【Article Interpretation】1. Power waveform distortion
GB/T15945-2008 "Power Quality Power System Frequency Deviation" promulgated by my country stipulates: "The normal deviation allowable value of the power system is ±0.2Hz.
When the system capacity is small, the deviation allowable value can be relaxed to +0.SHz." GB/T14549-1993 "Power Quality Public Grid Harmonics" promulgated by my country stipulates that the waveform distortion coefficient cannot exceed 5%.
2. The influence of power waveform distortion on measurement results
The measuring transformer works according to the principle of electromagnetic induction and is structurally composed of an iron core and a coil.
The iron core of the transformer is made of soft magnetic materials, mainly cold-rolled oriented silicon steel sheets.
Silicon steel sheets are conductive materials, which will induce eddy currents in the alternating magnetic field and generate eddy current losses and hysteresis losses, so the transformer has frequency characteristics.
Overall, the signal transmission frequency characteristics of the transformer are similar to those of a low-pass filter.
As the frequency increases, the signal output amplitude decreases and the phase shift increases. In the past decade, amorphous and microcrystalline alloy materials have been used in measuring transformers.
The magnetic permeability of this material is several times higher than that of silicon steel sheets, while the electrical conductivity is reduced by two orders of magnitude, which means that its cutoff frequency can be two orders of magnitude higher, reaching hundreds of kilohertz.
The cutoff frequency of amorphous and microcrystalline core transformers is mainly limited by the frequency characteristics of the winding itself, including the distributed capacitance and inductance of the winding.
The transformer windings used for hundreds of kilohertz need to adopt a segmented structure, and some need to add a power screen to minimize the time constant of the winding.
Transformers used for 50Hz measurement generally do not consider frequency characteristics, and the error of the transformer is only defined at the fundamental frequency.
The impact of harmonics in the test power supply on the transformer verification is mainly reflected in the error measurement.
In theory, if the frequency characteristics of the standard transformer and the transformer under test are exactly the same, most of the harmonics in the measurement circuit can be offset.
But in most cases, the accuracy level of the standard is higher than that of the transformer under test.
Their structures are largely different, and the frequency characteristics are also very different.
This makes it impossible for the harmonics in the primary circuit to be offset in the secondary circuit, while the fundamental wave is mostly offset, and the higher the accuracy, the greater the degree of fundamental wave offset.
Therefore, when calibrating a high-accuracy transformer, the fundamental wave component in the differential pressure or differential current is very small, and the harmonic component is relatively increased, which will interfere with the balance indicator or phase-sensitive rectifier circuit.
3. The influence of the neutral line voltage of the power supply to the ground on the measurement results
The power industry standard DLT 1082-2008 "Technical Conditions for High Voltage Laboratories" requires that high-voltage laboratories use isolation transformers to electrically isolate the laboratory power supply from the power grid, but transformer calibration laboratories generally do not use isolation transformers, so there may be a significant potential difference between the neutral point of the power grid and the laboratory grounding point.
The actual measurement results of some transformer laboratories show that the voltage between the neutral line of the power supply and the laboratory ground line sometimes exceeds 10V.
On the one hand, the neutral point voltage of the power supply can enter the secondary measurement circuit through the capacitive coupling with the grounding equipment and the measuring instrument, which will interfere with the measurement results.
On the other hand, the neutral point voltage of the power supply is not fixed.
After entering the secondary measurement circuit, the repeatability of the measurement results is reduced, and in severe cases, the measurement data cannot be read.
This phenomenon is more obvious when the power supply is unstable.
Therefore, JJG 313-2010 and JJG314-2010 specifically mention that when calibrating current and voltage transformers of level 0.01 and above, attention should be paid to the influence of the waveform distortion of the power supply and the neutral line voltage of the power supply on the measurement results.
When the measurement indication is found to be unstable or abnormal, a higher quality power supply should be considered.
【JJG313-2010】5.1.3 Standards
Standards include standard current transformers and current ratio standards.
The accuracy level and technical performance of the standards shall meet the following requirements: 5.1.3.1
The rated current ratio of the standard and the current transformer under test is the same, and the accuracy is at least two levels higher than the current transformer under test, and its actual error is not greater than 1/5 of the error limit of the current transformer under test.
When the standard does not meet the above conditions, a standard with one level higher accuracy than the current transformer under test can be selected, and the error introduced by the standard can be corrected according to the formula in Section 5.4.3.2.
