MODERN: Modular Distributed Energy Resource Network

Ashwin M. KhambadkoneDepartment of Electrical and Computer Engineering,

National University of Singapore

Project funded under A*STAR SERC IEDS programme

A Comprehensive Approach: Work packages synergize to achieve Objectives

Work Package 1 (WP1): Synthesis of Energy Source Mix for Distributed Energy Resource Network

Objectives• Software package for determining optimal Energy source

mix for a region (of any size).

• Performance assessment of Microgrids (Integration and Optimization)

Karimi (Co-PI), Ashwin (PI), Dipti (Co-PI), Sundar (RF), 3 (B.Eng)

Work package 2 (WP2): Grid Architecture Design and Control

Objectives• Methods to study and determine optimal grid architecture

Analysis of hybrid (AC+DC) microgrid structureTest the proposed hybrid microgrid structure using RTDS

• Multi-Agent based distributed Grid Control & ManagementOpen architecture for real time microgridOptimal microgrid operations through cooperative behavior of agents

Dipti (Co-PI), Ashwin (PI), Htay (RE), Zhuang (RE), Logenthiran (PhD)

Work Package 3 (WP3): Reconfigurable Intelligent Distributed Converters as Power electronic Building Block (PEBB)

Objectives • Power Electronic Building Block Architecture with interconnectivity and

reconfigurability

• Designs of Converter Systems using PEBB for Power Processing within DER

• Intelligent Control for Reconfigurable Converters to perform various power processing functions

Ashwin (PI), Xiaoxiao (PhD), Huanhuan (PhD), Terence (RE), Qingzhuang (RE)

Work Package 4 (WP4): Energy Storage Systems

Objectives• Feasibility studies on composite energy storage system

(CESS)• Prototype of Energy Storage system using varied energy

storage technologies is currently being implemented

Ashwin (PI), Erik (Co-PI), Balaya (Co-PI), Haihua (RA), Tran (PhD), Terence (RE)

Work Package 5 (WP5): DER Modeling Verification and System Integration.

Stefan (Co-PI), Erik (Co-PI), Stephen (Co-PI), Karimi (Co-PI), Karthik (PhD)

Objectives • Virtual Battery and Supercapacitors models for simulation• Models to assess BIPV & new PV technology and its impact

on DER

Microgrid is a LV network with distributed energy resourses, distributed storage and loads.

Loads

Distributed Energy Resource (DER)

Distributed Storage (DS)

Microgrid can be operated either in a grid-connected or an islanded operational mode.

Grid-connected Mode

Islanded Mode

Microgrid Challenges

EPS

Microgrid

EnergyStorage

Load

DistributedGeneration

How to Form the Electric Connection?

How to Interconnect?

Power Converter Systems are required.

Microgrids with LV AC BUS is a natural extension of distribution network

Many loads and Microsources are variable frequency AC or DC: DC Microgrids?

Hybrid of AC and DC Microgrids could be a transition stage

Hierarchy of Control from Distribution level to DER level 

Challenges for Microgrid Control in timescale

Real‐time control of Microgrids needs faster rates of control and data flow

Fast intermittencies in renewable sources pose dynamic control challenges

Fast storage system needed to compensate for intermittent source

19

Shortage of fossil fuel forces usto find sustainable energy sources

Sustainable sources such as PV and wind power are needed

Fossil fuel reserve will be zero after 75 years if energy demand increases 2.4% yearly

year

Rel

ativ

e re

serv

e

*

2.4% 75 years left

5% 50 years left

Intermittent and varying nature of renewable sources require energy storage 

Intermittent and quick fluctuation

20

Variation within 24 hours

24 hours PV profile 24 hours residential profile

Energy storage to meet both power and energy requirements are needed

A

B

BA

Single energy storage offers limited power and energy density profile 

21

High power density

Ragone chart

Hig

h en

ergy

den

sity No energy

storage here!

* Source US Defence Logistics Agency

Energy density is a term used for the amount of energy stored in unit volume or weight of energy storage

Power density is a term used to describe the amount of power that can be delivered from unit volume or weight of energy storage

Combining different energy storages offers flexibility and saves weight (1)

22

Example:

Energy storage should provide at least power 3000W and energy 6000Wh to satisfy the load requirement

Charging

Discharging

Maximum power

Combining different energy storages offers flexibility and saves weight (2)

23

Ultracapacitor (UC) Power density =2000W/kg, Energy density=10Wh/kgMinimum weight: 600kgCombined Power density =62.5 W/kg, Energy density=125Wh/kgMinimum weight: 48 kg 30kg from FC18kg from UC

Combining different energy storages can utilize advantages from individual energy storage and achieve best mix

Fuel cell (FC)Power density=20W/kg, Energy density=200Wh/kgMinimum weight:150kg

(kg)

Combining different energy storages achieve wide power and energy profile

24

High power density

Ragone chart

Hig

h en

ergy

den

sity energy storage profile

A

B

C

Composite Energy Storage System (CESS) offers high power and high energy storage in hybrid

micro-grid

25

Block diagram showing CESS interface with DC bus in hybrid grid

Challenges in designing and controlling power converters in CESS

26

• Meet individual energy storages’ requirements• Maximum utilization of energy storages• Meet load variation needs• Offer regulated DC bus

Proposed DC-DC converter to interface battery and ultracapacitor

27

1. Meet individual source requirements

2. Meet load requirements

3. Provide freedom for controlling output voltage from each converter

Advantages of proposed DC-DC converter

Case II: Battery replacement without interrupting the normal operation when CESS load is constant

28

Current in each converter

1st ultracapacitor interfacing converter current increase to compensate for the loss of first battery current

2nd to 4th ultracapacitor interfacing converter current decrease to compensate for the increase in 2nd to 4th battery currents

1st converter current when one battery power is reduced to zero

2nd to 4th converter currents for other three batteries to generate the lost power

t=t0

Ultracapacitor Fast dynamic response

BatterySlow dynamic response

Accelerated testReal Time Digital Simulation

Intermittent PV

Output from Storage

Fast storage system needed to compensate for intermittent source

30

Conclusion

Interleaved modular

converter

Back to back DC-DC converter

Flexible energy management

Composite Energy Storage

System

Meeting different source and load requirements

Topology for connecting ultracapacitor and battery

Flexible energy and power flow capability

One stop solution for any power and energy requirement

EDB A*STARUniversities, Polytechnics, Hospitals

Ministry of Trade and Industry

Biomedical (BMRC)

Science & Engineering (SERC) ETPLCCO CPADA*GA

ICES IMRE IHPC DSI IME I2R SIMTechTranslational

& Clinical Sciences

Biomedical Sciences & Technology

EPGC is a program under ICES which, like other A*STAR RIs, is part of MTI.

Chemicals  &Engineering

Computing Electronics Infocomm ManufacturingMaterials Memory

NMC

Metrology

Biomedical RIs and Consortia EPGC The Experimental Power Grid Centre is a

Program @ ICES

Technological Capabilities

iGrid

iDERS iEuse

Confidential

Capabilities existingin A*STAR• Information Processing• Cyber Security• Asset Management• Data Communications• Sensors• Fuels/BioFuels• Storage Materials• Packaging• Fuel Cells• StarHome• High Performance Computing

Capabilities within EPGC• Transition technologies• Flexible and self‐healing• Large complex systems• Storage systems• Diagnostics• Smart Demand Response• Decentralized Control• Renewable Energy Systems• Power Converter Systems• Plug and Play• Life cycle assessments• Smart user

World Class Research Facilities For Grid Technology

High speed link

@ Fusionopolis

@ Jurong Island

Rating1 MW

What is the EPGC facility?Initial Assets

600kW

Emulators

Building

PowerInfrastructure

EnergyAssets

Control andEnergy

Management +MicroturbinesFlow Batteries

Fuel Cell EV charging

What is the EPGC facility?

PowerInfrastructure

DAB converter is selected as basic cell of modular design

36

Positive Power transferTopology of DAB

Relative phase differences between VA and VB determine the amount and direction of the power flow

Primary leading secondary

Control strategy offers flexible power flow management

37

1. Regulate DC output voltage

2. Dynamic power allocation amongst energy storages

3. Flexible battery energy management

4. Ultracapacitor SOC control

Advantages of proposed controller

Control block diagram

Case I: Voltage regulation shows dynamic allocation between two storages

38

Fast dynamic response in ultracapacitor

Slow dynamic response in battery

Energy management for case I

Case II: Energy management of the batteries

39

SOC of all the batteries with equal discharge current

SOC of all the batteries with discharge current proportional to their SOCs

Low initial SOC depleted fast if equal discharge current is applied

Energy utilized fully if energy management scheme allocates current reference proportional to their SOCs

Experimental results

40

Output voltage ripple is reduced! Dynamic power allocation is achieved!

Waveforms of output voltage

Overview: Energy Mix Planning Tool

Choose Country/Region

Generic Formulation

Energy policyP1: Min ∑Total CostP2: Min ∑Total Cost

CO2 < TargetP3: Min ∑CO2 Emission.PN: Fuel diversity

Input DatabaseOptimization

Engine

Fuel Mix Technology MixCO2 EmissionTotal Cost

Advance Analysis

Attribute Details Policy-based Formulation

GAMS

Generic Energy Model formulation for DER

Diverse Energy policies and strategies

Optimal Energy Mix for diverse scenarios 

Multi‐period best energy mix for diverse scenarios 

Robust energy mix to remain stable against future uncertainties

Energy mix for Multiple integrated microgrids

Robust energy mix for Multiple integrated microgrids

Energy Mix Identification

Optimal (economical and reliable) sizing of DERs

CASE STUDY 1

Pulau Ubin Island

Area : 10.19 km2Peak Demand : 1.5 MWAverage Demand : 0.28 MW (2.5 GWhr/yr)Fuel Diversity : 70 %No of Fuels : 8 No of Technology : 13

1: Hydro2: Wind3: S olar Trough4: S olar Tower5: S olar Dish6: S olar PVC7: S olar PVF8: S olar PVC ON C9: F uel C ell10: Bio- diesel11: Biomass12: Micro Turbine13: Diesel

Energy mix planning for multiple integrated microgrids

MicrogridA

Commercial

MicrogridB

Industrail

MicrogridC

Residential

Optimal Sizing of DERs in Integrated Microgrid

WP1: Planning of Distributed power system Provides energy limits of DERs Provides emission limits of DERs

WP2: Operation of distributed power system Dealing with power ratings, constraints and available power of DERs

Optimal Sizing of DERs

Optimal Sizing Results

TechnologyApproach 1 Approach 2 Approach 3

MG 1 MG 2 MG 3 MG 1 MG 2 MG 3 MG 1 MG 2 MG 3Diesel (kW) 600 1000 500 550 900 450 550 900 450Natural Gas (kW) 500 500 430 400 450 400 400 450 400Fuel Cell (kW) 0 0 85 0 0 85 0 0 85Photovoltaic (kW) 1980(55) 2988(83) 1548(43) 1836(51) 2844(79) 1368(38) 1440(40) 2700(75) 1368(38)Wind Turbine (kW)

0 0 140(1) 0 0 140(1) 0 0 140(1)

Biodiesel (kW) 100 100 0 100 100 0 100 100 0Biomass (kW) 100 100 0 100 100 0 100 100 0Battery Bank (kW)

1000(5) 1800(9) 800(4) 400(2) 600(3) 200(1) 200(1) 400(2) 0(0)

LPSP Achieved 0.003 0.0002 0.00 0.001 0.0001 0.0001 0.00 0.001 0.00

Total Cost ($) 4.10x108

5.74x108

3.15x108

3.50x108

5.24x108

2.91x108

2.20x108

3.75x108

2.35x108

Best type and capacity is obtained in an optimal way for all DERs

Optimal Sizing Results

TechnologyIntegrated microgrids connected with

EPSMG 1 MG 2 MG 3

Diesel (kW) 550 900 450

Natural Gas (kW) 400 450 400

Fuel Cell (kW) 0 0 85

Photovoltaic (kW) 1440(40) 2700(75) 1368(38)

Wind Turbine (kW) 0 0 140(1)Biodiesel (kW) 100 100 0

Biomass (kW) 100 100 0

Battery Bank (kW)200(1) 400(2) 0(0)

LPSP Achieved 0.00 0.001 0.00

Total Cost ($)2.20x108 3.75x108 2.35x108

Best type and capacity is obtained in an optimal way for all DERs

Hybrid Microgrid: Getting best of both worlds

Power converter building block towards better asset management

PCC EPSMicrogrid AC Bus

DC Bus

PCBB

Modified DC Bus Interconnected Single-phase Micro-grid with PCBB Connecting to Electric Power System

if vf

if fv* *

Control For PCBB System

EnergyStorage

DCDC

DCDC

RenewableSources

Local Load

51

Droop Control of Paralleled Inverters

Output line Impedance: 13uH 0.02Ω

26uH 0.04Ω

52

Proposed Hybrid Microgrid Architecture

53

Dynamic Power Distribution Scheme for Paralleled Inverters located within a block

Equal Current Sharing

+

Dynamic Module Dropping

+

Time Sharing

50Hz

200Hz

1 1

DC

f

VL s R

1

1fC s

2 2

DC

f

VL s R

2

1fC s

2 2

DC

f

VL s R

2

1fC s

1 1

DC

f

VL s R

1

1fC s

2 2

DC

f

VL s R

2

1fC s

2 2

DC

f

VL s R

2

1fC s

54

Dynamic Electro-Thermal Modeling of the Paralleled Inverters System

PEBB shedding under low load to improve system

efficiency

55

Hardware-in-the loop test of the proposed hybrid control architecture --- System Set-up

RTDs(Real Time Digital Simulator): Partitioning a large electrical system and simultaneously simulate each part on different processors; manufactured by Manitoba HVDC Center

56

Hardware-in-the loop test of the proposed hybrid control architecture --- Results (1)

Inv Block1 output impedance: 13uH 0.02ΩInv Block2 output impedance: 26uH 0.04Ω

Vpcc

i load

i 1

i 2

Hardware-in-the loop test of the proposed hybrid control architecture --- Results (2)

i o i i nv;1

i o

Microgrid has to provide voltage and frequency control during islanding operation.

Islanding No frequency and voltage from utility grid.

Microgrid has to maintain frequency and voltage for its loads.

Islanding No frequency and voltage from utility grid.

Microgrid has to maintain frequency and voltage for its loads.

islanding?

Microgrid also has to balance the outputs from various DERs.

P1, Q1 P2, Q2 Pn, Qn

Pns and Qns are balanced among the DERs so that each can share a proportional amount of load.

Pns and Qns are balanced among the DERs so that each can share a proportional amount of load.

A droop controller can be used for balancing outputs and maintaining voltage and

frequency.

Droop controller

Droop controller

V1,ref V2,ref Vn,ref

LV Microgrids are more resistive than conventional transmission lines

Unlike Tradition Droop Control for High Power High Voltage Synchronous Generators.

A hybrid droop controller that controls both active and reactive power simultaneously .

[X. Yu, H. Wang, A. Khambadkone, and T. Siew “A Hybrid Control Architecture for Low Voltage Microgrid”, IEEE Energy Conversion Congress and Exposition (ECCE’10), 2010]

Simulation results(1) of hybrid controller

Output line Impedance:

13uH 0.02Ω

26uH 0.04Ω

Vpcc

i load

i 1

i 2

Load step

Simulation results(2) of hybrid controller

Output line Impedance:

13uH 0.02Ω

26uH 0.04Ω

i o i i nv;1

io

i i nv;2

i o

Hybrid Microgrid with improved power quality

P,Q

PQ

DC Renewable Sources

EnergyStorage

Elements

LocalLoads

Hybrid Micro-grid

EPS

DC Bus

AC BusPCC

PCBB

PQ

P

P,Q

Active filter function at PCC

Satisfy IEEE1547 Std, which allows a maximum 5% TDD

0 0.02s 0.04s

TDD=2.01% TDD=2.89% TDD=2.25%

0.1s 0.12s 0.14s 0.2s 0.22s 0.24s

0 0.02s 0.04s 0.1s 0.12s 0.14s 0.2s 0.22s 0.24s

0 0.02s 0.04s 0.1s 0.12s 0.14s0.2s 0.22s 0.24s

0 0.02s 0.04s0.1s 0.12s 0.14s 0.2s 0.22s 0.24s

t

t

t

iact*

ih*

icic*

ipcc

-0.50

1.50.5

-1.5

-0.60

0.6

-0.80

0.81.75

-1.75

-101

(pu)

(pu)

(pu)

(pu)

Only Q 50%PL + Q 100%PL + Q 50%PL to EPS

Active Current Reference，iact

*

Harmonic Current Reference，ih*

Shunt PCBB output Current reference , ic*

+

=

Summary• Microgrid control in real time needs to

maintain power balance• Power electronics can be used to for fast

response and power control• Microgrid architectures can be designed to

make best use of assets• Power quality control can be achieved using

Microgrid architectures