Abstract:
The aim of this project is to implement a Power System Stabilizer (PSS) and incorporate
it to a Generator to enhance its dynamic performance. When a generator is connected to a
power system the rotor angle oscillations due to small load changes can be observed This
is due to fact that once a generator is constructed the damping effect of it is fixed and it is
unable to offer additional damping to damp down these rotor angle oscillations. The
power system stabilizer (PSS) provides supplementary damping for low frequency rotor
angle oscillations ranging from 0.1 Hz to 3 Hz which covers local mode, Inter area mode
and inter unit mode power system oscillations. Here the PSS output is fed back to the
excitation system summing point to damp down rotor angle oscillations.//
In general, small load changes in a power system cause rotor angle oscillations. In order
to damp down these oscillations an electrical torque should be produced on the rotor
shaft, in phase with the speed deviation. Power system stabilizer adds a closed loop
auxiliary feedback signal to the reference voltage of Automatic voltage regulator (AVR)
in proportion to either speed deviation, frequency deviation or power deviation.//
Normally the terminal voltage of a generator is adjusted by the AVR after computing the
difference between reference voltage and feedback terminal voltage. In my project the
PSS is designed to operate as follows. A voltage signal in proportion to the frequency
deviation caused by small load changes is first produced. This voltage signal which
represents the frequency deviation is then added to the reference voltage of AVR. This in
turn causes an electrical torque to be added on the rotor shaft to enhance the small signal
stability performance.//
The developed PSS consists of a High pass filter (HPF), a Zero crossing detector (ZCD),
a Frequency to voltage conveitor (F/V), a Summer I and a Summer 2. The frequency
deviation is filtered by using the HPF in order to block steady changes in frequency. The
filtered signal is sent through the ZCD to maintain a constant amplitude signal to the F/V
converter because it responds to amplitude of input signal. The output of F/V converter
and the voltage corresponding to base load frequency which is injected separately using a
Calibration instrument, are summed in summer 1 to obtain a voltage deviation
corresponding to frequency deviation. The output of summer I and reference voltage of
AVR are summed in summer^ and fed back to LM723, which is the main control chip of
the AVR.//
In order to analyze the effect of PSS, following parameters were observed.
1 Output of frequency to voltage convertor
2 Generator Current
3 Induction Motor Current
Finally two MATLAB routines are used to extract actual low frequency oscillations from
observed signals.
1 A low pass filter is designed using ellip() and filter() functions and observed
signals are filtered with a cutoff frequency of 3 Hz. The outputs of the filter
clearly show the effect of PSS in enhancing the dynamic performance of the
generator.
2. Observed signals are analyzed using Fast Fourier transform technique in
MATLAB in order to observe the low frequency components.