Presentation by Andrew L. Isaacs ( pptx , 387 kB ) - Cigre

wistfultitleElectronics - Devices

Nov 24, 2013 (3 years and 10 months ago)

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Overview


What is a weak system?


Useful metric to identify weak systems: SCR


What could possibly go wrong?


Is a weak system so bad?


What can you do?


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?


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

Slide
5

Is the network
relatively

weak or strong?


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.


𝐶
=

𝐶𝑀 𝐴
𝑖𝑛𝑑

𝑃𝑙𝑎𝑛𝑡

𝑀



Relatively Weak = Low SCR


Relatively Strong = High SCR

Slide
6

Why is low SCR a problem?


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…


Slide
8

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

-
W
ind plant will likely perform predictably

Problems… SCR < 1.5

Slide
9

Real Power

Reactive Power

Simulation
starts at red
line…

Problems… 1.5 < SCR < 2.5

Slide
10

Wind plant
starts, but trips
after fault

Wind plant starts, but
interacts with system after
fault

Control tuning, or increased SCR?

Slide
11

Wind plant recovers
from fault, but hold
your nose!

Wind plant recovers
cleanly after fault

What to do? Manage Risk!


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?


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?

Slide
14

Sync Condensers may
introduce angular
instability…

What if you have a real problem?


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!


General trend toward weak systems, reduced inertia, complex electronics


Lack of Industry Awareness


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)


Problems with SCR metric


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 impedance
-
based metric rather?

Eg
… (V*V/Z
60
)/(Plant MW) Effective Strength Ratio (ESR)? Relative Strength Ratio (RSR)?


Model Quality and Availability


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