Building Control Systems

Assignment :

Read Chapt 3 in the TRACE User's Guide.  Scan Sections 6.4.5, 6.4.6, 6.4.8, and 6.4.9 to get a feel for the range of capabilities. 

The ability to use control blocks separates the amateur from the master analyst. This class forms a basis for the active control that you will introduce into later system models for homework and your final project.

• Basic Definitions
• Signal Variables are values of physical parameters (pressure, temperature, power, etc) that are extracted from user specified locations in the systems and provided as input to Trip and Control Block operations.
• Trips are simple tests that compare signal variables to thresholds and provide simple signals to the code (on, off, on reverse) indicating that thresholds have been crossed.
• Control blocks are in effect a programming language for the analyst. They accept output from signal variables and other control blocks as input, and perform some operation on the input to produce an output. Control block output may be used to drive component behavior such as pump speed, boundary condition properties, valve areas, and reactor power. 
  
• History and comparison of various codes
• The early development of best estimate codes (TRAC and RELAP5) was focused on analysis of Large Break Loss of Coolant Accidents. Requirements for modeling of plant control systems were relatively minimal. In TRAC this was handled with a combination of Trips and features such as rate factor tables in the component input.
• With the advent of TMI a much broader range of accidents were opened to best estimate analysis and a more complete control capability was required. The original Control system coding now seen in TRACE was developed at INEL for use in TRAC-B. Coding was picked up by LANL for use in TRAC-P and extended to include specific blocks for PI and PID controllers. The functional capabilities migrated within INEL to the RELAP5 effort, resulting in a very similar structure within that code.  TRACE captures all of the capabilities found in RELAP5, TRAC-B, and TRAC-P.
• Primary uses of trips and control systems
• Simulating real reactor control systems
• Remember the implementation in the code is digital not analog as are most real reactor control systems. Also keep in mind the digital sampling period is the time step for the integration of the flow and heat transfer equations. When this is too large it may introduce control instabilities that are not present in the real control system.
• Functionality of the reactor control system should be reproduced but usually you should not attempt to replicate the elements of a control system on a block by block basis as provided in a control system diagram.
• Driving the system to a steady state in preparation for transient analysis.
• Sometimes this involves a plant control system
• Usually it's a user created or code default control unrelated to a real control system
• Performing simple sanity checks such as giving pressure drops or calculating energy balances.
• Provide desirable indirect data such as enthalpies or integrated mass flows (can be plotted).
• Provide simulation of actual instrumentation response. Random number generators can simulate signal noise. Offsets and delays in the response of instruments to physical conditions can also be modeled.
• Provides a method for extracting key numbers in English rather than SI units.
• Control blocks are basically a programming language (unfortunately approximately at the assembly code level) available to the user to customize certain aspects of the code. Special control block functions are not required in the French code CATHARE, since the entire input is effectively a programming language.
• Control block Specifications
• Note alphabetic ordering, in the documentation.
• Two Broad classes of functions
• Actual Logic Circuits and elements in control circuits
• Useful functions and operations.
• The SNAP Model Editor uses a finer classification of control blocks:
• Arithmetic Blocks
• Calculus Blocks
• Controller Blocks
• Logical Blocks
• Manipulation Blocks
• Time Related Blocks
• Trignometric Blocks
  
• The General Rule is not to build detailed controller. Build a functional equivalent (diagrams)
• Capabilities are reproduced well in TRACE and RELAP5. RETRAN is much more limited.
• Special Steady State control included in TRACE
• Adjust pump speed to get desired flow.
  
• Adjust valve area to get desired upstream pressure or desired mass flow
  
• Adjust heat transfer area to give desired  temperature or volume fraction at the outlet of a heat exchanger

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