1. Benchmarking WECC Composite load model
Parag Mitra Sr. Engineer
Anish Gaikwad Sr. Project Manager

2. Load Model Benchmarking
Versions of software used
PSLF 19.02
PSS/E 33.9
PowerWorld 19
DSATools (TSAT) V (Release date Dec 14, 2016)
Uniform implementation across all platforms
Composite load model channels are now available in DSATools
Channel names straightforward in Powerworld

3. Test system 3 bus test system proposed by Siemens PTI
Generator and exciter use standard parameters
GENROU
IEEET1

4. Initialization of composite load model
Possibly due to leakage reactance Xa
Motor A
Motor B
Motor C
Slip P Q
PSSE
0.035
3.924
3.055
0.0195
3.347
1.962
1.673
PSLF
0.041
3.925
3.095
0.021
3.389
1.694
Powerworld
 0.041
0.021 
 3.926
 3.389
 1.962
 1.66
DSATools
3.045
0.02
3.246
1.623
Motor D
Electronic
ZIP P Q
PSSE
4.906
2.37
1.96
0.95
2.94
1.43
PSLF
4.91
Powerworld
 4.906
 2.37
DSATools
Swing Bus
Mid Bus
HV Bus/Substation bus
Internal Load Bus V θ
PSSE
1.01
1.00
-0.28
1.000
-0.84
0.999 -
PSLF
1.007
Powerworld
 1.01
DSATools  0
1.007 
 0.9986  - 

5. Performance of Motor A (Load torque=constant)
Responses are almost identical across the 4 platforms
Although similar in magnitude some fast oscillations can be seen in the TSAT output

6. Stalling of Motor A (WECC Recmd. Tripping/Reconnection)

7. Stalling of Motor A (No tripping)

8. Without trip and reconn.
Stalling of Motor A
With trip and reconn.
Without trip and reconn.
Had a discussion with Siemens. The difference is because the motor in PSSE starts up a little bit later.
This is possibly due to difference in the value of Xa between the different vendors. PSS/E assumes a different value than PSLF and others
TSAT recommends using a time step of 0.25 cycles or less for double cage IMs. Default time step is 0.5 cycles (This was causing TSAT to have numerical issues in our previous simulations)

9. Stalling/Restart of Motor A (Plots by Gary Thomann PTI)
Xa= 0.8 times sub-transient reactance
Xa= 0.6 times sub-transient reactance,
Results match PSLF closely
There is no standard documentation for this parameter

10. Reconnection of Motor B (Load torque α Speed2)
Voltage at load bus
Active power at load bus
100% motor tripping
Reactive power at load bus
Speed
PSLF does not reconnect.
TSAT does not let the motor to slow down as it did previously.
TSAT suggested using trapezoidal integration for this simulation

11. Performance of Electronic Load
The model response is identical across all the platforms in the current versions

12. Performance of ZIP Load
The model response is identical across all the platforms in the current versions

13. Low voltage performance of ZIP Load
a. Voltage at load bus
b. Active power at load bus
b. Reactive power at load bus
40% constant Z, 40% Constant I, 20% Constant P
The differences need to be sorted out
How is the low voltage behavior of the ZIP component modeled?
Is this implementation uniform across all the software platforms?

14. Active power at load bus Reactive power at load bus
Performance of Motor D
Active power at load bus
Reactive power at load bus
PSLF output is different than the other tools. The hump is missing.
Latest correspondence with Bill Price from PSLF indicates that this issue has been resolved and the updated model now matches with the other software tools

15. Motor D: Vstall and Vstallbrk
PSS/E and Powerworld both handles a Vstall = 0, replaces it with Vstllbrk
PSLF 19.0_02 has numerical issues if Vstall is small. However, if Vstall is set to 0, PSLF changes it to 0.6
TSAT generates the following message
Source: Powerworld Documentation
Problem is with Vstall and Tstall. Rstall and Xstall are ok

16. Some vendors may be doing this already (above figures from Powerworld)
EPRI suggestion
Some vendors may be doing this already (above figures from Powerworld)
Proposed change
If (Vstall<Vstallbrk)
Vstall=Vstallbrk
end
Instructions:
LMTF to supply recommended value of Vstall
To prevent stalling set Vstall as recommended value and set Tstall as 9999
Motor D does not stall
Motor moves on yellow marked path when voltage decreases
Motor moves on yellow marked path back when voltage recovers
Motor D stalls
Motor moves on yellow marked path when voltage decreases
Restartable part –yellow
Non restartable part – green (till they trip)

17.   Current status PSS®E PSLF™ Powerworld TSAT Initialization
Motor A, B, and C for a voltage dip
Motor A, B, and C during stalled condition
Tripping and reconnection capabilities of Motor A, B and C 
Electronic load
Static load
Motor D (performance with voltage dip)
Motor D (Vstall and Vstllbrk)