Frequency difference between two sources is shown in Fig.
4. As can be seen from diagram, the frequency difference is
not constant in the time of simulation especially after 1 s.
Consequently, period of oscillation cannot be constant.
By having an accurate look at oscillations in effect of Ferro-
resonance, it is obviously cleared that there are major
differences between a typical oscillations in effect of pole
slipping in generators and oscillations, which the network
experiences in effect of Ferroresonance in this simulation. As
it is shown in Fig. 5, in case of usual pole slipping in
generators envelopes of voltage swing leads the envelope of
current swings by 180
°
or vice versa. Therefore, the envelope
of voltage swing gets zero value reciprocally; envelope of
current swing gets maximum value at the same time. Whereas,
envelopes of voltage and current swing in Ferroresonance in
this simulation are in the same phase (Fig. 2). In another
difference, the magnitudes of voltage and current in out of step
condition traverse alternatively in both negative and positive
side of magnitude axis. They are changed symmetrically so
that the same envelope of swing is formed in both sides in
each period of swing. Whereas, envelope of swing in
Ferroresonance is formed only in negative or positive side
sequentially. In addition, in power swing in effect of out of
step condition, waveform of voltage and current are not
misshaped. Consequently, oscillations in effect of Ferro-
resonance in most cases can be distinguished with power
swing in case of pole slipping in the network. It must be noted
that, the mentioned differences are only obtained in this
simulation and compared with a typical pole slipping.
However, in some cases, the voltage and current swings in
effect of pole slipping in generator get different status
according to dynamics of generator and power system.