Module 4 : Voltage and Power Flow Control
Lecture 18a : HVDC converters
Objectives
In this lecture you will learn the following
AC-DC Converters used for HVDC applications.
Introduction to Voltage Source Converters.
Thyristors
In the previous lecture, we have introduced the use of controlled rectifiers (AC-DC converters) to provide
controllable field voltage to a synchronous generator. In the following lecture we shall consider their use again as a
part of of HVDC systems.
Therefore in this lecture, we revise the operation of a typical AC-DC converter used in these applications. The
treatment given in this lecture is not exhaustive, but is given to bring out some important functional characteristics
of these converters which are important from a power systems perspective. A reader is encouraged to read a power
electronics text for a rigorous analysis of AC-DC converters.
Power Electronic controllers are now very widespread and are present in computers and satellite power supplies,
drives used in traction etc. Their use in extremely high power applications is not so well known. In fact, in many
HVDC links, the total power handled ("converted") by a power electronic converter system exceeds 1 GW !
Power Electronic converters use semiconductor devices which are operated in " ON " and "OFF" states. In ON state,
the voltage across the device is negligible, while in an OFF state, current flow through it is negligible. The state of a
device is decided by the external circuit conditions and a (a low power) signal which is provided at the GATE
terminal of the device. Diodes do not have a GATE terminal and their state is wholly determined by the external
circuit (forward or reverse bias).
Currently, most power electronic converters used in power system applications, like HVDC, use a device known as a
thyristor. Thyristors are available at ratings in excess of 10 kV, 8000 A. Many thyristors can be connected in series
to yield larger equivalent voltage ratings which are required for power system applications.
An IDEAL thyristor has the following characteristics:
1. A thyristor turns ON if VAK > 0 (forward bias) and ig >0
(firing).
2. Once a thyristor is ON, it remains ON even if ig becomes 0.
3. A thyristor which is ON, switches OFF if iAK falls to 0.
A practical device has the following limitations:
1. A thyristor requires some time to turn on after the
application of the gating signal, known as turn on time, ton.
This is usually quite small.
2. Once iAK falls to 0, positive VAK should not be applied for
a duration, toff (called turn off time), otherwise the thyristor
may turn on again even without application of a gate signal !
3. A thyristor should be protected against high diAK/dt and
dvAK/dt otherwise it may damage the device.
, AC-DC converter control
Now suppose the thyristors are not fired as soon as they are forward biased, but fired after some delay.
Let us define delay angle for thyristor T1 as the angular delay from the point at which Vac becomes greater than zero.
Similarly the delay angle for the thyristors T2, T3..T6 is defined by the angular delay from the points at which Vbc, Vba,
... Vab become greater than zero respectively.
If all the thyristors are fired at a delay angle (a) of zero, then we get the waveforms as shown in the previous page.
However, if we fire all thyristors at a delay angle of 30° we get the waveform shown below.
Similarly the waveforms for a = 90° and 150° are also shown.
a = 30° a = 90° a = 150°
Lecture 18a : HVDC converters
Objectives
In this lecture you will learn the following
AC-DC Converters used for HVDC applications.
Introduction to Voltage Source Converters.
Thyristors
In the previous lecture, we have introduced the use of controlled rectifiers (AC-DC converters) to provide
controllable field voltage to a synchronous generator. In the following lecture we shall consider their use again as a
part of of HVDC systems.
Therefore in this lecture, we revise the operation of a typical AC-DC converter used in these applications. The
treatment given in this lecture is not exhaustive, but is given to bring out some important functional characteristics
of these converters which are important from a power systems perspective. A reader is encouraged to read a power
electronics text for a rigorous analysis of AC-DC converters.
Power Electronic controllers are now very widespread and are present in computers and satellite power supplies,
drives used in traction etc. Their use in extremely high power applications is not so well known. In fact, in many
HVDC links, the total power handled ("converted") by a power electronic converter system exceeds 1 GW !
Power Electronic converters use semiconductor devices which are operated in " ON " and "OFF" states. In ON state,
the voltage across the device is negligible, while in an OFF state, current flow through it is negligible. The state of a
device is decided by the external circuit conditions and a (a low power) signal which is provided at the GATE
terminal of the device. Diodes do not have a GATE terminal and their state is wholly determined by the external
circuit (forward or reverse bias).
Currently, most power electronic converters used in power system applications, like HVDC, use a device known as a
thyristor. Thyristors are available at ratings in excess of 10 kV, 8000 A. Many thyristors can be connected in series
to yield larger equivalent voltage ratings which are required for power system applications.
An IDEAL thyristor has the following characteristics:
1. A thyristor turns ON if VAK > 0 (forward bias) and ig >0
(firing).
2. Once a thyristor is ON, it remains ON even if ig becomes 0.
3. A thyristor which is ON, switches OFF if iAK falls to 0.
A practical device has the following limitations:
1. A thyristor requires some time to turn on after the
application of the gating signal, known as turn on time, ton.
This is usually quite small.
2. Once iAK falls to 0, positive VAK should not be applied for
a duration, toff (called turn off time), otherwise the thyristor
may turn on again even without application of a gate signal !
3. A thyristor should be protected against high diAK/dt and
dvAK/dt otherwise it may damage the device.
, AC-DC converter control
Now suppose the thyristors are not fired as soon as they are forward biased, but fired after some delay.
Let us define delay angle for thyristor T1 as the angular delay from the point at which Vac becomes greater than zero.
Similarly the delay angle for the thyristors T2, T3..T6 is defined by the angular delay from the points at which Vbc, Vba,
... Vab become greater than zero respectively.
If all the thyristors are fired at a delay angle (a) of zero, then we get the waveforms as shown in the previous page.
However, if we fire all thyristors at a delay angle of 30° we get the waveform shown below.
Similarly the waveforms for a = 90° and 150° are also shown.
a = 30° a = 90° a = 150°
