The function can be enabled and disabled with the Operation setting. The corresponding parameter values are "On" and "Off".
The operation of T1PTTR can be described using a module diagram. All the modules in the diagram are explained in the next sections.
The function uses ambient temperature which can be measured locally or remotely. Local measurement is done by the protection relay. Remote measurement uses analog GOOSE to connect AMB_TEMP input.
Max current selector
The max current selector of the function continuously checks the highest measured TRMS phase current value. The selector reports the highest value to the temperature estimator.
Temperature estimator
The final temperature rise is calculated from the highest of the three-phase currents according to the expression:
| I | the highest phase current |
| I ref | set Current reference |
| T ref | set Temperature rise |
The ambient temperature is added to the calculated final temperature rise estimation, and the ambient temperature value used in the calculation is also available in the monitored data as TEMP_AMB in degrees. If the final temperature estimation is higher than the set Maximum temperature, the START output is activated.
Current reference and Temperature rise setting values are used in the final temperature estimation together with the ambient temperature. It is suggested to set these values to the maximum steady state current allowed for the line or cable under emergency operation for a few hours per years. Current values with the corresponding conductor temperatures are given in cable manuals. These values are given for conditions such as ground temperatures, ambient air temperature, the way of cable laying and ground thermal resistivity.
Thermal counter
The actual temperature at the actual execution cycle is calculated as:
| Θ n | calculated present temperature |
| Θ n-1 | calculated temperature at previous time step |
| Θ final | calculated final temperature with actual current |
| Δt | time step between calculation of actual temperature |
| t | thermal time constant for the protected device (line or cable), set Time constant |
The actual temperature of the protected component (line or cable) is calculated by adding the ambient temperature to the calculated temperature, as shown above. The ambient temperature can be given a constant value or it can be measured. The calculated component temperature can be monitored as it is exported from the function as a real figure.
When the component temperature reaches the set alarm level Alarm value, the output signal ALARM is set. When the component temperature reaches the set trip level Maximum temperature, the OPERATE output is activated. The OPERATE signal pulse length is fixed to 100 ms.
There is also a calculation of the present time to operation with the present current. This calculation is only performed if the final temperature is calculated to be above the operation temperature:
Caused by the thermal overload protection function, there can be a lockout to reconnect the tripped circuit after operating. The lockout output BLK_CLOSE is activated at the same time when the OPERATE output is activated and is not reset until the device temperature has cooled down below the set value of the Reclose temperature setting. The Maximum temperature value must be set at least two degrees above the set value of Reclose temperature.
The time to lockout release is calculated, that is, the calculation of the cooling time to a set value. The calculated temperature can be reset to its initial value (the Initial temperature setting) via a control parameter that is located under the clear menu. This is useful during testing when secondary injected current has given a calculated false temperature level.
Here the final temperature is equal to the set or measured ambient temperature.
In some applications, the measured current can involve a number of parallel lines. This is often used for cable lines where one bay connects several parallel cables. By setting the Current multiplier parameter to the number of parallel lines (cables), the actual current on one line is used in the protection algorithm. To activate this option, the ENA_MULT input must be activated.
The ambient temperature can be directly connected through the AMB_TEMP input of T1PTTR using GOOSE connection.
The Env temperature Set setting is used to define the ambient temperature if the ambient temperature measurement value is not connected to the AMB_TEMP input. The Env temperature Set setting is also used when the ambient temperature measurement connected to T1PTTR is set to “Not in use” in the RTD function.
The temperature calculation is initiated from the value defined with the Initial temperature setting parameter. This is done in case the protection relay is powered up, the function is turned "Off" and back "On" or reset through the Clear menu. The temperature is also stored in the nonvolatile memory and restored in case the protection relay is restarted.
The thermal time constant of the protected circuit is given in seconds with the Time constant setting. Please see cable manufacturers manuals for further details.