Overview
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What is a weak system?
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What could possibly go wrong?
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Is a weak system so bad?
What can you do?
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Useful metric to identify weak systems: SCR
Risk management
Mitigation alternatives
Discussion and Questions
Slide 2
What is a strong system?
Slide 3
What is a weak system?
Slide 4
Is the network weak or strong?
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Imagine putting a fault on the system, and measuring the
current in the fault… strong systems will have lots of
current!
We calculate an MVA number based on this current, called
“Short Circuit MVA”, or SCMVA
STRONG SYSTEM = Big SCMVA
• Weak System = Little SCMVA
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Slide 5
Is the network relatively
weak or strong?
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The size of the wind farm relative to the strength of the
system is a useful metric…
We calculate a relative metric called “Short Circuit Ratio”, or
SCR. This is not a perfect metric… only a guide.
𝑆𝐶𝑀𝑉𝐴
𝑆𝐶𝑅 =
𝑊𝑖𝑛𝑑 𝑃𝑙𝑎𝑛𝑡 𝑀𝑊
Relatively Weak = Low SCR
• Relatively Strong = High SCR
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Slide 6
Why is low SCR a problem?
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If the wind plant is relatively large with respect to the
system strength (low SCR):
• Wind plant has a large capacity to affect, or move the system
• Fast power electronics require a steady voltage and frequency
to operate in a stable way
• Conventional power systems study tools may not be sufficiently
detailed to represent controls in weak systems
Slide 7
Why is low SCR a problem?
For example: for a typical Type 3 turbine manufacturer…
SCR Value
Concerns?
Less than 1.5
- Power electronics can’t maintain control… wind
plant will not run at full power.
- Conventional study tools (Transient stability) may
not run.
Less than 2.5
- Potential for control problems... Wind plant may
trip inappropriately, or interact badly with the
external network
- Conventional study tools may not be accurate
Higher than 2.5
- Wind plant will likely perform predictably
Slide 8
Problems… SCR < 1.5
Simulation
starts at red
line…
Real Power
Reactive Power
Slide 9
Problems… 1.5 < SCR < 2.5
Wind plant
starts, but trips
after fault
Wind plant starts, but
interacts with system after
fault
Slide 10
Control tuning, or increased SCR?
Wind plant recovers
from fault, but hold
your nose!
Wind plant recovers
cleanly after fault
Slide 11
What to do? Manage Risk!
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Find out SCR early in project planning. It’s an easy
calculation!
Guide interconnection study process to ensure case
lists represent potential problems
If necessary, do detailed studies. Electromagnetic
transient (EMT) tools can provide great visibility into
precise wind farm behavior. Manufacturers generally
have EMT capability, but may need some advance
notice. Avoid study delays!
Get manufacturer involved at an early stage to
optimize controls for low SCR
Slide 12
What if you have a real problem?
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If SCR is too low, and studies are showing concerns, you
can increase the SCMVA to increase the SCR.
• Synchronous condensers increase SCMVA and system inertia.
(BE CAREFUL… you can make new problems!)
• Additional network infrastructure
(More lines increase SCMVA… $$)
Slide 13
What if you have a real problem?
Sync Condensers may
introduce angular
instability…
Slide 14
What if you have a real problem?
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If new lines or sync condensers are out of the question:
• Lots of study and heroic control tuning measures
(You will make your consultants and manufacturers nervous)
• Special protection and remedial action
(Reliability standards may prevent this)
• Smaller wind farm!
(Directly increase SCR… it will work!)
Slide 15
Thank You!
Cigre B4-62 Addition:
Challenges as we see them!
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General trend toward weak systems, reduced inertia, complex electronics
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Lack of Industry Awareness
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Problems with SCR metric
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Planning departments and wind developers sometimes completely unaware
Utilities who are aware still have trouble imposing effective standards for planning studies
(lack of guidelines and standards)
Doesn’t properly account for nearby wind and power electronics (eg. STATCOMS)
Doesn’t measure detrimental effect of shunt capacitors. Could we use 60 Hz impedancebased metric rather?
Eg… (V*V/Z60)/(Plant MW) Effective Strength Ratio (ESR)? Relative Strength Ratio (RSR)?
Model Quality and Availability
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Transient stability algorithms/models may be inadequate to predict problems (Simplified
controls, protections, neglecting PLL, etc)
EMT type models are still poor quality industry wide, although getting better
Independent consultants are required (to meet NDA req’ts for EMT models)…
unacceptable for building EMT into regular planning practice.
Slide 17