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System Strength Discussion
Shun-Hsien (Fred) Huang
Electric Reliability Council of Texas
July 16, 2014
System Strength Workshop, Austin, Texas
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Outline
• Panhandle Renewable Energy Zone (PREZ) Study
• System Strength and Voltage Stability
• Short Circuit Ratio Method and System
Enhancement
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CREZ Implementation in Panhandle
• Original CREZ plan called
for ~5.5 GW of capacity in
Panhandle, but reactive
support equipment initially
developed for ~2.4 GW
• Panhandle transmission
remote from ERCOT load
and synchronous
generation
• Stability constrained
– Most of new wind plants
are at the edge of the
Panhandle system which
exacerbates the stability
constraints
Preliminary Results
PREZ Study
• To identify system constraints and upgrades to
accommodate future wind generation projects.
• To provide a project roadmap for both ERCOT and TSPs
to accommodate additional generation resources in the
study area.
– List of potential system upgrade projects.
– Triggers for when projects will be recommended.
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PREZ Study Results
• PREZ Study Report is available at ERCOT site
– Panhandle Weak Grid Characteristics – Low Short Circuit Ratio
– Voltage Ride-Through Requirement
– System Strength Index and Criteria
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Panhandle Weak Grid Characteristics
• Lack of local synchronous
generation and no local load
lead to grid strength
challenges and to voltage
stability
• Current wind generation
development
– > 5 GW of wind capacity in the
Panhandle with signed
interconnection agreements
– > 6 GW of additional wind
generation in the
interconnection study process
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In a Weak Grid
• Actual response
High Wind Output
Low Wind Output
• Study
7
Voltage Ride-Through Requirement
• ERCOT Operating Guide 2.9.1
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Short Circuit Ratio
• SCR is commonly used to evaluate the system strength
and it is defined as the ratio of the short circuit capacity
at the bus the device is located to the MW rating of the
device.
• This method can properly represent the system strength
when the studied power plant does not interact with
other plants in the system, which is considered as a valid
assumption for a strong system.
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In a Weak Grid
• Power plants can interact with each other and oscillate
together.
• SCR calculation under weak grid can result in optimistic
results.
• There is currently no industry-standard approach to
calculate the proper SCR index for a weak system with
high penetration of wind power plants.
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Weighted Short Circuit Ratio (WSCR)
• To take into account the effect of interactions between
wind plants and give a better estimate of the system
strength, a WSCR is used in the PREZ study, defined by:
WSCR 
Weighted
S SCMVA
N
P
RMW i
i

N
N
i
i
( S SCMVAi * PRMW i) /  PRMW i
N
P
RMW i
i
N

S
SCMVAi
* PRMW i
i
Where SSCMVAi is the short
circuit capacity at bus i before
the connection of wind plant i
and PRMWi is the MW rating of
wind plant i to be connected. N
is the number of wind plants
fully interacting with each other
and i is the wind plant index.
N
( PRMW i) 2
i
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WSCR
• The WSCR method gives a conservative estimate of the
system strength and is considered as a proper index to
represent the system strength for the Panhandle region.
• Based on the assumption of full interactions between
power plants.
• A WSCR value of 1.5 is proposed to provide a
reasonable minimum level of system strength for reliable
Wind Power Plant operation.
– Operational experience
– Information from various wind turbine manufactures
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Example
• Include 6.2 GW wind generation capacity in Panhandle
and the WSCR is close to 1.0.
• The power output of all wind plants oscillated together
after a three-phase fault with 4 cycles clearing time at a
345 kV bus close to the Panhandle area.
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System Enhancement Options
Option
Dynamic
Reactive
Support
System
Strength
Cost
Synchronous
Condenser
(SC)
Static Var
Compensator (SVC)
Variable Frequency
Transformer (VFT)
√
√
√
√
$$
−
$$
√
$$$
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