In laboratory tests, the focus is usually on instruments and test devices rather than complex relay protection systems. Among these, the microcomputer relay protection tester stands out as the most critical tool.
One straightforward way to test amplitude-frequency characteristics is by using a dual-channel oscilloscope. By having the test device generate harmonic states, one phase sends a power frequency voltage (U1), while another phase sends a high-frequency voltage (Un). Ideally, the computer should set both amplitudes to be equal (U1 = Un). If the amplitude-frequency characteristics are optimal, the waveforms of the two different frequencies should appear at the same height on the oscilloscope. At a high frequency of 1000Hz, if the amplitudes match and the waveform remains smooth, this indicates a sufficient number of points in the signal source, suggesting excellent amplitude-frequency characteristics. The JJC-1H device from Jiangxi Huadong Electric Power Instrument Factory boasts 180 points per cycle, showing excellent performance at frequencies below 350Hz. However, the Beijing Witt MRT-02B, with only 40 points per cycle, uses significant resistance smoothing, resulting in poorer amplitude-frequency characteristics. At the 5th harmonic, its amplitude error exceeds 50%.
Linearity and accuracy measurements are crucial for relay protection tests, particularly at 50Hz. These can be confirmed using standard current and voltmeters. To calibrate current accuracy, the test device should be evaluated from high currents down to lower values, which helps prevent overheating in devices with limited heat capacity. Limited D/A bit counts require careful attention to small voltage accuracy, focusing on the 0.2 to 30A current range. The OMICRON CMC156 demonstrates excellent accuracy and linearity due to its A/D feedback auto-correction, though its output capacity is somewhat limited. This affects small voltages and currents, leading to greater distortion and reduced accuracy in microcomputer readings. The stepping of the output waveform significantly impacts range accuracy. Domestic test equipment typically performs well at power frequency but struggles with smaller voltages due to fewer A/D bits.
Load capacity is another vital aspect, assessed by observing the oscilloscope and load resistance during undistorted output. Testing requires adjustable resistances under 1Ω with a current capacity up to 30A. Finding such resistors is challenging; multiple reactive transformers in series can serve as alternatives. Results show Jiangxi JJC-1H maintains undistorted voltage up to 10-11V at 30A, compared to Singapore’s VENUS330's 3.8V, highlighting the former’s superior load capacity.
The capability to superimpose harmonics evaluates both accuracy and amplitude-frequency characteristics. Foreign devices excel due to their excellent amplitude-frequency characteristics, with Jiangxi JJC-1H demonstrating strong harmonic synthesis and rapid response to step waves. For non-transformer output devices, square and step wave outputs can be observed using an oscilloscope.
Magnetic susceptibility testing of differential relays checks if the test device can automatically handle results. Similarly, impedance relay action zone tests assess the ability to superimpose harmonics or automatically fit curves. Seiko relay current tests examine dynamic capabilities, analyzing transient responses. Poor transient responses lead to larger discrepancies between precise current measurements, especially at low currents. Evaluating fitting line precision and data dispersion ensures accurate results without unreasonable oscillations.
This comprehensive approach ensures thorough evaluation of test devices, enhancing reliability and accuracy in relay protection applications.
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