Smoothing Reactor & Reactive power Control o HVDC System
y Prof. Bhim Singh Department of Electrical Engineering Hauz Khas, New DelhiDelhi-110016, -110016, India
[email protected],,
[email protected]
(+91)(+91) +91)-11 -1111-2659 -26592659-1045 -1045
Smoothing Reactor
Smoothing reactor is connected before the DC filter and in
It smoothin the DC current
IT also serves as a buffer between the converters and the DC line
including the reduction of ripple in the current
Smoothing Reactor
Smoothing reactor is connected before the DC filter and in
It smoothin the DC current
IT also serves as a buffer between the converters and the DC line
including the reduction of ripple in the current
Functions of Smoothing Reactor
They reduce the incidence of commutation failure in inverters cause y ps n t e vo tage at at t e co converter us
reducing the rate of rise of direct current in the bridge when the dirrect vo di volt ltaage of ano notthe herr ser eriies co conn nnec ecte ted d br briidge co coll llaaps psees
They smooth the ripple in the direct current in order to prevent
The decrease harmonics volta es and currents in the DC Line
Functions of Smoothing Reactor
They limit the crest current in the rectifier due to a short circuit on t e ne
pair operation, due to the discharge of shunt capacitance of the DC line
The last three functions are not relevant for Back-Back HVDC
Inductor value for DC transmission lines - 0.27 H to 1.5 H
Inductor value for Back-to-Back system - 12 mH to 200 mH
Smoothing Reactor
Sizing of the reactor is done not only from the above consideration
Also from the point of view of minimizing the effect of low order
It is necessar to avoid the series resonance of the DC s stem at fundamental frequency and also at the second harmonics
The inductor value must remain constant with variations in the direct current
The smoothing reactor also helps to limit the fault current in the DC line
Smoothing Reactor
It is also necessary to have to small reactor of the order of 5 to 10 , consequences of lightning strokes to the line
The advantage of having the reactor at the ground side is that it allows the converter ground faults to be cleared by converter
The insulation level of the reactor also is reduced for this location of the reactor
Location of Smoothing Reactor
Equivalent Circuit for calculating Ripple
eac ve ower
on ro
Reactive Power Control
The converters in HVDC are line commutated, which implies that the current initiation in the valve can only be delayed with re erence to t e zero cross ng o t e converter us vo tage
This results in LAGGING power factor operation of the converters
Requiring reactive power source connected at the convrter bus for better voltage control
Reactive Power Control
The reactive power source that are used vary from switched ca acitors to static VAR s stems
The operation with weak AC system can be problematic due to voltage instability and dynamic overvoltage
the converter as a static VAR compensator in addition to power conversion
Control strategies
The re ion of o eration of a converter brid e is bounded b the limits on the current and firing angle
The region is bounded by
Minimum α characteristics n mum γ c aracter st cs
Operating Region of Bridge om Pd -Qd p ane
Alternate Reactive power control c aracter st cs
There is need to modif the reactive ower characteristics of the converter station either by – Choice of the reactive power sources – Adjusting the converter control characteristics
The alternate control strategies are . 2. Constant Leading power factor characteristics
Alternate Reactive power control c aracter st cs
For constant Reactive ower
For Leading ower factor
constant
AC s stem
AC filters
Shunt Capacitors
Synchronous Condensers
Static VAR systems
-
Current Control in TCR
Variation Fundamental & THD
FC (fixed capacitor) - TCR
Thyristors Switched Capacitor
