08-10-2012, 01:09 AM
hello, my name is ahmed, iam operating engineer at gas power station. Motor bus is supplied by a Normal source but when a problem occur at this source so transfer is done to continue suppling the motor bus from auxiliary source and during this transfer, motor bus voltage is decying as shown in equation below, and want to compute the time constant (τ) of the decaying signal by this algorithm:
Voc =V e^(− t/τ)sin(ωt + θ) ---------(1)
where Voc is the open-circuit voltage at the motor bus, V is the
voltage at the terminals of the motor bus at t = 0, the instant
before interruption, τ is the open-circuit time constant of the
decay voltage of the motor bus, ω is the angular frequency
of the open-circuit voltage in radians, which is changing with
respect to the Auxiliary source, and θ is the phase angle of the motor bus in degrees with respect to the Auxiliary source.
The algorithm requires orthogonal discrete Fourier transform
(DFT) digital filters tuned to a higher harmonic not present in
the input signal. These orthogonal digital filters are referred
as the “τ estimation filters.” The input signal is sampled at a rate of
3840 Hz, a typical sampling rate for modern microprocessor
based relays. The anti-aliasing filter is designed with a cutoff
frequency of 1200 Hz. The “τ estimation filters” should be tuned
to a frequency that will not be present in the signal after passing
through the anti-aliasing filter. Hence, a frequency higher than
the cutoff frequency of the anti-aliasing filter has to be chosen
as the tuning frequency of the higher order digital filters. It
is assumed that the 1800-Hz signal (30th harmonic) will not
be present in the input signal and was chosen as the tuning
frequency of the higher order digital filters. The open-circuit
time constant is estimated by means of creating a lookup table
whose values are calculated off-line. The process of calculating
lookup table values is explained as follow:
1) Lookup Table: The look-up table is created from the outputs
of the “τ estimation filters.” The “τ estimation filters” are
given an input described by (1), with an initial magnitude of
1 pu and the time constants varying from 500 ms to 3 s in steps
of 1 ms. The ratios of the real part to imaginary part for each
time constant are stored in this lookup table. This ratio is fixed
for a given time constant, irrespective of the initial magnitude
of the decaying voltage.
finally, i want say that i built a model in simulink and have the decaying signal in equation (1) which is actual measured signal and i can see it by scope in simulink but dont know how can i use this signal to perform the algorithm.
iam waiting any reply for this algorithm and iam thankful for all.