Lesson 10 - Distribution Grid and Distributed Generation

8/13/2019 Lesson 10 - Distribution Grid and Distributed Generation

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Radial network

Energy supplied top-downfrom feeder

Traditional distribution grid:

From feeder to meter


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Distribution grid Flanders:

Facts and figures

3,075 feeders

29,546 km of medium-voltage lines

29,331 distribution transformers 24,820 distribution cabins

6,203 switching posts 52,473 km of low-voltage lines

1,796,083 connections 2,580,279 meters


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3 technological drivers Power electronics (PE) becomes ubiquitous in loads, generators

and grids

More power produced (and stored) near consumers: DistributedEnergy Resources (DER)

Increased importance of Power Quality (PQ): more disturbancesand more sensitive devices

3 socio-economic tendencies Liberalization of energy markets

More sustainable energy (renewable and high-quality) Non-guaranteed security of supply

Evolution in electrical energy


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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 IronToroids

ArmatureWinding

Holders

Housing

Endcap

FieldWinding

& Bobbin

Rotor


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Power electronic dominated grids

Source: KEMA


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Microgrid ?

Grids may even separate from central supply No net power exchange: total autonomy

Important aspect, characterizing a Microgrid

o Ancillary Services are all delivered internally

Balancing the active and reactive power

Stabilizing the grid: frequency, voltage

Providing quality and reliability: unbalance, harmonics,

Is a Microgrid new ? It all started that way, before interconnection

In fact, no: the grid behind certain UPS systems isdriven like a microgrid with one generator

H h l l


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DG%

t

How much local sources can

a distribution grid accept ?

Distribution grid was neverbuilt for local powerinjection, only top-down power delivery

Electrical power balance, anytime, in any grid:

Electricity produced - systemlosses= electricity consumed storage

Barriers to overcome: Power quality & reliability Control, or the lack of

Safety

Societal issues

Economic aspects


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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

DG units can be used as backupsupply


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Example: MV cable grid

Substation

connecting

to HV-grid

Location:Leuven-

Haasrode,

Brabanthal +

SME-zone


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Impact of wind turbine

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Control, or the lack of

Problem: Generators are NOT

dispatched in principle

o Weather-driven (manyrenewables)

o Heat-demand driven (CHP)

o Stabilising and balancing incable-dominated

distribution grids is not as

easy as in HV grids

reactive power voltage

active power frequency


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Networked system operations

Solutions: Higher level of control required to coordinate balancing, grid

parameters ?

Advande control technologies

Future technologies, under investigation Distributed stability control

o Contribution of power electronic front-ends

Market-based controlo Scheduling local load and production, by setting up a micro-

exchange (see example)

Management of power quality

o Customize quality and reliabili ty level Alternative networks

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

Rely heavily on intensified communication: interdependency


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Example: tertiary control on local market

DG units locally share loads dynamically based on marginal costfunctions, cleared on market

P P

MC MC

MC

P

P1old P1new P

2oldP

2new

equal marginal cost

translated

translated & mirrored


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Example: tertiary control on local market

Prices are set by marginal cost functions of generators: Real-time pricing

Real-time measurements

Network limitations introduce price-differences between nodes Safety and PQ issues similar to congestion in transmission grid

Demand side management Optimizing consumption

Load reacts to external signals as prices

Overall need for advanced metering


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Safety

Problem: Power system is designed for top-down

power flow

Local source contributes to the short-

circuit current in case of faulto Fault effects more severe

o Difficult to isolate fault location

Bidirectional flows

o Selectivity principle in danger: nobackup higher in the grid for failingprotection device

Conservative approach on unintentional

islanding Solution:

New active protection system necessary

G

G


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Societal issues

Problems: Environmental effects

o Global: more emissions due to

non-optimal operation oftraditional power plants

o Local effects as power isproduced on-the-spot, e.g.

visual pollution Making power locally often

requires transportinfrastructure for (more)primary energy

o Problem is shifted fromelectrical distribution grid to,for instance, gas distributiongrid!

Solution:

Multi-energyvectorapproach

Open debate onsecurity of supply

E i i


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Economic issues

Problems: Pay-back uncertain in liberalized

marketo Chaotic green and efficient power

productiono Reliability or PQ enhancement

difficult to quantify

System costso More complicated system operationo Local units offer ancillary services

System losses generally increase

Who pays for technologicaladaptations in the grid ? Who willfinance the backbone powersystem?

o Too much socialization causespublic resistance

Solution: Interdisciplinary

regulation, notonly legal

Need some realderegulation

S t l l


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System losses example

DG introductiondoes not mean

lowered losses

Optimum is 2/3power at 2/3distance

Other injections

generally causehigher system

losses

HV subst.

Load distributed along feeder cable

DG

Zero point

Af ter DG

Before DG

Power flow along cable

B l i ti i


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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 (DSM technology), also playing

on a market as mentioned in example? Boundary condition: minimize losses

H f ?


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How far can we go?

Large optimization exercise, considering thedifferent technical barriers:

Optimal proliferation, taking into account localenergetic opportunities, e.g. renewables options

Unit behavior towards grid: technology choice

control paradigm

Is the same level of reliability still desired ?

Level of introduction of new additionaltechnologies (storage, activated loads)

Optima are different, depending on stakeholder E.g. grid operator vs. client

O ti i ti l


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Optimization example

Total problem yields a huge mixed integer-continuous optimization problem Optimization goals: voltage quality penalty, minimum

losses, minimum costs Complexity: sample grid yields 240 siting options for

simple domestic CHP and PV scenario need

advanced maths Results are different hourly and vary with time of year,

e.g. during day: PV opportunities in peak hours: CHP helpful

Conclusion


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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 Special (technological) measures are necessary

E.g. in system control, mainly balancing

Not only technology push, but also customer pull

Conclusion


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Lesson 10 - Distribution Grid and Distributed Generation

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