kuching | jan-15 | feasibility of dc-microgrid for off-grid communities electrification

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!


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?


AC vs DC


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.


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)


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)


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]


Fig 4: (a) DC grid for data center (b) operation modes transition [5]


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.


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]


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%)


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.


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


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


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


Simulation of Dc-micro Grid

Simulated micro-grid topology

PV

Buck

boost

Buck-

boost

Battery

Buck

boost DC load


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])


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


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

)


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)


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.


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


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


kuching | jan-15 | feasibility of dc-microgrid for off-grid communities electrification

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