Fault models

Fault Models

Summary

HVAC system faults modeling plays key role in various aspects of the building technology development. A collection of component and operational faults will be developed in the Modelica Buildings Library. The aim is to provide basic models of common HVAC faults in building energy simulation to address the needs for:

• Modeling a realistic HVAC system with faults
• Fault detection and diagnosis
• Training and education

Possible Implementation

The implementation should be realized using the Modelica language as it is an open-source language that is well-posed to become the de-facto standard for modeling of dynamic systems. Models formulated in this language can then be used by different modeling and simulation environments for Modelica.

Leaking economizer dampers

Description: The economizer dampers include the return air damper, the outside air damper and the relief air damper. The leaks could cause by the aged seals on the edge of the damper blade or the loose linkages between the damper and the actuator. A leaking return air damper indicates that when the system is operating at 100% outside air condition, a certain amount of return air leaks through the return air damper and mixes with the outside air in the mixing box of an AHU resulting in reduced economizing capability. A leaking outside/relief air damper indicates that when the damper is commanded to be fully close, a certain amount of outdoor air leaks through the outside/relief air damper into the AHU. There are two major consequences causing by this: (1)In cold climate, when the system is off during unoccupied hours, the cold outside air can flow into the unit through the leaking outside/relief air damper and trigger the freezestat, and (2)If the leaking area is large, the outside air intake could be more than needed during non-economizer period.

Modeling approach: The flow coefficient of the air damper in the Modelica Buildings Library is modeled as a piecewise function. It consists of three pieces: (1) for yL < y < yU, the damper characteristics is k=exp(a+b(1-y)) (y is the control signal, and y=0 means the damper is closed, and y=1 means the damper is open. yL and yU are the low and high bounds that the damper characteristics holds.), and (2)for 0 <= y <= yL and yU <= y <= 1, the damper characteristics is defined by a quadratic polynomial that matches the damper resistance at y=0 and y=yL or y=yU and y=1. The polynomials are such that k(y) is differentiable in y and the derivative is continuous.

For loose linkage, the

Reverse acting damper

Stuck damper

Leaking coil valve

Fouled coil

Stuck coil valve

Undersized coil

Sensor faults

Actual Implementation

To be done.