8/12/2019 The challenge of protecting microgrids - Think Grid
n8
1/2
28 Alstom Grid///Spring-Summer 2011
Power storagefacility
Power facPhotovoltaic
Resid
Control centre
Divisionalswitch
Communications line
Power linePower receiving
equipment
In recent years, all eyes in the energybusiness have been
turning to microgrids,smaller versions of centralised systems
thatcan connect to the main grid or act aloneas an island.
Researchers are looking tomicrogrids for answers to changing
con-sumer demands and a need for increasedreliability.
Take the example of Japan. There, energysources are in short
supply, and researchersare paying particular attention to
renewableenergy, focusing on microgrid energy controland storage.
In Europe, power sources andloads are much closer to each other,
andresearchers are looking closely at intercon-necting microgrids
into smart grids. Thereis also research on microgrids in
Canada,Australia, China and many other countries,for different
power grid situations and dif-ferent demands.
The challengeof protectingmicrogrids
Microgrids are fastmultiplyingand will clearly play an
increasingly important role in the future
power supply system. However, they raise a
number of challenges, including a need fornew protection
techniques.
But why, exactly, are people so interestedin them? Clearly, a
system based on distrib-uted energy could improve reliability
andprovide service differentiation. However,connecting distributed
energy sources tothe existing networks does not magicallyprovide
the benets promised and can evenhave an adverse impact on power
quality
with, for example, power uctuations asso-ciated with renewable
energy sources.Microgrids, however, offer an efficientenergy
delivery and supply system basedon collocating distributed energy
sourcesand loads that can operate independentlyin case of outages
or energy crises.According to Dr Zhiqian Bo, China ResearchManager,
The concept of microgrids actu-ally varies from country to country,
but theyall share some common elements. Theyhave distributed energy
sources (DER),
renewable generation, bi-directional powerow, energy control,
and power electronicdevices. They offer a number of benets,making
them very smart grids. They do,however, raise a number of
challenges,among them the issue of protection.
New protection requirements
A number of questions arise when you con-nect microgrids to
conventional grids. Whenexactly, in view of the huge number of
abnor-malities a utility can experience, should youisland a
microgrid? How then, when themicrogrid is operating separately from
theutility, can you provide it with enough coor-dinated fault
protection? Once you createmultiple islands, how do you ensure the
rightselectivity to avoid unnecessary tripping?How do you ensure
the right sensitivity toavoid undetected faults or delayed
tripping?
MICROGRID SCHEME
MAIN FEATURE CHAPTER II AHEAD OF OUR TIME
8/12/2019 The challenge of protecting microgrids - Think Grid
n8
2/2
Alstom Grid///Spring-Summer 2011 2
One of the major challenges, says Dr Bo,is nding a protection
system that respondsto both main grid and microgrid faults.In
general, microgrids can operate in bothgrid-connected modeand
islanding mode.So protection func-tions are expected
to detect all types offault in the microgridfor both
operationmodes. A fast semi-conductor switch,called static switch,
is used to connect themicrogrid to the main power grid, and
thebasic approach to protection is to disconnectthe static switch
for all types of fault, includ-ing main grid faults and microgrid
faults.Most conventional feeder protectionsare based on short
circuit current sensing.
Over-current protection devices detect faultson the main power
grid, but power elec-tronic-controlled micro-sources cannotprovide
high enough levels of fault current.So, new algorithms will be
needed to detectmicrogrid faults.Furthermore, once you add sources,
energycan ow in either direction through protec-tion system sensing
devices, making it morelike a transmission line than a feeder.
Also,there are no bi-directional ows on mostradial systems.
Harmonics generated bypower electronic devices and
uncontrollable
energy sources like wind and solar createfurther challenges.To
resolve these issues, Dr Bo, says, micro-grid protection should be
independentof high-fault current, power-ow direction,unbalanced
load and plug-and-playgenerators. Relays should adapt to theenergy
source (wind, solar, etc.) change,and controls should limit
harmonics. Micro-
grid protection in thefuture will very likelyhave more
cross-con-tent with grid control,
to achieve both exi-bility and reliability.It will depend on
thelatest communicationstechnology and will
follow the development of microgrids,meaning we should see
reliable schemes inthe next ve to 10 years.There may still be many
solutions to nd,but one thing is sure: microgrids will playan
increasingly important role in the powersupply industry.
Hot water pipe
e
Public/commercial facility
Wind Biomass Fuel cell
MICROGRIDS:THE WAVE OF THE FUTURE
The concept of microgrids stemsfrom distributed energy
resources(DER), says Dr. Bo. Since the late1970s, intensive efforts
have beenmade to utilise renewable energysources, such as wind,
hydro, solarand tidal. However, the applicationof individual
distributed generatorscan cause as many problems as itmay solve. A
better way to realisethe emerging potential of
distributedgeneration is to take a system
approach that views generationand associated loads as a
subsystemor a microgrid. Microgrids are, infact, modern,
small-scale versionsof the centralised electricity system.They use
distributed energy, whichshortens the distance between powersource
and load, reducing transmissionloss. They use renewable
energysources, greatly decreasing carbonemissions. They use energy
storage,increasing power supply reliability.They are controlled by
power electronicdevices, so power sources can varysupply according
to demand.Plug-and-play power sources andloads make for a more
user-friendlygrid. And thanks to real-timemonitoring and control
technology,every disturbance on a microgridcan be detected and
adjusted.In fact, generation and loads can beisolated, keeping a
high level of servicewithout harming the grids integrity.
Dr Zhiqian Bo
M O R E
Microgrids offer
an efcient energydelivery and supplysystem.
