The necessity and challenges of modeling and coordinating microprocessor based thermal overload functions for device protection

Increased demand and economic constraints often cause the power system to be operated near its thermal limits and sometimes beyond. One way to mitigate this problem is to use the thermal overload protection functions built in to today's modern IED (Intelligent Electronic Device) relays that continuously monitor the thermal load in real time. These functions (ANSI 49) use very sophisticated algorithms that closely replicate the thermal image of the protected object (motor, transformer, feeder cable etc.). It is equally important that this function is set and coordinated properly to avoid mis-operations and equipment damage. Unfortunately, the operate time curves for these functions are often neglected in relay coordination software since modeling them is mathematically challenging. The steps required to express the curve equation in terms of prior load and a trip reference current are lengthy and complex. For transformer applications (using two time constants) the operate time must be solved iteratively. Even if a reduced single equivalent time constant is used to simplify the model there are still deviations. In addition to this, engineers often use typical time constant values since this data is not always provided from the manufacturer. As a result, time constant setting(s) may need adjustment to improve coordination. The ideal solution would be to use custom curves. However this is time consuming since points would have to be entered manually from third party tools. There's also an increased chance for error. This tutorial based paper describes what is required to overcome these modeling challenges followed by a detailed discussion on the coordination principles using motor and transformer project examples. The result, a single time constant equation derived from the first order thermal model that is easily adapted to specific relay settings (temperature, overload factor etc.). The equation, expressed in per unit of the trip reference current term, can be universally applied to any relay.