Current testing technology for high-precision power testing in power electronics

    Power conversion efficiency test of power conditioner, efficiency test of inverter and motor, loss test of reactor, etc., all aspects of power electronics require high-precision power (current and voltage) test.
    This article focuses on current test technology and introduces the development technology of current sensor and power analyzer that our company has independently owned in the past.
    2. About current test method
    The current test of power analyzer is generally carried out by direct measurement method (Fig.1(a)) and current sensor method (Fig.1(b)).
    The following will introduce the characteristics of each.

    Fig.1 Direct measurement method (a) and current sensor method (b)

    2.1 Direct measurement method
    The direct measurement method is a method of directly connecting the test line of the test object to the current terminal of the power analyzer for testing.
    This method has a relatively simple test principle and has been used for a long time because the power meter itself can measure current.
    However, connecting the current test line to the current input terminal of the power analyzer and directly inputting the current into the test circuit has the following disadvantages.
    (1) During the test, the state of the test object is different from the actual operating state.
    (2) The impedance of the test line used will increase the loss.
    (3) Capacitance is generated between the wiring and between the wiring and GND, increasing high-frequency leakage.
    For example, in (2) above, a 5m long AWG6 test line is used, and the wiring impedance is about 6.5mΩ. If the current of the test object is 30A, the loss caused by the wiring impedance is 5.85W.
    It is impossible to judge whether the value of 5.85W is large or small, but considering the power value of the test object, this part of the loss cannot be ignored.
    In addition, the direct measurement method generally uses a shunt resistor to measure the current.
    This shunt resistor method has the following disadvantages.
    (1) When current passes through a shunt resistor, it will generate Joule heat proportional to the square of the current.
    If this heat is calculated as the loss of the measuring device, it is because of the self-heating that the resistance of the shunt resistor changes,
    thus affecting the accuracy of the test.
    (2) In order to suppress the occurrence of Joule heat, a shunt resistor with a smaller resistance value can be selected. However, for a shunt resistor with a smaller resistance value, the extremely small induced component cannot be ignored, which will deteriorate the frequency characteristics.
    All of these are important factors that affect the accuracy of current and power testing, and should be paid special attention to when testing large currents.

    Fig2 Self-heating of shunt resistor

    Fig. 2 illustrates the phenomenon of self-heating when a 20A current passes through a 2mΩ shunt resistor.
    For comparison, our company's 50A rated current sensor CT6862 is connected to the wiring.
    The shunt resistor is heated by Joule heat, causing the temperature to rise to about 50°C.
    On the other hand, the current sensor is not affected by Joule heat and has almost no self-heating phenomenon. The loss of the measuring instrument and
    the temperature characteristics of the sensor itself have little effect on the test accuracy.
    Through the above discussion, the direct measurement method is very effective in testing the standby power of electronic equipment and the power consumption of LED lighting, because it is less affected by the Joule heat of the shunt resistor, when testing small currents (about 1A).
    2.2 Current sensor method
    Current sensor method is a method of connecting a current sensor to the wiring of the object to be measured, and inputting the output signal (current or voltage) of the sensor into the power analyzer for current testing. When
    using the current sensor method, the state of the object to be measured is the same as the actual operating state during the test. In addition, when the current is large, the self-heating is very small, which has no effect on the accuracy of the test. The
    current sensor method is generally used in the field of power electronics.
    In Fig. 3, the range and frequency range that can be tested with high accuracy by the direct test method and the current sensor method are shown.
    It should be noted that this does not mean that the parts outside the range shown in the figure cannot be tested by each method.

    Fig.3 The current value range and frequency range that can be tested with high accuracy using the direct measurement method and the current sensor method.
    (It does not mean that the parts outside the range in the figure cannot be tested)

    3 Use current sensor method to test power with high accuracy. As mentioned above, it is generally used when the current exceeds 5A.
    The current sensor method, like the linear measurement method, is not without flaws.
    There are also several points to note in order to perform high-precision current testing.
    This section will explain the points to note when using the current sensor method to perform high-precision power testing.
    3.1 Selection of a suitable current sensor.
    The premise of using the current sensor method to perform high-precision and high-repeat power testing is to select a suitable current sensor.
    As specific selection criteria, first give the following two examples:
    (1) The rated current value of the current sensor matches the current value of the test object.
    (2) The frequency range that the current sensor can test must include all the frequency components of the test object's current. In addition to the above two points,
    (3) The test accuracy of the frequency range covered by the current sensor must exceed the requirements of the test object.
    (4) The main causes of errors such as the output interference of the current sensor, temperature characteristics, the influence of the conductor position, the influence of the external magnetic field, the influence of magnetism, the influence of the common mode voltage, etc. must be above the specified basis or even smaller.
    Therefore, when choosing a current sensor, you must be very careful. Especially with regard to (3), the accuracy of a general current sensor is specified as DC or 50/60Hz, and the characteristics of other frequency ranges are often easily overlooked. It is necessary to pay attention to the fact that in order to perform high-precision current testing using the sensor method on the power analyzer, it is necessary to pay attention to the fact that the current sensor must have sufficient performance.
    3,2 Including the overall optimization of the current sensor power test system.
    In order to perform high-precision power testing using the current sensor method, as described in the previous chapter, not only must a suitable current sensor be selected, but the power test system including the current sensor must also be optimized. In other words, no matter how high the current sensor test accuracy is, if the output signal of the sensor cannot be transmitted normally to the power analyzer, then high-precision current testing cannot be performed.

    Fig.4 General power test system.

    Fig. 4 shows a general power test system including a current sensor. As mentioned above, the current sensor has a current output signal and a voltage output signal. Generally speaking, current sensors are more widely used than voltage sensors. Here, we will discuss the use of current sensors as a premise.
    The conditions for the correct transmission of the signal of the current sensor to the power analyzer are:
    (1) The power supply of the sensor must have good power quality. The method of obtaining GND must be appropriate.
    (2) The combined capacity between the wiring and between the wiring and GND should be small, and the anti-interference ability should be strong.
    (3) The frequency characteristics of the current input part of the power analyzer are good, the heat is small, the insulation performance (high CMRR, small leakage) is high,
    and the anti-interference performance is high. The conditions that can be listed include the appropriate method of obtaining GND, etc.
    Generally speaking, the current sensor, power supply for the sensor, and power analyzer are all from different manufacturers, and the type of test line and wiring method are carried out according to the customer's request. Under such circumstances, all the above conditions are met, and the output signal of the current sensor is normally transmitted to the power analyzer, ensuring the actual high-precision test current value. For the current sensor manufacturer, the power analyzer manufacturer, and the power supply manufacturer for the sensor, it is undoubtedly very difficult.
    On the other hand, our company has been developing current sensors and power analyzers with its own technology since the past. All elements of the power test system can be completed by one company. It is also the only measuring instrument manufacturer in the world that can do all related products. Our
    company's power test system has the following characteristics.
    (1) The current sensor is a voltage output type, and the full-band accuracy listed can be calibrated.
    (2) The voltage output type current sensor is used as the exclusive current input of the power analyzer, so that the output voltage level of the sensor and the input voltage level of the current input part of the power analyzer are best matched.
    (3) The power analyzer is equipped with a power supply for the sensor. The quality of the power supply for the sensor must be unified when our company specifies the accuracy. In addition, our company's power analyzer has the same power supply and GND for the sensor, and has eliminated the important cause of the ground loop. We have also repeatedly evaluated and improved the test accuracy and repeatability.
    (4) Shielded wire is used as the sensor output line to prevent interference and adjust the gain to offset the slight voltage drop caused by the line. In addition, our company's current sensor and power analyzer combination are evaluated for test accuracy and anti-interference tests in the company and at third-party certification agencies.
    Fig. 5 also shows the anti-interference test of the power test system of our company's current sensor (CT6862, CT6863, 9709, CT6841, CT6843, 3274) and power analyzer (PW6001) conducted at a third-party certification agency. In this way, in order to make the overall system most compatible, each component is designed and the adaptability of the component to the entire system is evaluated.

    Fig.5 The scene of our company's power test system anti-interference test conducted by a third-party certification agency.

    4 Conclusion
    This article introduced the high-precision power measurement required in various aspects of the power electronics field, especially focusing on the current measurement technology. It is part of the know-how of developing current sensors and power analyzers using original technology from the past.