The function can be enabled and disabled with the Operation setting. The corresponding parameter values are "On" and "Off".
The operation of MPTTR 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
Max current selector selects the highest measured TRMS phase current and reports it to Thermal level calculator.
Internal FLC calculator
Full load current ( FLC) of the motor is defined by the manufacturer at an ambient temperature of 40°C. Special considerations are required with an application where the ambient temperature of a motor exceeds or remains below 40°C. A motor operating at a higher temperature, even if at or below rated load, can subject the motor windings to excessive temperature similar to that resulting from overload operation at normal ambient temperature. The motor rating has to be appropriately reduced for operation in such high ambient temperatures. Similarly, when the ambient temperature is considerably lower than the nominal 40°C, the motor can be slightly overloaded. For calculating thermal level it is better that the FLC values are scaled for different temperatures. The scaled currents are known as internal FLC. An internal FLC is calculated based on the ambient temperature shown in the table. The Env temperature mode setting defines whether the thermal level calculations are based on FLC or internal FLC.
When the value of the Env temperature mode setting is set to the "FLC Only" mode, no internal FLC is calculated. Instead, the FLC given in the data sheet of the manufacturer is used. When the value of the Env temperature mode setting is set to "Set Amb Temp" mode, the internal FLC is calculated based on the ambient temperature taken as an input through the Env temperature Set setting. When the Env temperature mode setting is on "Use input" mode, the internal FLC is calculated from temperature data available through resistance temperature detectors ( RTDs) using the AMB_TEMP input.
| Ambient Temperature Tamb | Internal FLC |
|---|---|
| <20°C | FLC x 1.09 |
| 20 to <40°C | FLC x (1.18 - Tamb x 0.09/20) |
| 40°C | FLC |
| >40 to 65°C | FLC x (1 –[(Tamb -40)/100]) |
| >65°C | FLC x 0.75 |
The ambient temperature is used for calculating thermal level and it is available in the monitored data view from the TEMP_AMB output. The activation of the BLOCK input does not affect the TEMP_AMB output.
- If the ambient temperature measurement value is not connected to the AMB_TEMP input in ACT.
- When the ambient temperature measurement connected to 49M is set to "Not in use" in the RTD function.
- In case of any errors or malfunctioning in the RTD output.
Thermal level calculator
The module calculates the thermal load considering the TRMS and negative-sequence currents. The heating up of the motor is determined by the square value of the load current.
However, in case of unbalanced phase currents, the negative-sequence current also causes additional heating. By deploying a protection based on both current components, abnormal heating of the motor is avoided.
| I | TRMS value of the measured max of phase currents |
| Ir | set Current reference, FLC or internal FLC |
| I2 | measured negative sequence current |
| k | set value of Overload factor |
| K 2 | set value of Negative Seq factor |
| p | set value of Weighting factor |
| t | time constant |
The equation θ B is used when the values of all the phase currents are below the overload limit, that is, k x I r. The equation θ A is used when the value of any one of the phase currents exceeds the overload limit.
During overload condition, the thermal level calculator calculates the value of θ B in background, and when the overload ends the thermal level is brought linearly from θ A to θ B with a speed of 1.66 percent per second. For the motor at standstill, that is, when the current is below the value of 0.12 x I r, the cooling is expressed as:
| θ 02 | initial thermal level when cooling begins |
The required overload factor and negative sequence current heating effect factor are set by the values of the Overload factor and Negative Seq factor settings.
| Time constant (tau) in use | Phase current |
|---|---|
| Time constant start | Any current whose value is over 2.5 x Ir |
| Time constant normal | Any current whose value is over 0.12 x Ir and all currents are below 2.5 x Ir |
| Time constant stop | All the currents whose values are below 0.12 x Ir |
The Weighting factor p setting determines the ratio of the thermal increase of the two curves θ A and θ B.
The thermal level at the power-up of the protection relay is defined by the Initial thermal Val setting.
The temperature calculation is initiated from the value defined in the Initial thermal Val setting. This is done if the protection relay is powered up or the function is turned off and back on or reset through the Clear menu.
The calculated temperature of the protected object relative to the operate level, the TEMP_RL output, is available through the monitored data view. The activation of the BLOCK input does not affect the calculated temperature.
The calculated temperature of the protected object relative to the operate level, the TEMP_RL output, is available through the monitored data view or through the output signal TEMP_RL. The activation of the BLOCK input does not affect the calculated temperature.
The thermal level at the beginning of the start-up condition of a motor and at the end of the start-up condition is available in the monitored data view at the THERMLEV_ST and THERMLEV_END outputs respectively. The activation of the BLOCK input does not have any effect on these outputs.
Alarm and tripping logic
The module generates alarm, restart inhibit and tripping signals.
When the thermal level exceeds the set value of the Alarm thermal value setting, the ALARM output is activated. Sometimes a condition arises when it becomes necessary to inhibit the restarting of a motor, for example in case of some extreme starting condition like long starting time. If the thermal content exceeds the set value of the Restart thermal val setting, the BLK_RESTART output is activated. The time for the next possible motor start-up is available through the monitored data view from the T_ENARESTART output. The T_ENARESTART output estimates the time for the BLK_RESTART deactivation considering as if the motor is stopped.
When the emergency start signal START_EMERG is set high, the thermal level is set to a value below the thermal restart inhibit level. This allows at least one motor start-up, even though the thermal level has exceeded the restart inhibit level.
When the thermal content reaches 100 percent, the OPERATE output is activated. The OPERATE output is deactivated when the value of the measured current falls below 12 percent of Current reference or the thermal content drops below 100 percent.
The activation of the BLOCK input blocks the ALARM, BLK_RESTART and OPERATE outputs.