Table of Contents
When the power analyzer is directly connected to the circuit under test, the accuracy of the power analyzer is the system accuracy.
When the measured signal goes beyond the test range of the power analyzer, a voltage sensor and/or current sensor must be used to convert the measured signal into a signal that can be measured by power analysis, and then the signal is connected to the power analyzer.
In this case, voltage sensors, current sensors, transmission lines and power analyzers together constitute a power test system, then how to evaluate the accuracy of this kind of power test system?
Power tester is a title in recent years. It generally refers to instruments and meters that mainly measure active power, commonly known as power meters or wattmeters. Traditional wattmeters mainly measure electrical parameters as voltage RMS, current RMS and active power.
In recent years, the power electronics technology represented by frequency conversion speed regulation has developed rapidly. The measured electrical signal has evolved from the traditional 50Hz sine wave to a variable frequency electric quantity with a wide fundamental frequency range, rich harmonics, and complex waveforms. Traditional power meters cannot meet the test requirements of frequency conversion power. Under this circumstance, the power analyzer was developed to meet these requirements.
The power analyzer usually needs to adapt to the test needs of variable frequency power, and requires more functions and higher technical indicators.
Since there are many parameters to be measured, and some parameters need to be processed based on digital signals such as digital filters and Fourier transforms, power testers are mostly based on AC sampling technology, and various parameters that need to be measured are acquired by AC sampling. The digital sample sequence is obtained by correlation operation. For example, the true effective value can be obtained by performing root-mean-square operation on the digital sample sequence of one signal cycle of the measured voltage, and the Fourier transform can be used to obtain the fundamental wave effective value, harmonic amplitude and phase of the measured voltage, and further operation Harmonic distortion, harmonic content, and more can be obtained.
Main Factors Affecting the Accuracy of Power Test System
We are going to explain what factors will impact the system accuracy of power testing system.
Ratio error and phase angle error of voltage sensor and current sensor
The ratio difference refers to the ratio error, and the accuracy indicators of various voltage sensors and current sensors reflect the ratio difference. For example: a voltage sensor with an accuracy of 0.2% means that its ratio difference is not greater than ±0.2% at full scale.
The phase angle error is suitable for AC signals. For AC signals, the phases of the primary input and secondary output of an ideal voltage sensor are equal. The phases of the primary input and secondary output signals of the actual voltage sensor cannot be completely equal, and this difference is the angular error.
The angular difference directly affects the power measurement error of the power test system. When the angular difference is the same, the lower the power factor, the greater the power measurement error.
For voltage transformers and current transformers, the relevant standards have strict regulations on the angular difference, for example: at 50Hz for a voltage sensor of class 0.2, the angular difference under the rated voltage does not exceed 10′.
At present, most variable frequency voltage sensors and variable frequency current sensors (such as Hall voltage sensors and Hall current sensors) have not calibrated the angular difference index. When this type of sensor is used for power measurement, the power measurement accuracy of the test system cannot be judged simply based on the accuracy indicators of the sensor and analyzer.
Sensor to Analyzer Impedance Matching
In the power test system, impedance matching mainly refers to the matching between the output impedance of the sensor and the input impedance of the analyzer.
For voltage output sensors (not necessarily voltage sensors), when the input impedance of the analyzer is much greater than the output impedance of the sensor, it is generally considered that the impedance is matched.
For current output sensors (not necessarily current sensors), when the input impedance of the analyzer is much smaller than the output impedance of the sensor, it is generally considered that the impedance is matched.
The current electronic instruments can usually meet the above conditions. However, when the sensor is a voltage transformer or a current transformer, impedance matching has its special requirements!
“JJG313-2010 Verification Regulations for Current Transformers for Measurement” and “JJG314-2010 Verification Regulations for Voltage Transformers for Measurement” point out that the secondary circuit impedance should meet the requirement that the secondary load should not be lower than 25% of the rated load. That is to say, the impedance of the voltage input channel of the analyzer is not as large as possible, and the impedance of the current input channel is not as small as possible, but it is related to the rated secondary load of the transformer used!
The input impedance of most of the power analyzers currently used does not meet the secondary load matching requirements of the transformer, which will have a certain impact on the accuracy of the transformer!
Range matching of sensor and analyzer
Suppose a power analyzer has an accuracy of 0.05% of reading + 0.05% of full scale. Accuracy is 0.1% of reading when the input signal is near full scale and 0.55% of reading when the input signal is 10% of full scale.
The range matching between the sensor and the power analyzer has a great impact on the power test system!
At present, the voltage and current channels of most power analyzers are set with multiple ranges. As long as the range is selected properly, the nominal accuracy index close to the instrument can be obtained within a wide input range.
However, when sensors are used externally, especially active sensors such as Hall voltage sensors and Hall current sensors, since the secondary output signals of active sensors are generally small, generally there are only a small number of ranges or even no ranges to match.
Assume that the above power analyzer includes 8 voltage ranges: 1000V, 600V, 300V, 150V, 100V, 50V, 30V, 15V.
In direct measurement, higher measurement accuracy can be obtained within the range of 7.5V to 1000V.
The working voltage of active sensors is usually about 24V or plus or minus 12V, the maximum output voltage is within ±10V, and the maximum AC RMS voltage is about 7V.
In this way, for the power test system, there is actually only one effective range. The accuracy of the power test system at full scale is about 0.15% of the reading. When the input signal is 10% of the sensor’s rating, the accuracy of the power test system is about 1.05%.
transmission line loss
For current signal transmission, as long as the impedance matching of the power test system is reasonable, the transmission line has no loss;
For voltage signal transmission, when the line is long or the signal frequency is high, the transmission line loss cannot be ignored.
Interference introduced by the transmission line
The transmission line is like an antenna for receiving radio waves, and it is an important intrusion path for electromagnetic interference! The degree of influence of electromagnetic interference on the accuracy of the test system mainly depends on the size of the interfering signal and the size of the useful signal, which can usually be expressed by the signal-to-noise ratio.
In order to suppress the interference introduced by the transmission line, experienced test engineers will strictly control the material, form (such as twisted pair, coaxial cable, etc.), length, shielding, and grounding of the transmission line. Add a filter device at the end or instrument end. It is worth noting that the filter device can effectively suppress infection in many cases, and it will also directly affect the accuracy of the power test system.
Accuracy Evaluation of Power Test System
The first section comprehensively analyzes various factors that affect the system accuracy of the power test system. This section evaluates the system accuracy of the power-frequency power test system with examples.
A power-frequency power test system usually consists of a transformer and a traditional power meter. The transformers and wattmeters that constitute this type of power test system follow the relevant national standards, and the five factors that affect the accuracy of the power test system in the first section are fully considered.
1. The transformer has clear ratio difference and angle difference indicators, and the power meter is divided into full power factor power meter and low power factor power meter according to the angle difference index, and the system ratio difference and angle difference can be effectively controlled;
2. The transformer has a clear rated secondary load. Generally, the transformer supporting the secondary load can be selected according to the input impedance of the power meter;
3. The secondary output of the transformer has a standard value, the general voltage is 100V, the current is 5A or 1A, and the range of the power meter can well match the output of the transformer;
4. The loss of the transmission line can be evaluated more accurately based on the line impedance and the secondary load of the power meter;
5. The output voltage of the transformer is 100V, the current is 5A, the signal is larger, and the anti-interference ability is stronger.
Assuming that the accuracy of voltage transformers, current transformers, and power meters are all 0.2, generally speaking, after completing the above five links, the accuracy of the power test system can be evaluated in one of the following three ways:
Evaluated with root of sum of squares
The accuracy of the power test system is: √(0.2^2+0.2^2+0.2^2)=√3×0.2≈0.34%;
The accuracy index of FLUKE’s NORMA power analyzer (note, it is the instrument accuracy, not the system accuracy) adopts this evaluation method;
The basis of this approach is to understand errors as accidental errors (random errors).
Evaluated with arithmetic sums
The accuracy of the power test system is: 0.2+0.2+0.2=0.6%;
The basis of this method is to understand the error as a systematic error (note: the systematic error here is relative to the accidental error, which is different from the systematic error of the power test system described in this article).
Apparently, this method is more rigorous than the first method, and the accuracy of the evaluated system is lower.
IEC Comprehensive Evaluation Method
The IEC comprehensive evaluation method is the most stringent, basically similar to the second evaluation method, and on the basis of the second evaluation method, a 0.1% line error is added, and the system accuracy of the power test system is 0.7%.
Accuracy Evaluation of Variable Frequency Power Test System
Compared with the industrial frequency power test system, the system accuracy evaluation of the variable frequency power test system is much more complicated, and in many cases, even scientific evaluation cannot be carried out! The reason is that, in a variable frequency power test system:
1. The angular difference index of the voltage sensor and the current sensor is unknown;
At present, the power sensors suitable for frequency conversion power testing are mainly Hall voltage sensors, Hall current sensors, Rogowski coils, etc. The technical indicators of these sensors generally do not include angle difference indicators. The author found that the closed-loop Hall current sensor The angle difference index is better, and the angle difference index of the Hall voltage sensor and Rogowski coil, for example, the angle difference of the 6400V Hall voltage sensor LV200-AW/2/6400 is as high as 4°, which is 0.2 class voltage transformer angle 24 times the difference (10′)!
2. There is no standard for the secondary output of the sensor, and it is difficult to match the range of the power analyzer;
3. The output signal of the sensor is small, and the electromagnetic interference on site is large, and the influence of electromagnetic interference cannot be ignored.
However, RenAn Precision solved this problem by innovatively developed RAPS Power sensor, with clear indicator for phase angle difference and its front-end digitalization and fiber transmission capability.
How to ensure the accuracy of power test system
The IEC pointed out that the errors of all instruments and measuring devices must be actually measured, without measurement, only the errors calculated from other measurements and the combination of voltage, current and power factor cannot be used as the basis for evaluating the basic error of the device.
That is to say, for a power test system, especially a variable frequency power test system composed of a variable frequency power sensor and a variable frequency power analyzer, the system accuracy cannot be obtained by simple conversion based on the accuracy of the sensor and the instrument, but the power test system composed of the sensor and the instrument must be Carry out overall traceability (overall calibration), and obtain the system accuracy of the power test system through actual testing and calibration.