How are sensors in the EcoStruxure Transformer Expert calibrated?

Overview

• The EcoStruxure Transformer Sensor is a factory calibrated solid state sensor.
• The EcoStruxure Transformer Expert relies on algorithms and changes in measured trends to predict the life and condition of the transformer.
• As trend analysis is the focus of EcoStruxure Transformer Expert, the algorithms are not dependent on the absolute measured temperature and moisture values. As a result, the EcoStruxure Transformer Expert does not require highly specific measurements to complete the analysis.
• This means that in-field calibration is not required, and there is no calibration interval applicable for this product.
• The EcoStruxure Transformer Sensor is offered with a standard lifetime hardware warranty while covered by a subscription.
  
  

Method of Calibration

All sensors are individually calibrated. Before measurement, the sensors are allowed to stabilize to the conditions of the laboratory for at least 48 hours. A sample calibration certificate is attached at the bottom of this article.

1. External Tank Temperature Sensor

The external tank temperature sensor has a measurement range of -40°C to 100°C with a accuracy of +/- 3°C. The external temperature sensor on the printed circuit board assembly (PCBA) is calibrated by connecting the PCBA to a jig that simulates a range of resistances that covers -34°C to 100°C. The 11 data points are used to determine a 3rd order calibration polynomial. 

2. Internal Oil Temperature Sensor

The internal oil temperature sensor has a measurement range of -30°C to 125°C with a accuracy of +/- 0.5°C at 0 to 110°C, <+/-2 °C otherwise. The internal oil temperature sensor on the printed circuit board assembly (PCBA) is calibrated by connecting the PCBA to a jig that simulates a range of resistances that correspond to a temperature range that covers of 0°C to 95°C. The 9 data points are used determine a 3rd order calibration polynomial. 

3. Moisture Sensor

The moisture sensor reports a direct relationship between capacitance and humidity and a tolerance over
the 0 to 100% relative humidity (RH) working range of <2%RH therefore a two-point calibration is used.
One of the calibration conditions corresponds to 11.31%RH and is produced by exposing the sensor to a
closed equilibrium environment above a Lithium Chloride solution at 24°C. The second calibration point
corresponds to 33.07%RH and is produced by exposing the sensor to a closed equilibrium environment
above a magnesium chloride solution at 24°C. A data point from each calibration condition is used to
calculate the gain and offset parameters.

4. Partial Discharge Sensor

The partial discharge sensor is factory calibrated from the manufacturer. In addition to this, it has an integrated calibration process to improve the precision of its Received Signal Strength Indicators (RSSI) measurements. In this process, a known RF signal is injected at the Low Noise Amplifier (LNA) input which calibrates the receiver gain. This is automatically performed during the start-up.

5. Vibration Sensor

The vibration sensor is factory calibrated from the manufacturer. The device stores trim values for sensitivity (So) and zero-g level (TyOff). During active operation, these values are downloaded into the register for use. This process repeats each time the device is turned on, resulting in no need for further calibration.