Module 4 : Voltage and Power Flow Control
Lecture 18 : Reactive Power Characteristics of Special Devices
HVDC Terminals
You must have learnt about various kinds of AC/DC converters in a Power Electronics Course. Line
Commutated Converters (Thyristor Rectifier / Inverter Bridges) always absorb reactive power.The reactive
power absorbed is substantial and can be as much as 30-40% of the power transmitted by the HVDC link.
Usually shunt capacitors are connected to provide reactive power on the ac side of the converters.
If the thryistors are fired as soon as they
get forward biased, then we say that the
"delay angle" is zero. In such a case a
thyristor bridge is identical to a three
phase diode bridge. However, by delaying
the firing, we can control the average
voltage which appears on the DC side.
The delay angle associated with these
controlled rectifiers results in lag of the
fundamental of AC side current by
approximately the same angle as the delay
angle.
This is also true for inverter stations where
the firing angle is greater than 90 degrees
but less than 180 degrees. This is
illustrated by the figure given on the right.
You may simulate HVDC link behavior using the MATLAB/SIMULINK file (HVDC.mdl).
HVDC systems of lower ratings may also be implemented using Voltage Source Converters. These converters use
devices like Gate Turn off Thyristors which can be turned off by a gate signal. Unlike in a line commutated converter,
the voltage on the DC side is unidirectional while current can flow in either direction on the DC side. These
converters can absorb or supply reactive power independent of the real power flow in the line. As an
exercise, read about AC/DC converters in a book on Power Electronics (e.g. Power Electronics by M.H.Rashid, Prentice
Hall, 2002).
Static Var Compensators (contd..)
While the major means of control of reactive power and voltage is via the
excitation systems of synchronous generators, devices may be deployed
in a transmission network to maintain a good voltage profile in the
system.
Shunt connected devices like shunt capacitors or inductors or
synchronous condensers can be used for reactive power control. Shunt
Capacitors/Inductors may be fixed or switched (using circuit breakers).
Vernier or smooth control of reactive power is also possible by varying
effective susceptance characteristics, by use of power electronic devices.
As an example let us consider a "Static Var Compensator (SVC) " which
Lecture 18 : Reactive Power Characteristics of Special Devices
HVDC Terminals
You must have learnt about various kinds of AC/DC converters in a Power Electronics Course. Line
Commutated Converters (Thyristor Rectifier / Inverter Bridges) always absorb reactive power.The reactive
power absorbed is substantial and can be as much as 30-40% of the power transmitted by the HVDC link.
Usually shunt capacitors are connected to provide reactive power on the ac side of the converters.
If the thryistors are fired as soon as they
get forward biased, then we say that the
"delay angle" is zero. In such a case a
thyristor bridge is identical to a three
phase diode bridge. However, by delaying
the firing, we can control the average
voltage which appears on the DC side.
The delay angle associated with these
controlled rectifiers results in lag of the
fundamental of AC side current by
approximately the same angle as the delay
angle.
This is also true for inverter stations where
the firing angle is greater than 90 degrees
but less than 180 degrees. This is
illustrated by the figure given on the right.
You may simulate HVDC link behavior using the MATLAB/SIMULINK file (HVDC.mdl).
HVDC systems of lower ratings may also be implemented using Voltage Source Converters. These converters use
devices like Gate Turn off Thyristors which can be turned off by a gate signal. Unlike in a line commutated converter,
the voltage on the DC side is unidirectional while current can flow in either direction on the DC side. These
converters can absorb or supply reactive power independent of the real power flow in the line. As an
exercise, read about AC/DC converters in a book on Power Electronics (e.g. Power Electronics by M.H.Rashid, Prentice
Hall, 2002).
Static Var Compensators (contd..)
While the major means of control of reactive power and voltage is via the
excitation systems of synchronous generators, devices may be deployed
in a transmission network to maintain a good voltage profile in the
system.
Shunt connected devices like shunt capacitors or inductors or
synchronous condensers can be used for reactive power control. Shunt
Capacitors/Inductors may be fixed or switched (using circuit breakers).
Vernier or smooth control of reactive power is also possible by varying
effective susceptance characteristics, by use of power electronic devices.
As an example let us consider a "Static Var Compensator (SVC) " which
