1. Composed By: Angel Aquino, Ph.D. Presented by : Mark Laufenberg, Ph.D. WECC Joint User Group February 10-11, 2015 Colstrip Acceleration Trend Relay Model in PowerWorld Simulator angel@powerworld.com 217-384-6330 ext. 21

2. 2 ATR Model Acceleration Trend Relay (ATR) is a special relay designed to protect the customers of NorthWestern Energy from service interruptions and to prevent unstable events from disrupting the interconnected grid in Montana and Wyoming when faults and line outages occur on the Montana 500 kV transmission system. The ATR will trip Colstrip unit generators as necessary to restore the system to a stable operating condition when instability occurs –It tries to trip as few of the units as possible

3. 3 How had NorthWestern been modeling the ATR? They would run transient stability studies without the ATR modeled –They would take the rotor speed and power output of the simulation and create an input file for their ATR code to consume –Their ATR code represented the actual C source code that was used to implement the ATR 25 years ago Includes all the noise filtering Includes all the physical model to take into account the “tooth wheel” calculations This time-series information of speed and power for each of the 4 generators was then run through the ATR code –Determines if and when generators would be tripped by the ATR If the ATR would have tripped, then they would then go back and redo their transient stability simulation with tripping of particular units implemented at a particular time.

4. 4 Step #1 of this Project: Eric Bahr at NorthWestern Eric Bahr at NorthWestern Energy had to go through the ATR code and documentation and determine what it was doing Much of it was not relevant to a simulation –Noise filtering This is a simulation so there is no noise. –Tooth Wheel This is the mechanism used to directly measure the speed of the rotor Rotor speed is a state in transient stability equations so “measuring it” is not an issue. Again, this is a simulation so we don’t need to model it

5. 5 Overall Block Diagram of ATR Model

6. 6 Detailed Individual Unit Calculation Block Diagram

7. 7 Simplified Individual Unit Calculation Block Diagram Simply further to remove some very slow and some very fast time constants

8. 8 ATR Center of Mass and Look Ahead Angle Center of Mass Acceleration Trend Relay (ATR) is simply the appropriately weighted average of speed, angle, or acceleration of each online unit. Look Ahead Angle is a projected value of angle 6 cycles into the future using a constant acceleration assumption.

9. 9 ATR Trip Logic The ATR has 11 different algorithms it uses to detect instability 10 of them which use the center of mass data and 1 that uses individual unit data.

10. 10 ATR Update The ATR model has been implemented into Version 19.0 Beta of PowerWorld Simulator –The individual unit block diagrams are matching what Eric is simulating in Matlab pretty much exactly The model is being tested by Eric Bahr in NorthWestern Energy –Ensure that the tripping logic is working appropriately In the software Also to ensure NorthWestern has the right description of when and how much it trips Project has two primary deliverables –New generator model named “ATRRELAY” will be in PowerWorld Simulator Model has very few input parameters because this is all hard-coded –Complete document will be written in conjunction with NorthWestern that provides a description of all the trip logic and block diagrams in a manner that allows all other software vendors to implement this model