Colstrip Acceleration Trend Relay
Model in PowerWorld Simulator
Composed By: Angel Aquino, Ph.D.
Presented by : Mark Laufenberg, Ph.D.
WECC Joint User Group
February 10-11, 2015
angel@powerworld.com
217-384-6330 ext. 21
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
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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.
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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
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Overall Block Diagram of ATR
Model
Unit Calculations
Unit Relaying
Unit 1
Center of Mass Calculations
Unit 1
Unit 2
Speed
Unit 2
Unit 3
Acceleration
Unit 4
Angle
Look Ahead Angle
Calculation
Trip
Logic
Trip Arbitration
Unit 3
Unit 4
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Detailed Individual Unit
Calculation Block Diagram
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Simplified Individual Unit
Calculation Block Diagram
Simply further to remove some very slow
and some very fast time constants
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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.
𝑐𝑚𝑣𝑎𝑙 =
𝑢1𝑠𝑡𝑎𝑡 ∗ 𝑢1𝑣𝑎𝑙 + 𝑢2𝑠𝑡𝑎𝑡 ∗ 𝑢2𝑣𝑎𝑙 + 𝑢3𝑠𝑡𝑎𝑡 ∗ 2.75 ∗ 𝑢3𝑣𝑎𝑙 + 𝑢4𝑠𝑡𝑎𝑡 ∗ 2.75 ∗ 𝑢4𝑣𝑎𝑙
𝑢1𝑠𝑡𝑎𝑡 + 𝑢2𝑠𝑡𝑎𝑡 + 𝑢3𝑠𝑡𝑎𝑡 ∗ 2.75 + 𝑢4𝑠𝑡𝑎𝑡 ∗ 2.75
• Look Ahead Angle is a projected value of angle 6
cycles into the future using a constant
acceleration assumption.
𝑐𝑚𝑙𝑎6𝑎𝑛𝑔 = 𝑐𝑚𝑎𝑛𝑔 + 15.0 ∗ 𝑐𝑚𝑠𝑝𝑑 + 35.971 ∗ 𝑐𝑚𝑎𝑐𝑙
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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.
Center of Mass Algorithms
OACL
OSPD
OANG
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ACAN
SPAN
Peak Storage
ACSP
ANAC
Gross Enable
TDAC
2nd Peak
Over-speed
Dynamic
Oscillation
Individual Unit
Over-speed
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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
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