Distributed Generation: Benefits & Issues

(1)

Distributed Generation:

Benefits & Issues

Johan Driesen

K.U.Leuven

–

ESAT/ELECTA

http://

www.esat.kuleuven.be/electa

2nd Int. Conf. on Energy Innovation / 6-7-'06 2/52

Traditional low voltage grid

• Limited number of loads

• Energy supplied top-down

from central power station

• Increased loading

• Increased power quality &

reliability problems: due to

non-linear (power

electronic) and sensitive

loads

(2)

Why are grids changing?

•

3 technological drivers

Power Electronics

(PE) becomes ubiquitous in loads,

generators and grids

More power produced (and stored) near consumers:

Distributed Energy Resources

(DER)

Increased importance of

Power Quality

(PQ): more

disturbances and more sensitive devices

•

3 socio-economic tendencies

Liberalization

of energy markets: free choice of supplier

More

sustainable energy

(renewable and ‘high-quality’)

Less guaranteed

security of supply

Definition of Distributed Generation

• Distributed Generation (DG)

connected directly to the distribution

system or installed at the customer's

side of the meter;

non-dispatched by the network

operators;

not centrally planned;

small-scale generators (

≤

50 MW).

• Distributed Energy Resources

(DER)

(3)

2nd Int. Conf. on Energy Innovation / 6-7-'06 5/52 © K.U.Leuven - ESAT/Electa

DER technologies

• Distributed Generation:

Reciprocating engines

Gas turbines

Micro-turbines

Fuel cells

Photovoltaic panels

Wind turbines

CHP configuration

• Energy Storage

Batteries

Flywheels

Supercapacitors

Rev. fuel cells

Superconducting coils

Powder Iron Toroids Armature Winding Holders Housing Endcap Field Winding & Bobbin Rotor

Current DG penetration

Wind target in EU: 40,5 GW at present

to 75 GW in 2010 (EWEA)

Forecast: 7.2% DG share in 2004 to 14% in 2012

(4)

Grid of tomorrow ?

• Local generation

• Local storage

• Controllable loads, DSM

(‘

N

egaWatts’)

• Power quality and reliability

is a bigger issue

• System’s future structure & size?

Growth (?):

o

Consumption rises annually by several %

o

Investments in production: very uncertain due to

liberalisation

o

Limited or no grid expansions are accepted by the public

Balancing:

o

Short-term: make balance by introducing DG?

o

Long-term: more storage and/or ‘activate loads’?

Power electronic dominated grids

(5)

SmartGrids Vision

• EU Technology Platform

preparing FP7

• Involving specialists from

industry, academia, …

• Vision paper published in

2006

(6)

Enabling Technologies

• Active Distribution Networks

• Improved power flow: FACTS, WAMS, WAPS

• Power electronic technologies

• Smart Metering

• Communication for DSM, on-line services,

energy management

• Stationary energy storage

Concepts for the future

• Virtual Utilities:

Configure and

deliver ->

(7)

Concepts for the future

• Microgrids:

Low voltage

networks with DG

sources, local

storage and

controllable loads,

automatic islanding

Microgrid idea

• Technically, parts of the

grid may even

separate

from central supply

• A

Microgrid

is a collection

of small generators for a

collection of users in close

proximity

• No net power exchange,

total autonomy (

Energy

Island

) ?

(8)

Is a Microgrid new ?

• History of electricity networks:

It all started that way, before interconnection

• Today, are they a new concept?

No ?

o

The grid behind certain UPS systems are driven like a

microgrid with one generator

o

Multiple UPS units in parallel is not impossible, but not often

implemented due to complexity (control, earthing, …)

Yes !

o

Intended for longer term operation, not just emergencies

o

Scope of the idea is wider: from a single dwelling to parts of

the distribution network (longer connections, multitude of

connections, …)

What makes a Microgrid

difficult to operate?

• “Ancillary Services”

are all to be delivered

internally

Balancing the active and reactive power:

electricity produced - system losses

= electricity consumed – storage

Stabilizing the grid: frequency, voltage amplitude and

phase angle

Providing quality and reliability: unbalance, harmonics, …

Black-start capability

…

• Can a Microgrid become fully separate and deliver

these services?

How to organize (system operator

↔

fully distributed) ?

(9)

What is an optimal deployment

of DG technology ?

• Distribution grid was

never

built for local power

injection, only top-down power delivery

• Technical difficulties:

Power Quality & reliability

Efficiency

Safety

Control

Economic aspects

• Are there

optima

for the

size

and

location

of

DER units (generation, storage, active load) ?

DG%

t

Power quality & reliability

• Problem:

Bidirectional power flows

Distorted voltage profile

Vanishing stabilizing inertia

More harmonic distortion

More unbalance

• Technological solution:

Power electronics may be configured to

enhance PQ

(10)

2nd Int. Conf. on Energy Innovation / 6-7-'06 19/52 © K.U.Leuven - ESAT/Electa 0.99 1 1.01 1.02 1.03 1.04 1.05 1.06 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (min) Vo lt ag e (p .u .) 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 35 40 45 50 55 60 Time (min) P (kW) Q (kVar)

PV power is calculated from 5-s

average irradiance data measured

in Heverlee (B)

Power variation causes voltage

fluctuations

Voltage fluctuation with PV

Example: MV cable grid

Substation

connecting

to HV-grid

Location:

Leuven-Haasrode,

Brabanthal +

SME-zone

(11)

Impact of wind turbine

110 208 408 408 110 SUB SUB 1 ₁₀₄ 201 201 202 202 203 204 203 204 205 205 206 206 207 404 303 303 404 104 207 208 102 102 2 101 101 103 103 105 106 105 106 107 107 108 109 108 109 402 402 301 301 302 302 401 401 403 403 406 304 304 406 305 305 405 405 407 407 111 409 409 111 110 208 408 408 110 SUB SUB 1 ₁₀₄ 201 201 202 202 203 204 203 204 205 205 206 206 207 404 303 303 404 104 207 208 102 102 2 101 101 103 103 105 106 105 106 107 107 108 109 108 109 402 402 301 301 302 302 401 401 403 403 406 304 304 406 305 305 405 405 407 407 111 409 409 111 110 208 408 408 110 SUB SUB 1 ₁₀₄ 201 201 202 202 203 204 203 204 205 205 206 206 207 404 303 303 404 104 207 208 102 102 2 101 101 103 103 105 106 105 106 107 107 108 109 108 109 402 402 301 301 302 302 401 401 403 403 406 304 304 406 305 305 405 405 407 407 111 409 409 111 110 208 408 408 110 SUB SUB 1 ₁₀₄ 201 201 202 202 203 204 203 204 205 205 206 206 207 404 303 303 404 104 207 208 102 102 2 101 101 103 103 105 106 105 106 107 107 108 109 108 109 402 402 301 301 302 302 401 401 403 403 406 304 304 406 305 305 405 405 407 407 111 409 409 111

© K.U.Leuven - ESAT/Electa 401 201 301 301 201 101 1 SUB SUB 2 304 304 406 106 203 203 104 103 102 105 202 204 205 208 208 109 107 108 110 111 207 206 302 302 403 402 404 405 303 303 305 305 408 407 409

Voltage – Power factor relationship at node 406

9 Generally, power injected by DG improves

voltage profile

9 Over-voltage may occur at high level of power

injection (especially, synchronous machines)

• Connect one DG unit at node 406

→record system voltage profile at different power injections of DG unit Voltage profile along feeder 4

9 Little impact on the voltage rise with the

generators operating as induction machine

9 Synchronous DG raises system voltage

higher due to additional reactive power

injection

(12)

• Voltage at node 2 – branch 1-2 open

401

201

301

201

101

1 SUB

SUB

2

304

406

106

203

104

103

102

105

202

204

205

208

109 107 108

110 111

207

206

302

403

402 404 405

303

305

408

407

409 • An induction generator starts up (node 108)

A soft-start circuit is needed for large induction DG

A large induction DG starts up causes severe

voltage dips at connected node and nearby nodes

→

affect sensitivity loads

Voltage dips at node 2 with synchronous DG case Voltage dips at node 2 with induction DG case

Voltage dips

P

_load

+ jQ

_load

E

_s

Z

line

Z

_load

U

_r

I

U

r Critical Point Normal Operating Point

Voltage stability margin

P

_load

P

_max

(13)

9DG Location has significant impact on

voltage stability limit.

9Synchronous DG unit has better

contribution to voltage stability limits.

401

201

301

201

101

1 SUB

SUB

2

304

406

106

203

104

103

102

105

202

204

205

208

109 107 108

110 111

207

206

302

403

402 404 405

303

305

408

407

409 Synchronous DG cases

• DG unit: 3 MW

• Calculate voltage stability limit at node 111, is the furthest

point from substation (considered to be weakest bus in terms of

voltage stability)

401 201 301 301 201 101 1 SUB SUB 2 304 304 406 106 203 203 104 103 102 105 202 204 205 208 208 109 107 108 110 111 207 206 302 302 403 402 404 405 303 303 305 305 408 407 409

Induction DG cases

Voltage stability limit @ node 111,

impedance load

Control, or the lack of

• Problem:

Generators are

NOT

dispatched

in principle

o

Weather-driven (many

renewables)

o

Heat-demand driven (CHP)

o

Stabilising and balancing in

cable-dominated distribution

grids is not as easy as in HV

grids

reactive power voltage

active power frequency

(14)

Networked system operations

• Solutions:

Higher level of control required to

coordinate

balancing, grid

parameters ?

Advanced control technologies

• Future technologies, under investigation

Distributed stability control

o

Contribution of power electronic front-ends (see example)

Market-based control

o

Scheduling local load and production, by setting up a micro-exchange

(see example)

Management of power quality

o

Customize quality and reliability level

Alternative networks

o

E.g. stick to 50/60 Hz frequency ? Go DC (again) ?

• Rely heavily on intensified communication: interdependency

Example: fully decentralized control

• Standard method: “droop control”

• KUL method: Virtual Impedance method

Emulate a voltage source with internal

tunable impedance in the time domain

Ref.: K.De Brabandere et al. @ PESC’04

• Advantage: seamless transition from

grid-connected to island and reconnect

(15)

Experimental results: connection of

two independent grids (islands)

current before

voltage before

voltage after

current after

Example: tertiary control on local market

• DG units locally share loads dynamically based on marginal

cost functions, cleared on market

P P

MC MC

MC

∆P

P1_oldP1_new _P2_newP2_old equal marginal cost

translated

(16)

Safety

• Problem:

Power system is designed for top-down

power flow

Local source contributes to the

short-circuit

current

in case of fault

o

Fault effects more severe

o

Difficult to isolate fault location

Bidirectional flows

o

‘Selectivity’ principle in danger: no

backup ‘higher in the grid’ for failing

protection device

Conservative approach on unintentional

islanding

• Solution:

New

active

protection system

necessary

G G 110 208 408 408 110 SUB SUB 1 104 201 201 202 202 203 203 204 204 205 205 206 206 207 404 303 303 404 104 207 208 102 102 2 101 101 103 103 105 106 105 106 107 107 108 109 108 109 402 402 301 301 302 302 401 401 403 403 406 304 304 406 305 305 405 405 407 407 111 409 409 111 110 208 408 408 110 SUB SUB 1 104 201 201 202 202 203 203 204 204 205 205 206 206 207 404 303 303 404 104 207 208 102 102 2 101 101 103 103 105 106 105 106 107 107 108 109 108 109 402 402 301 301 302 302 401 401 403 403 406 304 304 406 305 305 405 405 407 407 111 409 409 111 99.5 100.0 100.5 101.0 101.5 102.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 s kA 99.5 100.0 100.5 101.0 101.5 102.0 0.10 0.15 0.20 0.25 0.30 0.35 0.40 s kA

Short-Circuit Current

- 3 syn. gen.: 3 MVA

(high penetration at a

feeder)

- 3-phase short circuit

With DG connection

Base case

With DG connection

(17)

Societal issues

• Problems:

Environmental effects

o

Global

: more emissions due to

non-optimal operation of

traditional power plants

o

Local

effects as power is

produced on-the-spot, e.g. visual

pollution

Making power locally often

requires

transport infrastructure

for (more) primary energy

o

Problem is shifted from electrical

distribution grid to, for instance,

gas distribution grid!

• Solution:

Multi-energy

vector

approach

Open debate on

security of supply

Economic issues

• Problems:

Pay-back

uncertain in liberalized

market

o

‘Chaotic’ green and efficient power

production

o

Reliability or PQ enhancement difficult

to quantify

System costs

o

More complicated system operation

o

Local units offer ‘ancillary services’

System losses generally increase

Who pays

for technological

adaptations in the grid ? Who will

finance the backbone power system?

o

Too much socialization causes public

resistance

• Solution:

Interdisciplinary

regulation, not

only legal

Need some real

(18)

System losses example

• DG

introduction

does not mean

lowered losses

• Optimum is 2/3

power at 2/3

distance

• Other

injections

generally

cause higher

system losses

HV subst.

Load distributed along feeder cable

DG

Zero point

After DG

Before DG

Power flow along cable

0 1 2 3 4 5 6 7 8 0 20 40 60 80 100 120 140 160 Penetration level (%) P lo ss ( % ) Induc PF 0.9 Con PF 1.0 Syn PF 0.9 0 10 20 30 40 50 60 70 80 0 20 40 60 80 100 120 140 160 Penetretion level (%) Re act iv e-volt age dr op (%) Induc PF 0.9 Con PF 1.0 Syn PF 0.9

Penetration level of DG

First:

PL

_DG

increases

→

Losses decrease

Then

PL

_DG

increases

→

Losses increase

PL

_DG

(%) =

DG

x 100

L

P

∑

9Synchronous DG units have the

largest influence on the reduction of

power losses.

(19)

Balancing question, again

• Fundamental electrical power balance,

at all times is the boundary condition:

Electricity produced - system losses

= electricity consumed – storage

• All sorts of reserves will decrease in the future

• Role of storage? Storage also means cycle losses!

• Next step in enabling technologies

Usable

storage

Activated intelligent loads (demand response technology), also

playing on a market?

Boundary condition: minimize losses

How far can we go?

• Large

optimization

exercise, making following

considerations:

Optimal proliferation, taking into account local

energetic opportunities, e.g. renewables options?

Unit behavior towards grid: technology choice?

Control responsibilities?

Is the same level of reliability still desired ?

Level of introduction of new additional

technologies (storage, activated loads)?

(20)

Looking for the optimal solution

Where to place DER?

Optimization goals:

o voltage quality penalty o minimum losses

o minimum costs

o …

Constraints:

o net balance

o load flow equations

o acceptable voltage profile

o …

A?

B?

C?

Optimization

non-convex multiple objectives mixed discrete-continuous

long term vs. short term interactions

DER Planning

• Single objective

Min (Energy Loss)

very straightforward

does not represent the real problem

• Multi-objective

Weight factors

Æ

single objective

Min (

α

Energy Loss +

β

Voltage penalty +

γ

Installation Costs)

value of weight factors?

useful if objectives are comparable, e.g. all costs (€)

True multi-objective

Æ

trade-offs

Min {Energy Loss; Voltage penalty; Installation Costs}

(21)

DER Planning

Long term planning problem efficiently handled by

using Genetic Algorithms

• Optimization based on evolutionary computing techniques

• Able to deal with mixed discrete (placement) – continuous (sizing) problem

• Global optimum not guaranteed!! Rather a target-oriented tool

• Often difficult to obtain sufficient data sets

Æ

use Markov models

Gen A Gen B Node i

Network topology

=

Test case

2 generator types used: PV & CHP

Æ

each represented by a 5-string bit

Æ

network chromosome = 20 x 2 x 5 =

200 bits

(22)

Test case

Load data are based on actual measurements

• residential buildings

• period of 1 year

• on a 15-minute basis

4 load scenario types are available

each scenario consists of 4 typical one-day load profiles

Æ

5 times repeated over the 20-node grid

Summer,

high load

Test case

summer

wi

n

te

r

high load

(e.g. working day)

low load

(e.g. weekend)

(23)

Test case

• Evolution of total power loss in the optimal solution of

each generation in the Summer–Low scenario

Æ

Optimal and stable solution reached after 100 generations

Test case

(24)

Test case

Cross-comparison test :

use solution of scenario

i

in fitness function of scenario

j

40.4

219.9

30.9

63.5 No DG

11.9

107.7

19.7

36.9 WL

74.7

48.1

52.9

59.1 WH

18.8

160.0

13.1

26.9 SL

13.9

130.2

17.5

23.4 SH

Using

optimal

DG of

scenario

WL

WH

SL

SH

Load scenarios

Total power loss

[kWh]

DER Planning

Instead of single-objective optimization, the use of true

multi-objective optimization is proposed

Single objective

• only one “optimal” solution

• significance?

Multi-objective

• pool of solutions

• trade-offs between objectives

• insight in the potential of the grid

• which objectives are conflicting?

Decreasing objective value with

(25)

DER Planning

What is a trade-off?

f

1

f

2

f

₁

f

₂

x

1

, x

2 feasible unfeasible trade-offs Pareto front translation from variable space to objective space

Rank topologies according to dominance:

If topology A is at least equal in all objectives compared to B

and better in at least one objective,

then topology A dominates topology B

Aim of the optimization:

Identify as much

non-dominated

topologies as possible

DER Planning

Fitness of a DER topology is calculated by running

a scenario with assumed loads and grid topology

What if

9 Storage is to be optimized?

Æ

Control?

9 Load has an elastic behavior?

Æ

Demand Response?

9 Production is dispatchable?

Æ

Forecasting?

Æ

What if Local Markets

exist?

Long-term planning interacts with short-term planning problem!!

primary control secondary control

(26)

Conclusion

• Current grid:

Interconnection

Higher PQ level required

DER looking around the corner

• History repeats: after 100 years the idea of locally supplied,

independent grids is back

Microgrids, being responsible for own ancillary services

• Maximum (optimal?) level of penetration of DER

= difficult optimization exercise

Whose optimum?

• Special (technological) measures are necessary

ancillary services are a challenge

o

e.g. in system control, balancing

role of loads?

• Not only technology push, but also customer pull required

more information:

http://www.esat.kuleuven.be/electa

check

publications

sections, e.g.:

Pepermans G., Driesen J., Haeseldonckx D., Belmans R., D’haeseleer W.:

“Distributed Generation: Definition, Benefits And Issues,” Energy Policy, Elsevier,

Distributed Generation: Benefits & Issues

Distributed Generation:

Benefits & Issues

Johan Driesen

Johan Driesen

K.U.Leuven

K.U.Leuven

–

–

ESAT/ELECTA

ESAT/ELECTA

http://

http://

www.esat.kuleuven.be/electa

www.esat.kuleuven.be/electa

Traditional low voltage grid

•

Limited number of loads

•

Energy supplied top-down

from central power station

•

Increased loading

•

Increased power quality &

reliability problems: due to

non-linear (power

electronic) and sensitive

loads

Why are grids changing?

•

3

technological drivers



Power Electronics

(PE) becomes ubiquitous in loads,

generators and grids



More power produced (and stored) near consumers:

Distributed Energy Resources

(DER)



Increased importance of

Power Quality

(PQ): more

disturbances and more sensitive devices

•

3

socio-economic tendencies



Liberalization

of energy markets: free choice of supplier



More

sustainable energy

(renewable and ‘high-quality’)



Less guaranteed

security of supply

Definition of Distributed Generation

•

Distributed Generation (DG)



connected directly to the distribution

system or installed at the customer's

side of the meter;



non-dispatched by the network

operators;



not centrally planned;



small-scale generators (

≤

50 MW).

•

Distributed Energy Resources

(DER)

DER technologies

•