 14 % loss for AC grid (b) 11 % loss for DC grid [6]
The 2nd Smart Villages Workshop – 27 Jan 2015
Advantages – higher efficiency
• First industry that involve is the 380 Vdc or +-190 Vdc data center [7]
• 8 % more eff (Intel),
• 15 % more eff (University of California),
• 10 % more eff ( Green.ch in Zurich)
- 7 % more eff (Lawrence Berkeley Naitional Laboratory) [15]
• DC grid in building(PV, battery, light, air cond., ac grid connection). 5
% more eff (Cologne Univerisity). [8]
• Individual dc/ac replaced by grid dc/ac (save 3 %)
• ac cable replaced by dc cable (save 2%)
The 2nd Smart Villages Workshop – 27 Jan 2015
Advantages
Better conductor and insulation utilization
Fig: Cross section area of dc and ac cable [7]
- For the same cable-cross sectional
area, the cable loss is higher in ac due
to the higher ac resistance over dc
resistance. (skin effect)
- Cable insulation for ac-grid needs to
be sized to the peak voltage, even
though the effective voltage (rms) that
transmit power is lower.
The 2nd Smart Villages Workshop – 27 Jan 2015
Advantages
No grid synchronization and less PQ problems
• There is no need for grid synchronization like in ac-grid. This facilitate
easier connection of renewable energy sources.
• Unlike ac grid, there is no reactive power (Q) or harmonics issues in
dc-grid.
• All these gives dc-grid better controllability
The 2nd Smart Villages Workshop – 27 Jan 2015
Additional
Dc-micro-grid can be more desirable for some specific
electrification problems in rural area.
Floating Hydro-generator
-Varying speed translate into varying
voltage magnitude and frequency
- Voltage magnitude can be mitigated (to a
certain extend, by voltage regulator), but
frequency variation causes flickering of
lights.
- The use of power electronic converter
can be a solution -> floating-hydro become
dc-source
The 2nd Smart Villages Workshop – 27 Jan 2015
Additional
3-phase to 1-phase Power
- Three-phase generators are easier to obtained (>1kW)
- House-holds are using single-phase ac-supply.
- Splitting the power into three separate single-phase supplies can cause
unbalance loading and low per-phase power.
- By rectifying the three-phase into dc (transmit via dc-grid) higher power
equipments can be used.
3-ph
Gen Rectifier
Inverter Load
The 2nd Smart Villages Workshop – 27 Jan 2015
Simulation of Dc-micro Grid
Simulated micro-grid topology
PV
Buck
boost
Buck-
boost
Battery
Buck
boost DC load
The 2nd Smart Villages Workshop – 27 Jan 2015
0 5 10 15 20 25 30 35 40 45 500.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.911 am solar radiation sampled at 1s
time(s)
kw
/m s
qr
time(s) 0-10 10-20
20-30
30-40
40-50
load (W)
10 110 210 310 10
pv 29 - 36.7 V 7.95 - 8.55 A PV Specs: 230 Wp, rated V=29 V, rated I=7.95 A, Uoc=36.7 V, Isc=8.55 A
battery 40 Ah 12 V
neglect line loss, efficiency, battery state of charge
average: 0.5657 kWh/m sqr
min: 0.00389 kWh/m sqr
max: 0.8443 kWh/m sqr
std. dev.:0.1432 kWh/m sqr (data from [16])
The 2nd Smart Villages Workshop – 27 Jan 2015
time (s) PV (ts=1 s) Battery(ts=1m s) load (ts=10 s)
remark
0-10 129.95 battery charge at max -10 A (-120 W)
10 129.95 - 120 -10 = 0 W
10-20 129.95 battery charge & discharge (0 W)
110 129.95 - 0 - 10 = 0 W
20-30 129.95 battery discharge at 6.667 A (80 W)
210 129.95 + 80 -210 = 0 W
30-40 129.95 battery discharge at max 10 A (120W)
310 129.95 +120 -310 = - 60 W
40-50 129.95 battery charge at max -10 A (-120 W)
10 129.95 - 120 -10 = 0 W
Load << PV
Avg Load = PV
Load > PV
Load >> PV
Load << PV
The 2nd Smart Villages Workshop – 27 Jan 2015
focus on bus voltage
0 5 10 15 20 25 30 35 40 45 5012.82
12.84
12.86
12.88
12.9
12.92
12.94
12.96
12.98
13battery terminal voltage
time(s)
voltage(V
)
0 5 10 15 20 25 30 35 40 45 500
5
10
15
20
25
30
35
40pv terminal voltage
time(s)
voltage(V
)
0 5 10 15 20 25 30 35 40 45 5012
14
16
18
20
22
24
26
28
30bus voltage
time(s)
voltage(V
)
0 5 10 15 20 25 30 35 40 45 5012
14
16
18
20
22
24
26
28
30bus voltage
time(s)
voltage(V
)
The 2nd Smart Villages Workshop – 27 Jan 2015
focus on battery i/o current
0 5 10 15 20 25 30 35 40 45 50-10
-5
0
5
10
15pv converter current
time(s)
curr
ent(
A)
0 5 10 15 20 25 30 35 40 45 50-15
-10
-5
0
5
10
15
20
time(s)
curr
ent(
A)
battery converter current
0 5 10 15 20 25 30 35 40 45 50-4
-2
0
2
4
6
8
time(s)
curr
ent(
A)
pv i/o current
0 5 10 15 20 25 30 35 40 45 50-15
-10
-5
0
5
10
15
20
25
30battery i/o current
time(s)
curr
ent(
A)
The 2nd Smart Villages Workshop – 27 Jan 2015
key points
• To prevent high cost of transmission, renewable sources are most suitable for off-grid application.
• DC micro-grid is favoured due to higher efficiency and lower copper cost.
• But DC micro-grid is lack of rotational inertia, high share of renewable and resistive lines.
• PV is unpredictable. Battery provides support for PV but battery behaves as a load and supply and has limited storage capacity. Standby generator (hydro/fuel cell) has to back up PV and battery.
The 2nd Smart Villages Workshop – 27 Jan 2015
Credits
Researchers:
Khaw Yan Ngee received Bachelor of Electrical Engineering (Hons) from Multimedia
University, Cyberjaya. He is currently working as research assistant in University of
Malaya.
Wooi-Ping Hew obtained his BEng and Masters (Electrical) degrees
from the University of Technology, Malaysia. He received his PhD
from the University of Malaya, Kuala Lumpur, Malaysia in 2000. He is
currently a Professor in UM Power Energy Dedicated Advanced
Centre (UMPEDAC), University of Malaya, Kuala Lumpur, Malaysia.
Dr. Hew is a Member of IET and a Chartered Engineer. His research
interests include electrical drives and electrical machine design
The 2nd Smart Villages Workshop – 27 Jan 2015
references • [1] Josep M. Guerrero, Juan C. Vasquez, Jose Matas, Luis Garcia de Vicuna, Miguel Castilla, “Hierarchical Control of Droop-Controlled AC
and DC Microgrids- A General Approach Toward Standardization”, IEEE Transactions on Industrial Electronics, Vol. 58, No.1, January 2011.
• [2] Nelson L. Diaz, Tomislav Dragicevic, Juan C. Vasquez, Josep M. Guerrero, “Intelligent Distributed Generation and Storage Units for DC Microgrids- A New Concept on Cooperative Control Without COmmuncations Beyong Droop Control”, IEEE Transactions on Smart Grid, Vol. 5, No. 5, September 2014.
• [3] Yunjie Gu, Xin Xiang, Wuhua Li, Xiangning He, “Mode-Adaptive Decentralized Control for Renewable DC Microgrid With Enhanced Reliability and Flexibility”, IEEE Transactions on Power Electronics, Vol. 29, NO. 9, September 2014.
• [4] Amir Khorsandi, Mojtaba Ashourloo, Hossein Mokhtari, “A Decentralized Control Method for a Low-Voltage DC Microgrid”,IEEE Transactions on Energy Conversion,Vol. PP, No.99, June 2014.
• [5] Daniel Salomonsson, Lennart Soder, Smbra Sannino, “An Adaptive Control System for a DC Microgrid for Data Centers”, IEEE Transactions on Industry Applications, Vol.44, No.6, November/December 2008
• [6] Ulrich Boeke, Roland Wei, ANton Mauder, Luc Hamilton, Leopold Ott, "Efficiency Advantages of +-380 V DC Grids in Comparison with 230 V/400 V AC Grids", ENIAC Joint Undertaking, May 2014
• [7] Guy AlLee and William Tschudi. "380 Vdc Brings reliability and Efficiency to Sustainable Data Centers", IEEE Power & Energy Magazine, Nov/Dec 2012.
• [8] Eberhard Waffenschmidt, Ulrich Boke, "Low Voltage DC Grids", Cologne Univerisity of Applied Sciences & Philips Research Eindhoven, Mar 2013.
• [9] Tine L. Vandoorn, Bart Meersman, Lieven Degroote, Bert Renders, Lieven Vandevelde, "A Control Strategy for Islanded Microgrids With DC-Link Voltage Control", IEEE Transactions on Power Delivery, Vol.26. No. 2, 2011.
• [10] Wenyuan Wang, Mike Barnes, "Power Flow Algorithms for Multi-Terminal VSC-HVDC With Droop Control", IEEE Transactions on Power Systems, Vol. 29, No. 4, July 2014
• [11] Boeke, Ulrich, and Matthias Wendt. "Comparison of low voltage AC and DC power grids." Philips Research, AE Eindhoven, The Netherlands, available online at: http://www. upn. se/htmlfiles/Glava/Referenser/Ref 201.
• [12] Arun Kumar et al, "Hydropower", Intergovermental panel on climate change(IPCC), Contrbution to Special Report Renewable Energy Sources(SRREN), Chatper 5, pp. 8, Dec 2009.
• [13] Andreas Ulbig, Theodor S. Borsche, Goran Andersson, "Impact of Low rotational Inertia on Power System Stability and Operation", IFAC World Congress, Dec 2014.
• [14] Maurizio Delfanti, Marco Merlo, Gabriele Monfredini, "Voltage Control on LV Distribution Network: Local Regulation Strategies for DG Exploitation", Research Journal of Applied Sciences, Engineering and Technology, June 2014
• [15] Annabelle Pratt, Pavan Kumar, Tomm V. ALdridge, "Evaluation of 400 Vdc distribution in Telco and Data Centers to Improve Energy Efficiency", 29th Telecommunication Energy Conference, Oct 2007.
• [16] CK Tang, Nic Chin,"Malaysia's Weather Data",Building Energy Efficiency Technical Guideline for Passive Design(Draft 1), Chapter 2, pp18-22,June 2012
The 2nd Smart Villages Workshop – 27 Jan 2015