Download CCVT in Power Line Communication

CCVT in Power Line Communication
CCVT is also an economical choice when the transmission line is used for power line
communication (refer Fig 8.6). High frequency RF signals can be coupled to the power
line for communication. Filtering of this RF signal is carried out by a parallel R-L-C
circuit which is also known as tuning pack. Normally, the frequency range of this RF
signal is 50 kHz-400 kHz. At this frequency the drainage reactor offers a high impedance
block to the RF signal; while for power frequency (50 Hz) it appears as a path to ground.
The high inductance of the reactor and the transformer provides a high impedance path
for the RF signal. Hence it prevents any leakage of RF signal into the transformer output
at 50Hz.
Ferro resonance Problem in CCVT
The iron cores of the reactor and transformer will not only introduce copper and core
losses but also can produce ferroresonance caused by the nonlinearity of the iron cores.
Hence a ferroresonance suppression circuit is also included in the secondary of the
transformer. The dangerous overvoltages caused by ferroresonance are eliminated by this
circuit. But it can aggravate CCVT transients.
Transient Response of CCVT
As seen in the fig8. CCVT equivalent circuit is a R-L-C circuit. If transformer is
considered ideal, it can be described by second order differential equation of the type,
1 t
di
V(t)=Ri+1/C ∫idt +L di/dt. V (t )  Ri   idt  L

C
dt
Thus, any transient including faults in the voltage signal will not be faithfully reproduced
by the CCVT. The response depends upon the damping α and ω and point on the voltage
waveform where the fault strikes. Fig 8.7 and 8.8 shows the possible subsistence transient
of CCVT. It can be seen that such a response can reduce the accuracy of distance relays.
Classification of CCVTs
CCVTs can be classified into two.
1. Class1 type
2. Class2 type
Table 8.1 shows the maximum limit for the ratio and phase angle errors. It can be seen
that errors of Class 2 type are double than that of class1 type.
VT Class
Maximum ratio error
Maximum phase angle error
Class 1
1%
40min
Class 2
2%
80min