bybyRon S,Y.HUI and Chi Kwan LEE

D t t f El t i l & El t i E i iDepartment of Electrical & Electronic EngineeringThe University of Hong Kong

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k d h d h llBackground & The Grand ChallengeElectric Springs & Hardware TestsSimulation Studies in Power SystemsRemarks

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The intermittent power is causing destabilization of the electric The intermittent power is causing destabilization of the electric grids, causing potential blackouts, weakening voltage and causing damage to industrial equipment.g g q p

• The Oahu-Hawaii power system collapsed in April 2013 when 17 %

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the wind power reached 17 % ! [EPRI]

Important points:1.Wind and solar power is the best way to cut CO2 emission.2.Rapid increase of Wind and Solar Power is possible, but….S p ,3.there is a urgent need to cope with stability issues.

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Existing Power Generation Future Power GenerationgCentralized Distributed

One-way power flow Bi-directional power flowOne way power flow Bi directional power flow

Existing Control Paradigm Future Control ParadigmPower Generation follows Load

DemandLoad Demand follows Power

Generation

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A power electronics device adaptive toA power electronics device adaptive to◦ fluctuating mains voltage and/or frequency

It is low cost and autonomous◦ No need for centralized control systems

It can therefore be ‘distributed” over the power grid◦ eg. Households and industrial sites to stabilize the mains voltageeg. Households and industrial sites to stabilize the mains voltage

and/or frequency in real-time.

l h h h ll ll dAlthough they are all small power devices, ◦ many “small” but distributed electric springs should provide a

collectively robust stabilizing effect.collectively robust stabilizing effect.

Hooke’s Law

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(1660)

Non-critical load power P1 follows the intermittent powerNon critical load power P1 follows the intermittent power generationMains voltage VS is regulated to its nominal valueS

Pin-

Compensator as Electric Spring vs

P1v s_reftive wer

+PWM Power

Inverter

P2voow

Load

222

Rv

Rvv

P sasin +

−=

time 

( ) ( )22

ReRe ZvZvP so⎟⎟⎞

⎜⎜⎛

+⎟⎟⎞

⎜⎜⎛

=

21

21

PPPRR

in +=

( ) ( )

21

22

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ReRe

PPP

ZZ

ZZ

P

in

in

+=

⎟⎟⎠

⎜⎜⎝

+⎟⎟⎠

⎜⎜⎝

=

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Signals feedback  Signals output to 

Vs , Is, ωs, Ps andQs

from the grid the converter

Gate PatternGenerator

Droop Control

Vs*(nominal) eg. 220V

From P and QM

Vs_ref

VsActive and

Reactive Power

Q

σ

Magnitude and Phase angle

VsIsError

Amplifier /

M

eVs ComputationsP

gControllerCompensator

Synchronization Circuit

Load Shedding (frequency) Control / Power Oscillation Damping Control

/ Transient Stability Control

Is, ωs, Ps andQs

Vs,ωs

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Thermal loads + Lighting systems = 40% ~ 50%Thermal loads + Lighting systems 40% 50%

Water heatingRefrigeration

Lighting

Air‐conditioning

Refrigeration

Air conditioning

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El t i S i b dd d Electric-Spring-embedded power supplies

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Stabilized mains voltage

Voltage of Electric Spring

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Load demand following power generation

Hui S.Y.R., Lee C.K. and Wu F.F., “Electric Springs: A New Smart Grid Technology”, IEEE Transactions on Smart Grid, 2012 15

2500

3000

G

1500

2000

2500

wer

(W

)

Total Input Power (PG+PR)

Battery Power (Ps)

Non-critical Load Power (P1)

Bidirectional ac-dc power converter

Non-critical load R1

Criticalload R2 0

500

1000

0 600 1200 1800 2400 3000 3600 4200

Po

Energy storage

-500

0 600 1200 1800 2400 3000 3600 4200

Time (sec)

18

27

600

900

PPPPP ++esSS EE ≥

PR 0

9

18

rgy

(Wh)

0

300

600

wer

(W

)

21 PPPPP RGS ++−−=

PG

PP1 P2

-18

-9

0 600 1200 1800 2400 3000 3600 4200E

ner

-600

-300

Po

Energy of Battery Storage (Es)

Power of Battery Storage (Ps)

PS-27

Time (sec)

-900

Pgen + Prenewable = Pcritical + Pnon-critical + Pstorage + PES Distributed Storage

Lee C.K. and Hui S.Y.R., “Reduction of energy storage requirements for smart grid using electric springs”, IEEE  Transactions on Smart Grid, 2013 16

Yan S., Tan S.C., et al. “Electric springs for reducing power imbalance in three‐phase power systems”, IEEE  Transactions on Power Electronics, 2015 17

Case study 2:Sa‐Lo Bay, Lantau Island, Hong Kong    (11kV ‐> 220V)

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The total reactive capacity required for the ESs is about 14 times less for under‐voltage

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The total reactive capacity required for the ESs is about 14 times less for under voltage condition and 30 times less for over‐voltage condition than that required by the STATCOM.

Without considering the stability of the power systems, existing grid-connected power inverters could be ‘destabilizing” the power systems.With the stability of power system in mind, our patent-pending technologies make grid-connected power inverters a “stabilizing force”connected power inverters a “stabilizing force”.Electric Springs – a New Technology to:

tame the intermittent nature of wind/solar power–tame the intermittent nature of wind/solar power–achieve the new control paradigm of having the load demand to follow thepower generationpower generation–potentially push the intermittent renewable power generation well above 20%.

With fast P and Q control, the Electric Springs is a technology with hugepotential for both voltage and frequency stabilities of power systems.

Prof. Ron S. Y. HUI, email: ronhui@eee.hku.hk

Dr Chi kwan LEE email: cklee@eee hku hk

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Dr. Chi-kwan LEE, email: cklee@eee.hku.hk