This component calculates the fundamental frequency (f), RMS magnitudes (V), and the phase angles (Θ) of a single-phase or a three-phase AC voltage input.
Input to this component is an electrical connection to a three-phase or a single-phase circuit. It measures the instantaneous three-phase or single-phase voltages at the input node. The measured instantaneous voltages are low-pass filtered, sampled at the specified sampling rate, and converted into per-unit, based on the specified, nominal voltage. The component then applies a Discrete Fourier Transform (DFT) to estimate the fundamental voltage magnitudes and phase angles. The frequency is estimated from the phase angle of the positive-sequence voltage (input voltage in the case of single-phase application). The frequency measurement is passed through a smoothing filter. There is an option to restrain the sudden frequency deviations that occur during transients for a specified duration.
NOTES:
It is important to properly specify the value of the nominal frequency. The nominal frequency is used to determine the initial sampling time. Thereafter, the sampling time is continuously adjusted based on the measured frequency so that the specified number of samples per cycle is maintained. If the specified nominal frequency is quite far from the actual frequency, it may take a long time for the output frequency to converge to the correct value, or the calculations may become unstable.
The phase angles cannot be accurately calculated when the input signal is very small or if the per-unit values become very small due to incorrectly specified nominal voltage. In these situations, a zero phase angle will be reported. Therefore, it is important to specify the nominal voltage correctly. Also, when the output voltage format is sequence components, the phase angles of the negative and zero sequence components reported under balanced conditions are not very reliable, as the magnitudes of these components are very small when the system is balanced.
The component is optimized to measure the power frequency signals (50 or 60 Hz), but it should be able to measure frequencies in the range 10-1000 Hz, provided that nominal frequency is correctly specified. If the component is used for measuring frequencies outside of this range, the smoothing filter time constants must be carefully specified.
There are also limitations to the frequency tracking performance when the frequency is changing at much faster than rates than those normally observed in power systems. Also, the results may be inaccurate if the harmonics in the system are too large.
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Name for Identification |
Text |
Optional text parameter for identification of the component. |
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Number of Phases |
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Choice |
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Select the type of input voltage: Three-Phase or Single-Phase. |
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Voltage Output Format |
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Choice |
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Select the format for reporting the output voltages if the input is three-phase: Phase voltages (order: A,B,C) or sequence voltages (order: positive/negative/zero). |
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Voltage Output Units |
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Choice |
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Select pu or kV as units of the output voltages. |
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Frequency Output Units |
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Choice |
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Select Hz or rad/s as units of the output frequncy. |
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Phase Output Units |
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Choice |
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Select Radians or Degrees as units of the output phase angles. |
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Nominal Voltage for Per-Unitizing (L-L) |
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REAL |
Constant |
Specify the nominal line-to-line voltage for three-phase inputs [kV]. |
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Nominal Voltage for Per-Unitizing (L-N) |
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REAL |
Constant |
Specify the nominal line-to-neutral voltage for single-phase inputs [kV]. |
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Nominal Frequency |
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REAL |
Constant |
Nominal frequency of the input signals [Hz]. |
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Anti-Aliasing Filter |
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Choice |
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Enable or Disable the anti-aliasing filter. |
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Sampling Rate (S/cycle) |
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Choice |
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Select the desired sampling rate in samples/cycle. The available options: 16, 32, 64, 96, 128, 256 samples/cycle. |
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Smoothing Filter Time Constant |
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REAL |
Constant |
Enter the desired value of the time constant for smoothing the frequency measurement [s]. |
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Transient Detection |
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Choice |
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Enable or Disable the transient detector. |
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Time to Restrain Frequency Output |
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REAL |
Constant |
Enter the desired time period for restraining the sudden deviations in the frequency after a transient is detected [s]. |