1. System Strength Discussion
Shun-Hsien (Fred) Huang
Electric Reliability Council of Texas
July 16, 2014
System Strength Workshop, Austin, Texas

2. Outline Panhandle Renewable Energy Zone (PREZ) Study
System Strength and Voltage Stability
Short Circuit Ratio Method and System Enhancement

3. 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

4. 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.

5. 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

6. 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

7. In a Weak Grid
Actual response
Study
High Wind Output
Low Wind Output

8. Voltage Ride-Through Requirement
ERCOT Operating Guide 2.9.1

9. 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.

10. 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.

11. 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:
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.

12. 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

13. 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.

14. System Enhancement Options
Option
Synchronous Condenser (SC)
Static Var Compensator (SVC)
Variable Frequency Transformer (VFT)
Dynamic Reactive Support √
System Strength −
Cost $$
$$$