Function of TCR
- The TCR not only controlled reactive power but also reduces total harmonic distortion ( THD ) as well as individual harmonic voltage.
- The single phase TCR branch consists of two anti parallel SCRs is connected in series with the air core inductor as shown in the Figure.
- The two anti parallel SCR is act as bidirectional switch. The SCR 1 conducts for positive half cycle and SCR 2 conducts for negative half cycle of the alternating supply respectively.
Firing angle control
- The firing angle control of TCR is done from angle 90 degree to 180 degree. When, the firing angle of SCR is taken as 90 degree, continuous sinusoidal current flow through TCR.
- The SCR turns off at natural zero voltage therefore it is called as line commutation
- The current reduces zero at firing angle 180 degree. When the firing angle sets at less than 90 degree, it introduces DC components in the current.
- Let supply voltage
V(s) =
Vm Sin ωt
Where Vm = Maximum supply voltage and
ω
= Angular frequency of the supply voltage
- The TCR current is
L ( di / dt ) –V( s )=0
Where L= Inductor of the TCR branch
By integrating above equation we get,
I ( t
) = 1 / L ∫ V( s ) dt + C
I ( t
) = 1 / L ∫ Vm Sin ωt dt + C
I ( t
) = – ( Vm / ωL ) Cos ωt +
C……. ( 1 )
Where C = Constant of integration
Current I = 0 for firing angle α = 0 degree
0 = – ( Vm / ωL ) Cos ωt + C
Therefore C = (
Vm / ωL ) Cos α……….. ( 2 )
From equation ( 1 ) and ( 2 )
I ( t ) = – (
Vm / ωL ) Cos ωt + ( Vm / ωL ) Cos α
I ( t ) = ( Vm
/ ωL )( Cos α – Cos ωt )
The TCR instantaneous current is given by
I( t
) = –V / ωL ( cos α – cos ωL )
- The current waveform is non – sinusoidal and it contains odd harmonic. These odd harmonics are function of conduction angle σ.
The fundamental component of TCR current is given by
I1(
σ ) = VBTCR( σ )
Where σ = Conduction Angle
V =
RMS Voltage
BTCR(σ )
= Susceptance of the TCR branch
BTCR(σ ) = ( σ – Sin σ ) / π ωL
The conduction angle σ = 2 ( π – α )
- The triple harmonic is eliminated on the line side when the TCR is connected in delta. The harmonic line current is of the order
n = 6k ± 1
Where k = Integer
- Some fixed capacitors are connected as series tuned filters in order to limit harmonic entering in the system.
- Use of twelve pulse configuration of TCR in which two sets of secondary winding of step down transformer is used. One set of secondary winding is connected in delta and other in star and in that case, the line current in the primary is in the order of
n = 12k ± 1
Where
k = Integer
- The TCR branches cancel generated harmonic of each other in the primary side.
- To connect TCR into two or more parallel connected branches and operate all branches except one branch as Thyristor switched reactor ( TSR ). The TSR do not contribute any harmonics.
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