Calibration Procedure to Test the Effects of Multiple Influence Quantities on Low-Power Voltage Transformers
Abstract
:1. Introduction
2. Low-Power Voltage Transformers
3. Calibration Procedure
3.1. Frequency Tests
3.2. External Field Tests
3.3. Temperature Tests
3.4. Combined Tests
- For the three frequencies described in Section 3.1 (49.5 Hz, 50 Hz, and 50.5 Hz);
- For the three frequencies when the electric field is acting as described in Section 3.2.
- Temperature + electric field. For the 11 measurement points of the thermal cycle, and are computed when the electric field acts on the LPVT (22 tests of 100 measurements each).
- Temperature + frequency. For the 11 measurement points of the thermal cycles, and are computed for the three frequencies of interest (33 tests of 100 measurements each).
- Frequency + electric field. For the three frequencies of interest, and are computed when the electric field is acting on the LPVT (6 tests of 100 measurements each).
4. Experimental Case Study
4.1. Introduction
4.2. Measurement Setup
- An Agilent 6813B power source. It provided the sinusoidal voltage, at the desired frequency, to the step-up transformer. Between the power source and the step-up transformer an isolating transformer with ratio 1:1 was placed to electrically separate the low and the medium voltage.
- The 0.1/15 kV step-up transformer. It took the low voltage (LV) from the isolating transformer and provided the rated voltage for the LPVTs, which for all of them was kV.
- A TVM-24-1C reference voltage transformer. It featured a selectable load of 0.25 or 1 VA, an accuracy class of 0.1, and nominal ratio of 20/0.1 kV. The transformer wa under metrological confirmation and was used as a reference for the accuracy performance evaluation of the three LPVTs under test. The transformer was used in series to a 11.0024:1 resistive divider to fit with the acquisition system adopted for the tests. The resistive divider was characterized by means of a Fluke 6105a Calibrator; from the results it emerged that the divider introduces a negligible phase displacement (compared to the quantity measured) and that it has a ratio of 11.0024 with a standard deviation of the mean of .
- A thermostatic chamber. Used for the temperature vs. accuracy tests, it was possible to vary its temperature in the range 5–70 °C.
- An acquisition system consisting of a NI-cDAQ 9174 and one NI-9239 data acquisition board (DAQ). The NI-9239 board is a 24-bit acquisition system with four channels at V input, 50 kSa/s per channel of maximum acquisition rate, and gain and offset errors of and , respectively.
- The three LPVTs under test. As mentioned above, they are referred to as A, B, and C and their main characteristics are summarized in Table 1. In particular, the table contains the type of each LPVT, their accuracy class (AC), and their rated primary and secondary voltage values, and , respectively. Note, that all LPVTs have the same accuracy class; hence, during the results evaluation a direct comparison is possible. Furthermore, the three LPVTs share very similar that fits with the full scale of the adopted acquisition board.
4.3. Experimental Tests and Results
4.3.1. Tests vs. Frequency
4.3.2. Tests vs. Electric Field
4.3.3. Tests vs. Temperature
- Technical limits of the thermostatic chamber adopted;
- Difficulties in the market to find thermostatic/climatic chambers with sufficient space to contain the MV equipment and to guarantee the distances for the electrical safety;
- A test performed at 5 °C, which is the average winter temperature in Italy [47], would have provided useful information: (i) what happens to Italian LPVTs working at their lowest average temperature; (ii) if anomalous behavior is reported at 5 °C, then at −5 °C the situation is even worse.
- As expected, the tested frequency values do not affect the LPVTs’ accuracy;
- The electric field instead, is a more disturbing quantity that influences the capacitive LPVT more, varying its up to the allowed limits. However, none of the and of the tested LPVTs exceeded the accuracy limits fixed by the accuracy class.
- The air temperature appears to be a really stressing influence quantity for all the tested LPVTs, no matter the diverse technologies used to manufacture them. Furthermore, the high temperature is critical for LPVTs B and C, while the low temperature is dangerous for A. This resulted in exceeding the accuracy limits in the different conditions mentioned, hence not guaranteeing the correct operation of the devices. Such a conclusion affects both the DSOs and the final costumers, depending on the sign of the and variations. In other words, the sign of the variation can make the DSOs and the customers save or pay money that they do not deserve.
4.3.4. Tests vs. Two Influence Quantities
- Overall, it was demonstrated in Section 4.3.1 that the frequency does not significantly affect the performance of the LPVTs; this behavior is confirmed also when such quantity is combined either with electric field or temperature (and for both and ). In fact, these last two influence quantities are far more critical than the frequency;
- Temperature has emerged as the main critical quantity. However, its combination with the electric field resulted in two main behaviors. One, where and worsened or were not affected at all at ambient or high temperatures. Another, where and improved by the presence of an electric field superimposed to the temperature effect, at low temperature. Furthermore, the beneficial effect of the electric field at 5 °C resulted in returned within the accuracy limits when they exceeded the effect of the temperature.
4.3.5. Tests vs. All Three Influence Quantities
5. Conclusions
Author Contributions
Funding
Conflicts of Interest
References
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LPVT | Type | AC | ||
---|---|---|---|---|
A | Resistive | 0.5 | ||
B | Capacitive | 1.35 | 0.5 | |
C | Active | 1 | 0.5 |
LPVT | [%] | |||
---|---|---|---|---|
A | −0.0083 | 0.1 | −0.001 | 0.002 |
B | −0.1047 | 0.1 | 0.000 | 0.002 |
C | −0.0258 | 0.1 | 0.000 | 0.002 |
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Mingotti, A.; Peretto, L.; Tinarelli, R. Calibration Procedure to Test the Effects of Multiple Influence Quantities on Low-Power Voltage Transformers. Sensors 2020, 20, 1172. https://doi.org/10.3390/s20041172
Mingotti A, Peretto L, Tinarelli R. Calibration Procedure to Test the Effects of Multiple Influence Quantities on Low-Power Voltage Transformers. Sensors. 2020; 20(4):1172. https://doi.org/10.3390/s20041172
Chicago/Turabian StyleMingotti, Alessandro, Lorenzo Peretto, and Roberto Tinarelli. 2020. "Calibration Procedure to Test the Effects of Multiple Influence Quantities on Low-Power Voltage Transformers" Sensors 20, no. 4: 1172. https://doi.org/10.3390/s20041172
APA StyleMingotti, A., Peretto, L., & Tinarelli, R. (2020). Calibration Procedure to Test the Effects of Multiple Influence Quantities on Low-Power Voltage Transformers. Sensors, 20(4), 1172. https://doi.org/10.3390/s20041172