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FACT SHEET:
FACT SHEET:
SYSTEM
SYSTEM
STRENGTH
STRENGTH
WHAT IS SYSTEM STRENGTH?
System strength is an inherent characteristic of any power
WHAT
system – itISis SYSTEM
a measure ofSTRENGTH?
the stability of a power system
under allstrength
reasonably
conditions.
System
System
is anpossible
inherentoperating
characteristic
of any power
strengthSystem
is important
as itiscan
materially
the way a
system.
strength
important
as itimpact
can materially
power system
impact
the wayoperates.
a power system operates.
System strength is usually measured by the available fault
System at
strength
usually measured
by thecircuit
available
current
a givenislocation
or by the short
ratiofault
– the
current
givencircuit
location
or byat
the
short in
circuit
ratio.with
Higher
ratio
of at
thea short
current
a point
the grid
the
fault current
levels
are
typically
a stronger and
power
current
at that
point
under
openfound
circuitinconditions
with
system,voltages.
while lower
faultfault
current
levelslevels
are representative
normal
Higher
current
are typically of
a weaker
system.
A system,
high fault
level,
or high
found
in apower
stronger
power
while
lower
faultcurrents
current
following
fault, could beofviewed
as the
generation
onhigh
the
levels
are arepresentative
a weaker
power
system. A
grid responding
strongly
tofollowing
the drop ainfault,
voltage
at the
fault
fault
level, or high
currents
could
be viewed
– trying
to restore on
thethe
situation.
Similarly astrongly
high short
circuit
as
the generation
grid responding
to the
ratio at
point in
measure
of the strength
of
drop
in avoltage
atthe
thegrid
faultis–atrying
to restore
the situation.
the response
toshort
any faults
that at
area.
Similarly
a high
circuitinratio
a point in the grid is a
measure of the strength of the response to any faults in
Faultarea.
currents vary around the grid both by location and by
that
voltage
level. The
currents
areboth
higher
areas close
to
Fault
currents
varyfault
around
the grid
byinlocation
and by
synchronous
and lower
generators.
voltage
level.generation
The fault currents
arefurther
higher from
in areas
close to
System strength
reducesand
withlower
increasing
of PEC1
synchronous
generation
furtheramounts
from generators.
connected
generation,
along
the displacement
System
strength
reduces
with with
increasing
amounts of of
PEC
synchronous
generationalong
whichwith
contribute
more to the
connected
generation,
the displacement
of
fault current. generation which contribute more to the
synchronous
fault current.
EXAMPLES OF GENERATION
SYNCHRONOUS:
Coal, gas, hydro.
POWER ELECTRONIC CONVERTER (PEC)
CONNECTED GENERATION:
Large-scale wind and solar.
WHAT ARE THE CHARACTERISTICS OF
STRONG AND WEAK SYSTEMS?
WHAT ARE THE CHARACTERISTICS OF
Power systems with a high quantity of on-line synchronous
STRONG
AND
generation and
very WEAK
little PECSYSTEMS?
connected generation
providesystems
larger fault
and are of
categorised
as strong
Power
with current
a high quantity
on-line synchronous
systems. This
is very
manifested
by connected
the ability of
the powerprovide
generation
and
little PEC
generation
systemfault
to maintain
stability
in response as
to strong
varioussystems.
types of
larger
current and
are categorised
disturbances.
This
is manifested by the ability of the power system to
maintain stability in response to various types of
Points of the power system with PEC connected generation
disturbances.
which are
distant
from
synchronous
more
Points
of the
power
system
with PECgeneration
connectedare
generation
likely to
weaker.
Low
system strength
generally
which
arebe
distant
from
synchronous
generation
are leads
more to
increased
network
system
normal
likely
to bevolatility
weaker.of
Low
systemvoltages
strengthduring
generally
leads
to
and disturbance
conditions.
Low
systemduring
strength
can also
increased
volatility
of network
voltages
system
normal
compromise
the conditions.
correct operation
of protection
and
disturbance
Low system
strengthsystems,
can also and
result in PECthe
connected
generationofsystems
disconnecting
compromise
correct operation
protection
systems,
during
disturbances.
and
result
in PEC1 connected generation systems
disconnecting during disturbances.
Some weak
weak systems
systems are
are easy
easy to
to identify,
identify, for
for example,
example, an
an
Some
isolated point
point in
in the
the system
system with
with no
no nearby
nearby synchronous
synchronous
isolated
generation. In
In other
other points
points in
in the
the system
system where
where there
there is
is
generation.
multiple concentrated
concentrated PEC
PEC connected
connected generation,
generation, weak
weak
multiple
systems can
can only
only be
be identified
identified through
through complex
complex power
power
systems
system studies
studies conducted
conducted by
by engineers
engineers using
using detailed
system
models. models.
detailed
VOLTAGE
MANAGEMENT IN
IN STRONG
STRONG
VOLTAGE MANAGEMENT
AND
AND WEAK
WEAK SYSTEMS
SYSTEMS
Strong
Strong power
power systems
systems exhibit
exhibit better
better voltage
voltage control
control in
response
to to
small
andand
large
system
disturbances.
Weak
in response
small
large
system
disturbances.
systems
are more
to voltage
instability
Weak systems
aresusceptible
more susceptible
to voltage
instability
or
or collapse.
collapse.
INCREASING CONNECTION OF PEC
GENERATION
Generation that is interfaced to the network using PEC’s
requires a minimum system strength to remain stable and
maintain continuous uninterrupted operation. Different types
of converters use different strategies to match their output
to the frequency of the system while maintaining voltage
levels and power flows. In a weak AC system, these can
fail to operate correctly through even relatively
minor disturbances.
Power electronic converters include rectifiers and inverters which convert alternating current (ac) to direct current (dc) and dc to ac, respectively.
Large scale wind generation is connected to the grid with complex converters whereas rooftop solar is connected to the grid with inverters.
1
Power electronic converters include rectifiers and inverters which converter
alternating current (ac) to direct current (dc) and dc to ac, respectively. Large
scale wind generation are connected to the grid with complex converters
whereas rooftop solar is connected to the grid with inverters.
TELEPHONE: 1300 858 724
1
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AEMO INFORMATION & SUPPORT HUB: 1300 236 600
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AEMO INFORMATION AND SUPPORT HUB: 1300 236 600
FACT SHEET:
SYSTEM STRENGTH
INCREASING CONNECTION OF
OPERATION OF PROTECTION EQUIPMENT
FACT
PEC GENERATION
IN WEAK SYSTEMS
SHEET: SYSTEM STRENGTH
Generation that is interfaced to the network using PEC’s
requires a minimum system strength to remain stable and
maintain continuous uninterrupted operation. Different types
of converters use different strategies to match their output
to the frequency of the system while maintaining voltage
levels and power flows. In a weak AC system, these can fail to
operate correctly through even relatively minor disturbances.
While weak systems are not new to system operators, they
are attracting greater attention following the rise of large
scale PEC connected generation in the power system.
Protection equipment within power systems work to clear
OPERATION OF PROTECTION EQUIPMENT
IN WEAK
faults, prevent
damage to SYSTEMS
network assets and mitigate risk
public
Protection
equipment
may be following
triggered the
While weak systems are not new to system operators, to
they
aresafety.
attracting
greater
attention
when the current following a fault exceeds the protection
rise of large scale PEC connected generation in the power
system.
activation
point, or by the impedance2 calculated from this
Protection equipment within power systems work to clear
faults,
damage
network
assetstoand
current.
Weakprevent
systems exhibit
lowerto
fault
current relative
the be
strong
networks.when
In a weak
protective
equipment
mitigate risk to public safety. Protection equipment may
triggered
thesystem,
current
following
a fault
which are2programmed to activate on measured current
exceeds the protection activation point, or by the impedance calculated from this current. Weak systems
or the ratio of measured voltage to current, could be
exhibit lower fault current relative to the strong networks.
In a weak
system,operation
protective
equipment
which
susceptible
to unintended
or failure
to operate.
are programmed to activate on measured current or the ratio of measured voltage to current, could be
susceptible to unintended operation or failure to operate.
Figure 1 – Current response from synchronous and PEC connected generation
Fault occurs
Fault occurs
Synchronous
PEC
Current
Protection activation
point
Time
2
The effective resistance in a power system to alternating current (ac).
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Electricity Market and interconnected power system in Australia’s eastern and south-eastern seaboard, and the Wholesale
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Market hubs in Adelaide, Sydney and Brisbane; the Wallumbilla Gas Supply Hub in Queensland; and the Moomba Gas
Supply Hub in South Australia. As Australia’s independent energy markets and power systems operator, AEMO provides
critical planning, forecasting and power systems security advice and services to deliver energy security for all Australians.
FURTHER INFORMATION
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www.aemo.com.au